Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study

Giovanni Ciampi; Yorgos Spanodimitriou; Michelangelo Scorpio; Antonio Rosato; Sergio Sibilio

doi:10.3390/buildings11040141

Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study

Ciampi, Giovanni;Spanodimitriou, Yorgos;Scorpio, Michelangelo;Rosato, Antonio;Sibilio, Sergio 2021-04-01 00:00:00 buildings Article Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study Giovanni Ciampi * , Yorgos Spanodimitriou * , Michelangelo Scorpio , Antonio Rosato and Sergio Sibilio * Department of Architecture and Industrial Design, University of Campania Luigi Vanvitelli, 81031 Aversa, Italy; michelangelo.scorpio@unicampania.it (M.S.); antonio.rosato@unicampania.it (A.R.) * Correspondence: giovanni.ciampi@unicampania.it (G.C.); yorgos.spanodimitriou@unicampania.it (Y.S.); sergio.sibilio@unicampania.it (S.S.) Abstract: Plastic materials are increasingly becoming used in the building envelope, despite a lack of investigation on their effects. In this work, an extruded Acrylonitrile-Butadiene-Styrene panel has been tested as a second-skin layer in a ventilated facade system using a full-scale facility. The experimental results show that it is possible to achieve performances very similar to conventional materials. A numerical model has then been developed and used to investigate the performances of plastic and composite polymer panels as second-skin layers. The experimental data has been Citation: Ciampi, G.; used to verify the behavior of the numerical model, from a thermal point of view, showing good Spanodimitriou, Y.; Scorpio, M.; reliability, with a root mean square error lower than 0.40 C. This model has then been applied Rosato, A.; Sibilio, S. Energy in different refurbishment cases upon varying: the polymer and the manufacturing technology Performances Assessment of (extruded or 3D-printed panels). Eight refurbishment case studies have been carried out on a typical Extruded and 3D Printed Polymers ofﬁce building located in Napoli (Italy), by means of a dynamic simulation software. The simulation Integrated into Building Envelopes results show that the proposed actions allow the reduction of the thermal and cooling energy demand for a South Italian Case Study. (up to 6.9% and 3.1%, respectively), as well as the non-renewable primary energy consumption (up Buildings 2021, 11, 141. https:// to 2.6%), in comparison to the reference case study. doi.org/10.3390/buildings11040141 Keywords: ventilated facade; second-skin materials; 3D printed materials; additive manufacturing; Academic Editors: Alessandro Cannavale, TRNSYS; full-scale facility; retroﬁt action; energy saving Francesco Martellotta and Francesco Fiorito Received: 18 February 2021 1. Introduction Accepted: 25 March 2021 Approximately 40% of the EU energy consumption can be directly attributed to Published: 1 April 2021 the building sector, which is also responsible for about 36% of the greenhouse gas emis- sions [1,2]. In addition, in the EU-28, only 3% of the ediﬁces have an efﬁcient building Publisher’s Note: MDPI stays neutral envelope [3], mainly due to the fact that about 35% of the EU’s buildings are over 50 years with regard to jurisdictional claims in old and only around 1% of them are renovated each year [1]. Certainly, the constraints published maps and institutional afﬁl- associated with the new buildings are fewer with respect to those associated with the iations. refurbishment of existing constructions, so the new one allows for better-optimized de- sign in terms of energy efﬁciency of the envelope [4]. However, in Italy, many buildings (about 4 million) were built in the early 1900s and about half of these have been classiﬁed as historical architectures and nowadays have been reused [5]. Therefore, in the Italian Copyright: © 2021 by the authors. scenario, the improvement of the energetic performances of the existing building enve- Licensee MDPI, Basel, Switzerland. lope represents a crucial aspect in the increasing of the building’s energy efﬁciency and This article is an open access article the indoor environmental quality on a large-scale [6]. In this context, different products distributed under the terms and and systems have been proposed to improve the buildings energy efﬁciency, visual and conditions of the Creative Commons thermal comfort, as well as their sustainability [7–12] and, in recent years, the interest of Attribution (CC BY) license (https:// the scientiﬁc community has seen an increase in the facade domain to improve the overall creativecommons.org/licenses/by/ building energy efﬁciency [13]. In particular, the use of passive systems is raising more 4.0/). Buildings 2021, 11, 141. https://doi.org/10.3390/buildings11040141 https://www.mdpi.com/journal/buildings Buildings 2021, 11, 141 2 of 27 and more interest in the building sector [7,8,14]. A passive building is one in which the indoor environment is not regulated by using mechanical heating and cooling systems, but by means of a conscious structure and architectural design of the envelope and its components [7]. In recent years, as part of a shift towards more energy-efﬁcient buildings, a lot of different new facade technologies and solutions have been proposed for the im- provement of their energy performance by the introduction of better insulation, shading devices, as well as a second-skin layer (double-skin facades [15,16], building integrated photovoltaic [17,18] and opaque ventilated facades [19,20]). Among these, the double-skin facade (DSF) and opaque ventilated facades (OVF) have been suggested as one of the best solutions, thanks to their ability to ensure better thermal performance and indoor environmental quality, as well as to improve the aesthetic appeal of buildings [15,16,19,20]. The concept of DSF was introduced in the early 1900s, but little progress was made until the 1990s [7,21]; it consists of a standard facade, an air cavity, and an additional external skin. The material used as a second-skin is usually glass [7,21]; however, a shading device can also be installed within the cavity between the two layers of the facade to control the solar radiation [7,21]. The OVFs are passive systems that consist of multiple layer construction (external second-skin, an intermediate air cavity, and an internal wall). The OVFs have been, more and more frequently, chosen by contractors, designers as well as architects for different typologies (ofﬁces, schools, residential) of new and renovated buildings as well as in different climates [19,20]. Several papers [22–29] investigated the energy performances of OVFs through simulation software (EnergyPlus [30] and TRNSYS [31], the most widely used), highlighting the beneﬁts achievable by these systems. In addition, the literature review [22–29] highlights that the materials usually used as second-skin layer are: glass, porcelain stoneware tile, natural stone, aluminum, OSB, and composite panels. Nowadays, there are always more innovative materials [32–35] used in architecture, also as a second-skin layer, even if evaluating their impact on the envelope’s energy performance is a complex task [6,11]. In this work, plastic and composite polymers have been investigated with the aim to evaluate their integration in the building facade and their potential beneﬁts achievable in refurbishment case studies. 1.1. Plastic and Composite Polymers in Building Facades In recent years, the use of polymers in building and engineering has increased substan- tially, thanks to their: (i) ease of production, (ii) ease of installation, (iii) durability, (iv) low maintenance requirements, (v) lightweight nature and (vi) ability to be formed into complex shapes [34,35]. Moreover, polymers form good thermal and electrical insulators that are not affected by chemical and biological risks [35]. They have not only been used to replace the traditional construction materials (cement, brick, concrete, wood, metal, and glass), but these materials have also been used in a complementary way to improve the building enve- lope performance to satisfy the modern demands of both new projects and refurbishment ones [32–35]. From an aesthetic point of view, these materials are available in several colors and texture alternatives. Several applications of plastic and composite polymer walls in buildings were reported in the literature [33]. In [33], 23 examples of architecture were re- ported, demonstrating that plastic and composite polymers can be used in dwelling domes, large-span volumes or envelop large facade surfaces, and transparent sky-lights on the roofs of industrial buildings. Another example can be found in [36,37], where the designers have realized a temporary pavilion with an envelope made of double-walled transparent corrugated sheets of PolyEthylene Terephthalate (PET) recycled plastic. Similarly, in [38], a massive pavilion, designed as an exhibition hall for the 2010 Taipei International Flora Expo, has been built using recycled PET for the building envelope, also proving strong resistance to ﬁres and earthquakes. As reported in [39], polycarbonate multi-sheet systems are increasing their share of the glazing market since they provide good performance while weighing and costing signiﬁcantly less than glass. For these reasons, several studies have been conducted to assess their characteristics upon varying geometries and installation typology. In [39], the authors assessed the thermal and optical characteristics of different Buildings 2021, 11, 141 3 of 27 polycarbonate sheets, highlighting a strong angular dependence in polycarbonate panels’ optical properties, signiﬁcantly different to the conventional multiple-layered glass, and a good overall thermal behavior. For these reasons, the polycarbonate could be used as a valid alternative, fulﬁlling the energy requirements and improving the visual comfort, reducing the glaring problem by diffusing the light, while providing more ﬂexibility in the design and appearance of the buildings. In [40], a detailed experimental analysis of the thermal behavior of different polycarbonate multi-sheet systems has been carried out by varying the installation angle. The results highlighted a very low incidence of the angle of inclination on the equivalent value of thermal conductivity, thus allowing the material to be equally used in every part of the building envelope. Thus, polycarbonate has already found large usage, as in the Bavaria Brewery Tocancipá headquarters, by Construcciones Planiﬁcadas [41], where the plastic has been used to get an industrial appearance while providing for thermal and visual comfort, or the property registration ofﬁces, by Irisarri + Piñera, where polycarbonate has been used to complement and balance the appearance of the existing structure. Across these materials already implemented in the traditional architectural paradigms, there is also a strong boost in the usage of plastics from the additive manufacturing (AM) sector, as several plastic products can also be utilized in AM processes, providing great freedom of form and enhancing designers’, architects’, and engineers’ freedom in creating complex designs [42–46]. In addition, the global 3D printing ﬁlament material market volume was 1.8 billion US$ in 2019, growing at a compound annual growth rate (CAGR) of 27% [47]. The most popular ﬁlament materials are PolyLactic Acid (PLA) and Acrylonitrile-Butadiene-Styrene (ABS), holding about 47% and 29% of the market, respectively. In contrast, materials as PolyEthylene Terephthalate Glycol-modiﬁed (PETG) can be useful alternatives, despite not being as popular, providing similar mechanical properties while also offering excellent recyclability and scalability [47]. The AM in the facade industry presents new signiﬁcant potential and requires relevant research to be conducted [46]. Indeed, more and more 3D printing materials are concurrently becoming utilized in contemporary architecture design [42–46], thanks to the lightness and effortless installation procedure, which results in a design solution useful for both new projects and refurbishment ones [44–46]. The 3D printing technology has been often used to create everything, from prototypes [42,43,46] and simple parts of facades, to give a distinctive signature to the constructions [44,45]. The 3D printing materials prove themselves to offer quite unique characteristics from an architectural and economic point of view [43–46]. Several scientiﬁc papers have been conducted into the loadbearing capacities and/or other essential qualities of AM products for the building industry, such as durability, water vapor diffusion resistance, thermal conductivity, or ﬁre-resistance [48–51]. The authors emphasized the potential applications of additive manufacturing to build honeycomb panels that optimize mechanical properties and heat transfer [48–51]. However, scientiﬁc research related to 3D printing materials in building energy efﬁciency applications is limited due to its relatively new nature as a technology. Only Sarakinioti et al. [46] aimed their research at developing an integrated 3D printed for thermal insulation and building physics. In particular, they presented a 3D printed facade panel design for thermal insulation and movable liquid heat storage [46], providing an overview of the development process. The authors tested the prototype and, at the same time, simulated to verify the thermal effects of the proposed facade system on indoor spaces in different climates. The simulation results reported in [46] showed the potential of the proposed 3D printed facade panel for reducing heating and cooling energy demand. Therefore, the effects of adopting 3D printing materials as a second-skin layer on the indoor environment have been scarcely investigated. Moreover, there is a lack of experimental testing and numerical model development of these materials in building simulation, even more, if considered in a second-skin in front of the building envelope, in a 3D printed composite facade arrangement. Indeed, in a facade arrangement realized with these innovative materials (ABS, PLA, PETG, etc.), the difﬁculties lie in predicting the behavior of the various facade sections, as the second-skin Buildings 2021, 11, 141 4 of 27 layer, the resulting air cavity, and ﬁnally, the effects on the indoor environment. Therefore, from an experimental point of view, standardization bodies, experts, and researchers are continually developing new methodologies or new procedures to correctly calculate the performances of these envelope components in simple and economical ways [6]. 1.2. Research Aims In this work, extruded ABS panels have been tested as second-skin materials in order to verify their performances in an OVF system. This novel material for building envelopes has been investigated through in-situ measurements by using two outdoor comparative test cells. The experimental data have been used to calibrate and validate a numerical model in TRNSYS 18 [31], also verifying the ability of the simulation software to effectively reproduce the behavior of a light material in an OVF system, which usually is made of materials as porcelain gres. Then, the validated numerical methodology has been used to implement different plastic materials in a set of refurbishment case studies, compared to a reference ofﬁce building, in order to assess the potential beneﬁts. The comparison has been performed in terms of (i) heating and (ii) cooling energy demands, as well as (iii) non-renewable primary energy consumption, upon varying the plastic material. Finally, additional refurbishment case studies have also been implemented considering 3D printed panels as a second-skin layer. The aims of this research can be summarized as reported below: investigate the performances of extruded ABS panels as a second-skin layer for inno- vative building envelopes with experimental tests in-situ; calibrate and validate a simulation model to predict the energy performance of the plastic and composite polymer panels used as a second-skin layer in an OVF system; assess the potential energy saving achievable in office building refurbishment using the proposed materials (extruded and 3D printed polymers) through numerical simulation. 1.3. Structure of the Research The research is structured as follows. Section 2 describes the methodology used to carry out the research, showing in detail how (i) the experimental data have been acquired, (ii) the numerical model has been implemented in TRNSYS 18, and how (iii) the experimental data have been used to calibrate and validate this numerical model. Section 3 reports the numerical results, in terms of the reduction of non-renewable primary energy consumption, achieved in an ofﬁce building refurbishment through the installation of an OVF system, upon varying the material used as a second-skin layer, considering both extruded and 3D printed ones. Finally, Section 4 discusses the integration capacity of the plastic and composite polymer panels in a second-skin layer of an OVF system, highlighting the advantages and limitations of such materials. 2. Methodology This section describes in detail the measurement methodologies and the experimental results obtained during the in-situ test as well as the methods and results related to the validation of the implemented numerical model. 2.1. Description of the Gemini Facilities, Experimental Results, and Discussion In this sub-section, a couple of experimental test cells and the experimental results are reported. Gemini facilities [11] are designed and built at the Ri.A.S.–Built Environment Control Laboratory [52] of the Department of Architecture and Industrial Design of the 0 00 0 00 University of Campania Luigi Vanvitelli in Aversa (40 59 39.1 N, 14 10 48.5 E). These full- size outdoor test cells have been designed to experimentally evaluate double-skin facade module performances, in real outdoor weather conditions. The test cells are designed as an identical couple in order to carry out comparative measurements. The Gemini’s internal Buildings 2021, 11, 141 5 of 27 dimensions are 2.20 m wide by 2.80 m deep and 2.40 m tall, oriented with the long side along the north-south axis. These dimensions correspond to the gross dimension of the main steel frame structure, on which the shell has been ﬁxed externally and seamlessly in order to avoid thermal bridges. The shell has been realized in a single layer of 10 mm thick sandwich panels consisting of two galvanized steel sheets and a polyurethane rigid foam ﬁlling, with a thermal transmittance (U ) value of 0.23 W/m K [11]. Then, for the ﬂoor, a wall 0.10 m air gap and a wood ﬂooring have been added above the structure, while, for the ceiling, a sheet metal roof has been placed 0.10 m above the outer panels, with a 2% slope, to allow a natural rainwater outﬂow. The Gemini’s facilities are designed to allow the in-situ characterization of innovative layers to be applied in double-skin facades with different geometries, layout, materials, and technologies. The acquired data can be used to evaluate the in-situ performances of the system under investigation and to realize, calibrate and validate simulation models. The Gemini is well-instrumented to acquire different indoor and outdoor physical quantities. Table 1 shows the measurement range, the accuracy, and the response time of the sensors used for outdoor and indoor climate characterization. In particular, with the aim to evaluate the real weather conditions, sensors for wind direction, wind speed, air temperature, air relative humidity, air pressure, global horizontal radiation, and diffuse horizontal radiation were placed at about 6.50 m from the ground, in the best position to minimize the inﬂuence of external obstructions (i.e., the obstructions angles are less than 10 ). In order to acquire diffuse horizontal radiation, one of the pyranometers is equipped with a shadow ring (diameter of 0.574 m and thickness equal to 0.052 m), and the data were corrected following the methodology proposed in [53], to take into account both the isotropic and anisotropic conditions. Figure 1 shows the weather station, with all the aforementioned sensors. Table 1. Installed Gemini sensor measurement range and accuracy. Number of Sensors Measured Quantity Type Range Accuracy ‘Pro First Class’ 1 Wind speed 0–50 m/s 0.01 m/s anemometer ‘Pro First Class’ 1 Wind direction 0–356.9 3 1 anemometer Temperature: Temperature: Air Temperature and Thermo-hygrometer 40–+60 C 0.2 C Relative humidity with precision transducer Rel. Humidity: Rel. Humidity: 0–100% 2% Atmospheric Barometer with 0.3 hPa 1 800–1100 hPa pressure piezo-resistive transducer at 20 C II class thermopile 2 2 3 Solar radiation 0–2000 W/m 10 V/(W/m ) pyranometer Hot wire air speed 0.2 m/s 2 Air cavity speed 0.2–40.0 m/s transmitter +3% f.s. 10 Temperature T-Type thermocouple 200–+350 C 1.5 C Buildings 2021, 11, x FOR PEER REVIEW 6 of 27 The air temperature inside the Gemini is monitored by a combined temperature-rel- Buildings 2021, 11, 141 6 of 27 ative humidity sensor placed in the middle of the room. Also, when the test cells are con- figured to test a second-skin system, this is monitored through a set of ten thermocouples, placed on the significant interfaces and in the cavity. Figure 1. The weather station used to monitor the real outdoor conditions. Figure 1. The weather station used to monitor the real outdoor conditions. The The sensor air temperatur layout follow e inside s the layout the Gemini shown in is Fig monitor ure 2a. In ed par by ticul a combined ar, it can be n temperatur oted e- that (i) six thermocouples are placed in the middle of both the inlet (T1, T2, and T3) and relative humidity sensor placed in the middle of the room. Also, when the test cells are the outlet (T8, T9, and T10) sections of the air cavity, (ii) four thermocouples are placed in conﬁgured to test a second-skin system, this is monitored through a set of ten thermocou- line at the center of the second-skin system (T4 on the back surface of the second-skin, T5 ples, placed on the signiﬁcant interfaces and in the cavity. in the middle of the cavity, T6 on the external surface of the south wall of the test cell, T7 The sensor layout follows the layout shown in Figure 2a. In particular, it can be noted on the internal surface of the south wall of the test cell, respectively); this last set of ther- that (i) six thermocouples are placed in the middle of both the inlet (T1, T2, and T3) and the mocouples falls in line with the same thermo-hygrometer which monitors the temperature outlet (T8, T9, and T10) sections of the air cavity, (ii) four thermocouples are placed in line of the air inside the test cell, in order to have all the sensors aligned at the center of the at the center of the second-skin system (T4 on the back surface of the second-skin, T5 in the system. middle of the cavity, T6 on the external surface of the south wall of the test cell, T7 on the In addition, all the thermocouples (Tx) have been shielded with high-reflective internal surface of the south wall of the test cell, respectively); this last set of thermocouples domes in order to avoid any direct solar radiation (Figure 2b). Buildings 2021, 11, x FOR PEER REVIEW 7 of 27 falls in line with the same thermo-hygrometer which monitors the temperature of the air The pyranometer (Ivert) has been installed to acquire the vertical solar radiation inci- inside dent on the thtest e south cell,sin urface order and, to have finally, all tw the o hot sensors -wire aligned anemomat etethe rs (center Win, and ofW the out) system. are placed in the air cavity, one in the inlet section and the other one in the outlet section, in order to monitor the airflow in the second-skin cavity. (a) (b) Figure 2. (a) Axonometric views of the sensor layout; (b) shielding devices used in the experimental setup to avoid any Figure 2. (a) Axonometric views of the sensor layout; (b) shielding devices used in the experimental setup to avoid any direct solar radiation on the thermocouples. direct solar radiation on the thermocouples. In order to verify the measurement methodologies and characterize each test cell from the thermal point of view, preliminary data has been acquired in a standard config- uration (both Gemini s without a second-skin system). These data are recorded with the aim of (i) verifying the operation of the different instruments and their correct positioning, (ii) comparing the thermal behavior of the two test cells, and (iii) defining a reference point for the following evaluation of the real performances of double-skin facades or smart win- dows. The preliminary experimental data were acquired and stored every 1 min on a pe- riod of 1 month (from 1 June to 30 June) and, later, averaged on an interval of 15 min. Figure 3 reports the indoor air temperature of Gemini 1 and Gemini 2, the external air temperature, and the global horizontal radiation for three typical days in June. This figure highlights that the difference between the indoor air temperature of Gemini 1 and the indoor air temperature of Gemini 2 (Tindoor, Gemini 1–Tindoor, Gemini 2) is negligible, varying within an interval with a maximum of 0.2 °C and a minimum of −0.2 °C. After the preliminary experimental campaign, a second-skin system has been mounted and tested on a test cell (Gemini 1) with an air cavity gap equal to 0.10 m, while the other cell has been left unequipped and used as a reference (Gemini 2). In particular, the investigated second-skin system has been realized with extruded ABS panels [54]. The experimental data were acquired and stored every 1 min over a period of 1 month (from 8 December to 31 December) and, later, averaged on an interval of 15 min. This experi- mental campaign aims to verify the performances of plastic panels as a second-skin layer for innovative envelopes. The extruded ABS panels have been selected with dimensions equal 600 mm × 1200 mm; such dimensions have been selected on the basis of the dimension of conventional panels in OVF systems, in order to guarantee an easy installation in a commercial OVF structure, as well as an easy substitution of conventional OVF system materials in a real- istic scenario. In Figure 4a, a detailed view of the analyzed extruded ABS panel before the installation is displayed, while Figure 4b shows the Gemini 1 equipped with the second- skin realized with six extruded ABS panels. Buildings 2021, 11, 141 7 of 27 In addition, all the thermocouples (Tx) have been shielded with high-reﬂective domes in order to avoid any direct solar radiation (Figure 2b). The pyranometer (I ) has been installed to acquire the vertical solar radiation in- vert cident on the south surface and, ﬁnally, two hot-wire anemometers (W , and W ) are out in placed in the air cavity, one in the inlet section and the other one in the outlet section, in order to monitor the airﬂow in the second-skin cavity. In order to verify the measurement methodologies and characterize each test cell from the thermal point of view, preliminary data has been acquired in a standard conﬁguration (both Gemini s without a second-skin system). These data are recorded with the aim of (i) verifying the operation of the different instruments and their correct positioning, (ii) comparing the thermal behavior of the two test cells, and (iii) deﬁning a reference point for the following evaluation of the real performances of double-skin facades or smart windows. The preliminary experimental data were acquired and stored every 1 min on a period of 1 month (from 1 June to 30 June) and, later, averaged on an interval of 15 min. Figure 3 reports the indoor air temperature of Gemini 1 and Gemini 2, the external air temperature, and the global horizontal radiation for three typical days in June. This ﬁgure highlights that the difference between the indoor air temperature of Gemini 1 and the Buildings 2021, 11, x FOR PEER REVIEW 8 of 27 indoor air temperature of Gemini 2 (T –T ) is negligible, varying indoor, Gemini 1 indoor, Gemini 2 within an interval with a maximum of 0.2 C and a minimum of 0.2 C. Indoor air temperature of Gemini 1 Indoor air temperature of Gemini 2 External air temperature Global horizontal solar radiation 31 850 30 800 29 750 28 700 27 650 26 600 25 550 24 500 23 450 22 400 21 350 20 300 19 250 18 200 17 150 16 100 15 50 14 0 Time (hh:mm) Figure 3. Gemini 1 and Gemini 2 preliminary experimental indoor air temperature trends for three Figure 3. Gemini 1 and Gemini 2 preliminary experimental indoor air temperature trends for three typical June days. typical June days. After the preliminary experimental campaign, a second-skin system has been mounted and tested on a test cell (Gemini 1) with an air cavity gap equal to 0.10 m, while the other cell has been left unequipped and used as a reference (Gemini 2). In particular, Gemini 1 the investigated second-skin system has been realized with extruded ABS panels [54]. The experimental data were acquired and stored every 1 min over a period of 1 month (from 8 December to 31 December) and, later, averaged on an interval of 15 min. This experimental campaign aims to verify the performances of plastic panels as a second-skin Gemini 2 layer for innovative envelopes. The extruded ABS panels have been selected with dimensions equal 600 mm 1200 mm; such dimensions have been selected on the basis of the dimension of conventional panels in OVF systems, in order to guarantee an easy installation in a commercial OVF structure, as well as an easy substitution of conventional OVF system materials in a realistic scenario. In Figure 4a, a detailed view of the analyzed extruded ABS panel before the installation is Extruded ABS panels (a) (b) Figure 4. (a) Extruded ABS panels [54] detail view; (b) Gemini 1 equipped with the OVF system. The second-skin system equipped during the tests has been realized by mounting the six extruded ABS panels on a steel frame, then hanging the whole system to the brackets on the south facade of the test cell Gemini 1. Finally, the sides of the second-skin system were covered and sealed by means of panels, similar to those used for the test cells’ enve- Temperature ( C) 600 mm Solar radiation (W/m ) Buildings 2021, 11, x FOR PEER REVIEW 8 of 27 Indoor air temperature of Gemini 1 Indoor air temperature of Gemini 2 External air temperature Global horizontal solar radiation 31 850 30 800 29 750 28 700 27 650 26 600 25 550 24 500 23 450 22 400 21 350 20 300 19 250 18 200 17 150 16 100 Buildings 2021, 11, 141 15 50 8 of 27 14 0 Time (hh:mm) displayed, while Figure 4b shows the Gemini 1 equipped with the second-skin realized Figure 3. Gemini 1 and Gemini 2 preliminary experimental indoor air temperature trends for three typical June days. with six extruded ABS panels. Gemini 1 Gemini 2 Extruded ABS panels (a) (b) Figure 4. (a) Extruded ABS panels [54] detail view; (b) Gemini 1 equipped with the OVF system. Figure 4. (a) Extruded ABS panels [54] detail view; (b) Gemini 1 equipped with the OVF system. Buildings 2021, 11, x FOR PEER REVIEW 9 of 27 The second-skin system equipped during the tests has been realized by mounting the The second-skin system equipped during the tests has been realized by mounting six extruded ABS panels on a steel frame, then hanging the whole system to the brackets the six on ext the r south uded fac ABS ade of panels the test cell on Gem a steel ini 1. frame, Finally, tthen he sides hanging of the second the-sk whole in system system to the were covered and sealed by means of panels, similar to those used for the test cells’ enve- brackets on the south facade of the test cell Gemini 1. Finally, the sides of the second-skin lope, in order to allow only for a vertical airflow in the cavity (Figure 4b). In this configu- system were covered and sealed by means of panels, similar to those used for the test cells’ ration, the acquisition period lasted for almost a month, in the wintertime. During the acqui- envelope, in order to allow only for a vertical airﬂow in the cavity (Figure 4b). In this sition period, the temperatures were monitored following the layout reported in Figure 2a. conﬁguration, the acquisition period lasted for almost a month, in the wintertime. During Fi the gure acquisition 5 shows period, an ovthe ervtemperatur iew of the es wh wer ole e monitor acquisitio ed n following period, the repo layout rting rth eported e external air in Figure 2a. Figure 5 shows an overview of the whole acquisition period, reporting the temperature, the global horizontal radiation, and the total vertical radiation on the south external air temperature, the global horizontal radiation, and the total vertical radiation on facade. the south facade. Figure 5. Overview of the whole winter acquisition period. Figure 5. Overview of the whole winter acquisition period. Figure 5 highlights that the external air temperature was quite warm, despite being the winter season. Also, the radiation values, both global horizontal and total vertical, show mostly high values, thus confirming good weather and clear sky across the whole acquisition period. Figure 6a,b report a focus for four typical days on the weather conditions during the measurements with the Gemini 1 equipped with the second-skin system, and the Gemini 2 left uncovered as a reference. In particular, Figure 6a shows the temperature and solar radiation data (global horizontal solar radiation, diffuse horizontal solar radiation, and total vertical solar radiation on the south facade), while Figure 6b reports the wind char- acteristics acquired during the analyzed days; on the left axis the wind speed is reported, while on the right axis the wind direction is displayed, considering 0° as north direction, 90° as east direction, 180° as south direction, and 270° as west direction. Wind speed Wind direction 2.4 360 2.2 2.0 300 1.8 270 1.6 1.4 210 1.2 180 1.0 0.8 120 0.6 90 0.4 60 0.2 0.0 0 Time (hh:mm) (a) (b) Figure 6. Weather conditions during four typical acquisition days: (a) solar radiation and temperature; (b) wind speed and wind direction. Temperature ( C) Speed (m/s) 600 mm Solar radiation (W/m ) Direction ( ) Buildings 2021, 11, x FOR PEER REVIEW 9 of 27 lope, in order to allow only for a vertical airflow in the cavity (Figure 4b). In this configu- ration, the acquisition period lasted for almost a month, in the wintertime. During the acqui- sition period, the temperatures were monitored following the layout reported in Figure 2a. Figure 5 shows an overview of the whole acquisition period, reporting the external air temperature, the global horizontal radiation, and the total vertical radiation on the south facade. Buildings 2021, 11, 141 9 of 27 Figure 5. Overview of the whole winter acquisition period. Figure 5 highlights that the external air temperature was quite warm, despite being Figure 5 highlights that the external air temperature was quite warm, despite being the winter season. Also, the radiation values, both global horizontal and total vertical, the winter season. Also, the radiation values, both global horizontal and total vertical, show mostly high values, thus confirming good weather and clear sky across the whole show mostly high values, thus conﬁrming good weather and clear sky across the whole acquisition period. acquisition period. Figure 6a,b report a focus for four typical days on the weather conditions during the Figure 6a,b report a focus for four typical days on the weather conditions during the measurements with the Gemini 1 equipped with the second-skin system, and the Gemini measurements with the Gemini 1 equipped with the second-skin system, and the Gemini 2 left uncovered as a reference. In particular, Figure 6a shows the temperature and solar 2 left uncovered as a reference. In particular, Figure 6a shows the temperature and solar radiation data (global horizontal solar radiation, diffuse horizontal solar radiation, and radiation data (global horizontal solar radiation, diffuse horizontal solar radiation, and total total vertical solar radiation on the south facade), while Figure 6b reports the wind char- vertical solar radiation on the south facade), while Figure 6b reports the wind characteristics acteristics acquired during the analyzed days; on the left axis the wind speed is reported, acquired during the analyzed days; on the left axis the wind speed is reported, while on while on the right axis the wind direction is displayed, considering 0° as north direction, the right axis the wind direction is displayed, considering 0 as north direction, 90 as east 90° as east dir ection, 180° as south direction, and 270° as west direction. direction, 180 as south direction, and 270 as west direction. Wind speed Wind direction 2.4 360 2.2 330 2.0 300 1.8 270 1.6 1.4 210 1.2 180 1.0 0.8 120 0.6 90 0.4 0.2 30 0.0 0 Time (hh:mm) (a) (b) Figure 6. Weather conditions during four typical acquisition days: (a) solar radiation and temperature; (b) wind speed Figure 6. Weather conditions during four typical acquisition days: (a) solar radiation and temperature; (b) wind speed and and wind direction. wind direction. Figure 6a better highlights that, during the measurement period, sunshine days were acquired with atypical temperatures for the period, ranging between a minimum of about 5.9 C and a maximum of about 19.4 C. Figure 6b shows a low wind speed in general during the measurement period and a slight wind predominance in the west/north-west direction. Also, the wind speed values acquired during the nighttime are equal to zero, due to a threshold value for the start/stop of the sensor equal to 0.15 m/s. Figures 7 and 8 report the experimental data associated with the cavity with a 1-h timestep for a single acquisition day. In more detail, Figure 7 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, while Figure 8 reports the airspeed at the inlet and the outlet of the air cavity, for the same day. The data reported in Figure 7 corresponds to the measures of the three thermocouples positioned in the middle point of the cavity of the second-skin system, more speciﬁcally T2, in the middle of the air cavity inlet, T5, in the middle of the air cavity geometrical center, and T9, in the middle of the air cavity outlet, as shown in Figure 2a. As a ﬁrst observation, the overall temperature distribution is directly related to solar radiation throughout the day, where the temperatures rise during the morning and drop during the afternoon. Also, during the day, the temperature trend seems to be substantially constant from the air cavity inlet to the middle of the facade, and then to increase to the air cavity outlet; this behavior is due to the chimney effect that is created thanks to the OVF system. Then, in the evening (starting from 16.00), the temperatures at the outlet of the cavity are only slightly higher than those at the center and the inlet; this is due to the reduction of solar radiation happening in the evening. Speed (m/s) Direction ( ) Buildings 2021, 11, x FOR PEER REVIEW 10 of 27 Buildings 2021, 11, x FOR PEER REVIEW 10 of 27 Figure 6a better highlights that, during the measurement period, sunshine days were Figure 6a better highlights that, during the measurement period, sunshine days were acquired with atypical temperatures for the period, ranging between a minimum of about acquired with atypical temperatures for the period, ranging between a minimum of about 5.9 ° 5.9 ° C C a and nd a a maximum maximum of of about about 19.4 ° 19.4 ° C. C. Fi Fig gure ure 6b 6b sho shows ws a a low low win wind d speed speed in g in ge enera neral l dur during ing tthe he meas measurem urement ent period period an and d a a sl sliight ght wind wind p predom redomin inance ance in in th the e west west/n /nort orth h--west west dir direct ection. ion. Al Also, so, tthe he wind wind spee speed d values values acquired during the nighttime are equal to zero, due to a threshold value for the start/stop acquired during the nighttime are equal to zero, due to a threshold value for the start/stop of the s of the sensor ensor equal t equal to 0. o 0.15 15 m/ m/s. s. Fi Fig gures ures 7 7 an and d 8 8 repo report rt th the e exp experiment erimental al dat data a assoc associiated ated with with th the e cavity cavity w with ith a a 1 1--h h timestep for a single acquisition day. In more detail, Figure 7 shows the trends of the daily timestep for a single acquisition day. In more detail, Figure 7 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, values of the air temperature inside the cavity upon varying the height from the ground, Buildings 2021, 11, 141 10 of 27 while while F Fig igu ure re 8 8 r repo eports rts th the e ai air rspeed speed at at th the e in inlet let and and th the e outl outlet et of of th the e air air cav cavity, ity, for for th the e s same ame day. day. 07:00 08:00 09:00 10:00 11:00 12:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 13:00 14:00 15:00 16:00 17:00 4 4..0 0 Thermocouples Thermocouples T T2 2//T T5 5//T T9 9 3 3..5 5 1 1.2 .20 0 m m 3.0 3.0 2.5 2.5 Thermocouple Thermocouple 2 2..0 0 ((T T9 9)) 1.5 1.5 Thermocouple Thermocouple (T5) (T5) 1.0 1.0 3 3.4 .40 0 m m 2.20 m 2.20 m 0 0..5 5 Thermocouple Thermocouple ((TT22)) 0 0..0 0 0 0.9 .90 0 m m 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 Temperature ( C) Temperature ( C) Figure 7. Daily values of the cavity air temperature. Figure Figure 7. 7. Dail Daily y val values ues of of the the c cavi avity ai ty air t r temp emperatur erature. e. C Cav aviitty y ai air r iin n lleett C Cav aviitty y ai air r o ou u ttlleett T T o ottal al v veert rtiiccal al s so ollar ar r rad adiiat atiio on n 0. 0 000 0. 0 000 0. 00 0. 00 0 0.. 0 0 00 00 1 1..2 20 0 m m IIn nllet et//Out Outllet et 0. 00 s sen ens so orrs s 0. 00 W1/W2 W1/W2 0 0.. 0 0 00 00 Outlet sensor Outlet sensor 0.2 00 0.2 00 ((W2 W2)) 0 0..20 20 00 00 0 0.. 00 00 3.30 m 3.30 m 0. 0 200 0. 0 200 Inlet sensor 0 0..0 0 00 00 Inlet sensor (W1) (W1) 0 0..00 00 0 0 1 1..0 00 0 m m i i e e hh hh:: Figure 8. Daily values of the airspeed at the inlet and the outlet of the air cavity. Figure Figure 8. 8. Dail Daily y val values ues of of the the a airs irspeed peed at at the the iinlet nlet and and the the outlet outlet of of the a the air ir ca cavi vity. ty. Figure 8 reports data acquired by the hot-wire anemometers placed based on the The data reported in Figure 7 corresponds to the measures of the three thermocou- The data reported in Figure 7 corresponds to the measures of the three thermocou- layout shown in Figure 2a, where W measured the airspeed value at the inlet of the cavity in ples ples po pos sition itioned ed in in th the e mid middle dle po point int of of tthe he cavity cavity o of f th the e second second--sk skiin n sys system tem, , mo more re speci specifi fi-- and W measured the value at the outlet of the cavity. This ﬁgure shows how, during the out day, the values acquired by W is higher than those measured by W , referable to a direct cally cally T T2, 2, in in th the e midd middle le o of f th the e air air c cavi avity ty in inlet, let, T T5, 5, in in th the e middle middle of of th the e ai air r cav cavity ity g geo eomet metrical rical out in effect of the solar radiation on the rising of the air temperatures along the air cavity, thus center, and T9, in the middle of the air cavity outlet, as shown in Figure 2a. As a first center, and T9, in the middle of the air cavity outlet, as shown in Figure 2a. As a first causing an increase in airspeed. After 15:00, the chimney effect in the air cavity is reduced observation, the overall temperature distribution is directly related to solar radiation observation, the overall temperature distribution is directly related to solar radiation because the temperatures are gradually decreasing over time due to the reduction of the th throughout roughout tthe he d day ay,, wher where e th the e tem temper perat atures ures ri rise se dur duriing ng th the e mo morni rning ng and and drop drop duri durin ng g th the e solar radiation on the south facade; this causes a signiﬁcant drop in the airspeed values measured in the cavity. Therefore, the analysis of the experimental results shows that a plastic material (i.e., ABS) can be used as a second-skin layer in OVF systems. Height from the ground (m) Height from the ground (m) it irs ee s it irs ee s ol r r i tio ol r r i tio Buildings 2021, 11, 141 11 of 27 2.2. Description of the Numerical Model The software TRNSYS 18 [31] has been used to model the Gemini test cells and to develop the second-skin model. TRNSYS software adopts a modular approach by using Fortran subroutines. Each Fortran subroutine is called a ‘Type’ and contains the model for a single system component. Several studies have been carried out in order to validate the numerical models developed in TRNSYS from the Colorado State University experimental houses and other researchers around the world [55–58]. In this study, the following main TRNSYS Types [31,59–61] have been used: Type 56 to simulate the Gemini test cells [59]; Type 1230 to model the second-skin system [61]; Type 16c to estimate the solar radiation on the Gemini and second-skin system sur- faces [59,60]; Type 69b to determine the sky temperature [59,60]; Type 33e to determine the moist air properties [59,60]. At ﬁrst, the Type 56 subroutine has been used to model the thermal behavior of the Gemini test cells; in particular, two thermal zones have been modeled, one for each test cell. Also, Type 56 contains information about the test cells’ surroundings (buildings, trees, bushes), which are described as ‘shading objects’. The geometrical modeling occurred in the SketchUp software [62], where it was possible to model the shapes and the position of each element accurately. Then, by means of the Trnsys3D plug-in, the geometries were imported into the Type 56 subroutine. The physical properties of each test cell’s external surface have been deﬁned, on the basis of the data provided by the manufacturers. Lastly, the internal thermal gains have been set for each test cell, which was determined on the basis of the equipment installed inside the facilities (notebook, data acquisition systems, and uninterruptible power supply units). The OVF system has been modeled using the Type 1230 subroutine, which effectively reproduces the behavior of an external second-skin layer with an air cavity behind it. Using this TRNSYS Type, the behavior of the OVF system has been correlated to that of the Gemini test cell modeled through the Type 56 subroutine. In particular, the last external layer of the Type 56 wall acts as an interface layer between the Type 1230 and the Type 56, by coupling its temperature and thermal resistance to model the wall heat transfer. Figure 9 shows a schematic of the boundaries of the two coupled Types (56 and 1230), highlighting the resistive interface layer. The Type 1230 parameters have been set following the data provided by the manufacturer of the extruded ABS panels [54], taking into account thickness, density, and thermal conductivity, speciﬁcally. During the simulations, Type 1230 takes into account: the solar radiation, the longwave radiation, and the air convection on the external surface of the outside layer; the energy storage and the conduction in the outside layer; radiation exchange between the outside layer and the air cavity; the convective exchanges from all the surfaces facing in the air cavity; the conduction through the interface layer. Type 16c has been implemented to model the solar radiation on all the external surfaces. This Type accepts global radiation, ambient temperature, and ambient relative humidity data as input, in order to output several quantities related to the position of the sun, as the diffuse radiation fraction on the horizontal, by estimating the cloudiness of the sky on the basis of the dry bulb temperature and the dew point temperature. Finally, the radiation on every external surface is computed, on the basis of their own orientation. Buildings 2021, 11, 141 12 of 27 Buildings 2021, 11, x FOR PEER REVIEW 12 of 27 Figure 9. Coupling between Type 56 and Type 1230 [61]. Figure 9. Coupling between Type 56 and Type 1230 [61]. The effective sky temperature is determined by means of Type 69b, which calculates During the simulations, Type 1230 takes into account: the long-wave radiation exchanges between the external surfaces and the atmosphere.  the solar radiation, the longwave radiation, and the air convection on the external Type 69b calculates the cloudiness factor as well, on the basis of the dry bulb and the dew surface of the outside layer; point temperatures.  the energy storage and the conduction in the outside layer; Finally, Type 33e has been implemented in order to calculate the properties of the  radiation exchange between the outside layer and the air cavity; moist air, in particular, by taking the air temperature, the relative humidity, and the air  the convective exchanges from all the surfaces facing in the air cavity; pressure as input; it returns the density of the air mixture for every timestep, which is  the conduction through the interface layer. then used to calculate the airﬂow at the OVF inlet. In this way, the inlet airﬂow is not a Type 16c has been implemented to model the solar radiation on all the external sur- ﬁxed value, but it corresponds to the experimental data acquired through the hot wire faces. This Type accepts global radiation, ambient temperature, and ambient relative hu- anemometers, placed as reported in Figure 2a. midity data as input, in order to output several quantities related to the position of the In this study, the experimental weather data acquired from 8 December to 31 December sun, as the diffuse radiation fraction on the horizontal, by estimating the cloudiness of the have been used as input data for the Type 16c, Type 69b, and Type 33e. sky on the basis of the dry bulb temperature and the dew point temperature. Finally, the During the simulation, both the time base used to solve the differential equations and radiation on every external surface is computed, on the basis of their own orientation. the simulation timestep has been set equal to 15 min in order to have a full correlation to The effective sky temperature is determined by means of Type 69b, which calculates the timestep of the experimental input data. the long-wave radiation exchanges between the external surfaces and the atmosphere. 2.3. Validation of the Numerical Model Type 69b calculates the cloudiness factor as well, on the basis of the dry bulb and the dew point temperatures. This sub-section reports the methods and results related to the validation of the nu- Finally, Type 33e has been implemented in order to calculate the properties of the merical model. The model reliability has been veriﬁed in terms of indoor air temperature moist air, in particular, by taking the air temperature, the relative humidity, and the air (T ) and the average temperature of the air cavity (T ) by comparing the experi- indoor cavity mental pressure values as input with ; it those returns obtained the density as anooutput f the air of mixt theusimulation re for every model timestabove ep, whic described, h is then deﬁning used to cthe alcu following late the ai per rflo centage w at the dif O fer VF ences inlet. DIn T this and way, Dth T e inlet : airflow is not a fixed indoor cavity value, but it corresponds to the experimental data acquired through the hot wire anemom- eters, placed as reported in Figure 2a. DT = T T /T (1) indoor indoor,ex p indoor,sim indoor,ex p In this study, the experimental weather data acquired from 8 December to 31 Decem- ber have been used as input data for the Type 16c, Type 69b, and Type 33e. DT = T T /T (2) cavity cavity,ex p cavity,sim cavity,ex p During the simulation, both the time base used to solve the differential equations and Figure 10a,b report the comparison between the simulation results and the experimen- the simulation timestep has been set equal to 15 min in order to have a full correlation to tal data acquired during the whole test period (from 8 December to 31 December) in terms the timestep of the experimental input data. of T and T , respectively. indoor cavity Buildings 2021, 11, x FOR PEER REVIEW 13 of 27 2.3. Validation of the Numerical Model This sub-section reports the methods and results related to the validation of the nu- merical model. The model reliability has been verified in terms of indoor air temperature (Tindoor) and the average temperature of the air cavity (Tcavity) by comparing the experi- mental values with those obtained as an output of the simulation model above described, defining the following percentage differences ΔTindoor and ΔTcavity: ΔT = T  T T (1)   indoor indoor,exp indoor,sim indoor,exp ΔT = T  T T (2)   cavity cavity,exp cavity,sim cavity,exp Figure 10a,b report the comparison between the simulation results and the experi- mental data acquired during the whole test period (from 8 December to 31 December) in terms of Tindoor and Tcavity, respectively. These figures highlight how the developed model is quite reliable, with values of Buildings 2021, 11, 141 ΔTindoor ranging between a minimum of − . 2%, and a maximum of .22%, as well as 13 thof e 27 values of ΔTcavity between a minimum of − . % and a maximum of . %. 21 23 14 13 7 3 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Experimental indoor air temperature ( C) Experimental air cavity temperature ( C) (a) (b) Figure 10. Comparison between the simulated values and the experimental values acquired during the whole test period Figure 10. Comparison between the simulated values and the experimental values acquired during the whole test period in in terms of (a) Tindoor and (b) Tcavity. terms of (a) T and (b) T . indoor cavity The accuracy of the model has also been validated by calculating the mean error These ﬁgures highlight how the developed model is quite reliable, with values of (ME), the mean absolute error (MAE), and the root mean square error (RMSE) as reported DT ranging between a minimum of 9.62%, and a maximum of 6.22%, as well as the indoor below: values of DT between a minimum of 8.51% and a maximum of 7.85%. cavity The accuracy of the model has also been validated by calculating the mean error (ME), ME  T  T N (3) the mean absolute error (MAE), and the root mean square err or (RMSE) as reported below: exp,i sim ,i i1 N N MAE  T  T N (4) ME =  T T /N (3) exp,i sim ,i å iexp, 1 i sim,i i=1 (5) RMSE T  T  ME N     exp,i sim ,i i1 M AE = T T /N (4) å exp,i sim,i i=1 where Texp,i is the experimental value at time step i, Tsim,i is the simulated value at time step i, and N is the number of measurement N s. Table 2 reports the values of the ME, MAE, and R MSE = T T ME /N (5) RMSE for both Tindoor and Tcavity. å exp,i sim,i i=1 where T is the experimental value at time step i, T is the simulated value at time exp,i sim,i step i, and N is the number of measurements. Table 2 reports the values of the ME, MAE, and RMSE for both T and T . indoor cavity Table 2. Values of ME, MAE, and RMSE obtained by comparing the simulated values and the experimental data acquired during the whole test period. T ( C) T ( C) indoor cavity ME MAE RMSE ME MAE RMSE 0.3 0.5 0.4 0.3 0.3 0.2 The values reported in Table 2 highlight that there is a slight difference between the measured and predicted results, in particular: (i) the ME associated to the T is equal indoor to 0.3 C, which means that the simulation model slightly overestimates the indoor air temperature, while that associated to the T is equal to 0.3 C; (ii) the values of RMSE cavity are equal to 0.4 C and 0.2 C for T and T , respectively. indoor cavity Therefore, the results show the ability of the Type 1230 to accurately predict the behavior of the extruded ABS panels in an OVF system. Thus, the same methodology is used to carry out a complete numerical campaign on a set of case studies upon varying the polymer (selecting the ones more used in architecture, as highlighted in the literature review) and manufacturing technology (extruded and 3D printed). Simulated idoor air temperature ( C) Simulated air cavity temperature ( C) Buildings 2021, 11, x FOR PEER REVIEW 14 of 27 Table 2. Values of ME, MAE, and RMSE obtained by comparing the simulated values and the ex- perimental data acquired during the whole test period. Tindoor (° C) Tcavity (° C) ME MAE RMSE ME MAE RMSE −0. 0.5 0.4 0.3 0.3 0.2 The values reported in Table 2 highlight that there is a slight difference between the measured and predicted results, in particular: (i) the ME associated to the Tindoor is equal to −0. °C, which means that the simulation model slightly overestimates the indoor air tem- perature, while that associated to the Tcavity is equal to 0.3 °C; (ii) the values of RMSE are equal to 0.4 °C and 0.2 °C for Tindoor and Tcavity, respectively. Therefore, the results show the ability of the Type 1230 to accurately predict the be- havior of the extruded ABS panels in an OVF system. Thus, the same methodology is used to carry out a complete numerical campaign on a set of case studies upon varying the polymer (selecting the ones more used in architecture, as highlighted in the literature re- view) and manufacturing technology (extruded and 3D printed). 3. Materials and Numerical Modeling Implementation The software TRNSYS 18 [24] is used to assess the potential energy saving achievable Buildings 2021, 11, 141 14 of 27 in an office building refurbishment using plastic and composite polymers as the second- skin layer material. The office building modeled in this work is the same for all configurations and it 3. Materials and Numerical Modeling Implementation consists of three identical fl Theoors. softwar Each e TRNSYS floor ha 18 [24s ] is a sur usedfto ac assess e of 45 the1 potential m and a vo energy l saving ume achievable equal to in an ofﬁce building refurbishment using plastic and composite polymers as the second-skin 3 2 2 2 1503 m , with a total window area (Aw, total) of 112.3 m (Aw, North = 24.5 m , Aw, South = 87.8 m ). layer material. It is located in Napoli (latitude = 40°51′ N; longitude = 14°16′ E), and as such, in order to The ofﬁce building modeled in this work is the same for all conﬁgurations and simulate the weather it consists condition, of thr ee the identical correspond ﬂoors. in Each g EnergyP ﬂoor has a lus surface weather of 451 d mata and ha as volume been 3 2 2 equal to 1503 m , with a total window area (A ) of 112.3 m (A = 24.5 m , w, total w, North used [30]. The office is firstly modeled in the SketchUp 3D modeling software (Figure 11). 2 0 0 A = 87.8 m ). It is located in Napoli (latitude = 40 51 N; longitude = 14 16 E), and w, South Then, the 3D model geometries were exported by means of the Trnsys3D plug-in and suc- as such, in order to simulate the weather condition, the corresponding EnergyPlus weather cessively imported into data T has RNS been YS used 18 [30 in]. or The der ofﬁce to is mo ﬁrstly del modeled the bui in ld the ing SketchUp envelope 3D modeling (stratig softwar raphy e (Figure 11). Then, the 3D model geometries were exported by means of the Trnsys3D plug- of the opaque wall and window typology), to define the infiltration, the internal gains, the in and successively imported into TRNSYS 18 in order to model the building envelope operation period of the heating and cooling systems as well as the operation of the electric (stratigraphy of the opaque wall and window typology), to deﬁne the inﬁltration, the equipment and lightinternal ing system gains,. the In par operation ticular, period the of sam the e heating TRNS and YS cooling Types systems described as well inas Sec the - operation of the electric equipment and lighting system. In particular, the same TRNSYS tion 4 have been used to simulate the case study. Types described in Section 4 have been used to simulate the case study. (a) (b) Figure 11. Ofﬁce building modeled in SketchUp 3D: (a) south view; (b) north view. Figure 11. Office building modeled in SketchUp 3D: (a) south view; (b) north view. The same typical three-story ofﬁce building is investigated upon, varying the insula- tion layer thickness on the south facade and the typology of second-skin material, for a total The same typical three-story office building is investigated upon, varying the insula- of eight case studies. Table 3 summarizes the eight simulation cases investigated in this tion layer thickness on the south facade and the typology of second-skin material, for a work. In particular, this table reports the reference case (Case 0) without the second-skin (Figure 12a) and seven refurbishment case studies with the OVF system (Figure 12b) upon total of eight case studies. Table 3 summarizes the eight simulation cases investigated in varying the second-skin material. These cases are: this work. In particular, this table reports the reference case (Case 0) without the second- Case 1, with an OVF system made of a conventional second-skin material (Porcelain gres); Cases 2–5, where the OVF systems have been implemented by using the extrude plastic and polymer materials more used in architecture (polycarbonate multi-wall sheets, ABS, PETG, and PLA); Cases 3_3D–5_3D, where the second-skin materials used in the OVF are the most popular 3D printed polymers (ABS, PETG, and PLA); Buildings 2021, 11, x FOR PEER REVIEW 15 of 27 Buildings 2021, 11, 141 15 of 27 skin (Figure 12a) and seven refurbishment case studies with the OVF system (Figure 12b) upon varying the second-skin material. These cases are:  Case 1, with an OVF system made of a conventional second-skin material (Porcelain Table 3. Summary of case studies investigated. gres); Case Study Second-Skin Material Insulation Thickness (m) Air Gap (m)  Cases 2–5, where the OVF systems have been implemented by using the extrude plas- Case 0 - - - tic and polymer materials more used in architecture (polycarbonate multi-wall Case 1 sheets, Por ABS celain , PET gresG, and PLA); 0.072 Case 2 Polycarbonate multi-wall sheets 0.063  Cases 3_3D–5_3D, where the second-skin materials used in the OVF are the most Case 3 Extruded ABS panels 0.070 popular 3D printed polymers (ABS, PETG, and PLA); Case 4 Extruded PETG panels 0.071 0.10 Case 5 Extruded PLA panels 0.069 In addition, an insulation layer has been added in each case study in order to reach Case 3_3D 3D printed ABS panels 0.065 the threshold values specified by the Italian Law [63] and equal to 0.36 W/m K for the Case 4_3D 3D printed PETG panels 0.067 climatic zone considered in this work. The different insulation thicknesses are also re- Case 5_3D 3D printed PLA panels 0.063 ported in Table 3, upon varying the simulation case. (a) (b) Figure Figure 12. 12. Section Section of of the the south south wall wall of of the the ofﬁce offibuilding: ce building (a):r ( efer a) reference cas ence case; (b) e retr ; (b oﬁt ) retrof cases itwith cases with the s the second-skin econd-skin s system. ystem. In addition, an insulation layer has been added in each case study in order to reach In all the retrofit cases, the cavity inlet airspeed is directly related to the wind speed the threshold values speciﬁed by the Italian Law [63] and equal to 0.36 W/m K for the and direction, as only the wind coming from a similar orientation as the second-skin sys- climatic zone considered in this work. The different insulation thicknesses are also reported tem (wind direction = 180° ± 45°) has been considered as input for the Type 1230. In addi- in Table 3, upon varying the simulation case. tion, the second-skin system has a control logic for the air cavity shutters, which are con- In all the retroﬁt cases, the cavity inlet airspeed is directly related to the wind speed sidered open during the cooling period and closed during the heating period. and direction, as only the wind coming from a similar orientation as the second-skin system (wind direction = 180 45 ) has been considered as input for the Type 1230. In addition, Table 3. Summary of case studies investigated. the second-skin system has a control logic for the air cavity shutters, which are considered open during the cooling period and closed during the heating period. Insulation Thickness Air Gap Table 4 shows the thermal-physical properties of the opaque walls of the envelope Case Study Second-Skin Material (m) (m) implemented in the case studies. Case 0 - - - Case 1 Porcelain gres 0.072 Case 2 Polycarbonate multi-wall sheets 0.063 Case 3 Extruded ABS panels 0.070 Case 4 Extruded PETG panels 0.071 0.10 Case 5 Extruded PLA panels 0.069 Case 3_3D 3D printed ABS panels 0.065 Case 4_3D 3D printed PETG panels 0.067 Case 5_3D 3D printed PLA panels 0.063 Buildings 2021, 11, x FOR PEER REVIEW 17 of 27 Buildings 2021, 11, 141 16 of 27 (considered equal to 12 W/m K [48]) and Vd is the volume fraction of the filler in the poly- Table 4. Thermal-physical properties of the opaque walls implemented in the reference case study. mer matrix (calculated as 0.69 of the specimens’ total volume). The internal geometries have been modeled as hexagons as suggested by [50], where hexagons specimens resulted Thickness Density Thermal Conductivity Thermal Capacity Surface Material as the most resilient to physical stress, 3 thus more suitable for a building envelope integra- (m) (kg/m ) (W/mK) (kJ/kgK) tion. Plaster 0.015 1400 0.70 1.01 Bricks 0.238 600 0.36 0.84 Vertical Walls Table 5. Simulation parameters used in this research. Mortar 0.015 1800 0.90 0.91 Parameter Detail Value Plaster 0.015 1400 0.70 1.01 Lighter concrete 0.027 500 0.17 0.88 Walls and South wall without insula- U = 1.15 W/m K Bricks 0.150 600 0.36 0.84 Roof tion (Case 0) Concrete 0.020 600 0.18 0.88 Roof U = 1.10 W/m K Bitumen 0.005 1200 0.17 1.47 Thermal Floor U = 0.94 W/m K Tiles 0.020 2000 1.00 1.00 Transmittance Windows (frame ratio of 15%) U = 2.95 W/m K Concrete 0.050 600 0.18 0.88 Floor Bricks South w 0.150 all with insulation 600 (Cases 1, 2, 0.36 0.84 U = 0.36 W/m K Lighter concrete 0.030 500 0.17 0.88 3, 4, 5, 3_3D, 4_3D and 5_3D) −1 Infiltration [72,73] Air changes per hour 0.6 h Table 5 reports the general simulation parameters adopted Set point in the = 20 ° eight C dif [74] fer ent case studies. In particular, this table highlights: (i) the values of the thermal transmittance for Operation period = 16 November/30 Heating system both opaque walls and windows, (ii) the air inﬁltration rate, (iii) the target of the indoor March [74] air temperature, the operation period, and the characteristics of the heating and cooling Heating and COP = 2.67 [64] system, (iv) the occupancy schedule and (v) the internal gains. As can be noticed from Cooling systems Set point = 26 °C [74] Table 5, in all the retroﬁt cases the values of thermal transmittance for the opaque surfaces Operation period = 1 April/15 Novem- Cooling system are equal to those of the reference case (Case 0), with the exception of the south wall, where ber [74] the OVFs have been implemented and the thermal transmittance has been set equal to the EER = 2.41 [64] threshold values speciﬁed by Italian law [63], considering the second-skin materials, the Occupancy Weekdays (8:00–18:00) air cavity and the insulation thicknesses reported in Table 3. Workweek schedule [75] Completely off on the weekends Two parallel-connected electric heat pump (EHP) devices, model CRA/K 91 [64], Operation = Occupancy schedule coupled with a multi-split type air conditioning system, have been used to cover both the Lighting systems Radiative = 11.13 W/m heating and cooling demands. The national grid has been used to cover all the electrical energy demand. Convective = 4.77 W/m The simulation timestep has been set to 30 min. Operation = Occupancy schedule Internal Finally, Type 1230 [61] has been used to model a second-skin layer in plastic 2 and Equipment Radiative = 1.4 W/m gains [76] composite polymers. The parameters required by Type 1230 for each material (i.e., density, Convective = 5.6 W/m thermal capacity, and thermal conductivity), for Cases 1–5, have been derived on the basis Operation = Occupancy schedule of the manufacturers or literature data [40,54,65–67]. With respect to the 3D printed panels Occupants Radiative/Convective = 2.5 W/m (Cases 3_3D–5_3D), several specimens have been printed (Figure 13), in order to measure Absolute humidity = 0.0055 kg/hm the ﬁnal dimensions and density. (a) (b) (c) Figure 13. The specimens made through the 3D printing process: (a) ABS [68]; (b) PETG [69]; (c) PLA [70]. Figure 13. The specimens made through the 3D printing process: (a) ABS [68]; (b) PETG [69]; (c) PLA [70]. Table 6 reports the parameters used to simulate the second-skin systems in the retro- fit cases. Buildings 2021, 11, 141 17 of 27 The thermal conductivity of the 3D printed materials (k ) has been calculated by 3D means of the equation expressed by [48,51] and reported below: k k k 2k d d d d 2 1 V + + + 2 k ah k ah m c m c k = k (6) 3D m k k k 2k d d d d 1 V + + + 2 k ah k ah m c m c where k is the thermal conductivity of the polymer as declared by their manufacturers [68–70], k is the thermal conductivity of the ﬁller consisting of the air in the hexagonal cavities of the printed panels (equal to 0.026 W/mK [71]), a is the ﬁller radius measured from the specimens (measured as 0.0045 m), h is the interfacial boundary conductance (considered equal to 12 W/m K [48]) and V is the volume fraction of the ﬁller in the polymer matrix (calculated as 0.69 of the specimens’ total volume). The internal geometries have been modeled as hexagons as suggested by [50], where hexagons specimens resulted as the most resilient to physical stress, thus more suitable for a building envelope integration. Table 5. Simulation parameters used in this research. Parameter Detail Value Walls and South wall without insulation (Case 0) U = 1.15 W/m K Roof U = 1.10 W/m K Thermal Floor U = 0.94 W/m K Transmittance Windows (frame ratio of 15%) U = 2.95 W/m K South wall with insulation (Cases 1, 2, 3, 4, 5, U = 0.36 W/m K 3_3D, 4_3D and 5_3D) Inﬁltration [72,73] Air changes per hour 0.6 h Set point = 20 C [74] Heating system Operation period = 16 November/30 March [74] COP = 2.67 [64] Heating and Cooling systems Set point = 26 C [74] Cooling system Operation period = 1 April/15 November [74] EER = 2.41 [64] Occupancy Weekdays (8:00–18:00) Workweek schedule [75] Completely off on the weekends Operation = Occupancy schedule Lighting systems Radiative = 11.13 W/m Convective = 4.77 W/m Operation = Occupancy schedule Internal Equipment Radiative = 1.4 W/m gains [76] Convective = 5.6 W/m Operation = Occupancy schedule Occupants Radiative/Convective = 2.5 W/m Absolute humidity = 0.0055 kg/hm Table 6 reports the parameters used to simulate the second-skin systems in the retroﬁt cases. Buildings 2021, 11, 141 18 of 27 Table 6. Summary of the main simulation parameters for the Type 1230, upon varying the materials used as a second-skin layer. Case Case Case Case Case Case Case Case Parameters 1 2 3 4 5 3_3D 4_3D 5_3D Polycarbonate Porcelain Extruded Extruded Extruded 3D printed 3D printed 3D printed Material multi-wall gres ABS PETG PLA ABS PETG PLA sheets Thickness 0.010 (m) Density 2000 300 1040 1300 1300 331 411 395 (kg/m ) Thermal capacity 0.840 1.05 1.40 1.20 1.80 1.21 1.07 1.25 (kJ/kgK) Thermal conductivity 1.20 0.0453 0.17 0.29 0.13 0.0548 0.0818 0.0448 (W/mK) Resistance of interface layer 0.486 0.427 0.472 0.479 0.467 0.438 0.455 0.427 (hm K/kJ) Convective heat transfer coefﬁcient of 2.06 2.34 2.12 2.09 2.14 2.28 2.20 2.35 interface layer (kJ/hm K) In conclusion, two additional parameters have been set for each case study, required by the Type 1230 as highlighted in Section 4: the resistance of interface layer, to be set in Type 1230 itself, and the convective heat transfer coefﬁcient of the interface layer, to be set in the construction south wall in Type 56 instead. The convective heat transfer coefﬁcients of the back of the south wall value have been set equal to the thermal transmittance value of the insulation layers, which act as interface layers between the two TRNSYS Types, while the thermal resistance values of the interface layer have been simply calculated as the inverse of the convective heat transfer coefﬁcients. These plastic and polymer materials do not differ only in terms of thermo-physical properties but also in terms of cost. In this work, the capital cost for each retroﬁt action has been neglected; however, Table 7 provides an overview of the costs per square meter associated with each plastic and polymer material implemented as a second-skin layer in the OVF system [54,77–79]. In general, the polycarbonate multi-wall sheets prove to be the cheaper material (10–25 /m ), being also the only one which is already used in the building sector; instead, the 3D printed panels are the more expensive ones (188–225 /m ). This cost limitation is typical for the 3D printing technology, especially compared to more traditional building materials. Despite this signiﬁcant difference in cost, it must be noted that 3D printing is an emerging technology which usage is still not so widespread. However, the 3D printing technology is the only one that would allow for the obtaining of complex panels’ shapes easily. Also, the price of the 3D printed panels reported in Table 7 are related to the brand-new spool, while it is possible to integrate also recycled ﬁlament spools in the production process [47]; indeed, the 3D printing manufacturing process is the only one where it’s easy to fully integrate eco-compatible materials, like PLA. Buildings 2021, 11, 141 19 of 27 Table 7. Costs per square meter of plastic and polymer materials used as a second-skin layer in this research [54,77–79]. Polycarbonate Extruded Extruded Extruded 3D Printed 3D Printed 3D Printed Multi-Wall Sheets ABS PETG PLA ABS * PETG * PLA * Cost 10–25 70–175 75–150 75–130 188–225 190–220 190–207 ( /m ) * Considering about 3.2 kg of material and including also the 3D printing cost (equal to 1.0 /h, for 130 h of the whole printing process), for a panel of 1 m . 3.1. Energy Analyses: Methods According to [12,75], the energy comparison between the proposed case (PC) and the reference case (RC) has been carried out considering the non-renewable primary energy consumption through the index PES (non-renewable primary energy saving): h i RC PC RC PES = E E /E 100 (7) p p p RC where E is the non-renewable primary energy associated with the reference case (Case 0, PC see Table 3), while E is the non-renewable primary energy associated with the eight proposed cases (Cases 1–5 and Cases 3_3D–5_3D, see Table 3). RC PC The values of the E and E are calculated as reported below: p p RC RC E E RC th cool E = + + E + E /h (8) p el,equi pment el,lighting PP COP EER PC PC E E PC th cool E = + + E + E /h (9) el,equi pment el,lighting PP COP EER where h is the Italian power plants’ average efﬁciency, including the transmission losses, PP and it is assumed equal to 0.42 [74]. A positive value of the index PES means that the proposed refurbishment allows reducing the non-renewable primary energy consumption with respect to the reference case. 3.2. Energy Analyses: Results In this section, the simulation results of the refurbishment case study are reported and commented on. Figure 14 reports the values of PES as a function of the proposed case studies, while Figures 15 and 16 show the main energy ﬂows of the building during the whole simulation period upon varying the simulation case. In particular, Figures 15 and 16 report the thermal energy ﬂows in positive values, while the cooling energy ﬂows in negative values. These ﬁgures highlight that: all the proposed OVF systems return positive PES values in comparison to the refer- ence case, which means a reduction of the non-renewable primary energy consump- tion ranging from 2.58% (Case 1) and 2.64% (Cases 2 and 5_3D); this is due to an average reduction of the thermal and cooling energy demands of about 6.9% and 3.0%, respectively; the retroﬁt actions where the plastic and composite polymers materials are used as a second-skin layer (Cases 2–5 and Cases 3_3D–5_5D, see Table 3) allow for a slight performance improvement with respect to those realized with a conventional second- skin material (Case 1), thanks to a reduction in the space cooling energy demand ranging from 31 kWh (Case 4) and 120 kWh (Case 5_3D); the results associated with the polycarbonate multi-wall sheets (Case 2) show a be- havior similar to the Cases 3_3D–5_5D, mostly due to the fact that the polycarbonate panels have a structure assimilable to the 3D printing logic; Buildings 2021, 11, x FOR PEER REVIEW 20 of 27 2.70 2.65 2.64 2.64 2.63 2.62 2.62 2.61 2.60 2.60 2.58 Buildings 2021, 11, 141 20 of 27 2.55 considering the cases with the same polymers (Case 3 vs. Case 3_3D, Case 4 vs. Case Buildings 2021, 11, x FOR PEER REVIEW 20 of 27 4_3D, and Case 5 vs. Case 5_3D), the 3D printed panels allow for a slight improvement 2.50 Case 1 Case 2 Case 3 Case 4 Case 5 Case 3_3D Case 4_3D Case 5_3D in performances. Simulation Cases Figure 14. Values of PES upon varying the case studies. 2.70 These figures highlight that:  all the proposed OVF systems return positive PES values in comparison to the refer- 2.65 ence case, which means a reduction of the non-renewable primary energy consump- 2.64 2.64 2.63 tion ranging from 2.58% (Case 1) and 2.64% (Cases 2 and 5_3D); this is due to an 2.62 2.62 average reduction of the thermal and cooling energy demands of about 6.9% and 2.61 3.0%, respectively; 2.60 2.60  the retrofit actions where the plastic and composite polymers materials are used as a 2.58 second-skin layer (Cases 2–5 and Cases 3_3D–5_5D, see Table 3) allow for a slight performance improvement with respect to those realized with a conventional sec- ond-skin material (Case 1), thanks to a reduction in the space cooling energy demand 2.55 ranging from 31 kWh (Case 4) and 120 kWh (Case 5_3D);  the results associated with the polycarbonate multi-wall sheets (Case 2) show a be- havior similar to the Cases 3_3D–5_5D, mostly due to the fact that the polycarbonate 2.50 panels have a structure assimilable to the 3D printing logic; Case 1 Case 2 Case 3 Case 4 Case 5 Case 3_3D Case 4_3D Case 5_3D  considering the cases with the same polymers (Case 3 vs. Case 3_3D, Case 4 vs. Case Simulation Cases 4_3D, and Case 5 vs. Case 5_3D), the 3D printed panels allow for a slight improve- Figure 14. Values of PES upon varying the case studies. Figur men e 14. t in per Values formances of PES up . on varying the case studies. These figures highlight that:  all the proposed OVF systems return positive PES values in comparison to the refer- ence case, which means a reduction of the non-renewable primary energy consump- tion ranging from 2.58% (Case 1) and 2.64% (Cases 2 and 5_3D); this is due to an average reduction of the thermal and cooling energy demands of about 6.9% and 3.0%, respectively;  the retrofit actions where the plastic and composite polymers materials are used as a second-skin layer (Cases 2–5 and Cases 3_3D–5_5D, see Table 3) allow for a slight performance improvement with respect to those realized with a conventional sec- -2 ond-skin material (Case 1), thanks to a reduction in the space cooling energy demand -4 ranging from 31 kWh (Case 4) and 120 kWh (Case 5_3D); -6 -8  the results associated with the polycarbonate multi-wall sheets (Case 2) show a be- -10 havior similar to the Cases 3_3D–5_5D, mostly due to the fact that the polycarbonate -12 panels have a structure assimilable to the 3D printing logic; Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec  considering the cases with the same polymers (Case 3 vs. Case 3_3D, Case 4 vs. Case Figure 15. Main energy flows of the building during the whole simulation period associated with Figure 15. Main energy ﬂows of the building during the whole simulation period associated with 4_3D, and Case 5 vs. Case 5_3D), the 3D printed panels allow for a slight improve- Case Case 0, 0,Case Case 1,1 and , and retr retrofit oﬁt Cases Case with s with extruded extrupanels. ded panels. ment in performances. -2 -4 -6 -8 -10 -12 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Figure 15. Main energy flows of the building during the whole simulation period associated with Case 0, Case 1, and retrofit Cases with extruded panels. Values of PES (%) Thermal and cooling energy associated with Thermal and cooling energy associated with Values of PES (%) the whole building (MWh) the whole building (MWh) 18.90 17.66 18.90 17.66 17.66 17.66 17.66 17.66 17.66 17.66 17.66 16.36 15.26 17.66 15.26 16.36 15.26 15.26 15.26 15.26 15.26 15.26 12.96 12.07 15.26 12.07 15.26 12.07 12.07 12.96 12.07 12.07 12.07 12.07 12.07 12.07 -0.46 -0.42 -0.41 -0.42 -0.42 -0.42 -0.46 -2.70 -2.66 -0.42 -2.63 -0.41 -2.65 -0.42 -2.65 -0.42 -2.64 -0.42 -7.05 -6.90 -2.70 -6.88 -2.66 -6.89 -2.63 -6.89 -2.65 -6.88 -2.65 -7.19 -2.64 -7.02 -6.99 -7.05 -7.01 -6.90 -7.01 -6.88 -7.00 -6.89 -3.50 -6.89 -3.38 -6.88 -3.36 -3.37 -7.19 -3.38 -7.02 -3.37 -6.99 -0.61 -7.01 -0.55 -7.01 -0.54 -7.00 -0.55 -0.55 -3.50 -0.54 -3.38 -0.01 -3.36 6.14 5.64 -3.37 5.64 -3.38 5.64 -3.37 5.64 5.64 -0.61 -0.55 15.70 14.57 -0.54 14.58 -0.55 14.58 -0.55 14.58 -0.54 14.58 -0.01 6.14 5.64 5.64 5.64 5.64 5.64 15.70 14.57 14.58 14.58 14.58 14.58 Buildings 2021, 11, x FOR PEER REVIEW 21 of 27 Buildings 2021, 11, 141 21 of 27 Buildings 2021, 11, x FOR PEER REVIEW 21 of 27 -2 -4 -6 -8 -10 -12 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Figure 16. Main energy flows of the building during the whole simulation period associated with Case 0, Case 1, and retrofit Ca 6ses with 3D printed panels. In order to better investigate the performance of the plastic and polymer materials used as a second-skin layer in the proposed OVF system, the trends of the values of the air tem- -2 -4 perature inside the cavity and the airspeed at the inlet and the outlet of the air cavity for the -6 Case 5_3D in a typical summer day (2 August) is also reported in Figures 17 and 18 with a -8 1 h timestep. In general, similar trends in the values of the temperatures in the cavity as -10 -12 well as the airspeed at the cavity inlet and the outlet have been predicted for all the other Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec cases. Figure 16. Main energy flows of the building during the whole simulation period associated with In more detail, Figure Figure 16. 17 Main show energy s th ﬂows e tren of the ds building of the during daily tva helu whole es osimulation f the air period temper associated ature with Case 0, Case 1, and retrofit Cases with 3D printed panels. Case 0, Case 1, and retroﬁt Cases with 3D printed panels. inside the cavity upon varying the height from the ground, and, in particular reports, the temperature of the air at the cavity inlet (about 0.15 m from the ground), the average tem- In order to better investigate the performance of the plastic and polymer materials used In order to better investigate the performance of the plastic and polymer materials perature of the air cavi as aty sec(h onig d-hl skiig n ht layed er in by th th e pe robl po ue sed reg OViF on sysin tem th , te he b tuil rending ds of tsection he values ) o and f the the air t em- used as a second-skin layer in the proposed OVF system, the trends of the values of the air perature inside the cavity and the airspeed at the inlet and the outlet of the air cavity for the temperature of the air temperatur at the cavity e insideo the utl cavity et (abo and ut the 10. airspeed 00 m from at the th inlet e ground and the );outlet Figure of the 18 air re-cavity Case 5_3D in a typical summer day (2 August) is also reported in Figures 17 and 18 with a for the Case 5_3D in a typical summer day (2 August) is also reported in Figures 17 and 18 ports the values of the airspeed at the cavity inlet (about 0.15 m from the ground) and the 1 h timestep. In general, similar trends in the values of the temperatures in the cavity as with a 1 h timestep. In general, similar trends in the values of the temperatures in the airspeed at the cavity outlet (about 10.00 m from the ground), as well as the total vertical well as the airspeed at the cavity inlet and the outlet have been predicted for all the other cavity as well as the airspeed at the cavity inlet and the outlet have been predicted for all radiation on the external surface of the second-skin layer. cases. the other cases. In more detail, Figure 17 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, and, in particular reports, the temperature of the air at the cavity inlet (about 0.15 m from the ground), the average tem- perature of the air cavity (highlighted by the blue region in the building section) and the temperature of the air at the cavity outlet (about 10.00 m from the ground); Figure 18 re- ports the values of the airspeed at the cavity inlet (about 0.15 m from the ground) and the airspeed at the cavity outlet (about 10.00 m from the ground), as well as the total vertical radiation on the external surface of the second-skin layer. Figure 17. Simulated daily values of the cavity air temperature for a typical summer day. Figure 17. Simulated daily values of the cavity air temperature for a typical summer day. Figure 17. Simulated daily values of the cavity air temperature for a typical summer day. Thermal and cooling energy associated with the whole building (MWh) 18.90 17.66 17.66 17.66 17.66 16.36 15.26 15.26 15.26 Thermal and cooling energy associated with 15.26 12.96 the whole building (MWh) 12.07 12.07 12.07 12.07 18.90 17.66 17.66 17.66 17.66 16.36 15.26 -0.46 15.26 -0.42 15.26 -0.42 15.26 -0.42 12.96 -0.41 12.07 12.07 -2.70 12.07 -2.66 12.07 -2.64 -2.64 -2.63 -7.05 -6.90 -0.46 -0.42 -6.88 -0.42 -6.88 -0.42 -6.88 -0.41 -7.19 -2.70 -2.66 -7.02 -2.64 -6.99 -2.64 -7.00 -2.63 -6.99 -7.05 -6.90 -3.50 -6.88 -3.38 -6.88 -3.36 -6.88 -3.37 -7.19 -3.36 -7.02 -6.99 -0.61 -7.00 -0.55 -6.99 -0.54 -3.50 -0.54 -3.38 -0.54 -3.36 -3.37 -0.01 -3.36 6.14 5.64 -0.61 -0.55 5.64 -0.54 5.64 -0.54 5.64 -0.54 15.70 -0.01 6.14 14.57 5.64 14.58 5.64 5.64 14.58 5.64 14.58 15.70 14.57 14.58 14.58 14.58 Buildings 2021, 11, x FOR PEER REVIEW 22 of 27 Buildings 2021, 11, 141 22 of 27 Cavity air inlet Cavity air outlet Total vertical solar radiation .0 00 Outlet air speed .0 00 2 .0 00 10.00 m 0 .0 00 .0 00 .0 00 .0 200 2.0 00 Inlet air speed 0.0 0 0.15 m i e hh: Figure 18. Simulated values of the airspeed at the inlet and the outlet of the air cavity for a typical summer day. Figure 18. Simulated values of the airspeed at the inlet and the outlet of the air cavity for a typical summer day. In more detail, Figure 17 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, and, in particular reports, The data reported in Figure 17 corresponds to the input and outputs of Type 1230. In the temperature of the air at the cavity inlet (about 0.15 m from the ground), the average particular, the inlet temperatur air tempera e ofture the air is cavit an input y (highlighted for Typ by e 1230, the blue wrhil egion e th in e the aver building age air section) tem- and the temperature of the air at the cavity outlet (about 10.00 m from the ground); Figure 18 perature in the whole cavity (Tcavity, the blue-edged markers in Figure 17) and the outlet reports the values of the airspeed at the cavity inlet (about 0.15 m from the ground) and the air temperature are returned as output results by Type 1230 itself. These values represent airspeed at the cavity outlet (about 10.00 m from the ground), as well as the total vertical the only two temperature values associated with the air cavity returned by Type 1230 [61]. radiation on the external surface of the second-skin layer. As a first observation, the overall temperature distribution is directly related to solar ra- The data reported in Figure 17 corresponds to the input and outputs of Type 1230. In particular, the inlet air temperature is an input for Type 1230, while the average air diation throughout the day, where the temperatures rise during the morning and drop temperature in the whole cavity (T , the blue-edged markers in Figure 17) and the cavity during the afternoon. Also, during the day, the temperature trend seems to be constantly outlet air temperature are returned as output results by Type 1230 itself. These values rising from the air cavity inlet to the air cavity outlet; this behavior is due to the chimney represent the only two temperature values associated with the air cavity returned by Type effect that is created thanks to the OVF system. 1230 [61]. As a ﬁrst observation, the overall temperature distribution is directly related Figure 18 report to s solar the radiation simulation throughout data cor the respon day, wher dine g the to th temperatur e cavity es inlet rise during airspeed the (i morning n- and drop during the afternoon. Also, during the day, the temperature trend seems to be put of the Type 1230) and the cavity outlet airspeed (output of the Type 1230), as well as constantly rising from the air cavity inlet to the air cavity outlet; this behavior is due to the the total vertical solar radiation. This figure shows how, during the day, the values pre- chimney effect that is created thanks to the OVF system. dicted at the cavity air outlet are always higher than those at the cavity air inlet (with a Figure 18 reports the simulation data corresponding to the cavity inlet airspeed (input difference between outlet and inlet ranging from 0.01 m/s and 1.46 m/s), gradually rising of the Type 1230) and the cavity outlet airspeed (output of the Type 1230), as well as the total vertical solar radiation. This ﬁgure shows how, during the day, the values predicted to a maximum peak at around 14:00. at the cavity air outlet are always higher than those at the cavity air inlet (with a difference Finally, in order to verify the potential benefits coming from the best case, an addi- between outlet and inlet ranging from 0.01 m/s and 1.46 m/s), gradually rising to a tional simulation case, not reported in Table 3, has been carried out. In this last simulation, maximum peak at around 14:00. the OVF system has been implemented on the whole building, following the same instal- Finally, in order to verify the potential beneﬁts coming from the best case, an additional lation methodology simulation of the prev case, ious not c reported ases. Th ineT mater able 3, ial hass been elected carried as a out. second In this-sk last in simulation, layer is the OVF system has been implemented on the whole building, following the same installation the 3D printed PLA, which proved to be one of the most effective in improving the non- methodology of the previous cases. The material selected as a second-skin layer is the 3D renewable primary energy saving. The proposed OVF system returned a PES value equal printed PLA, which proved to be one of the most effective in improving the non-renewable to 8.10% if compared to the reference case. primary energy saving. The proposed OVF system returned a PES value equal to 8.10% if compared to the reference case. 4. Conclusions The OVFs have been, more and more frequently, chosen for different building typol- ogies (offices, schools, residential) and in different climates. Nowadays, there are always more innovative materials used in architecture and as a second-skin layer, even if the eval- uation of their impact on the envelope’s energy performance is a complex task. In partic- ular, the use of polymers in building and engineering has increased substantially, thanks to their: (i) ease of production, (ii) ease of installation, (iii) durability, (iv) low maintenance requirements, (v) lightweight nature and (vi) ability to be formed into complex shapes. Several of these plastic products can also be utilized in additive manufacturing processes, providing excellent freedom of form, enhancing designers, architects, and engineers’ free- dom in creating complex designs. it irs ee s ol r i tio Buildings 2021, 11, 141 23 of 27 4. Conclusions The OVFs have been, more and more frequently, chosen for different building typolo- gies (ofﬁces, schools, residential) and in different climates. Nowadays, there are always more innovative materials used in architecture and as a second-skin layer, even if the evalu- ation of their impact on the envelope’s energy performance is a complex task. In particular, the use of polymers in building and engineering has increased substantially, thanks to their: (i) ease of production, (ii) ease of installation, (iii) durability, (iv) low maintenance require- ments, (v) lightweight nature and (vi) ability to be formed into complex shapes. Several of these plastic products can also be utilized in additive manufacturing processes, providing excellent freedom of form, enhancing designers, architects, and engineers’ freedom in creating complex designs. In this work, the numerical model of extruded ABS panels in an OVF system has been developed and validated. Then, the simulation methods, suggested by the authors, have been applied in different refurbishment cases upon varying: the polymer and the manufacturing technology, extrusion (polycarbonate multi-wall sheets, ABS, PETG, and PLA), and 3D printing (ABS, PETG, and PLA). The simulations have been carried out in order to assess the potential beneﬁts achiev- able in terms of non-renewable primary energy saving, as well as thermal and cooling energy demand reduction. The simulation results highlight that: (i) all the proposed retroﬁt cases allow to achieve a beneﬁt in terms of PES; (ii) the plastic and composite polymers materials allow for a slight performance improvement with respect to conven- tional second-skin material, such as porcelain gres; (iii) the best performances among the extruded polymers are returned by the polycarbonate multi-wall sheets (PES value equal to 2.64%); (iv) the best performances among the 3D printed polymers are achieved when using the PLA (PES value equal to 2.64%). However, the polymers’ results show very similar performances, thus allowing building contractors, designers, and architects to select the material based on other project requirements, as mechanical strength, weather resistance, environmental impact, etc. In this work, the thermal conductivity of the 3D printed materials has been calculated by means of an equation expressed in literature; therefore, in order to improve the accuracy of the model for the 3D printed material, in future works, the authors will carry out experimental investigations on full-scale 3D printed panels in OVF systems through the Gemini test cells. In addition, the capital cost for each retroﬁt action has been neglected. However, they represent an important parameter in the refurbishment typology choice; therefore, in future work, the authors will focus on a detailed economic analysis considering both the operating cost reduction and the simple payback period. Author Contributions: Conceptualization, G.C., Y.S., M.S., A.R., and S.S.; methodology, G.C., Y.S., M.S., A.R., and S.S.; software, G.C. and Y.S.; validation, G.C., Y.S., M.S., A.R., and S.S.; formal analysis, G.C., Y.S., M.S., and A.R.; investigation, G.C. and Y.S.; resources, G.C. and S.S.; data curation, G.C. and Y.S.; writing—original draft preparation, G.C., Y.S., and S.S.; writing—review & editing, G.C., Y.S., M.S., and A.R.; visualization, G.C., Y.S., M.S., A.R., and S.S.; supervision, G.C., M.S., and S.S.; project administration, G.C., M.S., and S.S.; funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was co-funded by a collaborative research and development project, number F/050405/01-03/X32 “WALLED: Smart LED&OLED per Lighting e MediaBuilding”–Fondo per la crescita sostenibile–Call Horizon 2020 PON I&C and by the European Union-PON for Research and Innovation 2014–2020. Data Availability Statement: The data presented in this study are available on request from the corresponding authors. Acknowledgments: The authors would like to thank the Academic Editors for their invitation to the Special Issue “Novel Technologies to Enhance Energy Performance and Indoor Environmental Quality of Buildings” and the Assistant Editor for his support. The authors would also like to thank the three anonymous reviewers for their insightful suggestions and careful reading of the manuscript. Buildings 2021, 11, 141 24 of 27 Conﬂicts of Interest: The authors declare no conﬂict of interest. Nomenclature Latin letters A surface area (m ) a ﬁller radius (m) ABS acrylonitrile-butadiene-styrene AM additive manufacturing CAGR compound annual growth rate COP Coefﬁcient of performance DSF double-skin facade E energy (kWh) EER energy efﬁciency ratio (-) EHP electric heat pump h interfacial boundary conductance (W/m K) I vertical pyranometer on the south wall (W/m ) vert k thermal conductivity of the 3D printed materials (W/mK) 3D k thermal conductivity of the ﬁller (W/mK) k thermal conductivity of the selected 3D printable polymers (W/mK) MAE Mean Absolute Error ( C) ME Mean Error ( C) N number of measurements (-) OVF opaque ventilated facades PC proposed case PES non-renewable primary energy saving (%) PET polyethylene terephthalate PETG polyethylene terephthalate glycol-modiﬁed PLA polylactic acid RC reference case RMSE Root Mean Square Error ( C) T thermocouple/temperature ( C) U transmittance value (m K/W) V volume fraction of the ﬁller W airspeed sensor Greeks D difference h efﬁciency (%) Subscripts/Superscripts cavity air cavity of the second-skin system cool cooling el electricity exp,i experimental value at time step i indoor indoor air p non-renewable primary energy PC proposed case PP power plant RC reference case sim,i simulated value at time step i th thermal w window References 1. 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DOI: 10.3390/buildings11040141
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Abstract

buildings Article Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study Giovanni Ciampi * , Yorgos Spanodimitriou * , Michelangelo Scorpio , Antonio Rosato and Sergio Sibilio * Department of Architecture and Industrial Design, University of Campania Luigi Vanvitelli, 81031 Aversa, Italy; michelangelo.scorpio@unicampania.it (M.S.); antonio.rosato@unicampania.it (A.R.) * Correspondence: giovanni.ciampi@unicampania.it (G.C.); yorgos.spanodimitriou@unicampania.it (Y.S.); sergio.sibilio@unicampania.it (S.S.) Abstract: Plastic materials are increasingly becoming used in the building envelope, despite a lack of investigation on their effects. In this work, an extruded Acrylonitrile-Butadiene-Styrene panel has been tested as a second-skin layer in a ventilated facade system using a full-scale facility. The experimental results show that it is possible to achieve performances very similar to conventional materials. A numerical model has then been developed and used to investigate the performances of plastic and composite polymer panels as second-skin layers. The experimental data has been Citation: Ciampi, G.; used to verify the behavior of the numerical model, from a thermal point of view, showing good Spanodimitriou, Y.; Scorpio, M.; reliability, with a root mean square error lower than 0.40 C. This model has then been applied Rosato, A.; Sibilio, S. Energy in different refurbishment cases upon varying: the polymer and the manufacturing technology Performances Assessment of (extruded or 3D-printed panels). Eight refurbishment case studies have been carried out on a typical Extruded and 3D Printed Polymers ofﬁce building located in Napoli (Italy), by means of a dynamic simulation software. The simulation Integrated into Building Envelopes results show that the proposed actions allow the reduction of the thermal and cooling energy demand for a South Italian Case Study. (up to 6.9% and 3.1%, respectively), as well as the non-renewable primary energy consumption (up Buildings 2021, 11, 141. https:// to 2.6%), in comparison to the reference case study. doi.org/10.3390/buildings11040141 Keywords: ventilated facade; second-skin materials; 3D printed materials; additive manufacturing; Academic Editors: Alessandro Cannavale, TRNSYS; full-scale facility; retroﬁt action; energy saving Francesco Martellotta and Francesco Fiorito Received: 18 February 2021 1. Introduction Accepted: 25 March 2021 Approximately 40% of the EU energy consumption can be directly attributed to Published: 1 April 2021 the building sector, which is also responsible for about 36% of the greenhouse gas emis- sions [1,2]. In addition, in the EU-28, only 3% of the ediﬁces have an efﬁcient building Publisher’s Note: MDPI stays neutral envelope [3], mainly due to the fact that about 35% of the EU’s buildings are over 50 years with regard to jurisdictional claims in old and only around 1% of them are renovated each year [1]. Certainly, the constraints published maps and institutional afﬁl- associated with the new buildings are fewer with respect to those associated with the iations. refurbishment of existing constructions, so the new one allows for better-optimized de- sign in terms of energy efﬁciency of the envelope [4]. However, in Italy, many buildings (about 4 million) were built in the early 1900s and about half of these have been classiﬁed as historical architectures and nowadays have been reused [5]. Therefore, in the Italian Copyright: © 2021 by the authors. scenario, the improvement of the energetic performances of the existing building enve- Licensee MDPI, Basel, Switzerland. lope represents a crucial aspect in the increasing of the building’s energy efﬁciency and This article is an open access article the indoor environmental quality on a large-scale [6]. In this context, different products distributed under the terms and and systems have been proposed to improve the buildings energy efﬁciency, visual and conditions of the Creative Commons thermal comfort, as well as their sustainability [7–12] and, in recent years, the interest of Attribution (CC BY) license (https:// the scientiﬁc community has seen an increase in the facade domain to improve the overall creativecommons.org/licenses/by/ building energy efﬁciency [13]. In particular, the use of passive systems is raising more 4.0/). Buildings 2021, 11, 141. https://doi.org/10.3390/buildings11040141 https://www.mdpi.com/journal/buildings Buildings 2021, 11, 141 2 of 27 and more interest in the building sector [7,8,14]. A passive building is one in which the indoor environment is not regulated by using mechanical heating and cooling systems, but by means of a conscious structure and architectural design of the envelope and its components [7]. In recent years, as part of a shift towards more energy-efﬁcient buildings, a lot of different new facade technologies and solutions have been proposed for the im- provement of their energy performance by the introduction of better insulation, shading devices, as well as a second-skin layer (double-skin facades [15,16], building integrated photovoltaic [17,18] and opaque ventilated facades [19,20]). Among these, the double-skin facade (DSF) and opaque ventilated facades (OVF) have been suggested as one of the best solutions, thanks to their ability to ensure better thermal performance and indoor environmental quality, as well as to improve the aesthetic appeal of buildings [15,16,19,20]. The concept of DSF was introduced in the early 1900s, but little progress was made until the 1990s [7,21]; it consists of a standard facade, an air cavity, and an additional external skin. The material used as a second-skin is usually glass [7,21]; however, a shading device can also be installed within the cavity between the two layers of the facade to control the solar radiation [7,21]. The OVFs are passive systems that consist of multiple layer construction (external second-skin, an intermediate air cavity, and an internal wall). The OVFs have been, more and more frequently, chosen by contractors, designers as well as architects for different typologies (ofﬁces, schools, residential) of new and renovated buildings as well as in different climates [19,20]. Several papers [22–29] investigated the energy performances of OVFs through simulation software (EnergyPlus [30] and TRNSYS [31], the most widely used), highlighting the beneﬁts achievable by these systems. In addition, the literature review [22–29] highlights that the materials usually used as second-skin layer are: glass, porcelain stoneware tile, natural stone, aluminum, OSB, and composite panels. Nowadays, there are always more innovative materials [32–35] used in architecture, also as a second-skin layer, even if evaluating their impact on the envelope’s energy performance is a complex task [6,11]. In this work, plastic and composite polymers have been investigated with the aim to evaluate their integration in the building facade and their potential beneﬁts achievable in refurbishment case studies. 1.1. Plastic and Composite Polymers in Building Facades In recent years, the use of polymers in building and engineering has increased substan- tially, thanks to their: (i) ease of production, (ii) ease of installation, (iii) durability, (iv) low maintenance requirements, (v) lightweight nature and (vi) ability to be formed into complex shapes [34,35]. Moreover, polymers form good thermal and electrical insulators that are not affected by chemical and biological risks [35]. They have not only been used to replace the traditional construction materials (cement, brick, concrete, wood, metal, and glass), but these materials have also been used in a complementary way to improve the building enve- lope performance to satisfy the modern demands of both new projects and refurbishment ones [32–35]. From an aesthetic point of view, these materials are available in several colors and texture alternatives. Several applications of plastic and composite polymer walls in buildings were reported in the literature [33]. In [33], 23 examples of architecture were re- ported, demonstrating that plastic and composite polymers can be used in dwelling domes, large-span volumes or envelop large facade surfaces, and transparent sky-lights on the roofs of industrial buildings. Another example can be found in [36,37], where the designers have realized a temporary pavilion with an envelope made of double-walled transparent corrugated sheets of PolyEthylene Terephthalate (PET) recycled plastic. Similarly, in [38], a massive pavilion, designed as an exhibition hall for the 2010 Taipei International Flora Expo, has been built using recycled PET for the building envelope, also proving strong resistance to ﬁres and earthquakes. As reported in [39], polycarbonate multi-sheet systems are increasing their share of the glazing market since they provide good performance while weighing and costing signiﬁcantly less than glass. For these reasons, several studies have been conducted to assess their characteristics upon varying geometries and installation typology. In [39], the authors assessed the thermal and optical characteristics of different Buildings 2021, 11, 141 3 of 27 polycarbonate sheets, highlighting a strong angular dependence in polycarbonate panels’ optical properties, signiﬁcantly different to the conventional multiple-layered glass, and a good overall thermal behavior. For these reasons, the polycarbonate could be used as a valid alternative, fulﬁlling the energy requirements and improving the visual comfort, reducing the glaring problem by diffusing the light, while providing more ﬂexibility in the design and appearance of the buildings. In [40], a detailed experimental analysis of the thermal behavior of different polycarbonate multi-sheet systems has been carried out by varying the installation angle. The results highlighted a very low incidence of the angle of inclination on the equivalent value of thermal conductivity, thus allowing the material to be equally used in every part of the building envelope. Thus, polycarbonate has already found large usage, as in the Bavaria Brewery Tocancipá headquarters, by Construcciones Planiﬁcadas [41], where the plastic has been used to get an industrial appearance while providing for thermal and visual comfort, or the property registration ofﬁces, by Irisarri + Piñera, where polycarbonate has been used to complement and balance the appearance of the existing structure. Across these materials already implemented in the traditional architectural paradigms, there is also a strong boost in the usage of plastics from the additive manufacturing (AM) sector, as several plastic products can also be utilized in AM processes, providing great freedom of form and enhancing designers’, architects’, and engineers’ freedom in creating complex designs [42–46]. In addition, the global 3D printing ﬁlament material market volume was 1.8 billion US$ in 2019, growing at a compound annual growth rate (CAGR) of 27% [47]. The most popular ﬁlament materials are PolyLactic Acid (PLA) and Acrylonitrile-Butadiene-Styrene (ABS), holding about 47% and 29% of the market, respectively. In contrast, materials as PolyEthylene Terephthalate Glycol-modiﬁed (PETG) can be useful alternatives, despite not being as popular, providing similar mechanical properties while also offering excellent recyclability and scalability [47]. The AM in the facade industry presents new signiﬁcant potential and requires relevant research to be conducted [46]. Indeed, more and more 3D printing materials are concurrently becoming utilized in contemporary architecture design [42–46], thanks to the lightness and effortless installation procedure, which results in a design solution useful for both new projects and refurbishment ones [44–46]. The 3D printing technology has been often used to create everything, from prototypes [42,43,46] and simple parts of facades, to give a distinctive signature to the constructions [44,45]. The 3D printing materials prove themselves to offer quite unique characteristics from an architectural and economic point of view [43–46]. Several scientiﬁc papers have been conducted into the loadbearing capacities and/or other essential qualities of AM products for the building industry, such as durability, water vapor diffusion resistance, thermal conductivity, or ﬁre-resistance [48–51]. The authors emphasized the potential applications of additive manufacturing to build honeycomb panels that optimize mechanical properties and heat transfer [48–51]. However, scientiﬁc research related to 3D printing materials in building energy efﬁciency applications is limited due to its relatively new nature as a technology. Only Sarakinioti et al. [46] aimed their research at developing an integrated 3D printed for thermal insulation and building physics. In particular, they presented a 3D printed facade panel design for thermal insulation and movable liquid heat storage [46], providing an overview of the development process. The authors tested the prototype and, at the same time, simulated to verify the thermal effects of the proposed facade system on indoor spaces in different climates. The simulation results reported in [46] showed the potential of the proposed 3D printed facade panel for reducing heating and cooling energy demand. Therefore, the effects of adopting 3D printing materials as a second-skin layer on the indoor environment have been scarcely investigated. Moreover, there is a lack of experimental testing and numerical model development of these materials in building simulation, even more, if considered in a second-skin in front of the building envelope, in a 3D printed composite facade arrangement. Indeed, in a facade arrangement realized with these innovative materials (ABS, PLA, PETG, etc.), the difﬁculties lie in predicting the behavior of the various facade sections, as the second-skin Buildings 2021, 11, 141 4 of 27 layer, the resulting air cavity, and ﬁnally, the effects on the indoor environment. Therefore, from an experimental point of view, standardization bodies, experts, and researchers are continually developing new methodologies or new procedures to correctly calculate the performances of these envelope components in simple and economical ways [6]. 1.2. Research Aims In this work, extruded ABS panels have been tested as second-skin materials in order to verify their performances in an OVF system. This novel material for building envelopes has been investigated through in-situ measurements by using two outdoor comparative test cells. The experimental data have been used to calibrate and validate a numerical model in TRNSYS 18 [31], also verifying the ability of the simulation software to effectively reproduce the behavior of a light material in an OVF system, which usually is made of materials as porcelain gres. Then, the validated numerical methodology has been used to implement different plastic materials in a set of refurbishment case studies, compared to a reference ofﬁce building, in order to assess the potential beneﬁts. The comparison has been performed in terms of (i) heating and (ii) cooling energy demands, as well as (iii) non-renewable primary energy consumption, upon varying the plastic material. Finally, additional refurbishment case studies have also been implemented considering 3D printed panels as a second-skin layer. The aims of this research can be summarized as reported below: investigate the performances of extruded ABS panels as a second-skin layer for inno- vative building envelopes with experimental tests in-situ; calibrate and validate a simulation model to predict the energy performance of the plastic and composite polymer panels used as a second-skin layer in an OVF system; assess the potential energy saving achievable in office building refurbishment using the proposed materials (extruded and 3D printed polymers) through numerical simulation. 1.3. Structure of the Research The research is structured as follows. Section 2 describes the methodology used to carry out the research, showing in detail how (i) the experimental data have been acquired, (ii) the numerical model has been implemented in TRNSYS 18, and how (iii) the experimental data have been used to calibrate and validate this numerical model. Section 3 reports the numerical results, in terms of the reduction of non-renewable primary energy consumption, achieved in an ofﬁce building refurbishment through the installation of an OVF system, upon varying the material used as a second-skin layer, considering both extruded and 3D printed ones. Finally, Section 4 discusses the integration capacity of the plastic and composite polymer panels in a second-skin layer of an OVF system, highlighting the advantages and limitations of such materials. 2. Methodology This section describes in detail the measurement methodologies and the experimental results obtained during the in-situ test as well as the methods and results related to the validation of the implemented numerical model. 2.1. Description of the Gemini Facilities, Experimental Results, and Discussion In this sub-section, a couple of experimental test cells and the experimental results are reported. Gemini facilities [11] are designed and built at the Ri.A.S.–Built Environment Control Laboratory [52] of the Department of Architecture and Industrial Design of the 0 00 0 00 University of Campania Luigi Vanvitelli in Aversa (40 59 39.1 N, 14 10 48.5 E). These full- size outdoor test cells have been designed to experimentally evaluate double-skin facade module performances, in real outdoor weather conditions. The test cells are designed as an identical couple in order to carry out comparative measurements. The Gemini’s internal Buildings 2021, 11, 141 5 of 27 dimensions are 2.20 m wide by 2.80 m deep and 2.40 m tall, oriented with the long side along the north-south axis. These dimensions correspond to the gross dimension of the main steel frame structure, on which the shell has been ﬁxed externally and seamlessly in order to avoid thermal bridges. The shell has been realized in a single layer of 10 mm thick sandwich panels consisting of two galvanized steel sheets and a polyurethane rigid foam ﬁlling, with a thermal transmittance (U ) value of 0.23 W/m K [11]. Then, for the ﬂoor, a wall 0.10 m air gap and a wood ﬂooring have been added above the structure, while, for the ceiling, a sheet metal roof has been placed 0.10 m above the outer panels, with a 2% slope, to allow a natural rainwater outﬂow. The Gemini’s facilities are designed to allow the in-situ characterization of innovative layers to be applied in double-skin facades with different geometries, layout, materials, and technologies. The acquired data can be used to evaluate the in-situ performances of the system under investigation and to realize, calibrate and validate simulation models. The Gemini is well-instrumented to acquire different indoor and outdoor physical quantities. Table 1 shows the measurement range, the accuracy, and the response time of the sensors used for outdoor and indoor climate characterization. In particular, with the aim to evaluate the real weather conditions, sensors for wind direction, wind speed, air temperature, air relative humidity, air pressure, global horizontal radiation, and diffuse horizontal radiation were placed at about 6.50 m from the ground, in the best position to minimize the inﬂuence of external obstructions (i.e., the obstructions angles are less than 10 ). In order to acquire diffuse horizontal radiation, one of the pyranometers is equipped with a shadow ring (diameter of 0.574 m and thickness equal to 0.052 m), and the data were corrected following the methodology proposed in [53], to take into account both the isotropic and anisotropic conditions. Figure 1 shows the weather station, with all the aforementioned sensors. Table 1. Installed Gemini sensor measurement range and accuracy. Number of Sensors Measured Quantity Type Range Accuracy ‘Pro First Class’ 1 Wind speed 0–50 m/s 0.01 m/s anemometer ‘Pro First Class’ 1 Wind direction 0–356.9 3 1 anemometer Temperature: Temperature: Air Temperature and Thermo-hygrometer 40–+60 C 0.2 C Relative humidity with precision transducer Rel. Humidity: Rel. Humidity: 0–100% 2% Atmospheric Barometer with 0.3 hPa 1 800–1100 hPa pressure piezo-resistive transducer at 20 C II class thermopile 2 2 3 Solar radiation 0–2000 W/m 10 V/(W/m ) pyranometer Hot wire air speed 0.2 m/s 2 Air cavity speed 0.2–40.0 m/s transmitter +3% f.s. 10 Temperature T-Type thermocouple 200–+350 C 1.5 C Buildings 2021, 11, x FOR PEER REVIEW 6 of 27 The air temperature inside the Gemini is monitored by a combined temperature-rel- Buildings 2021, 11, 141 6 of 27 ative humidity sensor placed in the middle of the room. Also, when the test cells are con- figured to test a second-skin system, this is monitored through a set of ten thermocouples, placed on the significant interfaces and in the cavity. Figure 1. The weather station used to monitor the real outdoor conditions. Figure 1. The weather station used to monitor the real outdoor conditions. The The sensor air temperatur layout follow e inside s the layout the Gemini shown in is Fig monitor ure 2a. In ed par by ticul a combined ar, it can be n temperatur oted e- that (i) six thermocouples are placed in the middle of both the inlet (T1, T2, and T3) and relative humidity sensor placed in the middle of the room. Also, when the test cells are the outlet (T8, T9, and T10) sections of the air cavity, (ii) four thermocouples are placed in conﬁgured to test a second-skin system, this is monitored through a set of ten thermocou- line at the center of the second-skin system (T4 on the back surface of the second-skin, T5 ples, placed on the signiﬁcant interfaces and in the cavity. in the middle of the cavity, T6 on the external surface of the south wall of the test cell, T7 The sensor layout follows the layout shown in Figure 2a. In particular, it can be noted on the internal surface of the south wall of the test cell, respectively); this last set of ther- that (i) six thermocouples are placed in the middle of both the inlet (T1, T2, and T3) and the mocouples falls in line with the same thermo-hygrometer which monitors the temperature outlet (T8, T9, and T10) sections of the air cavity, (ii) four thermocouples are placed in line of the air inside the test cell, in order to have all the sensors aligned at the center of the at the center of the second-skin system (T4 on the back surface of the second-skin, T5 in the system. middle of the cavity, T6 on the external surface of the south wall of the test cell, T7 on the In addition, all the thermocouples (Tx) have been shielded with high-reflective internal surface of the south wall of the test cell, respectively); this last set of thermocouples domes in order to avoid any direct solar radiation (Figure 2b). Buildings 2021, 11, x FOR PEER REVIEW 7 of 27 falls in line with the same thermo-hygrometer which monitors the temperature of the air The pyranometer (Ivert) has been installed to acquire the vertical solar radiation inci- inside dent on the thtest e south cell,sin urface order and, to have finally, all tw the o hot sensors -wire aligned anemomat etethe rs (center Win, and ofW the out) system. are placed in the air cavity, one in the inlet section and the other one in the outlet section, in order to monitor the airflow in the second-skin cavity. (a) (b) Figure 2. (a) Axonometric views of the sensor layout; (b) shielding devices used in the experimental setup to avoid any Figure 2. (a) Axonometric views of the sensor layout; (b) shielding devices used in the experimental setup to avoid any direct solar radiation on the thermocouples. direct solar radiation on the thermocouples. In order to verify the measurement methodologies and characterize each test cell from the thermal point of view, preliminary data has been acquired in a standard config- uration (both Gemini s without a second-skin system). These data are recorded with the aim of (i) verifying the operation of the different instruments and their correct positioning, (ii) comparing the thermal behavior of the two test cells, and (iii) defining a reference point for the following evaluation of the real performances of double-skin facades or smart win- dows. The preliminary experimental data were acquired and stored every 1 min on a pe- riod of 1 month (from 1 June to 30 June) and, later, averaged on an interval of 15 min. Figure 3 reports the indoor air temperature of Gemini 1 and Gemini 2, the external air temperature, and the global horizontal radiation for three typical days in June. This figure highlights that the difference between the indoor air temperature of Gemini 1 and the indoor air temperature of Gemini 2 (Tindoor, Gemini 1–Tindoor, Gemini 2) is negligible, varying within an interval with a maximum of 0.2 °C and a minimum of −0.2 °C. After the preliminary experimental campaign, a second-skin system has been mounted and tested on a test cell (Gemini 1) with an air cavity gap equal to 0.10 m, while the other cell has been left unequipped and used as a reference (Gemini 2). In particular, the investigated second-skin system has been realized with extruded ABS panels [54]. The experimental data were acquired and stored every 1 min over a period of 1 month (from 8 December to 31 December) and, later, averaged on an interval of 15 min. This experi- mental campaign aims to verify the performances of plastic panels as a second-skin layer for innovative envelopes. The extruded ABS panels have been selected with dimensions equal 600 mm × 1200 mm; such dimensions have been selected on the basis of the dimension of conventional panels in OVF systems, in order to guarantee an easy installation in a commercial OVF structure, as well as an easy substitution of conventional OVF system materials in a real- istic scenario. In Figure 4a, a detailed view of the analyzed extruded ABS panel before the installation is displayed, while Figure 4b shows the Gemini 1 equipped with the second- skin realized with six extruded ABS panels. Buildings 2021, 11, 141 7 of 27 In addition, all the thermocouples (Tx) have been shielded with high-reﬂective domes in order to avoid any direct solar radiation (Figure 2b). The pyranometer (I ) has been installed to acquire the vertical solar radiation in- vert cident on the south surface and, ﬁnally, two hot-wire anemometers (W , and W ) are out in placed in the air cavity, one in the inlet section and the other one in the outlet section, in order to monitor the airﬂow in the second-skin cavity. In order to verify the measurement methodologies and characterize each test cell from the thermal point of view, preliminary data has been acquired in a standard conﬁguration (both Gemini s without a second-skin system). These data are recorded with the aim of (i) verifying the operation of the different instruments and their correct positioning, (ii) comparing the thermal behavior of the two test cells, and (iii) deﬁning a reference point for the following evaluation of the real performances of double-skin facades or smart windows. The preliminary experimental data were acquired and stored every 1 min on a period of 1 month (from 1 June to 30 June) and, later, averaged on an interval of 15 min. Figure 3 reports the indoor air temperature of Gemini 1 and Gemini 2, the external air temperature, and the global horizontal radiation for three typical days in June. This ﬁgure highlights that the difference between the indoor air temperature of Gemini 1 and the Buildings 2021, 11, x FOR PEER REVIEW 8 of 27 indoor air temperature of Gemini 2 (T –T ) is negligible, varying indoor, Gemini 1 indoor, Gemini 2 within an interval with a maximum of 0.2 C and a minimum of 0.2 C. Indoor air temperature of Gemini 1 Indoor air temperature of Gemini 2 External air temperature Global horizontal solar radiation 31 850 30 800 29 750 28 700 27 650 26 600 25 550 24 500 23 450 22 400 21 350 20 300 19 250 18 200 17 150 16 100 15 50 14 0 Time (hh:mm) Figure 3. Gemini 1 and Gemini 2 preliminary experimental indoor air temperature trends for three Figure 3. Gemini 1 and Gemini 2 preliminary experimental indoor air temperature trends for three typical June days. typical June days. After the preliminary experimental campaign, a second-skin system has been mounted and tested on a test cell (Gemini 1) with an air cavity gap equal to 0.10 m, while the other cell has been left unequipped and used as a reference (Gemini 2). In particular, Gemini 1 the investigated second-skin system has been realized with extruded ABS panels [54]. The experimental data were acquired and stored every 1 min over a period of 1 month (from 8 December to 31 December) and, later, averaged on an interval of 15 min. This experimental campaign aims to verify the performances of plastic panels as a second-skin Gemini 2 layer for innovative envelopes. The extruded ABS panels have been selected with dimensions equal 600 mm 1200 mm; such dimensions have been selected on the basis of the dimension of conventional panels in OVF systems, in order to guarantee an easy installation in a commercial OVF structure, as well as an easy substitution of conventional OVF system materials in a realistic scenario. In Figure 4a, a detailed view of the analyzed extruded ABS panel before the installation is Extruded ABS panels (a) (b) Figure 4. (a) Extruded ABS panels [54] detail view; (b) Gemini 1 equipped with the OVF system. The second-skin system equipped during the tests has been realized by mounting the six extruded ABS panels on a steel frame, then hanging the whole system to the brackets on the south facade of the test cell Gemini 1. Finally, the sides of the second-skin system were covered and sealed by means of panels, similar to those used for the test cells’ enve- Temperature ( C) 600 mm Solar radiation (W/m ) Buildings 2021, 11, x FOR PEER REVIEW 8 of 27 Indoor air temperature of Gemini 1 Indoor air temperature of Gemini 2 External air temperature Global horizontal solar radiation 31 850 30 800 29 750 28 700 27 650 26 600 25 550 24 500 23 450 22 400 21 350 20 300 19 250 18 200 17 150 16 100 Buildings 2021, 11, 141 15 50 8 of 27 14 0 Time (hh:mm) displayed, while Figure 4b shows the Gemini 1 equipped with the second-skin realized Figure 3. Gemini 1 and Gemini 2 preliminary experimental indoor air temperature trends for three typical June days. with six extruded ABS panels. Gemini 1 Gemini 2 Extruded ABS panels (a) (b) Figure 4. (a) Extruded ABS panels [54] detail view; (b) Gemini 1 equipped with the OVF system. Figure 4. (a) Extruded ABS panels [54] detail view; (b) Gemini 1 equipped with the OVF system. Buildings 2021, 11, x FOR PEER REVIEW 9 of 27 The second-skin system equipped during the tests has been realized by mounting the The second-skin system equipped during the tests has been realized by mounting six extruded ABS panels on a steel frame, then hanging the whole system to the brackets the six on ext the r south uded fac ABS ade of panels the test cell on Gem a steel ini 1. frame, Finally, tthen he sides hanging of the second the-sk whole in system system to the were covered and sealed by means of panels, similar to those used for the test cells’ enve- brackets on the south facade of the test cell Gemini 1. Finally, the sides of the second-skin lope, in order to allow only for a vertical airflow in the cavity (Figure 4b). In this configu- system were covered and sealed by means of panels, similar to those used for the test cells’ ration, the acquisition period lasted for almost a month, in the wintertime. During the acqui- envelope, in order to allow only for a vertical airﬂow in the cavity (Figure 4b). In this sition period, the temperatures were monitored following the layout reported in Figure 2a. conﬁguration, the acquisition period lasted for almost a month, in the wintertime. During Fi the gure acquisition 5 shows period, an ovthe ervtemperatur iew of the es wh wer ole e monitor acquisitio ed n following period, the repo layout rting rth eported e external air in Figure 2a. Figure 5 shows an overview of the whole acquisition period, reporting the temperature, the global horizontal radiation, and the total vertical radiation on the south external air temperature, the global horizontal radiation, and the total vertical radiation on facade. the south facade. Figure 5. Overview of the whole winter acquisition period. Figure 5. Overview of the whole winter acquisition period. Figure 5 highlights that the external air temperature was quite warm, despite being the winter season. Also, the radiation values, both global horizontal and total vertical, show mostly high values, thus confirming good weather and clear sky across the whole acquisition period. Figure 6a,b report a focus for four typical days on the weather conditions during the measurements with the Gemini 1 equipped with the second-skin system, and the Gemini 2 left uncovered as a reference. In particular, Figure 6a shows the temperature and solar radiation data (global horizontal solar radiation, diffuse horizontal solar radiation, and total vertical solar radiation on the south facade), while Figure 6b reports the wind char- acteristics acquired during the analyzed days; on the left axis the wind speed is reported, while on the right axis the wind direction is displayed, considering 0° as north direction, 90° as east direction, 180° as south direction, and 270° as west direction. Wind speed Wind direction 2.4 360 2.2 2.0 300 1.8 270 1.6 1.4 210 1.2 180 1.0 0.8 120 0.6 90 0.4 60 0.2 0.0 0 Time (hh:mm) (a) (b) Figure 6. Weather conditions during four typical acquisition days: (a) solar radiation and temperature; (b) wind speed and wind direction. Temperature ( C) Speed (m/s) 600 mm Solar radiation (W/m ) Direction ( ) Buildings 2021, 11, x FOR PEER REVIEW 9 of 27 lope, in order to allow only for a vertical airflow in the cavity (Figure 4b). In this configu- ration, the acquisition period lasted for almost a month, in the wintertime. During the acqui- sition period, the temperatures were monitored following the layout reported in Figure 2a. Figure 5 shows an overview of the whole acquisition period, reporting the external air temperature, the global horizontal radiation, and the total vertical radiation on the south facade. Buildings 2021, 11, 141 9 of 27 Figure 5. Overview of the whole winter acquisition period. Figure 5 highlights that the external air temperature was quite warm, despite being Figure 5 highlights that the external air temperature was quite warm, despite being the winter season. Also, the radiation values, both global horizontal and total vertical, the winter season. Also, the radiation values, both global horizontal and total vertical, show mostly high values, thus confirming good weather and clear sky across the whole show mostly high values, thus conﬁrming good weather and clear sky across the whole acquisition period. acquisition period. Figure 6a,b report a focus for four typical days on the weather conditions during the Figure 6a,b report a focus for four typical days on the weather conditions during the measurements with the Gemini 1 equipped with the second-skin system, and the Gemini measurements with the Gemini 1 equipped with the second-skin system, and the Gemini 2 left uncovered as a reference. In particular, Figure 6a shows the temperature and solar 2 left uncovered as a reference. In particular, Figure 6a shows the temperature and solar radiation data (global horizontal solar radiation, diffuse horizontal solar radiation, and radiation data (global horizontal solar radiation, diffuse horizontal solar radiation, and total total vertical solar radiation on the south facade), while Figure 6b reports the wind char- vertical solar radiation on the south facade), while Figure 6b reports the wind characteristics acteristics acquired during the analyzed days; on the left axis the wind speed is reported, acquired during the analyzed days; on the left axis the wind speed is reported, while on while on the right axis the wind direction is displayed, considering 0° as north direction, the right axis the wind direction is displayed, considering 0 as north direction, 90 as east 90° as east dir ection, 180° as south direction, and 270° as west direction. direction, 180 as south direction, and 270 as west direction. Wind speed Wind direction 2.4 360 2.2 330 2.0 300 1.8 270 1.6 1.4 210 1.2 180 1.0 0.8 120 0.6 90 0.4 0.2 30 0.0 0 Time (hh:mm) (a) (b) Figure 6. Weather conditions during four typical acquisition days: (a) solar radiation and temperature; (b) wind speed Figure 6. Weather conditions during four typical acquisition days: (a) solar radiation and temperature; (b) wind speed and and wind direction. wind direction. Figure 6a better highlights that, during the measurement period, sunshine days were acquired with atypical temperatures for the period, ranging between a minimum of about 5.9 C and a maximum of about 19.4 C. Figure 6b shows a low wind speed in general during the measurement period and a slight wind predominance in the west/north-west direction. Also, the wind speed values acquired during the nighttime are equal to zero, due to a threshold value for the start/stop of the sensor equal to 0.15 m/s. Figures 7 and 8 report the experimental data associated with the cavity with a 1-h timestep for a single acquisition day. In more detail, Figure 7 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, while Figure 8 reports the airspeed at the inlet and the outlet of the air cavity, for the same day. The data reported in Figure 7 corresponds to the measures of the three thermocouples positioned in the middle point of the cavity of the second-skin system, more speciﬁcally T2, in the middle of the air cavity inlet, T5, in the middle of the air cavity geometrical center, and T9, in the middle of the air cavity outlet, as shown in Figure 2a. As a ﬁrst observation, the overall temperature distribution is directly related to solar radiation throughout the day, where the temperatures rise during the morning and drop during the afternoon. Also, during the day, the temperature trend seems to be substantially constant from the air cavity inlet to the middle of the facade, and then to increase to the air cavity outlet; this behavior is due to the chimney effect that is created thanks to the OVF system. Then, in the evening (starting from 16.00), the temperatures at the outlet of the cavity are only slightly higher than those at the center and the inlet; this is due to the reduction of solar radiation happening in the evening. Speed (m/s) Direction ( ) Buildings 2021, 11, x FOR PEER REVIEW 10 of 27 Buildings 2021, 11, x FOR PEER REVIEW 10 of 27 Figure 6a better highlights that, during the measurement period, sunshine days were Figure 6a better highlights that, during the measurement period, sunshine days were acquired with atypical temperatures for the period, ranging between a minimum of about acquired with atypical temperatures for the period, ranging between a minimum of about 5.9 ° 5.9 ° C C a and nd a a maximum maximum of of about about 19.4 ° 19.4 ° C. C. Fi Fig gure ure 6b 6b sho shows ws a a low low win wind d speed speed in g in ge enera neral l dur during ing tthe he meas measurem urement ent period period an and d a a sl sliight ght wind wind p predom redomin inance ance in in th the e west west/n /nort orth h--west west dir direct ection. ion. Al Also, so, tthe he wind wind spee speed d values values acquired during the nighttime are equal to zero, due to a threshold value for the start/stop acquired during the nighttime are equal to zero, due to a threshold value for the start/stop of the s of the sensor ensor equal t equal to 0. o 0.15 15 m/ m/s. s. Fi Fig gures ures 7 7 an and d 8 8 repo report rt th the e exp experiment erimental al dat data a assoc associiated ated with with th the e cavity cavity w with ith a a 1 1--h h timestep for a single acquisition day. In more detail, Figure 7 shows the trends of the daily timestep for a single acquisition day. In more detail, Figure 7 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, values of the air temperature inside the cavity upon varying the height from the ground, Buildings 2021, 11, 141 10 of 27 while while F Fig igu ure re 8 8 r repo eports rts th the e ai air rspeed speed at at th the e in inlet let and and th the e outl outlet et of of th the e air air cav cavity, ity, for for th the e s same ame day. day. 07:00 08:00 09:00 10:00 11:00 12:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 13:00 14:00 15:00 16:00 17:00 4 4..0 0 Thermocouples Thermocouples T T2 2//T T5 5//T T9 9 3 3..5 5 1 1.2 .20 0 m m 3.0 3.0 2.5 2.5 Thermocouple Thermocouple 2 2..0 0 ((T T9 9)) 1.5 1.5 Thermocouple Thermocouple (T5) (T5) 1.0 1.0 3 3.4 .40 0 m m 2.20 m 2.20 m 0 0..5 5 Thermocouple Thermocouple ((TT22)) 0 0..0 0 0 0.9 .90 0 m m 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 Temperature ( C) Temperature ( C) Figure 7. Daily values of the cavity air temperature. Figure Figure 7. 7. Dail Daily y val values ues of of the the c cavi avity ai ty air t r temp emperatur erature. e. C Cav aviitty y ai air r iin n lleett C Cav aviitty y ai air r o ou u ttlleett T T o ottal al v veert rtiiccal al s so ollar ar r rad adiiat atiio on n 0. 0 000 0. 0 000 0. 00 0. 00 0 0.. 0 0 00 00 1 1..2 20 0 m m IIn nllet et//Out Outllet et 0. 00 s sen ens so orrs s 0. 00 W1/W2 W1/W2 0 0.. 0 0 00 00 Outlet sensor Outlet sensor 0.2 00 0.2 00 ((W2 W2)) 0 0..20 20 00 00 0 0.. 00 00 3.30 m 3.30 m 0. 0 200 0. 0 200 Inlet sensor 0 0..0 0 00 00 Inlet sensor (W1) (W1) 0 0..00 00 0 0 1 1..0 00 0 m m i i e e hh hh:: Figure 8. Daily values of the airspeed at the inlet and the outlet of the air cavity. Figure Figure 8. 8. Dail Daily y val values ues of of the the a airs irspeed peed at at the the iinlet nlet and and the the outlet outlet of of the a the air ir ca cavi vity. ty. Figure 8 reports data acquired by the hot-wire anemometers placed based on the The data reported in Figure 7 corresponds to the measures of the three thermocou- The data reported in Figure 7 corresponds to the measures of the three thermocou- layout shown in Figure 2a, where W measured the airspeed value at the inlet of the cavity in ples ples po pos sition itioned ed in in th the e mid middle dle po point int of of tthe he cavity cavity o of f th the e second second--sk skiin n sys system tem, , mo more re speci specifi fi-- and W measured the value at the outlet of the cavity. This ﬁgure shows how, during the out day, the values acquired by W is higher than those measured by W , referable to a direct cally cally T T2, 2, in in th the e midd middle le o of f th the e air air c cavi avity ty in inlet, let, T T5, 5, in in th the e middle middle of of th the e ai air r cav cavity ity g geo eomet metrical rical out in effect of the solar radiation on the rising of the air temperatures along the air cavity, thus center, and T9, in the middle of the air cavity outlet, as shown in Figure 2a. As a first center, and T9, in the middle of the air cavity outlet, as shown in Figure 2a. As a first causing an increase in airspeed. After 15:00, the chimney effect in the air cavity is reduced observation, the overall temperature distribution is directly related to solar radiation observation, the overall temperature distribution is directly related to solar radiation because the temperatures are gradually decreasing over time due to the reduction of the th throughout roughout tthe he d day ay,, wher where e th the e tem temper perat atures ures ri rise se dur duriing ng th the e mo morni rning ng and and drop drop duri durin ng g th the e solar radiation on the south facade; this causes a signiﬁcant drop in the airspeed values measured in the cavity. Therefore, the analysis of the experimental results shows that a plastic material (i.e., ABS) can be used as a second-skin layer in OVF systems. Height from the ground (m) Height from the ground (m) it irs ee s it irs ee s ol r r i tio ol r r i tio Buildings 2021, 11, 141 11 of 27 2.2. Description of the Numerical Model The software TRNSYS 18 [31] has been used to model the Gemini test cells and to develop the second-skin model. TRNSYS software adopts a modular approach by using Fortran subroutines. Each Fortran subroutine is called a ‘Type’ and contains the model for a single system component. Several studies have been carried out in order to validate the numerical models developed in TRNSYS from the Colorado State University experimental houses and other researchers around the world [55–58]. In this study, the following main TRNSYS Types [31,59–61] have been used: Type 56 to simulate the Gemini test cells [59]; Type 1230 to model the second-skin system [61]; Type 16c to estimate the solar radiation on the Gemini and second-skin system sur- faces [59,60]; Type 69b to determine the sky temperature [59,60]; Type 33e to determine the moist air properties [59,60]. At ﬁrst, the Type 56 subroutine has been used to model the thermal behavior of the Gemini test cells; in particular, two thermal zones have been modeled, one for each test cell. Also, Type 56 contains information about the test cells’ surroundings (buildings, trees, bushes), which are described as ‘shading objects’. The geometrical modeling occurred in the SketchUp software [62], where it was possible to model the shapes and the position of each element accurately. Then, by means of the Trnsys3D plug-in, the geometries were imported into the Type 56 subroutine. The physical properties of each test cell’s external surface have been deﬁned, on the basis of the data provided by the manufacturers. Lastly, the internal thermal gains have been set for each test cell, which was determined on the basis of the equipment installed inside the facilities (notebook, data acquisition systems, and uninterruptible power supply units). The OVF system has been modeled using the Type 1230 subroutine, which effectively reproduces the behavior of an external second-skin layer with an air cavity behind it. Using this TRNSYS Type, the behavior of the OVF system has been correlated to that of the Gemini test cell modeled through the Type 56 subroutine. In particular, the last external layer of the Type 56 wall acts as an interface layer between the Type 1230 and the Type 56, by coupling its temperature and thermal resistance to model the wall heat transfer. Figure 9 shows a schematic of the boundaries of the two coupled Types (56 and 1230), highlighting the resistive interface layer. The Type 1230 parameters have been set following the data provided by the manufacturer of the extruded ABS panels [54], taking into account thickness, density, and thermal conductivity, speciﬁcally. During the simulations, Type 1230 takes into account: the solar radiation, the longwave radiation, and the air convection on the external surface of the outside layer; the energy storage and the conduction in the outside layer; radiation exchange between the outside layer and the air cavity; the convective exchanges from all the surfaces facing in the air cavity; the conduction through the interface layer. Type 16c has been implemented to model the solar radiation on all the external surfaces. This Type accepts global radiation, ambient temperature, and ambient relative humidity data as input, in order to output several quantities related to the position of the sun, as the diffuse radiation fraction on the horizontal, by estimating the cloudiness of the sky on the basis of the dry bulb temperature and the dew point temperature. Finally, the radiation on every external surface is computed, on the basis of their own orientation. Buildings 2021, 11, 141 12 of 27 Buildings 2021, 11, x FOR PEER REVIEW 12 of 27 Figure 9. Coupling between Type 56 and Type 1230 [61]. Figure 9. Coupling between Type 56 and Type 1230 [61]. The effective sky temperature is determined by means of Type 69b, which calculates During the simulations, Type 1230 takes into account: the long-wave radiation exchanges between the external surfaces and the atmosphere.  the solar radiation, the longwave radiation, and the air convection on the external Type 69b calculates the cloudiness factor as well, on the basis of the dry bulb and the dew surface of the outside layer; point temperatures.  the energy storage and the conduction in the outside layer; Finally, Type 33e has been implemented in order to calculate the properties of the  radiation exchange between the outside layer and the air cavity; moist air, in particular, by taking the air temperature, the relative humidity, and the air  the convective exchanges from all the surfaces facing in the air cavity; pressure as input; it returns the density of the air mixture for every timestep, which is  the conduction through the interface layer. then used to calculate the airﬂow at the OVF inlet. In this way, the inlet airﬂow is not a Type 16c has been implemented to model the solar radiation on all the external sur- ﬁxed value, but it corresponds to the experimental data acquired through the hot wire faces. This Type accepts global radiation, ambient temperature, and ambient relative hu- anemometers, placed as reported in Figure 2a. midity data as input, in order to output several quantities related to the position of the In this study, the experimental weather data acquired from 8 December to 31 December sun, as the diffuse radiation fraction on the horizontal, by estimating the cloudiness of the have been used as input data for the Type 16c, Type 69b, and Type 33e. sky on the basis of the dry bulb temperature and the dew point temperature. Finally, the During the simulation, both the time base used to solve the differential equations and radiation on every external surface is computed, on the basis of their own orientation. the simulation timestep has been set equal to 15 min in order to have a full correlation to The effective sky temperature is determined by means of Type 69b, which calculates the timestep of the experimental input data. the long-wave radiation exchanges between the external surfaces and the atmosphere. 2.3. Validation of the Numerical Model Type 69b calculates the cloudiness factor as well, on the basis of the dry bulb and the dew point temperatures. This sub-section reports the methods and results related to the validation of the nu- Finally, Type 33e has been implemented in order to calculate the properties of the merical model. The model reliability has been veriﬁed in terms of indoor air temperature moist air, in particular, by taking the air temperature, the relative humidity, and the air (T ) and the average temperature of the air cavity (T ) by comparing the experi- indoor cavity mental pressure values as input with ; it those returns obtained the density as anooutput f the air of mixt theusimulation re for every model timestabove ep, whic described, h is then deﬁning used to cthe alcu following late the ai per rflo centage w at the dif O fer VF ences inlet. DIn T this and way, Dth T e inlet : airflow is not a fixed indoor cavity value, but it corresponds to the experimental data acquired through the hot wire anemom- eters, placed as reported in Figure 2a. DT = T T /T (1) indoor indoor,ex p indoor,sim indoor,ex p In this study, the experimental weather data acquired from 8 December to 31 Decem- ber have been used as input data for the Type 16c, Type 69b, and Type 33e. DT = T T /T (2) cavity cavity,ex p cavity,sim cavity,ex p During the simulation, both the time base used to solve the differential equations and Figure 10a,b report the comparison between the simulation results and the experimen- the simulation timestep has been set equal to 15 min in order to have a full correlation to tal data acquired during the whole test period (from 8 December to 31 December) in terms the timestep of the experimental input data. of T and T , respectively. indoor cavity Buildings 2021, 11, x FOR PEER REVIEW 13 of 27 2.3. Validation of the Numerical Model This sub-section reports the methods and results related to the validation of the nu- merical model. The model reliability has been verified in terms of indoor air temperature (Tindoor) and the average temperature of the air cavity (Tcavity) by comparing the experi- mental values with those obtained as an output of the simulation model above described, defining the following percentage differences ΔTindoor and ΔTcavity: ΔT = T  T T (1)   indoor indoor,exp indoor,sim indoor,exp ΔT = T  T T (2)   cavity cavity,exp cavity,sim cavity,exp Figure 10a,b report the comparison between the simulation results and the experi- mental data acquired during the whole test period (from 8 December to 31 December) in terms of Tindoor and Tcavity, respectively. These figures highlight how the developed model is quite reliable, with values of Buildings 2021, 11, 141 ΔTindoor ranging between a minimum of − . 2%, and a maximum of .22%, as well as 13 thof e 27 values of ΔTcavity between a minimum of − . % and a maximum of . %. 21 23 14 13 7 3 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Experimental indoor air temperature ( C) Experimental air cavity temperature ( C) (a) (b) Figure 10. Comparison between the simulated values and the experimental values acquired during the whole test period Figure 10. Comparison between the simulated values and the experimental values acquired during the whole test period in in terms of (a) Tindoor and (b) Tcavity. terms of (a) T and (b) T . indoor cavity The accuracy of the model has also been validated by calculating the mean error These ﬁgures highlight how the developed model is quite reliable, with values of (ME), the mean absolute error (MAE), and the root mean square error (RMSE) as reported DT ranging between a minimum of 9.62%, and a maximum of 6.22%, as well as the indoor below: values of DT between a minimum of 8.51% and a maximum of 7.85%. cavity The accuracy of the model has also been validated by calculating the mean error (ME), ME  T  T N (3) the mean absolute error (MAE), and the root mean square err or (RMSE) as reported below: exp,i sim ,i i1 N N MAE  T  T N (4) ME =  T T /N (3) exp,i sim ,i å iexp, 1 i sim,i i=1 (5) RMSE T  T  ME N     exp,i sim ,i i1 M AE = T T /N (4) å exp,i sim,i i=1 where Texp,i is the experimental value at time step i, Tsim,i is the simulated value at time step i, and N is the number of measurement N s. Table 2 reports the values of the ME, MAE, and R MSE = T T ME /N (5) RMSE for both Tindoor and Tcavity. å exp,i sim,i i=1 where T is the experimental value at time step i, T is the simulated value at time exp,i sim,i step i, and N is the number of measurements. Table 2 reports the values of the ME, MAE, and RMSE for both T and T . indoor cavity Table 2. Values of ME, MAE, and RMSE obtained by comparing the simulated values and the experimental data acquired during the whole test period. T ( C) T ( C) indoor cavity ME MAE RMSE ME MAE RMSE 0.3 0.5 0.4 0.3 0.3 0.2 The values reported in Table 2 highlight that there is a slight difference between the measured and predicted results, in particular: (i) the ME associated to the T is equal indoor to 0.3 C, which means that the simulation model slightly overestimates the indoor air temperature, while that associated to the T is equal to 0.3 C; (ii) the values of RMSE cavity are equal to 0.4 C and 0.2 C for T and T , respectively. indoor cavity Therefore, the results show the ability of the Type 1230 to accurately predict the behavior of the extruded ABS panels in an OVF system. Thus, the same methodology is used to carry out a complete numerical campaign on a set of case studies upon varying the polymer (selecting the ones more used in architecture, as highlighted in the literature review) and manufacturing technology (extruded and 3D printed). Simulated idoor air temperature ( C) Simulated air cavity temperature ( C) Buildings 2021, 11, x FOR PEER REVIEW 14 of 27 Table 2. Values of ME, MAE, and RMSE obtained by comparing the simulated values and the ex- perimental data acquired during the whole test period. Tindoor (° C) Tcavity (° C) ME MAE RMSE ME MAE RMSE −0. 0.5 0.4 0.3 0.3 0.2 The values reported in Table 2 highlight that there is a slight difference between the measured and predicted results, in particular: (i) the ME associated to the Tindoor is equal to −0. °C, which means that the simulation model slightly overestimates the indoor air tem- perature, while that associated to the Tcavity is equal to 0.3 °C; (ii) the values of RMSE are equal to 0.4 °C and 0.2 °C for Tindoor and Tcavity, respectively. Therefore, the results show the ability of the Type 1230 to accurately predict the be- havior of the extruded ABS panels in an OVF system. Thus, the same methodology is used to carry out a complete numerical campaign on a set of case studies upon varying the polymer (selecting the ones more used in architecture, as highlighted in the literature re- view) and manufacturing technology (extruded and 3D printed). 3. Materials and Numerical Modeling Implementation The software TRNSYS 18 [24] is used to assess the potential energy saving achievable Buildings 2021, 11, 141 14 of 27 in an office building refurbishment using plastic and composite polymers as the second- skin layer material. The office building modeled in this work is the same for all configurations and it 3. Materials and Numerical Modeling Implementation consists of three identical fl Theoors. softwar Each e TRNSYS floor ha 18 [24s ] is a sur usedfto ac assess e of 45 the1 potential m and a vo energy l saving ume achievable equal to in an ofﬁce building refurbishment using plastic and composite polymers as the second-skin 3 2 2 2 1503 m , with a total window area (Aw, total) of 112.3 m (Aw, North = 24.5 m , Aw, South = 87.8 m ). layer material. It is located in Napoli (latitude = 40°51′ N; longitude = 14°16′ E), and as such, in order to The ofﬁce building modeled in this work is the same for all conﬁgurations and simulate the weather it consists condition, of thr ee the identical correspond ﬂoors. in Each g EnergyP ﬂoor has a lus surface weather of 451 d mata and ha as volume been 3 2 2 equal to 1503 m , with a total window area (A ) of 112.3 m (A = 24.5 m , w, total w, North used [30]. The office is firstly modeled in the SketchUp 3D modeling software (Figure 11). 2 0 0 A = 87.8 m ). It is located in Napoli (latitude = 40 51 N; longitude = 14 16 E), and w, South Then, the 3D model geometries were exported by means of the Trnsys3D plug-in and suc- as such, in order to simulate the weather condition, the corresponding EnergyPlus weather cessively imported into data T has RNS been YS used 18 [30 in]. or The der ofﬁce to is mo ﬁrstly del modeled the bui in ld the ing SketchUp envelope 3D modeling (stratig softwar raphy e (Figure 11). Then, the 3D model geometries were exported by means of the Trnsys3D plug- of the opaque wall and window typology), to define the infiltration, the internal gains, the in and successively imported into TRNSYS 18 in order to model the building envelope operation period of the heating and cooling systems as well as the operation of the electric (stratigraphy of the opaque wall and window typology), to deﬁne the inﬁltration, the equipment and lightinternal ing system gains,. the In par operation ticular, period the of sam the e heating TRNS and YS cooling Types systems described as well inas Sec the - operation of the electric equipment and lighting system. In particular, the same TRNSYS tion 4 have been used to simulate the case study. Types described in Section 4 have been used to simulate the case study. (a) (b) Figure 11. Ofﬁce building modeled in SketchUp 3D: (a) south view; (b) north view. Figure 11. Office building modeled in SketchUp 3D: (a) south view; (b) north view. The same typical three-story ofﬁce building is investigated upon, varying the insula- tion layer thickness on the south facade and the typology of second-skin material, for a total The same typical three-story office building is investigated upon, varying the insula- of eight case studies. Table 3 summarizes the eight simulation cases investigated in this tion layer thickness on the south facade and the typology of second-skin material, for a work. In particular, this table reports the reference case (Case 0) without the second-skin (Figure 12a) and seven refurbishment case studies with the OVF system (Figure 12b) upon total of eight case studies. Table 3 summarizes the eight simulation cases investigated in varying the second-skin material. These cases are: this work. In particular, this table reports the reference case (Case 0) without the second- Case 1, with an OVF system made of a conventional second-skin material (Porcelain gres); Cases 2–5, where the OVF systems have been implemented by using the extrude plastic and polymer materials more used in architecture (polycarbonate multi-wall sheets, ABS, PETG, and PLA); Cases 3_3D–5_3D, where the second-skin materials used in the OVF are the most popular 3D printed polymers (ABS, PETG, and PLA); Buildings 2021, 11, x FOR PEER REVIEW 15 of 27 Buildings 2021, 11, 141 15 of 27 skin (Figure 12a) and seven refurbishment case studies with the OVF system (Figure 12b) upon varying the second-skin material. These cases are:  Case 1, with an OVF system made of a conventional second-skin material (Porcelain Table 3. Summary of case studies investigated. gres); Case Study Second-Skin Material Insulation Thickness (m) Air Gap (m)  Cases 2–5, where the OVF systems have been implemented by using the extrude plas- Case 0 - - - tic and polymer materials more used in architecture (polycarbonate multi-wall Case 1 sheets, Por ABS celain , PET gresG, and PLA); 0.072 Case 2 Polycarbonate multi-wall sheets 0.063  Cases 3_3D–5_3D, where the second-skin materials used in the OVF are the most Case 3 Extruded ABS panels 0.070 popular 3D printed polymers (ABS, PETG, and PLA); Case 4 Extruded PETG panels 0.071 0.10 Case 5 Extruded PLA panels 0.069 In addition, an insulation layer has been added in each case study in order to reach Case 3_3D 3D printed ABS panels 0.065 the threshold values specified by the Italian Law [63] and equal to 0.36 W/m K for the Case 4_3D 3D printed PETG panels 0.067 climatic zone considered in this work. The different insulation thicknesses are also re- Case 5_3D 3D printed PLA panels 0.063 ported in Table 3, upon varying the simulation case. (a) (b) Figure Figure 12. 12. Section Section of of the the south south wall wall of of the the ofﬁce offibuilding: ce building (a):r ( efer a) reference cas ence case; (b) e retr ; (b oﬁt ) retrof cases itwith cases with the s the second-skin econd-skin s system. ystem. In addition, an insulation layer has been added in each case study in order to reach In all the retrofit cases, the cavity inlet airspeed is directly related to the wind speed the threshold values speciﬁed by the Italian Law [63] and equal to 0.36 W/m K for the and direction, as only the wind coming from a similar orientation as the second-skin sys- climatic zone considered in this work. The different insulation thicknesses are also reported tem (wind direction = 180° ± 45°) has been considered as input for the Type 1230. In addi- in Table 3, upon varying the simulation case. tion, the second-skin system has a control logic for the air cavity shutters, which are con- In all the retroﬁt cases, the cavity inlet airspeed is directly related to the wind speed sidered open during the cooling period and closed during the heating period. and direction, as only the wind coming from a similar orientation as the second-skin system (wind direction = 180 45 ) has been considered as input for the Type 1230. In addition, Table 3. Summary of case studies investigated. the second-skin system has a control logic for the air cavity shutters, which are considered open during the cooling period and closed during the heating period. Insulation Thickness Air Gap Table 4 shows the thermal-physical properties of the opaque walls of the envelope Case Study Second-Skin Material (m) (m) implemented in the case studies. Case 0 - - - Case 1 Porcelain gres 0.072 Case 2 Polycarbonate multi-wall sheets 0.063 Case 3 Extruded ABS panels 0.070 Case 4 Extruded PETG panels 0.071 0.10 Case 5 Extruded PLA panels 0.069 Case 3_3D 3D printed ABS panels 0.065 Case 4_3D 3D printed PETG panels 0.067 Case 5_3D 3D printed PLA panels 0.063 Buildings 2021, 11, x FOR PEER REVIEW 17 of 27 Buildings 2021, 11, 141 16 of 27 (considered equal to 12 W/m K [48]) and Vd is the volume fraction of the filler in the poly- Table 4. Thermal-physical properties of the opaque walls implemented in the reference case study. mer matrix (calculated as 0.69 of the specimens’ total volume). The internal geometries have been modeled as hexagons as suggested by [50], where hexagons specimens resulted Thickness Density Thermal Conductivity Thermal Capacity Surface Material as the most resilient to physical stress, 3 thus more suitable for a building envelope integra- (m) (kg/m ) (W/mK) (kJ/kgK) tion. Plaster 0.015 1400 0.70 1.01 Bricks 0.238 600 0.36 0.84 Vertical Walls Table 5. Simulation parameters used in this research. Mortar 0.015 1800 0.90 0.91 Parameter Detail Value Plaster 0.015 1400 0.70 1.01 Lighter concrete 0.027 500 0.17 0.88 Walls and South wall without insula- U = 1.15 W/m K Bricks 0.150 600 0.36 0.84 Roof tion (Case 0) Concrete 0.020 600 0.18 0.88 Roof U = 1.10 W/m K Bitumen 0.005 1200 0.17 1.47 Thermal Floor U = 0.94 W/m K Tiles 0.020 2000 1.00 1.00 Transmittance Windows (frame ratio of 15%) U = 2.95 W/m K Concrete 0.050 600 0.18 0.88 Floor Bricks South w 0.150 all with insulation 600 (Cases 1, 2, 0.36 0.84 U = 0.36 W/m K Lighter concrete 0.030 500 0.17 0.88 3, 4, 5, 3_3D, 4_3D and 5_3D) −1 Infiltration [72,73] Air changes per hour 0.6 h Table 5 reports the general simulation parameters adopted Set point in the = 20 ° eight C dif [74] fer ent case studies. In particular, this table highlights: (i) the values of the thermal transmittance for Operation period = 16 November/30 Heating system both opaque walls and windows, (ii) the air inﬁltration rate, (iii) the target of the indoor March [74] air temperature, the operation period, and the characteristics of the heating and cooling Heating and COP = 2.67 [64] system, (iv) the occupancy schedule and (v) the internal gains. As can be noticed from Cooling systems Set point = 26 °C [74] Table 5, in all the retroﬁt cases the values of thermal transmittance for the opaque surfaces Operation period = 1 April/15 Novem- Cooling system are equal to those of the reference case (Case 0), with the exception of the south wall, where ber [74] the OVFs have been implemented and the thermal transmittance has been set equal to the EER = 2.41 [64] threshold values speciﬁed by Italian law [63], considering the second-skin materials, the Occupancy Weekdays (8:00–18:00) air cavity and the insulation thicknesses reported in Table 3. Workweek schedule [75] Completely off on the weekends Two parallel-connected electric heat pump (EHP) devices, model CRA/K 91 [64], Operation = Occupancy schedule coupled with a multi-split type air conditioning system, have been used to cover both the Lighting systems Radiative = 11.13 W/m heating and cooling demands. The national grid has been used to cover all the electrical energy demand. Convective = 4.77 W/m The simulation timestep has been set to 30 min. Operation = Occupancy schedule Internal Finally, Type 1230 [61] has been used to model a second-skin layer in plastic 2 and Equipment Radiative = 1.4 W/m gains [76] composite polymers. The parameters required by Type 1230 for each material (i.e., density, Convective = 5.6 W/m thermal capacity, and thermal conductivity), for Cases 1–5, have been derived on the basis Operation = Occupancy schedule of the manufacturers or literature data [40,54,65–67]. With respect to the 3D printed panels Occupants Radiative/Convective = 2.5 W/m (Cases 3_3D–5_3D), several specimens have been printed (Figure 13), in order to measure Absolute humidity = 0.0055 kg/hm the ﬁnal dimensions and density. (a) (b) (c) Figure 13. The specimens made through the 3D printing process: (a) ABS [68]; (b) PETG [69]; (c) PLA [70]. Figure 13. The specimens made through the 3D printing process: (a) ABS [68]; (b) PETG [69]; (c) PLA [70]. Table 6 reports the parameters used to simulate the second-skin systems in the retro- fit cases. Buildings 2021, 11, 141 17 of 27 The thermal conductivity of the 3D printed materials (k ) has been calculated by 3D means of the equation expressed by [48,51] and reported below: k k k 2k d d d d 2 1 V + + + 2 k ah k ah m c m c k = k (6) 3D m k k k 2k d d d d 1 V + + + 2 k ah k ah m c m c where k is the thermal conductivity of the polymer as declared by their manufacturers [68–70], k is the thermal conductivity of the ﬁller consisting of the air in the hexagonal cavities of the printed panels (equal to 0.026 W/mK [71]), a is the ﬁller radius measured from the specimens (measured as 0.0045 m), h is the interfacial boundary conductance (considered equal to 12 W/m K [48]) and V is the volume fraction of the ﬁller in the polymer matrix (calculated as 0.69 of the specimens’ total volume). The internal geometries have been modeled as hexagons as suggested by [50], where hexagons specimens resulted as the most resilient to physical stress, thus more suitable for a building envelope integration. Table 5. Simulation parameters used in this research. Parameter Detail Value Walls and South wall without insulation (Case 0) U = 1.15 W/m K Roof U = 1.10 W/m K Thermal Floor U = 0.94 W/m K Transmittance Windows (frame ratio of 15%) U = 2.95 W/m K South wall with insulation (Cases 1, 2, 3, 4, 5, U = 0.36 W/m K 3_3D, 4_3D and 5_3D) Inﬁltration [72,73] Air changes per hour 0.6 h Set point = 20 C [74] Heating system Operation period = 16 November/30 March [74] COP = 2.67 [64] Heating and Cooling systems Set point = 26 C [74] Cooling system Operation period = 1 April/15 November [74] EER = 2.41 [64] Occupancy Weekdays (8:00–18:00) Workweek schedule [75] Completely off on the weekends Operation = Occupancy schedule Lighting systems Radiative = 11.13 W/m Convective = 4.77 W/m Operation = Occupancy schedule Internal Equipment Radiative = 1.4 W/m gains [76] Convective = 5.6 W/m Operation = Occupancy schedule Occupants Radiative/Convective = 2.5 W/m Absolute humidity = 0.0055 kg/hm Table 6 reports the parameters used to simulate the second-skin systems in the retroﬁt cases. Buildings 2021, 11, 141 18 of 27 Table 6. Summary of the main simulation parameters for the Type 1230, upon varying the materials used as a second-skin layer. Case Case Case Case Case Case Case Case Parameters 1 2 3 4 5 3_3D 4_3D 5_3D Polycarbonate Porcelain Extruded Extruded Extruded 3D printed 3D printed 3D printed Material multi-wall gres ABS PETG PLA ABS PETG PLA sheets Thickness 0.010 (m) Density 2000 300 1040 1300 1300 331 411 395 (kg/m ) Thermal capacity 0.840 1.05 1.40 1.20 1.80 1.21 1.07 1.25 (kJ/kgK) Thermal conductivity 1.20 0.0453 0.17 0.29 0.13 0.0548 0.0818 0.0448 (W/mK) Resistance of interface layer 0.486 0.427 0.472 0.479 0.467 0.438 0.455 0.427 (hm K/kJ) Convective heat transfer coefﬁcient of 2.06 2.34 2.12 2.09 2.14 2.28 2.20 2.35 interface layer (kJ/hm K) In conclusion, two additional parameters have been set for each case study, required by the Type 1230 as highlighted in Section 4: the resistance of interface layer, to be set in Type 1230 itself, and the convective heat transfer coefﬁcient of the interface layer, to be set in the construction south wall in Type 56 instead. The convective heat transfer coefﬁcients of the back of the south wall value have been set equal to the thermal transmittance value of the insulation layers, which act as interface layers between the two TRNSYS Types, while the thermal resistance values of the interface layer have been simply calculated as the inverse of the convective heat transfer coefﬁcients. These plastic and polymer materials do not differ only in terms of thermo-physical properties but also in terms of cost. In this work, the capital cost for each retroﬁt action has been neglected; however, Table 7 provides an overview of the costs per square meter associated with each plastic and polymer material implemented as a second-skin layer in the OVF system [54,77–79]. In general, the polycarbonate multi-wall sheets prove to be the cheaper material (10–25 /m ), being also the only one which is already used in the building sector; instead, the 3D printed panels are the more expensive ones (188–225 /m ). This cost limitation is typical for the 3D printing technology, especially compared to more traditional building materials. Despite this signiﬁcant difference in cost, it must be noted that 3D printing is an emerging technology which usage is still not so widespread. However, the 3D printing technology is the only one that would allow for the obtaining of complex panels’ shapes easily. Also, the price of the 3D printed panels reported in Table 7 are related to the brand-new spool, while it is possible to integrate also recycled ﬁlament spools in the production process [47]; indeed, the 3D printing manufacturing process is the only one where it’s easy to fully integrate eco-compatible materials, like PLA. Buildings 2021, 11, 141 19 of 27 Table 7. Costs per square meter of plastic and polymer materials used as a second-skin layer in this research [54,77–79]. Polycarbonate Extruded Extruded Extruded 3D Printed 3D Printed 3D Printed Multi-Wall Sheets ABS PETG PLA ABS * PETG * PLA * Cost 10–25 70–175 75–150 75–130 188–225 190–220 190–207 ( /m ) * Considering about 3.2 kg of material and including also the 3D printing cost (equal to 1.0 /h, for 130 h of the whole printing process), for a panel of 1 m . 3.1. Energy Analyses: Methods According to [12,75], the energy comparison between the proposed case (PC) and the reference case (RC) has been carried out considering the non-renewable primary energy consumption through the index PES (non-renewable primary energy saving): h i RC PC RC PES = E E /E 100 (7) p p p RC where E is the non-renewable primary energy associated with the reference case (Case 0, PC see Table 3), while E is the non-renewable primary energy associated with the eight proposed cases (Cases 1–5 and Cases 3_3D–5_3D, see Table 3). RC PC The values of the E and E are calculated as reported below: p p RC RC E E RC th cool E = + + E + E /h (8) p el,equi pment el,lighting PP COP EER PC PC E E PC th cool E = + + E + E /h (9) el,equi pment el,lighting PP COP EER where h is the Italian power plants’ average efﬁciency, including the transmission losses, PP and it is assumed equal to 0.42 [74]. A positive value of the index PES means that the proposed refurbishment allows reducing the non-renewable primary energy consumption with respect to the reference case. 3.2. Energy Analyses: Results In this section, the simulation results of the refurbishment case study are reported and commented on. Figure 14 reports the values of PES as a function of the proposed case studies, while Figures 15 and 16 show the main energy ﬂows of the building during the whole simulation period upon varying the simulation case. In particular, Figures 15 and 16 report the thermal energy ﬂows in positive values, while the cooling energy ﬂows in negative values. These ﬁgures highlight that: all the proposed OVF systems return positive PES values in comparison to the refer- ence case, which means a reduction of the non-renewable primary energy consump- tion ranging from 2.58% (Case 1) and 2.64% (Cases 2 and 5_3D); this is due to an average reduction of the thermal and cooling energy demands of about 6.9% and 3.0%, respectively; the retroﬁt actions where the plastic and composite polymers materials are used as a second-skin layer (Cases 2–5 and Cases 3_3D–5_5D, see Table 3) allow for a slight performance improvement with respect to those realized with a conventional second- skin material (Case 1), thanks to a reduction in the space cooling energy demand ranging from 31 kWh (Case 4) and 120 kWh (Case 5_3D); the results associated with the polycarbonate multi-wall sheets (Case 2) show a be- havior similar to the Cases 3_3D–5_5D, mostly due to the fact that the polycarbonate panels have a structure assimilable to the 3D printing logic; Buildings 2021, 11, x FOR PEER REVIEW 20 of 27 2.70 2.65 2.64 2.64 2.63 2.62 2.62 2.61 2.60 2.60 2.58 Buildings 2021, 11, 141 20 of 27 2.55 considering the cases with the same polymers (Case 3 vs. Case 3_3D, Case 4 vs. Case Buildings 2021, 11, x FOR PEER REVIEW 20 of 27 4_3D, and Case 5 vs. Case 5_3D), the 3D printed panels allow for a slight improvement 2.50 Case 1 Case 2 Case 3 Case 4 Case 5 Case 3_3D Case 4_3D Case 5_3D in performances. Simulation Cases Figure 14. Values of PES upon varying the case studies. 2.70 These figures highlight that:  all the proposed OVF systems return positive PES values in comparison to the refer- 2.65 ence case, which means a reduction of the non-renewable primary energy consump- 2.64 2.64 2.63 tion ranging from 2.58% (Case 1) and 2.64% (Cases 2 and 5_3D); this is due to an 2.62 2.62 average reduction of the thermal and cooling energy demands of about 6.9% and 2.61 3.0%, respectively; 2.60 2.60  the retrofit actions where the plastic and composite polymers materials are used as a 2.58 second-skin layer (Cases 2–5 and Cases 3_3D–5_5D, see Table 3) allow for a slight performance improvement with respect to those realized with a conventional sec- ond-skin material (Case 1), thanks to a reduction in the space cooling energy demand 2.55 ranging from 31 kWh (Case 4) and 120 kWh (Case 5_3D);  the results associated with the polycarbonate multi-wall sheets (Case 2) show a be- havior similar to the Cases 3_3D–5_5D, mostly due to the fact that the polycarbonate 2.50 panels have a structure assimilable to the 3D printing logic; Case 1 Case 2 Case 3 Case 4 Case 5 Case 3_3D Case 4_3D Case 5_3D  considering the cases with the same polymers (Case 3 vs. Case 3_3D, Case 4 vs. Case Simulation Cases 4_3D, and Case 5 vs. Case 5_3D), the 3D printed panels allow for a slight improve- Figure 14. Values of PES upon varying the case studies. Figur men e 14. t in per Values formances of PES up . on varying the case studies. These figures highlight that:  all the proposed OVF systems return positive PES values in comparison to the refer- ence case, which means a reduction of the non-renewable primary energy consump- tion ranging from 2.58% (Case 1) and 2.64% (Cases 2 and 5_3D); this is due to an average reduction of the thermal and cooling energy demands of about 6.9% and 3.0%, respectively;  the retrofit actions where the plastic and composite polymers materials are used as a second-skin layer (Cases 2–5 and Cases 3_3D–5_5D, see Table 3) allow for a slight performance improvement with respect to those realized with a conventional sec- -2 ond-skin material (Case 1), thanks to a reduction in the space cooling energy demand -4 ranging from 31 kWh (Case 4) and 120 kWh (Case 5_3D); -6 -8  the results associated with the polycarbonate multi-wall sheets (Case 2) show a be- -10 havior similar to the Cases 3_3D–5_5D, mostly due to the fact that the polycarbonate -12 panels have a structure assimilable to the 3D printing logic; Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec  considering the cases with the same polymers (Case 3 vs. Case 3_3D, Case 4 vs. Case Figure 15. Main energy flows of the building during the whole simulation period associated with Figure 15. Main energy ﬂows of the building during the whole simulation period associated with 4_3D, and Case 5 vs. Case 5_3D), the 3D printed panels allow for a slight improve- Case Case 0, 0,Case Case 1,1 and , and retr retrofit oﬁt Cases Case with s with extruded extrupanels. ded panels. ment in performances. -2 -4 -6 -8 -10 -12 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Figure 15. Main energy flows of the building during the whole simulation period associated with Case 0, Case 1, and retrofit Cases with extruded panels. Values of PES (%) Thermal and cooling energy associated with Thermal and cooling energy associated with Values of PES (%) the whole building (MWh) the whole building (MWh) 18.90 17.66 18.90 17.66 17.66 17.66 17.66 17.66 17.66 17.66 17.66 16.36 15.26 17.66 15.26 16.36 15.26 15.26 15.26 15.26 15.26 15.26 12.96 12.07 15.26 12.07 15.26 12.07 12.07 12.96 12.07 12.07 12.07 12.07 12.07 12.07 -0.46 -0.42 -0.41 -0.42 -0.42 -0.42 -0.46 -2.70 -2.66 -0.42 -2.63 -0.41 -2.65 -0.42 -2.65 -0.42 -2.64 -0.42 -7.05 -6.90 -2.70 -6.88 -2.66 -6.89 -2.63 -6.89 -2.65 -6.88 -2.65 -7.19 -2.64 -7.02 -6.99 -7.05 -7.01 -6.90 -7.01 -6.88 -7.00 -6.89 -3.50 -6.89 -3.38 -6.88 -3.36 -3.37 -7.19 -3.38 -7.02 -3.37 -6.99 -0.61 -7.01 -0.55 -7.01 -0.54 -7.00 -0.55 -0.55 -3.50 -0.54 -3.38 -0.01 -3.36 6.14 5.64 -3.37 5.64 -3.38 5.64 -3.37 5.64 5.64 -0.61 -0.55 15.70 14.57 -0.54 14.58 -0.55 14.58 -0.55 14.58 -0.54 14.58 -0.01 6.14 5.64 5.64 5.64 5.64 5.64 15.70 14.57 14.58 14.58 14.58 14.58 Buildings 2021, 11, x FOR PEER REVIEW 21 of 27 Buildings 2021, 11, 141 21 of 27 Buildings 2021, 11, x FOR PEER REVIEW 21 of 27 -2 -4 -6 -8 -10 -12 Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Figure 16. Main energy flows of the building during the whole simulation period associated with Case 0, Case 1, and retrofit Ca 6ses with 3D printed panels. In order to better investigate the performance of the plastic and polymer materials used as a second-skin layer in the proposed OVF system, the trends of the values of the air tem- -2 -4 perature inside the cavity and the airspeed at the inlet and the outlet of the air cavity for the -6 Case 5_3D in a typical summer day (2 August) is also reported in Figures 17 and 18 with a -8 1 h timestep. In general, similar trends in the values of the temperatures in the cavity as -10 -12 well as the airspeed at the cavity inlet and the outlet have been predicted for all the other Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec cases. Figure 16. Main energy flows of the building during the whole simulation period associated with In more detail, Figure Figure 16. 17 Main show energy s th ﬂows e tren of the ds building of the during daily tva helu whole es osimulation f the air period temper associated ature with Case 0, Case 1, and retrofit Cases with 3D printed panels. Case 0, Case 1, and retroﬁt Cases with 3D printed panels. inside the cavity upon varying the height from the ground, and, in particular reports, the temperature of the air at the cavity inlet (about 0.15 m from the ground), the average tem- In order to better investigate the performance of the plastic and polymer materials used In order to better investigate the performance of the plastic and polymer materials perature of the air cavi as aty sec(h onig d-hl skiig n ht layed er in by th th e pe robl po ue sed reg OViF on sysin tem th , te he b tuil rending ds of tsection he values ) o and f the the air t em- used as a second-skin layer in the proposed OVF system, the trends of the values of the air perature inside the cavity and the airspeed at the inlet and the outlet of the air cavity for the temperature of the air temperatur at the cavity e insideo the utl cavity et (abo and ut the 10. airspeed 00 m from at the th inlet e ground and the );outlet Figure of the 18 air re-cavity Case 5_3D in a typical summer day (2 August) is also reported in Figures 17 and 18 with a for the Case 5_3D in a typical summer day (2 August) is also reported in Figures 17 and 18 ports the values of the airspeed at the cavity inlet (about 0.15 m from the ground) and the 1 h timestep. In general, similar trends in the values of the temperatures in the cavity as with a 1 h timestep. In general, similar trends in the values of the temperatures in the airspeed at the cavity outlet (about 10.00 m from the ground), as well as the total vertical well as the airspeed at the cavity inlet and the outlet have been predicted for all the other cavity as well as the airspeed at the cavity inlet and the outlet have been predicted for all radiation on the external surface of the second-skin layer. cases. the other cases. In more detail, Figure 17 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, and, in particular reports, the temperature of the air at the cavity inlet (about 0.15 m from the ground), the average tem- perature of the air cavity (highlighted by the blue region in the building section) and the temperature of the air at the cavity outlet (about 10.00 m from the ground); Figure 18 re- ports the values of the airspeed at the cavity inlet (about 0.15 m from the ground) and the airspeed at the cavity outlet (about 10.00 m from the ground), as well as the total vertical radiation on the external surface of the second-skin layer. Figure 17. Simulated daily values of the cavity air temperature for a typical summer day. Figure 17. Simulated daily values of the cavity air temperature for a typical summer day. Figure 17. Simulated daily values of the cavity air temperature for a typical summer day. Thermal and cooling energy associated with the whole building (MWh) 18.90 17.66 17.66 17.66 17.66 16.36 15.26 15.26 15.26 Thermal and cooling energy associated with 15.26 12.96 the whole building (MWh) 12.07 12.07 12.07 12.07 18.90 17.66 17.66 17.66 17.66 16.36 15.26 -0.46 15.26 -0.42 15.26 -0.42 15.26 -0.42 12.96 -0.41 12.07 12.07 -2.70 12.07 -2.66 12.07 -2.64 -2.64 -2.63 -7.05 -6.90 -0.46 -0.42 -6.88 -0.42 -6.88 -0.42 -6.88 -0.41 -7.19 -2.70 -2.66 -7.02 -2.64 -6.99 -2.64 -7.00 -2.63 -6.99 -7.05 -6.90 -3.50 -6.88 -3.38 -6.88 -3.36 -6.88 -3.37 -7.19 -3.36 -7.02 -6.99 -0.61 -7.00 -0.55 -6.99 -0.54 -3.50 -0.54 -3.38 -0.54 -3.36 -3.37 -0.01 -3.36 6.14 5.64 -0.61 -0.55 5.64 -0.54 5.64 -0.54 5.64 -0.54 15.70 -0.01 6.14 14.57 5.64 14.58 5.64 5.64 14.58 5.64 14.58 15.70 14.57 14.58 14.58 14.58 Buildings 2021, 11, x FOR PEER REVIEW 22 of 27 Buildings 2021, 11, 141 22 of 27 Cavity air inlet Cavity air outlet Total vertical solar radiation .0 00 Outlet air speed .0 00 2 .0 00 10.00 m 0 .0 00 .0 00 .0 00 .0 200 2.0 00 Inlet air speed 0.0 0 0.15 m i e hh: Figure 18. Simulated values of the airspeed at the inlet and the outlet of the air cavity for a typical summer day. Figure 18. Simulated values of the airspeed at the inlet and the outlet of the air cavity for a typical summer day. In more detail, Figure 17 shows the trends of the daily values of the air temperature inside the cavity upon varying the height from the ground, and, in particular reports, The data reported in Figure 17 corresponds to the input and outputs of Type 1230. In the temperature of the air at the cavity inlet (about 0.15 m from the ground), the average particular, the inlet temperatur air tempera e ofture the air is cavit an input y (highlighted for Typ by e 1230, the blue wrhil egion e th in e the aver building age air section) tem- and the temperature of the air at the cavity outlet (about 10.00 m from the ground); Figure 18 perature in the whole cavity (Tcavity, the blue-edged markers in Figure 17) and the outlet reports the values of the airspeed at the cavity inlet (about 0.15 m from the ground) and the air temperature are returned as output results by Type 1230 itself. These values represent airspeed at the cavity outlet (about 10.00 m from the ground), as well as the total vertical the only two temperature values associated with the air cavity returned by Type 1230 [61]. radiation on the external surface of the second-skin layer. As a first observation, the overall temperature distribution is directly related to solar ra- The data reported in Figure 17 corresponds to the input and outputs of Type 1230. In particular, the inlet air temperature is an input for Type 1230, while the average air diation throughout the day, where the temperatures rise during the morning and drop temperature in the whole cavity (T , the blue-edged markers in Figure 17) and the cavity during the afternoon. Also, during the day, the temperature trend seems to be constantly outlet air temperature are returned as output results by Type 1230 itself. These values rising from the air cavity inlet to the air cavity outlet; this behavior is due to the chimney represent the only two temperature values associated with the air cavity returned by Type effect that is created thanks to the OVF system. 1230 [61]. As a ﬁrst observation, the overall temperature distribution is directly related Figure 18 report to s solar the radiation simulation throughout data cor the respon day, wher dine g the to th temperatur e cavity es inlet rise during airspeed the (i morning n- and drop during the afternoon. Also, during the day, the temperature trend seems to be put of the Type 1230) and the cavity outlet airspeed (output of the Type 1230), as well as constantly rising from the air cavity inlet to the air cavity outlet; this behavior is due to the the total vertical solar radiation. This figure shows how, during the day, the values pre- chimney effect that is created thanks to the OVF system. dicted at the cavity air outlet are always higher than those at the cavity air inlet (with a Figure 18 reports the simulation data corresponding to the cavity inlet airspeed (input difference between outlet and inlet ranging from 0.01 m/s and 1.46 m/s), gradually rising of the Type 1230) and the cavity outlet airspeed (output of the Type 1230), as well as the total vertical solar radiation. This ﬁgure shows how, during the day, the values predicted to a maximum peak at around 14:00. at the cavity air outlet are always higher than those at the cavity air inlet (with a difference Finally, in order to verify the potential benefits coming from the best case, an addi- between outlet and inlet ranging from 0.01 m/s and 1.46 m/s), gradually rising to a tional simulation case, not reported in Table 3, has been carried out. In this last simulation, maximum peak at around 14:00. the OVF system has been implemented on the whole building, following the same instal- Finally, in order to verify the potential beneﬁts coming from the best case, an additional lation methodology simulation of the prev case, ious not c reported ases. Th ineT mater able 3, ial hass been elected carried as a out. second In this-sk last in simulation, layer is the OVF system has been implemented on the whole building, following the same installation the 3D printed PLA, which proved to be one of the most effective in improving the non- methodology of the previous cases. The material selected as a second-skin layer is the 3D renewable primary energy saving. The proposed OVF system returned a PES value equal printed PLA, which proved to be one of the most effective in improving the non-renewable to 8.10% if compared to the reference case. primary energy saving. The proposed OVF system returned a PES value equal to 8.10% if compared to the reference case. 4. Conclusions The OVFs have been, more and more frequently, chosen for different building typol- ogies (offices, schools, residential) and in different climates. Nowadays, there are always more innovative materials used in architecture and as a second-skin layer, even if the eval- uation of their impact on the envelope’s energy performance is a complex task. In partic- ular, the use of polymers in building and engineering has increased substantially, thanks to their: (i) ease of production, (ii) ease of installation, (iii) durability, (iv) low maintenance requirements, (v) lightweight nature and (vi) ability to be formed into complex shapes. Several of these plastic products can also be utilized in additive manufacturing processes, providing excellent freedom of form, enhancing designers, architects, and engineers’ free- dom in creating complex designs. it irs ee s ol r i tio Buildings 2021, 11, 141 23 of 27 4. Conclusions The OVFs have been, more and more frequently, chosen for different building typolo- gies (ofﬁces, schools, residential) and in different climates. Nowadays, there are always more innovative materials used in architecture and as a second-skin layer, even if the evalu- ation of their impact on the envelope’s energy performance is a complex task. In particular, the use of polymers in building and engineering has increased substantially, thanks to their: (i) ease of production, (ii) ease of installation, (iii) durability, (iv) low maintenance require- ments, (v) lightweight nature and (vi) ability to be formed into complex shapes. Several of these plastic products can also be utilized in additive manufacturing processes, providing excellent freedom of form, enhancing designers, architects, and engineers’ freedom in creating complex designs. In this work, the numerical model of extruded ABS panels in an OVF system has been developed and validated. Then, the simulation methods, suggested by the authors, have been applied in different refurbishment cases upon varying: the polymer and the manufacturing technology, extrusion (polycarbonate multi-wall sheets, ABS, PETG, and PLA), and 3D printing (ABS, PETG, and PLA). The simulations have been carried out in order to assess the potential beneﬁts achiev- able in terms of non-renewable primary energy saving, as well as thermal and cooling energy demand reduction. The simulation results highlight that: (i) all the proposed retroﬁt cases allow to achieve a beneﬁt in terms of PES; (ii) the plastic and composite polymers materials allow for a slight performance improvement with respect to conven- tional second-skin material, such as porcelain gres; (iii) the best performances among the extruded polymers are returned by the polycarbonate multi-wall sheets (PES value equal to 2.64%); (iv) the best performances among the 3D printed polymers are achieved when using the PLA (PES value equal to 2.64%). However, the polymers’ results show very similar performances, thus allowing building contractors, designers, and architects to select the material based on other project requirements, as mechanical strength, weather resistance, environmental impact, etc. In this work, the thermal conductivity of the 3D printed materials has been calculated by means of an equation expressed in literature; therefore, in order to improve the accuracy of the model for the 3D printed material, in future works, the authors will carry out experimental investigations on full-scale 3D printed panels in OVF systems through the Gemini test cells. In addition, the capital cost for each retroﬁt action has been neglected. However, they represent an important parameter in the refurbishment typology choice; therefore, in future work, the authors will focus on a detailed economic analysis considering both the operating cost reduction and the simple payback period. Author Contributions: Conceptualization, G.C., Y.S., M.S., A.R., and S.S.; methodology, G.C., Y.S., M.S., A.R., and S.S.; software, G.C. and Y.S.; validation, G.C., Y.S., M.S., A.R., and S.S.; formal analysis, G.C., Y.S., M.S., and A.R.; investigation, G.C. and Y.S.; resources, G.C. and S.S.; data curation, G.C. and Y.S.; writing—original draft preparation, G.C., Y.S., and S.S.; writing—review & editing, G.C., Y.S., M.S., and A.R.; visualization, G.C., Y.S., M.S., A.R., and S.S.; supervision, G.C., M.S., and S.S.; project administration, G.C., M.S., and S.S.; funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was co-funded by a collaborative research and development project, number F/050405/01-03/X32 “WALLED: Smart LED&OLED per Lighting e MediaBuilding”–Fondo per la crescita sostenibile–Call Horizon 2020 PON I&C and by the European Union-PON for Research and Innovation 2014–2020. Data Availability Statement: The data presented in this study are available on request from the corresponding authors. Acknowledgments: The authors would like to thank the Academic Editors for their invitation to the Special Issue “Novel Technologies to Enhance Energy Performance and Indoor Environmental Quality of Buildings” and the Assistant Editor for his support. The authors would also like to thank the three anonymous reviewers for their insightful suggestions and careful reading of the manuscript. Buildings 2021, 11, 141 24 of 27 Conﬂicts of Interest: The authors declare no conﬂict of interest. Nomenclature Latin letters A surface area (m ) a ﬁller radius (m) ABS acrylonitrile-butadiene-styrene AM additive manufacturing CAGR compound annual growth rate COP Coefﬁcient of performance DSF double-skin facade E energy (kWh) EER energy efﬁciency ratio (-) EHP electric heat pump h interfacial boundary conductance (W/m K) I vertical pyranometer on the south wall (W/m ) vert k thermal conductivity of the 3D printed materials (W/mK) 3D k thermal conductivity of the ﬁller (W/mK) k thermal conductivity of the selected 3D printable polymers (W/mK) MAE Mean Absolute Error ( C) ME Mean Error ( C) N number of measurements (-) OVF opaque ventilated facades PC proposed case PES non-renewable primary energy saving (%) PET polyethylene terephthalate PETG polyethylene terephthalate glycol-modiﬁed PLA polylactic acid RC reference case RMSE Root Mean Square Error ( C) T thermocouple/temperature ( C) U transmittance value (m K/W) V volume fraction of the ﬁller W airspeed sensor Greeks D difference h efﬁciency (%) Subscripts/Superscripts cavity air cavity of the second-skin system cool cooling el electricity exp,i experimental value at time step i indoor indoor air p non-renewable primary energy PC proposed case PP power plant RC reference case sim,i simulated value at time step i th thermal w window References 1. 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Journal

Buildings – Multidisciplinary Digital Publishing Institute

Published: Apr 1, 2021

Keywords: ventilated facade; second-skin materials; 3D printed materials; additive manufacturing; TRNSYS; full-scale facility; retrofit action; energy saving

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Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study

Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study

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Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study

Energy Performances Assessment of Extruded and 3D Printed Polymers Integrated into Building Envelopes for a South Italian Case Study

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