Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efficiency in Building

Heangwoo Lee; Sowon Han; Janghoo Seo

doi:10.3390/buildings12010081

Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efficiency in Building

Lee, Heangwoo;Han, Sowon;Seo, Janghoo 2022-01-15 00:00:00 buildings Article Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efﬁciency in Building 1 1 2 , Heangwoo Lee , Sowon Han and Janghoo Seo * College of Design, Sangmyung University, Cheonan-si 31066, Korea; 2hw@smu.ac.kr (H.L.); hkghkdfyd123@naver.com (S.H.) School of Architecture, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul 02707, Korea * Correspondence: seojh@kookmin.ac.kr; Tel.: +82-02-910-4593 Abstract: Some recent research in the area of light shelves has been focused on applying photovoltaic modules to light shelves to save building energy. However, due to the modules installed on the light shelf reﬂectors, most such light shelves have failed to improve both daylighting and gener- ation efﬁciency. This study proposes a folding technology to improve light shelves’ daylighting and generation efﬁciency that uses photovoltaic modules and validates their performance using a testbed. The major obtained ﬁndings are as follows: (1) The proposed folding technology has a struc- ture in which reﬂectors and photovoltaic modules fold alternately by modularizing the light shelf. The reﬂector and photovoltaic modules are controlled by adjusting the degree of folding. (2) Because light shelf angles for improving daylighting and generation differed depending on the application of the photovoltaic module, the optimal light shelf speciﬁcations differed. (3) Compared to previous light shelf technologies, the light shelf with folding technology and a photovoltaic module reduced energy use by 31.3% to 38.2%. This demonstrates the efﬁcacy of the proposed system. (4) Applying a photovoltaic module can lower the indoor uniformity ratio, which means that the daylighting Citation: Lee, H.; Han, S.; Seo, J. performance of the light shelf is degraded due to the reduction of the area occupied by the reﬂector. Light Shelf Development Using Folding Technology and Photovoltaic Keywords: light shelf; photovoltaic module; folding technology; performance evaluation; energy efficiency Modules to Increase Energy Efﬁciency in Building. Buildings 2022, 12, 81. https://doi.org/10.3390/ buildings12010081 1. Introduction Academic Editors: Zhenjun Ma, Recently, research on daylighting and shading systems such as light shelves, light pipes, Alessandro Cannavale and blinds, louvers, and awnings has been increasing to reduce the consumption of light- Jianhui Hu ing energy in indoor spaces and create a comfortable indoor light environment [1–5]. Received: 29 November 2021 Among these systems, a light shelf is a type of reﬂector that contributes to lighting energy Accepted: 13 January 2022 savings by reﬂecting and introducing natural light deep into a room [6–9]. It can also Published: 15 January 2022 increase daylighting efﬁciency by responding to external environmental factors like solar altitude [10] by controlling the angle of the reﬂector. Several studies on light shelves have Publisher’s Note: MDPI stays neutral been conducted, indicating that their efﬁciency is widely recognized. Recent studies on with regard to jurisdictional claims in light shelves [11,12] have discovered that applying photovoltaic modules that convert published maps and institutional afﬁl- sunlight into electricity to the light shelf can increase building energy savings. However, iations. most of the approaches studied [12] have involved the application of photovoltaic modules to part of the light shelf reﬂector. When photovoltaic modules are attached to the light shelf reﬂector, the two components end up having the same angle, which is not suitable Copyright: © 2022 by the authors. for maximizing daylighting and generation performance at the same time. This is because Licensee MDPI, Basel, Switzerland. light shelves and photovoltaic modules require different angles to maximize daylighting This article is an open access article and generation performance. distributed under the terms and As a result, this study proposes and validates a method for simultaneously improving conditions of the Creative Commons the daylighting and generation efﬁciency of light shelves that use photovoltaic modules Attribution (CC BY) license (https:// using a full-scale testbed. creativecommons.org/licenses/by/ 4.0/). Buildings 2022, 12, 81. https://doi.org/10.3390/buildings12010081 https://www.mdpi.com/journal/buildings Buildings 2022, 12, x FOR PEER REVIEW 2 of 19 Buildings 2022, 12, 81 2 of 18 1.1. The Light Shelves Concept and Operation Technologies 1.1. The Light Shelves Concept and Operation Technologies As shown in Figure 1, a light shelf is one of the most prevailing daylighting systems As shown in Figure 1, a light shelf is one of the most prevailing daylighting systems installed on windows (inside or outside) that saves lighting energy by introducing natural installed on windows (inside or outside) that saves lighting energy by introducing natural light light in inside sidea abuilding building(r (ro oom) om) by by reﬂecting reflecting sunlight sunligh thr t throu oughgthe h the light light shelf shel reﬂector f reflector [8– [ 10 8]. – Light 10]. Li shelves ght shel can ves c also an also help help to to solvesolve indoor indoor il illuminance lumina imbal nce im an ba ces lances cau caused by sed by dif differences fer- in ences in il illuminance luminance between bet ar w eas een are nearaand s nefar ar and from far windows from window by preventing s by preven entry tin of g en some try of of the som excessive e of the ex natural cessivelight natufr ral om ligh the t from window the window . On the . other On the hand, othe it r can hand, intr itoduce can introd natural uce light deeper into an indoor space by reﬂecting natural light from the ceiling, and reﬂector, natural light deeper into an indoor space by reflecting natural light from the ceiling, and so reﬂections from the reﬂector and ceiling surface are typically considered. The variables reflector, so reflections from the reflector and ceiling surface are typically considered. The such as angle, height, reﬂectance, and width of light shelves determine its performance. variables such as angle, height, reflectance, and width of light shelves determine its per- Similarly, the light shelf angle is a primary variable to respond to external environmental formance. Similarly, the light shelf angle is a primary variable to respond to external en- factors such as the solar altitude, as shown in Figure 1 [10,12]. vironmental factors such as the solar altitude, as shown in Figure 1 [10,12]. Figure 1. Light shelf concept and operation: (a) Concept and variables, (b) Inflow of the natural light Figure 1. Light shelf concept and operation: (a) Concept and variables, (b) Inﬂow of the natural light by manipulating angle of the light shelf. by manipulating angle of the light shelf. Several studies have been conducted on light shelves to improve their daylighting Several studies have been conducted on light shelves to improve their daylighting per- performance, and some of these are listed in Table 1. Researchers have attempted to en- formance, and some of these are listed in Table 1. Researchers have attempted to enhance the hance the light shelf reflectors’ shape and also used multiple building envelope compo- light shelf reﬂectors’ shape and also used multiple building envelope component technolo- gies nent techno such as blinds logies such as blinds and awnings toan impr d awning ove light s to she im lfprove li daylighting ght sperformance helf daylight[ in 8,g perfo 9,11–21r]. - Some mance [8 recent ,9,11– studies, 21]. Some in particular recent studi , have es, in pa concentrated rticular, have concen on movabletrated on mov light shelves a using ble light in- formation technologies such as user recognition and location awareness [10,22]. However, shelves using information technologies such as user recognition and location awareness these studies controlled the light shelf angle using a rotating shaft (see Figure 1). Previous [10,22]. However, these studies controlled the light shelf angle using a rotating shaft (see studies Figure 1 on ). Pr light evious shelves studie with s on photovoltaic light shelves w modules ith photovol [11,12]ta have ic mo attached dules [11, photovoltaic 12] have at- modules to the front or part of the light shelf reﬂector. Installing photovoltaic modules tached photovoltaic modules to the front or part of the light shelf reflector. Installing pho- on the part of the light shelf reﬂector was more advantageous in saving building energy tovoltaic modules on the part of the light shelf reflector was more advantageous in saving than applying them to the front due to enabling daylighting and concentrating light at building energy than applying them to the front due to enabling daylighting and concen- the same time [12]. Previous studies that used photovoltaic modules on light shelves [12] trating light at the same time [12]. Previous studies that used photovoltaic modules on encountered difﬁculties in maximizing daylighting and generation at the same time because light shelves [12] encountered difficulties in maximizing daylighting and generation at the reﬂector that reﬂects natural light and the photovoltaic module that concentrates light the same time because the reflector that reflects natural light and the photovoltaic module maintain the same angle. that concentrates light maintain the same angle. Buildings 2022, 12, 81 3 of 18 Table 1. Previous studies on light shelves. Photovoltaic Module Consideration of Author Purpose Application Operation Technologies Proposal and performance Lim and Heng [8] evaluation of dynamic internal light shelf in high-rise ofﬁce buildings Not considered Performance evaluation according Claros and Soler [13] (Fixed light shelf) to light shelf reﬂectance Indoor visual comfort analysis Warrier and Raphael [14] according to the presence of light shelves Performance evaluation of Lee [9] perforated light shelves in response to external wind pressure Performance evaluation of light Lee et al. [15] shelves with diffusion sheets Proposal of a prism sheet Lee and Seo [16] application method for improving light shelf performance Parametric design study of light No Mangkuto et al. [17] shelves for application to hospital buildings Light shelf angle control by Performance evaluation of light a single rotating shaft Lee et al. [18] shelves by applying curvature Evaluation of the light shelf Meresi [19] performance based on the application of the external blinds Development and performance Lee [20] evaluation of a light shelf that can change the reﬂectivity AmirEbrahimi-Moghadam Performance evaluation of interior et al. [21] light shelves Development and performance Kim et al. [10] evaluation of light shelves based on user-awareness technology Performance evaluation of light Light shelf and light shelf Lee et al. [22] shelves with angle control by multiple location-awareness technology rotating shafts Performance evaluation of Not considered Hwang et al. [11] photovoltaic-integrated light (Fixed light shelf) shelf systems Yes Performance evaluation of light Light shelf angle control by Lee [12] shelves according to photovoltaic a single rotating shaft module attachment ratio 1.2. Concept and Power Generation Principle of Photovoltaic Modules As shown in Figure 2, a photovoltaic module is a structure of photovoltaic cells connected by a ribbon to generate the required energy [23,24]. A photovoltaic cell is the smallest unit that converts solar energy into electrical energy and has p-n semiconductor junction structures. When photovoltaic cells absorb photons from the outside, electrons and holes are generated inside the photovoltaic cells, as shown in Figure 2. These electrons and holes migrate to n-type and p-type semiconductors. This movement drives the load of the photovoltaic cells, generating electrical energy. The generation process allows the Buildings 2022, 12, x FOR PEER REVIEW 4 of 19 1.2. Concept and Power Generation Principle of Photovoltaic Modules As shown in Figure 2, a photovoltaic module is a structure of photovoltaic cells con- nected by a ribbon to generate the required energy [23,24]. A photovoltaic cell is the small- est unit that converts solar energy into electrical energy and has p-n semiconductor junc- tion structures. When photovoltaic cells absorb photons from the outside, electrons and Buildings 2022, 12, 81 4 of 18 holes are generated inside the photovoltaic cells, as shown in Figure 2. These electrons and holes migrate to n-type and p-type semiconductors. This movement drives the load of the photovoltaic cells, generating electrical energy. The generation process allows the photovoltaic cell to transform the solar energy into electrical energy. Temperature is a photovoltaic cell to transform the solar energy into electrical energy. Temperature is a factor that has a signiﬁcant impact on the power generation efﬁciency of photovoltaic cells. factor that has a significant impact on the power generation efficiency of photovoltaic This efﬁciency decreases as the temperature rises [25–30]. In addition, the photovoltaic cells. This efficiency decreases as the temperature rises [25–30]. In addition, the photovol- cells should be perpendicular to the sun to increase power generation efﬁciency, and the taic cells should be perpendicular to the sun to increase power generation efficiency, and efﬁciency decreases as the sunlight deviates from a vertical angle [31–33]. the efficiency decreases as the sunlight deviates from a vertical angle [31–33]. Figure 2. Photovoltaic module concept and power generation principle of photovoltaic cells: (a) Figure 2. Photovoltaic module concept and power generation principle of photovoltaic cells: Photovoltaic module concept, (b) Principle of the power generation. (a) Photovoltaic module concept, (b) Principle of the power generation. 1.3. Indoor Illuminance Standards for Lighting Control 1.3. Indoor Illuminance Standards for Lighting Control Ma Maintaining intaining optim optimal al indo indoor or ililluminance luminance can can inc incr reaease se the the effef icienc ﬁciency y of vis of u visual al wor work k by crea by cr ting eating a co am comfortable fortable liglight ht env envir ironm onment ent fofor r occup occupants ants and andsav saving ing build building ing en ener ergy by gy by pr pre eventing venting un unnecessary necessary llighting ighting c contr ontro ol l [34 [34]]. . The op The optimal timal ran range ge o of f indoo indoor r illuminance illuminance is is determ determined ined by by the type the type of ofworkplace workplace or or the the level level ofo visual f visual work. worThis k. This study study consider consider ed the ed optimal illuminance standards in the United States [35], Japan [36], and Korea [37] based on the optimal illuminance standards in the United States [35], Japan [36], and Korea [37] the grade of visual work, as shown in Table 2. The illuminance standards in these countries, based on the grade of visual work, as shown in Table 2. The illuminance standards in these however, differ. As a result, this study established the optimal indoor illuminance standard countries, however, differ. As a result, this study established the optimal indoor illumi- at 500 lx based on the intersection for general visual work in the United States, Japan, nance standard at 500 lx based on the intersection for general visual work in the United and Korea and used this standard to assess the performance of light shelves. States, Japan, and Korea and used this standard to assess the performance of light shelves. Table 2. Indoor illuminance standards for visual work in the US, Korea, and Japan. Table 2. Indoor illuminance standards for visual work in the US, Korea, and Japan. Illuminance Range (lx) Illuminance Range (lx) Optimal Indoor Illuminance Stand- Country Task Grade Optimal Indoor ards Minimum-Standard-Maximum Country Task Grade Minimum-Standard- Illuminance Standards General 500- Maximum 750-1000 USA IES [35] Simple 200-300-500 General 500-750-1000 USA IES [35] General 300-500-600 Simple 200-300-500 Japan JIS Z 9110 [36] Simple 150-200-300 General 300-500-600 Japan JIS Z 9110 [36] General 300-400-600 Simple 150-200-300 Republic of Korea KS A 3011 [37] Simple 150-200-300 General 300-400-600 Republic of Korea KS A 3011 [37] Simple 150-200-300 2. Methods 2.1. Proposal of Light Shelf That Applies Folding Technology and Photovoltaic Modules This study adopted folding technology to propose a way to simultaneously improve the daylighting and generation performance of light shelves that apply photovoltaic mod- ules, and the details are as follows. First, the light shelf was designed with a folding structure to improve daylighting and generation performance, as shown in Figure 3. The light shelf was divided and modularized in a horizontal direction with the daylighting window to implement such a folding structure, and a hinge structure connected the divided light shelf modules. Second, Buildings 2022, 12, x FOR PEER REVIEW 5 of 19 2. Methods 2.1. Proposal of Light Shelf That Applies Folding Technology and Photovoltaic Modules This study adopted folding technology to propose a way to simultaneously improve the daylighting and generation performance of light shelves that apply photovoltaic mod- ules, and the details are as follows. Buildings 2022, 12, 81 5 of 18 First, the light shelf was designed with a folding structure to improve daylighting and generation performance, as shown in Figure 3. The light shelf was divided and mod- ularized in a horizontal direction with the daylighting window to implement such a fold- reﬂectors and photovoltaic modules were installed alternately from the window side of ing structure, and a hinge structure connected the divided light shelf modules. Second, refl theector light s and shelf, photovo which lta applies ic modu ales folding were ins technology talled altern and atelphotovoltaic y from the windo modules. w side of As a result, the light shelf, which applies a folding technology and photovoltaic modules. As a result, folding the light shelf made the reﬂector angle symmetrical with the photovoltaic module folding the light shelf made the reflector angle symmetrical with the photovoltaic module angle (see Figure 3). This principle enables the proposed system to outperform conventional angle (see Figure 3). This principle enables the proposed system to outperform conven- ﬂat light shelves in terms of daylighting and generation. Third, the proposed system folds tional flat light shelves in terms of daylighting and generation. Third, the proposed system and unfolds the light shelf by moving along a rail, unlike previous methods in which the folds and unfolds the light shelf by moving along a rail, unlike previous methods in which light shelf rotates around a rotating shaft. the light shelf rotates around a rotating shaft. Figure 3. The concept and principle of the light shelf that applies folding technology and photovol- Figure 3. The concept and principle of the light shelf that applies folding technology and photovoltaic taic modules: (a) Structure of proposed system, (b) Daylighting and generation by the proposed modules: (a) Structure of proposed system, (b) Daylighting and generation by the proposed system. system. 2.2. Environment for Performance Evaluation A full-scale testbed including an artiﬁcial climate chamber was built to evaluate the performance of the proposed light shelf that applies folding technology and photovoltaic modules, and the details are as follows. First, as shown in Figures 4 and 5, the dimensions of the internal space of the testbed were 4.9 m 6.6 m 2.5 m (W D H). The reﬂectance of the ﬂoor, wall, and ceiling was set to 25%, 46%, and 86%, respectively. The window used to install the light shelf measured 1.9 m 1.7 m (W H) and was made of 24 mm thick pair glass with an 80 percent transmittance. Second, eight illuminance sensors were installed to measure the change in illuminance of the indoor space caused by the light shelf. Because of the height of the work surface, they were placed 0.85 m from the ﬂoor. Third, four lights were installed in the testbed using the IES 4-point method [35]. These LED lights were capable of 8-level dimming control (excluding lights off). Fourth, the testbed had an artiﬁcial climate chamber installed adjacent to the outside of the window. An artiﬁcial solar irradiation apparatus was installed in the chamber that would stimulate the brightness and altitude of the sun by regulating the intensity and angle of the natural light. The performance evaluation Buildings 2022, 12, 81 6 of 18 was carried out in an artiﬁcial environment due to its advantages in implementing a consistent external environment. The Grade-A artiﬁcial solar irradiation apparatus also ensured measurement uniformity following ASTM E927-85, resulting in valid results across performance evaluations. Due to mechanical limitations, this apparatus could not simulate the sun’s azimuth. The temperature range of the artiﬁcial climate chamber was also adjustable in light of the ﬁndings of related works [25–27] that the generation efﬁciency of photovoltaic modules was signiﬁcantly affected by temperature. Fifth, the current study develops an energy monitoring system to more precisely estimate lighting energy Buildings 2022, 12, x FOR PEER REVIEW 7 of 19 consumption (see Figure 4 for more detail). Figure 4. The layout, cross-section, and equipment in the testbed for performance evaluation. Figure 4. The layout, cross-section, and equipment in the testbed for performance evaluation. Figure 5. Environment for performance evaluation: (a) Testbed, (b) Artificial Solar Light Radiation Apparatus, (c) Chamber thermostat, (d) Chamber temperature controller, (e) Light dimming con- troller, (f) Energy monitoring system. Buildings 2022, 12, x FOR PEER REVIEW 7 of 19 Buildings 2022, 12, 81 7 of 18 Figure 4. The layout, cross-section, and equipment in the testbed for performance evaluation. Figure 5. Environment for performance evaluation: (a) Testbed, (b) Artificial Solar Light Radiation Figure 5. Environment for performance evaluation: (a) Testbed, (b) Artiﬁcial Solar Light Radiation Apparatus, (c) Chamber thermostat, (d) Chamber temperature controller, (e) Light dimming con- Apparatus, (c) Chamber thermostat, (d) Chamber temperature controller, (e) Light dimming controller, troller, (f) Energy monitoring system. (f) Energy monitoring system. Buildings 2022, 12, x FOR PEER REVIEW 8 of 19 2.3. Methods of Performance Appraisal The performance appraisal was conducted to prove the effectiveness of the daylight- ing and generation performance of the light shelf that applies folding technology and 2.3. Methods of Performance Appraisal photovoltaic modules. The performance appraisal was conducted to prove the effectiveness of the daylight- First, as shown in Table 3, this study set up three scenarios based on whether or ing and generation performance of the light shelf that applies folding technology and pho- not photovoltaic modules were used and how they worked. Case 1 was a standard tovoltaic modules. light shelf that did not include a photovoltaic module. Case 2 was a light shelf with First, as shown in Table 3, this study set up three scenarios based on whether or not a photovoltaic module attached to the reﬂector, resulting in the photovoltaic module and photovoltaic modules were used and how they worked. Case 1 was a standard light shelf reﬂector that did no having the t include same angle. a photovoltaic However module , the. C ara ea se wher 2 was a e the ligh photovoltaic t shelf with a p module hotovoltaic was module attached to the reflector, resulting in the photovoltaic module and reflector having attached in Case 2 had the same size as the reﬂector where reﬂection occurs, considering the same angle. However, the area where the photovoltaic module was attached in Case the previous study ﬁndings [12], in which installing photovoltaic modules on the part of 2 had the same size as the reflector where reflection occurs, considering the previous study the light shelf reﬂector was found to provide advantages in terms of saving building energy findings [12], in which installing photovoltaic modules on the part of the light shelf reflec- by enabling daylighting and generation at the same time. As shown in Figure 1, a single tor was found to provide advantages in terms of saving building energy by enabling day- rotating shaft was used to change the angles of the light shelves in Cases 1 and 2. In Case 2, lighting and generation at the same time. As shown in Figure 1, a single rotating shaft was the light shelf angle increased from 70 to 30 in 10 increments while considering the used to change the angles of the light shelves in Cases 1 and 2. In Case 2, the light shelf photovoltaic module’s generation function. Case 3 was designed around a light shelf that angle increased from −70° to 30° in 10° increments while considering the photovoltaic employs folding technology as well as photovoltaic modules. As shown in Table 4, the light module’s generation function. Case 3 was designed around a light shelf that employs fold- shelf is folded in stages. Each stage of folding changed the light shelf width, reﬂector angle, ing technology as well as photovoltaic modules. As shown in Table 4, the light shelf is and photovoltaic module angle. The photovoltaic cells used in the photovoltaic module folded in stages. Each stage of folding changed the light shelf width, reflector angle, and are speciﬁed in Table 5. Finally, as shown in Figure 6, this study used a proﬁle to make the photovoltaic module angle. The photovoltaic cells used in the photovoltaic module are specified in Table 5. Finally, as shown in Figure 6, this study used a profile to make the light shelf for a performance evaluation. light shelf for a performance evaluation. Table 3. Case settings for performance evaluation. Table 3. Case settings for performance evaluation. Light Shelf Photovoltaic Module Light Shelf Photovoltaic Module Folding Application Operation Application Folding Technol- Operation Case Technology Light Shelf Angle Case Light Shelf Angle (# of Photovoltaic Cells Method Width Angle (# of Photovoltaic Cells ogy Application Method Width Angle Application Applied) Applied) 10 , 0−10°, 0° , 10 ,, 10°, 1 1 NotNot applied applied (0)(0) 20 , 3020°, 30° 70 , −70°, 60 ,−60°, Rotation Rotation by a by a Not applied Not applied 50 , −50°, 40 ,−40°, rotating rotating shaft shaft 2 2 30 , −30°, 20 ,−20°, Applied Applied (33 (33 *) *) 0.6 m 0.6 m 10 , 0−10°, 0° , 10 ,, 10°, 20 , 3020°, 30° Applied Operates Applied Operates 3 0 (ﬁxed) Applied (33 *) (divided into along a rail 3 0° (fixed) Applied (33*) (divided into 6 along a rail 6 modules) axis modules) axis * The efﬁciency * The effici decreases ency decreases at at rate of 6.1% rate of 6.1% when the Photovoltaic when the Photovolt module a was ic m applied odule wa using s applie 33 photovoltaic d using 33 pho- cells. tovoltaic cells. Table 4. Folding shape, light shelf angle, and photovoltaic module angle according to the width of Case 3. Light Shelf Reflector Module Photovoltaic Mod- Folding Stage Width (W) Angle (α) ule Angle (β) 1 (Straight, no folding) 0.60 m 0° 180° 2 0.58 m 14.8° 165.2° 3 0.56 m 21.0° 159° 4 0.54 m 25.8° 154.2° 5 0.52 m 29.9° 150.1° Buildings 2022, 12, 81 8 of 18 Table 4. Folding shape, light shelf angle, and photovoltaic module angle according to the width of Case 3. Light Shelf Reﬂector Module Photovoltaic Folding Stage Width (W) Angle () Module Angle ( ) 1 (Straight, no folding) 0.60 m 0 180 Buildings 2022, 12, x FOR PEER REVIEW 9 of 19 2 0.58 m 14.8 165.2 3 0.56 m 21.0 159 Buildings 2022, 12, x FOR PEER REVIEW 9 of 19 4 0.54 m 25.8 154.2 6 0.50 m 33.6° 146.4° 5 0.52 m 29.9 150.1 6 6 0.50 m 0.50 m 33.6° 33.6 146.4° 146.4 Table 5. Photovoltaic cell specifications. Table 5. Photovoltaic cell specifications. Table 5. Photovoltaic cell speciﬁcations. Item Specifications Item Specifications Item Specifications Item Specifications Item Speciﬁcations Item Speciﬁcations Max. Power 2 W Max. Current (Impp) 670 mA Max. Power 2 W Max. Current (Impp) 670 mA Max. Power 2 W Max. Current (Impp) 670 mA Max. Voltage (Vmpp) 3 V Size 165 mm × 100 mm Max. Voltage (Vmpp) 3 V Size 165 mm × 100 mm Max. Voltage (Vmpp) 3 V Size 165 mm 100 mm Efficiency Efficiency 16.3% 16.3% Reflectance Reflectance 1–6% 1–6% Efﬁciency 16.3% Reﬂectance 1–6% Figure 6. Light shelf fabrication for performance evaluation. Figure 6. Light shelf fabrication for performance evaluation. Figure 6. Light shelf fabrication for performance evaluation. Secondly, monitored the distribution of indoor illuminance according to the cases set for performance evaluation to derive the minimum illuminance, average illuminance, and Secondly, monitored the distribution of indoor illuminance according to the cases Secondly, monitored the distribution of indoor illuminance according to the cases set uniformity ratio. The uniformity ratio was the ratio of the minimum illuminance to the set for performance evaluation to derive the minimum illuminance, average illuminance, for performance evaluation to derive the minimum illuminance, average illuminance, and average. and uniformity ratio. The uniformity ratio was the ratio of the minimum illuminance to uniformity ratio. The uniformity ratio was the ratio of the minimum illuminance to the the average. Thirdly, the study determined the dimming level and lighting energy consumption average. for eThir ach ca dlyse , the to ach study ieve o determined ptimal in the doo dimming r illuminance level and , and the lighting deener tails are as fol gy consumption lows. As for Thirdly, the study determined the dimming level and lighting energy consumption shown in Figure 7, dimming control was only used when the minimum value measured each case to achieve optimal indoor illuminance, and the details are as follows. As shown for each case to achieve optimal indoor illuminance, and the details are as follows. As in by the Figur eie gh 7t , il dimming luminance contr sensors w ol wasas only less used than 5when 00 lx. If the the m minimum inimum val value uemeasur measured ed by by shown in Figure 7, dimming control was only used when the minimum value measured the the il eight lumina illuminance nce sensors wa sensors s grwas eater less than 500 l than 500 x, all light lx. If the s we minimum re turned of value f witmeasur hout dim- ed by the illuminance sensors was greater than 500 lx, all lights were turned off without by the ming con eight trol. The system illuminance sensors w monitore asd th lese va s than lues measured by th 500 lx. If the minimu e illum m val inance ue mea sensors sured by dimming control. The system monitored the values measured by the illuminance sensors while increasing the dimming levels sequentially from the light closest to the illuminance the illuminance sensors was greater than 500 lx, all lights were turned off without dim- while increasing the dimming levels sequentially from the light closest to the illuminance sensor with the minimum value. During this process, dimming control ended when all ming control. The system monitored the values measured by the illuminance sensors sensor with the minimum value. During this process, dimming control ended when all measurements by the illuminance sensors reached 500 lx. Finally, the performance of each while increasing the dimming levels sequentially from the light closest to the illuminance measurements by the illuminance sensors reached 500 lx. Finally, the performance of each case was compared by calculating the lighting energy consumption based on the level of sensor with the minimum value. During this process, dimming control ended when all dimming control. measurements by the illuminance sensors reached 500 lx. Finally, the performance of each case was compared by calculating the lighting energy consumption based on the level of dimming control. Buildings 2022, 12, 81 9 of 18 Buildings 2022, 12, x FOR PEER REVIEW 10 of 19 Buildings 2022, 12, x FOR PEER REVIEW 10 of 19 case was compared by calculating the lighting energy consumption based on the level of dimming control. Figure 7. Lighting dimming control flow chart for performance evaluation. Figure 7. Lighting dimming control ﬂow chart for performance evaluation. Figure 7. Lighting dimming control flow chart for performance evaluation. Four Fourth, th, the e the n ener ergy prod gy produced uced by bythe ph the photovol otovoltaic mo taic module’s dule’s generation perform generation performance ance Fourth, the energy produced by the photovoltaic module’s generation performance was was mea measur sureed d in in this this s study tudy. The . The photo photovoltaic voltaic modu module’s le’s ener energy gy was wascalculated calculated by by mu multiply- lti- was measured in this study. The photovoltaic module’s energy was calculated by multi- ing the module’s maximum voltage (Vmp) and maximum current (Imp) while producing plying the module’s maximum voltage (Vmp) and maximum current (Imp) while produc- plying the module’s maximum voltage (Vmp) and maximum current (Imp) while produc- power. Table 6 shows the speciﬁcations of the photovoltaic module used for performance ing power. Table 6 shows the specifications of the photovoltaic module used for perfor- ing power. Table 6 shows the specifications of the photovoltaic module used for perfor- evaluation and the equipment used to measure the voltage and current. mance evaluation and the equipment used to measure the voltage and current. mance evaluation and the equipment used to measure the voltage and current. Table 6 Table . 6. SpecSpeciﬁcations ifications of the ofvoltag the e volt and age current m and curr easu ent ring measuring device (Equ device ipment nam (Equipment e: MULL name: ER Table 6. Specifications of the voltage and current measuring device (Equipment name: MULLER 3201). MULLER 3201). 3201). Item Item Spec Speciﬁcationsifications ImageImage Item Specifications Image Measurement item DC Voltage (0~600 V), Measurement item (measurement DC Voltage (0 V ~ 600 V), DC Current (0 A Measurement item (measurement DC Voltage (0 V ~ 600 V), DC Current (0 A (measurement capacity) DC Current (0~60 A) capacity) ~ 60 A) capacity) ~ 60 A) Error rate (0.5% + 3) Error rate ±(0.5% + 3) Error rate ±(0.5% + 3) Fifth, the artiﬁcial climate chamber of the testbed created the external environment Fifth, the artificial climate chamber of the testbed created the external environment Fifth, the artificial climate chamber of the testbed created the external environment of the outdoor space, where the performance evaluation was conducted for summer, mid- of the outdoor space, where the performance evaluation was conducted for summer, mid- of the outdoor space, where the performance evaluation was conducted for summer, mid- season, and winter, as shown in Table 7. The experiment was performed under three season, and winter, as shown in Table 7. The experiment was performed under three ex- season, and winter, as shown in Table 7. The experiment was performed under three ex- external conditions based on seasonal variation (i.e., summer, middle season, and winter). ternal conditions based on seasonal variation (i.e., summer, middle season, and winter). ternal conditions based on seasonal variation (i.e., summer, middle season, and winter). More speciﬁcally, each condition was controlled hour by hour to reﬂect potential change More specifically, each condition was controlled hour by hour to reflect potential change More specifically, each condition was controlled hour by hour to reflect potential change in external illuminance and solar radiation during a 5 h session between 10 am and in external illuminance and solar radiation during a 5 h session between 10 am and 3 pm. in external illuminance and solar radiation during a 5 h session between 10 am and 3 pm. 3 pm. The external environment’s characteristics were speciﬁcally based on Seoul, Korea, The external environment’s characteristics were specifically based on Seoul, Korea, which The external environment’s characteristics were specifically based on Seoul, Korea, which which has four distinct seasons. The outdoor temperature for each season was determined has four distinct seasons. The outdoor temperature for each season was determined by has four distinct seasons. The outdoor temperature for each season was determined by by considering the Korea Meteorological Administration’s average climate data for the past considering the Korea Meteorological Administration’s average climate data for the past considering the Korea Meteorological Administration’s average climate data for the past thirty years [38]. However, the solar irradiation for each season was determined by thirty years [38]. However, the solar irradiation for each season was determined by vary- ing the intensity of the artificial solar irradiation apparatus rather than by observing actual Buildings 2022, 12, 81 10 of 18 thirty years [38]. However, the solar irradiation for each season was determined by varying the intensity of the artiﬁcial solar irradiation apparatus rather than by observing actual climate data. This limitation was due to the performance evaluation being conducted in an artiﬁcial environment. Table 7. Climatic settings for performance evaluation based on geographical speciﬁcation. Outdoor Meridian External Illuminance (lx)/Solar Radiation (W/m ) Season Temperature Altitude 10:00–11:00 11:00–12:00 12:00–13:00 13:00–14:00 14:00–15:00 Summer 76.5 70,000/530 80,000/638 80,000/638 80,000/638 70,000/530 27.1 C Middle 52.5 50,000/414 50,000/414 60,000/476 60,000/476 50,000/414 17.2 C season Winter 29.5 20,000/289 30,000/332 30,000/332 30,000/332 20,000/289 3.2 C Sixth, the optimal speciﬁcations (optimal angle and folding stage) were derived for each case. These were derived by considering lighting energy saving as a priority. When multiple speciﬁcations saved the same amount of energy, the one with the highest unifor- mity was deemed to be optimal. Conditions that would result in the glare as a result of introducing natural light directly into the room via the light shelf without bouncing it off the ceiling, on the other hand, were excluded from the optimal speciﬁcations. 3. Results and Discussion 3.1. Performance Evaluation Results This study conducted a performance evaluation to validate the effectiveness of the light shelf that applies folding technology and photovoltaic modules. The results are as follows. Firstly, Figure 8 illustrates the performance evaluation results of Case 1 (light shelf with no photovoltaic module), which shows that light shelf angle affects the daylighting performance. Increasing the light shelf angle during the summer was beneﬁcial in saving lighting energy and improving the indoor uniformity ratio. Increasing the light shelf angle was also helpful during the middle season, but the uniformity ratio deteriorated when the light shelf angle was 30 . As shown in Figure 9, setting the angle at 30 allows high illuminance light to reach a speciﬁc area only by reﬂecting off the light shelf, resulting in an illuminance imbalance in the indoor space. In winter, the increment in the light shelf angle was suitable for saving lighting energy by increasing the amount of natural light entering the room through light shelf reﬂection, but adjusting the light shelf angle to 30 was inappropriate for saving lighting energy and improving the indoor uniformity ratio. This is because the solar altitude is lower in the summer compared to the winter and middle seasons, allowing natural light to enter deep into the indoor space through the daylighting window. Furthermore, during the winter, the solar altitude is 27.5 , so when the light shelf angle is 30 , the light shelf only acts as a shade, as shown in Figure 9. A light shelf angle of 20 was also excluded from the optimal speciﬁcations during the winter because, like using a 30 angle during the middle season, it could reduce the uniformity ratio and cause glare. As a result, the optimal light shelf angles for Case 1 during the summer, mid-season, and winter were 30 , 20 , and 10 , respectively, with lighting energy consumption of 0.471 kWh, 0.309 kWh, and 0.134 kWh. Buildings 2022, 12, x FOR PEER REVIEW 12 of 19 Buildings 2022, 12, 81 11 of 18 Buildings 2022, 12, x FOR PEER REVIEW 12 of 19 Figure 8. Indoor uniformity and lighting energy consumption according to the light shelf angle in Figure 8. Indoor uniformity and lighting energy consumption according to the light shelf angle in Figure 8. Indoor uniformity and lighting energy consumption according to the light shelf angle in Case 1: (a) Summer, (b) Middle season, (c) Winter. Case 1: (a) Summer, (b) Middle season, (c) Winter. Case 1: (a) Summer, (b) Middle season, (c) Winter. Figure 9. The inflow of natural light according to the light shelf angle in Case 1: (a) Middle season, Angle 30°, (b) Winter, Angle 20°, (c) Winter, Angle 30°. Figure 9. The inflow of natural light according to the light shelf angle in Case 1: (a) Middle season, Figure 9. The inﬂow of natural light according to the light shelf angle in Case 1: (a) Middle season, Angle 30°, ( Seco bn ) Winter, dly, FiguAngle re 10 s 20°, how (s t c) Winter he outp , Angle 30° ut of a per . formance evaluation of Case 2 (light Angle 30 , (b) Winter, Angle 20 , (c) Winter, Angle 30 . shelf applying photovoltaic module). In terms of saving lighting energy, the optimal spec- ifications during summer, mid-season, and winter were 30°, 20°, and 10°, respectively, the Secondly, Figure 10 shows the output of a performance evaluation of Case 2 (light Secondly, Figure 10 shows the output of a performance evaluation of Case 2 (light shelf applying photovoltaic module). In terms of saving lighting energy, the optimal spec- shelf applying photovoltaic module). In terms of saving lighting energy, the optimal ifications during summer, mid-season, and winter were 30°, 20°, and 10°, respectively, the speciﬁcations during summer, mid-season, and winter were 30 , 20 , and 10 , respectively, the same as Case 1. However, in Case 2, the area of the reﬂector used for daylighting was reduced by 50% compared to Case 1, reducing the amount of natural light entering the room through light shelf reﬂection and deteriorating uniformity, as shown in Figure 11. Case 2 Buildings 2022, 12, x FOR PEER REVIEW 13 of 19 Buildings 2022, 12, 81 12 of 18 same as Case 1. However, in Case 2, the area of the reflector used for daylighting was reduced by 50% compared to Case 1, reducing the amount of natural light entering the room through light shelf reflection and deteriorating uniformity, as shown in Figure 11. also has a higher lighting energy consumption than Case 1. Meanwhile, the photovoltaic Case 2 also has a higher lighting energy consumption than Case 1. Meanwhile, the photo- module in Case 2 generated the most power at light shelf angles of 10 , 40 , and 60 , voltaic module in Case 2 generated the most power at light shelf angles of −10°, −40°, and which proves that the closer the incident angle of natural light is to vertical, the higher −60°, which proves that the closer the incident angle of natural light is to vertical, the the power generation efﬁciency. However, it is difﬁcult to maximize both the daylighting higher the power generation efficiency. However, it is difficult to maximize both the day- and generation performance at the same time in Case 2 because it controls the reﬂector lighting and generation performance at the same time in Case 2 because it controls the for daylighting and the photovoltaic module for concentrating light at the same angle. reflector for daylighting and the photovoltaic module for concentrating light at the same Therefore, the optimal speciﬁcations for Case 2 during summer, mid-season, and winter angle. Therefore, the optimal specifications for Case 2 during summer, mid-season, and were 10 , 10 , and 20 , respectively, and the lighting energy consumption was 0.406 kWh, winter were 10°, −10°, and 20°, respectively, and the lighting energy consumption was 0.314 kWh, and 0.100 kWh, respectively. 0.406 kWh, 0.314 kWh, and 0.100 kWh, respectively. Figure 10. Figure 10. Lighti Lighting ng energy c energy ons consumption umption and the power ge and the power nerated by the p generated by th hotovoltai e photovoltaic c module ac- module cording to the light shelf angle in Case 2: (a) Summer, (b) Mid-season, (c) Winter. according to the light shelf angle in Case 2: (a) Summer, (b) Mid-season, (c) Winter. Buildings 2022, 12, 81 13 of 18 Buildings 2022, 12, x FOR PEER REVIEW 14 of 19 Figure 11. Case 1 and Case 2 indoor uniformity analysis: (a) Summer, (b) Mid-season, (c) Winter. Figure 11. Case 1 and Case 2 indoor uniformity analysis: (a) Summer, (b) Mid-season, (c) Winter. Thirdly, Thirdly, Figur Figure 12 shows e 12 shows the the r re esults sults of of a a performance performance apprais appraisal al o of f Case Case 3 (ligh 3 (light t shelf shelf applying folding technology and photovoltaic module). The optimal speciﬁcations for applying folding technology and photovoltaic module). The optimal specifications for Case Case 3, con 3, considering sidering on only ly lighting lighting ener energ gy y sa savings vings an and d improving improving indoor l indoor light ight unif uniformity ormity during summer, mid-season, and winter were folding stages 6, 4, and 3(4), respectively. during summer, mid-season, and winter were folding stages 6, 4, and 3(4), respectively. During the winter, however, as shown in Table 4, folding stages 3 and 4 reduce the light During the winter, however, as shown in Table 4, folding stages 3 and 4 reduce the light shelf angle to 21 and 25.8 , respectively. These angles, like a light shelf angle of 20 in shelf angle to 21° and 25.8°, respectively. These angles, like a light shelf angle of 20° in the the winter, cause glare by allowing the direct ﬂow of natural light into the interior space winter, cause glare by allowing the direct flow of natural light into the interior space by reflecting off the light shelf, so they were excluded from the optimal specifications. Taking Buildings 2022, 12, 81 14 of 18 Buildings 2022, 12, x FOR PEER REVIEW 15 of 19 by reﬂecting off the light shelf, so they were excluded from the optimal speciﬁcations. Taking these factors into account, the best speciﬁcations for saving lighting energy and these factors into account, the best specifications for saving lighting energy and improving improving indoor uniformity in Case 3 were folding stages 6, 4, and 2 for summer, mid- indoor uniformity in Case 3 were folding stages 6, 4, and 2 for summer, mid-season, and season, and winter, respectively. The optimal speciﬁcations for generating power by the winter, respectively. The optimal specifications for generating power by the photovoltaic photovoltaic module in Case 3 were folding stages 3, 6, and 6 for summer, mid-season, module in Case 3 were folding stages 3, 6, and 6 for summer, mid-season, and winter, and winter, respectively. Therefore, the optimal speciﬁcations for Case 3 during summer, respectively. Therefore, the optimal specifications for Case 3 during summer, mid-season, mid-season, and winter w and ere fol winter dingwer stage e folding s 6, 4, and stages 4, resp 6, e 4, ctand ively 4, , and respectively the lighting , and enthe ergy lighting consum ener p- gy consumption tion was 0.307 was kWh, 0. 0.307 22kWh, 4 kWh, and 0.224 kWh, 0.034 kWh and ,0.034 respectiv kWh, ely. respectively . Figure 12. Lighting energy consumption and the power generated by the photovoltaic module ac- Figure 12. Lighting energy consumption and the power generated by the photovoltaic module cording to the folding stage in Case 3: (a) Summer, (b) Mid-season, (c) Winter. according to the folding stage in Case 3: (a) Summer, (b) Mid-season, (c) Winter. Buildings 2022, 12, x FOR PEER REVIEW 16 of 19 Buildings 2022, 12, 81 15 of 18 3.2. Performance Evaluation Discussion 3.2. Performance Evaluation Discussion This study proposed a folding technology to improve light shelves’ daylighting and generation efficiency that incorporates photovoltaic modules and validated its effective- This study proposed a folding technology to improve light shelves’ daylighting and ness through a performance evaluation. A discussion of the results follows. generation efﬁciency that incorporates photovoltaic modules and validated its effectiveness First, the optimal specifications for Cases 1, 2, and 3 were derived through evaluating through a performance evaluation. A discussion of the results follows. their performance. Figure 13 shows the energy consumption based on these results. Case First, the optimal speciﬁcations for Cases 1, 2, and 3 were derived through evaluating 2 red their u performance. ced energy consumption by 10.3% Figure 13 shows the ener compared gy consumption to Case 1based , demonstra on these ting the results. effe Case ctive- 2 ness o reduced f the photovoltaic m energy consumption odule by 10.3% used o compar n light ed shelve to Case s. Case 1, demonstrating 3 reduced ene the rgy effectiveness consump- of the photovoltaic module used on light shelves. Case 3 reduced energy consumption by tion by 31.3% compared to Case 2, due to improved daylighting and generation efficiency achieve 31.3% compar d by adju ed to sting t Caseh 2, e refl dueect to o impr r and oved phot daylighting ovoltaic module and generation angles th ef rough ﬁciency foachieved lding. In by adjusting the reﬂector and photovoltaic module angles through folding. In particular, particular, although the proposed light shelf that applies folding technology and photo- although the proposed light shelf that applies folding technology and photovoltaic modules voltaic modules (Case 3) had an operating range of only 0.1 m, it reduced building energy (Case 3) had an operating range of only 0.1 m, it reduced building energy by a signiﬁcant by a significant amount compared to the conventional light shelf. These results prove the amount compared to the conventional light shelf. These results prove the effectiveness of effectiveness of the proposed system (Case 3). the proposed system (Case 3). Figure 13. Energy consumption analysis (by Case). Figure 13. Energy consumption analysis (by Case). Second, Case 3 uses folding technology to cope with external wind pressure and snow Second, Case 3 uses folding technology to cope with external wind pressure and load by completely folding the light shelf. For example, if you are concerned about the snow load by completely folding the light shelf. For example, if you are concerned about damage caused by wind pressure exceeding a certain level, you can fold the light shelf the damage caused by wind pressure exceeding a certain level, you can fold the light shelf to avoid any damage caused by protruding outside. Case 3, in particular, connects each to avoid any damage caused by protruding outside. Case 3, in particular, connects each module with a hinge structure, resulting in gaps between each module. Compared to module with a hinge structure, resulting in gaps between each module. Compared to con- conventional movable light shelves, this structural feature responds quickly to external ventional movable light shelves, this structural feature responds quickly to external envi- environmental factors such as wind pressure. ronmental factors such as wind pressure. Third, installing photovoltaic modules on light shelves reduces the area occupied by Third, installing photovoltaic modules on light shelves reduces the area occupied by reﬂectors to perform daylighting, which reduces the amount of natural light ﬂowing indoors reflectors to perform daylighting, which reduces the amount of natural light flowing in- through light shelf reﬂection. In this context, the light shelf with a photovoltaic module doors through light shelf reflection. In this context, the light shelf with a photovoltaic may cause issues, such as increasing the amount of lighting energy required to maintain module may cause issues, such as increasing the amount of lighting energy required to optimal indoor illuminance and reducing indoor uniformity. Therefore, further research maintain optimal indoor illuminance and reducing indoor uniformity. Therefore, further should be conducted on adjusting the width of the light shelf according to the area occupied research should be conducted on adjusting the width of the light shelf according to the by the photovoltaic module in the light shelf. area occupied by the photovoltaic module in the light shelf. 4. Conclusions 4. Conclusions A folding technology was proposed to improve the daylighting and generation perfor- A folding technology was proposed to improve the daylighting and generation per- mance of light shelves that apply photovoltaic modules, and its performance was evaluated formance of light shelves that apply photovoltaic modules, and its performance was eval- through a full-scale testbed. The main ﬁndings are as follows. uated through a full-scale testbed. The main findings are as follows. First, the proposed light shelf that employs folding technology and photovoltaic mod- First, the proposed light shelf that employs folding technology and photovoltaic ules has a structure in which reﬂectors and photovoltaic modules are installed alternately modules has a structure in which reflectors and photovoltaic modules are installed alter- by modularizing the light shelf. A hinge structure connects each module, allowing the nately by modularizing the light shelf. A hinge structure connects each module, allowing system to be folded. This structure enables the reﬂector module and photovoltaic module to be symmetrical and operate at different angles depending on the degree of folding. Due to Buildings 2022, 12, 81 16 of 18 such structural features, the proposed light shelf that applies folding technology and pho- tovoltaic modules can improve both daylighting and generation efﬁciency. This system also employs a novel operation method based on rails instead of conventional light shelves, which control the light shelf angle via a rotating shaft. Second, the optimal light shelf angle for each case was derived. The optimal light shelf angles were based on the lighting energy consumption and uniformity ratio to maintain the optimal indoor illuminance. Angles that may cause glare were excluded, even if they were excellent in terms of saving energy. The optimal angles for a light shelf without a photovoltaic module during the summer, mid-season, and winter were 30 , 20 , and 10 , respectively, indicating that the angles must be controlled by operating or moving the light shelf to improve performance. In contrast, the optimal angles for a light shelf with a photovoltaic module during the summer, mid-season, and winter were 10 , 10 , and 20 , respectively, compared to a light shelf without a photovoltaic module. The photovoltaic module and light reﬂector module require different angles to increase power generation efﬁciency and daylighting efﬁciency. Third, the light shelf that applies folding technology and photovoltaic modules can reduce energy consumption by 38.2% and 31.3%, respectively, compared to light shelves with no photovoltaic modules and light shelves with photovoltaic modules but no folding technology. These results validate the effectiveness of the application of photovoltaic mod- ules to light shelves and prove that the folding technology can improve both daylighting and generation efﬁciency. Fourth, the light shelf that applies a photovoltaic module was unsuitable in terms of improving the indoor uniformity compared to the light shelf with no photovoltaic module because the area occupied by the reﬂector decreases due to installing the photovoltaic module, which leads to reducing the amount of natural light ﬂowing into the room through the light shelf. This aspect should be considered when designing light shelves that apply photovoltaic modules. This study is signiﬁcant because it proposes and validates a new technology related to light shelves to save building energy, a primary current concern. However, due to the use of a testbed, the performance evaluation was carried out in a restricted external environment. Other limitations include the fact that various other light shelf variables, such as width and height, were not considered. As a result, additional in-depth studies should be conducted in further research in this sector. Author Contributions: Methodology, H.L.; writing—original draft preparation, H.L. and S.H.; visualization, S.H.; writing—review and editing, H.L. and J.S.; supervision, J.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2020R1C1C1004704) (NRF-2021R1A2B5B02001469). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No additional data available. Acknowledgments: None stated. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Heng, C.Y.S.; Lim, Y.W.; Ossen, D.R. Horizontal light pipe transporter for deep plan high-rise ofﬁce daylighting in tropical climate. Build. Environ. 2020, 171, 106645. [CrossRef] 2. Jenkins, D.; Muneer, T. Modelling light-pipe performances—A natural daylighting solution. Build. Environ. 2003, 38, 965–972. [CrossRef] 3. Lee, H.; Jang, H.; Seo, J. A preliminary study on the performance of an awning system with a built-in light shelf. Build. Environ. 2018, 131, 255–263. [CrossRef] Buildings 2022, 12, 81 17 of 18 4. 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ISSN: 2075-5309
DOI: 10.3390/buildings12010081
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Abstract

buildings Article Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efﬁciency in Building 1 1 2 , Heangwoo Lee , Sowon Han and Janghoo Seo * College of Design, Sangmyung University, Cheonan-si 31066, Korea; 2hw@smu.ac.kr (H.L.); hkghkdfyd123@naver.com (S.H.) School of Architecture, Kookmin University, 77, Jeongneung-ro, Seongbuk-gu, Seoul 02707, Korea * Correspondence: seojh@kookmin.ac.kr; Tel.: +82-02-910-4593 Abstract: Some recent research in the area of light shelves has been focused on applying photovoltaic modules to light shelves to save building energy. However, due to the modules installed on the light shelf reﬂectors, most such light shelves have failed to improve both daylighting and gener- ation efﬁciency. This study proposes a folding technology to improve light shelves’ daylighting and generation efﬁciency that uses photovoltaic modules and validates their performance using a testbed. The major obtained ﬁndings are as follows: (1) The proposed folding technology has a struc- ture in which reﬂectors and photovoltaic modules fold alternately by modularizing the light shelf. The reﬂector and photovoltaic modules are controlled by adjusting the degree of folding. (2) Because light shelf angles for improving daylighting and generation differed depending on the application of the photovoltaic module, the optimal light shelf speciﬁcations differed. (3) Compared to previous light shelf technologies, the light shelf with folding technology and a photovoltaic module reduced energy use by 31.3% to 38.2%. This demonstrates the efﬁcacy of the proposed system. (4) Applying a photovoltaic module can lower the indoor uniformity ratio, which means that the daylighting Citation: Lee, H.; Han, S.; Seo, J. performance of the light shelf is degraded due to the reduction of the area occupied by the reﬂector. Light Shelf Development Using Folding Technology and Photovoltaic Keywords: light shelf; photovoltaic module; folding technology; performance evaluation; energy efficiency Modules to Increase Energy Efﬁciency in Building. Buildings 2022, 12, 81. https://doi.org/10.3390/ buildings12010081 1. Introduction Academic Editors: Zhenjun Ma, Recently, research on daylighting and shading systems such as light shelves, light pipes, Alessandro Cannavale and blinds, louvers, and awnings has been increasing to reduce the consumption of light- Jianhui Hu ing energy in indoor spaces and create a comfortable indoor light environment [1–5]. Received: 29 November 2021 Among these systems, a light shelf is a type of reﬂector that contributes to lighting energy Accepted: 13 January 2022 savings by reﬂecting and introducing natural light deep into a room [6–9]. It can also Published: 15 January 2022 increase daylighting efﬁciency by responding to external environmental factors like solar altitude [10] by controlling the angle of the reﬂector. Several studies on light shelves have Publisher’s Note: MDPI stays neutral been conducted, indicating that their efﬁciency is widely recognized. Recent studies on with regard to jurisdictional claims in light shelves [11,12] have discovered that applying photovoltaic modules that convert published maps and institutional afﬁl- sunlight into electricity to the light shelf can increase building energy savings. However, iations. most of the approaches studied [12] have involved the application of photovoltaic modules to part of the light shelf reﬂector. When photovoltaic modules are attached to the light shelf reﬂector, the two components end up having the same angle, which is not suitable Copyright: © 2022 by the authors. for maximizing daylighting and generation performance at the same time. This is because Licensee MDPI, Basel, Switzerland. light shelves and photovoltaic modules require different angles to maximize daylighting This article is an open access article and generation performance. distributed under the terms and As a result, this study proposes and validates a method for simultaneously improving conditions of the Creative Commons the daylighting and generation efﬁciency of light shelves that use photovoltaic modules Attribution (CC BY) license (https:// using a full-scale testbed. creativecommons.org/licenses/by/ 4.0/). Buildings 2022, 12, 81. https://doi.org/10.3390/buildings12010081 https://www.mdpi.com/journal/buildings Buildings 2022, 12, x FOR PEER REVIEW 2 of 19 Buildings 2022, 12, 81 2 of 18 1.1. The Light Shelves Concept and Operation Technologies 1.1. The Light Shelves Concept and Operation Technologies As shown in Figure 1, a light shelf is one of the most prevailing daylighting systems As shown in Figure 1, a light shelf is one of the most prevailing daylighting systems installed on windows (inside or outside) that saves lighting energy by introducing natural installed on windows (inside or outside) that saves lighting energy by introducing natural light light in inside sidea abuilding building(r (ro oom) om) by by reﬂecting reflecting sunlight sunligh thr t throu oughgthe h the light light shelf shel reﬂector f reflector [8– [ 10 8]. – Light 10]. Li shelves ght shel can ves c also an also help help to to solvesolve indoor indoor il illuminance lumina imbal nce im an ba ces lances cau caused by sed by dif differences fer- in ences in il illuminance luminance between bet ar w eas een are nearaand s nefar ar and from far windows from window by preventing s by preven entry tin of g en some try of of the som excessive e of the ex natural cessivelight natufr ral om ligh the t from window the window . On the . other On the hand, othe it r can hand, intr itoduce can introd natural uce light deeper into an indoor space by reﬂecting natural light from the ceiling, and reﬂector, natural light deeper into an indoor space by reflecting natural light from the ceiling, and so reﬂections from the reﬂector and ceiling surface are typically considered. The variables reflector, so reflections from the reflector and ceiling surface are typically considered. The such as angle, height, reﬂectance, and width of light shelves determine its performance. variables such as angle, height, reflectance, and width of light shelves determine its per- Similarly, the light shelf angle is a primary variable to respond to external environmental formance. Similarly, the light shelf angle is a primary variable to respond to external en- factors such as the solar altitude, as shown in Figure 1 [10,12]. vironmental factors such as the solar altitude, as shown in Figure 1 [10,12]. Figure 1. Light shelf concept and operation: (a) Concept and variables, (b) Inflow of the natural light Figure 1. Light shelf concept and operation: (a) Concept and variables, (b) Inﬂow of the natural light by manipulating angle of the light shelf. by manipulating angle of the light shelf. Several studies have been conducted on light shelves to improve their daylighting Several studies have been conducted on light shelves to improve their daylighting per- performance, and some of these are listed in Table 1. Researchers have attempted to en- formance, and some of these are listed in Table 1. Researchers have attempted to enhance the hance the light shelf reflectors’ shape and also used multiple building envelope compo- light shelf reﬂectors’ shape and also used multiple building envelope component technolo- gies nent techno such as blinds logies such as blinds and awnings toan impr d awning ove light s to she im lfprove li daylighting ght sperformance helf daylight[ in 8,g perfo 9,11–21r]. - Some mance [8 recent ,9,11– studies, 21]. Some in particular recent studi , have es, in pa concentrated rticular, have concen on movabletrated on mov light shelves a using ble light in- formation technologies such as user recognition and location awareness [10,22]. However, shelves using information technologies such as user recognition and location awareness these studies controlled the light shelf angle using a rotating shaft (see Figure 1). Previous [10,22]. However, these studies controlled the light shelf angle using a rotating shaft (see studies Figure 1 on ). Pr light evious shelves studie with s on photovoltaic light shelves w modules ith photovol [11,12]ta have ic mo attached dules [11, photovoltaic 12] have at- modules to the front or part of the light shelf reﬂector. Installing photovoltaic modules tached photovoltaic modules to the front or part of the light shelf reflector. Installing pho- on the part of the light shelf reﬂector was more advantageous in saving building energy tovoltaic modules on the part of the light shelf reflector was more advantageous in saving than applying them to the front due to enabling daylighting and concentrating light at building energy than applying them to the front due to enabling daylighting and concen- the same time [12]. Previous studies that used photovoltaic modules on light shelves [12] trating light at the same time [12]. Previous studies that used photovoltaic modules on encountered difﬁculties in maximizing daylighting and generation at the same time because light shelves [12] encountered difficulties in maximizing daylighting and generation at the reﬂector that reﬂects natural light and the photovoltaic module that concentrates light the same time because the reflector that reflects natural light and the photovoltaic module maintain the same angle. that concentrates light maintain the same angle. Buildings 2022, 12, 81 3 of 18 Table 1. Previous studies on light shelves. Photovoltaic Module Consideration of Author Purpose Application Operation Technologies Proposal and performance Lim and Heng [8] evaluation of dynamic internal light shelf in high-rise ofﬁce buildings Not considered Performance evaluation according Claros and Soler [13] (Fixed light shelf) to light shelf reﬂectance Indoor visual comfort analysis Warrier and Raphael [14] according to the presence of light shelves Performance evaluation of Lee [9] perforated light shelves in response to external wind pressure Performance evaluation of light Lee et al. [15] shelves with diffusion sheets Proposal of a prism sheet Lee and Seo [16] application method for improving light shelf performance Parametric design study of light No Mangkuto et al. [17] shelves for application to hospital buildings Light shelf angle control by Performance evaluation of light a single rotating shaft Lee et al. [18] shelves by applying curvature Evaluation of the light shelf Meresi [19] performance based on the application of the external blinds Development and performance Lee [20] evaluation of a light shelf that can change the reﬂectivity AmirEbrahimi-Moghadam Performance evaluation of interior et al. [21] light shelves Development and performance Kim et al. [10] evaluation of light shelves based on user-awareness technology Performance evaluation of light Light shelf and light shelf Lee et al. [22] shelves with angle control by multiple location-awareness technology rotating shafts Performance evaluation of Not considered Hwang et al. [11] photovoltaic-integrated light (Fixed light shelf) shelf systems Yes Performance evaluation of light Light shelf angle control by Lee [12] shelves according to photovoltaic a single rotating shaft module attachment ratio 1.2. Concept and Power Generation Principle of Photovoltaic Modules As shown in Figure 2, a photovoltaic module is a structure of photovoltaic cells connected by a ribbon to generate the required energy [23,24]. A photovoltaic cell is the smallest unit that converts solar energy into electrical energy and has p-n semiconductor junction structures. When photovoltaic cells absorb photons from the outside, electrons and holes are generated inside the photovoltaic cells, as shown in Figure 2. These electrons and holes migrate to n-type and p-type semiconductors. This movement drives the load of the photovoltaic cells, generating electrical energy. The generation process allows the Buildings 2022, 12, x FOR PEER REVIEW 4 of 19 1.2. Concept and Power Generation Principle of Photovoltaic Modules As shown in Figure 2, a photovoltaic module is a structure of photovoltaic cells con- nected by a ribbon to generate the required energy [23,24]. A photovoltaic cell is the small- est unit that converts solar energy into electrical energy and has p-n semiconductor junc- tion structures. When photovoltaic cells absorb photons from the outside, electrons and Buildings 2022, 12, 81 4 of 18 holes are generated inside the photovoltaic cells, as shown in Figure 2. These electrons and holes migrate to n-type and p-type semiconductors. This movement drives the load of the photovoltaic cells, generating electrical energy. The generation process allows the photovoltaic cell to transform the solar energy into electrical energy. Temperature is a photovoltaic cell to transform the solar energy into electrical energy. Temperature is a factor that has a signiﬁcant impact on the power generation efﬁciency of photovoltaic cells. factor that has a significant impact on the power generation efficiency of photovoltaic This efﬁciency decreases as the temperature rises [25–30]. In addition, the photovoltaic cells. This efficiency decreases as the temperature rises [25–30]. In addition, the photovol- cells should be perpendicular to the sun to increase power generation efﬁciency, and the taic cells should be perpendicular to the sun to increase power generation efficiency, and efﬁciency decreases as the sunlight deviates from a vertical angle [31–33]. the efficiency decreases as the sunlight deviates from a vertical angle [31–33]. Figure 2. Photovoltaic module concept and power generation principle of photovoltaic cells: (a) Figure 2. Photovoltaic module concept and power generation principle of photovoltaic cells: Photovoltaic module concept, (b) Principle of the power generation. (a) Photovoltaic module concept, (b) Principle of the power generation. 1.3. Indoor Illuminance Standards for Lighting Control 1.3. Indoor Illuminance Standards for Lighting Control Ma Maintaining intaining optim optimal al indo indoor or ililluminance luminance can can inc incr reaease se the the effef icienc ﬁciency y of vis of u visual al wor work k by crea by cr ting eating a co am comfortable fortable liglight ht env envir ironm onment ent fofor r occup occupants ants and andsav saving ing build building ing en ener ergy by gy by pr pre eventing venting un unnecessary necessary llighting ighting c contr ontro ol l [34 [34]]. . The op The optimal timal ran range ge o of f indoo indoor r illuminance illuminance is is determ determined ined by by the type the type of ofworkplace workplace or or the the level level ofo visual f visual work. worThis k. This study study consider consider ed the ed optimal illuminance standards in the United States [35], Japan [36], and Korea [37] based on the optimal illuminance standards in the United States [35], Japan [36], and Korea [37] the grade of visual work, as shown in Table 2. The illuminance standards in these countries, based on the grade of visual work, as shown in Table 2. The illuminance standards in these however, differ. As a result, this study established the optimal indoor illuminance standard countries, however, differ. As a result, this study established the optimal indoor illumi- at 500 lx based on the intersection for general visual work in the United States, Japan, nance standard at 500 lx based on the intersection for general visual work in the United and Korea and used this standard to assess the performance of light shelves. States, Japan, and Korea and used this standard to assess the performance of light shelves. Table 2. Indoor illuminance standards for visual work in the US, Korea, and Japan. Table 2. Indoor illuminance standards for visual work in the US, Korea, and Japan. Illuminance Range (lx) Illuminance Range (lx) Optimal Indoor Illuminance Stand- Country Task Grade Optimal Indoor ards Minimum-Standard-Maximum Country Task Grade Minimum-Standard- Illuminance Standards General 500- Maximum 750-1000 USA IES [35] Simple 200-300-500 General 500-750-1000 USA IES [35] General 300-500-600 Simple 200-300-500 Japan JIS Z 9110 [36] Simple 150-200-300 General 300-500-600 Japan JIS Z 9110 [36] General 300-400-600 Simple 150-200-300 Republic of Korea KS A 3011 [37] Simple 150-200-300 General 300-400-600 Republic of Korea KS A 3011 [37] Simple 150-200-300 2. Methods 2.1. Proposal of Light Shelf That Applies Folding Technology and Photovoltaic Modules This study adopted folding technology to propose a way to simultaneously improve the daylighting and generation performance of light shelves that apply photovoltaic mod- ules, and the details are as follows. First, the light shelf was designed with a folding structure to improve daylighting and generation performance, as shown in Figure 3. The light shelf was divided and modularized in a horizontal direction with the daylighting window to implement such a folding structure, and a hinge structure connected the divided light shelf modules. Second, Buildings 2022, 12, x FOR PEER REVIEW 5 of 19 2. Methods 2.1. Proposal of Light Shelf That Applies Folding Technology and Photovoltaic Modules This study adopted folding technology to propose a way to simultaneously improve the daylighting and generation performance of light shelves that apply photovoltaic mod- ules, and the details are as follows. Buildings 2022, 12, 81 5 of 18 First, the light shelf was designed with a folding structure to improve daylighting and generation performance, as shown in Figure 3. The light shelf was divided and mod- ularized in a horizontal direction with the daylighting window to implement such a fold- reﬂectors and photovoltaic modules were installed alternately from the window side of ing structure, and a hinge structure connected the divided light shelf modules. Second, refl theector light s and shelf, photovo which lta applies ic modu ales folding were ins technology talled altern and atelphotovoltaic y from the windo modules. w side of As a result, the light shelf, which applies a folding technology and photovoltaic modules. As a result, folding the light shelf made the reﬂector angle symmetrical with the photovoltaic module folding the light shelf made the reflector angle symmetrical with the photovoltaic module angle (see Figure 3). This principle enables the proposed system to outperform conventional angle (see Figure 3). This principle enables the proposed system to outperform conven- ﬂat light shelves in terms of daylighting and generation. Third, the proposed system folds tional flat light shelves in terms of daylighting and generation. Third, the proposed system and unfolds the light shelf by moving along a rail, unlike previous methods in which the folds and unfolds the light shelf by moving along a rail, unlike previous methods in which light shelf rotates around a rotating shaft. the light shelf rotates around a rotating shaft. Figure 3. The concept and principle of the light shelf that applies folding technology and photovol- Figure 3. The concept and principle of the light shelf that applies folding technology and photovoltaic taic modules: (a) Structure of proposed system, (b) Daylighting and generation by the proposed modules: (a) Structure of proposed system, (b) Daylighting and generation by the proposed system. system. 2.2. Environment for Performance Evaluation A full-scale testbed including an artiﬁcial climate chamber was built to evaluate the performance of the proposed light shelf that applies folding technology and photovoltaic modules, and the details are as follows. First, as shown in Figures 4 and 5, the dimensions of the internal space of the testbed were 4.9 m 6.6 m 2.5 m (W D H). The reﬂectance of the ﬂoor, wall, and ceiling was set to 25%, 46%, and 86%, respectively. The window used to install the light shelf measured 1.9 m 1.7 m (W H) and was made of 24 mm thick pair glass with an 80 percent transmittance. Second, eight illuminance sensors were installed to measure the change in illuminance of the indoor space caused by the light shelf. Because of the height of the work surface, they were placed 0.85 m from the ﬂoor. Third, four lights were installed in the testbed using the IES 4-point method [35]. These LED lights were capable of 8-level dimming control (excluding lights off). Fourth, the testbed had an artiﬁcial climate chamber installed adjacent to the outside of the window. An artiﬁcial solar irradiation apparatus was installed in the chamber that would stimulate the brightness and altitude of the sun by regulating the intensity and angle of the natural light. The performance evaluation Buildings 2022, 12, 81 6 of 18 was carried out in an artiﬁcial environment due to its advantages in implementing a consistent external environment. The Grade-A artiﬁcial solar irradiation apparatus also ensured measurement uniformity following ASTM E927-85, resulting in valid results across performance evaluations. Due to mechanical limitations, this apparatus could not simulate the sun’s azimuth. The temperature range of the artiﬁcial climate chamber was also adjustable in light of the ﬁndings of related works [25–27] that the generation efﬁciency of photovoltaic modules was signiﬁcantly affected by temperature. Fifth, the current study develops an energy monitoring system to more precisely estimate lighting energy Buildings 2022, 12, x FOR PEER REVIEW 7 of 19 consumption (see Figure 4 for more detail). Figure 4. The layout, cross-section, and equipment in the testbed for performance evaluation. Figure 4. The layout, cross-section, and equipment in the testbed for performance evaluation. Figure 5. Environment for performance evaluation: (a) Testbed, (b) Artificial Solar Light Radiation Apparatus, (c) Chamber thermostat, (d) Chamber temperature controller, (e) Light dimming con- troller, (f) Energy monitoring system. Buildings 2022, 12, x FOR PEER REVIEW 7 of 19 Buildings 2022, 12, 81 7 of 18 Figure 4. The layout, cross-section, and equipment in the testbed for performance evaluation. Figure 5. Environment for performance evaluation: (a) Testbed, (b) Artificial Solar Light Radiation Figure 5. Environment for performance evaluation: (a) Testbed, (b) Artiﬁcial Solar Light Radiation Apparatus, (c) Chamber thermostat, (d) Chamber temperature controller, (e) Light dimming con- Apparatus, (c) Chamber thermostat, (d) Chamber temperature controller, (e) Light dimming controller, troller, (f) Energy monitoring system. (f) Energy monitoring system. Buildings 2022, 12, x FOR PEER REVIEW 8 of 19 2.3. Methods of Performance Appraisal The performance appraisal was conducted to prove the effectiveness of the daylight- ing and generation performance of the light shelf that applies folding technology and 2.3. Methods of Performance Appraisal photovoltaic modules. The performance appraisal was conducted to prove the effectiveness of the daylight- First, as shown in Table 3, this study set up three scenarios based on whether or ing and generation performance of the light shelf that applies folding technology and pho- not photovoltaic modules were used and how they worked. Case 1 was a standard tovoltaic modules. light shelf that did not include a photovoltaic module. Case 2 was a light shelf with First, as shown in Table 3, this study set up three scenarios based on whether or not a photovoltaic module attached to the reﬂector, resulting in the photovoltaic module and photovoltaic modules were used and how they worked. Case 1 was a standard light shelf reﬂector that did no having the t include same angle. a photovoltaic However module , the. C ara ea se wher 2 was a e the ligh photovoltaic t shelf with a p module hotovoltaic was module attached to the reflector, resulting in the photovoltaic module and reflector having attached in Case 2 had the same size as the reﬂector where reﬂection occurs, considering the same angle. However, the area where the photovoltaic module was attached in Case the previous study ﬁndings [12], in which installing photovoltaic modules on the part of 2 had the same size as the reflector where reflection occurs, considering the previous study the light shelf reﬂector was found to provide advantages in terms of saving building energy findings [12], in which installing photovoltaic modules on the part of the light shelf reflec- by enabling daylighting and generation at the same time. As shown in Figure 1, a single tor was found to provide advantages in terms of saving building energy by enabling day- rotating shaft was used to change the angles of the light shelves in Cases 1 and 2. In Case 2, lighting and generation at the same time. As shown in Figure 1, a single rotating shaft was the light shelf angle increased from 70 to 30 in 10 increments while considering the used to change the angles of the light shelves in Cases 1 and 2. In Case 2, the light shelf photovoltaic module’s generation function. Case 3 was designed around a light shelf that angle increased from −70° to 30° in 10° increments while considering the photovoltaic employs folding technology as well as photovoltaic modules. As shown in Table 4, the light module’s generation function. Case 3 was designed around a light shelf that employs fold- shelf is folded in stages. Each stage of folding changed the light shelf width, reﬂector angle, ing technology as well as photovoltaic modules. As shown in Table 4, the light shelf is and photovoltaic module angle. The photovoltaic cells used in the photovoltaic module folded in stages. Each stage of folding changed the light shelf width, reflector angle, and are speciﬁed in Table 5. Finally, as shown in Figure 6, this study used a proﬁle to make the photovoltaic module angle. The photovoltaic cells used in the photovoltaic module are specified in Table 5. Finally, as shown in Figure 6, this study used a profile to make the light shelf for a performance evaluation. light shelf for a performance evaluation. Table 3. Case settings for performance evaluation. Table 3. Case settings for performance evaluation. Light Shelf Photovoltaic Module Light Shelf Photovoltaic Module Folding Application Operation Application Folding Technol- Operation Case Technology Light Shelf Angle Case Light Shelf Angle (# of Photovoltaic Cells Method Width Angle (# of Photovoltaic Cells ogy Application Method Width Angle Application Applied) Applied) 10 , 0−10°, 0° , 10 ,, 10°, 1 1 NotNot applied applied (0)(0) 20 , 3020°, 30° 70 , −70°, 60 ,−60°, Rotation Rotation by a by a Not applied Not applied 50 , −50°, 40 ,−40°, rotating rotating shaft shaft 2 2 30 , −30°, 20 ,−20°, Applied Applied (33 (33 *) *) 0.6 m 0.6 m 10 , 0−10°, 0° , 10 ,, 10°, 20 , 3020°, 30° Applied Operates Applied Operates 3 0 (ﬁxed) Applied (33 *) (divided into along a rail 3 0° (fixed) Applied (33*) (divided into 6 along a rail 6 modules) axis modules) axis * The efﬁciency * The effici decreases ency decreases at at rate of 6.1% rate of 6.1% when the Photovoltaic when the Photovolt module a was ic m applied odule wa using s applie 33 photovoltaic d using 33 pho- cells. tovoltaic cells. Table 4. Folding shape, light shelf angle, and photovoltaic module angle according to the width of Case 3. Light Shelf Reflector Module Photovoltaic Mod- Folding Stage Width (W) Angle (α) ule Angle (β) 1 (Straight, no folding) 0.60 m 0° 180° 2 0.58 m 14.8° 165.2° 3 0.56 m 21.0° 159° 4 0.54 m 25.8° 154.2° 5 0.52 m 29.9° 150.1° Buildings 2022, 12, 81 8 of 18 Table 4. Folding shape, light shelf angle, and photovoltaic module angle according to the width of Case 3. Light Shelf Reﬂector Module Photovoltaic Folding Stage Width (W) Angle () Module Angle ( ) 1 (Straight, no folding) 0.60 m 0 180 Buildings 2022, 12, x FOR PEER REVIEW 9 of 19 2 0.58 m 14.8 165.2 3 0.56 m 21.0 159 Buildings 2022, 12, x FOR PEER REVIEW 9 of 19 4 0.54 m 25.8 154.2 6 0.50 m 33.6° 146.4° 5 0.52 m 29.9 150.1 6 6 0.50 m 0.50 m 33.6° 33.6 146.4° 146.4 Table 5. Photovoltaic cell specifications. Table 5. Photovoltaic cell specifications. Table 5. Photovoltaic cell speciﬁcations. Item Specifications Item Specifications Item Specifications Item Specifications Item Speciﬁcations Item Speciﬁcations Max. Power 2 W Max. Current (Impp) 670 mA Max. Power 2 W Max. Current (Impp) 670 mA Max. Power 2 W Max. Current (Impp) 670 mA Max. Voltage (Vmpp) 3 V Size 165 mm × 100 mm Max. Voltage (Vmpp) 3 V Size 165 mm × 100 mm Max. Voltage (Vmpp) 3 V Size 165 mm 100 mm Efficiency Efficiency 16.3% 16.3% Reflectance Reflectance 1–6% 1–6% Efﬁciency 16.3% Reﬂectance 1–6% Figure 6. Light shelf fabrication for performance evaluation. Figure 6. Light shelf fabrication for performance evaluation. Figure 6. Light shelf fabrication for performance evaluation. Secondly, monitored the distribution of indoor illuminance according to the cases set for performance evaluation to derive the minimum illuminance, average illuminance, and Secondly, monitored the distribution of indoor illuminance according to the cases Secondly, monitored the distribution of indoor illuminance according to the cases set uniformity ratio. The uniformity ratio was the ratio of the minimum illuminance to the set for performance evaluation to derive the minimum illuminance, average illuminance, for performance evaluation to derive the minimum illuminance, average illuminance, and average. and uniformity ratio. The uniformity ratio was the ratio of the minimum illuminance to uniformity ratio. The uniformity ratio was the ratio of the minimum illuminance to the the average. Thirdly, the study determined the dimming level and lighting energy consumption average. for eThir ach ca dlyse , the to ach study ieve o determined ptimal in the doo dimming r illuminance level and , and the lighting deener tails are as fol gy consumption lows. As for Thirdly, the study determined the dimming level and lighting energy consumption shown in Figure 7, dimming control was only used when the minimum value measured each case to achieve optimal indoor illuminance, and the details are as follows. As shown for each case to achieve optimal indoor illuminance, and the details are as follows. As in by the Figur eie gh 7t , il dimming luminance contr sensors w ol wasas only less used than 5when 00 lx. If the the m minimum inimum val value uemeasur measured ed by by shown in Figure 7, dimming control was only used when the minimum value measured the the il eight lumina illuminance nce sensors wa sensors s grwas eater less than 500 l than 500 x, all light lx. If the s we minimum re turned of value f witmeasur hout dim- ed by the illuminance sensors was greater than 500 lx, all lights were turned off without by the ming con eight trol. The system illuminance sensors w monitore asd th lese va s than lues measured by th 500 lx. If the minimu e illum m val inance ue mea sensors sured by dimming control. The system monitored the values measured by the illuminance sensors while increasing the dimming levels sequentially from the light closest to the illuminance the illuminance sensors was greater than 500 lx, all lights were turned off without dim- while increasing the dimming levels sequentially from the light closest to the illuminance sensor with the minimum value. During this process, dimming control ended when all ming control. The system monitored the values measured by the illuminance sensors sensor with the minimum value. During this process, dimming control ended when all measurements by the illuminance sensors reached 500 lx. Finally, the performance of each while increasing the dimming levels sequentially from the light closest to the illuminance measurements by the illuminance sensors reached 500 lx. Finally, the performance of each case was compared by calculating the lighting energy consumption based on the level of sensor with the minimum value. During this process, dimming control ended when all dimming control. measurements by the illuminance sensors reached 500 lx. Finally, the performance of each case was compared by calculating the lighting energy consumption based on the level of dimming control. Buildings 2022, 12, 81 9 of 18 Buildings 2022, 12, x FOR PEER REVIEW 10 of 19 Buildings 2022, 12, x FOR PEER REVIEW 10 of 19 case was compared by calculating the lighting energy consumption based on the level of dimming control. Figure 7. Lighting dimming control flow chart for performance evaluation. Figure 7. Lighting dimming control ﬂow chart for performance evaluation. Figure 7. Lighting dimming control flow chart for performance evaluation. Four Fourth, th, the e the n ener ergy prod gy produced uced by bythe ph the photovol otovoltaic mo taic module’s dule’s generation perform generation performance ance Fourth, the energy produced by the photovoltaic module’s generation performance was was mea measur sureed d in in this this s study tudy. The . The photo photovoltaic voltaic modu module’s le’s ener energy gy was wascalculated calculated by by mu multiply- lti- was measured in this study. The photovoltaic module’s energy was calculated by multi- ing the module’s maximum voltage (Vmp) and maximum current (Imp) while producing plying the module’s maximum voltage (Vmp) and maximum current (Imp) while produc- plying the module’s maximum voltage (Vmp) and maximum current (Imp) while produc- power. Table 6 shows the speciﬁcations of the photovoltaic module used for performance ing power. Table 6 shows the specifications of the photovoltaic module used for perfor- ing power. Table 6 shows the specifications of the photovoltaic module used for perfor- evaluation and the equipment used to measure the voltage and current. mance evaluation and the equipment used to measure the voltage and current. mance evaluation and the equipment used to measure the voltage and current. Table 6 Table . 6. SpecSpeciﬁcations ifications of the ofvoltag the e volt and age current m and curr easu ent ring measuring device (Equ device ipment nam (Equipment e: MULL name: ER Table 6. Specifications of the voltage and current measuring device (Equipment name: MULLER 3201). MULLER 3201). 3201). Item Item Spec Speciﬁcationsifications ImageImage Item Specifications Image Measurement item DC Voltage (0~600 V), Measurement item (measurement DC Voltage (0 V ~ 600 V), DC Current (0 A Measurement item (measurement DC Voltage (0 V ~ 600 V), DC Current (0 A (measurement capacity) DC Current (0~60 A) capacity) ~ 60 A) capacity) ~ 60 A) Error rate (0.5% + 3) Error rate ±(0.5% + 3) Error rate ±(0.5% + 3) Fifth, the artiﬁcial climate chamber of the testbed created the external environment Fifth, the artificial climate chamber of the testbed created the external environment Fifth, the artificial climate chamber of the testbed created the external environment of the outdoor space, where the performance evaluation was conducted for summer, mid- of the outdoor space, where the performance evaluation was conducted for summer, mid- of the outdoor space, where the performance evaluation was conducted for summer, mid- season, and winter, as shown in Table 7. The experiment was performed under three season, and winter, as shown in Table 7. The experiment was performed under three ex- season, and winter, as shown in Table 7. The experiment was performed under three ex- external conditions based on seasonal variation (i.e., summer, middle season, and winter). ternal conditions based on seasonal variation (i.e., summer, middle season, and winter). ternal conditions based on seasonal variation (i.e., summer, middle season, and winter). More speciﬁcally, each condition was controlled hour by hour to reﬂect potential change More specifically, each condition was controlled hour by hour to reflect potential change More specifically, each condition was controlled hour by hour to reflect potential change in external illuminance and solar radiation during a 5 h session between 10 am and in external illuminance and solar radiation during a 5 h session between 10 am and 3 pm. in external illuminance and solar radiation during a 5 h session between 10 am and 3 pm. 3 pm. The external environment’s characteristics were speciﬁcally based on Seoul, Korea, The external environment’s characteristics were specifically based on Seoul, Korea, which The external environment’s characteristics were specifically based on Seoul, Korea, which which has four distinct seasons. The outdoor temperature for each season was determined has four distinct seasons. The outdoor temperature for each season was determined by has four distinct seasons. The outdoor temperature for each season was determined by by considering the Korea Meteorological Administration’s average climate data for the past considering the Korea Meteorological Administration’s average climate data for the past considering the Korea Meteorological Administration’s average climate data for the past thirty years [38]. However, the solar irradiation for each season was determined by thirty years [38]. However, the solar irradiation for each season was determined by vary- ing the intensity of the artificial solar irradiation apparatus rather than by observing actual Buildings 2022, 12, 81 10 of 18 thirty years [38]. However, the solar irradiation for each season was determined by varying the intensity of the artiﬁcial solar irradiation apparatus rather than by observing actual climate data. This limitation was due to the performance evaluation being conducted in an artiﬁcial environment. Table 7. Climatic settings for performance evaluation based on geographical speciﬁcation. Outdoor Meridian External Illuminance (lx)/Solar Radiation (W/m ) Season Temperature Altitude 10:00–11:00 11:00–12:00 12:00–13:00 13:00–14:00 14:00–15:00 Summer 76.5 70,000/530 80,000/638 80,000/638 80,000/638 70,000/530 27.1 C Middle 52.5 50,000/414 50,000/414 60,000/476 60,000/476 50,000/414 17.2 C season Winter 29.5 20,000/289 30,000/332 30,000/332 30,000/332 20,000/289 3.2 C Sixth, the optimal speciﬁcations (optimal angle and folding stage) were derived for each case. These were derived by considering lighting energy saving as a priority. When multiple speciﬁcations saved the same amount of energy, the one with the highest unifor- mity was deemed to be optimal. Conditions that would result in the glare as a result of introducing natural light directly into the room via the light shelf without bouncing it off the ceiling, on the other hand, were excluded from the optimal speciﬁcations. 3. Results and Discussion 3.1. Performance Evaluation Results This study conducted a performance evaluation to validate the effectiveness of the light shelf that applies folding technology and photovoltaic modules. The results are as follows. Firstly, Figure 8 illustrates the performance evaluation results of Case 1 (light shelf with no photovoltaic module), which shows that light shelf angle affects the daylighting performance. Increasing the light shelf angle during the summer was beneﬁcial in saving lighting energy and improving the indoor uniformity ratio. Increasing the light shelf angle was also helpful during the middle season, but the uniformity ratio deteriorated when the light shelf angle was 30 . As shown in Figure 9, setting the angle at 30 allows high illuminance light to reach a speciﬁc area only by reﬂecting off the light shelf, resulting in an illuminance imbalance in the indoor space. In winter, the increment in the light shelf angle was suitable for saving lighting energy by increasing the amount of natural light entering the room through light shelf reﬂection, but adjusting the light shelf angle to 30 was inappropriate for saving lighting energy and improving the indoor uniformity ratio. This is because the solar altitude is lower in the summer compared to the winter and middle seasons, allowing natural light to enter deep into the indoor space through the daylighting window. Furthermore, during the winter, the solar altitude is 27.5 , so when the light shelf angle is 30 , the light shelf only acts as a shade, as shown in Figure 9. A light shelf angle of 20 was also excluded from the optimal speciﬁcations during the winter because, like using a 30 angle during the middle season, it could reduce the uniformity ratio and cause glare. As a result, the optimal light shelf angles for Case 1 during the summer, mid-season, and winter were 30 , 20 , and 10 , respectively, with lighting energy consumption of 0.471 kWh, 0.309 kWh, and 0.134 kWh. Buildings 2022, 12, x FOR PEER REVIEW 12 of 19 Buildings 2022, 12, 81 11 of 18 Buildings 2022, 12, x FOR PEER REVIEW 12 of 19 Figure 8. Indoor uniformity and lighting energy consumption according to the light shelf angle in Figure 8. Indoor uniformity and lighting energy consumption according to the light shelf angle in Figure 8. Indoor uniformity and lighting energy consumption according to the light shelf angle in Case 1: (a) Summer, (b) Middle season, (c) Winter. Case 1: (a) Summer, (b) Middle season, (c) Winter. Case 1: (a) Summer, (b) Middle season, (c) Winter. Figure 9. The inflow of natural light according to the light shelf angle in Case 1: (a) Middle season, Angle 30°, (b) Winter, Angle 20°, (c) Winter, Angle 30°. Figure 9. The inflow of natural light according to the light shelf angle in Case 1: (a) Middle season, Figure 9. The inﬂow of natural light according to the light shelf angle in Case 1: (a) Middle season, Angle 30°, ( Seco bn ) Winter, dly, FiguAngle re 10 s 20°, how (s t c) Winter he outp , Angle 30° ut of a per . formance evaluation of Case 2 (light Angle 30 , (b) Winter, Angle 20 , (c) Winter, Angle 30 . shelf applying photovoltaic module). In terms of saving lighting energy, the optimal spec- ifications during summer, mid-season, and winter were 30°, 20°, and 10°, respectively, the Secondly, Figure 10 shows the output of a performance evaluation of Case 2 (light Secondly, Figure 10 shows the output of a performance evaluation of Case 2 (light shelf applying photovoltaic module). In terms of saving lighting energy, the optimal spec- shelf applying photovoltaic module). In terms of saving lighting energy, the optimal ifications during summer, mid-season, and winter were 30°, 20°, and 10°, respectively, the speciﬁcations during summer, mid-season, and winter were 30 , 20 , and 10 , respectively, the same as Case 1. However, in Case 2, the area of the reﬂector used for daylighting was reduced by 50% compared to Case 1, reducing the amount of natural light entering the room through light shelf reﬂection and deteriorating uniformity, as shown in Figure 11. Case 2 Buildings 2022, 12, x FOR PEER REVIEW 13 of 19 Buildings 2022, 12, 81 12 of 18 same as Case 1. However, in Case 2, the area of the reflector used for daylighting was reduced by 50% compared to Case 1, reducing the amount of natural light entering the room through light shelf reflection and deteriorating uniformity, as shown in Figure 11. also has a higher lighting energy consumption than Case 1. Meanwhile, the photovoltaic Case 2 also has a higher lighting energy consumption than Case 1. Meanwhile, the photo- module in Case 2 generated the most power at light shelf angles of 10 , 40 , and 60 , voltaic module in Case 2 generated the most power at light shelf angles of −10°, −40°, and which proves that the closer the incident angle of natural light is to vertical, the higher −60°, which proves that the closer the incident angle of natural light is to vertical, the the power generation efﬁciency. However, it is difﬁcult to maximize both the daylighting higher the power generation efficiency. However, it is difficult to maximize both the day- and generation performance at the same time in Case 2 because it controls the reﬂector lighting and generation performance at the same time in Case 2 because it controls the for daylighting and the photovoltaic module for concentrating light at the same angle. reflector for daylighting and the photovoltaic module for concentrating light at the same Therefore, the optimal speciﬁcations for Case 2 during summer, mid-season, and winter angle. Therefore, the optimal specifications for Case 2 during summer, mid-season, and were 10 , 10 , and 20 , respectively, and the lighting energy consumption was 0.406 kWh, winter were 10°, −10°, and 20°, respectively, and the lighting energy consumption was 0.314 kWh, and 0.100 kWh, respectively. 0.406 kWh, 0.314 kWh, and 0.100 kWh, respectively. Figure 10. Figure 10. Lighti Lighting ng energy c energy ons consumption umption and the power ge and the power nerated by the p generated by th hotovoltai e photovoltaic c module ac- module cording to the light shelf angle in Case 2: (a) Summer, (b) Mid-season, (c) Winter. according to the light shelf angle in Case 2: (a) Summer, (b) Mid-season, (c) Winter. Buildings 2022, 12, 81 13 of 18 Buildings 2022, 12, x FOR PEER REVIEW 14 of 19 Figure 11. Case 1 and Case 2 indoor uniformity analysis: (a) Summer, (b) Mid-season, (c) Winter. Figure 11. Case 1 and Case 2 indoor uniformity analysis: (a) Summer, (b) Mid-season, (c) Winter. Thirdly, Thirdly, Figur Figure 12 shows e 12 shows the the r re esults sults of of a a performance performance apprais appraisal al o of f Case Case 3 (ligh 3 (light t shelf shelf applying folding technology and photovoltaic module). The optimal speciﬁcations for applying folding technology and photovoltaic module). The optimal specifications for Case Case 3, con 3, considering sidering on only ly lighting lighting ener energ gy y sa savings vings an and d improving improving indoor l indoor light ight unif uniformity ormity during summer, mid-season, and winter were folding stages 6, 4, and 3(4), respectively. during summer, mid-season, and winter were folding stages 6, 4, and 3(4), respectively. During the winter, however, as shown in Table 4, folding stages 3 and 4 reduce the light During the winter, however, as shown in Table 4, folding stages 3 and 4 reduce the light shelf angle to 21 and 25.8 , respectively. These angles, like a light shelf angle of 20 in shelf angle to 21° and 25.8°, respectively. These angles, like a light shelf angle of 20° in the the winter, cause glare by allowing the direct ﬂow of natural light into the interior space winter, cause glare by allowing the direct flow of natural light into the interior space by reflecting off the light shelf, so they were excluded from the optimal specifications. Taking Buildings 2022, 12, 81 14 of 18 Buildings 2022, 12, x FOR PEER REVIEW 15 of 19 by reﬂecting off the light shelf, so they were excluded from the optimal speciﬁcations. Taking these factors into account, the best speciﬁcations for saving lighting energy and these factors into account, the best specifications for saving lighting energy and improving improving indoor uniformity in Case 3 were folding stages 6, 4, and 2 for summer, mid- indoor uniformity in Case 3 were folding stages 6, 4, and 2 for summer, mid-season, and season, and winter, respectively. The optimal speciﬁcations for generating power by the winter, respectively. The optimal specifications for generating power by the photovoltaic photovoltaic module in Case 3 were folding stages 3, 6, and 6 for summer, mid-season, module in Case 3 were folding stages 3, 6, and 6 for summer, mid-season, and winter, and winter, respectively. Therefore, the optimal speciﬁcations for Case 3 during summer, respectively. Therefore, the optimal specifications for Case 3 during summer, mid-season, mid-season, and winter w and ere fol winter dingwer stage e folding s 6, 4, and stages 4, resp 6, e 4, ctand ively 4, , and respectively the lighting , and enthe ergy lighting consum ener p- gy consumption tion was 0.307 was kWh, 0. 0.307 22kWh, 4 kWh, and 0.224 kWh, 0.034 kWh and ,0.034 respectiv kWh, ely. respectively . Figure 12. Lighting energy consumption and the power generated by the photovoltaic module ac- Figure 12. Lighting energy consumption and the power generated by the photovoltaic module cording to the folding stage in Case 3: (a) Summer, (b) Mid-season, (c) Winter. according to the folding stage in Case 3: (a) Summer, (b) Mid-season, (c) Winter. Buildings 2022, 12, x FOR PEER REVIEW 16 of 19 Buildings 2022, 12, 81 15 of 18 3.2. Performance Evaluation Discussion 3.2. Performance Evaluation Discussion This study proposed a folding technology to improve light shelves’ daylighting and generation efficiency that incorporates photovoltaic modules and validated its effective- This study proposed a folding technology to improve light shelves’ daylighting and ness through a performance evaluation. A discussion of the results follows. generation efﬁciency that incorporates photovoltaic modules and validated its effectiveness First, the optimal specifications for Cases 1, 2, and 3 were derived through evaluating through a performance evaluation. A discussion of the results follows. their performance. Figure 13 shows the energy consumption based on these results. Case First, the optimal speciﬁcations for Cases 1, 2, and 3 were derived through evaluating 2 red their u performance. ced energy consumption by 10.3% Figure 13 shows the ener compared gy consumption to Case 1based , demonstra on these ting the results. effe Case ctive- 2 ness o reduced f the photovoltaic m energy consumption odule by 10.3% used o compar n light ed shelve to Case s. Case 1, demonstrating 3 reduced ene the rgy effectiveness consump- of the photovoltaic module used on light shelves. Case 3 reduced energy consumption by tion by 31.3% compared to Case 2, due to improved daylighting and generation efficiency achieve 31.3% compar d by adju ed to sting t Caseh 2, e refl dueect to o impr r and oved phot daylighting ovoltaic module and generation angles th ef rough ﬁciency foachieved lding. In by adjusting the reﬂector and photovoltaic module angles through folding. In particular, particular, although the proposed light shelf that applies folding technology and photo- although the proposed light shelf that applies folding technology and photovoltaic modules voltaic modules (Case 3) had an operating range of only 0.1 m, it reduced building energy (Case 3) had an operating range of only 0.1 m, it reduced building energy by a signiﬁcant by a significant amount compared to the conventional light shelf. These results prove the amount compared to the conventional light shelf. These results prove the effectiveness of effectiveness of the proposed system (Case 3). the proposed system (Case 3). Figure 13. Energy consumption analysis (by Case). Figure 13. Energy consumption analysis (by Case). Second, Case 3 uses folding technology to cope with external wind pressure and snow Second, Case 3 uses folding technology to cope with external wind pressure and load by completely folding the light shelf. For example, if you are concerned about the snow load by completely folding the light shelf. For example, if you are concerned about damage caused by wind pressure exceeding a certain level, you can fold the light shelf the damage caused by wind pressure exceeding a certain level, you can fold the light shelf to avoid any damage caused by protruding outside. Case 3, in particular, connects each to avoid any damage caused by protruding outside. Case 3, in particular, connects each module with a hinge structure, resulting in gaps between each module. Compared to module with a hinge structure, resulting in gaps between each module. Compared to con- conventional movable light shelves, this structural feature responds quickly to external ventional movable light shelves, this structural feature responds quickly to external envi- environmental factors such as wind pressure. ronmental factors such as wind pressure. Third, installing photovoltaic modules on light shelves reduces the area occupied by Third, installing photovoltaic modules on light shelves reduces the area occupied by reﬂectors to perform daylighting, which reduces the amount of natural light ﬂowing indoors reflectors to perform daylighting, which reduces the amount of natural light flowing in- through light shelf reﬂection. In this context, the light shelf with a photovoltaic module doors through light shelf reflection. In this context, the light shelf with a photovoltaic may cause issues, such as increasing the amount of lighting energy required to maintain module may cause issues, such as increasing the amount of lighting energy required to optimal indoor illuminance and reducing indoor uniformity. Therefore, further research maintain optimal indoor illuminance and reducing indoor uniformity. Therefore, further should be conducted on adjusting the width of the light shelf according to the area occupied research should be conducted on adjusting the width of the light shelf according to the by the photovoltaic module in the light shelf. area occupied by the photovoltaic module in the light shelf. 4. Conclusions 4. Conclusions A folding technology was proposed to improve the daylighting and generation perfor- A folding technology was proposed to improve the daylighting and generation per- mance of light shelves that apply photovoltaic modules, and its performance was evaluated formance of light shelves that apply photovoltaic modules, and its performance was eval- through a full-scale testbed. The main ﬁndings are as follows. uated through a full-scale testbed. The main findings are as follows. First, the proposed light shelf that employs folding technology and photovoltaic mod- First, the proposed light shelf that employs folding technology and photovoltaic ules has a structure in which reﬂectors and photovoltaic modules are installed alternately modules has a structure in which reflectors and photovoltaic modules are installed alter- by modularizing the light shelf. A hinge structure connects each module, allowing the nately by modularizing the light shelf. A hinge structure connects each module, allowing system to be folded. This structure enables the reﬂector module and photovoltaic module to be symmetrical and operate at different angles depending on the degree of folding. Due to Buildings 2022, 12, 81 16 of 18 such structural features, the proposed light shelf that applies folding technology and pho- tovoltaic modules can improve both daylighting and generation efﬁciency. This system also employs a novel operation method based on rails instead of conventional light shelves, which control the light shelf angle via a rotating shaft. Second, the optimal light shelf angle for each case was derived. The optimal light shelf angles were based on the lighting energy consumption and uniformity ratio to maintain the optimal indoor illuminance. Angles that may cause glare were excluded, even if they were excellent in terms of saving energy. The optimal angles for a light shelf without a photovoltaic module during the summer, mid-season, and winter were 30 , 20 , and 10 , respectively, indicating that the angles must be controlled by operating or moving the light shelf to improve performance. In contrast, the optimal angles for a light shelf with a photovoltaic module during the summer, mid-season, and winter were 10 , 10 , and 20 , respectively, compared to a light shelf without a photovoltaic module. The photovoltaic module and light reﬂector module require different angles to increase power generation efﬁciency and daylighting efﬁciency. Third, the light shelf that applies folding technology and photovoltaic modules can reduce energy consumption by 38.2% and 31.3%, respectively, compared to light shelves with no photovoltaic modules and light shelves with photovoltaic modules but no folding technology. These results validate the effectiveness of the application of photovoltaic mod- ules to light shelves and prove that the folding technology can improve both daylighting and generation efﬁciency. Fourth, the light shelf that applies a photovoltaic module was unsuitable in terms of improving the indoor uniformity compared to the light shelf with no photovoltaic module because the area occupied by the reﬂector decreases due to installing the photovoltaic module, which leads to reducing the amount of natural light ﬂowing into the room through the light shelf. This aspect should be considered when designing light shelves that apply photovoltaic modules. This study is signiﬁcant because it proposes and validates a new technology related to light shelves to save building energy, a primary current concern. However, due to the use of a testbed, the performance evaluation was carried out in a restricted external environment. Other limitations include the fact that various other light shelf variables, such as width and height, were not considered. As a result, additional in-depth studies should be conducted in further research in this sector. Author Contributions: Methodology, H.L.; writing—original draft preparation, H.L. and S.H.; visualization, S.H.; writing—review and editing, H.L. and J.S.; supervision, J.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF-2020R1C1C1004704) (NRF-2021R1A2B5B02001469). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: No additional data available. Acknowledgments: None stated. Conﬂicts of Interest: The authors declare no conﬂict of interest. References 1. Heng, C.Y.S.; Lim, Y.W.; Ossen, D.R. Horizontal light pipe transporter for deep plan high-rise ofﬁce daylighting in tropical climate. Build. Environ. 2020, 171, 106645. [CrossRef] 2. Jenkins, D.; Muneer, T. Modelling light-pipe performances—A natural daylighting solution. Build. Environ. 2003, 38, 965–972. [CrossRef] 3. Lee, H.; Jang, H.; Seo, J. A preliminary study on the performance of an awning system with a built-in light shelf. Build. Environ. 2018, 131, 255–263. [CrossRef] Buildings 2022, 12, 81 17 of 18 4. 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Buildings – Multidisciplinary Digital Publishing Institute

Published: Jan 15, 2022

Keywords: light shelf; photovoltaic module; folding technology; performance evaluation; energy efficiency

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Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efficiency in Building

Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efficiency in Building

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Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efficiency in Building

Light Shelf Development Using Folding Technology and Photovoltaic Modules to Increase Energy Efficiency in Building

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