Optimal Sizing of Solar-Assisted Heat Pump Systems for Residential Buildings

Alessandro Franco; Fabio Fantozzi

doi:10.3390/buildings10100175

Franco, Alessandro;Fantozzi, Fabio

2020-10-04 00:00:00

buildings Article Optimal Sizing of Solar-Assisted Heat Pump Systems for Residential Buildings Alessandro Franco * and Fabio Fantozzi Department of Energy, Systems, Territory and Constructions Engineering (DESTEC), University of Pisa, Largo Lucio Lazzarino, 56122 Pisa, Italy; fabio.fantozzi@unipi.it * Correspondence: alessandro.franco@ing.unipi.it Received: 1 September 2020; Accepted: 27 September 2020; Published: 4 October 2020 Abstract: This paper analyzes the optimal sizing of a particular solution for renewable energy residential building integration. The solution combines a photovoltaic (PV) plant with a heat pump (HP). The idea is to develop a system that permits the maximum level of self-consumption of renewable energy generated by using a small-scale solar array installed on the same building. The problem is analyzed using data obtained from an experimental system installed in a building in Pisa, Italy. The residential house was equipped with a PV plant (about 3.7 kW of peak power), assisting a HP of similar electrical power (3.8 kW). The system was monitored for eight years of continuous operation. With reference to the data acquired from the long-term experimental analysis and considering a more general perspective, we discuss criteria and guidelines for the design of such a system. We focus on the possibility of exporting energy to the electrical grid, from the perspective of obtaining self-consumption schemes. Considering that one of the problems with small-scale PV plants is represented by the bidirectional energy ﬂows from and to the grid, possible alternative solutions for the design are outlined, with both a size reduction in the plant and utilization of a storage system considered. Dierent design objectives are considered in the analysis. Keywords: renewable energy systems; smart grid; photovoltaic plants; ground heat pumps; experimental analysis; optimization 1. Introduction During the last decade, the development of intermittent renewable energy systems (RES)—mainly photovoltaic (PV) systems—and increasingly decentralized production have been observed in many countries. Accordingly, the number of small to medium scale plants has increased, and they are often installed in residential buildings [1]. In principle, this is considered to be a good result. However, in complex energy systems, maintaining the balance between energy production and energy demand when large ﬂuctuations in renewable energy occur is a dicult task. Therefore, increasing the penetration of RES needs to be combined with the development of strategies aimed at increasing direct utilization of the energy produced, in order to achieve eective energy savings for the whole system. Without this, the signiﬁcant eort expended to promote these energy systems will not lead to real reductions in the dependence on fossil fuel and carbon dioxide emissions. Several studies have shown that the penetration of RES, in particular PV plants, seemed to be limited to an upper level of technical considerations. A further increase in the level could be possible by integrating various energy uses (thermal, mobility, and electrical), which could be obtained mainly at the local level, for example, in civil and residential sectors. This requires an important shift from thermal energy production to the ﬁeld of electricity. From this perspective, for example, a heat pump (HP) for hot and cold temperature production and electric Buildings 2020, 10, 175; doi:10.3390/buildings10100175 www.mdpi.com/journal/buildings Buildings 2020, 10, 175 2 of 18 mobility can directly and indirectly play a relevant role in the shift from fossil fuel use to renewable source supply, as the necessary electricity for a HP in an electric vehicle can be produced with RES [2–4]. Franco et al. [5] analyzed the problem in connection with the Italian energy system, which was characterized by an important modiﬁcation to the structure from 2005 to 2020 as a consequence of an increase in the number of PV plants, other renewable energies, and the number of energy producers. In a complex energy system, one method to increase the system’s capability of introducing an increased share of intermittent RES, such as PV plants, depends on the possibility of increased electricity demand and the promotion of self-consumption schemes. For some years, at small power-generating facilities, with all the electricity produced from RES, a form of priority dispatch has been granted via a speciﬁc priority order. However, in the near future, this may not be maintained, and in any case, it must be compatible with participation in the electricity market. The simultaneous utilization of HP and PV modules yields a positive synergy, with respect to a shift from using fuel-based systems (such as natural gas in Italy) to electricity produced by means of renewable energies, as well as a consistent reduction in local pollution and carbon dioxide (CO ) emissions. This would guarantee the economical sustainability of investments in renewable energy sources without the need for subsidiary mechanisms [6]. The system involves the favorable integration of PV modules with a HP, and thereby generates heat and electricity in self-consumption schemes, with a reasonably high overall eciency [7]. Regarding the typology of heat pumps, both air source and ground source heat pumps should be considered. One of the main limitations of air source heat pumps (ASHP) is the fact that the thermal power delivery curve is opposite to the environmental conditions, and the maximum PV electricity production occurs during daytime hours with higher ambient temperatures, while there is no production during evening hours when the maximum thermal power demand occurs. The use of more stable seasonal temperature ground source heat pumps (GHP) oers, in principle, considerable opportunities for reducing global energy consumption, due to their potentially higher eciency. The eciency of a HP is represented by the coecient of performance (COP), which can move from values of 3–4 to even higher values, even though the installation is more complicated and signiﬁcantly increases the investment cost [8,9]. However, the shift from PV production to a GHP energy use proﬁle still remains, occurring mainly during the cold season. In all cases, the promotion of integrated solutions of solar-assisted heat pumps (both the air type and the geothermal type) and a PV plant to support a system producing electricity to supply the HP can, in principle, add “ﬂexibility” to the system. In this case, a relevant problem is represented by the bidirectional energy ﬂows of the external power grid, due to the electricity exported to the grid from a renewable-based system and the electricity imported from the grid depending on the electricity demand of a building. For public buildings, this is a minor problem because the peak energy consumption occurs during the daytime when peak energy is typically generated by the PV plant. It is a more important issue in the case of residential buildings, as discussed in [10]. If a building is equipped with photovoltaic (PV) modules, the use of electric storage could be particularly important to mitigate the eects of the time mismatch between the electrical production peak, which occurs between 11:00 and 15:00, and the thermal power demand peak, which occurs in the early morning or the late evening. In principle, a further increase in the number of PV plant installations is expected in the near future. Notably, during summertime, heat pumps typically contribute to an increase in the ﬂexibility of the system, as they can consume electricity during hours of excess production, while they can also be eective during the winter. For this reason, it is important to consider the possible connection between the production of electricity and use of thermal energy, as obtained in the case of integrated PV and HP systems for building services [11,12]. Over recent years, several studies have proposed designs for solar-assisted HPs that have been tested using modelling. This has especially been the case for smart energy systems, which are considered an important element of the development of net zero-energy building (nZEB) systems— systems able to produce all of the energy required by a building using renewable energies. Buildings 2020, 10, 175 3 of 18 A recently published review article, [13], has been published in recent years on the topic, considering the dierent system boundaries and the main performance indicators used for assessing energetic and economic optimization, including economic assessment of solar photovoltaic and heat pump systems. The possibility of experimentally monitoring the behavior of dierent types of building–plant systems that cover the most widespread typologies, such as residential buildings, or that are considered “strategic” by the various energy eciency programs, such as oce buildings, supermarkets and educational buildings, is of fundamental importance to evaluate heat pump (HP) system performance in real conditions. Depending on the building typology and envelope characteristics, lay-out and use, it is important to properly select the size of each single component. The development of design methods for these systems could be particularly relevant for engineers and designers. The sizing of PV systems was considered in some recent papers [14,15], and it is usually based on various technical and economic criteria. The problem of energy ﬂows from and to the grid was only considered occasionally and no boundary conditions were considered for energy ﬂows to the grid. In this paper, we analyze data obtained from a real case in which a PV plant was used in a residential building to produce electricity for the operation of an HP, which provides heating and cooling, and for the miscellaneous energy loads of the house. Moving from the analysis of the data acquired during the long-term experimental analysis of the system, we then focus our analysis on the speciﬁc problem of energy ﬂows to and from the power grid. In the second part of the paper, considering that in the near future plants will be sized to maximize the self-consumption capacity of a system, we propose guidelines for an optimum design strategy of the system under analysis to increase the share of energy produced by the PV plant and directly used in the building. The objective is to deﬁne the size of a PV plant necessary to maximize the direct use of the energy produced and minimize energy ﬂows. The possibility of using a “small size” storage system is also considered. Starting from the analysis of the particular system considered here, some general guidelines for the design of such systems are discussed. The data analyzed in this paper can also provide insights both on the operation of the PV system and the operation of the GHP. General criteria and guidelines for the optimum design of PV plants for buildings, providing the use of HP for heating and cooling service are considered in the ﬁnal part. While in general the possibility of using the electrical grid as a “buer” for the energy produced is deemed optimal, we attempt to reconsider the perspective of obtaining eective self-consumption conﬁgurations. In this case, the electrical grid can be used for energy import and not for exporting the excess energy. This will support further increases in RES while avoiding state interventions, which are often designed in an uncoordinated manner and have led to increasing distortions of the wholesale electricity market, with negative consequences for investors. 2. Integrated PV-HP System for Residential Building: A General Description One of the methods used to transform a complex energy system and increase the amount of energy produced with intermittent RES, such as PV plants, is to increase the electricity demand and promote the in situ strategies for the direct utilization of the energy produced, minimizing the amount of energy ﬂowing from and to the grid. An interesting solution is the use of solar-assisted heat pumps for heating and cooling purposes in residential buildings. The Energy Performance of Buildings Directive (EPBD) is the main legislative instrument at the European Union level for improving the energy eciency of buildings. A fundamental element of this directive is represented by the requirements relating to zero-energy buildings (ZEB) or net zero-energy buildings (nZEB) and the related integration of renewable sources in the implementation of the relevant provisions. Buildings 2020, 10, 175 4 of 18 At the level of deﬁnition of nZEB buildings, the European Directive does not contain speciﬁc prescriptions in terms of the minimum absorbed energy, but provides a generic deﬁnition, that is, an nZEB is considered a building with very high energy performance, characterized by very low or almost zero energy needs, which should be covered to a very signiﬁcant extent by energy from renewable sources present on site. The concept of a nearly zero-energy building therefore contains the notion of a synergy of interventions in terms of energy production from renewable sources and energy eciency. Energy eciency refers to the amount of energy, calculated or measured, necessary to meet the energy needs associated with normal use of the building, including, in particular, the reduction in energy used for heating, cooling, ventilation, the production of domestic hot water and lighting. In this deﬁnition, important factors come into play: the type of heating and conditioning system, the use of energy from renewable sources, the passive elements of heating and cooling, the shading systems, the quality of indoor air, adequate natural lighting, and the architectural features of the building. Considering the concept of nearly zero-energy buildings, interesting theoretical studies have been proposed based on experimental data [16,17]. Some articles deal with the topic of nearly zero-energy buildings from the perspective of regulatory interpretation [18], while others provide a descriptive collection of exemplary cases [19–21]. One solution considered in various cases as fundamental to the concept of nZEBs is the solar-assisted heat pump, consisting of a HP system assisted by a PV plant installed on the building. This solution enables the direct use of excess energy produced to operate the HP. During the last decade, a number of studies have investigated the design, modelling and testing of solar-assisted HP systems [22,23]. A recent review on the topic presents problems and perspectives of this solution [24]. The design criteria dier among papers and are often based on economic criteria. In general, the technical solutions proposed are not optimized from the point of view of the “energy system”. In particular, the size of the PV plant is not always correctly deﬁned in this perspective. This problem is a consequence of the fact that, until some years ago, in many countries the electricity produced by renewable systems from small power-generating facilities had priority dispatch. In the near future, it is expected that priority dispatch should be deemed to be compatible with the participation in the electricity market of power-generating facilities using renewable energy sources. Plants can be optimized with the utilization of a consistent storage energy system. Moreover, the joint operation of a PV plant and HP does not permit a proﬁtable use of energy during the winter period, therefore in a lot of cases an oversizing of the PV plant is observed, thus it is necessary for energy to be exported to the electrical grid. It is expected that in the near future, the utilization of such solutions could be connected with self-consumption schemes, in which only the import of energy will be possible, as shown in Figure 1. Consequently, energy produced and not directly used or stored inside can be considered wasted energy, and it is not possible (or convenient) to export excess energy to the grid. The ideal condition is clearly the one in which all of the energy required by the building can be produced within, so that the level of energy imported approaches zero, as required in the ZEB conﬁguration. Taking into account two of the most diused technologies (i.e., ground source heat pumps (GHP) and the small size PV system) and the recent developments in terms of economic support policies, there has been strong growth in the use of both PV plants (determined by the eect of the feed-in tari) and HP systems (aided by the eect of ﬁscal incentives) [25,26]. The typical integration proposed by [27] may contribute in the medium to long term to a relevant reduction in the use of conventional fossil fuels (e.g., natural gas) for heating purposes and to an increase in the production of solar energy. Buildings 2020, 10, x FOR PEER REVIEW 5 of 19 Buildings 2020, 10, 175 5 of 18 National Electrical grid It is not possible It is possible to to export energy import energy in excess to the from the grid grid nZEB-Building (RES based plants + Heat Pump + Storage systems + Miscellaneous Energy Devices) Figure 1. Logic of a net zero-energy building (nZEB). RES: renewable energy system. Figure 1. Logic of a net zero-energy building (nZEB). RES: renewable energy system. 3. Integrated PV-GHP System for Residential Buildings: Experimental Analysis of a Speciﬁc Case Taking into account two of the most diffused technologies (i.e., ground source heat pumps The possibility of monitoring the behavior of dierent types of building–plant systems to cover (GHP) and the small size PV system) and the recent developments in terms of economic support the most widespread typologies, such as residential buildings, is of fundamental importance to be policies, there has been strong growth in the use of both PV plants (determined by the effect of the able to analyze the performance of those systems under real operating conditions. For this reason, feed-in tariff) and HP systems (aided by the effect of fiscal incentives) [25,26]. we began with the analysis of data obtained from an experimental system. The typical integration proposed by [27] may contribute in the medium to long term to a relevant The experimental system under analysis (described in [4]) is located in Pisa, Italy, a town with a reduction in the use of conventional fossil fuels (e.g., natural gas) for heating purposes and to an typical Mediterranean climate. The building has a ﬂoor surface area of about 150 m and a volume of increase in the production of solar energy. 450 m . The heat pump installed is a typical “three circuit” type ground heat pump (GHP) system, in which the ﬂuid (water and salt) that circulates in the borehole heat exchanger is dierent to the ﬂuid 3. Integrated PV-GHP System for Residential Buildings: Experimental Analysis of a Specific Case circulating in the HP (R407) and the water that circulates inside the house. The possibility of monitoring the behavior of different types of building–plant systems to cover The nominal electric power of the HP is 3.8 kW and it provides up to 15.2 kWh of space heating the most widespread typologies, such as residential buildings, is of fundamental importance to be for the entire building, while reverse cycle technology will also provide cooling during summer with able to analyze the performance of those systems under real operating conditions. For this reason, we a maximum load of 11.4 kWh. The plant is composed of the HP and ground probes as well as a PV began with the analysis of data obtained from an experimental system. system directly connected to the power grid by means of an inverter. The plant was realized in the The experimental system under analysis (described in [4]) is located in Pisa, Italy, a town with a framework of the “Conto-Energia” support policy, active in Italy from 2006 to 2015. No storage systems typical Mediterranean climate. The building has a floor surface area of about 150 m and a volume of had been installed at the time because the priority dispatch policy active in Italy permitted the export 450 m . The heat pump installed is a typical “three circuit” type ground heat pump (GHP) system, in of excess electricity produced by the PV plant to the electric grid. The PV plant was sized according to which the fluid (water and salt) that circulates in the borehole heat exchanger is different to the fluid the maximum output power required by the heat pump; thus, the peak power of the PV plant (3.7 kW) circulating in the HP (R407) and the water that circulates inside the house. and peak electric power required by the heat pump (3.8 kW) are similar. The main characteristics and The nominal electric power of the HP is 3.8 kW and it provides up to 15.2 kWh of space heating nominal data of the modules used for the PV plant are provided in Table 1. As additional data, it is for the entire building, while reverse cycle technology will also provide cooling during summer with possible to include the temperature coecient of the plant, that is, the value of 0.38%/ C is declared a maximum load of 11.4 kWh. The plant is composed of the HP and ground probes as well as a PV for the modules if the operating temperature of the module is above 25 C. Speciﬁcally, the nominal system directly connected to the power grid by means of an inverter. The plant was realized in the operating temperature of the module is 46 C. framework of the “Conto-Energia” support policy, active in Italy from 2006 to 2015. No storage systems had been installed at the time because the priority dispatch policy active in Italy permitted Table 1. Nominal data of the photovoltaic (PV) plant. the export of excess electricity produced by the PV plant to the electric grid. The PV plant was sized Nominal Power Number of Area of a Total Plant Nominal Power Nominal according to the maximum output power required by the heat pump; thus, V the peak power of th I e PV mpp mpp of the PV Plant Modules Module Surface of a PV Module Eciency plant (3.7 kW) and peak electric power required by the heat pump (3.8 kW) are similar. The main 2 2 3.74 17 220 41.0 V 5.37 A 17% 1.24 m 21.13 m characteristics and nominal data of the modules used for the PV plant are provided in Table 1. As additional data, it is possible to include the temperature coefficient of the plant, that is, the value of The typical climatic conditions of Pisa are shown in Figures 2 and 3 for the heating and cooling 0.38%/°C is declared for the modules if the operating temperature of the module is above 25 °C. period, respectively. Figure 2 provides the distribution of the external temperature, showing the Specifically, the nominal operating temperature of the module is 46 °C. number of hours the selected temperature can be observed. The value of the temperature is provided considering a range of variation of 1 C, while on the ordinate axis, the number of hours is characterized Table 1. Nominal data of the photovoltaic (PV) plant. Buildings 2020, 10, x FOR PEER REVIEW 6 of 19 Buildings 2020, 10, x FOR PEER REVIEW 6 of 19 Nominal Power of Number of Area of a Total Plant Nominal Power of Nominal Vmpp Impp Nominal Power of Number of Area of a Total Plant Nominal Power of Nominal the PV Plant Modules Module Surface a PV Module Efficiency Vmpp Impp the PV Plant Modules Module Surface a PV Module Efficiency 41.0 5.37 2 2 3.74 17 1.24 m 21.13 m 220 17% 41.0 5.37 V A 2 2 3.74 17 1.24 m 21.13 m 220 17% V A The typical climatic conditions of Pisa are shown in Figures 2 and 3 for the heating and cooling The typical climatic conditions of Pisa are shown in Figures 2 and 3 for the heating and cooling period, respectively. Figure 2 provides the distribution of the external temperature, showing the period, respectively. Figure 2 provides the distribution of the external temperature, showing the Buildings 2020, 10, 175 6 of 18 number of hours the selected temperature can be observed. The value of the temperature is provided number of hours the selected temperature can be observed. The value of the temperature is provided considering a range of variation of 1 °C, while on the ordinate axis, the number of hours is considering a range of variation of 1 °C, while on the ordinate axis, the number of hours is characterized by a well-defined average temperature value. The period from 15 October to 15 April by a well-deﬁned average temperature value. The period from 15 October to 15 April is considered. characterized by a well-defined average temperature value. The period from 15 October to 15 April is considered. Figure 3 provides the same distribution for the hot season (considering the period from Figure 3 provides the same distribution for the hot season (considering the period from 15 May to is considered. Figure 3 provides the same distribution for the hot season (considering the period from 15 May to 30 September). 30 September). 15 May to 30 September). 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 T (°C) T (°C) Figure 2. Typical distribution of median outdoor temperature during the cold season (15 October–15 Figure 2. Typical distribution of median outdoor temperature during the cold season (15 October– Figure 2. Typical distribution of median outdoor temperature during the cold season (15 October–15 April). 15 April). April). 0 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 4041 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 4041 T (°C) ext T (°C) ext Figure 3. Typical distribution of average outdoor temperature during the hot season (15 May– Figure 3. Typical distribution of average outdoor temperature during the hot season (15 May–30 30 September). Figure 3. Typical distribution of average outdoor temperature during the hot season (15 May–30 September). September). Pisa is characterized by a total annual irradiance of 1500 kWh/m , for a surface with a south-facing Pisa is characterized by a total annual irradiance of 1500 kWh/m , for a surface with a south- aspect. Using modules of high quality that are well exposed, the expected 2 PV value could correspond Pisa is characterized by a total annual irradiance of 1500 kWh/m , for a surface with a south- facing aspect. Using modules of high quality that are well exposed, the expected PV value could to 1100–1150 kWh for each kW of peak power installed for each year of operation for a poly-crystalline facing aspect. Using modules of high quality that are well exposed, the expected PV value could correspond to 1100–1150 kWh for each kW of peak power installed for each year of operation for a high-quality PV plant. Considering the HP used, this is a commercial heat pump that uses R-407 as the correspond to 1100–1150 kWh for each kW of peak power installed for each year of operation for a poly-crystalline high-quality PV plant. Considering the HP used, this is a commercial heat pump that refrigerant; the heating COP in nominal conditions is four, while the cooling COP is approximately poly-crystalline high-quality PV plant. Considering the HP used, this is a commercial heat pump that uses R-407 as the refrigerant; the heating COP in nominal conditions is four, while the cooling COP three. In terms of the HP, the condenser temperature was 35/30 C while the evaporator temperature uses R-407 as the refrigerant; the heating COP in nominal conditions is four, while the cooling COP is approximately three. In terms of the HP, the condenser temperature was 35/30 °C while the was considered 0 C. The geothermal heat exchanger is of the borehole type with a total depth of 50 m. is approximately three. In terms of the HP, the condenser temperature was 35/30 °C while the evaporator temperature was considered 0 °C. The geothermal heat exchanger is of the borehole type The total length of the borehole heat exchanger is 200 m, this value was obtained using a reference value evaporator temperature was considered 0 °C. The geothermal heat exchanger is of the borehole type with a total depth of 50 m. The total length of the borehole heat exchanger is 200 m, this value was of 50 W/m for the heat transfer value. The criterion used for the design was that the energy output with a total depth of 50 m. The total length of the borehole heat exchanger is 200 m, this value was from the PV plant should be able to completely assist the operation of the GHP (for cooling purposes) and the consumption of all remaining electrical devices during the hot period. In the remaining part of the year, a relevant exchange of energy (from and to the power grid) could be possible. A schematic diagram of the system and its operational logic are provided in Figure 4. According to the description, the energy produced by the PV plant, as measured by a counter, can be directly used by the various electrical devices or exported to the electrical grid. Energy used by the HP is measured with an Buildings 2020, 10, 175 7 of 18 additional counter. Other relevant additional data can be measured including the internal and external Buildings 2020, 10, x FOR PEER REVIEW 8 of 19 temperature (using thermocouples) and relative humidity inside the house. Figure 4. A schematic description of the plant composed by a ground source heat pump (GHP) for Figure 4. A schematic description of the plant composed by a ground source heat pump (GHP) for house heating and cooling, assisted by a PV system and connected to the electrical grid. house heating and cooling, assisted by a PV system and connected to the electrical grid. Some relevant data concerning the operation of the system can be directly measured or derived. The energy consumption, energy required by the HP, EGHP, and energy required by the rest of The total energy imported from the power grid, E , corresponds to the whole energy used in TOT the household devices, EDEV, as well as the size of the operation, are reported in Table 2. the house. The energy imported from the grid, E , corresponds to the periods during which no IMP In a reference period of operation of the plant, it is possible to understand how the total production of the PV plant was available, or there was less energy than the total required. The energy production of the PV plant, EPV, is approximately the same as that required both for the operation of exported to the power grid, E , reﬂects periods of excess energy production. A speciﬁc counter EXP GHP and of the other devices (1399 kWh and 1448 kWh, respectively). While during the period from enabled us to directly measure the energy used by the GHP (E ) and another counter measured the GHP April to June the PV production was higher than the total energy consumption, during the other energy produced by the PV plant, E , and directly exported to the power grid. PV periods (autumn: from October–December, and winter: January–March) the PV production was The energy produced can be easily compared with the value that was estimated as the product insufficient to power the GHP and all the other devices. of the annual solar irradiation typical for the speciﬁc place at a given exposure, H , of a balance of SN In order to obtain a kind of parity, considering the energy totally produced along the whole year, system eciency, , taking into account the various electrical losses in the system, and the peak BOS an oversizing of the PV plant of a factor 1.5 (with a peak power of 5.5 kW corresponding to a 25 power of the PV plant, P . PV modules), while in the perspective of covering the electricity required in the period from October to E = (H )P (1) PV PV SN BOS March, could be necessary to increase the size of the PV plant. In this last case, it would be necessary to in The stallmaximum a power plpr an oduction t with a power over 10 of a PV system kW expr (for essed examin ple, kWh 13.4 k /kW W—eq in the uiv speciﬁc alent to 61 place modu and lethe s). annual In this c solar aseirradiation , the energy is ex a consequence ported to the of gr the id wo factuld that drastically in the nominal power crease, of cathe usin PV g a plant n imis pacalculated ct on the considering power grid. a speciﬁc solar power of 1000 W/m , a value referred to in the standard test conditions. The operation of the HP during the year, including both the heating mode and cooling mode, is Considering the two values of the energy used in the house, E , and energy used by the GHP, TOT described in Figure 4. The electricity consumption values range from a lower value of 2–3 kWh/day, E , it is easy to evaluate the energy used by the other electrical devices. GHP typical of the mid-season period, when the heat pump is used only for the production of sanitary hot E = E E (2) water, up to values just below 35 kWh/day, typical of the winter season, when there may be 10 hours DEV TOT GHP of operation per day. A good operation of the heat pump was evaluated in the winter phase, while For each day and each period of the year, using the results of the measurement, it was possible during the summer cooling period, the COP value was slightly lower than the nominal one. During to evaluate the energy exchanged with the power grid to understand how close the system is to the the heating phase, a monthly average power value was found to be very close to the nominal value, which does not exist in the cooling phase when the heat pump works frequently in ON–OFF mode and the transient phase is therefore not negligible. Buildings 2020, 10, 175 8 of 18 model of a zero-energy building (ZEB) and the amount of energy exported, E , or imported from the EXP power grid, E , according to the following deﬁnitions: IMP E = E E 0 (3) EXP PV TOT E = E E 0 (4) IMP TOT PV The value of the energy produced with the PV plant and directly used, E , can be calculated by E = E E 0 (5) D TOT IMP The share of the energy produced by the PV plant and directly used in the building, measured with the dimensionless coecient, I , can be deﬁned as = I (6) PV where values range from zero to one. The ideal system would be the one in which the energy produced is directly used in the building, so that I approaches unity. The energy consumption, energy required by the HP, E , and energy required by the rest of the GHP household devices, E , as well as the size of the operation, are reported in Table 2. DEV Table 2. Energy balance of the system (values of year 2009, ﬁrst one of operation). E E E E GHP DEV TOT PV Period of the Year (kWh) (kWh) (kWh) (kWh) January–March 1658 671 2329 850 April–June 377 550 927 1436 July–September 796 652 1448 1399 October–December 1005 622 1627 580 Total year 3836 2495 6331 4265 In a reference period of operation of the plant, it is possible to understand how the total production of the PV plant, E , is approximately the same as that required both for the operation of GHP and of PV the other devices (1399 kWh and 1448 kWh, respectively). While during the period from April to June the PV production was higher than the total energy consumption, during the other periods (autumn: from October–December, and winter: January–March) the PV production was insucient to power the GHP and all the other devices. In order to obtain a kind of parity, considering the energy totally produced along the whole year, an oversizing of the PV plant of a factor 1.5 (with a peak power of 5.5 kW corresponding to a 25 modules), while in the perspective of covering the electricity required in the period from October to March, could be necessary to increase the size of the PV plant. In this last case, it would be necessary to install a power plant with a power over 10 kW (for example, 13.4 kW—equivalent to 61 modules). In this case, the energy exported to the grid would drastically increase, causing an impact on the power grid. The operation of the HP during the year, including both the heating mode and cooling mode, is described in Figure 4. The electricity consumption values range from a lower value of 2–3 kWh/day, typical of the mid-season period, when the heat pump is used only for the production of sanitary hot water, up to values just below 35 kWh/day, typical of the winter season, when there may be 10 h of operation per day. A good operation of the heat pump was evaluated in the winter phase, while during the summer cooling period, the COP value was slightly lower than the nominal one. During the heating phase, a monthly average power value was found to be very close to the nominal value, which does not exist in the cooling phase when the heat pump works frequently in ON–OFF mode and the transient phase is therefore not negligible. Buildings 2020, 10, x FOR PEER REVIEW 9 of 19 Table 2. Energy balance of the system (values of year 2009, first one of operation). EGHP EDEV ETOT EPV Period of the Year (kWh) (kWh) (kWh) (kWh) January–March 1658 671 2329 850 April–June 377 550 927 1436 July–September 796 652 1448 1399 October–December 1005 622 1627 580 Total year 3836 2495 6331 4265 Figure 5 provides the daily consumption and corresponding operating hours of the GHP throughout the year. The energy required for summer air conditioning is approximately a third of the energy required for the winter period, and the maximum energy required for the operation of the HP occurs during the winter period, when the production of the PV plant is at its lowest levels. A further element of discussion concerns the results of the data obtained during long-term monitoring of a building–plant system. Table 3 shows the distribution of the various energy flows in the different years of operation of the experimental plant to verify the weak points and possible improvements that could be obtained for the integrated system under analysis. It can be observed that, while during winter (mainly in December and January) PV production was insufficient for supporting the operation of GHP, in summer (from June to September) the production of PV is sometimes excessive. It is interesting to analyze the behavior of the system during the period between the hot and cold seasons, that is, from February to May and from October to November. The data reported in Table 3 provide a summary of the various electricity consumptions within the system. We evaluated the electricity consumption of the house and the production of the PV system, during the years from 2013 to 2017, to compare them with 2009, the first year of operation. Considering the energy produced by the PV plant, the values are similar to those calculated using Equation (1), considering a value of ηBOS approximately equal to 0.75–0.8. As is evident from the analysis of the data, the PV plant has operated in quite a satisfactory way, even though some differences among the various years can be observed. Differences can be measured by the value of the difference between the maximum and the minimum value of the production, divided by the average production rate, defined as EE − PV,, MAX PV MIN I = (7) PV PV ,avg Buildings 2020, 10, 175 9 of 18 Based on the data in Table 4, this value was about 8.5%, which could be connected to the different weather conditions and the degradation of the modules over time. Figure 5 provides the daily consumption and corresponding operating hours of the GHP With regard to the HP, it appears to be oversized for the particular operating conditions. As throughout the year. The energy required for summer air conditioning is approximately a third of the discussed before, the longest daily running time is 12 h during the coldest day of the year, but on energy required for the winter period, and the maximum energy required for the operation of the HP most days the heat pump operates for a reduced number of hours (usually less than two). occurs during the winter period, when the production of the PV plant is at its lowest levels. Buildings 2020, 10, x FOR PEER REVIEW 10 of 19 (a) 0 30 60 90 120 150 180 210 240 270 300 330 360 day of the year (b) Figure Figure 5. 5. Daily Daily ener energ gy consumption y consumption of the of the GHP GHP in a in a typical typic year al y(e aar ( ) and a) an hours d hours of operat of operation (ion ( b). b) A further element of discussion concerns the results of the data obtained during long-term Table 3. Annual energy flows of the plant under various conditions. monitoring of a building–plant system. Table 3 shows the distribution of the various energy ﬂows ED EDEV EPV (kWh) EIMP (kWh) EEXP (kWh) ETOT (kWh) EGHP (kWh) in the dierent years of operation of the experimental plant to verify the weak points and possible (kWh) (kWh) improvements that could be obtained for the integrated system under analysis. It can be observed that, 2009 4265 4923 2857 1408 6331 3836 2495 2013 3825 4518 2622 1203 5721 3370 2351 while during winter (mainly in December and January) PV production was insucient for supporting 2014 4059 4133 2982 1077 5210 2924 2286 the operation of GHP, in summer (from June to September) the production of PV is sometimes excessive. 2015 3947 4971 2784 1110 6081 3685 2396 It is interesting to analyze the behavior of the system during the period between the hot and cold 2016 3957 4938 2804 1153 6091 3210 2881 seasons, that is, from February to May and from October to November. The data reported in Table 3 2017 4084 4510 2833 1251 5761 3382 2379 provide a summary of the various electricity consumptions within the system. We evaluated the electricity Table 4. consumption PV modof ule the pro house ductioand n in t the he m pro oduction nths betwof een the thPV e hot and system, coduring ld season in the years different years from 2013 to 2017, (data in to compar kWh). e them with 2009, the ﬁrst year of operation. Considering the energy produced by the PV plant, the values are similar to those calculated using Equation (1), considering a value of BOS PV Prod February March April May October November approximately equal to 0.75–0.8. As is evident from the analysis of the data, the PV plant has operated [kWh] 2009 220 350 413 575 316 161 in quite a satisfactory way, even though some dierences among the various years can be observed. 2013 221 335 403 487 240 160 Dierences can be measured by the value of the dierence between the maximum and the minimum 2014 186 388 427 470 310 101 value of the production, divided by the average production rate, deﬁned as 2015 210 312 445 582 256 111 2016 147 311 441 498 256 156 E E PV,MAX PV,MIN 2017 184 413 469 458 313 163 I = (7) PV Average (2013–17) 190 352 437 614 275 139 PV,avg The data, acquired over more than eight years of operation, demonstrate that the energy consumed by the GHP and the energy produced by the PV array are similar if the total amount is considered, and that the energy exchange with the electricity grid is relevant. Considering an average year of the five from 2013 to 2017, the average values of the energy exported to and imported from the grid in the various months of the year are provided in Figure 6. Table 5 provides the average values of the energy flows from January to December. Considering the data of Table 5, the index of direct utilization of the energy produced by the PV plant, IU, can be approximately estimated with a maximum level (0.55–0.56) in the winter and the minimum value occurring in May (average, 0.21). GHP operating hours Buildings 2020, 10, 175 10 of 18 Table 3. Annual energy ﬂows of the plant under various conditions. E E E E E E E PV IMP EXP D TOT GHP DEV (kWh) (kWh) (kWh) (kWh) (kWh) (kWh) (kWh) 2009 4265 4923 2857 1408 6331 3836 2495 2013 3825 4518 2622 1203 5721 3370 2351 2014 4059 4133 2982 1077 5210 2924 2286 2015 3947 4971 2784 1110 6081 3685 2396 2016 3957 4938 2804 1153 6091 3210 2881 2017 4084 4510 2833 1251 5761 3382 2379 Based on the data in Table 4, this value was about 8.5%, which could be connected to the dierent weather conditions and the degradation of the modules over time. Table 4. PV module production in the months between the hot and cold season in dierent years (data in kWh). PV Prod [kWh] February March April May October November 2009 220 350 413 575 316 161 2013 221 335 403 487 240 160 2014 186 388 427 470 310 101 2015 210 312 445 582 256 111 2016 147 311 441 498 256 156 2017 184 413 469 458 313 163 Average (2013–17) 190 352 437 614 275 139 With regard to the HP, it appears to be oversized for the particular operating conditions. As discussed before, the longest daily running time is 12 h during the coldest day of the year, but on most days the heat pump operates for a reduced number of hours (usually less than two). The data, acquired over more than eight years of operation, demonstrate that the energy consumed by the GHP and the energy produced by the PV array are similar if the total amount is considered, and that the energy exchange with the electricity grid is relevant. Considering an average year of the ﬁve from 2013 to 2017, the average values of the energy exported to and imported from the grid in the various months of the year are provided in Figure 6. Table 5 provides the average values of the energy ﬂows from January to December. Considering the data of Table 5, the index of direct utilization of the energy produced by the PV plant, I , can be approximately estimated with a maximum level Buildings 2020, 10, x FOR PEER REVIEW 11 of 19 (0.55–0.56) in the winter and the minimum value occurring in May (average, 0.21). Figure 6. Monthly energy ﬂow from and to the grid, with a 3.74 kW PV plant. Figure 6. Monthly energy flow from and to the grid, with a 3.74 kW PV plant. Table 5. Average monthly values of the energy flows in the period 2009–2017. EPV EEXP EIMP EGHP EDEV ETOT ED Month IU (kWh) (kWh) (kWh) (kWh) (kWh) (kWh) (kWh) January 105 46 761 622 198 820 59 0.561 February 195 108 644 519 211 731 87 0.446 March 335 229 498 394 210 604 106 0.316 April 432 322 258 166 203 369 111 0.256 May 504 398 187 103 190 293 106 0.210 June 491 375 196 122 190 312 116 0.236 July 552 372 358 316 221 538 180 0.326 August 519 319 395 327 268 595 200 0.385 September 370 282 199 114 173 287 88 0.238 October 262 191 271 140 201 341 70 0.267 November 133 80 492 304 241 545 53 0.398 December 109 48 763 511 313 824 61 0.559 Total 4007 2771 5021 3639 2618 6257 1236 0.308 4. The Problem of Defining a Correct Size for PV-GHP and n-ZEB The particular solution proposed here is often considered one of the possible solutions for ZEBs, and the analysis conducted under real operating conditions have shown some interesting aspects and some critical elements concerning the modifications required. The solution experimentally analyzed was designed with the same values of power for the HP and the PV plants. The electrical grid is used as an “energy buffer” to decouple energy generation and energy use, mainly during the spring and autumn. The possibility of exporting energy to the electrical grid (often the national electrical grid) was a good method in principle to promote the installation of PV plants directly connected to residential building. This option is not available today because of the problems induced by the relevant penetration of such plants, already discussed in [5], with reference to an Italian case, where in 2020 more than 800,000 PV plants were present in the territory [28]. It is important to test different cases, and to ensure that excess energy can be stored using an electrochemical system, so that unused energy is not wasted. For this reason, in the present section, the system under analysis is modified. In particular, the objective was to maximize the energy production of the PV plant from the perspective that surplus energy cannot be exported to the power grid, although energy can be imported from the grid. Considering the schematic in Figure 4, this would result in a size reduction in PV power plants, or, if the introduction of a storage system was considered, the size of the PV system could be similar to those experimentally analyzed. The possibility of introducing an appropriately sized electrical storage unit could help reduce the amount of energy exchanged with the electrical grid. 4.1. A Reduction in the Size of PV Plants Buildings 2020, 10, 175 11 of 18 Table 5. Average monthly values of the energy ﬂows in the period 2009–2017. E E E E E E E PV EXP IMP GHP DEV TOT D Month I (kWh) (kWh) (kWh) (kWh) (kWh) (kWh) (kWh) January 105 46 761 622 198 820 59 0.561 February 195 108 644 519 211 731 87 0.446 March 335 229 498 394 210 604 106 0.316 April 432 322 258 166 203 369 111 0.256 May 504 398 187 103 190 293 106 0.210 June 491 375 196 122 190 312 116 0.236 July 552 372 358 316 221 538 180 0.326 August 519 319 395 327 268 595 200 0.385 September 370 282 199 114 173 287 88 0.238 October 262 191 271 140 201 341 70 0.267 November 133 80 492 304 241 545 53 0.398 December 109 48 763 511 313 824 61 0.559 Total 4007 2771 5021 3639 2618 6257 1236 0.308 4. The Problem of Deﬁning a Correct Size for PV-GHP and n-ZEB The particular solution proposed here is often considered one of the possible solutions for ZEBs, and the analysis conducted under real operating conditions have shown some interesting aspects and some critical elements concerning the modiﬁcations required. The solution experimentally analyzed was designed with the same values of power for the HP and the PV plants. The electrical grid is used as an “energy buer” to decouple energy generation and energy use, mainly during the spring and autumn. The possibility of exporting energy to the electrical grid (often the national electrical grid) was a good method in principle to promote the installation of PV plants directly connected to residential building. This option is not available today because of the problems induced by the relevant penetration of such plants, already discussed in [5], with reference to an Italian case, where in 2020 more than 800,000 PV plants were present in the territory [28]. It is important to test dierent cases, and to ensure that excess energy can be stored using an electrochemical system, so that unused energy is not wasted. For this reason, in the present section, the system under analysis is modiﬁed. In particular, the objective was to maximize the energy production of the PV plant from the perspective that surplus energy cannot be exported to the power grid, although energy can be imported from the grid. Considering the schematic in Figure 4, this would result in a size reduction in PV power plants, or, if the introduction of a storage system was considered, the size of the PV system could be similar to those experimentally analyzed. The possibility of introducing an appropriately sized electrical storage unit could help reduce the amount of energy exchanged with the electrical grid. 4.1. A Reduction in the Size of PV Plants Starting from the results obtained from the real plant and analyzed, in this section we propose alternative conﬁgurations for the system. The introduction of a storage system is considered together with the deﬁnition of an optimal size of both PV system and HP system. Table 6 provides the results obtained in ﬁve dierent cases, considering as a ﬁrst case the actual conﬁguration of the plant, with a PV plant of 3.74 kW power. In this case, the amount of energy wasted was about 70% of the energy produced. By reducing the size of the plant, the amount of energy wasted is reduced, but the size of the plant is then too small. In the last case, the peak power of the PV plant is reduced to a level of 1.1 kW, the energy produced with the PV plant is approximately 1200 kWh, less than 20% of the energy required by the house, although the amount of energy wasted is reduced to less than 10%. In this case, it can be observed that the energy imported from the grid is similar to that of the reference case (5193 kWh and 5021 kWh, respectively). A reasonable compromise could be achieved by installing 10 modules of the same type, instead of 17, that is, with a total power rate of 2.2 kW (case four in Table 6). Buildings 2020, 10, x FOR PEER REVIEW 12 of 19 Starting from the results obtained from the real plant and analyzed, in this section we propose alternative configurations for the system. The introduction of a storage system is considered together with the definition of an optimal size of both PV system and HP system. Table 6 provides the results obtained in five different cases, considering as a first case the actual configuration of the plant, with a PV plant of 3.74 kW power. In this case, the amount of energy wasted was about 70% of the energy produced. By reducing the size of the plant, the amount of energy wasted is reduced, but the size of the plant is then too small. In the last case, the peak power of the PV plant is reduced to a level of 1.1 kW, the energy produced with the PV plant is approximately 1200 kWh, less than 20% of the energy required by the house, although the amount of energy wasted is reduced to less than 10%. In this case, it can be observed that the energy imported from the grid is similar to that of the reference case Buildings 2020, 10, 175 12 of 18 (5193 kWh and 5021 kWh, respectively). A reasonable compromise could be achieved by installing 10 modules of the same type, instead of 17, that is, with a total power rate of 2.2 kW (case four in Table 6). Using this configuration, the amount of energy imported from the grid is approximately the Using this conﬁguration, the amount of energy imported from the grid is approximately the same as same as that observed for the original solution, while the energy wasted is 1121 kWh instead of 2771 that observed for the original solution, while the energy wasted is 1121 kWh instead of 2771 kWh. kWh. Figure 7 provides the energy flows from the grid and the energy to the grid (or wasted). A Figure 7 provides the energy ﬂows from the grid and the energy to the grid (or wasted). A relevant relevant increase in self-consumption of the PV plant was obtained even if the total amount of energy increase in self-consumption of the PV plant was obtained even if the total amount of energy is less is less than 50% of the total energy amount necessary in the house. than 50% of the total energy amount necessary in the house. Table 6. Level of energy wasted with a reduced size of the PV pant. Table 6. Level of energy wasted with a reduced size of the PV pant. PPV EIMP EW P (kW) No. of Modules E (kWh) E (kWh) No. of Modules PV IMP W (kW) (kWh) (kWh) BASIC 3.74 17 5021 2771 BASIC 3.74 17 5021 2771 Case 1 2.2 Case 1 2.2 10 10 5021 5021 1121 1121 Case 2 1.54 Case 2 1.54 7 7 5042 5042 435 435 Case 3 1.32 Case 3 1.32 6 6 5107 5107 263 263 Case 4 1.1 Case 4 1.1 5 5 5193 5193 114 114 energy to the grid energy from the grid jan feb mar apr may jun jul aug sep oct nov dec Figure 7. Monthly energy (kWh) from and to the grid, with a PV plant of 2.2 kW power. Figure 7. Monthly energy (kWh) from and to the grid, with a PV plant of 2.2 kW power. 4.2. A Reduction in the Size of the PV Plant with the Introduction of a Storage System 4.2. A Reduction in the Size of the PV Plant with the Introduction of a Storage System The conﬁguration of the plant analyzed in the previous section, characterized by a reduction in the size of the PV plant, clearly shows the real problem connected with the development of such a The configuration of the plant analyzed in the previous section, characterized by a reduction in solution. In general, the amount of energy exchanged with the power grid is important and if the the size of the PV plant, clearly shows the real problem connected with the development of such a producer cannot export energy to the grid, the amount of energy wasted is also important. We tried to solution. In general, the amount of energy exchanged with the power grid is important and if the establish the optimization criteria to solve this issue, considering the installation of an electrochemical producer cannot export energy to the grid, the amount of energy wasted is also important. We tried storage and based on the deﬁnition of the optimal size of HP and PV. to establish the optimization criteria to solve this issue, considering the installation of an As the building is equipped with a PV plant, the installation of electric storage mitigates the eect electrochemical storage and based on the definition of the optimal size of HP and PV. of the time mismatch between electricity production and peak thermal load. The scheme of the system As the building is equipped with a PV plant, the installation of electric storage mitigates the is provided in Figure 8, where it is shown that only a unidirectional ﬂow is possible. A critical analysis effect of the time mismatch between electricity production and peak thermal load. The scheme of the of the experimental data collected from the existing systems, combined with the optimum design system is provided in Figure 8, where it is shown that only a unidirectional flow is possible. A critical strategy, permitted us to develop guidelines for the optimal combination between the power rating of analysis of the experimental data collected from the existing systems, combined with the optimum PV generators and the energy capacity of thermal and electrochemical storage systems, and to deﬁne an appropriate sizing of the HP and the other components of the system. [kWh] Buildings 2020, 10, x FOR PEER REVIEW 13 of 19 design strategy, permitted us to develop guidelines for the optimal combination between the power rating of PV generators and the energy capacity of thermal and electrochemical storage systems, and Buildings 2020, 10, 175 13 of 18 to define an appropriate sizing of the HP and the other components of the system. Figure 8. A schematic description of the plant consisting of a GHP for house heating and cooling and Figure 8. A schematic description of the plant consisting of a GHP for house heating and cooling and assisted by a PV system with a storage system (it is not possible to export energy to the grid). assisted by a PV system with a storage system (it is not possible to export energy to the grid). A possible modiﬁcation of the system from the perspective of increasing the utilization of the A possible modification of the system from the perspective of increasing the utilization of the energy energy produced consists of the introduction of a storage system. A series of attempts have been produced consists of the introduction of a storage system. A series of attempts have been made to made to deﬁne the correct size of the storage system. We analyzed these dierent options and Table 7 define the correct size of the storage system. We analyzed these different options and Table 7 provides provides the results. We found that installing a storage system of 6 kWh/day capacity would reduce the the results. We found that installing a storage system of 6 kWh/day capacity would reduce the energy energy wasted to approximately zero, if exporting energy to the power grid is not possible. With the wasted to approximately zero, if exporting energy to the power grid is not possible. With the installation of this storage system, energy losses are reduced to a low level. The particular capacity of installation of this storage system, energy losses are reduced to a low level. The particular capacity of the storage system corresponds to about 35%–40% of the total energy amount produced for each kW of the storage system corresponds to about 35%–40% of the total energy amount produced for each kW PV power installed. For example, just with the introduction of a storage system of 6 kWh/day capacity of PV power installed. For example, just with the introduction of a storage system of 6 kWh/day the electricity transfer from and to the grid decreased drastically. In particular, in the months of May capacity the electricity transfer from and to the grid decreased drastically. In particular, in the months and June, a general self-production proﬁle can be obtained, while from July to September, the energy of May and June, a general self-production profile can be obtained, while from July to September, the imported from the grid is small, as shown in Figure 9. energy imported from the grid is small, as shown in Figure 9. Table 7. Energy imported from the grid and wasted in ﬁve dierent conﬁgurations with storage system. E E Storage Capacity P PV Plant IMP PV (kWh) (kWh) (kWh) (kW) No. of Modules CASE 1 2800 551 13 3.74 17 CASE 2 3189 232 10 3.08 14 CASE 3 3348 156 9 2.86 13 CASE 4 3703 39 7 2.42 11 CASE 5 3903 0 6 2.2 10 Buildings 2020, 10, x FOR PEER REVIEW 14 of 19 Table 7. Energy imported from the grid and wasted in five different configurations with storage system. EIMP EW PPV PV Plant Storage Capacity (kWh) (kWh) (kWh) (kW) no. of Modules CASE 1 2800 551 13 3.74 17 CASE 2 3189 232 10 3.08 14 CASE 3 3348 156 9 2.86 13 CASE 4 3703 39 7 2.42 11 CASE 5 3903 0 6 2.2 10 Buildings 2020, 10, 175 14 of 18 energy from the grid energy to the grid jan feb mar apr may jun jul aug sep oct nov dec Figure 9. Monthly energy from the grid (PV plant of 2.2 kW, and PV plant of 2.2 kW with 6 kWh Figure 9. Monthly energy from the grid (PV plant of 2.2 kW, and PV plant of 2.2 kW with 6 kWh storage). storage). 4.3. System with Higher Level of Self-Consumption 4.3. System with Higher Level of Self-Consumption One of the problems associated with the results provided in Sections 4.1 and 4.2 is that in all of the One of the problems associated with the results provided in Sections 4.1 and 4.2 is that in all of cases analyzed, a reduction in the size of the PV plant was necessary. In this case, it is possible to reach the cases analyzed, a reduction in the size of the PV plant was necessary. In this case, it is possible to the objective of reducing the production of surplus energy, but the total amount of energy produced by reach the objective of reducing the production of surplus energy, but the total amount of energy the PV plant is often quite low (considering, for example, the results of Table 5, the minimum energy produced by the PV plant is often quite low (considering, for example, the results of Table 5, the wasted can be obtained with a plant of 1.1 kW peak power). It could be argued that there is a reduced minimum energy wasted can be obtained with a plant of 1.1 kW peak power). It could be argued that overall utility associated with this solution. there is a reduced overall utility associated with this solution. In this ﬁnal section, we analyze the various possible optimal solutions, obtained according to In this final section, we analyze the various possible optimal solutions, obtained according to dierent objective functions. The availability of a storage system is always considered. Table 8 provides different objective functions. The availability of a storage system is always considered. Table 8 the results of the most important cases, and it can be seen that some of the self-consumption situations provides the results of the most important cases, and it can be seen that some of the self-consumption require unrealistically large PV plants (e.g., a PV plant of about 36 kW of peak power is required situations require unrealistically large PV plants (e.g., a PV plant of about 36 kW of peak power is to achieve the minimum energy import). However, there are some interesting cases for which the required to achieve the minimum energy import). However, there are some interesting cases for condition of relevant energy production and reduced values of energy waste are available. In particular, which the condition of relevant energy production and reduced values of energy waste are available. the case in which the minimum of the sum of energy ﬂows from and to the grid (correspondent to a size In particular, the case in which the minimum of the sum of energy flows from and to the grid of 4.6 kW of the PV plant), and the case in which the plant provides the maximum self-consumption (correspondent to a size of 4.6 kW of the PV plant), and the case in which the plant provides the level (about 99%), corresponding to a PV plant size of 2.6 kW and a 9 kWh of storage system capacity, maximum self-consumption level (about 99%), corresponding to a PV plant size of 2.6 kW and a 9 can be considered two interesting solutions. kWh of storage system capacity, can be considered two interesting solutions. Table 8. Optimized size of the PV plant for dierent design objectives. Storage System Energy Produced with Energy Produced with PV Objective of the Design Capacity PV Plant and Directly Respect to the Total (kW) (kWh) Used (%) Required (%) Experimental System 3.7 - 30 19 PV plant minimizing energy 36 26 20 100 imported from the grid (ZEB case) PV plant minimizing the sum of 4.6 17 87 55 energy imported and wasted PV plant producing the same 5.9 11 65 52 amount of energy as that required PV plant with maximum 2.6 9 99 37 self-consumption level 5. Discussion and Guidelines for the Sustainable Sizing of a PV-HP System As a result of the analysis we carried out, here we pick out the general elements of discussion and criteria that may help set guidelines for the design of similar systems, that is, an integrated system including a heat pump (HP) and photovoltaic (PV) system, with the possible addition of storage (either electrical or thermal). [kWh] Buildings 2020, 10, 175 15 of 18 An important preliminary step is the deﬁnition of the thermal load of the building and its ﬂuctuations over the annual cycle. This can be achieved by means of a dynamic simulation or considering a typical value of the heat transmission, which deﬁnes the type of building and the test reference year (TRY), which deﬁnes the climate in the area, given by an average value of the heat trasmittance of the building (UA expressed in W/K). This information facilitates the determination of the average power of the heat pump and it is possible to calculate the electricity that is needed to supply the building to meet its thermal demand. Once the size of the HP has been determined, it is possible to deﬁne various options to satisfy the project’s speciﬁc objectives: (1) The nominal power of the PV plant is similar to the nominal power of the HP: in this case, a relevant amount of the energy produced needs to be exported to the grid or will be wasted. (2) The nominal power of the PV plant can be considered as about 70% of the peak power of the GHP to reduce the amount of energy exported to the grid (or wasted), without increasing the energy imported from the grid. (3) The nominal power of the PV plant must be about 70% of the peak power of the GHP and an electricity storage system of 3 kWh for each kW of PV plant installed would be required to reduce energy waste to a level below 10%. (4) The energy produced with the PV plant is approximately the same as that required by the GHP and the other devices: in this case, the peak power of the PV plant is approximately 50% over the electric power of the GHP and a storage system of 2 kWh for each kW of PV plant installed would be necessary. (5) The PV plant energy production needs to cover the total energy use of the house: in this case, an oversizing of the PV plant is necessary (9–10 times the peak power of the HP), and a storage system (about 0.7 kWh for each kW installed) must be introduced. Obviously, the previous statements are fairly speciﬁc to the case under analysis. To discuss more general elements, it could be necessary to take into account the variation of solar radiation, the type of PV module selected (and its relative performances), and all the other constraints. In general, two dierent optimization criteria can be considered. The ﬁrst leads us to minimize energy losses by introducing an additional penalty to the surplus energy produced by the modules, which cannot be directly used in the building. In this case, a reduction in the PV plant size is desirable. An alternative criterion considers not the relevant the energy losses and the overproduction of the PV plant, but is particularly dependent on the cost of the electricity imported from the grid. In both cases, the deﬁnition of the size of the storage system is particularly important. It should be noted that these considerations are primarily connected with the speciﬁc climatic condition under analysis. While one might discuss the potential change to our ﬁndings in the case of dierent climatic conditions, it seems clear that the considerations outlined here will be quite similar regardless of climatic conditions. 6. Conclusions We ﬁrst analyzed data obtained from the real operation of a building–plant system consisting of a domestic PV plant of reduced size that was supporting a GHP for heating and cooling. We attempted to develop criteria and guidelines for the correct sizing of such systems from the perspective of obtaining eective self-production of energy. The experimental analysis in Pisa, Italy, with typical Mediterranean-climate conditions, covers a period of about ten years of operation (2009–2018), with particular attention to the last ﬁve years. From the analysis, it is possible to conclude that it is dicult to obtain self-consumption schemes and real net zero-energy building conﬁgurations due to the time between energy production and energy use. Considering the experimental data acquired during the 10 years of operation, it was observed that 30% of the energy produced by the PV plant was directly used by the GHP and the other miscellaneous devices during the year (a minimum of 26.5% was recorded in 2014 and a maximum of Buildings 2020, 10, 175 16 of 18 33% was recorded in 2009). A large amount of the energy required by the GHP was imported from the electrical grid, mainly during winter, while an important amount of energy produced by the PV plant (67%–73.5%) was exported or lost, depending on whether or not electricity could be exported to the national electric grid. In the second part of the paper, we concentrated on ﬁnding a solution to mitigate the eects of the time between the electrical production peak and the thermal power demand peak through the reduction in the size of the PV plant, and by the introduction of a storage system. A reduction of about 30%–35% in the peak power of the PV plant (in this case using a plant with a peak power of between 2.2 and 2.6 kW ), combined with using a storage system of 6 kWh capacity, could reduce the need for exporting energy to the grid to almost zero. In this case, the majority of energy produced by the PV plant was directly used by the house appliances and energy was only imported from the grid. Solutions that considered using larger PV plants were also discussed. Excluding the case of a completely self-sucient system (with a PV power of about 36 kW and a storage system of 26 kWh), an interesting compromise can be obtained with a PV plant of a larger size, in particular from 4.5 to 6 kW peak power. In all those cases, a combined use of a storage system of appropriate dimensions would be necessary. For a ﬁnal comparison among the various optimized solutions, an economic analysis is required. Considering the pre-covid-19 pandemic scenario, the plant with a reduced size may be preferred for general development and promotion. Nevertheless, the new ﬁnancial support for renewables applied to buildings (for example, Italy’s recent 110% “Ecobonus” ﬁnancial support mechanism) leads us to consider the beneﬁts of larger sized PV plants. From a critical analysis of the experimental data collected from an existing system and based on our further analytical consideration, we provided some guidelines for the optimal combination of power rating of PV generators and energy capacities of thermal and electrochemical storage systems, and we outlined how to determine the size of the system in order to pursue speciﬁc design objectives. In any case, it will be important to make use of the energy produced, reducing the energy wasted. Author Contributions: A.F. deﬁnes the methodology, A.F. and F.F., conceived and designed the experiments and acquire the data; A.F. developed software and analyze the data, A.F., wrote the paper, A.F. reviewed and edited tbe paper; A.F. supervised the paper and acquire funds for publication. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the University of Pisa (PRA 2018–19, project no. 2018_38). Acknowledgments: Enrico Ciulli of the University of Pisa is acknowledged by the authors for the support given during experimental analysis. Conﬂicts of Interest: The authors declare no conﬂict of interest with respect to the research, authorship, and/or publication of this article. Nomenclature A Surface of the building envelope (m ) ASHP Air source heat pump COP Coecient of performance in cooling mode COP Coecient of performance in heating mode E Electrical energy (kWh) E Electricity produced by the PV plant and directly used in the house (kWh) E Electricity used for the various house devices (kWh) DEV E Electrical energy exported to the grid (kWh) EXP E Electricity used by the ground source heat pump (kWh) GHP E Electrical energy imported from the grid (kWh) IMP E Electricity produced by the PV plant (kWh) PV E Minimum value of the energy monthly produced by the PV plant (kWh) PV,min E Maximum value of the energy month produced by the PV plant (kWh) PV,max E Average value of the energy monthly produced by PV plant in the years (kWh) PV,avg E Electricity used in the house considering GHP and other devices (kWh) TOT Buildings 2020, 10, 175 17 of 18 Ew Electricity produced by PV plant, not used and wasted (kWh) GHE Ground neat exchanger GHP Ground (source) heat pump H Annual solar irradiance (kWh/m ) SN HP Heat pump I Short circuit current (A) SC I Current of maximum power (A) mpp I Index of utilization of energy produced with the PV plant n Number of hours in which a well-deﬁned outside temperature is observed nZEB net zero energy building P Power of PV system PV PV PhotoVoltaic Q Solar gain (kWh) in,solar RES Renewable energy system STC Standard test conditions t time (h) T internal temperature ( C) in T average external temperature ( C) ext UA average value of the transmittance of the building (W/K) V Voltage of maximum power for the PV module (V) mpp V Voltage of open circuit for the PV module (V) op Balance of system eciency for the PV plant BOS eciency of the PV module PV ZEB Zero energy building References 1. Terna: Statistical Data on Electricity in Italy—2006 to 2012: Section Power Plants. Available online: http://www.terna.it/default/home_en/electric_system/statistical_data.aspx (accessed on 17 March 2014). 2. Lund, H. Renewable Energy Systems the Choice and Modeling of 100% Renewable Solutions, 3rd ed.; Elsevier: Amsterdam, The Netherlands, 2010. 3. Franco, A.; Salza, P. Strategies for optimal penetration of intermittent renewables in complex energy systems based on techno-operational objectives. Renew. Energy 2011, 36, 743–753. [CrossRef] 4. Franco, A.; Fantozzi, F. Experimental analysis of a self consumption strategy for residential building: The integration of PV system and geothermal heat pump. Renew. Energy 2016, 86, 1075–1085. [CrossRef] 5. Antonelli, M.; Desideri, U.; Franco, A. Eects of large scale penetration of renewables: The Italian case in the years 2008–2015. Renew. Sustain. Energy Rev. 2018, 81, 3090–3100. [CrossRef] 6. Facci, A.L.; Krastev, V.K.; Falcucci, G.; Ubertini, S. Smart integration of photovoltaic production, heat pump and thermal energy storage in residential applications. Sol. Energy 2019, 192, 133–143. [CrossRef] 7. Vaishak, S.; Bhale, P.V. Photovoltaic/thermal-solar assisted heat pump system: Current status and future prospects. Sol. Energy 2019, 189, 268–284. [CrossRef] 8. Fischer, D.; Madani, H. On heat pumps in smart grids: A review. Renew. Sustain. Energy Rev. 2017, 70, 342–357. [CrossRef] 9. Ratnam, E.L.; Weller, S.R.; Kellett, C.M. An optimization-based approach to scheduling residential battery storage with solar PV: Assessing customer beneﬁt. Renew. Energy 2015, 75, 123–134. [CrossRef] 10. Roselli, C.; Diglio, G.; Sasso, M.; Tariello, F. A novel energy index to assess the impact of a solar PV-based ground source heat pump on the power grid. Renew. Energy 2019, 143, 488–500. [CrossRef] 11. Carli, D.; Ruggeri, M.; Bottarelli, M.; Mazzer, M. Grid assisted photovoltaic power supply to improve self sustainability of ground-source heat pump systems. In Proceedings of the 2013 International Conference on Industrial Technology (ICIT), Cape Town, South Africa, 25–28 February 2013; IEEE: Cape Town, South Africa, 2013; pp. 1579–1584. 12. Manfroi, G.; Maistrello, M.; Tagliabue, L.C. Synergy of geothermal heat pumps and PV plant for buildings block. In Proceedings of the 2011 International Conference on Clean Electrical Power (ICCEP), Ischia, Italy, 14–16 June 2011; IEEE: Ischia, Italy, 2011; pp. 466–473. Buildings 2020, 10, 175 18 of 18 13. Poppi, S.; Sommerfeldt, N.; Bales, C.; Madani, H.; Lundqvist, P. Techno-economic review of solar heat pump systems for residential heating applications. Renew. Sustain. Energy Rev. 2018, 81, 22–32. [CrossRef] 14. Lazzarin, R.; Noro, M. Photovoltaic/Thermal (PV/T)/ground dual source heat pump: Optimum energy and economic sizing based on performance analysis. Energy Build. 2020, 211, 109800. 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[CrossRef] 20. Garcia, J.F.; Kranzl, L. Ambition levels of nearly Zero Energy Buildings (nZEB) deﬁnitions: An approach for cross-country comparison. Buildings 2018, 8, 143. [CrossRef] 21. Tamašauskas, R.; Šadauskiene, ˙ J.; Krawczyk, D.A.; Medeliene, ˙ V. Analysis of primary energy factors from photovoltaic systems for a nearly Zero Energy Building (NZEB): A case study in Lithuania. Energies 2020, 13, 4099. [CrossRef] 22. Ozgener, O.; Hepbasli, A. A review on the energy and exergy analysis of solar assisted heat pump systems. Renew. Sustain. Energy Rev. 2007, 11, 482–496. [CrossRef] 23. Kotarela, F.; Kyritsis, A.; Papanikolaou, N. On the implementation of the nearly Zero Energy Building concept for jointly acting renewables self-consumers in Mediterranean climate conditions. Energies 2020, 13, 1032. [CrossRef] 24. Wang, X.; Xia, L.; Bales, C.; Zhang, X.; Copertaro, B.; Pan, S.; Wu, J. A systematic review of recent air source heat pump (ASHP) systemsassisted by solar thermal, photovoltaic and photovoltaic/thermalsources. Renew. Energy 2020, 146, 2472–2487. [CrossRef] 25. ECOFYS. Financing Renewable Energy in the European Energy Market; ECOFYS: Utrecht, The Netherlands, 2011; Available online: https://ec.europa.eu/energy/sites/ener/ﬁles/documents/2011_ﬁnancing_renewable.pdf (accessed on 3 October 2020). 26. Nykamp, S.; Andor, M.; Hurink, J.L. ‘Standard’ incentive regulation hinders the integration of renewable energy generation. Energy Policy 2012, 47, 222–237. [CrossRef] 27. Cao, S.; Hasan, A.; Sirén, K. Matching analysis for on-site hybrid renewable energy systems of oce buildings with extended indices. Appl. Energy 2014, 113, 230–247. [CrossRef] 28. Gigoni, L.; Betti, A.; Crisostomi, E.; Franco, A.; Tucci, M.; Bizzarri, F.; Mucci, D. Day-ahead hourly forecasting of power generation from photovoltaic plants. IEEE Trans. Sustain. Energy 2017, 9, 831–842. [CrossRef] © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Optimal Sizing of Solar-Assisted Heat Pump Systems for Residential Buildings

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DOI: 10.3390/buildings10100175
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