Investigation of Inorganic Phase Change Material for a Semi-Transparent Photovoltaic (STPV) Module

The semi-transparent photovoltaic (STPV) module is an emerging technology to harness the solar energy in the building. Nowadays, buildings are turning from energy consumers to energy producers due to the integration of the STPV module on the building envelopes and facades. In this research, the STPV module was integrated on the rooftop window of the experimental room at Kovilpatti (9◦10′0′′ N, 77◦52′0′′ E), Tamil Nadu, India. The performance of the STPV modules varies with respect to the geographical location, incident solar radiation, and surface temperature of the module. The surface temperature of the STPV module was regulated by the introduction of the mixture of graphene oxide and sodium sulphate decahydrate (Na2SO4·10H2O). The various concentration of the graphene oxide was mixed together with the Na2SO4·10H2O to enhance the thermal conductivity. The thermal conductivity of the mixture 0.3 concentration was found to be optimum from the analysis. The instantaneous peak temperature of the semi-transparent photovoltaic phase change material (STPV-PCM) module was reduced to 9 ◦C during summer compared to the reference STPV. At the same time, the energy conversion efficiency was increased by up to 9.4% compared to the conventional STPV module. Due to the incorporation of the graphene oxide and Na2SO4·10H2O, the daily output power production of the STPV module was improved by 12.16%.


Introduction
The depletion of fossil fuel leads to a search for renewable energy sources (RES) such as solar, wind, and fuel cells. Earlier, these energy sources were widely used for a large scale power generation sector. However, in recent years, due to the advancements seen in the distributed energy generation, the use of these RES has become much popular in various sectors, especially the integrated application in residential and other commercial buildings [1]. The RES integration into the building converts the building from energy consumer to energy producer [2]. A building can be considered a zero hexahydrate. It was concluded that the Copper Indium diselenide module surface temperature was reduced to 9 • C [26]. The incorporation of the PCM and finned PCM in the PV module has enhanced the performance of the module. The PV-PCM system has improved the power production by 7% of daily production while 8% by finned PCM [27]. The feasibility of the incorporation of the PCM on the STPV module was analyzed and concluded that the incorporation of the PCM enhanced the power production and life span of the PV module. It is a cost-effective technique to cool the PV module [28].
The study was carried out to improve the performance of the BIPV system by employing the eutectic binary mixture of PCM which was developed using Na 2 SO 4 ·10H 2 O and N 2 O 6 Zn·6H 2 O to regulate the surface temperature of the BIPV module. It was concluded that the instantaneous surface temperature of the BIPV module was reduced to 12 • C [29]. The building integrated solar water heating system was investigated by incorporating the nano-SiO 2 composite PCM for thermal management. The results reported that the nanocomposite PCM enhanced the system efficiency by 16.05% [30,31]. The paraffin wax was incorporated on the PV module to regulate the surface temperature. It was concluded that the electrical efficiency was improved by 6.34% [32].
According to the literature study, it is evident that the performance of the STPV module gets affected by the rise in temperature and incident solar radiation. The studies of [20,29] reported that Glauber salt has the potential to regulate the surface temperature of the STPV modules. Moreover, it was suggested that the increase in thermal conductivity could enhance the performance of the PCM. The inorganic PCM with the graphene oxide nanoparticles is needed for the hot and humid climatic conditions in the Southern part of India. The graphene oxide is capable to enhance the thermal conductivity of the inorganic PCM.
In this work, Glauber salt was mixed with graphene oxide nanoparticles to augment the thermal conductivity of the PCM. The developed PCM is incorporated in the STPV module and tested in the experimental room at the Kovilpatti region, Tamil Nadu, India, where the climatic condition is hot and humid.

Materials
The experimentation was carried out at the hot and humid climatic condition of the Kovilpatti region, Tamil Nadu, India. The schematic view of the STPV module is shown in Figure 1.
Energies 2020, 13, x FOR PEER REVIEW 3 of 12 reduced to 9 °C [26]. The incorporation of the PCM and finned PCM in the PV module has enhanced the performance of the module. The PV-PCM system has improved the power production by 7% of daily production while 8% by finned PCM [27]. The feasibility of the incorporation of the PCM on the STPV module was analyzed and concluded that the incorporation of the PCM enhanced the power production and life span of the PV module. It is a cost-effective technique to cool the PV module [28].
The study was carried out to improve the performance of the BIPV system by employing the eutectic binary mixture of PCM which was developed using Na2SO4·10H2O and N2O6Zn·6H2O to regulate the surface temperature of the BIPV module. It was concluded that the instantaneous surface temperature of the BIPV module was reduced to 12 °C [29]. The building integrated solar water heating system was investigated by incorporating the nano-SiO2 composite PCM for thermal management. The results reported that the nanocomposite PCM enhanced the system efficiency by 16.05% [30][31]. The paraffin wax was incorporated on the PV module to regulate the surface temperature. It was concluded that the electrical efficiency was improved by 6.34% [32]. According to the literature study, it is evident that the performance of the STPV module gets affected by the rise in temperature and incident solar radiation. The studies of [20,29] reported that Glauber salt has the potential to regulate the surface temperature of the STPV modules. Moreover, it was suggested that the increase in thermal conductivity could enhance the performance of the PCM. The inorganic PCM with the graphene oxide nanoparticles is needed for the hot and humid climatic conditions in the Southern part of India. The graphene oxide is capable to enhance the thermal conductivity of the inorganic PCM.
In this work, Glauber salt was mixed with graphene oxide nanoparticles to augment the thermal conductivity of the PCM. The developed PCM is incorporated in the STPV module and tested in the experimental room at the Kovilpatti region, Tamil Nadu, India, where the climatic condition is hot and humid.

Materials
The experimentation was carried out at the hot and humid climatic condition of the Kovilpatti region, Tamil Nadu, India. The schematic view of the STPV module is shown in Figure 1. The selection of PCM is the major criterion in the passive PCM cooling, and for this reason, we analyzed the ambient temperatures at the experimental location. The PCM melting point was selected as 32 °C as per the observed range variation in ambient temperature, i.e., from 29 °C to 36 °C. The The selection of PCM is the major criterion in the passive PCM cooling, and for this reason, we analyzed the ambient temperatures at the experimental location. The PCM melting point was selected as 32 • C as per the observed range variation in ambient temperature, i.e., from 29 • C to 36 • C. The inorganic PCM was selected because its melting point is lower and has a higher latent heat. Hence, the inorganic Glauber salt (Na 2 SO 4 ·10H 2 O) with a melting temperature of 32 • C was selected. The graphene oxide nanoparticle was mixed using a sonicator with Na 2 SO 4 ·10H 2 O to form a mixture that enhances the thermal conductivity. The sonification was carried out at 40 kHz to prevent the sedimentation of the graphene oxide in the Glauber salt. The prepared nano-PCM was tested for the thermal conductivity to confirm the enhancement of thermal conductivity compared to that of PCM. The thermal conductivity and specific heat capacity of nano-PCM were assessed using a decagon make KD2-Pro type thermal analyzer. The stability and sedimentation of the nano-PCM dispersion were analyzed using the gravity sedimentation method [33]. The thermophysical properties of the Glauber salt and graphene oxide are listed in Table 1.

Experimentation
The experimentation was conducted during May 2019, placing the STPV at the rooftop window of the experimental room at National Engineering College, Kovilpatti, Tamil Nadu, India. The linear dimension of the experimental room length and height was 6 m and 3 m, respectively. Two STPV modules were fabricated with a length, breadth, and width of 0.45 m, 0.70 m, and 0.01 m, respectively. Six series connected spaced type polycrystalline solar cells were sandwiched between the two-glass to form the STPV module. The other module (STPV-PCM) was fabricated by keeping the provision of the PCM incorporation at the rear side of every cell. The STPV system was monitored from 6:00 to 18:00. The voltage, current, and surface temperature parameters of both modules were measured every 60 min. The various temperatures were measured to analyze the thermal performance. The specification of the STPV module is listed in Table 2. The STPV module with and without PCM is shown in the figure photographic view of the STPV, and the front and rearview of the STPV-PCM systems are shown in Figures 2 and 3, respectively. The experimental room was fabricated to test the STPV module by placing them on the rooftop window as shown in Figure 4. The instrument used for the analysis is presented in Table 3.

Results and Discussion
The performance of the STPV module was analyzed by integrating it on the building roof. The various parameters were analyzed, such as temperature, voltage, and current of the STPV module. To regulate the surface temperature of the STPV module, Glauber salt with the nanocomposite of graphene oxide was added. The addition of graphene oxide into Glauber salt enhanced the thermal conductivity of the salt. The various concentrations (0.1, 0.2, 0.3, and 0.4) of the graphene oxide was mixed together. The thermal conductivity of the Glauber salt mixture was measured. In this mixed proportion, 0.3% of the concentration mixture was found to be the optimum in the enhanced thermal conductivity. The thermal conductivity of the mixture without adding the nanocomposite PCM was 0.51 W/mK. The addition of the concentration mixture and the enhancement is shown in Figure 5. The increase in the concentration of the nanoparticles above 0.3% was found to be not suitable in the thermal conductivity as well as the sedimentation, which was observed in thermal cycling. The thermal conductivity of the material reached a maximum of 0.94 W/mK for the concentration of 0.3% of the weight, while the minimum was 0.75 W/mK. The Differential scanning calorimetry curve for the sodium sulphate decahydrate is shown in the Figure 6.

Results and Discussion
The performance of the STPV module was analyzed by integrating it on the building roof. The various parameters were analyzed, such as temperature, voltage, and current of the STPV module. To regulate the surface temperature of the STPV module, Glauber salt with the nanocomposite of graphene oxide was added. The addition of graphene oxide into Glauber salt enhanced the thermal conductivity of the salt. The various concentrations (0.1, 0.2, 0.3, and 0.4) of the graphene oxide was mixed together. The thermal conductivity of the Glauber salt mixture was measured. In this mixed proportion, 0.3% of the concentration mixture was found to be the optimum in the enhanced thermal conductivity. The thermal conductivity of the mixture without adding the nanocomposite PCM was 0.51 W/mK. The addition of the concentration mixture and the enhancement is shown in Figure 5. The increase in the concentration of the nanoparticles above 0.3% was found to be not suitable in the thermal conductivity as well as the sedimentation, which was observed in thermal cycling. The thermal conductivity of the material reached a maximum of 0.94 W/mK for the concentration of 0.3% of the weight, while the minimum was 0.75 W/mK. The Differential scanning calorimetry curve for the sodium sulphate decahydrate is shown in the Figure 6.

Results and Discussion
The performance of the STPV module was analyzed by integrating it on the building roof. The various parameters were analyzed, such as temperature, voltage, and current of the STPV module. To regulate the surface temperature of the STPV module, Glauber salt with the nanocomposite of graphene oxide was added. The addition of graphene oxide into Glauber salt enhanced the thermal conductivity of the salt. The various concentrations (0.1, 0.2, 0.3, and 0.4) of the graphene oxide was mixed together. The thermal conductivity of the Glauber salt mixture was measured. In this mixed proportion, 0.3% of the concentration mixture was found to be the optimum in the enhanced thermal conductivity. The thermal conductivity of the mixture without adding the nanocomposite PCM was 0.51 W/mK. The addition of the concentration mixture and the enhancement is shown in Figure 5. The increase in the concentration of the nanoparticles above 0.3% was found to be not suitable in the thermal conductivity as well as the sedimentation, which was observed in thermal cycling. The thermal conductivity of the material reached a maximum of 0.94 W/mK for the concentration of 0.3% of the weight, while the minimum was 0.75 W/mK. The Differential scanning calorimetry curve for the sodium sulphate decahydrate is shown in the Figure 6.   The STPV module converts the light energy into electrical energy due to the photovoltaic effect. The incident solar radiation plays a vital role in the power production of the STPV module. The solar radiation varies with respect to the geographical conditions and the seasonal variation. The experimentation was carried out during the summer season for two typical days in a location that experiences hot and humid weather. Results were plotted for the hottest day of the month and the average day of the month. In Figure 7, the incident solar radiation is shown, and it reached a maximum of 975 W/m 2 at 13:00 and gradually started to decrease. The maximum average solar radiation was 725 W/m 2 .
Energies 2020, 13, x FOR PEER REVIEW 7 of 12 The STPV module converts the light energy into electrical energy due to the photovoltaic effect. The incident solar radiation plays a vital role in the power production of the STPV module. The solar radiation varies with respect to the geographical conditions and the seasonal variation. The experimentation was carried out during the summer season for two typical days in a location that experiences hot and humid weather. Results were plotted for the hottest day of the month and the average day of the month. In Figure 7, the incident solar radiation is shown, and it reached a maximum of 975 W/m 2 at 13:00 and gradually started to decrease. The maximum average solar radiation was 725 W/m 2 .

Electrical Performance of STPV
The electrical performance of the STPV module is an important parameter that is to be analyzed. For analyzing the electrical performance, a variable electrical load in the form of a rheostat was considered. The voltage and current were measured by varying the load.
The output power was measured and plotted in Figure 8a. The energy conversion efficiency of the module was calculated using the output power and solar radiation of the system, which is shown in Figure 8b. The output power of the STPV module on the hottest day during summer is illustrated in Figure 8a. It is inferred from the graph that the output power increased from the morning and reached its maximum at 13:00 and gradually started decreasing. Due to the photons present in the solar radiation strikes on the surface of the STPV module, flow electron takes place during the bright sunshine hour. For diffused sunlight, the presence of the photons in the sunlight is less. Thus, the energy production from the module became low which is seen after 16:00. The STPV module with PCM yielded a higher output power compared to the module without PCM. STPV module without PCM yielded a maximum of 18.5 W at 13:00 while STPV-PCM obtained a maximum of 20.35 W. The maximum energy conversion efficiency of the module with PCM was 20.43% while the module without PCM reached a maximum of 18.58%. A similar pattern was obtained in a typical day of summer during 15 May 2019 as shown in Figure 9a,b. The module with and without PCM generated 148 Whr and 130 Whr, respectively, for the 24 WP module. The output power of the STPV-PCM module was increased by 12.16% due to the use of PCM.

Electrical Performance of STPV
The electrical performance of the STPV module is an important parameter that is to be analyzed. For analyzing the electrical performance, a variable electrical load in the form of a rheostat was considered. The voltage and current were measured by varying the load.
The output power was measured and plotted in Figure 8a. The energy conversion efficiency of the module was calculated using the output power and solar radiation of the system, which is shown in Figure 8b. The output power of the STPV module on the hottest day during summer is illustrated in Figure 8a. It is inferred from the graph that the output power increased from the morning and reached its maximum at 13:00 and gradually started decreasing. Due to the photons present in the solar radiation strikes on the surface of the STPV module, flow electron takes place during the bright sunshine hour. For diffused sunlight, the presence of the photons in the sunlight is less. Thus, the energy production from the module became low which is seen after 16:00. The STPV module with PCM yielded a higher output power compared to the module without PCM. STPV module without PCM yielded a maximum of 18.5 W at 13:00 while STPV-PCM obtained a maximum of 20.35 W. The maximum energy conversion efficiency of the module with PCM was 20.43% while the module without PCM reached a maximum of 18.58%. A similar pattern was obtained in a typical day of summer during 15 May 2019 as shown in Figure 9a,b. The module with and without PCM generated 148 Whr and 130 Whr, respectively, for the 24 W P module. The output power of the STPV-PCM module was increased by 12.16% due to the use of PCM.

Thermal Performance of STPV
To enhance the thermal performance of the STPV system, PCM was incorporated. The increase in STPV surface temperature reduced the output power of the STPV module due to a rise in the internal charge carrier recombination rate of the PV cell. The flow of electrons reduced and decreased the output power. Hence, the surface temperature of the module was controlled and monitored. In Figure 10a, it is inferred that the surface temperature of the STPV module with the PCM was less than the module without PCM. The PCM absorbed the incident solar light and heat energy on the surface of the module and maintained the surface temperature of the module close to the melting temperature of the PCM. When the energy storing capacity reached the saturation state of the latent heat, the PCM started emitting energy to the atmosphere [20,29]. The instantaneous surface temperature was reduced to 9 °C. The incorporation of the PCM has reduced the surface temperature addition, and thus indirectly improving the electrical performance of the system. In Figure 10a,b, the surface temperature of the STPV module with PCM and without PCM is plotted. The ambient temperature of the location ranged from 29 °C to 35 °C during the hottest day in the summer. The freezing point of the selected PCM is 4 °C. Due to the ambient temperature, active methods are required to regain the energy storage capacity of the PCM. Hence, cold water was flowed rear side of the module to solidify the PCM. The room temperature varied between 27 °C to 33 °C. The maximum PV surface temperature reached 55 °C without the PCM module, while PCM reached 46

Thermal Performance of STPV
To enhance the thermal performance of the STPV system, PCM was incorporated. The increase in STPV surface temperature reduced the output power of the STPV module due to a rise in the internal charge carrier recombination rate of the PV cell. The flow of electrons reduced and decreased the output power. Hence, the surface temperature of the module was controlled and monitored. In Figure 10a, it is inferred that the surface temperature of the STPV module with the PCM was less than the module without PCM. The PCM absorbed the incident solar light and heat energy on the surface of the module and maintained the surface temperature of the module close to the melting temperature of the PCM. When the energy storing capacity reached the saturation state of the latent heat, the PCM started emitting energy to the atmosphere [20,29]. The instantaneous surface temperature was reduced to 9 °C. The incorporation of the PCM has reduced the surface temperature addition, and thus indirectly improving the electrical performance of the system. In Figure 10a,b, the surface temperature of the STPV module with PCM and without PCM is plotted. The ambient temperature of the location ranged from 29 °C to 35 °C during the hottest day in the summer. The freezing point of the selected PCM is 4 °C. Due to the ambient temperature, active methods are required to regain the energy storage capacity of the PCM. Hence, cold water was flowed rear side of the module to solidify the PCM. The room temperature varied between 27 °C to 33 °C. The maximum PV surface temperature reached 55 °C without the PCM module, while PCM reached 46

Thermal Performance of STPV
To enhance the thermal performance of the STPV system, PCM was incorporated. The increase in STPV surface temperature reduced the output power of the STPV module due to a rise in the internal charge carrier recombination rate of the PV cell. The flow of electrons reduced and decreased the output power. Hence, the surface temperature of the module was controlled and monitored. In Figure 10a, it is inferred that the surface temperature of the STPV module with the PCM was less than the module without PCM. The PCM absorbed the incident solar light and heat energy on the surface of the module and maintained the surface temperature of the module close to the melting temperature of the PCM. When the energy storing capacity reached the saturation state of the latent heat, the PCM started emitting energy to the atmosphere [20,29]. The instantaneous surface temperature was reduced to 9 • C. The incorporation of the PCM has reduced the surface temperature addition, and thus indirectly improving the electrical performance of the system. In Figure 10a,b, the surface temperature of the STPV module with PCM and without PCM is plotted. The ambient temperature of the location ranged from 29 • C to 35 • C during the hottest day in the summer. The freezing point of the selected PCM is 4 • C. Due to the ambient temperature, active methods are required to regain the energy storage capacity of the PCM. Hence, cold water was flowed rear side of the module to solidify the PCM. The room temperature varied between 27 • C to 33 • C. The maximum PV surface temperature reached 55 • C without the PCM module, while PCM reached 46 • C. In the hot and humid climatic region, the room temperature was always less than the ambient temperature due to the insulation provided.
Energies 2020, 13, x FOR PEER REVIEW 9 of 12 °C. In the hot and humid climatic region, the room temperature was always less than the ambient temperature due to the insulation provided.
(a) (b) The solar heat gain of the building was obtained using the surface temperature of the window glass and the room temperature. The solar heat energy performance of the double clear glass (DCG) window, STPV window, and STPV-PCM window was compared and is shown in Figure 11. The DCG window allowed solar heat gain with a maximum of 325 W during noon. At the same time, STPV without PCM reached the maximum, i.e., 75 W, while STPV with PCM obtained 70 W. The traditional transparent DCG window allowed higher solar energy into a building's interior because of its higher transmission (>70%) [34][35]. Opaque silicon-based spaced type STPV has potential to restrict the solar energy transmission as silicon has high absorption [36][37]. Due to latent heat capacity, STPV-PCM slowed down the increment of a building's interior room temperature in a higher order compared to other two systems [38][39]. The utilization of STPV with the PCM module will reduce the cooling load of the building by minimizing the solar heat gain into the ZEB buildings.

Conclusions
The main objective of the study was to enhance the performance of the STPV module. In this research, two special prototypes of the STPV modules were fabricated, and among the two, one The solar heat gain of the building was obtained using the surface temperature of the window glass and the room temperature. The solar heat energy performance of the double clear glass (DCG) window, STPV window, and STPV-PCM window was compared and is shown in Figure 11. The DCG window allowed solar heat gain with a maximum of 325 W during noon. At the same time, STPV without PCM reached the maximum, i.e., 75 W, while STPV with PCM obtained 70 W. The traditional transparent DCG window allowed higher solar energy into a building's interior because of its higher transmission (>70%) [34,35]. Opaque silicon-based spaced type STPV has potential to restrict the solar energy transmission as silicon has high absorption [36,37]. Due to latent heat capacity, STPV-PCM slowed down the increment of a building's interior room temperature in a higher order compared to other two systems [38,39]. The utilization of STPV with the PCM module will reduce the cooling load of the building by minimizing the solar heat gain into the ZEB buildings.
Energies 2020, 13, x FOR PEER REVIEW 9 of 12 °C. In the hot and humid climatic region, the room temperature was always less than the ambient temperature due to the insulation provided.
(a) (b) The solar heat gain of the building was obtained using the surface temperature of the window glass and the room temperature. The solar heat energy performance of the double clear glass (DCG) window, STPV window, and STPV-PCM window was compared and is shown in Figure 11. The DCG window allowed solar heat gain with a maximum of 325 W during noon. At the same time, STPV without PCM reached the maximum, i.e., 75 W, while STPV with PCM obtained 70 W. The traditional transparent DCG window allowed higher solar energy into a building's interior because of its higher transmission (>70%) [34][35]. Opaque silicon-based spaced type STPV has potential to restrict the solar energy transmission as silicon has high absorption [36][37]. Due to latent heat capacity, STPV-PCM slowed down the increment of a building's interior room temperature in a higher order compared to other two systems [38][39]. The utilization of STPV with the PCM module will reduce the cooling load of the building by minimizing the solar heat gain into the ZEB buildings.

Conclusions
The main objective of the study was to enhance the performance of the STPV module. In this research, two special prototypes of the STPV modules were fabricated, and among the two, one

Conclusions
The main objective of the study was to enhance the performance of the STPV module. In this research, two special prototypes of the STPV modules were fabricated, and among the two, one module had the provision of incorporating PCM, and the one acted as a reference module. The experimentation was carried out during the summer season of a hot and humid southern India location. After a thorough investigation, the following conclusions were made: • The integration of the PCM reduced the instantaneous peak PV temperature to 9 • C during summer compared to the reference STPV module; • The conversion efficiency of the STPV-PCM was improved by 9.4% compared to the reference STPV module; • The STPV-PCM output power production was increased by 12.16% compared to the reference STPV module; • The instantaneous peak solar heat gain of the building through the DCG window was observed as 325 W, while the STPV-PCM and STPV windows were 70 W and 75 W, respectively.
The STPV module with Glauber salt and graphene oxide is an optimum solution to regulate the surface temperature of the STPV module and enhances the power production. In addition, it reduces the energy consumption in the ZEB buildings.