Investigation of decoupling of thermal and electrical performance of semi-transparent photovoltaic windows based on the external quantum efficiency

Semi-transparent photovoltaic (STPV) windows have great potential in building energy saving due to their power generation and reduction of building energy consumption. STPV windows have a complex optical-thermal-electrical coupling mechanism. However, previous studies have rarely considered the coupling mechanism when simulating the thermal and electrical performance of STPV windows, which has resulted in the power generation being incorrectly treated as waste heat. As has been mentioned in EN 50583:2016 and IEC 63092:2020, the calculation of thermal performance of STPV windows should consider the mechanism that part of absorbed solar energy is transformed into electricity rather than heat. With considering the mechanism, based on the external quantum efficiency (EQE) of solar cells, a decoupled optical, thermal, and electrical performance simulation method for STPV windows was proposed in this study. Firstly, the optical characteristics of STPV windows and the EQE of the integrated solar cells were measured by a spectrophotometer and a monochromatic incident photon-to-electron conversion efficiency (IPCE) testing device, respectively. The EQE was used to calculate the spectral photoelectric conversion efficiency (SPCE) of STPV windows. Then, the SPCE was subtracted from the original absorptance and added to the front reflectance at corresponding wavelength, thereby decoupling the thermal and electrical performance. To verify the importance of the decoupling, the thermal and electrical performance of STPV windows with and without decoupling were investigated using EnergyPlus and WINDOW programs. An outdoor experimental rig was used to validate the simulation results of EnergyPlus. It was found that the decoupled results match well with experimental results. The simulation results show that in the coupled case, the window temperature and cooling load were overestimated, while the heating load was underestimated. However, the impact on power generation was negligible due to the low temperature coefficient of efficiency. Besides, for the analyzed STPV glazing configuration, the solar heat gain coefficient (SHGC) of the coupled case is relatively 9.3% higher than that of the decoupled case, when the solar cell coverage ratio is 70%. Moreover, the decoupled calculation method provides a theoretical basis for more accurate simulation and overall performance evaluation of STPV windows in maximum power point (MPP) state.