Theoretical And Experimental Study Of Thermal And Electrical Perfor-mance Of Semi-transparent Photovoltaic Windows Operating At Different States

Semi-transparent photovoltaic(STPV)window as a building energy saving technology,which is important for achieving carbon peaking and carbon neutrality goals.STPV windows have a complex optical-thermalelectrical coupling mechanism,and the optical,thermal and electrical performance interact with each other.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,and have been mentioned in EN50583:2016 and IEC 63092:2020.For example,the thermal performance simulation of STPV windows operating at the open circuit(OC)state was always considered as that of STPV windows operating at the maximum power point(MPP)state.Thus,it is urgent to propose a thermoelectric decoupling method for correctly simulating the thermal and electrical performance of STPV windows.The decoupled thermal and electrical performance calculation corresponds to MPP state,and the non-decoupled thermal performance calculation corresponds to OC state.However,the non-decoupled electrical performance calculation only corresponds to the case which ignoring the coupling 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)device,respectively.The EQE was used to calculate the spectral photoelectric conversion efficiency(SPCE)of the STPV windows.Then,the SPCE was subtracted from the original absorptance and added to the front reflectance at the corresponding wavelength,thereby decoupling the thermal and electrical performance.Thus,the decoupling of the optical,thermal,and electrical parameters of the STPV windows was achieved.To verify the importance of the decoupling,the thermal and electrical performances of STPV windows with and without decoupling were investigated using Energy Plus and WINDOW programs.It was found that in the non-decoupled 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 of the solar cells,the PV generation of decoupled case is only about 2% higher than that of non-decoupled case.In addition,this paper analyzes the influence of solar cell coverage ratio(CR)and solar cell types on the difference in solar heat gain coefficient(SHGC)with and without decoupling.For the monocrystalline silicon STPV windows analyzed in this paper,it was found that the SHGC of the non-decoupled case is 9.28% higher than that of the decoupled case,when the solar cell coverage ratio is 70%.Besides,For the analyzed PV windows of three different PV cells types,the ranking of the influence of the decoupled method on SHGC is as follows:amorphous silicon STPV window<Cd Te STPV window<silicon STPV window.The maximum relative difference of SHGC between coupled and decoupled case,occurs in silicon STPV windows,which can reach 13%.To verify the authenticity of the above simulation results,outdoor experimental tests for STPV windows were carried out.Before the experiment,9 main parameters which affecting the SHGC of STPV windows were extracted through theoretical analysis.Then,A simple outdoor experimental method for testing SHGC has been proposed in this paper.Thus,the comparative studies of SHGC of STPV windows,which operating at different states,were carried out.It was found that the SHGC of STPV windows operating at OC condition is significantly higher than that of STPV windows operating at maximum power point,and the maximum difference in SHGC between the two can reach up to 11.5%.Secondly,the reliability of the outdoor experimental method is proved through uncertainty analysis,and its uncertainty is 5.9%.Moreover,the decoupled calculation method provides a theoretical basis for more accurate simulation and overall performance evaluation of STPV windows.The outdoor experimental method proposed in this paper will provide guidance for the thermal performance tests of photovoltaic windows operating in real conditions.The results of this paper will push forward the development of building integrated photovoltaics technologies.