International inter-laboratory comparison of solar heat gain coefficient of building-integrated photovoltaic modules results of tests with or without power generation and tests with PV cell coverage ratios

Abstract

The solar heat gain coefficient is one of the important indicators to evaluate the performance of the building envelope components. In a BIPV module, secondary heat transfer to the indoors is reduced, compared to conventional glazing, due to the shielding effect of the PV cells and energy conversion by power generation. There is a difference in SHGC between power-generating (MPP) and effectively open-circuit (OC) states. In this paper, in order to prepare a calorimetric SHGC evaluation methodology for BIPV modules, we documented the current status of the experimental calorimetric test methods and test apparatus adopted by test laboratories in four different countries and identified the differences. Differences between the test laboratories were found in to the applied test methods, solar simulator types, spectral distributions, DC or AC power supplies for the solar simulator, irradiation inhomogeneity on the light-receiving surface of the test sample, temperature conditions between the test chamber and the metering box, and surface heat transfer coefficients. Especially, the round-robin test results obtained in an interlaboratory comparison clarified that the differences in the characteristics of the Calorimetic Hot Box and the Heat Flux Metering methodologies with a cooled plate, the differences in irradiation inhomogeneity on the light-receiving surface of the test sample and differences in surface heat transfer coefficients significantly affect the SHGC evaluation for BIPV modules. In addition, for the SHGC test in the MPP state, it was confirmed that an absolute SHGC reduction effect of 0.02 to 0.04 was obtained for a PV laminate with 81 % cell coverage, for all SHGC test methods and test apparatus. Pe characterises the effect of converting part of the sunlight absorbed by the PV cell into electric power and extracting it from the BIPV module, reducing the heat re-radiated indoors compared to the OC state. Due to this mechanism, the reduction effect Pe always occurs during power generation, regardless of the type of PV cell technology and whether the PV cells are opaque or transparent. In addition, the decrease in SHGC for a given glazing configuration was found to be proportional to the increase in PV cell coverage ratio. SHGC tests with four different cell coverage ratios confirmed that the relationship between PV cell coverage ratio and SHGC is linear to a high degree of accuracy when no electric power is extracted (OC state), provided that the incident radiation is spatially homogeneous. The results obtained will be useful when proposing a calorimetric SHGC evaluation methodology for international standardization, as they document the differences due to a range of test facilities and testing conditions, differences caused by power generation and extraction, and the effect of varying PV cell coverage ratios. The insights gained were valuable in identifying which aspects of test methodology and boundary conditions must be specified with particular attention to detail when calorimetric testing standards are extended to explicitly address the SHGC determination of BIPV modules.