Experimental Study to Characterize the Performance of Combined Photovoltaic/Thermal Air Collectors

Combined photovoltaic/thermal (PV/T) collectors show great potential for reaching the objective of net-zero energy consumption in buildings, but the number of products on the market is still very limited. One of the reasons for the slow market uptake of PV/T collectors is the absence of standardized methods to characterize their performance. Performance characterization is a challenge for PV/T collectors because of the interaction between the thermal and electrical yield. This study addresses this particular issue for PV/T air collectors used in either closed-loop or open-loop configurations. In particular, it presents the potential of the equivalent cell temperature method to determine the temperature of the PV cells in a PV/T air collector and validates models to predict the thermal performance and cell temperature for this particular type of solar collector. Indoor and outdoor experimental tests were performed on two c-Si unglazed PV/T modules. The indoor part of this procedure provided the thermal diode voltage factor and the open-circuit voltage temperature coefficient, two parameters that are essential in the calculation of the equivalent cell temperature. The outdoor procedure consisted of acquiring simultaneous electrical and thermal measurements at various inlet temperatures and flowrates. For the collector used in a closed-loop configuration, thermal efficiency models using the fluid inlet, outlet, or average temperature in the calculation of the reduced temperature provided similar results. For an open-loop configuration, a thermal efficiency model as a function of the fluid outlet flowrate was found to be more appropriate. Using selection of variable methods, it was found that a multiple linear regression model using the fluid inlet temperature, the irradiance, and the fluid outlet temperature as predictive variables could be used to estimate both the PV module back surface average temperature and the equivalent cell temperature. When using the PV temperature predicted by these models in the electrical efficiency model, both PV temperatures showed similar performance. In collectors where the PV back surface temperature is not accessible for temperature sensors mounting, the equivalent cell temperature provides a valuable alternative to be used as the PV temperature. The PV/T collector thermal and electrical performance in either closed-loop or open-loop configurations was found to be encapsulated with a series of five-plots.