Experimental Testing of Solar Photovoltaic/Thermal Collectors for Low Temperature Heat Pump Integration

Abstract: A hybrid photovoltaic/thermal (PVT) collector enables simultaneous electricity and heat production from a single solar module. Integrating PVT collectors with a ground source heat pump (GSHP) improves the seasonal performance of the heating system and prevents from temperature degradation of the ground. However, the integration of PVT and GSHP requires more research, for example, to discover the optimal characteristics of a PVT collector for low operating temperatures and varying weather conditions. Thus, the thermal performance of two commercial unglazed flat-plate PVT collectors was experimentally characterized during low-temperature operation and under dynamic outdoor conditions, including various solar irradiance levels and working fluid flow rates as well as frost formation and condensation on the absorber surface. The thermal performance coefficients were obtained from a simplified version of the ISO 9806:2017 standard steady-state model for solar thermal collectors and used as performance indicators for the tested collectors and for comparison with other PVT collectors found in the literature. During nighttime operation, the PVT collector with a box-channel absorber and the one with a sheet-and-tube absorber have a similar thermal performance, shown as almost the same heat loss coefficients. The effect of flow rate on the heat output of the collectors appears to be negligible when there is no solar irradiance. During humid nights, the heat gains from frost formation and/or condensation on the absorber surface can double the nighttime heat production of the collectors when compared to a dry night with no frost formation or condensation. As the irradiance increases, the box-channel collector starts to outperform the sheet-and-tube collector due to a more effective cooling of the PV module by the box-channel absorber design, which is shown as a higher zero-loss efficiency. The flow rate dependence of the heat output is increased with an increasing irradiance, higher flow rates leading to higher heat outputs. According to the yearly simulations, both tested collectors provide a higher annual heat output under Stockholm weather conditions than the other box-channel and sheet-and-tube collectors available in the Solar Keymark database, when the annual mean fluid temperature is below 7 °C. However, the fin-tube collectors designed for low-temperature heat pump integration outperform the tested collectors on annual basis with mean fluid temperatures below 10 °C.