Performance and control of a CO2 dual source solar assisted heat pump with a photovoltaic-thermal evaporator

This work presents a double source transcritical CO2 heat pump that features hybrid photovoltaic-thermal collectors as evaporators. The heat pump can work in two different modes using alternatively air or solar radiation as thermal sources. In air-source mode, a conventional finned coil heat exchanger is used as the evaporator, whereas in solar-source mode, an innovative solar hybrid collector (photovoltaic-thermal) is used as the direct expansion evaporator. The photovoltaic-thermal collector has a double effect: it allows to evaporate the CO2 that flows in the tubes and to cool down the photovoltaic cells improving the photovoltaic conversion efficiency. An experimental comparison between the two operative modes is presented in terms of evaporation temperature and performance coefficient, which can increase up to 30% in the solar-source mode as compared to the air-source mode. Moreover, due to the cooling of the photovoltaic cells, the photovoltaic electric production can be 10 % higher compared to that estimated if the cells were not cooled. The present design of the refrigerant loop allows to avoid the presence of superheated vapor at the outlet of the evaporators and thus it regulates the evaporation pressure always at the highest possible value using efficiently all the available evaporator area to vaporize the CO2 mass flow rate. The present measurements have been used to develop and validate a mathematical model of the heat pump, able to predict the performance when varying the ambient conditions or the photovoltaic-thermal evaporator area. The iso-COP maps obtained from the model can be used for the switching strategy between the air-source and the solar-source but also to determine the minimum evaporator area in the design of the system.