Experiments and dynamic modelling of dry expansion and flooded evaporators in a CO2 solar assisted heat pump

In the present study, a dynamic model of evaporators in a CO2 solar assisted heat pump is presented and experimentally validated. The evaporators are PV-T (photovoltaic-thermal) solar collectors and they can work both in direct expansion mode and flooded mode. Thanks to the PV-T evaporators we convert the solar radiation into electricity in the PV cells, ensure the evaporation of the CO2 and cool the solar cells to improve the PV conversion efficiency. The evaporators of the heat pump can work under two operative modes, dry expansion and flooded evaporation. When working in dry-expansion mode, the refrigerant mass flow rate after the throttling valve is directly sent to the solar collectors. Alternatively, when operating in flooded mode, the collectors are fed with saturated liquid refrigerant coming from a low-pressure receiver. In real applications, we can have multiple collectors that are fed in parallel and thus the advantage and the main reason for studying the flooded solar evaporators is that, with this schematic, we can feed them with saturated liquid avoiding possible distribution problems that may occur when distributing the liquid-vapor mixture (direct expansion mode). In this article, the operating mechanism of the heat pump is presented and an experimental comparison between the dry expansion and flooded evaporation is discussed. Since the system operation strongly depends on the solar irradiance and its variation, a comprehensive dynamic numerical model of the system is developed and validated. The results of the model show the regulation mechanism of the evaporators and the whole system and allow to study the solution to possible issues that are encountered in this application.