Dual source (air-solar) heat pump: thermo-economic analysis of sizing factors depending on climate conditions

The building sector, especially for heating and cooling systems, represents approximately the 28% of the global CO2 emissions, and has to be de-carbonized according to the European greenhouse gas emissions target of 2050. Indirect and direct environmental impacts can be reduced by both increasing the use of renewable energy sources and employing natural fluids such as propane or CO2 instead of high-GWP fluids, especially for small capacity machines. In this context, solar-assisted heat pumps have been developed to take advantage of solar energy to increase the evaporation level and therefore the system performance. This paper deals with the modelling of a propane (R290) dual source (air/solar) heat pump in commercial applications, operating in heating mode, in which evaporation can take place alternatively through a fin-and-tube heat exchanger and directly inside solar collectors. Through the model, a control strategy to maximise system performances has been developed, defining a threshold for solar irradiance for switching evaporation through air or solar collectors. Furthermore, a dynamic performance analysis has been carried out in different climates conditions of Naples, Berlin, Bergen, for which higher SCOP up to respectively 14.2%, 8.9% and 7.8% are obtained. Finally, an economic analysis shows that, despite the high conveniences in term of seasonal performances, total costs of solar assisted heat pumps are still higher than only-air evaporation technology, without the employment of any incentives.