Air-to-water heat pumps: the impact of climate, compressor technology, water output temperature and sizing on the seasonal coefficient of performance for heating

Air-to-water heat pumps (AWHPs) are devices that will broadly replace heating systems based on fossil fuels. The performance of AWHPs in heating operation is strongly influenced by various parameters such as the climate of the location, the hot water output temperature to the heat emitters, the oversizing or downsizing of the HP with respect to the thermal load of the building, the control system, and the heat pump’s compressor technology (fixed-capacity and inverter-driven HPs). The aim of this work is to study the impact of those parameters on the seasonal efficiency for heating of AWHPs in buildings at representative cities of the four Greek climatic zones. The active mode seasonal coefficient of performance (SCOPon) was estimated in various case studies, with the method proposed in European standard EN 14825, by using climatic data of the four locations. The results show the positive effect of lowering the water supply temperature to the heating system and of using inverter technology and control system with compensation on AWHPs efficiency. Additionally, it is demonstrated that the optimal performance of inverter-driven HPs appears when they are sized at the design outdoor air temperature, while in the case of the fixed-capacity HPs when they are down-sized.