Comparison Between a Single Stage and a Two Stage Air to Water Domestic Heat Pump with one Variable Speed Compressor.

Replacing conventional boilers by air to water heat pumps represents a major market opportunity and a contribution to more sustainable heating approaches. However, such retrofit situation impose tough technical requirements which must be met in order to demonstrate reasonable efficiencies at high temperature lifts together with a moderate heat rate decrease at lower air temperatures. During the last decade major improvements in heat exchanger and compressor efficiencies have permitted significant COP increase in single stage domestic heat pumps, bringing the technology to a point where further progress can only be achieved with more complex cycles. Two stage cycles common in large heat pump or refrigeration systems have not been exploited so far in the domestic heat pump range, although they potentially meet some of the requirements mentioned above. This paper describes the results obtained with a laboratory prototype allowing operation with both single and two stage with economizer cycles, as well as the speed control of one of the two compressors. At A-7/W50 the two stage configuration outperformed the single stage by up to 15% in terms of COP and up to more than 70% in terms of heat supply. Exergetic efficiencies (discounted for fan power and defrosting) up to 49% at A-10/W60 have been measured.