Experimental demonstration of an air-source heat pump application using an integrated phase change material storage as a desuperheater for domestic hot water generation

Heat pumps with a three-media refrigerant/phase change material (PCM) water heat exchanger (RPW-HEX), integrated in the hot superheated section after the compressor, have a promising potential for electric energy savings. The RPW-HEX operates as a desuperheater that stores the sensible energy provided by the hot gas during heating and cooling operation for later heat transfer to domestic hot water (DHW) storage devices. So far, such a system has not yet been implemented and analysed in an overall system suitable for heating, cooling and DHW generation. In the present work, the operation of a prototypical heat pump with integrated RPW-HEX connected to three artificial apartments, was demonstrated in the laboratory under controlled ambient conditions. For this purpose, two RPW-HEX modules with a total storage capacity of about 5 kWh were integrated into an R32 air-source heat pump with a heating power of about 7.7 kW at -10 degrees C ambient temperature and a feed water temperature of 45 degrees C. Technical feasibility and operation with rule-based control strategies have been successfully demonstrated for realistic use cases. Besides individual tests, the heat pump was operated over 48 hours with and without RPW-HEX at an ambient temperature of -2 degrees C, a feed water temperature for the heating system of 40 degrees C. Both systems, achieved the same average COP, but the system with RPW-HEX was able to provide a 10 K higher average feed water temperature for DHW generation compared to the system without RPW-HEX. For the same feed water temperatures for DHW generation, an enhancement of about 3.1% of the average COP can be expected with the current system. This is about 60% of the theoretically possible value. Furthermore, for a low feed water temperature for heating of about 32 degrees C at -2 degrees C, an enhancement of the average COP up to 9.4% can be expected for the analysed heating and DHW scenario with an improved design.