Theoretical and experimental analysis of a CO2 heat pump for domestic hot water

This paper describes the development of a CO2 air/water heat pump for the production of tap hot water in a residential building. The basic design consists of a single-stage piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator and a low pressure receiver. The heat pump is combined with a storage tank designed to maintain internal water stratification. The gas cooler pressure optimisation in the case of fixed water delivery temperature was theoretically analysed. A new control method for the upper cycle pressure was developed to maximise the COP of the heat pump, while the water mass flow was adjusted to maintain the set water temperature at the gas cooler exit. Before commissioning, the heat pump was factory tested to verify its energy performance and to validate the high pressure control logic.