Research on refrigerant flow characteristics and performance of a multi-functional heat pump system

Multi-functional domestic heat pump systems (MDHPS) can reduce operating costs, lessen thermal pollution to the environment, and improve equipment utilization rate. Compared with the traditional heat pump system, the MDHPS has a wider range and more dramatic changes in operating conditions. This paper incorporates the refrigerant flow characteristic and performance analysis of an electronic expansion valve (EEV)-controlled MDHPS which operating in four typical modes (sanitary water heating mode, space cooling mode, space cooling combined with condensing heat recovery mode, radiant floor heating mode). The calculating results demonstrate that there are considerable differences in the refrigerant mass flow range when the MDHPS operates in the different modes. Moreover, sudden changes in working conditions due to mode switching easily result in large-scale fluctuations of system parameters. Therefore, to ensure the MDHPS to work more reliably and efficiently, a control scheme of the EEV specialized for the MDHPS is proposed and applied in the presented experimental system. The experimental results indicate that the system is able to perform efficiently and stably under various conditions by this control method. In the sanitary water heating mode, the average heating coefficient of performance (COP) reaches 2.2, 3.2 and 4.1 when the ambient temperature is 5 °C, 20 °C and 35 °C, respectively. In the condensing heat recovery mode, besides yielding free sanitary hot water, the average cooling COP can be improved 9% when the water temperature ranges from 30 °C to 55 °C. In the radiant floor heating mode, the COP is 3.3 at 6 °C ambient temperature.

Research highlights
► We compare refrigerant flow ranges under different working modes for a multi-functional heat pump.
► We propose control methods of electronic expansion valve specialized for such a system.
► Although some loss in the cooling capacity maybe occur in mode 3, average cooling COP can be improved.