Experimental study on operating characteristics and control strategy of CO2 heat pump in circulating heating mode

Abstract

The CO2 heat pump is a promising low carbon technology for domestic hot water production due to its potential heating advantages and the natural properties of CO2. However, the change pattern of operational parameters, heating time, and energy efficiency of CO2 heat pump in circulating heating mode remains to be fully revealed. Meanwhile, the matching control strategies to improve the system performance have not been developed. Firstly, we experimentally revealed the operating characteristics of CO2 heat pump in circulating heating mode. Results demonstrated that it is quite challenging to simultaneously maintain the lower input power, shorter heating time, and higher coefficient of performance (COP) without taking control measures. It is found that the circulating heating process could be divided into stable and upward periods, depending on the water temperature at the inlet of gas cooler. The deterioration of the system performance appears during the upward period, where the instantaneous COP drops from 4.1 to 3.1. This is because input power rises, while the heating capacity drops as the water temperature at the inlet of gas cooler increases. Secondly, we experimentally developed a matching control strategy to improve the system performance during the upward period. The control method is the correlation of optimal electronic expansion valve opening, and the regulation frequency is regulating for 1 °C per rise of the water temperature at the inlet of gas cooler. As a result, the overall COP increased by 3.71%, and the overall input power reduced by 3.74% while the heating time remained constant.