Compressor control strategy for CO2 heat pump toward efficient and stable domestic hot water production: An experimental investigation

The CO2 heat pump has proven to be a low-carbon solution for domestic hot water production owing to its eco-friendliness and high efficiency. However, it requires real-time control to maintain high performance due to the special characteristics of the trans-critical cycle. Most existing studies on the control of the CO2 heat pump focus on the compressor discharge pressure, but there has been little concern about the rotation speed. The influence of rotation speed on the CO2 heat pump is still not fully understood. Moreover, the synergistic control strategy for rotation speed and discharge pressure has rarely been investigated. Given this, experiments are conducted in this paper to address the issues. The experiments verified that increasing the rotation speed could greatly increase the heating capacity (Qh) with a slight decrease in the heating coefficient of performance (COPh). Qh is increased by a maximum of 10.8 %, and COPh is decreased by a maximum of 0.9 %. The compressor rotation speed exhibited a significant linear variation rule at different return water and heat source temperatures, with a correlation co-efficient of at least 0.98. The outlet water temperature is well maintained with a deviation of no more than 0.5 degrees C by the control strategy developed. Furthermore, the average Qh could be increased by up to 6.6 % with no degradation in the average COPh. The present study may help to understand the working characteristics of the CO2 heat pump at variable rotation speeds and may provide engineers with a simple and economical compressor control strategy.