Experimental Investigation on the Influence of Refrigerant Charge on the Performance of Trans-critical CO2 Water-Water Heat Pump

For the environmentally aware, natural refrigerant such as CO2 is recommended and pursued by researchers for refrigerators. In this paper, a transcritical CO2 heat pump system was set up to investigate the influence of refrigerant charge on the performance of a small-sized heat pump water heater. The transcritical CO2 heat pump system was composed of a rotary compressor, a tube-in-tube evaporator and gas cooler, an electronic expansion valve (EEV), and an internal heat exchanger (IHX). Based on the experimental results, the effects of refrigerant charge on the key pressures, mass flow rate, power consumption, discharge temperature, superheat, heating capacity and coefficient of performance (COP) were analyzed with different EEV openings. The pressure in the evaporator and gascooler increased with the rise of refrigerant charge. The mass flow rate increased with the refrigerant charge range from 0.6kg to 2.2kg. The experimental results showed that the COP of the system was strongly related to CO2 mass charge and had a maximum value at a specific CO2 mass charge. Undercharged CO2 systems could result in a fast decrease of the heating capacity and COP. The COP reduced more significantly at undercharged conditions than at overcharged conditions as the refrigerant charge further deviated from optimal charge. As EEV opening increased from 40% to 60%, the optimal COP decreased. And with the increase of EEV opening, the corresponding charge amount of optimal COP reduced.