CO2, heat pump water heater, refrigerant charge, performance
Natural refrigerant CO2 is widely used in refrigeration and heat pump systems. It is of great significance for reductions of ozone depletion and global warming. The efficiency of heat pump system is affected by many factors. The amount of refrigerant charge in the heat pump is a primary parameter that influences the energy efficiency. Undercharge or overcharge of refrigerant degrade its performance and deteriorate system reliability. Therefore, heat pump should be charged with an optimum amount of refrigerant to achieve high performance. However, it is difficult to calculate the optimum charge accurately because of the various components and operating parameters. Â In this paper, a trans-critical CO2Â heat pump system is set up to investigate the influence of refrigerant charge on the performance of a small-sized heat pump water heater. The trans-critical CO2Â heat pump system is composed of a rotary compressor, a tube-in-tube evaporator and gas cooler, an electronic expansion valve (EEV), and an internal heat exchanger (IHX). The objective of this study is to analyze the characteristics of the CO2 heat pump under various refrigerant charging conditions. Therefore, the performance of the CO2Â system was measured and discussed on the basis of refrigerant charge amount. Â Based on the experimental results, the effects of refrigerant charge on the power of system, rejection pressure, evaporating pressure, mass flow rate and coefficient of performance (COP) were analyzed with different EEV openings. The experimental results show that the COP was strongly related to CO2 mass charge and that the formation of the trans-critical cycle depended on CO2 mass charge greatly. The pressures in the gas cooler and the evaporator increased with the rise of refrigerant charge. The compression ratio decreased with the increase of the refrigerant charge. The COP has a maximum value at a specific CO2 mass charge. Undercharged CO2 systems could result in a fast decrease of the heating capacity and COP. However, overcharged CO2 systems could cause an abrupt increase of compressor power consumption. The heating cycle displayed different characteristics with different CO2 mass charges. The optimum CO2 mass charges varied with different EEV openings, and it could be determined according to several parameters.