Effects of refrigerant charge in the output of a CO2 heat pump

Abstract

Many of the refrigerants currently being used in heating, ventilation, air conditioning and refrigeration systems have high global warming potential. One potential, environmentally friendly replacement refrigerant is carbon dioxide (CO2). In this study, a CO2 trans-critical water to water test bed was used to study the performance of a typical heat pump by varying the amount of refrigerant charge. Initial experimental results and thermo-physical properties were analysed by NIST REFROP and plotted in a temperature-entropy (T-S). The heat energy output in the gas cooler was compared to the mechanical work input in the compressor to obtain the efficiency of the system in terms of coefficient of performance (COP). It was observed that the refrigerant charge has a great effect on the heat output and efficiency of the system. Furthermore, the maximum amount of charge the system can take is not necessarily the optimum amount of charge. This was related to the effect of the charge amount on the equipment involved and flow process variables like pressure loss and friction which affect the heat transfer process.