CO2, R744, Transcritical cycle, Optimum COP
For more than a decade, carbon dioxide (R744) has been revived as a natural environmentally friendly refrigerant. Compared to HFC refrigerants with a global warming potential (GWP) in the order of 1300-1900, R744 has a GWP of 1. As it is relevant for R744 heat pumps, a transcritical cycle has an extra degree of freedom with the gas cooler pressure and outlet temperature being thermodynamically independent of each other. Utilizing MATLAB integrated with the NIST REFPROP thermodynamic database, a single stage transcritical R744 heat pump cycle is modeled and simulated. The isentropic and volumetric efficiency correlations of a commercial semi-hermetic reciprocating compressor are generated as a function of pressure ratio, based on simulated data obtained from the manufacturer’s software. Developed with the cycle model, an optimized control correlation is presented that relates the gas cooler pressure to the gas cooler outlet temperature. The correlations are compared to correlations available in the literature. The range of the gas cooler pressure varies from 75 to 140 bar, the gas cooler outlet temperature from 32 °C to 53 °C, and the evaporation temperature from -30 °C to 15 °C. The developed correlations are for maximizing the coefficient of performance (COP) of the cycle during operation in the range of the above operating conditions.