The effect of the refrigerant charge amount on single and cascade cycle heat pump systems

The refrigerant charge amount is a key factor for heat pump system optimization, and normally determines the condensing pressure, which affects the subcooling at the exit of the condenser. Heating capacity increases as subcooling increases, however, there is an optimum charge amount for the best coefficient of performance (COP), since the compressor work increases as the charge amount increases. Many studies have focused on optimization of the charge amount for a single-stage heat pump system, while few investigations have been carried out on the charge amount for a cascade heat pump system. The cascade cycle has been suggested as an alternative to a single cycle in extremely cold regions. In this study, air-to-water single and cascade heat pumps were studied to experimentally and numerically verify the effect of the charge amount on the heat pumps. Numerical results showed a good accuracy of experimental results in terms of optimal subcooling.