Blends of CO2 and N2O as working fluids in cascade cycles.

The performance of a cascade refrigeration cycle operated with blends comprising carbon dioxide (CO2 or R744), nitrous oxide (N2O or R744A), with ethane (C2H6 or R170) as the low-temperature working fluid, was analyzed over a wide range of compositions and intermediate temperatures. To conduct the analysis, a property software based on the Carnahan-Starling-De Santis (CSD) equation of state was developed, using binary interaction parameters derived from our experimental data for CO2 + N2O and N2O + C2H6 systems, and from VLE data for the amply-studied CO2 + C2H6 system. The properties of the investigated blends were used to simulate the behaviour of a cascade cycle using ammonia (R717) as the high-temperature-circuit working fluid at the condensing temperature of 30 °C and operating at a low-temperature-circuit evaporating temperature of -70 °C. In the simulation, the value of improving the COP by using a suction-liquid heat exchanger on the low-temperature side of the circuit was also studied, as were the sensitivity of the COP to subcooling on the low-temperature condenser, and the temperature differences between the high-temperature evaporator and the low-temperature condenser. The results show that the N2O blends are an attractive option for the low-temperature circuit of cascade systems operating at temperatures approaching 200 K.