Performance analysis of a modified subcritical zeotropic mixture recuperative high-temperature heat pump

The heat sinks with a finite capacity and a large temperature glide bring about new challenges of efficient heat pump. This paper presents a modified subcritical zeotropic mixture recuperative high-temperature heat pump cycle for the above heat sinks by introducing a modified recuperation process of zeotropic mixtures with a large temperature glide in gas-liquid phase change region. Performance analysis and optimizations are performed by genetic algorithm in MATLAB connecting HYSYS considering COP as an objective function for hot water supply from 15 degrees C to 99 degrees C with an air source using five candidate binary zeotropic mixtures. The study presents the effects of pressures and mixture compositions on cycle COP and exergy utilization. The results show that the modified cycles using n-hexane/propene and R4310mee/R32 efficiently improve COP and exergy utilization by enabling a good temperature match of heat sink with refrigerant under the optimal conditions. Furthermore, compared with CO2 transcritical cycle, CO2/R32, CO2/R143a and CO2/R41 result in about 8.88%, 6.55% and 4.95% increases of COP, while n-hexane/propene and R4310mee/R32 recuperative cycles bring about 4.35% and 3.28% increases of COP. Interestingly, the operating pressures of n-hexane/propene (1409.5 kPa/704.0 kPa) and R4310mee/R32 (1968.0 kPa/980 kPa) are far lower than those of pure CO2 and CO2 blends, which are consistent with those of conventional air-conditioning and refrigeration. A preliminary cost comparison result reveals the low-cost advantage of the modified zeotropic mixture recuperative cycle because off-the-shelf air-conditioning and refrigeration products can be used. It is concluded that n-hexane/propene and R4310mee/R32 subcritical recuperative heat pump cycles can be superior alternatives to CO2 heat pump cycle.