A unified approach for the thermodynamic comparison of heat pump cycles

AbstractThe flexible heat pump cycle introduces a heat storage device into the Evans-Perkins cycle to recover, store, and reuse part of the sensible heat carried by the hot liquid refrigerant from the condenser, achieving a higher coefficient of performance than the latter. In this paper, we develop a unified approach, namely cycle superposition to allow comparison of the flexible heat pump cycle with other performance-enhancing cycle layouts including two-stage cycles with intercooling, subcooling, flash gas removal, or their combinations. We show that under ideal conditions, the flexible heat pump cycle is thermodynamically similar to two-stage heat pump cycles with full subcooling or flash gas removal, but no intercooling. From the energy recovery perspective, the two-stage cycles recover and reuse some sensible heat carried by hot liquid refrigerant simultaneously using their high-stage compressor, whereas the flexible heat pump cycle decouples the recovery and reuse of such heat in time using a heat storage. However, the irreversible heat transfer via real heat exchangers during charging and discharging processes will reduce the benefits of the flexible heat pump cycle. The ef