Multi-temperature heat pumps: A literature review
This review highlights the major advantages and challenges of mechanically driven heat pumps and refrigeration systems with focus on multi-temperature applications. Different design strategies are presented, including cycles with multi-stage compressors, ejectors, ex-pansion valves, cascades, and separated gas coolers. Most multi-temperature heat pump cycles use two heat sources and one heat sink. In supermarket applications, multi-stage compressor cycles with transcritical CO2 is an es-tablished key technology. Cascades with secondary loops are another frequently applied type of system. Expansion valve cycles are applied in household refrigeration and air condition-ing. Cycles with ejectors seem to be a promising modification for system performance improvement. Separated gas coolers for space heating and hot water production have re-cently attracted attention due to the possible combination with supercritical CO2 cycles. Thermodynamic simulations reveal that multi-stage compressor cycles have the highest COPs and second law efficiencies, followed by cascade, ejector, and expansion valve cycles. The baseline cycle consisting of two single-stage heat pumps in parallel shows lower second law efficiency than the multi-stage compressor and cascade cycles, and higher efficiency than the ejector and expansion valve cycles.