High-Temperature Industrial-Scale CO2 Heat Pumps: Thermodynamic Analysis and Pilot-Scale Testing

Abstract

Electrification of heat-generating processes is a key means to decarbonize industrial emissions, and heat pumps significantly improve the efficiency of electrified heat. Carbon dioxide is one of the oldest known heat pump working fluids, but its use is presently limited to low-temperature (<120 °C) applications and, when operating in a transcritical mode, for heating liquid water and similar single-phase materials. Conventional subcritical hydrofluorocarbon (HFC) or hydrofluoroolefin (HFO) refrigerants also have temperature limitations due to thermal degradation and are subject to eventual phase-out. Finally, most smaller heat pump systems use oil-lubricated positive displacement compressors, which also impose temperature limitations due to the thermal stability of the entrained oil in the refrigerant. By taking advantage of oil-free, turbomachinery-based equipment and using CO2 as the refrigerant, the temperature limitations of existing heat pump solutions can be eliminated. Novel cycle architectures, partially derived from sCO2 power cycle concepts, have been developed that significantly improve the performance of CO2 heat pumps relative to conventional vapor compression architectures, both for single-phase material heating and for medium pressure steam generation. Cycle simulations of these new heat pump cycles that cover a wide range of conditions and applications have been completed. Pilot-scale (<50 kWth) demonstration systems have been built and tested, and their measured performance provides validating data for the simulated results.