Energy Investigation of Reverse Brayton High-Temperature Heat Pump Operating with Supercritical CO2 Mixtures

The electrification of the industrial sector is an important pathway to decarbonizing the industry and achieving a sustainable society. High-temperature heat pumps (HTHPs) are critical devices for providing industrial heat and consuming green electricity. The goal of the present work is the theoretical thermodynamic analysis of a reverse Brayton HTHP that operates with novel working fluids. Specifically, the idea of using mixtures of working fluids with CO2 is studied for the first time with the aim of suggesting new candidates to increase the performance of industrial HTHPs. A model of an HTHP with an internal heat exchanger is developed and verified in the MATLAB programming language. Nine different mixtures are studied: CO2/R152a, CO2/R1234ze(E), CO2/Propane, CO2/Butane, CO2/Isobutane, CO2/Pentane, CO2/Isopentane, CO2/Hexane and CO2/Heptane. The examined industrial heat production temperatures are 150 °C, 200 °C and 250 °C, while the waste heat stream temperatures that drive the heat pump are considered to be 80 °C and 100 °C. The results prove that the application of the mixtures can enhance the COP, especially in the case of lower temperature lifts. CO2/R152a seems to be a promising choice compared to pure CO2, presenting performance enhancements ranging from 4.12% to 64.02% among the studied scenarios.