High-Temperature Heat Pump Using CO₂-Based Mixture for Simultaneous Heat and Cold Energy Reservation

To leverage temperature glide in evaporation, a transcritcal heat pump using a CO₂-based mixture is investigated from a perspective of simultaneous heat and cold energy storage. Coefficient of performance for heating (COP_h) and exergy efficiency are used to evaluate system performance. A parametric investigation on the heat pump is conducted, and the coupling behavior of the cycle with thermal energy storage (TES) material is investigated in view of stored exergy of TES. Optimization and comparative studies are carried out among various mixtures. The results reveal that maximum cycle temperature is mainly affected by high pressure and superheating degree, while minimum cycle temperature, as well as cold exergy, is highly dependent on evaporating temperature glide, with little influence from high pressure. The total exergy efficiency can reach up to 60%. The temperature of low temperature TES could reach as low as −32.4 °C for CO₂/R601, providing the largest proportion of cold exergy to total exergy, up to 30.1%.