Optimal temperature matching in high-temperature heat pumps

Heat transfer within heat exchangers is the main source of exergy destruction in high-temperature heat pumps [Hu et al., 2017]. Matching the temperature profile of refrigerant and secondary medium is an effective approach to reduce exergy destruction. One way to achieve temperature matching in the heat exchangers is selecting refrigerants with an appropriate critical temperature. Depending on the refrigerant’s critical temperature and the boundary conditions applied, three operational modes exist, namely: subcritical, transcritical and supercritical. Above the critical pressure, the refrigerant transfers heat associated to a temperature change. Whereas below the critical pressure, the temperature is constant during condensation or evaporation. A second approach is to make use of zeotropic mixtures that achieve non-isothermal phase change. In this paper, both transcritical and supercritical operation as zeotropic mixtures will be studied to achieve optimal temperature matching for heat sink outlet temperatures in the range of 160-200°C. Keywords: High temperature heat pump, Zeotropic mixtures, Subcritical cycle, Transcritical cycle, Supercritical cycle