On the efficiency of caloric materials in direct comparison with exergetic grades of compressors

Abstract

Efficiency improvements in heat pump can drastically reduce global energy demand. Caloric heat pumps are currently being investigated as a potentially more efficient alternative to vapor compression systems. Caloric heat pumps are driven by solid-state materials that exhibit a significant change in temperature when a field is applied, such as a magnetic or an electric field as well as mechanical stress. For most caloric materials, the phase transition results in a certain amount of power dissipation, which drastically impacts the efficiency of a caloric cooling system. The impact on the efficiency can be expressed by a figure of merit (FOM), which can directly be deduced from material properties. This FOM has been derived for 36 different magneto-, elasto-, electro and barocaloric material classes based on literature data. It is found that the best materials can theoretically attain second law efficiencies of over 90%. The FOM is analogous to the isentropic efficiency of idealized compressors of vapor compression systems. The isentropic efficiency can thus be directly linked to the theoretically achievable efficiency of a compressor-based refrigeration system for a given refrigerant. In this work a theoretical comparison is made between efficiency of caloric heat pumps and vapor compression systems based on the material losses for the caloric heat pump and the efficiency of the compressor for vapor compression systems. The effect of heat regeneration is considered in both cases. In vapor compression systems, the effect of the working fluid on the efficiency is also studied.