Performance overview of caloric heat pumps: magnetocaloric, elastocaloric, electrocaloric and barocaloric systems

Abstract

Heat pumps are an excellent solution to supply heating and cooling for indoor space conditioning and domestic hot water production. Conventional heat pumps are typically electrically driven and operate with a vapour-compression thermodynamic cycle of refrigerant fluid to transfer heat from a cold source to a warmer sink. This mature technology is cost-effective and achieves appreciable coefficients of performance (COP). The heat pump market demand is driven up by the urge to improve the energy efficiency of building heating systems coupled with the increase of global cooling needs for air-conditioning. Unfortunately, the refrigerants used in current conventional heat pumps can have a large greenhouse or ozone-depletion effect. Alternative gaseous refrigerants have been identified but they present some issues regarding toxicity, flammability, explosivity, low energy efficiency or high cost. However, several non-vapour-compression heat pump technologies have been invented and could be promising alternatives to conventional systems, with potential for higher COP and without the aforementioned refrigerant drawbacks. Among those, the systems based on the so-called “caloric effects” of solid-state refrigerants are gaining large attention. These caloric effects are characterized by a phase transition varying entropy in the material, resulting in a large adiabatic temperature change. This phase transition is induced by a variation of a specific external field applied to the solid refrigerant. Therefore, the magnetocaloric, elastocaloric, electrocaloric and barocaloric effects are adiabatic temperature changes in specific materials when varying the magnetic field, uniaxial mechanical stress, electrical field or hydrostatic pressure, respectively. Heat pump cycle can be built from these caloric effects and several heating/cooling prototypes were developed and tested over the last few decades. Although not a mature technology yet, some of these caloric systems are well suited to become new efficient and sustainable solutions for indoor space conditioning and domestic hot water production. This technical report (and the paper to which this report is supplementary materials) aims to raise awareness in the building community about these innovative caloric systems. It sheds some light on the recent progress in that field and compares the performance of caloric systems with that of conventional vapour-compression heat pumps for building applications.