Electrocalorics (EC) is potentially more efficient than refrigeration and heat pumps based on compressors and does not need detrimental fluids. Current EC-prototypes use solid-state contact or forced convection with liquids to transfer the heat generated from the EC-material, which inhibits high cycle frequencies and thus limits power density. Here we present a heatpipe system solution, where the heat transfer is realized through condensation and evaporation of ethanol as a heat transfer fluid. Our prototype with lead scandium tantalate (PST) EC-material working at 5 Hz shows a specific cooling power of 1.5 W g⁻¹. This is one order of magnitude more than previously reported for ceramic EC-prototypes. Overcoming the limits of slow heat transfer is essential to reach high specific cooling powers enabling a future commercial success of the technology.

Hysteresis and the associated production of dissipative heat during first order phase transitions are often major contributors to thermodynamic losses in caloric heat pumps. The figure of merit (FOM), defined as the ratio of adiabatic temperature change and the thermal hysteresis of the caloric material, quantifies these losses, and can also be used to calculate the maximum potential efficiency of a caloric material in a thermodynamic cycle. This paper presents a novel and simple method to determine the heat loss and thus the FOM can be determined from self-heating of the caloric material during repeated field cycling. As this method mainly requires temperature readings and the ability to cycle the caloric material in a field, most test setups that directly measure the adiabatic temperature change should already be able to perform dissipative heat measurements with this technique. With the presented method, we were able to determine the efficiency of a commercial LaFeSiMnH-sample with a high degree of accuracy. A maximum FOM of 37 ± 1 was determined for the selected LaFeSiMnH-sample. In an ideal cascaded magneto caloric system, this corresponds to a system efficiency of 90%, with an ideal heat regeneration this could theoretically even be increased to 97%.

In this study, we characterize electrocaloric lead scandium tantalate (PST) samples by means of the adiabatic temperature change $\Delta T_{\mathrm{ad}}$ and the dissipative heat $q_{\mathrm{diss}}$ with a direct thermal method. The figure of merit ($FOM$), defined as the ratio between the adiabatic temperature change and the thermal hysteresis, quantifies the losses of the material. Additionally, it is also possible to draw conclusions on the efficiency of a caloric cooling system based on the regenerator or cascaded approach. The maximum adiabatic temperature change of the measured samples results in $\Delta T_{\mathrm{ad},\max }=(1.39\pm 0.02)$ K and the dissipative heat yields $q_{\mathrm{diss}}=(0.39\pm 0.05) \mathrm{J}/(\mathrm{k}\mathrm{g} \mathrm{K})$, resulting in an $FOM=(5.1\pm 0.2)$. The efficiency for an ideal cascaded system is given by ${\eta }_{\mathrm{cas}}=0.56$, and for the ideal regenerator, the efficiency is given by ${\eta }_{\mathrm{reg}}=0.84$. The results demonstrate that the PST material in this study exceeds the maximum $FOM$ in the literature by 34%.

Electrocalorics (EC) is potentially more efficient than refrigeration and heat pumps based on compressors and does not need detrimental fluids. Current EC-prototypes use solid-state contact or forced convection with liquids to transfer the heat generated from the EC-material, which inhibits high cycle frequencies and thus limits power density. Here we present a heatpipe system solution, where the heat transfer is realized through condensation and evaporation of a heat transfer fluid. Our prototype with lead scandium tantalate (PST) EC-material working at 5 Hz shows a specific cooling power of 1.5 W g-1. This is 36 times more than previously reported for ceramic EC-prototypes. Overcoming the limits of slow heat transfer is essential to reach high specific cooling powers enabling a future commercial success of the technology.