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.

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.

When it comes to covering the growing demand for cooling power worldwide, elastocalorics offer an environmentally friendly alternative to compressor-based cooling technology. The absence of harmful and flammable coolants makes elastocalorics suitable for energy applications such as battery cooling. Initial prototypes of elastocaloric systems, which transport heat by means of thermal conduction or convection, have already been developed. A particularly promising solution is the active elastocaloric heat pipe (AEH), which works with latent heat transfer by the evaporation and condensation of a fluid. This enables a fast and efficient heat transfer in a compression-based elastocaloric cooling system. In this publication, we present a simulation model of the AEH based on MATLAB-Simulink. The model showed very good agreement with the experimental data pertaining to the maximum temperature span and maximum cooling power. Hereby, non-measurable variables such as efficiency and heat fluxes in the cooling system are accessible, which allows the analysis of individual losses including the dissipation effects of the material, non-ideal isolation, losses in heat transfer from the elastocaloric material to the fluid, and other parasitic heat flux losses. In total, it can be shown that using this AEH-approach, an optimized system can achieve up to 67% of the material efficiency.

This work analyzes how an over 99% efficient charging circuit and electrical offset fields enhance the coefficient of performance (COP) of electrocaloric heat pumps, an emerging technology with zero global warming potential. The COP is studied for Carnot-like cycles regarding the material’s permittivity and dissipation factor, and the system’s charging efficiency. Compared to the Carnot limit, a relative material COP of 50.4% is calculated for a lead magnesium niobate (PMN) ceramic, and enhanced to 87.4% by an offset field. The offset avoids high loss at low fields, where the non-linear permittivity-related dissipative loss is highest. A 99.2% efficient gallium nitride half-bridge switched-mode converter is used as charging circuit. Including the charging loss, the calculated relative system COP of 11.9% is significantly enhanced to 28.6% by an offset field. The 0.8% external loss exceeds the material loss (dissipation factor below 0.2%), reducing the system COP from the material COP. Compared to 80% efficient state-of-the-art resonant circuits for electrocalorics, this work’s approach reduced charging loss 20 times. The work contributes to transfer the high COP of electrocaloric materials also to electrocaloric heat pump systems.

This work combines a 99.2% efficient GaN-based low-voltage fast-switching half-bridge converter with a Si-based high-voltage slow-switching and almost lossless switched capacitor multiplexer using a partial power processing approach to a six-level prototype with 99.74% efficiency. A loss breakdown shows how the ideal partial power processing efficiency is theoretically increased to 99.84% by using four additional voltage levels, but then reduced to the measured efficiency by the additional static and dynamic losses of the multiplexer. For electrocaloric heat pumps, an emerging technology for cooling and heating applications with zero global warming potential, such high charging efficiencies enable a high heat pump system coefficient of performance (COP). Based on available data of electrocaloric lead magnesium niobate (PMN)-based samples, and based on a first-principle and best-case analysis for Carnot-like cycles, it is predicted that the 99.74% electrical charging efficiency in combination with the electrocaloric material data enables to surpass 50% of the thermal Carnot limit (for cooling with a heat pump). Ultra-high efficiency of power converters thus pave the way towards future electrocaloric heat pumps of competitive system performance.

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%.