How Highly-Efficient Power Electronics Transfers High Electrocaloric Material Performance to Heat Pump Systems

Electrocaloric heat pumps are an emerging solution for emission-free cooling and heating with high performance. The material performance of some known electrocaloric ceramics or polymers can exceed that of today’s vapor compression systems. However, the system performance of published prototypes still lacks significantly behind. This talk presents ultra-efficient power electronics as a solution to this fundamental problem: First, a simple half-bridge switched-mode power converter topology based on gallium nitride transistors is used to demonstrate 99.3% efficient charging of capacitors. Second, an advanced multilevel and partial-power-processing approach is demonstrated, which achieves over 99.7% charging efficiency. This is a significant reduction of electrical charging losses (by multiple 10-times) compared to a resonant circuit approach previously used for electrocaloric prototypes. It is shown how the combination of low-loss electrocaloric materials and highly-efficient power electronics, both existing already today, will enable a high electrocaloric heat pump performance. For example, for available PMN material data, a best-case relative system coefficient of performance COPr (including the external charging losses) over 50% for Carnot-like cycles and over 75% for Ericsson cycles with temperature regeneration is projected, which is close to the material limit of over 80%. The flexibility of the proposed power electronics is demonstrated: The switched-mode approach allows to efficiently charge also very large capacitive loads which are required for future large-scale heat pumps. On the one hand, over 99% efficient charging of 384 commercial capacitors with a very low cycle frequency below 0.02 Hz is demonstrated, which allows heat transfer between thick electrocaloric elements. On the other hand, over 1 kHz cycle frequencies are efficiently realized, which is useful for characterization of the samples’ dielectric losses but is beyond the usable thermal time constants of today’s prototypes. The trade-off between performance and power density is analyzed. To increase the system efficiency (COPr), but at the expense or reduced power density, it is shown how an offset-field significantly reduces the effective electrocaloric capacitance while the temperature effect is only reduced slightly. The effect of increasing system frequency is discussed. It is discussed how it improves the power density of the system by reduction of electrocaloric material required for the same cooling power. However, it is also shown that it does not reduce the required power rating of the charging circuit and thus does not reduce the size of the electronics. The control system has an inner current-control for adjustable constant-current charging, and an outer voltage-control for waveform generation (arbitrary field variation). A trigger signal allows synchronization of for example servo-control signals, which are useful to realize complete heat pump prototypes. While maintaining a high charging efficiency, the voltage waveform generation is used with an electrocaloric heat-pump prototype to experimentally demonstrate Carnot-like cycles, further verified by temperature and heat flux measurements. Finally, it is shown how the highly-efficient power electronics will be applied to an electrocaloric prototype in the ongoing Fraunhofer lighthouse project “ElKaWe – Electrocaloric heat pumps”, which is targeting a high cycle frequency by using a heat-pipe system approach. The discussed multi-disciplinary approach combines ideas from material research, mechanical engineering and electrical engineering. This allows to accelerate the development of electrocaloric heat pumps, and the transition towards emission-free heat pumps for efficient cooling and heating applications.