Electrocaloric Cooling Cycles in Lead Scandium Tantalate with True Regeneration via Field Variation

Abstract

There is growing interest in heat pumps based on materials that show thermal changes when phase transitions are driven by changes of electric, magnetic, or stress field. Importantly, regeneration permits sinks and loads to be thermally separated by many times the changes of temperature that can arise in the materials themselves. However, performance and parameterization are compromised by net heat transfer between caloric working bodies and heat-transfer fluids. Here, we show that this net transfer can be avoided—resulting in true, balanced regeneration—if one varies the applied electric field while an electrocaloric (EC) working body dumps heat on traversing a passive fluid regenerator. Our EC working body is represented by bulkPbSc0.5Ta0.5O3near its first-order ferroelectric phase transition, where we record directly measured adiabatic temperature changes of up to 2.2 K. Indirectly measured adiabatic temperature changes of similar magnitude are identified, unlike normal, from adiabatic measurements of polarization, at nearby measurement set temperatures, without assuming a constant heat capacity. The resulting high-resolution field-temperature-entropy maps of our material, and a small clamped companion sample, are used to construct cooling cycles that assume the use of an ideal passive regenerator in order to span≤20K. These cooling cycles possess well-defined coefficients of performance that are bounded by well-defined Carnot limits, resulting in large (>50%) well-defined efficiencies that are not unduly compromised by a small field hysteresis. Our approach permits the limiting performance of any caloric material in a passive regenerator to be established, optimized, and compared; provides a recipe for true regeneration in prototype cooling devices; and could be extended to balance active regeneration.

Alternate abstract:

Plain Language Summary

In traditional refrigeration, pressure changes drive a cooling cycle mediated by a fluid refrigerant such as Freon, which is harmful to the environment. New methods for cooling could save energy, permit miniaturization, and allow for faster startup times. One such approach is based on electrocaloric techniques, where voltage plays the role of pressure and the fluid refrigerant is replaced by a solid material whose temperature can be changed via an electric field. However, it is unclear how efficient such heat pumps can be in light of the limitations of the active electrocaloric materials at their hearts. Using detailed experimental data on an archetypal electrocaloric material, we calculate that the potential performance of such a cooling device is encouragingly high.

Based on dense electrical polarization and heat-capacity data, we construct thermodynamic maps, which track measurements of thermodynamic properties against temperature and voltage. These maps allow us to visualize, in high resolution, the transition at which we drive electrocaloric effects, and they obviate the standard practice of assuming a constant heat capacity. To permit the construction of cooling cycles over a wide range of temperatures, we assume that our material is used to develop a temperature gradient along a lossless column of heat-transfer fluid by dumping heat at the hot end and absorbing heat towards the cold end. We find that cooling cycles with well-defined energy efficiency require close control of voltage.

In the future, our approach should permit researchers to optimize and compare the performance of all caloric materials, including those driven by magnetic fields or mechanical stresses.