1. Introduction

For almost 150 years, refrigeration applications were solved by means of vapour compression. While the most efficient fluids for this approach are based on chlorofluorocarbons, hydrochlorofluorocarbons and hydrofluorocarbons, they come with the severe drawback of contributing to global warming and ozone depletion. Therefore, in 1987, the Montreal Protocol issued a ban on these chemicals providing regulations for phasing them out. Promising natural alternative substances are impractical due to their toxicity (ammonia) or—in particular—their flammability (propane) [1].

Vapour compression refrigerators (VCRs) are operated as reverse Rankine cycles. They use a circulating liquid refrigerant as a medium. The refrigerant is: (i) adiabatically compressed, (ii) condensed at constant pressure undergoing a phase transition (thereby rejecting heat to the heat sink), (iii) adiabatically throttled in an expansion valve and (iv) evaporated at constant

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pressure undergoing the reverse phase transition (thereby absorbing heat from the load). The amount of transferred heat is determined by the latent heat of the first-order phase transition. Similarly, in solid-state electrocaloric (EC) cooling, the adiabatic compression/expansion of the refrigerant is analogous to adiabatic polarization/depolarization, while the isobaric processes are replaced by isofield ones. Contrary to VCR, where the adiabatic expansion of the vapour is thermodynamically irreversible, the EC and the magnetocaloric (MC) effects are thermodynamically reversible processes that could reach the limit of the Carnot efficiency. This is another aspect making them promising for future application.

Electric fields required for the EC refrigeration cycle can be supplied much easier and less expensively than the high magnetic fields required for the MC refrigeration [2]. Other advantages in comparison with MC cooling are higher power densities due to potentially higher cycle frequencies, smaller mass of the device, compactness, potential cost reduction, independence on risks of rare-earth materials supply, etc. [3]. Moreover, electrical energy for EC cooling can be provided by stationary or mobile solar cells and by electric vehicle batteries. This opens up completely new possibilities for an environment-friendly industrialization of developing countries.

EC materials provide a solid-state cooling technology without polluting liquid refrigerants and no or almost absent moving parts (pump and motion of a pumped heat transfer fluid). Generally, EC material (refrigerant) converts the electrical input work

$$\mathcal{W} = \int \mathcal{E}d\mathcal{D},\tag{1}$$

into cooling or heating. Here, E is the electric field and D is the dielectric displacement. The latter is a vector field describing the electrical effect of free and bound charges in materials. Compared to VCR, the E plays the role of pressure and D plays the role of volume in vapour compression.

More detailed descriptions can be found in a number of recent reviews of the EC effect [2, 4, 5] and its application in refrigerators [3, 6, 7], and a book on this topic [8].
