The general energy and ecological situation on the planet requires further substantial increases in the efficiency of cryogenic and refrigeration units. While ozone depletion is being addressed, the battle against global warming is just at a beginning. As a big user of power, the cooling industry is in the spotlight in the drive to reduce carbon emissions and tackle climate change. 

Electrocaloric Effect: In polar crystals the net dipole moment and, consequently, the spontaneous polarization increases with the application of an external electric field. This causes a decrease in entropy of the system and, in turn, an adiabatic increase in temperature. This phenomenon is called the electrocaloric effect . The effect is reversible. When the external field is switched off the entropy is increased and temperature decreases. A general theory of the EC effect has not been developed yet and a phenomenological approach is generally used to describe the relationship between the polarisation, strength of external electric field and temperature change.

EC Materials: The first experimental measurement of the EC effect was performed on Rochelle salt and reported in 1930. Because the temperature change was not large it was only in 1956 that the possibility of the EC effect was first discussed. The EC-induced temperature change under an external field of 25 kV/cm was generally lower than one degree. This has been the main reason that the EC effect was not considered as an alternative for a realization of a cooling process.
      In recent years the research has focused on ferroelectrics and it was shown that around Curie point the EC effect can be significantly greater compared to the effect far away from the Curie temp. The temperature change of 2.3K was obtained for modified
PbSc1/2Ta1/2O3 in a temperature range of ±5K. However, it is impossible to organize a cooling cycle in such a narrow temperature range; therefore, a further research has focused into systems with a diffused ferroelectric transition such as disordered crystals, polycrystalline samples and relaxors. Recently, a paper was published on a giant EC effect found in the PbZr0.95Ti0.05O3 thin film. The authors described the EC effect of ~12K. The results were obtained through an indirect method by a measurement of a temperature dependence of polarization. The results are still under the intensive scientific discussion because the EC effect was calculated and not confirmed with a direct caloricimetric measurement.

Applications: An experimental EC cooling unit was built in 1985 at the Moscow Power Engineering Institute. This was the first attempt of feasible EC cooling and it was made for air-condition purposes. Yet, at the moment a reliable cooling unit has yet not been produced. Sinyavskii took the development of EC cooling in to a more applied engineering stage rather than a fundamental material science research. The EC cooling unit was based on a closed system with recirculation of cooling media. The device was mainly comprised of power supplied alternating EC converters, heat emitters and heat receptors. The efficiency of such system was directly related to the intesity of EC effect rather than to other factors such as number of stages or volume of the EC material. The same researchers had developed a model of a very simple EC device not using any thermal switches or pumping of heat as transfer agent. Its operating principle was based on the thermophysical change of the EC. This occurs upon an action of an electric field; where an asymmetric heat-flux aroused, leading to cooling of one end and heating of one end. The investors never came further than rough calculations of ideal values up to 40% efficiency when optimal EC material was used. The Moscow investigation left the material science community with three unsolved future goals:
1) higher efficient EC materials as this was proven to improve efficiency most
2) development of the EC unit dimensions to optimise the field strength, kV/cm, by introducing multistage structures
3) development of a high-efficient heat transfer agent with no mechanically moving parts.

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Updated on 1.12.2007 ..................... matjaz.valant@p-ng.si