Energy Harvesting from Temperature: Use of Pyroelectric and Electrocaloric Properties

The previous chapters were devoted to the introduction to the electrocaloric effect and an extensive presentation of different materials. The main application target is the solid-state cooling, and its practical implementation was discussed in chap. 8. In this case, the electrical energy is converted into thermal energy. But electrocaloric materials are also pyroelectric, and the thermal energy can be transformed into electrical energy using the same materials. It can be utilized for power supply of autonomous devices (also called energy harvesting), or even for energy production. In this chapter, the principles of electrical energy production from thermal sources using pyroelectric materials are presented. Linear pyroelectric properties may be utilized as a straightforward transfer from piezoelectric energy harvesting. In particular, energy harvesting figure of merit, as well as electrothermal coupling factor are presented. Then, materials properties and their optimization are discussed. Beyond linear materials, nonlinear ones exhibit the highest known electrocaloric properties. Performing thermodynamic cycles (such as Olsen / Ericsson cycles, or Stirling cycles), it is possible to obtain much larger output energies when working in the vicinity of phase transitions. Finally, the correlation between electrocaloric effect and energy harvesting ability is established. It is shown that the best materials for electrocaloric cooling are also the best candidates for energy harvesting as well. Some predictions are then shown with ultra high output energy densities and efficiencies related to Carnot cycle.