From ferroic cooling to ferroic energy harvesting

Ferroic transitions enable energy efficient cooling trough magnetocaloric, elastocaloric and electrocaloric effects, and combinations of thereof. After briefly generalizing the underlying thermodynamic cycle of ferroic cooling, here we will focus on a complementary thermodynamic cycle suitable for ferroic energy harvesting. In this process low grade waste heat is used to change the ferroic order parameter, which drives a heat engine, which converts waste heat to electrical energy.

As a first example, thermomagnetic devices are discussed, where the temperature dependent change of magnetization within a magnetic field is used to build a motor, oscillator or generator. For the generator we discuss optimum magnetic field topologies and their impact on output power etc. We derive material selection guidelines for functional materials, which reach maximum efficiency and minimum price per watts. These guidelines illustrate, that a different paradigm is necessary for the development of thermomagnetic compared to magnetocaloric materials. When waste heat is available only at low grade, the best thermomagnetic materials can already compete today with thermoelectric materials.

As a second example, we present thermoelastic devices, which use prestrained shape memory alloy wires to convert waste heat to mechanical energy. A protagonist-antagonist setup allows recovering the mechanical energy required for prestraining, and an elaborated fluid management allows reaching a high cycle frequency. Even with commercial NiTi shape memory alloys a higher power density and efficiency compared to thermomagnetic materials is obtained.

As summary, we generalize the concept towards ferroic energy harvesting and sketch guidelines for future material development with even higher conversion efficiency. As today there is hardly any efficient method to recover low grade waste heat, ferroic energy harvesting can contribute to reduce the climate change.