A miniature piezoelectric-thermomagnetic generator for low-grade waste heat recovery

This paper presents the experimental and model-based investigation of a compact miniature-size generator for conversion of low-grade waste heat at temperatures near 100 °C to electricity leveraging the combined transducer effects of a thermomagnetic (TM) and a piezoelectric (P) layer. TM energy conversion is achieved via the recently developed effect of resonant self-actuation of a cantilever using the large temperature-induced change of magnetization at the ferromagnetic transition of a Ni-Mn-Ga film. The resulting kinetic energy is converted into electrical energy via the piezoelectric effect of a thin lead-zirconium-titanate (PZT) layer, which enables rectification of the harvested energy using standard electrical circuitry. The performance of the generator is characterized with respect to the effects of tip mass and length of the piezoelectric layer as well as heat transfer and damping. A lumped element model (LEM) of the coupled system shows that heat intake and heat dissipation have to be balanced to achieve resonant self-actuation, while damping limits the oscillation frequency of the cantilever. A single P-TM generator generates an average power output of 0.8 µW at 105 °C. After rectification, this corresponds to 1.25 µW/cm² of DC power at a voltage of 2 V DC.