A Thermal Energy Harvester Based on Bistable SMA Microactuation

We present the design, fabrication, and characterization of a miniature-scale thermal energy harvester based on bistable shape memory alloy (SMA) microactuation. This device consists of two free-standing Ti53.9Ni30.4Cu15.7 bridge structures with curved memory shape that are coupled by a spacer with a pick-up coil. The device undergoes a periodic oscillatory motion between two stable equilibrium positions, when placed in between two heat sources. An analytical model is implemented to simulate and optimize the force-displacement characteristics. The inhomogeneous field of a miniature permanent magnet induces a voltage in the pick-up coil. The resulting power output is investigated as a function of bridge geometry, heat source temperature and load resistance of the energy harvesting circuit. For a spacing between SMA bridges of 5 mm, the output voltage reaches 48 mV at heat source temperature of 160 deg C corresponding to a maximum output power of 4.5 muW.