Energy Harvesting with Thermally Induced Vibrations in Shape Memory Alloys by a Constant Temperature Heater

The paper reports a theoretical and experimental study of a novel thermal energy harvester with heat-induced vibrations in an environment with a nonzero temperature gradient. The energy harvester comprises a Shape Memory Alloy wire from Nitinol and two elastic cantilever beams with deposited Lead Zirconium Titanate piezoelectric layers. The Shape Memory Alloy wire is pre-strained by the free ends of the cantilever beams connected in a bow-similar structure. The environment temperature gradient is obtained from the difference in temperatures of a heater and colder air in the room. When the wire approaches the heater, it heats up and shortens, causing it to move away from the hot zone and entering into a colder zone. This causes the wire to cool and to approach the heater again. The cyclic change of the length of the Shape Memory Alloy wire causes vibrations in the cantilever beams due to which electricity is produced by the piezoelectric layers. A dynamical model, based on the theory of Lagrange and Maxwell, combining mechanical, piezoelectric and thermal domains, is derived and used to prove the concept of the developed novel device and for theoretical investigation of the energy harvester performance. The hysteretic behaviour of the Shape Memory Alloy is involved in the model. The theoretical results have been proved experimentally.