Design, modeling and evaluation of a thermo-magnetically activated piezoelectric generator

Thermal energy harvesting can be realized by numerous techniques of energy transduction. Direct conversions of thermal to electrical energy are typically the most popular technologies used. When miniaturized generators are required, direct conversion methods present difficulties, including the need of bulky heat sinks or the strong dependence to rapid temperature fluctuations. Therefore, indirect conversion methods, like thermal-to-mechanical-to-electrical energy are presented as an alternative to thermal energy harvesters towards powering autonomous sensors. This disruptive technology opens up a new approach to overcome the limitations of miniaturized thermal energy harvesting systems. Even if having a relatively low efficiency due to losses linked to energy conversion steps, energy harvesters based on thermo-magnetic effect show a large power density upon miniaturization. Nevertheless, little research on thermo-magnetic energy harvesting at miniature scale has been conducted and no competitive electrical output has been reported until now.This work presents the design of a generator able to convert small and slow ambient temperature fluctuations into electricity. It exploits the thermo-magnetic effect of a soft magnetic material, namely, iron nickel alloy (FeNi) and piezoelectricity. Thermo-magnetization of FeNi is driving the conversion of thermal energy, in the form of temporal fluctuations, into mechanical vibrations of a structure. The structure consists in a piezoelectric bimorph (PZT) cantilever beam. The generator has two stable positions; open position and closed one. Curie temperature of FeNi being near to ambient temperature, applications like wearable connected devices may be targeted. By changing the temperature of the soft magnetic alloy, the interaction between counterbalance forces (magnetic and mechanical forces) leads the generator to one of its two commutations.Analytical model is developed in order to predict generator performance. Making use of this model, a rapid design of generator is conducted to fit custom requirements such as: temperature of operations, temperature range of operation, thermal response, piezoelectric energy conversion capabilities, etc.Additionally, main design rules were derived from the design parameters of the generator. Special attention was paid on how scaling down size affects the generator performance by using the analytical model.Finite element models are developed through ANSYS software in order to validate the analytical simplified model. They couple the thermal to magnetic field and then mechanical to electrical energy conversion is solved. This model allows designers to explore other materials and do improvements by using design optimization processes.First generation energy harvesting demonstrators achieve power densities of 0.6µWcm^-3 during opening commutations around 40°C and 0.02µWcm^-3 at closing commutations around 28°C. By reducing the generator’s size opening commutations at 31°C while closing commutations at 27°C are achieved. By modifying design parameters such as initial distance of separation between the permanent magnet and soft magnetic alloy is identified as a key to boost the energy conversion capability of the generator. Finally, electrical equivalent model of this thermo-magnetically activated piezoelectric generator is developed to design an energy extraction circuit and power management module. This circuit is developed in a unique software PSpice, to implement losses linked to materials (mechanic and dielectric losses). Making use of curve fitting processes, this model is able to find losses values. A variability analysis of the design is conducted by using the analytical model through Matlab in order to explore the feasibility of producing such a generator industrially. Thus, thermo-magnetic energy harvesting can compete for the first time with the state-of-the-art thermos-electrics.