Analysis of the thermal impact of a bimetal on the dynamic behavior of a thermal energy harvester

International audienceThis paper reports on the dynamic analysis of the bimetallic strip of a thermal energy harvester based on a two-step conversion mechanism. The system is primarily based on a thermo-mechanical transduction ensured by a bimetal that is then coupled to an electro-mechanical transduction ensured by a piezoelectric membrane. Previous studies have shown that the bimetal that acts as a shuttle between the hot source and the cold surface does not impact the stationary thermal behavior of the device. However, this bimetal considered as a self-oscillating heat engine does impact the functioning of the structure by shifting its corresponding temperature range. The present study consequently deals with a dynamic model of the harvester using a lumped parameter representation that confirms the temperature shifts observed experimentally. A bimetal that is bounded to start snapping at 70 °C and to snap down at 67 °C only starts snapping at 84 °C once mounted in the structure. Thus, the purpose of this paper is about to explain this phenomenon and propose a model to predict the real snap temperature of any bimetal once it is introduced in a harvester, as this would be a huge advance. This paper thus solves the problem of experimentally finding the snap and the snap-back temperature of any bimetal once mounted in a harvester and proposes many applications to the established dynamic model. For example, one is able to predict the intrinsic snap-up and -down temperatures of a bimetal as a function of its desired snapping temperature once it is introduced in the harvester. Moreover, experimental measurements have been carried out on various bimetals thus making it possible to experimentally validate the dynamic model. The model finally gives the main guidelines for bimetal design from a thermal point of view