A bimetal and electret-based converter for thermal energy harvesting
  This paper presents a new device able to turn thermal gradients into electricity by using a bimetal-based heat engine coupled to an electrostatic converter. A two-steps conversion is performed: (i) a curved bimetallic strip turns the thermal gradient into a mechanical movement (thermal-to-mechanical conversion) that is (ii) then converted into electricity thanks to an electret-based electrostatic converter (mechanical-to-electrical conversion). An output power up to 5.5uW on a hot source at 50{\deg}C has already been reached, validating this new concept. 
S. Boisseau, G. Despesse, S. Monfray, O. Puscasu, T. Skotnicki
physics.class-ph