Low-cost, high output Peltier cell.
Peltier Junctions are thermo-electric heating and cooling devices that feature solid state, reliable temperature control. Uses include cooling elements for mini fridges, cooling laboratory instruments and cooling hot running ICs.

Peltier cells can also be used to generate electricity by heating one surface and cooling the other. With a sufficient temperature differential a cell will output up to 1 Amp at 5V - sufficient to drive a small electric motor.