Hermoelectric generators (also called thermogenerators) are devices which convert heat (temperature differences) directly into electrical energy, using a phenomenon called the “Seebeck effect” (or “thermoelectric effect”). Their typical efficiencies are around 5-10%. Older Seebeck-based devices used bimetallic junctions and were bulky while more recent devices use bismuth telluride (Bi2Te3) semiconductor p-n junctions and can have thicknesses in the millimeter range. These are solid state devices and unlike dynamos have no moving parts, with the occasional exception of a fan. Radioisotope thermoelectric generators can provide electric power for spacecraft. Automotive thermoelectric generators are proposed to recover usable energy from automobile waste heat.