In a few very special situations, it may be viable to use miniature heat pumps to provide a larger heat gradient that can then be harvested by a thermoelectric generator. A heat pump is a device that moves heat from one location to another by using mechanical work. Most heat pump technology moves heat from a low temperature heat source to a higher temperature heat sink and does this with liquids and forced evaporation but solid state versions have also been demonstrated. The opportunity to use them for energy harvesting is limited by size and cost.