Heat pump

A heat pump does what it's name implies, it pumps heat from one point to another. A refrigerator uses this same principle.

A liquid requires energy to evaporate, and it drains that energy from it's surroundings, in the form of heat. This is why sweating cools down the body. Alternatively, the condensing proccess releases the energy, again in the form of heat.

A heat pump employs a liquid with a low boiling point, once typically the ozone unfriendly freon (CFC), now generally liquid ammonia. First this liquid is allowed to evaporate on the side that is to cool down, extracting heat. The gas is pumped to the other side, where it is compressed into a liquid, causing it to release it's heat. A heatpump uses only the energy needed to run the compressor, and therefor has the potential to be very efficient.