The first electric welding transformer, a device which uses electricity to weld joints of metal using resistance heating. (Corbis Corporation)

A heat pump is a thermodynamic heating/refrigerating system used to transfer heat. Cooling and heating heat pumps are designed to utilize the heat extracted at a low temperature and the heat rejected at a higher temperature for cooling and heating functions, respectively.

The household refrigerator can provide a simple analogy. A refrigerator is actually a one-way heat pump that transfers heat from the food storage compartment to the room outside. In so doing, the inside of the refrigerator becomes progressively cooler as heat is taken out. In a closed room, the heat coming out of the refrigerator would make the room warmer. The larger the refrigerator is, the greater the potential amount of heat there is to transfer out. If the refrigerator door were open...