A heat pump is a device that transfers thermal energy from a source to a sink that is at a higher temperature than the source. Thus, heat pumps move thermal energy in a direction which is opposite to the direction of spontaneous heat flow. The heat pump uses some form of low entropy energy to accomplish the desired transfer of thermal energy from source to sink.
Compressor-driven air conditioners and freezers are examples of heat pumps. However, the term "heat pump" is more general and applies to devices which are used to heat a conditioned-space (i.e., a confined space such as a building), that must be warmer than a cold environment. A heat pump can provide either heating or cooling of a given conditioned-space, depending upon whether the surrounding environment is cooler or warmer than the conditioned-space. When a heat pump is used for heating, it uses the same basic refrigeration-type cycle employed by an air conditioner or a refrigerator, but releasing heat into the conditioned-space rather than into the surrounding environment. In this use, heat pumps generally draw heat from the cooler external air or from the ground.
Heat pumps are used to provide heating because less high-grade (i.e., low-entropy) energy is required for their operation, than appears in the released heat. In general, most of the energy for heating in such systems comes from the external environment, and only a fraction comes from the high-grade energy source. For example, in an electrically powered heat pump, the heat power released to the conditioned environment can be typically two or three times larger than the electrical power consumed, making the system efficiency 200 or 300%, as opposed to the 100% efficiency of a conventional electrical heater, in which all heat is produced from input electrical energy.
Some heat pumps are able to accomplish either conditioned-space cooling or warming, depending on operational setting. Reversible-cycle heat pumps are devices designed to work in either thermal direction, in order to provide heating or cooling of the same conditioned environment. These devices operate by changing which coil is the condenser and which coil is the evaporator, rather than physically turn the device around. Such a function is achieved by a "reversing valve." In heating and air conditioning (HVAC) applications, the term heat pump usually refers to easily reversible vapor-compression refrigeration devices that are optimized for high efficiency in both directions of thermal energy transfer.