Heat Pump

A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the class includes many other types of devices. During the operation, some of the thermal energy must be transformed to another type of energy in the process, before reappearing as thermal energy in the sink.

The heat pump uses mechanical work, or some source of thermodynamic work (such as much higher–temperature heat source dissipating heat to lower temperatures) to accomplish the desired transfer of thermal energy from source to sink. In the classical thermodynamic sense, a heat pump does not actually move heat, which by definition cannot flow from cold to hot temperatures. However, since the effect of the device in moving thermal energy is the same as if heat were flowing (albeit in the incorrect direction with regard to temperature difference), the "heat pump" is named by analogy.

A heat pump always moves thermal energy in the opposite direction from temperature, but a heat pump that maintains a thermally conditioned–space can be used to provide either heating or cooling, depending upon whether the environment is cooler or warmer than the conditioned–space. When pumps are used to provide heating, they are used because less input from a commercial–energy source is required than is required for newly–creating thermal energy by transforming heat–free sources of energy (for example, electricity) or low–entropy sources of energy (for example, a gas flame) directly into the required heating. This is because the heat pump utilizes some thermal energy from the environment for part of the delivered–heating, increasing the "efficiency" of the process. In cooler climates, it is common for heat pumps to be designed only to provide heating.

Even when a heat pump is used for heating, it still uses the same basic refrigeration cycle to do the job (merely changing operation so that the warm end of the device is inside). Rather than physically turn the device around, a reversible–cycle heat pump simply operates in a way that changes which coil is the condenser, and which the evaporator. This is normally achieved by a "reversing valve." Common examples of non–reversible (unidirectional) heat pumps are air conditioners, food refrigerators, and freezers. Reversible–cycle heat pumps are often seen in providing building–space heating in high latitude climates that are much warmer than comfortable in one season, but colder in another season. In heating, ventilation, and air conditioning (HVAC) applications, the term heat pump normally refers to a vapor–compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of thermal energy flow may be changed without loss of efficiency. Most commonly, when used in heating, heat pumps draw heat from the air or from the ground

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