Overview
Heat pumps can move thermal energy against a thermal gradient at the expense of an external source of power, usually a mechanical compressor.
Heat spontaneously flows from warmer places to colder spaces. A heat pump can move heat in the opposite direction. A heat pump can absorb heat from a cold space and release it to a warmer one. "Heat" is not conserved in this process and it is augmented by the high grade energy expended.
The name heat pump is by analogy with a conventional fluid pump that pumps a fluid against its natural flow from higher to lower elevation.
Heat pumps use a refrigerant as an intermediate fluid to absorb heat where it vaporizes, in the evaporator, and then to release heat where the refrigerant condenses, in the condenser. The refrigerant flows through insulated pipes between the evaporator and the condenser, allowing for efficient thermal energy transfer at relatively long distances.
In reversible heat pumps, a reversing valve allows the flow direction of the refrigerant to be changed.
- In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser. The refrigerant flowing from the evaporator (outdoor coil) carries the thermal energy from outside air (or ground) indoors, after the fluid's temperature has been augmented by compressing it. The indoor coil then transfers thermal energy (including energy from the compression) to the indoor air, which is then moved around the inside of the building by an air handler. Alternatively, thermal energy is transferred to water, which is then used to heat the building via radiators or underfloor heating. The heated water may even be used for domestic hot water consumption. The refrigerant is then allowed to expand, cool, and absorb heat to reheat to the outdoor temperature in the outside evaporator, and the cycle repeats. This is a standard refrigeration cycle, save that the "cold" side of the refrigerator (the evaporator coil) is positioned so it is outdoors where the environment is colder.
- In cooling mode the cycle is similar, but the outdoor coil is now the condenser and the indoor coil (which reaches a lower temperature) is the evaporator. This is the familiar mode in which air conditioners operate.
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