A heat pump is a machine or device that moves heat from one location (the 'source')
to another location (the 'sink' or 'heat sink') using mechanical work. Most
heat pump technology moves heat from a low temperature heat source to a higher
temperature heat sink.[1] Common examples are food refrigerators and freezers,
air conditioners, and reversible-cycle heat pumps for providing thermal comfort.
Heat
pumps can be thought of as a heat engine which is operating in
reverse. In heating, ventilation, and air conditioning (HVAC) applications, a
heat pump normally refers to a vapor-compression refrigeration device that includes
a reversing valve and optimized heat exchangers so that the direction of
heat flow may be reversed. Most commonly, heat pumps draw heat from the air or
from the ground. Some air-source heat pumps do not work as well when temperatures
fall below around -5 °C (23 °F).
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