Heat Pump

A heat pump moves heat from a colder place to a warmer one. Heat naturally flows from hot to cold, so pushing it the other way takes work; see Entropy. But moving heat takes much less energy than creating it, which is why heat pumps can deliver three or four times as much heat as the electricity they use.

A refrigerator is a heat pump. It takes heat from inside the cabinet and releases it into the kitchen. An air conditioner moves heat from a room to the outdoors. A heat pump used for heating does the same thing in reverse, pulling heat from cold outdoor air, the ground, or water and releasing it indoors. Many units can switch direction and do both.

Most heat pumps use the vapor-compression cycle:

1. A refrigerant at low pressure evaporates in a coil exposed to the cold source. Evaporation absorbs heat even when the source is below freezing, because the refrigerant boils at a very low temperature.
2. A compressor squeezes the vapor, raising its pressure and temperature.
3. The hot vapor condenses in a coil indoors, releasing heat.
4. An expansion valve drops the pressure, and the cycle repeats.

Efficiency is measured by the coefficient of performance, heat delivered divided by electricity used. An electric resistance heater has a coefficient of exactly one: every joule of electricity becomes a joule of heat. A heat pump typically achieves three or more, because most of the heat it delivers was already present outside.

The coefficient falls as the temperature gap grows. Thermodynamics sets a ceiling that depends on the indoor and outdoor temperatures, and a heat pump working against a large difference does more work per unit of heat. A heat pump is essentially a heat engine run backward. Modern cold-climate units with variable-speed compressors keep working well below freezing, though output drops. Ground-source heat pumps draw from soil or groundwater, which stays at a steady temperature year-round, for higher and more stable efficiency.

Heat pumps have become central to plans for cutting emissions from buildings, since they can run on electricity from low-carbon sources instead of burning gas or oil. Refrigerant choice matters too. Older refrigerants damaged the ozone layer, and many current ones are potent greenhouse gases if they leak.

Other devices move heat without compressors. Solid-state thermoelectric coolers pump heat with an electric current but are less efficient. Buildings with high thermal mass and good insulation need smaller pumps.