In our article Geothermal Heat Pump we introduced the ground source heat pump, a device which uses the heat stored in the earth to heat water for a home efficiently.
Air Source Heat Pump
An air source heat pump works in exactly the same way as a ground source heat pump, only it extracts heat from the air (instead of the ground) outside a building, and uses it to heat water in the building (air-water system), or the air in the building (air-air).
The advantage of an air source heat pump over a ground source heat pump is that an air source heat pump requires far less space to install, and no excavations. This makes air source heat pumps far more suitable for the average urban home.
How Does an Air Source Heat Pump Work?
An air source heat pump is made up of three main components – an evaporator coil, a compressor, and a heat exchanger.
The evaporator coil is fitted to the outside of an external wall. Here it absorbs heat from the outside air. The compressor pushes the refridgerant gas through the system compressing it until it is at the desired temperature (typically up to 35-40 degrees Celcius). The hot refridgerant then passes through the heat exchanger where the heat from the refridgerant is transferred to water or air.
In the schematic above the evaporator coil is labelled as outdoor coil, and the heat exchanger is the indoor coil since the system illustrated is an air-air heating system.
Note that the operation of a heat pump can be reversed and used to cool down the air in a building by radiating it outside – behaving in exactly the same way as a refrigerator:
Coefficient of Performance
Heat pumps are measured by their coefficient of performance (CoP). The CoP for air source heat pumps is very similar to that for ground source heat pumps at approximately 2-3. With a CoP for instance of 3, 3 units of heat are produced for every 1 unit of electricity consumed. (The two free units of heating were extracted from the outside air). Even with ambient air temperatures of -10 to -15 degrees Celcius, an air source heat pump can extract useful heat (i.e. > 1 unit of heat generated per unit of electricity consumed).
For more information on air-source heat pumps and their emission reducing and cost saving benefits over the latest super-efficient gas-powered condensing boilers read: Compare Heat Pump to Condensing Boiler.