Heat pumps are simply devices designed to use work to move heat in the opposite direction it typically would flow and thus do not create heat. There are several types of heat pumps and while each of these heat pumps achieves the same end result, they operate on different engineering principles.
Mechanical heat pump
The pressure of a refrigerant is increased via a compressor which thereby raises the boiling temperature. Two types of mechanical heat pumps exist. They either operate on the principle of direct expansion or operate by using a vessel to separate gaseous and fluid refrigerant known as the pump system. Mechanical heat pumps are the most common type of heat pump and are usually the most feasible to install.
Direct Expansion System
Gas Engine heat pump
This heat pump operates on the same principles as a mechanical heat pump, however, rather than using an electrical pump to drive the compressor, a gas powered engine is used to drive the compressor.
Absorption heat pump
These heat pumps work by using thermal energy to evaporate refrigerant which is then absorbed into a medium. Examples of absorption heat pumps include Lithium-Bromide and Water and Ammonia. This type of heat pump is useful when both heating and cooling are necessary.
Adsorption heat pump
This type of heat pump is based around the same principles as the absorption heat pump, but uses a solid medium for absorption rather than a fluid. Some examples of this would be Silica gel and water, zeolite and water, and active carbon/salt and ammonia.
Trans-critical CO2 heat pump
When above 31 degrees Celsius, Carbon Dioxide liquid and gas are indistinguishable. This is called a trans-critical range. This phenomenon allows heat to be released at a range of temperatures.
The blue line shows the transcritical cycle. The steps are as follows:
1 – 2 Compression to transcritical pressure
2 – 3 Gas cooling in transcritical area
3 – 4 Expansion
4 – 5 Evaporation
5 – 1 Superheating
Hybrid Heat pump
These pumps are a combination of the mechanical and absorption heat pumps. They use a mixture such as Ammonia and Water. Changes in the composition of this mixture allow heat to be emitted at a range of temperatures. This can lead to higher efficiency.
Thermoacoustic heat pump
This type of heat pump is not yet commercially available. It works on the principle that sound waves and temperature are related and one can generate the other.