Types of heat pumps
The two main types of heat pumps are compression heat pumps and absorption heat pumps. Compression heat pumps always operate on mechanical energy (through electricity), while absorption heat pumps may also run on heat as an energy source (through electricity or burnable fuels).
A number of sources have been used for the heat source for heating private and communal buildings.
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air source heat pump (extracts heat from outside air)
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air–air heat pump (transfers heat to inside air)
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air–water heat pump (transfers heat to a tank of water)
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geothermal heat pump (extracts heat from the ground or similar sources)
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geothermal–air heat pump (transfers heat to inside air)
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ground–air heat pump (ground as a source of heat)
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rock–air heat pump (rock as a source of heat)
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water–air heat pump (body of water as a source of heat)
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geothermal–water heat pump (transfers heat to a tank of water)
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ground–water heat pump (ground as a source of heat)
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rock–water heat pump (rock as a source of heat)
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water–water heat pump (body of water as a source of heat)
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Heat sources
Most commonly, heat pumps draw heat from the air (outside or inside air) or from the ground (groundwater or soil). The heat drawn from the ground is in most cases stored solar heat, and it should not be confused with geothermal heat, though the latter will contribute in some small measure to all heat in the ground. Other heat sources include water; nearby streams and other natural water bodies have been used, and sometimes domestic waste water which is often warmer than the ambient temperature.
Air-source heat pumps
Air-source heat pumps are relatively easy (and inexpensive) to install and have therefore historically been the most widely used heat pump type. However, they suffer limitations due to their use of the outside air as a heat source or sink. The higher temperature differential during periods of extreme cold or heat leads to declining efficiency, as explained above. In mild weather, COP may be around 4.0, while at temperatures below around −8 °C (17 °F) an air-source heat pump can achieve a COP of 2.5 or better, which is considerably more than the COP that may be achieved by conventional heating systems. The average COP over seasonal variation is typically 2.5-2.8, with exceptional models able to exceed 6.0 (2.8 kW).
Air-source heat pumps
Ground source heat pumps
Ground source heat pumps, which are also referred to as Ground source heat pumps, typically have higher efficiencies than air-source heat pumps. This is because they draw heat from the ground or groundwater which is at a relatively constant temperature all year round below a depth of about eight feet (2.5 m). This means that the temperature differential is lower, leading to higher efficiency. Ground-source heat pumps typically have COPs of 3.5-4.0 at the beginning of the heating season, with lower COPs as heat is drawn from the ground. The tradeoff for this improved performance is that a ground-source heat pump is more expensive to install due to the need for the digging of wells or trenches in which to place the pipes that carry the heat exchange fluid. When compared versus each other, groundwater heat pumps are generally more efficient than heat pumps using heat from the soil.
Ground source heat pump
Solid state heat pumps
Main article: Magnetic refrigeration
In 1881, the German physicist Emil Warburg put a block of iron into a strong magnetic field and found that it increased very slightly in temperature. Some commercial ventures to implement this technology are underway, claiming to cut energy consumption by 40% compared to current domestic refrigerators. The process works as follows: Powdered gadolinium is moved into a magnetic field, heating the material by 2 to 5 °C (4 to 9 °F). The heat is removed by a circulating fluid. The material is then moved out of the magnetic field, reducing its temperature below its starting temperature.