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Renewable heat
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Heat pump heating (and cooling) systems

A heat pump is a system which is able to extract low grade heat from the air, ground or water and concentrate it to provide useful heat for space and water heating. Heat pumps (in spite of their name) can also produce space cooling during the summer. In this process high grade heat is extracted from a building and converted into a lower grade of heat which can then be diffused back into the earth or atmosphere. Thus a single system can produce space heating, space cooling and hot water.

Advantages

  • Electricity and low grade heat are available from a variety of sources
  • Systems are available in all sizes from heating one room to heating a single or multiple dwellings
  • Some systems are reversible in that they can both heat and cool
  • Electricity to operate the heat pump can come from renewable sources
  • Natural refrigerants are being used which have zero or very low environmental impact

Disadvantages

  • Space if a horizontal ground loop collector is used
  • Efficiency drops with decreasing air temperature for air source systems
  • Higher initial costs than conventional heating systems

Efficiency
Energy is only required to concentrate the heat stored directly or indirectly from the sun, not produce it so this type of system has a very high efficiency compared with fossil fuel boilers whose efficiency lies in the range of 0.8 to 0.9.

The ratio of heat out to electricity consumed is called the coefficient of performance and this can vary from 3.0 to 5.0 depending upon –

  • the type of heat pump
  • the difference in temperature between the heat source and the desired output temperature (uplift temperature)

Air source systems
These extract heat from the ambient air and so their efficiency will vary depending upon the outdoor temperature. This poses a problem in very cold weather as the uplift temperature increases and so the overall efficiency will decrease and consequently an auxiliary form of heating may be required.

Ground source systems
These extract heat from the ground via a collector system which transfers heat into a pipe in which a collector fluid flows. The collector pipe may either be laid in a horizontal trench or vertically by inserting into a specially drilled bore hole up to 200m in depth. The drilling of a bore hole requires access for a drilling rig whilst for a horizontal trench, access is needed to a garden or open space. A collector pipe of 100m in length will provide sufficient geothermal heat for an individual heat pump to concentrate the heat and provide between 4 and 6 kW of heat output.

The horizontal trench needs to have sufficient depth, typically 1.2 m in the UK, so that the ground temperature is not influenced to any extent by the ambient air temperature. For northern Europe the trench depth will need to be deeper.

Open loop systems
With these systems, water is extracted from a river, lake, dam or underground aquifer and circulated through a collector pipe to the input side of the evaporator heat exchanger. After transferring heat to the refrigerant, the cooled water is returned to its source.



 

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