Heat pumps: harvest latent heat from your environment

Heat pumps extract ambient heat from the external air or ground and concentrate it for space or water heating. In effect they can be seen as reverse refrigerators or air-conditioners. These systems require energy to operate the condensers and pumps, therefore even though the latent heat extracted is renewable, the electrical load required is generally supplied by the grid.

heat_pump.jpgHeat pump (courtesy of Valliant)

Air source heat pumps use a small external heat exchanger and a circulating fan, operating like an air-conditioning unit in reverse. Ground source heat pumps use a large heat exchanger in the form of tubing typically laid 1-2m underground. These tubes when laid horizontally can take up a large area and is therefore generally not suitable for homes without a reasonable amount of land. However vertical systems can be used but are more
expensive and generally require planning permission. [1]

How much do they cost?

Air source heat pumps cost typically around £6,000 to £10,000, whereas ground source heat pumps can be up to £17,000 (excluding heat distribution costs such as fitting underfloor heating) [2]. Smaller air-to-air room heaters of around 4 kWth are available for around £300 (excluding installation) [1]. Annual running costs of a ground source heat pump can be around £650 and £750 for an air source [2]. Savings of around £450 per year could be achieved if replacing electrical storage heating, although minimal savings are seen versus natural gas [3].

How efficient are they?

The efficiency of heat pumps is based on the amount of electricity consumed versus the heat output and is known as the coefficient of performance (COP). A typical ground source heat pump will generate 3 to 4 times more heat energy than the electricity consumed, whereas an air source heat pump is around 2 to 3 times [4]. However in practice the difference attained is much smaller, around 2.3 for ground and 2.2 for air [5].

What are the CO2 savings?

A 8kW capacity heat pump could provide 70kg CO2/yr savings, assuming 6,250kWh space and water heating including backup immersion heating of 1,750kWh [4].

How long do they last?

Heat pumps are generally more durable than standard gas central heating boilers, with an expected lifetime of 40 years or more, although the compressors may require replacing after around 20 years [1; 2].

 

 

References

[1] Staffell, I., Baker, P., Barton, J. P., Bergman, N., Blanchard, R., Brandon, N. P., Brett, D. J. L., Hawkes, A., Infield, D., Jardine, C. N., Kelly, N., Leach, M., Matian, M., Peacock, A. D., Sudtharalingam, S. and Woodman, B. (2010), "UK microgeneration. Part II: Technology overviews", Proceedings of Institution of Civil Engineers: Energy, vol. 163, no. 4, pp. 143-165.

[2] Energy Saving Trust, ( 2011), A buyer’s guide to renewable and low carbon technologies, Energy Saving Trust, London.

[3] Energy Saving Trust (2011), Microgeneration and renewables media fact sheet 2011–2012, Energy Saving Trust, London.

[4] Energy Saving Trust (2010), Domestic Low and Zero Carbon Technologies - Technical and practical integration in housing, CE317, Energy Saving Trust, London.

[5] Energy Saving Trust (2010), Getting warmer: a field trial of heat pumps, Energy Saving Trust, London.