GEO-THERMAL ENERGY

Iceland has used Geo-Thermal energy for decades to heat outdoor swimming pools and provide domestic hot water and heating, as well as generate power. The domestic possibilities in the UK are more constrained, but companies which promote what has come to be known as Ground-Source Heat extol the apparent 'free energy' to be extracted from the earth or from a body of water. It's not free of course and what is referred to is the claim that for each 1 unit of power used to run a Ground-Source system between 3 and 5 units of heat are produced. This may be true but the running costs are not insignificant and the capital cost is large.

Two sources can be tapped for energy by this means:

• Heat stored in the earth itself, most often as a result of the sun's energy but in some locations (as in Iceland) because of geological conditions. The earth is much slower to warm up than the air above, and also much slower to cool down. The frequently quoted figure is a temperature of 10deg.C all year round at a depth of 1 metre.
• Heat stored in a body of water in the same way as in the earth. For this to be useful it seems that the volume of water should be quite large and fairly still. A garden pond is not much use but a village pond or a lake would be. As with the earth, at a depth of 1 metre the temperature varies much less that that of the air above.

I am not commenting on 'Air-to-Air' technology which is effectively a reversible air conditioning unit, as I don't consider it sustainable energy. Some manufacturers make both types of unit.

In both situations the principle is the same: pipes carrying a fluid are immersed or buried, the fluid is pumped around a circuit at the other side of which is a compressor and heat-exchange unit. The fluid picks up heat from the earth or the water and transfers it to the heat exchanger. The compressor serves to concentrate the heat which can then be used for space heating. It is usually described as a refrigerator in reverse: a 'fridge extracts heat from the inside, concentrates it and expels it to the outside; a Ground-Source Heat Pump extracts weak but plentiful heat from the outside (ground or water), concentrates it and supplies it to the inside (of a building).

There are constraints to this method of tapping into renewable energy:

• The available heat, even after concentration, is at a lowish temperature and is considered best used for Underfloor Heating systems rather than for DHW. The Co-efficient of Performance - COP - is at its highest where the output tempeature is 35-45deg.C, fine for UFH but not for DHW. Some other heat source is required to raise GSHP water temperature for DHW use.
• The compressor and pumps used to run a system have a fairly high electrical loading, I have read figures of up to 2Kw for the compressor. It is also suggested that a GSHP system should be run continuously during the seasons when space heating is required, probably because a system will take a fairly long time to stabilise at the required temperature and may deteriorate if not operated frequently (c.f. vehicle aircon systems). Consequently the availability and cost of electricity has to be taken into account when assessing whether GSHP is financially worthwhile. It may not be 'sustainable energy' if the electricity used to run GSHP derives from fossil fuels or (in my opinion) from nuclear generation.
• According to method chosen, either a large flattish area of ground is required, or a smaller but easily bored area of ground, or a largish body of water under the owner's control. The large flat ground area is trenched to a depth of 1 metre and at least 50metres in length, into which are run the flexible pipes carrying the heat-transfer fluid. A smaller area of ground may be used for boreholes up to 30-40 metres deep, into which are inserted flow and return pipes: this utilises the higher temperature found at greater depth. Fluid transfer pipes may be lowered onto the bed of a lake. A variation on the trenching method is to sink heat-absorbing panels horizontally or vertically in the soil, this takes up a much smaller area of ground.

SUMMARY

• Available continuously without influence of sun or wind
• Potentially very useful for underfloor heating systems, less so for DHW
• Equipment is fairly large, may cause intrusive background noise and will require occasional maintenance
• Costing is difficult as a grid-connected electricity supply is likely to be required
• An open area of land is needed, large or modest depending on the equipment chosen
• Important to obtain a proper unbiased assessment of whether a specific location is suitable

LINKS

ICE Energy:    www.iceenergy.co.uk

Kensa:    www.kensaengineering.com

GeoHeat:    www.geoheat.co.uk

Worcester-Bosch    www.worcester-bosch.co.uk

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