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Going underground for true sustainability

The ever-increasing emphasis on sustainable heating and ventilating means geothermal energy ought to be on everyone's agenda, says Stephen Laws, technical director of Clyde Energy Solutions
Going underground for true sustainability
THERE are inherent impracticalities with most types of sustainable energy sources. Solar and wind power cannot be relied upon for constant levels of output, and not many people have the possibility of harnessing wave or hydro-electric power.

Systems burning biomass, such as pellets, vegetable oils and organic waste fall into the renewable energy category. There are good arguments for using these fuels to reduce net carbon emissions, though they are not strictly speaking sustainable.

A more reliable approach is to use heat pumps to extract energy from sources that would otherwise remain untapped. A heat pump provides a way of concentrating low-grade heat, and works like a refrigerator in reverse. Although any source of heat can be used, the most common are the atmosphere (air source) and the ground (geothermal).

The air or ground does not have to be particularly hot, it just needs to be warm enough to evaporate a refrigerant - either directly in the case of air, or via a circulating fluid system in the case of geothermal. The process is energy efficient, with just a small amount of power required to drive a pump and a compressor.

Heat pumps are only truly sustainable if the electricity needed to drive the heat pump comes from a sustainable source. But, by that definition, none of these technologies could truly be considered sustainable unless the energy and raw materials used in their construction (the embedded energy) exceeded the return.

Underground energy About 4m down, the earth is at a pretty constant temperature, so energy extracted is always available, day or night, winter or summer.

Strictly speaking, geothermal energy is the energy generated by the earth itself, through processes such as radioactive decay. But it is taken to mean all energy extracted from the earth and, in particular, that which is directly absorbed from the sun.

There are two basic elements in an indirect geothermal heating system: a way of transferring the heat in the ground to a circulating liquid system (generally using water or brine); and a heat pump to convert that heat into a form in which it can be used for space heating.

There are a number of alternative ways of extracting the heat from the ground and the best choice will depend on the size of the site, its structure, and the amount of heat that needs to be extracted. If there is a large area of land available, a horizontal ground collector can be used. This is an array of tubes a few feet below the surface through which the heat-collecting liquid is circulated.

Where space is at a premium, the most appropriate solution is often to dig a borehole and install a heat-collecting circuit of pipes.
Horizontal arrays are generally the cheapest systems to install but require a significant amount of ground area, preferably with direct sunlight, and have the lowest heat extraction rate - around 20W per metre of tube.

Boreholes are more expensive to install, but take up much less space. They are best suited to sites where the underlying geology is consolidated rock, and give a greater heat extraction rate of 25W per metre of tube.

If a relatively large body of water such as a pond, lake or river is available, the tubes can simply be laid in the water. Alternatively water itself can be abstracted from a subterranean source and used as a source of heat. In this case though the available heat is totally dependent on the source water flow rate, and water abstraction and re-injection licences will be required.

Underfloor heating

Whatever the heat source, the most effective way to use geothermal energy is in low-temperature systems, such as underfloor heating.
A heat pump will be at its most efficient with a system (heat to water) flow temperature of around 35˚C. This is too low for domestic hot water, which requires primary heat flow temperatures of up to 60˚C. Raising the system flow temperature from a heat pump will reduce its efficiency, so a supplementary heat source might be more appropriate for hot-water generation.

If underfloor heating is installed, it is also possible to operate the system in reverse and cool the building without the need for energy intensive air conditioning. This type of passive cooling system can take heat from the building on hot days and transfer it back into the ground. When the building needs to be heated up again in the evening, the ground is hotter and so the rate of heat regeneration is improved.

In practice, the most convenient, energy-efficient and cost-effective approach is to combine the sustainable geothermal energy with a supplementary source of top-up heating. And there is no reason why the supplementary heat cannot come from a sustainable or renewable source - such as solar collector or biomass boiler - rather than a conventional fossil fuel.

Clyde offers systems suitable for small-, medium- and large-scale geothermal installations, as well as air source heat pumps. It also offers a consultancy service on aspects of energy efficiency and the suitability of renewable solutions in specific applications.
1 December 2007


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