Geothermal energy is derived from beneath the earth's surface. There are a variety of different thermal resources, each of which creates its own engineering challenge as to the way that the energy can be tapped. In some cases current technologies are not practical or economic for successfully extracting the thermal energy. To all intents and purposes geothermal energies are renewable because the latent quantities of energy are so large we cannot imagine them running out.

The environmentally-friendly potential is enormous because, in principle, well designed plants could be cost effective and reliable as well as being clean in terms of emissions. Also they need not produce local environmental visual scarring. Unfortunately the emphasis is on the word 'potential' because much more research and development and capital investment is required to make a wide-scale success of some of these various sources.

Everywhere under the earth's surface there is hot material but it occurs in different forms and at very different depths. Four types of resource are recognised which could be used for substantial amounts of energy transfer and used, for example, to drive electric power stations, in theory at least. A fifth method can be used for smaller applications but, despite the modest energy transfers per installation, this method can be easily engineered and therefore is quite valuable in practice. These five resources are briefly discussed.

The main one, applicable to power generation, is referred to as the hydrothermal reservoir and this has some track record of success. The reservoir is water or steam at a high temperature and the way that the heat exchange is engineered depends on the temperature. It is the fact that the heat carrier (water) is already present (and water is very convenient to handle) which makes this source reasonably accessible. The steam, or hot water flashed to steam, is used to drive turbines to generate electricity.

The other three powerful resources are hot dry rock, geopressure brines and magma (molten rock) and although they have the potential to provide energy the current technologies are not sufficiently developed to make them commercially viable. It does not require much imagination to recognise some of the problems. For example the dry rock must be fractured and liquid forced through the cracks; the geopressurised liquids are rich in methane and exist at great depths; the magma is too hot for conventional processes to be used. It should be possible, eventually, to provide large proportions of our energy requirements using these sources but that state of affairs is a long way off.

The fifth and more modest resource is to extract heat from the ground just under the surface and this is a technique that has been used for ages. It relies on the sun's radiation warming the ground which then behaves as a giant storage medium. Where such heat at low temperatures is available at a shallow depth, a water-circulating scheme with heat pumps can be used to transfer the heat to where it is required. Applications include heating of houses, greenhouses etc but you've got to be lucky to have suitable back garden.

As an added feature where heat pumps are used the heat transfer can be used in reverse so providing cooling in summer. This method of heat transfer is not suitable for large scale power generation and since it is dependent on the sun, its applicability is limited in cooler districts. Some installations are available in the UK, although we doubt it would be economical for an individual domestic plot, but may be cost-effective for small community schemes.

The environmental pollution caused by geothermal installations is small because there are few emissions. Visually a geothermal site need not be offensive because of its construction which only requires a small profile and can easily be screened, by trees for example. Nevertheless, there can be a few problems caused by solids produced where salts carried up in the water must be disposed of and there have been cases of subsidence due to the drillings. Perhaps the worst scenario is when magma has unexpectedly found its way to the surface through the drillings. None of these drawbacks are insuperable.

Successful schemes are in operation around the world and some have been continuously productive for about 100 years, although they tend to be located in specific areas. Fairly obviously, location is dependent on the amount of geothermal activity and its depth, something which is related to the earth's plate tectonics. Countries which have taken advantage of geothermal energy include the US, Italy, Germany, Switzerland, Belgium, Portugal, Iceland, Mexico, Canada and New Zealand.

There are many more, the International Geothermal Association has more than 60 members. Even the UK has three experimental sites in Southampton, Cleethorpes and Penryn although we cannot see geothermal energy becoming a large scale contributor of renewable energy here (wind, water and possibly solar being more likely contenders q.v.). Of the developing countries, maybe half of them have the potential to develop geothermal sites.

In summary, internationally the quantity of geothermal energy is virtually infinite and the environmental benefits are beyond reproach. Set against this are the disadvantages that considerable more Research and Development is needed to take advantage of the buried wealth and even when a commercially viable site is identified the initial investment cost can be a serious deterrent. Maybe if some of the multinationals who have the resources to invest in oil exploration could channel them into geothermal exploration, research and development we might see geothermal energy being tapped on a significant scale. But then we're prejudiced aren't we?