Geothermal energy stems from impacts that occurred during the accretive formation of our planet, the radioactive decay of its constitu­ents and incident sunlight. Its radioactive component is estimated [2] as about 30 TW, which is about half the total and twice the present global electricity demand. However, commercial access is achievable only at relatively few locations along the boundaries of tectonic plates and where the geology is porous or fractured. Though hot springs and geysers occur naturally, commercial extraction for district heating, horticulture or electric power involves deep drilling into bedrock with one hole to extract hot water and another thermally distant to inject its necessary replenishment. There are presently no commercial geothermal generation sites in the United Kingdom, but a 41/2 km deep 10 MW station near Truro is under active consideration.

The Second Law of Thermodynamics [3] by Lord Kelvin asserts that a heat engine must involve a heat source at a temperature T1 and a cooler heat sink at a temperature T0. In 1824, Carnot proved that the maximum efficiency r* by which heat could be converted into mechanical work is

Г* _ 1 _ t0=t 1 with T1, T0 in Kelvin (1.2)

A typical electric kettle consumes 2kW.

Given a relatively hot geothermal source of 200° C and a condensing temperature of 40°C, the above efficiency bound evaluates as 34%, but intrinsic thermodynamic irreversibilities [3] allow practical values [2] of only between 10 and 23%. Because the majority of geothermal sources have temperatures below 175°C they are economic only for district and industrial space heating or as tourist spectacles in areas of outstanding beauty (e. g., Yosemite National Park, USA). Exploitation of the higher temperature sources for electric power is engineered by means of a Binary Cycle system, in which extracted hot water vaporizes butane or pentane in a heat exchanger to drive a turbo-alternator. Replenishment water for the geothermal source is provided by the colder outlet, and district or industrial space heating is derived from recompression of the hydro­carbon. The largest geothermal electricity units are located in the United States and the Philippines with totals of 3 and 2 MW, respectively, but these countries with others intend further developments.

According to the US Department of Energy an 11 MW geothermal unit of the Pacific Gas and Electric Company had from 1960 an operational life of 30 years, which matches those for some fossil and nuclear power stations. Because geothermal generation involves drilling deep into bedrock with only a 25 to 80% chance of success, development is both risky and capital intensive and so it incurs a high discount rate. Moreover, despite zero fuel charges, low thermal conversion efficiencies reduce the rate of return on invested capital, which further increases interest rate repayments. That said, nations with substantial geothermal resources are less dependent on others for their electricity which is an important political and economic advantage. Construction costs for a recent 4.5 MW unit in Nevada, the United States were $3.2M per installed MW.

Geothermal water contains toxic salts of mercury, boron, arsenic and antimony. Their impact on a portable water supply is minimized by replenishments at similar depths to the take-off points. These sources deep inside the earth’s crust also contain hydrogen sulfide, ammonia and methane, which contribute to acid rain and global warming. Otherwise with an equivalent carbon emission of just 122 kg per MWh, geothermal generation’s “footprint” is small compared with fossil-fired production. However, the extraction process fractures rock strata that has caused subsidence around Wairakei, NZ, and at Basel CH small Richter-scale 3.4 earth tremors led to suspension of the project after just 6 days.

Geothermal energy for domestic and small-scale industrial space heating can be provided without an environmental impact by heat pumps [3,15]. An early 1920’s example is the public swimming pool at Zurich CH which used the River Limmat as its heat source. Finally, some recently built UK homes have heat pumps whose input is accessed from coils buried in their gardens.