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Underground energy

Geothermal energy is coming back in favour in the UK as an energy option, after some years in the wilderness. A major geothermal “hot dry rock” test project in Cornwall was abandoned in the 1980s after it was assessed as not being likely to produce sufficient energy for electricity production, and although Southampton persevered with a more conventional aquifer heat-based system, geothermal was basically left to other countries.

There is now over 11 GW of installed geothermal aquifer electricity generation capacity around the world and even more heat supplying capacity, but deep “hot rock” geothermal technology has recently had a renaissance, in part due to the availability of improved drilling techniques developed in the oil industry.

Enhanced geothermal systems (EGS) as they are now called, are beginning to move towards commercialization. A 2.9 MWe plant is operating commercially in Landau, western Germany, while projects are now being developed in Australia, the US and Japan, and plans are taking shape for a 3 MWe plant in Cornwall, at the Eden Project.

In its recent Renewable Energy Strategy, the UK government said that it would “commit up to £6 m to explore the potential for deep geothermal power in the UK helping companies carry out exploratory work needed to identify viable sites”.

Depths of 3,000 to 10,000 metres can now be reached, with water pumped down to be heated by the hot rocks to around 200 degrees centigrade. Feeding back to the surface, this water can then be used to drive turbines to generate electricity.

Martin Culshaw of the Geological Society’s engineering group, said: “Cooling one cubic kilometre of rock by one degree provides the equivalent energy of 70,000 tonnes of coal. This has the potential of equalling the nuclear industry in providing 10–20% of Europe’s energy.” Geothermal systems have the big advantage over many other renewables of supplying “firm” continuous power, although they aren’t strictly 100% renewable, in that heat wells exhaust the local heat resource over time. But it’s topped up eventually by the heat from deeper inside the earth- derived from nuclear isotope decay. That makes it one form of nuclear power that seems benign.

However there can still be problems. Iceland has been developing geothermal electricity production on a large scale, but as Lowana Veal has reported in IPS News, there have been concerns about emissions of hydrogen sulphide gas. Moss in the area was being effected. Levels were well below what was thought have any health risks for humans, but it is being monitored. H₂S can be filtered out of the steam and water vapour that is emitted by geothermal plants, or it can be reinjected back into the well in a closed loop binary system. But that all adds to the cost.

Perhaps more importantly, carbon dioxide gas is also present. To deal with this a “Carb Fix” programme is being developed.  The idea is to dissolve the CO₂ in water under high pressures and then pump the solution into layers of basalt about 400–700 m underground, in the expectation that the dissolved CO₂ will react with calcium in the basalt to form solid calcium carbonate. The project is a form of carbon capture and storage (CCS). But rather than filling empty oil or gas wells with CO₂ gas under pressure, mineral storage offers a safer bet, since there is less chance of leakage.

There may also be a potential problem with earthquake risks from deep drilling in some locations. Drilling kilometres down and then pressurising the system can lead to release of geological stresses, lubrication of fissures and small earth tremors. There were some recorded for example with a geothermal project in Switzerland in 2006, when water was injected at high pressure into to 5 km deep borehole. A shock measuring 3.4 on the Richter scale was detected, which caused local alarm, but evidently no injuries or serious damage, although further work was halted. There was a 3.1 scale tremor subsequently. This issue has recently led to concerns about some of the new German projects.

Problems like this apart, the prospects for geothermal seem good. The USA is in the lead in terms of geothermal electricity production, and has around 4,000 MW of new capacity under development. Google.org recently put £5.4 m into enhanced “hot rock” geothermal systems , supporting three new projects in the USA, and Obama allocated $350 m to geothermal work under the new economic stimulus funding.

The resource potential is very large. The US Department of Energy has suggested that in theory the US could ultimately have at least 260,000 MW of geothermal capacity. Large resources also exist elsewhere in the world and there are many projects in operation or being developed. As already mentioned, Iceland is a leading user, but the Philippines, which generates 23% of its electricity from geothermal energy, is the world’s second biggest producer after the US the United States. It aims to increase its installed geothermal capacity by 2013 by more than 60%, to 3,130 MW. Indonesia, the world’s third largest producer, plans to have 6,870 MW of new geothermal capacity over the next 10 years – equal to nearly 30% of its current electricity generating capacity from all sources. Kenya has announced a plan to install 1,700 MW of new geothermal capacity within 10 years – 13 times greater than the current capacity and one-and-a-half times greater than the country’s total electricity generating capacity from all sources.

www.earthpolicy.org/Updates/2008/Update74.htm

Finally, the use of ground-source heat-pump technology is also expanding rapidly, with perhaps 200,000 units having been installed in domestic and commercial buildings around the world. This is also sometimes labelled as “geothermal”, not really completely correctly, since at least for surface based heat pipe extraction, the heat is mostly ambient heat ultimately derived from the sun, not from deep in the earth. But some heat pumps do use deeper pipes and they can also be used to upgrade the value of geothermal heat. In addition, heat can be stored in the earth via underground piping, creating local underground heat stores.

Posted by Dave Elliott on September 26, 2009 11:20 AM | Permalink