Geothermal power is a clean way of producing energy. It uses the heat from deep in the earth's bowels, geothermal literally means "heat from the earth". It currently accounts for 0.26% of worldwide electricity production.
Heat from the earth's centre spread outwards, no matter where you are in the world, if you drill deep enough you will eventually find heat. However with today's technology it is only feasible (and efficient) to use this heat in certain conditions. A combination of a hot spot relatively close to the surface, and water infiltrating the surrounding rocks are necessary. If you bore a well in the right place then this heated fluid will spurt out, at temperatures of up to 370 C. This does restrict the sites at which geothermal power can be used, however with further research it is likely that it will become possible to use sources such as hot dry rock.
There are 3 widespread ways of converted the stream of hot pressurised water into electricity, usually one is preferable other the others for a given site. All involve harnessing the power of the water to drive a turbine.
- Dry steam: If the output is mainly steam, then this can be passed directly into a turbine. This is the oldest type of geothermal power, first used in Lardarello in Italy in 1904. The Geysers in northern California use this type of power generation.
Flash steam: If the output is water above 200C then this can be transformed into steam by a sudden pressure drop. This steam can then be used to drive a turbine.
Binary cycle: If the water is below 200C (by far the most common case), the heat of the water is transferred to some other fluid with a lower boiling point via a heat exchanger. The resulting steam drives a turbine
In all cases once the steam or water has driven the turbine, it is pumped back into the ground to maintain reservoir
pressure (although this has not always been the case in the past).
- Clean: The only byproducts of geothermal power are leaked steam and trace amounts of gases or chemicals dissolved in the water.
- Reliable: existing geothermal plants have availabilities around 95%. This compares favourably to fossil fuel or nuclear power plants which have availabilities of around 70% and renewable energies such as solar or wind power which rely on appropriate climatic conditions.
- Small: Unlike solar or wind power, which need vast arrays of panels or windmills, geothermal plants are relatively compact. This also compares favourably with fossil fuel plants which beyond the actual plant need a large infrastructure dedicated to the extraction and transport of fuel.
- Price competitive: In the US costs range between $0.05 to $0.08 per kilowatt-hour, and the US Department of Energy believes that the cost can be brought down to $0.03 per kilowatt hour. Some geothermal producers sell electricity as cheap as $0.015 /kWh. Current costs in the USA are 3.73 cents/kWh for nuclear, 3.27 cents/kWh for coal and 5.87 cents/kWh for gas (including capital costs). Oil and gas (and to a much lesser degree coal and nuclear power) are vulnerable to changes in fuel costs. Geothermal power does have the disadvantage that almost all costs are upfront and vary according to the site (in particular how deep the well needs to be).
The most obvious one is that geothermal power is limited to certain areas. It is hoped that in the future we will be able to tap into hot dry rock geothermal energy, available potentially anywhere in the world, but currently we lack the technology to drill the 6 to 15 km boreholes needed.
A less obvious disadvantage is that geothermal power is not as renewable as one might think. While the heat source at the centre of the earth isn't going anywhere (at least on the timescale of human existence), the same is not true of the actual reservoirs that the power plants draw their heat from. These receive heat at a finite rate from the surrounding rocks, if plants draw energy faster than the reservoirs recharge they will eventually cool down. How fast this happens is not well known, nor is the time that a site would need to "recharge" once use had ceased.
A second limitation is due to the actual water or steam being drawn from the production wells. Early plants operated on the assumption that there was an essentially infinite supply of steam, however this has been shown to be incorrect. The Geysers installation has seen steam pressure (and as a direct consequence, electricity production) decline since 1987, initially around 500 psi it is now below 200 psi. The rate of pressure decline has slowed considerably in recent years and it is believed that this can be at least partially avoided by reinjecting fluids after they have been used to drive the turbines. Current predictions show that the site will remain viable for at least 50 years.
Lastly, corrosion of machinery can be a problem. This depends largely on the chemicals present in the water/steam, but some of the culprits are hydrogen sulphides, chloride ions, carbon dioxide (in various dissolved forms), hydrogen ions and oxygen. While corrosion can happen in a fairly uniform way (which is fairly easy to monitor), it can happen in an unpredicatable and localized fashion (known as pitting), which is very difficult to allow for in designs. Some alloys may crack when under stress and exposed to certain compounds in the right conditions and others are vulnerable to dealloying which is happens when a specific constituent element is attacked (for example deplumbification: removal of the lead from a lead-tin solder). Corrosion is an important factor in the design of any geothermal plant, as bad choices of techniques and materials used will result in significantly increased maintenance work and shorter plant lifetime.
When it is not necessary or viable to use geothermal energy to generate electricity it can also be used to produce hot water, saving energy. In Iceland this is used to heat greenhouses in order to grow flowers and vegetables all year round (did you know that Iceland is Europe's biggest banana producer?). In France around 200,000 homes are heated in this fashion and even the Romans used geothermal power to heat buildings in Pompei.
Another possibility is the use of geothermal ground source heat pumps. To cut a long story short these use the fact that ground temperature stays more or less constant during the year, and so can be used as an aid to cooling in summer and heating in winter. 500,000 US homes are equipped with such devices and they are around 50-70% more efficient at heating and 25-60% more efficient at cooling than traditional installations.
How widespread is geothermal power?
Unfortunately not very. Worldwide there are around 8000 MW of electrical geothermal power (0.26% of overall production), and 4000 MW of thermal power.
In the US around 2700 megawatts are produced yearly by geothermal plants (roughly equivalent to 60 million barrels of oil, enough for around 2.8 million homes, 0.4% of overall production), but it expected that in the next decade an extra 15000 MW will become available.
This of course varies from country, in Iceland 86% of residential heating is provided by geothermal energy.
When used with care, geothermal power has great potential as a reliable and clean power source. Hopefully research will enable us both to expand the number of sites where geothermal power can be used, and ensure the long term efficiency of these sites.