Geothermal Systems

NB in the UK this does not include the Ground Source Loop system described under Heat Exchange, whereas in other parts of the world this too is considered to be one of the Geothermal systems. The notes below apply only to boreholes drilled deep into the earth's crust - 1+ miles down - depending on the proximity of a hot spot to the surface.

It is interesting that as yet there is a distinct paucity of companies worldwide who specialize in installing Geothermal Power Stations, but in reality the requirements of Geothermal power are supplied by a combination of the deep mining industry and the usual fossil fuel Power Stations.

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How might it work for you

The expense of installing a Geothermal System and the amount of energy recoverable, mean that this is ideally suited as a power source for a town and it is not suitable for individual use! These are Power Station sized developments, in which the primary super hot fluid in the geothermal borehole system is used to heat - preferably 'cleaned' - water from the environment to produce super hot steam. This in turn is then used to turn the turbines to produce electricity. The resulting hot water has normally, in the past, been cooled in Cooling Towers before being returned to the environment. Many of the current installations have had problems due to using the vaporized water from the boreholes to pass through the steam turbines. This has resulted in corrosion due to the high percentage of Sulphide and Chloride contamination from the bedrock. However, current research has come up with ways of reducing this and improving the electrical output.

The first Geothermal Power Plant was built in 1911 for the town of Larderello in Italy. It is still producing 4,800GW/ann; enough power to provide all the electricity for 1 million homes, at a rate of 4.8MW/home/ann.

In 2008: 26% of Iceland and the Philippine's electricity and 5% of California's electricity was supplied by Geothermal at a price that was competitive with Electricity from Coal.

Facts - Energy Produced and Costs of Production

An idea of the potential energy production from each installation is as given for the Larderello Power Station. One of the modern power stations (presumably smaller) is achieving 26.325MW after anti-corrosion modifications have been applied; and still others are achieving an output of 35MW after modification.

The costs of bringing such a project to production will vary with the financial climate at the time of set-up, and can only realistically be calculated for specific projects.

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Pros and Cons

Pros

A Geothermal Power Plant performs better than either Hydrocarbon or Nuclear Power Plants in that:

  • That it runs 24 hours/day with an uptime of over 90%. This compares with 75% and 65% for Coal and Nuclear Plants respectively due to the much greater amount of Maintenance downtime that these require.
  • It is non-polluting.
  • It does not require any fuel.
  • It does not produce any noxious Waste products.
  • Price-wise it competes well with Coal, as its running costs are small. Once the borehole is drilled, there is no further mining, nor vehicular transportation of fuel - so far the Larderello borehole(s) are said to have lasted for nearly 100 years.
  • Taking set-up and running costs over the life span of a borehole, mean that this is considerably cheaper than setting up and running a Power Station fuelled by Hydrocarbons.

Cons

  • It is not suitable for individual houses or small communities.
  • It is expensive to set up.
  • When vaporized water from the borehole is used, then corrosion is a problem. However, this can be overcome by modifying the surface metal of the turbine; and could also be overcome if the water bringing the heat to the surface was kept separate from the water used to provide the steam for the turbines.
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Variations on the theme

Researchers in China have recently reported on the water heating potential of old mine shafts and oil wells. Since the temperature of earth's rocks increases by 25 - 50°C with every kilometre of depth; and since these shafts are often several kilometres in depth; they propose to heat water by circulating it through concentric pipes. It is predicted that they would only produce ca. 54kWh electricity, but would be relatively low cost to install compared to the drilling of a dedicated geothermal borehole. Time will tell how efficient these installations will be; but since there are approx. 2.5 million oil and gas shafts in the United States alone, the additive production could be huge. It is also the sort of installation that could be carried out routinely as wells came out of production.

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How and Why it works

These systems make use of the heat generated by Atomic decay, which emanates from the molten outer Core of the Earth at ca. 5,500ºC. Occasionally the high temperature of the inner Crust shows up on the surface in the form of Hot Springs e.g. those found in Iceland and New Zealand. But in most parts of the world one can only tap into it by drilling into hot rock near the base of the Earth's Crust.

From the facts given below, it can be seen that providing that we curtail ourselves to the Crust, deep shafts could be drilled down to areas where the rock/mineral temperature is above 500ºC.

Some Earth Facts

  • The radius of the earth varies between 6,378.39 km (3,963.34 miles) at the Equator and 6,356.91 km (3,949.99 miles) at the Poles.
  • The deepest place on the surface is the Marianas Trench at 36,198 ft. (ca. 6.9 miles below the surface of the sea).
  • The highest point on the surface is the summit of Mount Everest at 29,028 ft. (ca 5.5 miles above the surface of the sea).
  • The Earth's Crust is an average of 17 km (10.56 miles) thick. Depending on the morphology of the surface this can vary from 10 - 50 miles thick.
  • Earthquakes seem to be caused by movements in the Mantle acting on weakened areas of the Crust, and can originate as deep as 700 km below the surface.
  • Most volcanoes emanate from the Mantle, which forms a shell of 2,883km depth around the Outer and Inner Cores.
  • The Mantle has been deduced to be solid, but its minerals are likely to be close to their melting points.
  • Measurements of temperatures in boreholes have shown that the temperature of the rocks increases by 1ºC for every 30 metres drilled down. Therefore at a depth of 17 km it would be reasonable to expect the temperature to be ca. 566.6 ºC. The Melting Points of the following elements put this temperature in perspective: Sulphur - 113ºC; Tin - 232ºC; Calcium - 851ºC; Gold - 1,063ºC; Silica - 1,400ºC; and Iron - 1,530ºC.

The heat of the pressurized fluid from Geothermal Sources is so great that this energy is used to produce steam to drive a turbine in the same way as high temperature steam is used in a Coal-fired Power station. The result is that the heat is converted directly to Electrical Energy and is not used in conjunction with a Heat Pump.