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Tapping into the energy that lies deep underground

The Champagne Pool at Wai-O-Tapu, New Zealand: hot water for free. Rebecca Naden/PA

Geothermal energy is derived from heat produced by the decay of radioactive elements within the Earth’s molten core, where temperatures reach 6000°C around 6000km below the ground. This heat naturally dissipates upwards towards the earth’s surface.

Geothermal energy is usually associated with countries that have active volcanoes, for example Iceland, where, geothermal energy provides two thirds of the country’s primary energy, mainly used for heating homes, and 25% of its electricity generation.

In volcanically active regions of the globe, geothermal power is produced from water or steam at temperatures in excess of 220°C that is used to drive turbines to make electricity, with heat being produced as a valuable by-product. These systems are referred to as high temperature geothermal systems.

While the UK does not possess any active volcanoes it does have several geological features at depths of between 2-4km that are potential geothermal targets, albeit with lower temperatures of around 60-80°C. For example, in Southampton a geothermal heating plant operated by Cofely District Energy has supplied homes and businesses with heat for the past 25 years. This is built around a borehole that extends 1.8km into water-bearing sandstone that supplies water at around 60°C to a district heating network.

In response to the oil crisis of the late 1970s, the British Geological Survey mapped the geothermal potential of the whole of the UK. The conclusion was that there were significant geothermal resources that could meet the entire UK’s heat demand, and potentially generate power too. Despite this the Southampton scheme remains the only system in the country.

Cheaper gas prices in the 1980s and early 1990s did not provide favourable economics for more widespread deep geothermal exploitation in the UK. However more recent concerns relating to climate change, carbon emissions and energy security, and advances in drilling and electricity generation technology, has led to renewed interest. The economic case for geothermal energy is even better with combined heat and power generation, but this depends upon the temperature of the resource being sufficient (at least 70°C). The temperature is governed by depth, which in turn has implications on drilling costs that increase with depth. Where the resource is located in relation to those who will use it also determines its usefulness, especially if it is used solely for direct heat, which is easily lost.

Geothermal energy is accessed by drilling a deep well into the target area. For those that contain water, hot geothermal fluids are pumped to the surface where heat is removed and the cooled water returned below ground, usually through another borehole. Dry wells have fluid circulated through the well to be heated before carrying the heat to the surface. Geothermal fluids can either be used directly with a heat exchanger for hot water production, or passed through a power plant to produce electricity if temperatures are sufficient.

BritGeothermal, a research collaboration between the Universities of Durham, Glasgow and Newcastle and the British Geological Survey, has been set up to promote the potential of deep geothermal energy to the UK government, industry and society. Areas of interest the group is studying include the potential use of hot water produced as a by-product from oil extraction, or the geothermal potential of granites, geological faults and sedimentary basins deep underground at several locations throughout the UK. The group has also examined the potential for water within abandoned mineworkings as a heat source for cities and towns above. So far, deep geothermal boreholes have been drilled at Eastgate, County Durham and also in central Newcastle upon Tyne, and the group is collaborating with other geothermal projects in India and Kenya.

The UK’s comparatively low temperature deep geothermal resources are best suited to heating. With the potential to provide a massive 100GW of heat, this could theoretically satisfy the entire space heating demand in the UK, saving carbon dioxide emissions of around 120 million tonnes - that’s something worth looking deeply into.

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