Geothermal energy is derived from the decay of radioactive elements within the Earth's molten core, where temperatures reach 6000°C at depths of around 6000 km. This heat naturally dissipates towards the earth's surface. Geothermal energy is usually associated with countries that have active volcanoes, one example being Iceland, where geothermal energy provides two-thirds of the country's primary energy demand and provides and 25 per cent of its electricity demand.
In volcanically active regions of the globe, geothermal power is produced from water or steam at temperatures in excess of 220°C. This is used to drive turbines to produce electricity, with heat being produced as a valuable by-product. These systems are referred to as high temperature or high enthalpy geothermal systems.
While the UK does not possess any active volcanoes it does have several geological features at depths of between 2 – 4 km that are potential geothermal targets, albeit with lower temperatures of around 60 – 80°C. These low temperature or low enthalpy targets include:
The UK has a geothermal heating plant at Southampton which is operated by Cofely District Energy and has supplied homes and businesses with heat for the past 25 years. This scheme centres upon a borehole that extends 1.8 km into water-bearing sandstone and 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 entire UK, concluding that there are significant geothermal resources that could meet the UK's entire heat demand, with the potential for some power generation. Despite this, the Southampton scheme remains the only operational system in the UK.
Cheaper gas prices in the 1980s and early 1990s did not provide favourable economics for more widespread uptake of deep geothermal energy in the UK. More recent concerns relating to climate change, carbon emissions and energy security, combined with advances in drilling and electricity generation technology, have renewed interest in deep geothermal energy. The economic case for geothermal energy is improved with combined heat and power generation, however 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. The proximity of the resource to the end user also determines its usefulness, especially when used solely for direct heat.
Geothermal energy is accessed by drilling a deep well into the target area. Where the formation at depth contains hot 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 which is 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 partnership between the Universities of Durham, Glasgow and Newcastle and BGS, has been established to research and promote the potential of deep geothermal energy to the UK government, industry and society. The partnership undertakes research into the geothermal potential of hot water produced as a by-product from oil extraction,radiogenic granites, geological faults, deep sedimentary basins and abandoned, flooded mineworkings.
The partnership has experince of drilling deep geothermal boreholes at Eastgate, County Durham and also in central Newcastle upon Tyne, and the group is collaborating with other geothermal projects in India and Kenya.