Geothermal energy is heat energy generated and stored in the Earth, which originates from radioactive decay and the Earth’s formation. The global installed capacity of geothermal power is 12,635MW across 26 countries and an additional 28GW of direct geothermal heating capacity is installed for district heating, space heating, spas, industrial processes, desalinisation and agricultural applications. The ease with which this energy can be accessed varies from place to place. In some locations, such as Iceland or New Zealand, high temperatures are found at shallow depths and thus electricity generation is very easy. In the UK, higher temperatures are found at greater depths and thus it is possible to exploit power, but it is more complicated and riskier, and therefore more expensive.
Geothermal power is reliable, sustainable and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size of viable resources, especially for applications such as home heating, opening a potential for widespread exploitation. Some argue that geothermal wells release greenhouse gases trapped deep within the earth, but these emissions are much lower per energy unit than those of fossil fuels. But, geothermal energy produces more CO2 (at 38gCO2eq/kWh) than wind and hydropower. However, despite the embodied emissions in the plant fabric, geothermal energy remains a very low carbon energy source.
As with many forms of green energy, capital costs are significant. Drilling accounts for over half the costs, and exploration of deep resources entails significant risks. But once these costs have been overcome, a reliable source of renewable energy can be maintained indefinitely. At $0.05/kWh as of 2015, this is the cheapest form of energy.