Geothermal energy is increasingly recognised as one of the most important renewable sources of energy available. It has one of the lowest carbon emissions of any renewable energy source.
It is one of the most efficient sources of power in the world; it has a low surface footprint and, unlike most renewable energy types, is non-intermittent and thus available 24/7. Able to provide heat as well as electricity, it is a technology that is growing fast and will play an ever increasing part in delivering clean, affordable renewable energy.
Geothermal energy derives from a variety of sources. The shallowest layers are heated mainly by the sun, and the deep interior of the Earth is heated by the slow leakage of the planet’s vast internal heat reserves by conduction through the cooler outer rocks. Developers like us seek to extract this heat by finding reservoirs of water, or brine, at depths that vary from 1000m to 5000m. The geothermal fluids are pumped to the surface, the heat exploited directly or to produce electricity, and then pumped back down into the ground to maximise sustainability.
Lower temperature geothermal reservoirs, and waste water from geothermal power plants, can be used directly for heating domestic and commercial buildings, for industrial and agricultural applications, and even for cooling buildings using absorption chillers.
Total global geothermal power output is currently around 12.8GW. This extends over 25 countries including Indonesia, Ethiopia, Kenya, The Philippines, Iceland, Mexico and the USA.
In those parts of the world which are blessed with very high temperature geothermal resources – mainly volcanic regions – boreholes can directly encounter high-pressure steam, or else very hot water (usually >150°C) which will spontaneously turn to steam on being brought to the surface via the boreholes. In either of those situations, it is possible to pass the high-pressure steam directly through conventional steam turbines – very similar to those used in fossil-fuel power stations – to generate electricity. Power plants that convert ultra-hot water to steam are called ‘flash’ steam turbine plants.
New technological developments, in particular progressive “binary cycle” systems, enable power generation from geothermal fluids with temperatures as low as 100°C. This is achieved by transferring the heat from the hot water into another substance which boils at a lower temperature than water, with the resulting vapour then used to drive a turbine. This opens up the geothermal potential of regions which are not as ‘hot’ as those in volcanic areas.