Frequently Asked Questions

It has been done before – though only rarely in the UK. In areas with the best geological resource it is a mainstream technology: Iceland gets the majority of its heat from geothermal sources, while enough geothermal power is generated in California to supply one million households. Several deep geothermal power plants already exist in Continental Europe, and there is one existing UK scheme: the geothermal borehole in Southampton which has successfully fed heat into that city’s heat network since 1986. In the 70s and 80s the UK was in fact the world leader in ‘EGS’ technology, before very low fossil fuel prices caused the ‘Hot Rocks’ project to be suspended.

As the input fuel for geothermal energy is the essentially inexhaustible heat of the earth, this is not a relevant question for geothermal energy. It is worth noting that for a typical geothermal power station, large amounts of heat are typically produced along with the electricity. Geothermal energy is thus highly suited to being operated as combined heat and power (CHP).

The cost of geothermal energy is highly dependent on the specific generation technology, and the location of the plant. In general the deeper the drilling required, the higher the cost of the power or heat produced. In sites where high temperatures are accessible close to the surface geothermal power can be highly competitive against fossil fuels without the exposure to fuel price volatility.

Geothermal energy is an exceptionally clean technology, involving power plants with a small surface footprint and little or no direct venting of greenhouse or other gases. As such it is one of the lowest carbon forms of power generation available, comparable to hydroelectric or wind power. In a closed loop system water loss is also very low. While there is little academic work on the carbon intensity of Geothermal heat systems, it is regarded as being even lower carbon still. One source suggests that generating 1 MWh of geothermal heat will produce less than 1 Kg of CO2 - the comparable figure for gas central heating is around 185 Kg of CO2 per MWh.

No. ‘Fracking’ involves fracturing - artificially generating cracks - in the rock at depth to increase permeability, generally using high pressure water or another suitable liquid. The technique is used in the extraction of shale gas, which is unrelated to geothermal energy production. The projects that Cluff Geothermal are developing in the UK are targeted on finding permeable rock at depth, and therefore will not involve fracking, while our East African projects will rely on energetic hydrothermal resources where fracking is entirely unnecessary.

One of the positive features of deep geothermal power is that the surface footprint is very small – typically [1 or 2 acres], and the only gases produced in any quantity will be steam. As the heat comes from underground, there are also no deliveries of fossil fuel. The largest impact is from the temporary presence of the drill rig, which is typically in place for a matter of weeks. [During the drilling of the 1800m deep borehole at Science Central – in the centre of Newcastle, and close to residential properties – there was only one complaint about noise, which was satisfactorily resolved.

No. The heat in deep masses of granite has been caused by radioactive decay of the rocks, but this has taken place slowly over millions of years. Water which has passed through the rock (or even the rock itself) is no more radioactive than a granite kitchen top.

No. There have been allegations that EGS geothermal plants have in the past caused small earthquakes (known as ‘microseismics’) but this is associated with fracking techniques, which will not be necessary for Cluff’s projects.

No. The amounts of heat in the interior of the Earth are unimaginably vast, and any heat removed to generate heat or electricity would be microscopic in comparison.

Over a period of a few hundred years geothermal energy is fully renewable. Running a geothermal power station does cause a small reduction in the temperature of the host rock or aquifer, but this is a small effect given the time frame of geological change. The temperature in an aquifer might drop by a few degrees centigrade over a few decades of exploitation, for example – though note that the world’s first geothermal power station, at Lardinello in Italy, has been running continuously since 1909! Once the power station is shut down the temperature will recover. For this reason geothermal energy is sometimes described as ‘heat mining’.

The distance from the surface to the centre of the earth is around 6370 km, or 3,960 miles - the distance from London to Chicago. Geothermal boreholes are less than 5 km deep: the distance between Trafalgar Square and Paddington Station.

With a small surface footprint and near-zero greenhouse gas emissions, geothermal drilling has a very low environmental impact. Also, any extraction or injection of water for use in a generating plant is subject to close monitoring by the Environment Agency; for example, they would prevent any unregulated discharge of brine-rich water into the surface environment. This issue would not even arise in a closed loop system or a borehole doublet where one was used for reinjection.