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Engineered geothermal systems


An engineered geothermal system (EGS) harnesses the energy in the earth to produce commercial quantities of electricity.  The basic principle of an EGS is to access the high temperature available at depth and manipulate the underground rock mass to enhance permeability so that cooled water can be injected from one well and steam or hot water is returned from other wells, at an acceptable cost.

Temperatures from the earth’s hot core are curtailed from reaching the surface by the blanketing effect of layers of sedimentary geology (volcanic activity is a notable exception).  This blanket generally allows heat to flow at a rate of 60mW/m² with temperatures from the surface increasing at an average rate of 17-30°C/km.

Hot igneous rocks can be at considerable depths in the earth’s crust, as in the UK, and so reaching them with conventional drilling equipment is costly and difficult.  First, there needs to be exploration to locate high-temperature rocks within achievable drilling distance.   Second, the rocks need to be sufficiently fractured or demonstrate potential for artificial fracturing to allow hot water to be circulated and produced at sufficiently high rates.

How an EGS plant works

EGS technology involves drilling two wells several hundred meters apart down 3-5km into a rock system that has the right characteristics to be manipulated to produce power at the surface.  Water is pumped down into one borehole and returns through the other, heated up via the artificially created heat exchanger.

At the surface, the hot water (150+°C) can be used to drive a conventional turbine, with each two well module generating between 3-6MW.  The main parameters that determine the heat energy that is exploited are: the size of the reservoir in the rock; the rate at which water circulates through the reservoir; the distance between the faults through which the water flows; and distribution of the flow within the reservoir.

The key to the technology is the skill in enhancing the fracture permeability in the rock through which the water travels in order to be heated up.  The width of the aperture needs to be wide enough to let water pass through it with less resistance but encompass a large number of fractures to sustain heat extraction for years at a commercially viable rate.

Google.org has both invested significant amounts in EGS companies in the US and also produced an informative animation of what is involved.  See:

http://blog.google.org/2008/08/enough-geothermal-energy-to-power-globe.html

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