Geoscientist Online

Peak Oil is here and our Government, very belatedly, is thinking about replacing fossil fuels with renewable energy. Wind energy is the current favourite. Ambitious proposals are being aired, like generating 15% of our energy (not just electricity) from wind by 2020. This would involve erecting many thousands of wind turbines (several every day) on- and off-shore, although they are in short supply; and producing their steel components requires massive amounts of fossil fuel.

The problems associated with wind energy are well known. Wind speed is so variable that, according to German and Danish experience, a turbine rated at 2 megawatts typically produces little more than 0.5 megawatts when averaged over a year. Seriously large amounts of electrical energy can only be stored in exceptional circumstances (such as when pumped storage like Dinorwig in Snowdonia is available) in spite of experiments with compressed air and giant flywheels. So the vital, and in my view fatal, drawback to wind energy proposals is that conventional power stations must be available, ticking over, ready to cut in and supply the nation’s electricity when the wind fails. Moreover, the National Grid cannot easily accept the huge slugs of electricity that wind power produces at unpredictable intervals.

Electricity storage on a grand scale is required. It occurs to me that hydrogeologists might be able to help. Steam from geothermal boreholes can drive turbines when temperature exceeds 150°C. Natural groundwater temperatures in UK experimental boreholes fall far short of this figure (Southampton, 1.8km deep, 76°C; Eastgate, 1km deep, 46°C). But why not store wind energy as superheated water at the bottom of a deep borehole? When powerful immersion heaters are available, using intermittent input from a wind farm, water could be injected into a suitably permeable stratum at, say, 2km depth and heated until it and the country rock reach 200°C or more. Steam could then be released and used to drive turbines. I visualise one or more boreholes, cased as necessary, carrying the heaters and cold water input down to the permeable stratum. Steam extraction would be from an adjacent borehole, in much the same way as oil can be mobilised by heat and removed from tar sands.

In this way, one of the great disadvantages of wind energy could be overcome. No doubt much energy would be lost in changing electricity to heat and back again, but set against this would be the huge advantage of reliable continuous energy supply, removing the need to have conventional power stations as backup for when the wind stops blowing. The mimic geothermal borehole would differ from ‘hot dry rock’ ones in the UK in that their sources of heat gradually cool down, whereas that of the mimic would be topped up indefinitely.

No doubt there are practical problems with this scheme, such as steam leakage to surface; but the advantages, if after comprehensive research it could be made to work, would surely be worth the effort.