As much as a third of the heat needed to keep Scotland warm could be provided by tapping geothermal energy from old coal mines across the central belt, a major new study for the Scottish Government has concluded.
Warm water piped up from abandoned mine shafts between Glasgow and Edinburgh and in Ayrshire and Fife could help heat many thousands of homes and other buildings for decades, researchers said. They are urging ministers to embark on an ambitious attempt to make geothermal energy a major new source of clean, renewable power within a few years.
The two-volume, 345-page study was conducted by the US energy firm Aecom and the British Geological Survey, and has been published by the Scottish Government. Geothermal energy from deep underground has "the potential to play a significant role in Scotland's future energy provision", it said.
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The most promising source is the water that has flooded the hundreds of disused mine shafts that underlay large areas of the Central Belt. Heated by the warmth of the Earth, it averages 17C, with higher temperatures at deeper levels.
According to the study, the water can be pumped to the surface and, in combination with modern heat exchange technology, used to supply central heating and hot water to buildings. It estimates that water from Scotland's 600 cubic kilometres of old mine workings could provide a "maximum accessible heat resource" of 12 gigawatts.
The study said: "On this basis, mine waters could theoretically provide the equivalent of approximately one third of Scotland's heat demand." It points out, however, that the actual contribution is likely to be less because of the problems of transporting heat over long distances and the possibility that some mines might not be suitable for use.
The study recommends a series of actions by Scottish ministers in the next three years, including the development of a national geothermal energy strategy. It suggests two major new "demonstrator" projects, at the Clyde Gateway in eastern Glasgow and at Shawfair in Midlothian, by 2016.
It points out that two small geothermal schemes in Scotland that tap the warmth of mine water have been running since 2000. One is at Shettleston in Glasgow and the other at Lumphinnans in Fife, each serving fewer than 20 homes.
The study authors urged the Scottish Government to introduce a presumption in favour of geothermal developments in its current planning policy. They say a proposed renewable heat subsidy of 5p per kilowatt hour "has been broadly welcomed by developers".
For the longer term, the study calls on ministers to launch a national geothermal exploration programme aimed at uncovering the potential for extracting heat from "hot rocks" deeper underground. It also mentions that hydraulic fracturing - fracking - may be needed to help extract the heat.
If the heat is used to generate electricity, the study suggested the existing price subsidy would need to be doubled. It also recommends new legislation by 2020 to clarify legal ownership of geothermal resources.
The proposal to tap the heat from old mines has been backed by environmental groups.
Dr Sam Gardner, head of policy at WWF Scotland, said it should be seriously examined and "taken forward in every suitable Scottish city and town".
Dr Richard Dixon, director of Friends of the Earth Scotland, said: "It is a nice irony that some homes that used to be heated by coal are now being heated by water from old mine workings, and it would be great to see this idea deployed on a very wide scale."
The renewable energy industry agreed that geothermal energy had "significant potential" in Scotland. Joss Blamire from Scottish Renewables said: "We have the chance to add another excellent asset to our renewable energy mix."
The Scottish Government described the potential for geothermal energy as very welcome news, with a spokeswoman saying it showed Scotland's renewable energy potential "could be even greater than we imagined".
She added: "We would like to see geothermal energy play a significant role in our energy future."
"A number of recommendations have been provided in the report that may assist in offering the necessary certainty to exploit our geothermal potential, and the Scottish Government will consider these recommendations carefully."