‘Hot Dry Rock’ (HDR) Geothermal Energy

In the last issue of Around Langley I explained that certain areas of the earth are better suited for the exploitation of geothermal energy, for the simple reason that one finds higher temperatures close to the surface of the earth. Cornwall was identified then as the region in the UK with the highest temperature gradient in the near-surface.

The reason why the heatflow is enhanced in Cornwall is the several km thick granite, with its radioactive, heat- producing elements, that forms the peninsula. What is special about the Cornish granite is that its radioactivity is virtually constant throughout its thickness. In other granites of the UK, present in Northern Ireland, Scotland and the Lake District for example, the surface measurements of radioactivity roughly match those of Cornwall, but it decreases rapidly with depth, so the total heat generated due to the granite mass is small compared to that in SW England.

In order to exploit this heat energy, we need it to be brought to the surface, and the way that happens is through water. If the rocks are naturally water-bearing like some sedimentary formations, then if we sink a borehole, the higher pressure underground will allow hot water to gush out. But the water that comes out will be rich in minerals and will fur up pipes and other instruments if used directly. So a heat exchanger is used to extract heat, and the cooled down geothermal water is ‘put back’ into the ground through another borehole. The ‘clean’ water that has been heated up can now be used, mostly for space heating.

Granites, however, are ‘dry’ rocks and contain no groundwater. Therefore in order to get the heat out, we need to introduce water at depth, by drilling boreholes, through so-called ‘injection wells’. Having ‘poured down’ water into the ground we have to ensure that it gets heated up and is then brought back up again by other boreholes, called ‘production wells’. So each site of geothermal energy needs to have a ‘twin borehole system, consisting of an injection well and a productions well.

At the start of the HDR project, it was totally unknown whether any hot water would be recoverable. It was known that granites have cracks and joints underground and the expectation was that the water coming out at depth from the injection borehole would percolate through the cracks and faults, get heated up and eventually find its way to the production borehole. To facilitate easier circulation of water at depth, explosives are set off to expand the cracks which would normally be pretty well closed with the pressure of the rocks above.

Los Alamos in America first pioneered the project, and to begin with, there was no return flow from the production well. This made the engineers set off more and bigger explosives, to no avail, because the result was to melt the rocks and seal up the cracks! With smaller amounts of explosives, they were in fact able to show that this was a viable project. There will also be cracking and expansion of the joints by the injection of cold water. That was the beginning of the HDR geothermal project. Our own experiment in Cornwall started around that time, in the 80s, and there was good collaboration between the engineers of the two projects.

Twin boreholes at five or six kilometres depth can produce ‘superheated’ water at 200o C and more, our modelling had showed. The hot water coming up this way also would have so much dissolved minerals in it that it needs to have a heat exchanger before it can be taken to drive turbines in an electrical power generator. But heating and lighting a small town using this technique is by no means beyond the realms of technical feasibility.

This is the last of the three articles on geothermal energy as a Renewable Energy source, and the potential for geothermal energy in the UK. In summary we can say that if there is the political will and economic necessity to do so, we could indeed use the heat from the earth as a renewable energy source. In some areas, it can be used as low grade energy, applicable in farming, heating of dwellings and so forth, while in a few areas it would even be possible to fulfil the electricity and heating needs for whole towns.