Deep mining for heat on a giant scale

A grandiose scheme for electrical generation from hot rock via a gigantic underground heat exchanger has been unveiled in London

Innovative proposals for an advanced form of large-scale geothermal generation have been made public at London’s Imperial College by the Mining for Heat Consortium. The project would be extremely costly, but if successfully developed it could reach a size of 10 GW and generate electricity for as little as 1.4 cents per kWh. The scheme entails sinking a series of shafts to a depth of around 12 km to allow water to be heated to 400°C at 150 bar, a technology that has only been made feasible by recent advances in mining.

Unlike conventional geothermal plant, the system does not rely on naturally occurring fissures within the rock strata to allow water flow between the inlet and exit vents. Instead, a series of shafts of perhaps 2 km depth with spurs or haulages, each of which will hold a heat exchanger and a series of pressure control valves, would be constructed. Subsequent shafts would then be sunk from the haulage tunnels, and so on to the required depth.

Heat exchanger

As the figure (right) shows, the water treatment system pumps desalinated water into the main reservoir; filling it is expected to take as long as two years. The system will use large volumes of water at modest flow rates with an estimated through-flow duration of around 24 hours for the entire series of heat exchangers and small reservoirs. Steam will then be sent through the steam riser to the subterranean generator room before going on to the dump condensers, ready to be recirculated.

There are several geothermal sources but in this case the heat comes from hot dry rock, usually deep crystalline basement rocks such as granite and basalt characterised by low water content combined with high temperature. The source is renewable, (ie it is replenished faster than it is extracted) and of enormous potential but in the past commercialisation has been ruled out by the very high capital cost, especially where drilling is involved.

The system outlined here is inappropriate for regions which are not tectonically stable, but in any case is not dependent on close contact with high temperature rock. The temperature gradient in the earth’s crust is typically 30 to 40°C per km. The heat source for this project is warm rock (at 400°C plus) owing its high temperature at least partly to the accumulated heat of radioactive decay. Granite is a suitable rock for this kind of heat source. In the UK a sizeable inclusion of granite at high temperature is located in Cornwall, in a region stretching 70 miles across the county. Other regions suitable for such developments are the east coast of the United States, Germany and northern Russia.

Commercialisation

Costs for initial stages of research and development are estimated at around £60 million ($86 million) and would see an exploration hole drilled down to the required depth to determine the structure of the rock strata and the heat gradient through the rock. Initial R&D is expected to last 3-5 years, but development of the full-scale system is expected to take as much as 25-30 years, and at a cost estimated at £80 billion using conventional mining techniques. The justification for time and costs on such a scale is a projected lifetime for the plant of 50 years, and, backers suggest, possibly double that. Some of the technology necessary, especially the deep level mining, is not likely to exist for ten years yet. And the scheme depends on a single fact – that running costs would be very low. Despite the large initial investment, generation costs conservatively estimated at 1p/kWh are expected, which would keep the cost lead time to a minimum, with a return on development costs calculated rather optimistically at 3-5 years.

Additional benefits are expected to accrue as side effects from the technical development required, particularly in drilling technology, robotics, micro turbine development and mine tailing utilisation. The originators of the scheme – mainly academics and mining engineers with a couple of entrepreneurs – accept the extremely long term (probably 25 years), nature of the project and extraordinarily high cost (estimated at £200 billion) but liken it to the Channel tunnel or the moon race – and contrast it with the apocalyptic alternative of fossil fuel exhaustion.