UK goes back to the Batholith to develop geothermal energy

There are plans afoot to generate geothermal electricity from Cornwall's hot granite. It has been tried unsuccessfully before but new technologies for enhanced geothermal energy have given it a new lease of life.

Frustrating stuff, energy - there's masses of it about; unfortunately most of it tends to be in the wrong place and in the wrong form. Geothermal energy is a good case in point. The legacy of the kinetic energy of the colliding debris that formed the Earth some 4.5 billion years ago, plus the heat generated by radioactive decay within the crust, means that the Earth is a thermal energy source of, quite literally, astronomical proportions.

Generally speaking, this vast energy store is buried out of reach; only becoming accessible along tectonic fault lines, where disruptions in the crust mean the heat can come much closer to the surface. In such instances, and where the heat is combined with precipitation filtered down through the local rocks to produce steam, we have the basis for a geothermal generating plant.

Around the world there are currently some 5,000 such power stations, with a combined capacity of around 10,000MWe. It's a useful figure, but it only amounts to 0.3 per cent of the world's total electricity demand. What's required is some means of accessing the Earth's store of geothermal energy in regions - the great bulk of the Earth's surface - far away from tectonic fault lines. In the early 1970s, around the time of the first energy crisis, scientists at the Los Alamos Laboratory in New Mexico reckoned they'd come up with the answer.

The plan

As you drill down into the Earth the rock gets hotter. Away from tectonic plate boundaries, the thermal gradient is around 25-30°C/km. What the Los Alamos scientists were proposing was to drill two boreholes, several kilometres deep, into the basement rock, and then fracture the rock to create a permeable heat exchanger linking the bottom of the two boreholes. Cold water would then be pumped down one borehole, heated as it passed through the heat exchanger, and drawn up through the second borehole at, hopefully, a sufficiently high temperature to form a basis for electricity generation. It was assumed the basement rocks would be hot dry and homogeneous, and the approach was quickly labelled HDR (hot dry rocks).

To fracture the rock, the scientists proposed hydraulic fracturing - a technique widely used to boost production rates in oil and gas wells. The principle is fairly simple. Large quantities of water, at high pressure, are pumped down a borehole where it forces the rock apart, creating fractures extending from the borehole into the rock formation, improving flow rates for the oil or gas. The fractures are typically kept open by a material such as grains of sand (a 'proppant'), which stops them closing when the pressure is released. First deployed commercially in 1949, hydraulic fracturing - or fracking as it's informally known, is now used in thousands of oil and gas wells each year.

Early days

The world's first HDR project was started in 1974, at Fenton Hill on the edge of Valles Caldera in New Mexico - just 35 miles west of Los Alamos. In 1977, the UK got in on the act with its own HDR research programme, based at the Rosemanowes granite quarry, near Penryn in Cornwall.

South-west England, Cornwall and Devon, rests on a geological formation called the Cornubian Batholith - a huge mass of interconnected granite intrusions, between 10 and 20km deep, 40-60km wide at its base, and extending some 200km from Dartmoor to the Isles of Scilly.

Granite, Cornish granite in particular, tends to be slightly radioactive, and the heat generated from the decay of radioactive isotopes, notably uranium, thorium and potassium, makes for a relatively high surface heat flow and a high temperature gradient. Across the Cornubian Batholith surface heat flows on average more than 110mW/m2, compared with a UK average of 54mW/m2, while the temperature gradients can reach 40°C/km. The higher the temperature gradient, the less you have to drill to reach a given temperature - a key consideration with drilling costs of up to £1m per km. Surface heat flows at Rosemanowes are 120mW/m2, which, along with its proximity to the Camborne Schools of Mines, responsible for supervising the UK HDR programme, made it an ideal location.

Many a slip

While the originators of the HDR concept envisaged drilling into uniform homogeneous rock, nobody anticipated such conditions at Rosemanowes. As a mass of molten granite cools from the top downwards, huge internal pressures are created, resulting in the formation of a network of fractures permeating the body of the granite mass.

It was the flow of hot fluids along such fractures in Cornish granite, leaching out tin and copper to create lodes of tin and copper ore, that formed the basis of the Cornish mining industry. So the Rosemanowes researchers certainly knew they were drilling into fractured rock, what they didn't appreciate was how it would respond to hydraulic fracturing.

It was assumed the fractures would be prised apart under the effects of pressure - in line with experience in oil and gas wells. Instead, before this could happen, the opposite sides of the fracture slipped relative to each other, shearing through a few millimetres. The opposite sides of the fracture no longer match, so the fracture remains open, increasing the overall permeability of the rock mass when the pressure is removed.

The direction of expansion of the heat exchanger formed by hydraulic fracturing depends on the interaction between the direction of pre-existing stresses and the orientation of the natural fracture network. The key consequence of the unexpected fracture mechanism uncovered at Rosemanowes was to overthrow prior expectations of how this interaction would work, so that the heat exchanger expanded in a totally unexpected direction.

The first two major boreholes were drilled to a depth of 2km, 300m apart. Contrary to expectations, subsequent hydraulic fracturing produced a heat exchanger that expanded downwards, away from the space between the bottom of the boreholes, so that the hydraulic connection was poor. Flow rates were much improved by drilling a third borehole to a depth around 600m below the other two. Unfortunately, this positive result was offset by a loss of thermal performance, and between 1985 and 1988, the temperature of the water at the top of the production borehole fell from 80 to 55°C.

Closure

A key objective of the UK HDR programme was to determine the feasibility of creating a viable HDR subsurface heat exchanger - the acid test of viability being lots of high-temperature water produced without the need to consume excessive quantities of energy in the process.

To get lots of hot water you need a heat exchanger with a large surface area of ten million square metres. The principal energy demand in an HDR plant comes from the effort required to pump the water across the heat exchanger. Flow resistance is measured in MPa/l/s (the fluid equivalent of electrical resistance, with pressure replacing voltage, and flow rate replacing current) and the target flow impedance for viability was a figure less than 0.1MPa/l/s. These are exacting requirements; they have to be realised in rock only accessible via a hole several kilometres deep and just eight-and-a-half inches wide at the bottom.

The UK HDR programme made major contributions to what is now referred to as engineered geothermal systems (EGS) - rather than HDR - but when the project was reviewed in 1990, it was concluded that: "A satisfactory procedure for creating an underground HDR heat exchanger has not been demonstrated." In 1991 the UK Department of Energy closed the project, announcing that future UK research in this area would be conducted in the context of a collaborative European partnership.

Never say die

The closure of Rosemanowes was a disappointment, but it certainly didn't put paid to EGS developments. Major research programmes have continued, notably at Soultz in France (the focus of European research), the world's first commercial EGS plant has started operating in Landau in Germany, and across the world, EGS start-up companies are attracting significant levels of investment. In Australia, which has over a thousand square kilometres of promising hot granite, Geodynamics has raised close to AUS$300m in pursuit of its goal of eventually supplying 25 per cent of Australia's generating capacity. EGS also looks set for a comeback in Cornwall.

In June this year, EGS Energy announced plans to build a 3MWe plant, to supply electricity and heat to the Eden Project. Drilling of the first borehole for this project is expected to begin next year, with electrical power due to start flowing in 2012. A second company, Geothermal Engineering, also expects to be drilling a pilot-project borehole in 2010. Both companies are anticipating going on to build large-scale EGS installations - producing tens of mega watts and more - using assemblies of the basic EGS module demonstrated in their pilot projects. The obvious question is what has happened since the closure of Rosemanowes in 1991 to transform the prospect for EGS-based generating plant in Cornwall?

Yes we can

The answer comes in two distinctly different parts. Everyone agrees that there have been significant and important advances in much of the associated technology. Drilling is faster and cheaper, borehole data logging has improved, microseismic monitoring (used to plot the location of the heat exchanger) is much better, and the temperature at which electricity can be generated has fallen, meaning we can use shallower and cheaper boreholes. Where opinions differ is over the extent to which we can engineer viable heat exchangers on a more or less routine basis, across a board spectrum of geological conditions - the key issue responsible for the demise of Rosemanowes.

Roy Baria, technical director of EGS Energy, previously deputy project director at Rosemanowes and chief scientist and project coordinator at Soultz, in very much in the 'yes we can' camp.

"For the Eden Project, we need a flow rate of around 35l/s, at a temperature of about 165°C, from a depth of around 4km, but this doesn't mean we need highly specific or especially favourable geological conditions. What we have to do is determine what we've got in terms of existing stresses and fracture network.

"Then we use our toolbox of techniques to do whatever is necessary to engineer a heat exchanger to get the heat out. That's why the name has changed to engineered geothermal systems. By exploiting our understanding of slip stress from Rosemanowes, we can increase permeability by a factor of 20 to 30."

Not everyone is quite so optimistic. As a young mining engineer, Peter Ledingham began his working life at Rosemanowes. Today he's operations director at Falmouth-based Geoscience Limited - consultants to Geothermal Engineering. "The key issue is whether we do, or do not know how to make a heat exchanger of adequate size and performance," Ledingham says. "Our view is that the connectivity of what you start with is crucial, and, basically, Rosemanowes wasn't good enough."

In identifying a suitable location for Geothermal Engineering's pilot plant, Geoscience has concentrated on locations offering potentially higher natural permeability. "We're not going to drill through undisturbed granite," says Ledingham. "Instead, we're getting nature to give us a helping hand by picking a hot spot on a natural fault line."

Heat and power

EGS power plants will produce huge quantities of heat - the relatively low operating temperature means the thermal efficiency is around 10 per cent.

Both EGS Energy and Geothermal Engineering will thus be keen, initially at least, on finding markets for copious quantities of heat energy. The great attraction of the Eden Project from the EGS perspective is that its famous biomes (greenhouses) use over 2,800,000kWh of heat each year - supplied predominately via gas-fired boilers, augmented by a bio-mass back-up boiler. However, once the viability of the pilot plants has been demonstrated, the intention is to move on to larger EGS plants, focused on electricity generation.

"The Eden Project is a test for us," Baria says. "If it all works out, then we'll be flying. There will be a number of additional projects in the St Austell area, but then we'll be heading for the Land's End granite, going deeper for higher temperatures, and building 25-50MWe plants, supplying baseload electricity into the grid."

It's a great vision - a vast store of green electricity, totally unaffected by the vagaries of wind, sun and tide. In just a few years we'll know if it's for real.

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