Harnessing hot rocks

A twin-borehole system in Durham will heat water for use in a geothermal power plant by cycling it 1km underground.

Newcastle University engineers are planning to develop a power plant on former cement works in County Durham that will harness heat from deep underground.

The twin-borehole geothermal system will allow water to be continually cycled through a maze of granite fractures as deep as 1km, where it will be heated up to temperatures of 30°C or more, then brought up to the surface and passed through heat exchangers before being returned underground to be re-heated.

According to project leader Prof Paul Younger of Newcastle University, the system will provide renewable energy for homes and businesses in the planned Eastgate eco-village in Weardale, County Durham, complementing four other forms of renewable energy – solar, wind, biomass and hydro – that are also harnessed there.

Aside from heating homes and businesses, some of the natural hot water will also be used in a spa – the first such development in the UK since the Romans tapped the hot springs at Bath. It will also be used to provide an environment for farming Tilapia fish, an alternative to cod that thrives in warm water.

Funding of £461,000 from the Department of Energy and Climate Change (DECC) will allow the researchers to drill a second re-injection borehole at the site to complement a 995m deep exploration borehole that was originally drilled there three years ago.

Prof Younger said the DECC funding to drill the second borehole will solve problems that have hindered other attempts to develop such direct geothermal systems.

Using two boreholes for both the primary and secondary water-injection stages removes the need for a packer arrangement to isolate two sections of a single borehole. This would ensure that the differences in water temperature and pressure did not adversely affect the operation of the system.

Not only would that be a cumbersome approach, it would also require additional maintenance personnel and mean that the single borehole would become cluttered with equipment.

The latter is important because ideally Prof Younger and his team would like to use directional drilling tools to drill still deeper into the existing borehole to around 2-3km, to intersect the main permeable structure of the granite – a depth that would produce water that would then be hot enough to generate electricity.

Water Cycle
Water Cycle: The twin-borehole geothermal system will cycle water to 1km depths

’There is every reason to suppose that if we drill even deeper here in future, we will find water at boiling point,’ said Prof Younger. ’Once the twin set of boreholes is complete, we will be in a position to explore this possibility.’

Should they discover water at such a temperature, the researchers may then deploy what is known as a binary-cycle power plant, an indirect geothermal system that would be capable of generating electricity from the water.

In such a binary-cycle system, heat from the water causes a secondary fluid to flash to vapour that can then drive a turbine. Moderate-temperature water is by far the more common geothermal resource and most geothermal power plants in the future will be of the binary-cycle type. Newcastle University’s Prof David Manning said the plan was to build a geothermal prototype that could be used at other ’hotspots’ across the UK.

“There is reason to suppose that if we drill even deeper here in future, we will find water at boiling point”

’Water deep underground gets heated by the naturally occurring low-level radiation that is found in all rocks,’ he explained. ’But some rocks are far better at producing heat than others – especially granite of the kind we drilled into at Eastgate. This makes it one of the country’s “hotspots” – where water starts warming up quite close to the surface.’

Because the researchers were only granted planning permission for the site late last year, it is too early to say what sort of power the plant may produce.

’Until we have finished the injection borehole, it is hard to say what sort of power we may be able to generate,’ said Prof Younger. ’The limiting factor will not be the production well – because it has the highest yield of any borehole ever drilled into granite anywhere in the world -but the amount of water that we can re-inject back into the well. Nevertheless, we hope that we can achieve a 5MW power output over the next few years.’

Production essentials

The key facts to take away from this article

  • Water will be continually cycled through granite fractures 1km deep
  • It will be heated to 30°C, then be brought up to the surface and passed through heat exchangers
  • There are plans to drill to 3km, at which level water could reach boiling point
  • Granite produces more heat than most rock types