Everyone interested in renewable energy has a favourite, which he or she may feel is unjustifiably neglected in the competition between alternative sources. With solar, wind, wave, tidal, hydro, biomass, geothermal and many others vying for investment, I have long believed the heat beneath our feet provides a huge energy store that deserves more public attention.
So I was delighted to attend a briefing organised by the Royal Academy of Engineering in London, at which geologists and engineers called for more investment both in ground source heat pumps, which extract energy from near the surface for individual buildings, and in deeper geothermal projects, which involve drilling down to 1,000 metres.
“We are a sitting on top of a gigantic free reservoir of natural heat in the ground – ubiquitous and potentially available to all,” says David Banks, a heat source expert. “All we need to access it is a hole in the ground (a trench or borehole) and a pump, to lift it from a low temperature, typically 10-14ºC in the UK, to a high temperature at which it can be distributed and used to provide space heating.”
Banks says heat pumps are beginning to take off in Britain, with the market doubling every year. But the total installed base in the UK, which he estimates at 6,000 units, is still far smaller than in Scandinavia and north America.
In Sweden 350,000 units are installed, providing more than 10 per cent of heating in homes and offices. The US accounts for about half the world’s total ground source heating capacity, with 80,000 units installed every year.
Heat pumps have struggled because the initial capital costs of installation are high – typically £10,000-£15,000 for a 6 kilowatt domestic system – and natural gas is still relatively cheap in Britain. But they could pay off within 10 or 15 years for households in rural areas who have no mains gas supplies and enough space to lay the 100 metres of pipes required to extract heat from the ground. For commercial and public buildings, the economic returns are better.
While heat pumps extract low-level heat always available in the ground for use in buildings, some parts of the world have easy access to much more intense heat. This may be a blessing in energy terms but it often comes at a price because such places are vulnerable to volcanic activity and earthquakes. Iceland and the Geysers geothermal field in California are good examples.
Britain’s geology is not so favourable for geothermal energy but hot rocks are down there if you drill deep enough. The country’s only operational scheme has provided hot water to homes and offices in the centre of Southampton for more than 20 years, but David Manning, director of Newcastle University’s Institute for Research on Environment and Sustainability, says a recent project in Weardale, County Durham, shows the potential for geothermal energy in north-east England.
Prof Manning and colleagues supervised the drilling of a test borehole 1 km deep at Eastgate, which produced plentiful supplies of hot water at 40ºC – see picture below. It is expected to create a spa and heat buildings in a new “renewable energy village“.
The same geological formation of hot granite hundreds of metres deep extends up to the Tyne Valley. Geologists are now planning another borehole on the old Newcastle Brewery development site.