As one of our advisory panel members succinctly put it, “the plethora of geological environments and technologies associated with geothermal energy development can be confusing, even to geologists”. Articles in this edition cover different aspects of geothermal energy, from shallow, lower-temperature through to deep, superhot resources, as well as a rough cost comparison for different types, but it’s impossible to cover all ground in one issue, and we welcome further discussion.
Resources including the Society’s policy and technology explainer, Decarbonising heat with geothermal energy, and a newly launched geothermal glossary from Clean Air Task Force, as well as concise overviews from the BGS (www.bgs.ac.uk), help clarify and standardise concepts. In simple terms, geothermal energy is the heat beneath our feet, generated largely by Earth’s natural radioactive decay. Typically, subsurface temperatures increase at a rate of about 25-30°C per kilometre, so the deeper we go, the hotter the resource we can tap. Magmatism, mostly in regions close to plate boundaries, raises that geothermal gradient so that hot rocks exist at shallower, more accessible depths. To utilise the heat, we need a fluid, usually water, to circulate through the rocks and transfer the heat back to the surface. In the absence of a naturally occurring aquifer and existing permeability and porosity, we can fracture the rocks and pump water through in so-called enhanced or engineered systems. The ‘cooler’ heat delivered from shallow depths can be amplified to more useful temperatures via a heat pump (or used to cool buildings in warm months), while heat delivered from great depth is sufficient to convert water to steam, powering turbines that generate electricity.
Given its location far from a plate boundary, the UK does not have access to the copious geothermal energy enjoyed by countries like Iceland. But we still have an abundance of geothermal resources the likes of which are successfully exploited in countries with similar geology, including the Netherlands, France, and Germany. We’re far behind our European counterparts but efforts are accelerating. A recent report, Dig Deep: Opportunities To Level Up Through Deep Geothermal Heat & Energy On The Way To Net Zero, was commissioned by MP Dr Keiran Mullan and based largely on research carried out by a team at the Durham Energy Institute (including our President-designate, Jon Gluyas). The report identifies 45 sites in the UK – many in regions with ‘low economic resilience’ – with great potential for deep geothermal plants and concludes that with support (including data acquisition programmes), the UK could have 360 geothermal plants producing 15,000 GWh annually by 2050. The report recognises that past success with oil and gas in the North Sea means the UK has the technology and skills to develop a deep geothermal industry. It also highlights the potential for repurposing the warm water held in disused flooded coal mines as a lower-cost alternative to deep geothermal, with work by the Coal Authority showing that 25% of all properties and 9 out of the 10 largest cities in Great Britain lie above coalfields. At time of writing, the government is debating whether there is a bigger role for geothermal energy in the UK. It will be interesting to see what they conclude.
Amy Whitchurch, Executive Editor