Jon Busby, Team Leader Renewables, Energy Storage and Clean Coal at the British Geological Survey details the benefits of deep geothermal heat sources in the UK…
In simple terms ‘geothermal energy is the energy stored in the form of heat beneath the earth’s surface’. Most people will associate it with steam emanating from mineral encrusted fumaroles, but even in non-volcanic regions such as the UK there is great potential for geothermal to contribute to our energy mix. It is carbon free, sustainable and is not subject to intermittent supply as are some renewables, e.g. solar, wind and wave.
The temperature increases in the ground with depth at an average rate of 26°C per km. At depths of 4-5km power generation is possible, but this is mainly consigned to regions with large masses of high heat producing granitic rocks. These granites have very slightly raised levels of the natural radiogenic isotopes of potassium, uranium and thorium, whose radioactive decay leads to a small heat anomaly at depth. Cornwall is the most prospective region for power generation and 2 companies, with the support of the county council, are actively pursuing power projects with generation capacities of between 3 and 10 MWe.
Of greater (and more widespread) potential is the utilisation of geothermal heat for district heating and agricultural purposes. There are 3 fundamentals required for exploiting deep geothermal heat. The rocks must contain water (or brine) within the voids (pores) between the mineral grains; the pores must be interconnected so that the water can flow through the rock (the rock is permeable) and the rocks must be at a sufficient depth for the temperature to be high enough for the direct use application. At a depth of 1km temperatures of around 36ºC would be expected (sufficient for agricultural heating), but depths of 2-3km (temperatures of around 60-90ºC) are needed for district heating. A group of rocks, referred to as the Permo-Triassic sandstones, are known to have favourable porosities and permeabilities and therefore, at depth, are a potential direct heat use geothermal resource. These rocks underlie a number of regions at favourable depths, most notably east Yorkshire and Lincolnshire, Cheshire, Gloucestershire, Dorset and the eastern counties of Northern Ireland. The UK’s only deep geothermal facility is at Southampton where a legacy borehole from the early 1980s was brought into production in 1987. It exploits the Permo-Triassic sandstones in a depth interval of 1,725-1,749m. The brine is extracted at a temperature of 76ºC and provides part of the heat to a city centre district heating scheme.
The distribution of Permo-Triassic sandstones at depth is limited and does not coincide with many of the main urban centres. However, many regions are underlain by older sandstone rocks that still have favourable porosities and permeabilities, albeit less so than the Permo-Triassic sandstones. Water can also flow through fractures in the rock and many of these older rocks contain ancient fractures. Evidence of this can be found at the 2 localities where there are warm springs; Bath/Bristol and the Peak District. Here, water flows through fractures in limestone relatively rapidly from depth, at Bath the temperature of the water is 46ºC and at Buxton it is 27ºC. The possibility of finding similarly fractured limestone elsewhere is being explored as a potential heat source for district heating. In Manchester, planning permission has now been granted for boreholes that would intersect limestone at 3km depth. There is also evidence that water may flow laterally along ancient fractures within the crust. Buried granitic rocks to the west-southwest of Newcastle-upon-Tyne are known to have high heat producing characteristics. Warmer water around the granite may migrate eastward along an ancient fault zone. This water may then recharge sandstone beneath Newcastle which may have favourable porosities and permeabilities. This has been investigated recently beneath a brownfield site in the centre of Newcastle where a measured temperature of 73°C at a depth of 1,767m was recorded, indicating a geothermal gradient of 36°C/km. The higher than expected temperature at depth clearly suggests that heat may have been transported by migrating groundwater.
At the end of 2012 the UK’s exploitation of direct use geothermal heat was a paltry 0.01TWh/yr (from the Southampton scheme). By contrast, in mainland France the direct use of geothermal heat was 1TWh/yr and Germany 0.7TWh/yr. Neither of these countries has a fundamental geological advantage when it comes to geothermal. Now that a deep geothermal tariff of 5p/kWh has been introduced for the Renewable Heat Incentive, perhaps it is time for the UK to catch-up on its exploitation of geothermal heat.