Geothermal Energy: Origins, Exploitation and Energy Targets

By Dr Chris Rochelle; Senior Geochemist, British Geological Survey

Swirls of steam and the loud hiss of a large fumarole greeted me on my first visit to a geothermal area in the mid 1980s during my PhD studies. Heat energy was literally pouring out of the ground, and the visit opened my eyes to the potential of geothermal energy as a renewable energy resource. Now, some 30 years later, that single geothermal area in Costa Rica produces some 14% of that country’s electricity.  This, together with a large hydroelectric capacity (approximately 80%), allows 99% of Costa Rica’s electricity generation to be from renewable sources, and the country aims to be carbon neutral by 2021 [1]

Achieving 100% renewable energy generation is akin to the Holy Grail of national energy targets, not least because it would signify a huge shift away from our dependence on CO2-producing fossil fuels which have been the mainstay of our energy-generating technologies since the Industrial Revolution. However, we must make this transition, as climate change driven by release of greenhouse gasses to the atmosphere is one of the biggest challenges we currently face. And Costa Rica is not the only country with high aims. I write this blog on the outward leg of a long-haul flight to Mexico to help start a €20M joint EU-Mexico collaborative project which aims to enhance understanding of, two geothermal systems east of Mexico City. Developing such systems will help Mexico achieve its own target of 50% of its energy from ‘clean energy sources’ by 2050 [2]. The knowledge learned will also help in Europe’s geothermal development, which could provide an important contribution towards the European target of at least 20% of its total energy needs being via renewable sources by 2020 [3].

The author at a geothermal well site in Mexico – on a cold and wet November day.

Geothermal energy encompasses a range of techniques that extract heat energy from the ground [4] [5]. Key to these is water, which transfers the heat from hot rocks to surface power plant. Unlike other renewable energy options that are dependant upon sunshine, wind and tides, geothermal is able to reliably supply power 24 hours a day, which makes it very attractive. However, it does have two limitations. Firstly, the hot rocks are where they are, and this may not coincide with large centres of population or industry that would utilise that energy. So energy transmission may be required. Secondly, the hottest waters are found deep underground, and the boreholes needed to access them are expensive. Not only are these the largest expenditure of a geothermal plant, but this is an up-front cost. In addition, for new developments, there may be significant uncertainties because the production characteristics of the rocks may not be fully known. Geothermal development would thus be helped through the development of lower cost drilling methods, and through new high resolution geophysical techniques that could identify the best geothermal target rocks prior to drilling.

Now there are two basic ways in which we can use geothermal water produced from geothermal boreholes.
  • The first is simply to use the hot water to heat homes and offices. This is especially important in higher latitude regions – for example, in the UK some 60% of domestic power usage goes to heating our homes. Geothermal waters from about 70-100°C can be used directly, though if they are saline a heat exchanger may be needed to extract the heat into clean water. These geothermal waters could come from boreholes drilled into deep aquifers. One such example is in Southampton, where water at 70°C is used as part of the energy supply for a district heating scheme (the rest of the energy comes from fossil fuels) [6]
  • But for shallow, only slightly warm conditions, heat pumps are needed to concentrate the energy into useful amounts of heat. Such systems can be quite small, with buried pipes for ground source heat pumps being practical in suburban gardens [5] [7]
Power station at Krafla, Iceland

When geothermal waters produced from boreholes exceed about 100°C, and especially, 150°C, they are hot enough to be used for electricity production. For 100-150°C the waters are put through a heat exchanger and used to vaporise an organic liquid such as butane, and it is that which turns turbines connected to generators to produce electricity. However, waters under pressure at above about 150°C can flash to steam if the pressure is reduced, and this steam can be used to turn turbines. In some situations, zones of high pressure steam are intercepted underground, and this steam can be used to turn turbines directly. It is this high temperature (‘high enthalpy’) water and steam that are used in most geothermal systems around the world, typically in regions of volcanic activity [4].

Many countries possess such high temperature geothermal resources, but only a few of them are currently exploited. There is much potential therefore, to expand this important source of renewable energy, and in the process, reduce our dependence on fossil fuels.

The hottest geothermal well in the world
And in terms of future developments, the next step for geothermal energy is already beginning – exploiting ‘super-hot’ systems. These systems are above 350°C, but to access them requires drilling boreholes into rocks that are not just very hot, they contain fluids that may also be corrosive and contain a high dissolved load. So, to exploit such a powerful natural energy resource will require; advances in drilling technologies, corrosion-resistant materials, and accurate surveying techniques to direct drilling close to magma chambers. One such well was the Iceland Deep Drilling Project borehole number 1 (IDDP-1) well drilled at Krafla, Iceland. This drilled through the superhot zone and into a magma chamber, becoming the World’s hottest geothermal borehole, which produce superheated steam at 450°C [8]. Most importantly however, fluids from these wells carry more energy than other geothermal waters, and a superhot borehole can produce ten times the energy of a conventional geothermal well. To illustrate the importance of this, consider the Icelandic domestic power requirement (c. 150MW). It would take only three 50MW super hot geothermal boreholes to power all the houses in Iceland. If the technology can be developed to harness this natural energy resource successfully, then there is great potential for the future, and it is actively being investigated in countries such as Iceland, Italy, Japan, Mexico and the US.

IDDP-1 well under test (Landsvirkjun Power Company photo)
So geothermal energy has an exciting future ahead of it. Not only will its renewable credentials help its expansion as we move away from fossil fuels, but a revolution may be about to happen in terms of the amount of energy it can produce

Dr Rochelle gave a lecture on geothermal energy as part of the GERC Invited Lecture Series. You can watch it on our GERC YouTube channel below or on the GERC website

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