Why I’m excited about geothermal energy
Clean, flexible baseload power — what’s not to like?
This post follows on from a previous one about managing renewable intermittency. If you’re not clear on why clean, flexible power is valuable to the electricity system, it’s worth going back and giving that post a read.
What is geothermal?
The centre of the Earth is hot. Real hot.
I’m not going to go into why it’s so hot. All you need to know is that it starts to get hot a couple of kilometers below the surface and then gets hotter as you go deeper.
Geothermal power plants extract this heat, then use it to create steam to drive a turbine and generate electricity. In some cases, the heat can be used directly in buildings or industrial processes, but electricity generation is what this article focuses on.
And there’s a lot of energy down there. This poster from Stanford University provides estimates of the size of different energy sources available on Earth. It puts geothermal energy at 15m zetajoules, over 20,000 times all the energy stored in fossil fuels, and 170 times all the solar energy reaching the Earth’s surface.
Geothermal is interesting because it’s able to provide baseload power without requiring any fuel and without emitting any CO2 — something that only hydropower (and maybe nuclear?) can claim to deliver today. Modern geothermal technologies are also flexible — power output can be increased or decreased as needed, allowing it to operate alongside variable renewables like solar and wind. Finally, geothermal can be developed with limited surface footprint, given almost all the magic happens underground (see image further down the article for an example of this). This allows deployment of geothermal plants with minimal disruption to existing land users.
I’ve been interested in geothermal ever since I read a new year (or new decade) blog post from Eli Dourado. In the post, Eli tries to predict the technology breakthroughs that will occur in the 2020s. In a section on energy, he states: “advanced geothermal could reach 2¢/kWh [which is incredibly cheap] and scale to become viable just about anywhere on the planet”. I started reading up on the space and have been following what companies like Eavor and Fervo have been up to ever since. I decided to write this article after I became aware of developers exploring, and in some cases constructing, geothermal projects in the UK.
The rest of this article lays out the constraints on geothermal, and the advances in technology looking to overcome them. It concludes with an overview of what’s going on with geothermal in the UK.
Traditional geothermal
Our World in Data estimates that over 14 GW of geothermal electricity production has been deployed around the world. The vast majority of this is what I’ll call traditional geothermal. This is geothermal deployed in locations where there are high temperatures not too far below the surface and where there is permeable or fractured bedrock. Where these two features (high temperatures and permeable rock) coincide, water can be injected into the rock, will heat up and will emerge at the surface as steam that can be used to drive a turbine.
Traditional geothermal is only possible in places with surface-level tectonic activity — i.e. where there are volcanoes and geysers. It’s viable in the likes of Iceland, Indonesia and parts of the US, but not in the UK. (Whilst it may not be exhaustive, this map gives a good overview of where geothermal plants are located today).
The locational limitations on geothermal described above could soon change with upcoming advances based on two simple ideas:
- If we dig deep enough, it’ll get hot.
- If the rock isn’t permeable to water, we can fracture it.
Beyond these locational limitations, there are 3 key environmental challenges associated with geothermal. Firstly, injecting water into the geothermal wells runs the risk of depleting local water supplies. Secondly, extracting fluids from wells risks contaminating aquifers, damaging the local ecology and releasing gases trapped underground (such as carbon dioxide, methane or sulphur dioxide) into the atmosphere. Finally, extracting water from underground reservoirs can lead to the collapse of structures in the rock and subsidence at the surface. This subsidence can have a devastating effect on the geothermal well and plant itself, and also on other land users located near the well.
‘Closed loop systems’ that recycle the extracted fluids and gases back into the well can help address each of these issues but can’t eliminate them.
There are three newer types of geothermal technology worth discussing. These technologies begin to eliminate the locational constraints and may help with the environmental challenges:
- Deep geothermal,
- Enhanced geothermal systems (EGS), and
- Advanced geothermal systems (AGS).
Deep geothermal
Deep geothermal is a natural continuation of what has proven effective at shallower depths, only deeper. Whilst it involves drilling further to reach high temperatures, it still relies on existing fractures or permeability within the rock so that injected fluids can pick up heat and then make their way back to the surface. Deep geothermal therefore relaxes one of the two key constraints on traditional geothermal — projects can be located where high temperatures are further below the surface.
In addition, as wells get deeper and temperatures get higher, the amount of energy that can be extracted increases significantly. This is because water at high temperature (over about 375 °C ) reaches what’s called a ‘supercritical’ state. The physics at play here are complicated and I’m not going to pretend to understand them, but all we need to know is that supercritical water contains far more energy for each unit of mass and is capable of generating far more electricity. As wells are drilled deeper and get hotter, we should be able to generate far more electricity per well.
There is ongoing exploration and development of deep geothermal projects in both Cornwall and Scotland. While I’m no geologist, a quick look at a geological map of the UK indicates plenty of spots where the bedrock is described as “Unnamed Igneous Intrusion” in both Cornwall and Scotland. This type of rock is what makes deep geothermal possible.
Deep geothermal technologies and techniques allow the development of geothermal electricity projects in a wider range of areas, where the bedrock is porous or fractured but where high temperatures are further below the surface. EGS and AGS take geothermal a step further and relax the second constraint on traditional geothermal as well, related to the permeability of the bedrock.
Enhanced Geothermal Systems (EGS)
EGS technologies allow deployment of geothermal in locations where the bedrock is not porous or fractured. Deep wells are drilled into the rock and fluid is injected at high pressure in order to create fractures. These fractures allow water to pick up heat from the surrounding rock, and then allow the resulting steam to escape to the surface where it can be used to drive a turbine.
EGS allows development of geothermal resources in more places around the world. As long as you can drill deep enough to find the heat, and as long as you can make the bedrock porous or fractured, you can extract the required energy. Progress will be gradual as there are still technical challenges to be addressed, but deeper drilling combined with EGS technologies will allow geothermal electricity plants to move further and further from their traditional ‘hotspots’ over time.
EGS technologies suffer from the same challenges described above relating to water injection and extraction. In fact, the fluids that are pumped into the rock in order to cause fractures, along with the fluids and gases released when fractures occur, will be harder to control and account for given it’s not possible to perfectly predict where fractures will happen. Therefore more of them may leak out into aquifers or into the atmosphere, something which we need to avoid in order to make claims about geothermal being a ‘clean’ generation technology.
In addition to these issues, the intentional fracturing of the bedrock can cause seismic activity — i.e. small earthquakes. Testing of a geothermal well at the Eden Project in Cornwall was paused after a 1.7 magnitude earthquake was detected in March 2022.
These combined issues with geothermal may sound familiar, because they’re the exact same issues cited by opponents to fracking (alongside the fact that fracking will lead to more burning of fossil fuels). More on this later.
Advanced Geothermal Systems (AGS)
Advanced geothermal systems take geothermal to a new level again, opening up impermeable and unfractured rock by drilling straight through it. Using drilling technologies from the oil and gas industries, AGS involves drilling a continuous borehole deep into the bedrock, horizontally underground and then back up to the surface again.
This borehole can be lined or sealed, creating a closed loop system where fluid is injected on one side and then recovered, hotter, on the other. The cold fluid will sink, while the heated fluid will rise back to the surface, creating a thermally driven cycling of the water through the borehole, no pumping required. The emerging hot fluid can be used to heat water via a heat exchanger and the resulting steam can be used to drive a turbine.
As advanced geothermal systems are closed loop systems, they reduce the environmental risks related to water usage, pollution and subsidence. They also allow the use of fluids apart from water to transfer heat to the surface— given the system is sealed and fluids can’t escape, it’s possible to use fluids with better thermal properties than water but which would be dangerous if they leaked out into the rock and contaminated local water supplies. In addition, given AGS involves drilling through rock rather than fracturing it under pressure, seismicity is both less likely and likely to be less intense.
AGS might be the holy grail when it comes to geothermal. But it’s still got a long way to go. Someone who knows geothermal far better than I do explained to me that risk-averse investors like pension funds might be willing to fund a proven technology like traditional or even deep geothermal. But it’s only venture capital firms who are willing to fund AGS projects right now.
The only operational AGS project that I’m aware of is a demonstration project constructed by Eavor, a Canadian startup. Whilst this project is an important milestone, demonstrating the drilling techniques required and proving the physics of the closed loop system, it doesn’t reach the temperatures required for electricity generation.
UPDATED TO ADD: In November 2022, Eavor announced that it had started construction of its first commercial Eavor-Loop project in Germany. Drilling is expected to start in summer 2023 with the first energy being produced in 2024. When at full capacity, the site is expected to produce enough heat and power to supply 30,000 local homes.
Key challenges facing geothermal
Geothermal will only play a meaningful role in the future energy system if it can overcome its locational constraints, allowing it to move outside areas with surface-level tectonic activity. Next-generation geothermal technologies like deep geothermal, EGS and AGS aim to do this. But additional challenges remain.
One of the most pressing challenges is related to drilling and requires solving if next-generation geothermal projects are to be developed economically. Current drilling techniques struggle at the depths and temperatures required for next-generation geothermal projects. The oil and gas industry has been developing more advanced drilling technologies (e.g. horizontal drilling) to allow access to those hard to reach oil and gas reserves. However, oil and gas wells have to worry less about high temperatures given they’re seeking out hydrocarbons rather than heat itself. Developing drilling technologies that work at temperatures of 200 °C or more (and ideally up to 500 °C) will be important for the geothermal sector.
Another key area where progress will be required to make geothermal cost-effective is the surveying and modelling techniques used to predict site (or well) potential and performance. If a new geothermal project requires drilling tens of exploratory wells before the right one is found, costs will be sky high. AGS may help with this given it doesn’t rely on identifying the right sort of rock that will fracture as required (it relies solely on finding heat), but even AGS will need the ability to plot a course avoiding the hardest rock to make drilling as quick (and therefore cheap) as possible. Again, technologies from the oil and gas industry should help and at a minimum form the basis of what’s required.
The fact that there are technologies ready to be borrowed from the oil and gas industry should be seen as a huge positive. Moving away from fossil fuels will result in a reduction in jobs supported by the oil and gas sector — geothermal gives these people a place to continue using their skills. It also means that the geothermal industry can hit the ground jogging (if not running) given it won’t need to develop all these technologies and skills from scratch. Last month, the US Department of Energy (DoE) announced a $10m programme “to develop a roadmap for addressing technology and knowledge gaps in geothermal energy, based on best practices used within the oil and gas industry”. Once the roadmap is developed, the DoE plans to fund $155m of research.
As well as addressing engineering challenges, geothermal will need to do some work on its public image. This will be important if geothermal wants government support (including subsidies or perhaps a simplified planning process) and if geothermal plants are ever to be located close to where people live and work.
Geothermal will be met with scepticism by the public, given the process of drilling wells is viewed as being similar to fracking. The drilling, fracturing and operation of geothermal wells may be far safer and less environmentally damaging than the equivalent processes used in fracking (although this remains to be seen). However, if the public’s perception is that they’re no different, then geothermal is likely to face serious resistance and NIMBYism. There’s plenty of work required to distance geothermal from fracking and brand geothermal as safe and sustainable.
What next?
Geothermal must reduce its costs to allow next-generation projects to generate power at a similar cost to other clean baseload technologies (which means nuclear or large hydropower).
As noted above, the US DoE is supporting this effort by funding research, and the development of EGS demonstration projects. The DoE’s hope is that, with some support, geothermal can move down its learning curve, reducing costs along the way. And these cost reductions are needed — a study published in March 2022 suggests that, for AGS to become economic, drilling costs still need to fall by over 50%.
In the UK, the government allows geothermal projects to participate in its Contract for Difference (CfD) scheme. The CfD scheme guarantees a price for the electricity produced by a renewable energy project over fifteen years, de-risking the project and helping it raise funding. The CfD scheme played a key role in driving down the cost of UK offshore wind over the past two decades.
Inclusion in the CfD scheme allows geothermal to access a government-guaranteed electricity price of up to £154 per MWh (in current prices), roughly 3 times the price guaranteed under the same scheme for several offshore wind projects in July 2022. No geothermal projects have been awarded a CfD contract to date, possibly because none have applied. Maybe we’ll see our first geothermal project with a CfD contract in the auction in the first half of 2023.
Oil majors are also getting involved at the riskier end of the spectrum, hoping to bring about cheap geothermal in the form of AGS that can be deployed anywhere. Eavor announced $40m in funding in 2021 from the likes of BP and Chevron (Michael Liebreich of BNEF fame is also an investor).
Could geothermal make use of existing power generation infrastructure to bring costs down further? In another article on geothermal, Eli Dourado suggests repowering coal and gas plants using geothermal as a source of high temperature steam, as opposed to a furnace powered by fossil fuels. He writes: “These generators take supercritical steam as an input and use it to produce electricity. The generators don’t care whether the steam comes from a boiler fired with coal or from 15 km underground. Piping steam from a geothermal production well straight into a coal plant turbine would allow the power plant to produce the same amount of electricity as it did under coal, except with no fuel costs and no carbon emissions”. As well as making use of existing turbines, this approach would also allow the use existing grid connections and grid capacity, further reducing costs.
Geothermal’s potential in the UK
I thought I’d finish this article with a quick summary of geothermal’s progress in the UK.
Could geothermal be the next offshore wind? Or will it go the way of the hydrogen car? It’s hard to predict, but I’d like to believe it’ll be somewhere in between the two.
According to the UK government’s renewable energy planning data, there are 4 geothermal projects seeking planning permission (all in Cornwall) and 1 that has received planning permission but is yet to start construction (at the Eden Project, also Cornwall). I’m also aware of a couple of different companies exploring geothermal in Scotland but these projects are yet to reach planning, so they’re at an early stage. So all in all, we’re off to a slow start.
The British Geological Survey (BGS) doesn’t leave me confident that we’re about to witness an explosion of geothermal electricity generation. The BGS states subsurface temperatures in the UK “are below the economic threshold for conventional (steam turbine) power generation (which is >160°C)”. However, they may be high enough “to provide heat for direct-use space heating as well as for a variety of heat-intensive industrial processes and agricultural applications”. It’s not clear to me whether the BGS’s assessment above considers only traditional geothermal, or whether it takes into account the fact that wells are getting deeper all the time and technologies like EGS and AGS are on the horizon.
But who knows? These sorts of technologies have a habit of surprising everyone with how rapidly they improve, how quickly their costs fall and how widespread they become. They also have a habit of fizzling out without anyone noticing.
Maybe we will have a geothermal boom in Cornwall and parts of Scotland. And maybe AGS will prove itself as the holy grail of geothermal and will end up being deployed in other parts of the country.
If none of this happens, we might have to wait for the proposed Icelink interconnector to start delivering geothermal-abundant power from Iceland before we feel the benefits of geothermal electricity in the UK. However, that’s not looking too promising either.
