Geothermal – the firm renewable energy
If I asked you to imagine renewable energies, what would you see before your eyes?
Wind mills, towering over the land and casting their shadows wide? Or are you thinking of fields and fields of flat, blue panels of photovoltaic covering the ground, slightly tilted towards the sun?
These are the pictures you have seen. Wind and solar are often referred to as “variable renewable energies” (VRE), because, well their energy output is variable.
A lot of the discussion about the capabilities of these energy forms to replace fossil fuels is exactly about this variability. It’s not enough to be able to power the grid “on average”, supply and demand have to be matched at all times – or there are blackouts.
This either means that some form of firm baseload, flexible backup power and/or storage has to be present in the grid, which increases system costs. What options do we have?
Currently, most advocates of VRE favor more energy storage. But storage is still far too expensive for this purpose. Other possibilities for creating firm electricity supply without carbon emissions are fossil power with carbon capture and sequestration and of course nuclear power.
But not all forms of renewable energies are variable though. There are also: biomass, hydro and the neglected form of renewable energy and today’s topic: geothermal power.
Geothermal generates only about 0.4% of US electricity in 2019. Amazingly, this represents 20% of geothermal’s entire global electricity production. Geothermal energy uses the earth’s natural heat to create electricity. This heat is thought to be generated by the decay of radioactive elements in the earth’s core. The flow rate is roughly 30 TW. This is double of all human energy consumption. It’s estimated that this process will continue for quite some time. Billions of years is in the ballpark, although the power level will decrease.
There are several ways to extract that heat. Which we will look at shortly.
According to the ARPA-E “0.1% of the heat content of Earth could supply humanity’s total energy needs for 2 million years.” It is a huge, untapped resource.
Geothermal is renewable and inexhaustible. It is capable of baseload power and can be dispatched flexibly. It is almost ubiquitously available, if the newer technologies are used. It is resilient to weather events and hard to sabotage. Geothermal energy offers another advantage that VREs are lacking. It offers an easy way to deliver heating for commercial and residential applications. It’s estimated that the geothermal resources are large enough to satisfy that demand for several thousand years at current consumption levels.The catch? Geothermal power is too expensive at moment. But if it could be made to work reliably and economically, it could become a perfect companion to wind and solar.
In 2008, paper on geothermal predicted 140GW of global geothermal capacity in 2050. But is also states the estimates for the potential may be “orders of magnitude higher based on enhanced geothermal systems (EGS)-technology”. This assessment was mirrored in an paper by the IPCC. It also estimates between 50 and 200 GW of geothermal electricity in 2050. But it also states that between 2,000 and 4,000 GW are possible. The potential is not constraint by availability, but by “economics, demand, material constraints, and social factors.”
Geothermal has gained an important ally lately. The drilling industry. Oil and gas exploration has taken a serious downturn during the Covid19 induced drop in demand. A lot people with experience in drilling technologies are out of work.
Technologies developed for and during the shale revolution are also thought to enable a lot of the more advanced approaches towards geothermal energy. Experts in drilling are hopeful that learning form the past decades can be deployed to geothermal systems.
In a sense geothermal energy lacks opponents in the US. The 24/7 carbon-free electricity is appealing to Democrats and the usage of drilling technologies conforms well to the Republican narrative about American ingenuity, jobs and fuel security.
Oil and gas majors might be interested in buying up some geothermal startups to shelter their portfolio in a world of low oil prices. It would also be a more natural match for these companies than wind and solar, because it neatly ties into the their specific core knowledge: exploration and drilling.
So how can this energy be made useful for us? There are three conditions for a geothermal reservoir to be able to provide geothermal energy: Heat, coolant and permeability. Heat is obvious, but why the other two? Well, because you need a way to extract the heat from the rock. For that you need some form of coolant (mostly water) and a way to get it out of the rock, which means you need flow paths for the water out of the rock (permeability).
Conventional hydrothermal resources
There are a few places on earth, like parts of Iceland, California or Indonesia, where the temperature close to the surface is really hot, the rock is relatively permeable and fractured and water or even stream is present. In these places, the hot water or steam rises through the crust and gets captured under less permeable rock formations. This creates huge, pressurized reservoirs that are only waiting to be tapped into. Well, sometimes the water is not waiting and finds a way out on its own. We can detect its presence via hot springs or fumaroles.
To tap into these reservoirs, a suitable production well is found via trial and error. So called exploratory wells are drilled until one of these proves to be good enough for production purposes. The hot, pressurized water has a temperature of up to 300°C, but only very special locations offer such temperature levels. Once heat is extracted, cooled fluid is returned to the reservoir via an injection well. This is crucial to keep the pressure up.
Almost all of the currently productive geothermal projects are using such a high-quality hydrothermal resource. Unfortunately, these are confined to to rather specific geographic locations, like volcanic formations. Of course all of the obvious spaces, with a lot of fumaroles etc., have long been utilized. Unfortunately finding new ones without these obvious cues is far more difficult and an area of active research.
This does not scale well. So what can be done?
Enhanced Geothermal Systems (EGS)
If not all of the three conditions for a geothermal system are there naturally, what can we do change that? That’s what EGS tries to answer.
There is a plenty of heat in the rock in a lot of places. Not all of is is porous enough. Not all of it is wet enough. So how to you make rock more permeable and get more coolant in there? Here, the experience of the shale revolutions comes in handy. Vertical wells are drilled down and high-pressure liquids are used to fracture the rock. The details differ on which liquids to use and what the operating pressures are, but at least superficially, these processes are pretty close to each other.
The vertical wells allow the injection of the desired amount of coolant during the operation of such a geothermal system, while the fissures in the rock, created by the fracking process, increase the permeability to sufficient levels.
Temperature, depth and permeability are the variables that characterize a site’s desirability for EGS. The better the technology gets, the more of the sites with suboptimal variables can be utilized. As a rule of thumb: the deeper the reservoir, the hotter and less permeable it is.
The plan for EGS is to start in already explored and existing geothermal formations, branch out horizontally from there to places near these formations and get vertically down after that. At least in theory, advances in technology would enable the next phases.
While it is thought to be possible to inherit some of the technology of the shale revolution, it might be impossible to avoid inheriting a rather troublesome heirloom: the public’s fear of fracking. While the proponents will try to convince with certain differences, like lower volumes and pressures of injected fluids, but it remains to be seen whether this is enough to persuade voters and legislators.
Super-hot-rock geothermal
If EGS gets deep enough, there is a threshold, where the performance of such a system increases significantly. The gain and the challenges of these depths is great enough to give these systems a special name: super-hot-rock geothermal.
If the temperature of the reservoir gets over 374°C and the pressure increases over 221 bar water becomes supercritical. That is a special phase, which is not quite a liquid and not quite a gas. In this state, the water can hold much more energy than in water or steam per unit mass, somewhere in the range of 4 to 10 times as much.
Additionally, the higher temperature enables higher efficiencies in the conversion process. There is a real, physical limit for how efficient a heat conversion system can be. This is the famous Carnot efficiency, 1-Tcold/Thot.
This means that the efficiency of a 400°C reservoir is approximately 50% higher than that of a reservoir at 200°C. Additionally, the same permeability of rock would give you a roughly 6 fold increase of heat flow out of the ground.
You get more energy out of the ground and you can convert more of that energy into electricity. The same well gives you almost 10 times as much electricity.
This doesn’t come for free, of course. These reservoirs are deep, 10km and more. The pressures, weird chemistry of supercritical water and temperatures make different materials for casings necessary than the oil and gas industry is used to. Corrosion concerns are higher, because drilling equipment has not yet been developed with this use case in mind; and frankly, it wasn’t needed in the oil sector until now.
There are research projects to develop contact-free drilling technologies. which replace drilling bits with high energy beams. These melt and vaporize rocks. It is hoped that this approach could increase drilling speed by 10 times or more, while reducing costs and reaching higher temperatures and greater depths.
Advanced Geothermal Systems (AGS)
A certain type of geothermal system that gets rid of some of the associations with fracking is called Advanced Geothermal Systems (AGS). Unlike EGS, which is an open system, where the working fluid in not guided through any artificial ducts, but instead through fissures in the reservoir, a AGS is a closed loop systems.
The coolant is guided through artificial tubes and picks up heat from the surrounding rock, while flowing through the tubes.
To do this, multiple, precisely drilled horizontal connections are established between vertical wells. The ability to dill horizontally is an achievement of the shale revolution. A Canadian company called Eavor, wants to build the equivalent of up to 100 km of sealed well bore per AGS. Their catchy slogan is: “The future truly is wind, solar and Eavor-Loop”
In a different configuration, even more lateral pipes can be attached to two vertical boreholes. Controlling the horizontal drilling seems more challenging, though.
Interestingly enough, differences in the density of the working fluids are eliminating a lot of the pumping needs for this technology via the thermosyphon effect.
This technology only relies on hot rock and brings the other 2 crucial components: a coolant, which does not even have to be water, and the permeability via the horizontal pipes. This increases the number of potential reservoirs significantly.
A plan to proceed
The developers of geothermal systems have a game plan to increase the capabilities and applicability in a piecemeal fashion, which seems solid.
What can be done to accelerate these endeavors? As I have often stressed, R&D can really help us in finding solutions to climate change. Until now, there has been inconsistent public funding for geothermal technologies. To change this, the Breakthrough Institute calls for 5 to 10 projects to fail and innovate to improve the technology. What would that cost? A billion? Maybe 3? Half a billion?
Another obvious policy to get a new technology going: subsidies. Clean energy credits and feed-in tariffs have been used in several countries successfully to bring the cost of technologies down. Of course, care has to be taken to ween the industry and the customers soon enough to prevent some of the effects that Germany seems to be experiencing right now: run-away costs and increasing concerns about grid-stability.
A predictable schedule for these subsidies would create certainty for the developers and limit the risk to the tax payers.
Given that a lot of the most promising reservoirs are in the West US, where a lot of land is held by the federal government, preferential leasing of this land to clean energy projects seems like another possible avenue to aid the development of geothermal systems. Absent of a price on carbon, clean energy standards are another mighty tool to shape the energy infrastructure of the future, which would obviously benefit deployment of geothermal systems.
Regulatory burdens seems to be higher for geothermal systems than for drilling projects at the moment and building permits are only granted sluggishly. It seems very possible to level this disparity – by treating a geothermal project as an oil & gas drilling project.
Research money for the demonstration and development of crucial technologies, a predictable schedule for subsidies, sites on federal land for the demonstrations and early deployments and easing of the regulatory burden don’t all come for free. But we know the financial risks and what our losses are in the worst case. Let’s look at the upside: The promise of geothermal is huge. It could help to bring firm, carbon-free energy to the grid. It is a given that it does not make sense to electrify everything and that the demand makes the challenge to reach zero or even negative emissions more costly and difficult. Thus, there will likely always be a huge need for chemical energy and process heat; geothermal can satisfy that need and make an even larger contribution to combat climate change.
In my book, this looks like a very good bet. Any takers?
Further Reading:
https://thebreakthrough.org/issues/energy/take-geothermal-seriously