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Converting coal plants to geothermal plants

Published by , Assistant Editor
Energy Global,

As the world moves away from fossil fuels in the battle against climate change, could coal plants be repurposed to produce renewable geothermal energy? Panelists addressing the topic at PIVOT2022, a recent weeklong geothermal conference, were excited about that potential, citing the number of enabling technologies that are coming online and the escalating interest in the heat beneath our feet.

Converting coal plants to geothermal plants

After a wide-ranging discussion that covered everything from the size of the potential market to a novel drilling tool that could allow access to the mother lode of geothermal energy, moderator Alex Fitzsimmons, a Senior Director with ClearPath, brought the group back to the title of the session, “Converting Coal Plants to Geothermal: On the Horizon or Science Fiction?”

“A lot of the technology advances [needed] are coming into that proof step where you’ll have physical proof that they work. So I would say we are ready to launch, if we can just bring together the right utility, the right contract and engineering expertise, and the right site to launch the proofs to show that this can be done,” said Ken Wisian, a geothermal geophysicist and Associate Director of the Bureau of Economic Geology at the University of Texas, Austin, US. “The picture could be accelerating dramatically over the next few years. We just need the proof projects to land.”

The opportunity

Neeraj Nandurdikar is a Senior Vice President with Wood Mackenzie, a global energy research and consulting firm. He began the session by outlining the potential opportunity, noting that there are about 4200 coal plants around the world, of which approximately 1000 appear to be retired. Most (some 2200, of which 2000 are operational) are in China, which also has 84 plants under development.

The US is next with approximately 750 plants, of which approximately 200 are operating (no new projects are under construction). India, Indonesia, and Germany round out the top five countries with the most coal plants. Many plants in the US, he said, are idle or shut down because of the lack of economic incentive. “So from our vantage point, the opportunity is big if something can be done with these assets.”

John Kosub is a Senior Director at CPS Energy, a utility serving the San Antonio area in Texas, US. He made a similar point. CPS has made huge investments in the equipment associated with producing power from coal, such as steam turbines and grid interconnection, “so reusing those investments has a significant cost advantage,” he said. Continuing to use the workforce “would be [another] big advantage”.

Coal plant conversions and greenfield geothermal operations, or those built from scratch, could also “take advantage of the existing oil and gas workforce and completely pivot it to this purpose,” said Kevin Bonebrake, CFO for Quaise Energy. Bonebrake noted that the majority of Quaise employees have an oil and gas background, including co-founder and CEO Carlos Araque. (Araque was among the winners of a PIVOT2022 Five on Fire award recognising geothermal catalysts of the year).

Where to start?

Where are the most promising areas for a coal to geothermal conversion?

“The hotter the better,” said Wisian. That means focusing first on coal plants that are close to the superhot rock relatively near the Earth’s surface that can be found in places like the Western US, Iceland, and near volcanoes.

For most of the world, however, such superhot rock is not accessible. It is too far down, at 10 - 20 km (approximately 12 miles). The conventional drill bits used in the oil and gas industry cannot withstand the extreme temperatures and pressures at those depths.

Bonebrake explained how Quaise’s new drilling technology could change that “by enabling you to get deeper and hotter than anybody has been able to go before.”

Earlier, Wisian noted that “getting to the level of energy density of a coal power plant is going to be difficult for geothermal…unless you get to super deep, supercritical situations.”

To reach those conditions, Quaise replaces the conventional drill bits that mechanically break up the rock with millimetre-wave energy (cousins to the microwaves many of us cook with). Those millimetre waves literally melt then vaporise the rock to create ever deeper holes. An inert gas that accompanies the millimetre waves brings the vaporised rock back up to the surface.

Bonebrake explained that a single module of the final system would consist of three deep wells: one that injects water, and two others to collect the heated water and carry it to the surface. The heated water is then used to generate electricity in the power plant, after which it is recirculated back down the injection well to repeat the cycle.

Other enablers

Wisian applauded the many geothermal technology developments described at the conference, but stressed that “you cannot ignore the geology. It is just like, no matter how great a new jet you have, if you are going somewhere you need as detailed a forecast of the weather at your landing site as possible.” Fortunately, “that old-fashioned geologic detective work” is common to the oil and gas industry. “I am interested in transitioning that methodology and mindset into…geothermal.”

A member of the audience asked if there is a need for a screening tool with technical, financial, and regulatory aspects that can help utilities assess their options. “Yes,” said Wisian. “A publicly accessible or shareable modelling tool would be a significant step forward.”

Nandurdikar of Wood Mackenzie said that such a tool is being developed by his firm and Quaise. Among other things, they have already mapped all the coal plants in the US with a pinpointed assessment of the geothermal attractiveness at each site.

Fitzsimmons concluded the session by noting that “finally geothermal is starting to get some of the attention that it deserves. We just have to keep pushing forward.”




For more news and technical articles from the global renewable industry, read the latest issue of Energy Global magazine.

Energy Global's Summer 2022 issue

The Summer 2022 issue of Energy Global hosts an array of technical articles focusing on wind, solar, biofuels, green hydrogen, geothermal, and more. This issue also features a regional report discussing the outlook for renewables in the US.

Read the article online at:

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