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Flexible geothermal power combines clean energy with built-in ‘battery’

Professional Engineering

(Credit: Shutterstock)
(Credit: Shutterstock)

Flexible operation of new geothermal plants could provide the best value for the grid, a new study has found, instead of viewing them as ‘around the clock’ energy sources.

By using the inherent energy storage properties of new enhanced geothermal projects, which have different features to conventional geothermal, researchers at Princeton University in New Jersey and start-up Fervo Energy found that flexible geothermal power and falling costs for drilling technology could provide over 100GW of capacity in the western US – a greater capacity than the entire US nuclear fleet.

“Such an innovation would transform geothermal energy from its niche status on the grid today into a major component of a decarbonised future,” the researchers said.

Conventional geothermal power plants require a specific set of conditions: hot, permeable rocks close to the Earth’s surface, and fluid to transport heat from underground. These requirements limit traditional geothermal installations to specific locations with naturally occurring geysers, volcanoes and hot springs, the team said.

“Advances in drilling and hydraulic fracturing technologies have unlocked the enhanced geothermal approach, which removes the need for permeable rocks and greatly expands access to the heat that already exists far beneath our feet,” the announcement said.

In the newer approach, engineers dig two or more boreholes into impermeable rock that extend thousands of feet below the Earth’s surface, then drill similar distances horizontally. After creating fracture networks in the horizontal sections to connect those holes deep underground, they inject a fluid into one borehole, which heats up as it travels through the fractures. That heated fluid can be pulled up through the other wells and subsequently used to generate electricity.

“Enhanced geothermal is much less geographically dependent than conventional geothermal, which is really only possible in a small number of ideal spots,” said Wilson Ricks, first author of the study. “With enhanced geothermal, you can open up wide swathes of the country – wherever you can dig down and find hot rocks close to the Earth’s surface.”

By operating enhanced geothermal power plants flexibly – using the inherent energy storage capabilities offered by enhanced geothermal reservoirs to generate more or less energy as needed – the team found that the value of geothermal energy dramatically increased because it could complement and compensate for intermittent energy sources such as solar and wind.

The impermeable rock required for enhanced geothermal also functions as a self-contained underground reservoir for heated fluid, which acts like a giant energy storage system.

To ‘charge’ the system, engineers would inject more fluid underground than they pull out, filling up the reservoir with heated fluid and building up pressure. When clean energy is most needed, this heated, pressurised fluid would be discharged to generate electricity.

“Because enhanced geothermal comes with its own variation of energy storage, you’re able to shift how you operate the plant to generate as much power as possible at the times when it’s most valuable to the grid,” said Ricks. “That added flexibility allows enhanced geothermal to avoid generating energy when wind and solar are active and electricity prices are extremely low, something many alternative energy sources struggle to do.”

The researchers published their analysis in Nature Energy.

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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