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What Is An Enhanced Geothermal System?

We need cleaner, more sustainable sources of energy, right?

molten rock

Photo: Alan Sterling (Flickr)

Cooled, molten rock.

Time to go again to the A Moment of Science Mailbag. A listener writes:

Dear A Moment of Science,

We need cleaner, more sustainable sources of energy, right? So here’s an idea: Dig down deep enough into the earth and you eventually hit molten rock, right? So why not just drill down to the hot rocks, pump in water to pick up the heat, and then channel it up to the surface and transfer the heat to a liquid with a low boiling point to make steam to run turbines? Why wouldn’t that work?

Enhanced Geothermal System

It does work. In fact, what you’ve just described is called an enhanced geothermal system, or hot dry rock system.

Scientists have been experimenting with it since the 1970s. The catch is that working with hot rock thousands of feet underground is tricky. Those rocks are not permeable, so you have to pump water down at high pressure to create a
network of fractures for the water to flow through.

But it’s not easy to create a precise system of fractures that allow the water to flow precisely in the direction of the well that returns the water to the surface. If the cracks don’t line up the right way, the water may not head in the right direction.

Fracturing The Cracks

The most important thing geologists have discovered is that fracturing doesn’t create a new system of cracks in
hot rocks.

Instead, it re opens cracks that are already there but fused shut over millions of years. So a lot of the research goes towards mapping the native cracks and understanding how water flows through them.

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