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Sustainable Energy

Geothermal without the Earthquakes

An underground “heat nest” design avoids the need to fracture the rock.

A startup in Connecticut says it has a way to improve the reach of enhanced geothermal energy, without the financial or geological risks associated with such projects.

Enhanced geothermal systems (EGS) represents a promising source of clean power generation in geographies that lack the ideal combination of underground heat, water, and rock permeability needed for conventional geothermal. But with EGS, developers typically have to engineer the conditions they require, and this involves fracturing solid rock by pumping fluids into wells at high pressure, an approach that has raised concerns about the potential to trigger earthquakes and contaminate aquifers.

The problem, called “induced seismicity,” led to the cancellation in 2009 of a project in Basel, Switzerland, after the high-pressure fracturing of rock around the well caused hundreds of seismic events, some large enough to damage property. In North America, EGS developer AltaRock Energy has been caught up in a similar controversy.

“You can get seismic events with any kind of fracturing,” said Herbert Einstein, a professor of rock mechanics in MIT’s department of civil and environmental engineering. “If you do it close to a city, it’s an issue.”

GTherm, founded in 2008, says it has come up with an approach that doesn’t require any fracturing or water cooling. It uses a kind of solid-state heat exchanger—what the company calls a “heat nest”— at the bottom of wells. The nest draws heat away from the surrounding rock more efficiently, with the help of a highly conductive grout that encases the heat exchanger.

To generate power, fluid travels the length of the well in a closed loop and carries the heat from the nest back to the surface, where a secondary fluid within a separate closed loop is turned into gas to drive an electricity-generating turbine. To further enhance heat recovery and increase power output, thin bore holes about 100 feet long and lined with heat-conducting material can be drilled off the main vertical well. “We’re basically a heat pump on steroids,” says Michael Parrella, CEO and founder of GTherm.

The company has completed 3-D modeling and is doing early validation and commercial feasibility testing with the Electric Power Research Institute (EPRI). Demonstration projects could begin as early as 2012.

Luis Cerezo, a technical executive within EPRI’s renewable generation program, says there is significant potential to use GTherm’s single-well design in areas that have been off-limits to geothermal development. “We’re looking at depths of about five kilometers with down-the-hole temperatures of between 250 °F and 300 °F,” says Cerezo. “With this, we’re aiming to produce one megawatt net from each well.”

One megawatt isn’t much, but GTherm envisions a more distributed and scalable model of geothermal generation—from installations of a few megawatts to large clusters of wells totaling hundreds of megawatts.

Thousands of depleted oil and gas wells across the United States and Canada are prime candidates for development, Parrella says. Temperature data is already known in these fields, significantly reducing exploration costs. Parrella is convinced GTherm can deliver clean power for less than 10 cents per kilowatt-hour.

“The approach is definitely beneficial,” says Einstein. But he questions whether GTherm has truly eliminated the risk of triggering seismic events. “The grout must be a viscous liquid before it hardens, so I don’t see why putting it in the well wouldn’t cause similar (seismic) problems. So they’ll have to show it actually works.”

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