GTherm: Cutting Cost and Quakes From Geothermal Power

A three-year-old startup has a new approach to tapping the Earth’s heat for power generation at sites where conventional geothermal technologies fall short. GTherm, based in Connecticut, says its innovation offers a safer and less costly alternative to so-called enhanced geothermal systems, and can even turn depleted or under performing oil and gas wells into sources of clean energy.
Michael Parrella, founder and chief executive of GTherm, told us in an interview that the bright idea for the company’s innovation came when he realized that they could move heat instead of water. Three years ago, he set about reading hundreds of patents related to geothermal energy, and realized much of the geothermal industry was stuck on moving water. Parrella’s knowledge of thermodynamics, however, led him to ponder: why not move heat?
“It was a Eureka moment,” Parrella said.
The geothermal power industry’s laser-like focus on moving water was partly the result of a federally funded report released in 2007 by analysts at the Massachusetts Institute of Technology, which looked at the future of engineered, or enhanced, geothermal systems (EGS) in the U.S. The report found promise in geothermal systems using large amounts of water pumped through an area of fractured rock, and after the report came out “everybody stopped looking at other ways of doing it,” Parrella said.
The same landmark MIT study also recognized the potential seismic risk that could come with fracturing hot rock using huge amounts of water under high pressure (called “fracking”), and noted that some regions would have difficulty supplying enough water for these thirsty geothermal plants. But the GTherm concept, dubbed Single-Well Engineered Geothermal System, or SWEGS, is meant to circumvent those challenges.

Smarter Geothermal

The SWEGS design, which has yet to be demonstrated at scale, calls for the creation of a “heat nest,” made up of a heat exchanger at the bottom of a well, a specialized thermal grout, and what Parrella calls “heat highways.” These are horizontal drilling bores extending several hundred feet out from the well into hot rock.
Rather than piping water through the rock, GTherm proposes using Duratherm, a fluid that transfers heat. The fluid (“environmentally inert,” according to Parrella) is circulated in a closed-loop system through the heat nest and up to the ground level. It heats fluid circulating through a second loop, driving a turbine connected to an electric generator.
As the Electric Power Research Institute (EPRI), which is working with GTherm and Dartmouth University to analyze the SWEGS concept, noted earlier this year, GTherm has a ways to go on the road to large-scale commercial deployment. “Ultimately, down-hole heat exchanger designs, grouts, and working fluids will need to be tested and optimized in field environments over increasingly large scales,” EPRI wrote.
Yet if it works as planned, GTherm’s approach could offer several advantages over competing geothermal technology, eliminating the need for large amounts of water, limiting exposure to corrosive, mineral-laden brine, reducing seismic risk, and avoiding the cost of injection and production wells. “It’s a much less dangerous situation on the topside,” said Parrella.
Maintenance is minimal, required only for above-ground valves, pumps, and generators — or as Parrella calls them: “the spinning things on the surface of the power plant.” Each SWEGS plant requires about two acres of land, but the projects needn’t monopolize the surface. In Jamaica, for example, the company is working to develop a 10-megawatt plant underneath an airport.
“We pick up the heat, bring it up top, and use it to generate electricity,” Parella said. And the “amount of heat we extract equals the refresh rate of the rock.” That means the “well will never deplete,” he said. The system could be put to work at “dead geothermal wells,” in the deep rock of the Northeastern U.S., and at abandoned oil and gas wells around the world. According to EPRI, however, this type of single-well system will likely be most effective when deployed in a “fractured or porous medium” that is highly saturated.
The company is applying for several grants from the Department of Energy and EPRI. Already, EPRI has helped GTherm complete mathematical modeling, Parrella said.
In addition to its U.S. staff, GTherm has subsidiaries in the Dominican Republic, Jamaica, Costa Rica and Chile, employing a total of about 35 to 40 people. Citing job estimates from the Department of Energy, Parrella emphasized GTherm’s plants could employ hundreds more if they come online as planned.
GTherm has a handful of plants in the pipeline, including a 2 MW plant in the Dominican Republic; a 4 MW plant in Southern California; a pair of 10 MW plants in Costa Rica and Jamaica; and a 30 MW plant in Texas. Each plant is expected to cost less than $4 million per megawatt, according to Parrella.
It takes eight weeks to “put a hole in the ground,” for GTherm’s system, and for a project on the scale of the Texas plant, GTherm’s contractors will need to drill as many as 36 well holes. These projects are modular in the sense that “as each two holes are drilled, a nest is built,” and it can start generating electricity, said Parrella.
GTherm has been “self-funded” to date, according to Parrella. It is in the “final stages of negotiation” to secure power-purchase agreements for the projects, but Parrella said the company is fully committed to completing them, adding, “We’re ready to rock and roll.”