One of the first enhanced geothermal systems (EGS) was connected to the U.S. electric grid this April, marking a major milestone for the fledgling technology that seeks to tap the enormous terrestrial heat potential deep within Earth’s crust using directional drilling and pressurized water. Reno, Nev.–based Ormat Technologies, the U.S. Department of Energy (DOE) and Schlumberger subsidiary GeothermEx said they had stimulated an existing injection well to increase power output from brine by 1.7 MW at Ormat’s grid-connected 26-MW Desert Peak 2 geothermal plant in the Brady complex in Nevada’s Churchill County (Figure 4).
|4. Digging deep. Ormat Technologies, the U.S. Department of Energy, and GeothermEx in April said they had successfully produced 1.7 additional megawatts from an enhanced geothermal system (EGS) project inside an existing well field at Ormat’s Desert Peak 2 geothermal power plant in Churchill County, Nev. This image, taken in 2008, shows an EGS wellhead at Ormat’s Desert Peak project. Courtesy: NREL
EGS, a method also referred to as a hot dry rock or hot fractured rock system, has been around for more than three decades. Around the world, several large-scale EGS field projects have reached varying degrees of success, but only one project—the 2007-commissioned 3.2-MW Landau project in Germany—has sustained commercial production rates. The method has been stalled by a variety of issues, foremost among them an exponentially higher power cost than for fossil-fueled generation, owing to expenses associated with drilling of deep geothermal wells, experts say.
EGS is essentially an engineered heat exchanger designed to extract geothermal energy under circumstances in which conventional geothermal production is uneconomic or inefficient. It involves “enhancing” the permeability of deep hot rock by hydrothermal fracturing, high-rate water injection, and/or chemical dissolution of minerals by drilling production wells to depths of 10,000 feet and beyond where temperatures reach upwards of 350F. A cold working fluid—water, typically—is then allowed to flow through the deep openings in the rock to further crack it and to mine its heat energy. When the water is pumped back to the surface, the resulting steam is used to power a turbine to generate power. The water is cooled again into a liquid and injected back into the ground to repeat the cycle in a closed-loop system.
Ormat’s Desert Peak EGS project uses a production well at a previously built geothermal site in Churchill County, and its developers say that since beginning power production, it has increased power output at the site by nearly 38%. The project was of particular significance to Ormat because it helped the company demonstrate how EGS could be employed on sub-commercial wells. “This could enable us to use unproductive wells to generate more power and new revenue,” explained Lucien Bronicki, Ormat’s founder and chief technology officer. Conducted under a “stringent induced seismicity protocol” developed by Lawrence Berkeley National Laboratory (LBNL) and the DOE, the project used Ormat’s air-cooled power plants, so no water was consumed in the conversion of energy into power. “We achieved an increased injection rate up to 1,600 gallons per minute without consuming or discharging water at the surface and using only existing geothermal brine returned to the original aquifer,” Bronicki said.
The project, which has received $5.4 million in direct DOE funding (and $2.6 million matched by Ormat), got its start in 2002, and a boost in 2008 as several entities—including the U.S. Geological Survey (USGS), LBNL, and Sandia National Laboratories—joined the operation. It is currently one of a handful of projects in the U.S. focused on demonstrating the commercial viability of EGS. Other DOE-sponsored projects include a Calpine demonstration project at The Geysers in Middletown, Calif., and an AltaRock demonstration project at the Newberry Volcano near Bend, Ore.
Seattle-based AltaRock Energy this January announced a milestone of its own, claiming it had created “multiple stimulated zones from a single wellbore” at its project—an achievement that could “dramatically increase the flow and energy output per well for the completed system,” it said. AltaRock this year expects to test for permeability, flow rates, and heat-capturing properties of created reservoirs before it drills production wells about 1,500 feet from the injection well.
The DOE, meanwhile, plans to widen its investment in EGS. In February, the Lawrence Livermore National Laboratory released a technology roadmap for strategic development of EGS systems in the U.S., citing a 2008 USGS projection that EGS could be exploited to meet projected capacities on the order of 100-plus GW. The potential for EGS has especially heightened, the roadmap notes, because current practices in unconventional oil and gas development—particularly technology advancements for drilling horizontal wells and for fracturing fluids—demonstrate that “rapid technology advancement correlates with sector growth by improving project economics and decreasing risk.”
—Sonal Patel is POWER’s senior writer.