Renewable energy can be a tricky business. If you’re not dealing with the intermittent flow of solar or wind energy, you may be struggling with some of the more complex problems of geothermal energy. For example, older geothermal plants rely on steam production that can diminish over time or damage the plant’s turbine components. Or a geothermal plant can damage the underground aquifer from which it draws hot water (brine). Or, if the geothermal plant is air-cooled, a particularly hot day can reduce the plant’s efficiency.
To combat all of these problems, Italian renewable energy company Enel Green Power has been working to make its geothermal resources in Fallon, Nevada, and Cove Fort, Utah, more efficient by combining them with other renewables. In its most recent effort at Cove Fort, Enel cleverly combines hydropower with geothermal energy to provide more electricity for the plant’s operation.
Typically, geothermal plants pump up mineral-rich brine from areas of hot rock below the Earth’s surface, convert that heat into energy, and then inject that water back into the ground to warm up again. The reinjection process is usually as simple as it sounds: just put the water back into the soil where you found it once it cools. Instead, Enel harnesses the power of all that falling water. Brian Stankiewicz, Enel’s senior operations manager for geothermal and solar power, told Ars that the company realized it had “an exponential amount of hydraulic energy that could be harnessed” shortly after buying the defunct plant in 2007.
Enel enlisted the expertise of oilfield service provider Baker Hughes to help install a downhole generator in the injection well, opposite the production well where the water is pumped. The generator is placed at the bottom of the well and receives the recycled geothermal water, connected to a turbine above that rotates as the re-injected water is pulled down by gravity. The generator converts that mechanical energy into electrical energy, producing an additional 1,008 megawatt hours for the plant. Enel says the hydroelectric addition has improved Cove Fort’s efficiency by 8.8 percent.
That may seem like a small amount for a geothermal plant generating 160,000 MWh annually, but the hydroelectric component also helps protect Cove Fort’s aquifer. Many geothermal projects limit the amount of brine that can be re-injected to prevent damage to underground rock formations, Stankiewicz told Ars, adding, “If the flows get too big for the well, it could create a lot of turbulence.” Enel’s downhole generator at Cove Fort controls the injection of the brine back into the aquifer, limiting those damaging “turbulent eddies,” as geothermal engineers call them.
The hydroelectric component is an interesting addition to what was still an old, non-operational geothermal plant in 2007. Construction of Cove Fort in southwestern Utah began in 1984 and between 1990 and 2003 it was a fully operational steam turbine geothermal plant. But steam turbines at geothermal plants don’t always have the longest life — minerals in the brine can wear out turbine components. faster than a steam engine using plain water. Stankiewicz also attributed the degradation of the original plant to aspects of the reinjection process. “With a conventional flash installation, you can’t re-inject 100 percent of the brine,” he said. “There’s a lot of evaporation.”
So when Enel bought Cove Fort in 2007, it converted the plant into a more efficient Rankine binary cycle plant – instead of using direct steam to drive turbines, Enel pumps the hot water to a heat exchange site where a secondary fluid with a lower boiling point than water (in this case pentane) is heated by the water and evaporates, creating hot vapor that can power the plant’s generators.
“In recent decades, geothermal development has shifted to using fluid-dominated resources and away from steam as new power generation technologies allow lower-temperature resources to be developed and used economically,” Stankiewicz explained in a follow-up email.
Enel reopened Cove Fort in 2013, and since then the 25 MW plant has powered approximately 13,000 homes served by the Salt River Project, an Arizona utility cooperative.
Check in on some 16ft high transoms
The combination of geothermal and hydroelectric power at Cove Fort is a first for North American geothermal power plants. But it is the second hybrid power station that Enel has built by combining geothermal energy with other renewable sources. Enel’s Stillwater plant in Fallon, Nevada, is the only triple-hybrid renewable power plant in the U.S., according to the Department of Energy (DOE), combining geothermal power with a 26 MW array of photovoltaic solar panels and concentrated solar power (CSP ). ) system that uses mirrors to add heat from the sun to already hot geothermal water. In fact, Ars toured Stillwater way back in 2014, when the company was still installing the 16-foot-long parabolic mirrors that currently concentrate heat on a 5-inch pipe full of geothermal brine.
The CSP system finally came online in 2015, and in October of this year, the U.S. Department of Energy and Enel announced they would be collaborating to conduct some studies on ways to optimize Stillwater’s three energy sources.
Currently, the CSP offers some interesting efficiencies to Stillwater. Shortly after opening the geothermal portion of the plant in 2009, Enel realized that ambient temperatures in the Nevada desert reduced the rate at which the air-cooled geothermal system could dissipate heat, detracting from the amount of energy the plant in the center could generate. of the process could produce. day. While Enel can’t make a hot desert afternoon cooler, it realized it could make the geothermal brine it pumped from the earth hotter. Its parabolic mirrors can heat the air in front of them to about 600 degrees Fahrenheit, while the brine inside goes from 300 degrees Fahrenheit out of the ground to 390 degrees Fahrenheit.
According to the DOE, this strategy has worked for Enel: “Between the months of March and December 2015, the CSP component increased total output by an average of 3.6 percent.” That, combined with the solar PV array, means that Stillwater is a renewable power plant that produces 24/7 – during the day when geothermal energy production may decrease, the solar array produces energy and the CSP system boosts the weakened geothermal energy. performance. At night, when the solar panels are not producing anything, geothermal energy functions well and the cool night air makes the CSP system unnecessary.
What does the future hold?
Enel couldn’t say if more projects like this are in the works, but Stankiewicz did say Cove Fort’s Down Hole generator scheme could likely work at other geothermal sites. “The Cove Fort plant presented unique geological conditions that proved to be a good initial testing facility for this technology,” he wrote to Ars in an email. “We are actively looking at ways in which this technology can be applied to other locations around the world with similar geological conditions. An analysis of individual injection wells must be made to ensure there is sufficient head pressure and flow to allow the installation of the Down Hole Generator.”
And despite the new Trump administration’s aversion to renewables, the company is publicly optimistic that a renewable energy market will continue to exist in the US. In addition to the geothermal plants, Enel has a handful of solar and wind farms and in October announced a deal to build a 300 MW wind farm in Missouri. “Our company is well positioned to continue its growth in the US, as evidenced by more than 1 GW under construction and continued investment in both renewables and the US market,” said a company spokesperson.
Correction: The story originally said that Cove Fort was the first geothermal plant to use hydro and geothermal power together, but a Northern California Power Agency project actually predates Enel’s project.