Champagne and strawberries, chocolate and peanut butter, geothermal and solar energy -- some things just go nicely together. Enel Green Power North America’s Stillwater plant, outside the desert town of Fallon, Nevada, came online in 2009 as a conventional geothermal plant and later added solar panels. After adding concentrating solar power in 2015, it became the world’s first triple hybrid renewable energy plant.
Many renewable energy sources, including geothermal energy, experience fluctuations in power output -- a perhaps surprising fact given that geothermal energy draws upon ever-present heat from deep in the earth. The fluctuations stem from the system’s difficulty discarding heat on hot days.
A quick primer: The geothermal module of the Stillwater plant pumps up geothermal brine -- scalding hot water found deep underground -- where its heat is transferred to a second fluid, isobutane, which flash-boils and spins turbines as it expands. Having imparted its heat energy, the brine is returned into the earth while the isobutane is cooled and condensed back into a liquid for another cycle.
Because the Stillwater plant relies on ambient air to cool and condense the isobutane, its cooling system struggles to shed excess heat on hot days, degrading the plant’s performance as a result. Unable to turn down the thermostat on the ambient air, engineers turned up the temperature of the incoming geothermal brine using a technology called concentrating solar power, or CSP.
When Enel Green Power North America approached the Energy Department’s Geothermal Technologies Office to identify collaborative opportunities for optimizing this hybrid technology, DOE put the company in touch with engineers from INL and NREL.
“NREL has CSP analysis and characterization capabilities, and INL has geothermal power plant simulation and optimization expertise,” said Dan Wendt, the INL engineer who led the project’s analysis. “It was the ideal mix of resources and skills necessary for the analysis.”
The concept behind CSP is simple: rows of parabolic mirrors focus the sun’s rays onto tubes of demineralized water. Heat from this water is transferred to incoming geothermal brine, increasing the amount of energy available to boil the isobutane, resulting in more power to spin the electricity-producing turbines. And it works: between the months of March and December 2015, the CSP component increased the amount of overall output by 3.6 percent, on average.
“It can often be difficult to obtain commercial power plant operating data for model validation purposes,” said Wendt, “but the partnership gave us the chance to analyze detailed operating data from a state-of-the-art power plant with a unique configuration.”
The Stillwater plant also draws upon the power of sun to generate electricity directly via 240 acres of adjacent solar photovoltaic panels that work in tandem with the geothermal system, each gaining strength just as the other falters. The sunniest part of the day, when the geothermal system’s output dips, is precisely when the solar cells shine. Likewise, the geothermal energy output strengthens throughout the cool desert night when the solar cells aren’t producing a single watt.
Watch a video about the Stillwater plant to learn more.