Sunset on Oil Rigs: CA’s Solar GeoTES Revolutionizes Energy Storage

Down on the south end of California’s San Joaquin Valley, oil rigs have plumbed the earth like flocks of blackened drinking birds for more than a century. Now, they’re fast becoming vestiges of a bygone era. Kern County still produces nearly three-quarters of the state’s oil and gas, but California’s push to decarbonize its energy grid is putting these rigs and the people who operate them out of work. Their disappearance might come as a death knell to the communities that have long depended on bringing energy up to the surface; but now, those same oilfields could offer new life by sending energy back underground.

Ample sunlight and tens of thousands of abandoned oil wells and experienced oilfield workers have made Kern County the focus of a new battery-storage technology. The plan is to retrofit depleted oil wells to store concentrated solar energy in super-heated groundwater for long periods of time, then use that heat to drive turbines when energy demand rises. If it works as planned, the project — which is being run by the National Renewable Energy Laboratory (NREL) and a private investment group and has been dubbed GeoTES, for geological thermal energy storage — has the potential to overcome some of the renewable energy transition’s greatest hurdles.

Six years ago, California announced one of the world’s most aggressive climate goals: carbon-neutral energy production by 2045. But it soon became clear that producing clean energy would not, by itself, suffice to meet the mark. The sun and wind provide power only when the sun shines and the wind blows, and they sometimes produce too much power for the grid to handle. A full energy transition will require storing that power for when it is needed — whether that is hours, days, or even months from when it is produced. The project is believed to be the world’s first attempt to store solar energy in a natural geologic reservoir.

Over the last five years, California has increased its energy storage capacity tenfold to more than 10 gigawatts, and on April 16, in a notable first, batteries provided the largest source of supply in the California grid, if only for two hours. This is huge, but it is still a long way from the 52 gigawatts of stored energy that the California Energy Commission predicts the state will need to meet its ambitious climate goals. So far, storage gains have relied on batteries that use lithium, which is in short supply and is often produced in nations known for human-rights and environmental abuses.

Faced with fast-approaching decarbonization deadlines, and the challenges of lithium battery production, the California Energy Commission has earmarked $330 million to develop energy storage alternatives. A wave of nascent technology is storing energy in chemical, electrochemical, and mechanical forms. Like GeoTES, some of these methods also store power as heat but do it in aboveground silos filled with molten salt or sand. GeoTES offers something different, said Guangdong Zhu, the senior researcher overseeing the long-duration energy storage program at NREL, which focuses on systems capable of retaining power for 10 or more hours. The project is believed to be the world’s first attempt to store solar energy in a natural geologic reservoir, and it aims to store that energy for more than 1,000 hours. “We are looking to find the battery, not build it,” he said.

The GeoTES project occupies a dusty patch of ground nestled between well-plumbed oilfields with hundreds of wells — some idle, some still pumping. Just 1,200 feet below its surface — shallow by drilling standards — are pockets of permeable sandstone, now emptied of oil, into which highly brackish water has seeped. Like the oil it is replacing, GeoTES begins with the sun. At the surface, parabolic mirrors gather solar energy, which is used to heat a silicon oil flowing through an aboveground loop to 700 degrees Fahrenheit. Comparatively cold groundwater is pumped up, heated by the oil, then sent back down. As long as the sun is shining, the system heats the water in the reservoir. At night, the water continues circulating, maintaining a temperature of about 500 degrees. When called upon to make electricity, the groundwater is brought to the surface through an extraction well, where its heat spins a turbine.

By themselves, none of the pieces that make up GeoTES are cutting edge, Zhu said. “It’s a new concept, but it’s a combination of different mature technologies. It doesn’t require any kind of scientific or material breakthrough.” What is novel is the process of putting them together. “The basic concept of geothermal heat transfer is used all the time in geothermal heat pump systems in locations around the world,” said Kenneth Gillingham, an environmental and energy economist at the Yale School of the Environment. “Those are proven to be effective and often cost effective, too.” If the project works and can scale up, it could power

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