Pioneering grid battery nudges California closer to 24/7 clean energy

On June 1, the Tumbleweed project in California’s Kern County became the first major battery installation in the U.S. that can discharge power for up to eight hours at a time — twice as long as typical energy-storage facilities.

The U.S. power sector now builds more battery storage capacity than any other form of on-demand power, like gas, nuclear, or geothermal. But battery developers typically design their projects to discharge at maximum capacity for four hours before running out of juice; that’s what has made sense, so far, given equipment costs and market opportunities. Analysts have concluded that longer-duration storage is needed to cost effectively power the grid with clean energy 24/7.

Consequently, California regulators in 2021 ordered power companies to procure longer-duration storage as part of the state’s planned transition to zero-carbon energy. California Community Power, a consortium of local nonprofit power providers, issued a contract for Tumbleweed back in 2022 to fulfill this obligation.

“This was one of our first eight-hour contracts in the country for batteries, and now it’s one of the first projects online, and it’s a complex deal with a bunch of members coming together,” said Alex Morris, general manager of California Community Power. ​“It’s designed to be part of the clean energy mix, helping capture the solar and discharge that later when they need it.”

Granted, the system’s 125 megawatts of instantaneous capacity are modest compared with the multi-hundred-megawatt batteries getting built elsewhere in the West. But Tumbleweed is far bigger than the 6-megawatt, eight-hour battery installed in Nantucket in 2019 for a special-case island power role, and larger than the 50-megawatt, eight-hour battery that went live in Australia in May. Tumbleweed has finally delivered eight-hour storage at a meaningful scale to test what this emerging resource means for the grid.

How to build an eight-hour grid battery

The patch of inland Southern California that surrounds Tumbleweed resembles Texas, said Cody Hill, who leads storage development for Rev Renewables, which built the project. It’s a flat, scrubby desert that’s hosted ample oil production for decades and more recently turned into a hot spot for wind turbines, vast solar arrays, and batteries. Rev picked up a parcel there that was too small to fit serious solar capacity but just right for an energy-dense battery installation.

Rev built the site in two phases, first activating 125 megawatts with four hours of duration back in the summer of 2024. Ava Community Energy, the locally governed nonprofit that secures electricity for customers in Alameda and San Joaquin counties, paid for 50 megawatts to serve its capacity obligations. For two years, Rev used the rest of the available capacity as a merchant power plant, bidding into the markets run by the California Independent System Operator.

To convert this very regular four-hour battery into a groundbreaking eight-hour battery, Rev ​“literally doubled the number of battery boxes on the site,” Hill said. ​“Technology-wise, the differences are pretty trivial.”

Rev hired the same construction firm, Mortenson, to build both phases sequentially, so lessons learned during the first stage ensured that ​“the expansion went really smoothly,” Hill added. Now, Ava controls 50 megawatts with eight hours’ duration, and California Community Power can use 75 megawatts with eight hours’ duration.

Any grid needs power 24/7, and by law, California is working to provide that without burning fossil fuels by 2045. At the highest level, the state’s strategy is to build as much solar power as possible and add enough storage to spread that throughout the day (and then hope that a major floating offshore wind complex materializes sometime in the 2030s, but that’s another story).

At this time of year, California gets strong solar generation from around 8 a.m. to 6 p.m.; Tumbleweed can fill up on that very cheaply, given its proximity to the sun-drenched solar fields, and then push that clean energy back onto the wires for another eight hours, say from 6 p.m. to 2 a.m. That leaves six low-demand hours while most people are sleeping; California currently serves the 2 a.m. to 8 a.m. period with a mix of wind, nuclear, geothermal, hydropower, and some fossil gas. If this eight-hour battery format takes off, California would have a clear path to serving clean electricity for nearly all of a typical 24-hour cycle.

That’s the theory, anyway. California Community Power and Ava are deciding exactly how to operate their portions of Tumbleweed in California’s wholesale markets in order to fulfill their capacity obligations and maybe even generate savings for their customers. The real run-time data will tell the full story.

The outlook for long-duration grid batteries

People generally agree that at some point between now and a 100% clean grid, renewables will produce such an abundance that they’ll effectively require longer-duration bulk storage to distribute that power through the day and night. But experts and grid planners have not formed a consensus on when exactly that tipping point will hit.

Regulators at the California Public Utilities Commission originally mandated the state’s utilities to start obtaining some long-duration storage by 2026, to figure out what it takes to build this new kind of project. Later, the commission punted its own deadline to 2031. This happens sometimes when investor-owned utilities, armed with vast capital budgets and legions of in-house experts, fail to deliver on deadlines they’ve known about for years. The scrappy teams at Ava and California Community Power plowed ahead with the project even as state officials took their feet off the pedal.

So now the eight-hour battery is online even though it no longer technically has to be.

The project wouldn’t make financial sense without the anchor contracts spurred by California’s long-duration procurement policy, Hill noted. In other words, the additional merchant market revenue from doubling the size of the battery wouldn’t justify the additional costs on its own. But the goal of the mandate was to start building things the grid will need soon, so the state doesn’t have to scramble to keep pace with a rapidly changing market.

“This is proactive and not driven by the short-term energy markets,” he said.

With the battery fully operational, though, the customers are going to make the most of the opportunity. California is already tapping batteries as the biggest power source for two- to three-hour stretches after sunset. Tumbleweed can keep that discharge going into the night. Batteries are very inexpensive to run, since their fuel can be low-priced midday solar power, and they have few moving pieces. As long as more-expensive gas-powered plants are setting the market price through the night, Tumbleweed can displace them with its cheaper, cleaner power.

Scores of entrepreneurs have raised billions of dollars from venture capitalists on the presumption that lithium-ion batteries — the kind used at Tumbleweed and nearly every other existing battery facility — cannot meet the needs of shifting renewables to round-the-clock energy delivery. In the last decade or two of trying to come up with an alternative, though, this long-duration startup sector has delivered a raft of bankruptcies and hardly any utility-scale projects.

Tumbleweed, by its very existence, suggests that all this investment in novel technologies may have been a massive waste — at least for the ones purporting to reach up to eight hours.

Hill said he takes calls from startups pitching new storage devices, but he chose lithium-ion phosphate cells from Chinese energy giant BYD as a bankable technology that was already in high-volume production. Lithium-ion, he added, keeps getting better.

“It improves along every metric, and it gets cheaper,” Hill said. ​“It is totally ready to be deployed at infrastructure scale today.”

That doesn’t preclude the potential for alternative devices to serve multiday storage, like Form Energy’s iron-air batteries and Noon Energy’s carbon-based system. At the 100-hour level, the material costs of lithium-ion look prohibitively expensive. But many startups launched to beat 2010-era projections for lithium pricing at four-, six-, or eight-hour durations, and they’ve now been overtaken before they ever got to scale.

If Tumbleweed can do eight-hour grid storage today, lithium-ion won’t stop there. It forces the question of how much further this tech can push with additional cost declines and improvements, if more regions realize a need for longer-lasting storage.