Will this startup finally crack the code on flow battery tech?

Germany’s CMBlu just pulled in a 100-million-euro investment, is ramping up manufacturing and has contracts with major utilities. Can it succeed where others have failed?
By Julian Spector

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A room full of large rectangular metal boxes attached to mechanical equipment

Flow batteries, a long-promised solution to the vicissitudes of renewable energy production, boast an outsize ratio of hype to actual performance. These batteries, which store electricity in a liquid electrolyte pumped through tanks, have been kicking around in labs for ages and in startup pitch decks for the last couple of decades. The technology’s promoters insist it excels where lithium-ion batteries are weak, but grid-storage buyers keep failing to notice.

But the flow-battery sector’s slow roll didn’t stop a relative newcomer, Germany’s CMBlu, from pulling in 100 million euros last month. European construction-services firm Strabag made that investment while promising to lend its infrastructural expertise to help install the batteries around the world.

To contextualize the scale of this funding, it comes close to what was once the largest-ever equity investment for a grid-storage hardware startup, announced just four years ago (that was for Energy Vault’s block-stacking robo-crane, since discontinued). These days, the check sizes have gotten bigger — see Form Energy’s $450 million round last fall — but CMBlu may well have pulled in the largest venture investment for the subcategory of flow-battery startups. Fellow flow-battery-maker ESS, which already has a real factory and real installations, raised a total of $57 million from venture investors before going public via a special-purpose acquisition company (SPAC) in 2022.

So what is CMBlu doing differently to break through the flow-battery malaise and make a technology that really helps decarbonize the electric grid?

Ben Kaun, president of CMBlu’s U.S. division, told Canary Media earlier this fall that the company stands apart from the flow-battery pack by augmenting liquid electrolytes with solid components to squeeze more energy storage into its physical footprint. And by choosing organic components, CMBlu avoids having to use the often-controversial minerals that mainstream clean-energy technologies rely on.

We’re transitioning from a fossil fuel economy to a renewables and metals economy — there’s so much metal” needed to manufacture wind, solar and conventional batteries, Kaun said. In contrast, What we have here at CMBlu is a rechargeable polymer, a rechargeable plastic. We can make the plastic [and] recycle the plastic.”

That pitch has won two large-scale contracts with U.S. utilities and a couple in Europe, demonstrating some commercial traction for the unusual liquid-solid batteries. CMBlu is building an automated pilot factory in Alzenau, Germany and plans to install that system at larger-scale factories in one or more parts of the U.S. in 2025.

Bodily inspiration for energy-dense flow batteries 

CMBlu’s U.S. chief brings an informed perspective to the role — Kaun spent the previous 12 years researching the whole swath of long-duration storage contenders for the nonprofit Electric Power Research Institute. CMBlu managed to lure him from analyzing the field to competing in it.

CMBlu founder Peter Geigle previously worked on energy management within brain cells as a medical doctor and biotech entrepreneur, Kaun told me at the RE+ trade show in Las Vegas this September. The human brain, incidentally, runs on electricity but doesn’t require piles of lithium and cobalt. Geigle’s understanding of how the body stores and transmits energy via organic materials propelled CMBlu’s unique approach to flow batteries.

CMBlu fills its storage tanks with a solid polymer that holds a charge, and then transfers it to and from the liquid electrolyte, which is pumped into an electrode stack for charging and discharging cycles. Kaun likened that system to how the human body stores energy in fat, and then transfers that energy to blood sugar when it needs to be delivered to cells.

This mimicking of human anatomy could give CMBlu an edge in the metric of energy density, or how much storage capacity can be packed into the physical footprint of the product.

Generally speaking, flow batteries and other non-lithium-ion storage devices sprawl out a bit — they don’t have to squeeze into the chassis of a sports car, so companies are willing to try less-dense approaches. But even though power plants can accommodate more spacious battery technologies, there are costs to letting things get bloated.

With anything where you have low energy density, you’re putting in a lot of extra infrastructure to make it happen,” Kaun said. There’re a lot of costs that you don’t see immediately. But once you try to build a project, everything’s bigger.”

Avoiding the use of metals distinguishes CMBlu’s technology from that of conventional lithium-ion batteries or certain chemistries that are popular in the flow-battery field, like vanadium redox. When the price of vanadium shoots up, companies that rely on that ingredient feel the pain. And while mining supplies haven’t constrained the growth of lithium-ion batteries thus far, there are real concerns about the pace of new mine openings keeping up with projected demand growth in the coming decades. CMBlu dodges that impending challenge by working with plastics; ESS does it by using iron, an abundant and inoffensive commodity.

Some demand now: Select utilities test flow batteries

All investments make a bet on the future, but that’s particularly apparent with flow batteries because their actual sales and construction today are so limited. This makes it hard to evaluate the relative success of various types of flow batteries — even the category leaders have little to show by way of built and operating storage projects.

Flow batteries’ commercial appeal increases as power companies need to store energy for longer periods of time, Kaun explained. That’s because it’s cheaper to extend storage duration by installing a larger tank than it is to add more lithium-ion battery packs.

There really hasn’t been demand for those solutions until recently,” he said. For now, the economic adoption of 4-hour lithium-ion batteries largely depends on capacity payments, a mechanism to ensure that utilities always have enough power to meet demand at peak times. Grid planners and regulators in places like California have recognized the value of 4-hour storage, so battery developers build 4-hour batteries to qualify for capacity payments.

But in the future, it’s evolving,” Kaun said. He predicts demand ramping up for 6- and 8-hour batteries before too long.

CMBlu’s U.S. sales strategy, then, has been to seek out utilities whose coal plant retirements are forcing them to figure out a clean-energy replacement sooner rather than later. When they realize they need something more than what lithium-ion can give them, CMBlu offers an alternative.

Two major U.S. utilities have taken up the offer. First came Wisconsin utility WEC, which chose CMBlu in February for a pilot of 1 to 2 megawatt-hours at the Valley Power Plant in Milwaukee, Wisconsin. That’s a modest size, but already past the megawatt-hour threshold.

Then, Phoenix-area public power company Salt River Project chose CMBlu from a cattle call of proposals submitted by emerging long-duration storage companies. This pilot,” announced in late August, will have a capacity of 5 megawatts and 50 megawatt-hours, making it, to use a technical term, pretty big actually. The warehouse-like structure will be built in 2025 to go online by the end of that year; it will help SRP stretch its growing solar production into the more valuable nighttime hours. Kaun’s former colleagues at EPRI will study both projects to document how well the technology performs in real life.

The biggest flow battery in the world is reportedly a 100-megawatt/400-megawatt-hour vanadium redox flow system in Dalian, China. Other major flow-battery projects include ESS’ multiyear contract to install 2 gigawatt-hours of iron flow batteries in Sacramento to help the municipal utility reach zero carbon by 2030. Invinity, formed by the merger of two flow-battery startups, is selling a new-generation product and recently clinched Department of Energy grants to support 84 megawatt-hours installed across several sites in the U.S.

In other words, the flow-battery market is still in its trial period, and no clear winners have swept the field yet. CMBlu still has plenty of time to establish itself before the alternative grid-battery market gets going, and now it has the money to stick around, too.

Julian Spector is a senior reporter at Canary Media. He reports on batteries, long-duration energy storage, low-carbon hydrogen and clean energy breakthroughs around the world.