With the impending passage of the Inflation Reduction Act, renewables are about to get a fresh jolt in the U.S. They’re already some of the cheapest sources of electricity to build and run, but they haven’t taken over because they’re often dependent on the weather.
The simple solution is to store any excess power produced, but that raises the overall cost of renewable power. That’s set off a race among startups to find the cheapest way to do it, from batteries to compressed air and even giant concrete blocks.
The front runner so far appears to be batteries, many of which use the same lithium-ion chemistries found in EV batteries. The scale of EV battery production has made lithium-ion easy to obtain, allowing it to get a foothold in the sector, but its long-term prospects for grid-scale storage are murkier given its high cost of materials.
Competition for battery materials is intensifying, and there are many uses for batteries beyond EVs, which is why some companies, like Germany’s VoltStorage, are trying to build batteries using the cheapest, most widely available materials possible — chiefly, iron.
“We don’t want to make the same mistakes as we did with lithium-ion batteries and being dependent on certain supply chains,” said VoltStorage CTO Michael Peither.
Its launch product was a vanadium-redox flow battery tailored for industrial users that need to store enough energy to last five to 10 hours, enough for some industrial and agricultural users. (All batteries charge and discharge at certain rates, and different flow battery chemistries can be optimized for different charge and discharge durations.) The chemistry, which VoltStorage still sells, works well for those use cases. But as the company was beginning to investigate scaling it to store surplus wind and solar power, the U.S.-China trade war and COVID upended supply chains.
“A lot of metals and raw materials got stupendously expensive for a period of time. In this case, it probably was five months where the prices were like six times the normal baseline price. For a commodity material to be going into production, that is crazy,” Peither said. “At that time, we were looking for a pivot, because we needed to do an alternative.”
That’s when the team landed on iron. They’re not the first company to explore using iron as a battery material. As one of Earth’s most abundant elements and one with a massive global supply chain, it’s an attractive alternative to rarer and pricier metals like lithium, cobalt and vanadium.
“We’ve come to the point that the iron-salt chemistry is close enough to the vanadium batteries that we already produce. It’s still the same basic technology. We still use a lot of parts of the development of a small battery, reconfiguring that and using it for the iron,” he said.
VoltStorage recently raised a Series C worth $24 million from engine manufacturer Cummins. Its previous rounds included investments from SOSV and EIT InnoEnergy.
It isn’t the first company to explore using iron in batteries. ESS Inc., based in Oregon, makes iron-flow batteries that store six to 12 hours of electricity. Form Energy, based in Massachusetts, is working on an iron-air chemistry that’ll store 100 hours of electricity.
The iron-redox chemistry that VoltStorage is developing will be optimized to land in between those. “You can discharge the energy for days or weeks or in extreme cases, months. But to be honest, the use case will be something like a discharge duration of 12, 24, 48 hours,” Peither said. The batteries, he added, are about 75% efficient before the power is converted to the more widely used AC format. That’s lower than lithium-ion batteries, but on par or better than competing long-duration storage options.
The modules will each hold 50 kWh that can be discharged at a rate of 1 to 5 kW, netting 10 to 50 hours of electricity. Five modules fit inside a 20-foot shipping container, and the containers can be stacked up to three high. They can also be installed inside buildings, but Peither predicts that shipping containers will be the most popular format.
Flow batteries work somewhat differently from lithium-ion or alkaline batteries. The charge-holding materials are two different liquids rather than two different solids, and they each flow through a stack where ions can migrate between positive and negative sides. Flow batteries tend to have very long lifespans, and they don’t often include flammable electrolytes.
That’s not to say they’re without problems. In VoltStorage’s chemistry, there are some side reactions, one of which produces hydrogen, Peither said. To keep the system functioning, they’ve had to devise an additional, continuously running loop to collect the hydrogen and inject it back into the positive solution.
The iron inside the battery is also subject to rust, which the company handles with a cleaning cycle that’s required every 50 to 60 cycles, or about every six months. The cleaning cycle can be scheduled according to predicted demand, just like how traditional power plants are taken offline periodically for maintenance. VoltStorage’s cleaning cycle only takes 30 to 45 minutes, though, while large power plants are often in maintenance mode for a week or more.
Plus, cells don’t all have to be cleaned at the same time. Each module contains multiple systems, Peither said. “If you have hundreds of systems, and you just shut down one, you only lose 1% in power capabilities for a period of time. You can run the other 99% perfectly well, and you can schedule that accordingly. So in the end, you might lose 1% or 2% of your power output for a certain period of time every half year.”
VoltStorage is using the current fundraise to finish the development of its next-generation vanadium-redox flow battery and commercialize its iron-salt battery. If VoltStorage can make that happen, it’ll be competitive with new natural gas power plants in the U.S. The combination, Peither said, would be “less than coal, gas, peaking, nuclear,” making it the cheapest option for utilities looking to add capacity. Getting to that point will take near gigawatt-hour scale production, which he said will happen in the next five years.
The goal is to get the cost of the battery coupled with wind and solar power to below $70 per MWh. That’s currently more than it costs to run existing natural gas power plants, but as the world has discovered recently, the price of fossil fuels is anything but stable. The certainty of costs for grid-scale storage is undoubtedly worth a premium, one that could close the gap with existing power plants if gas prices go much higher.
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