When it comes to storing lots of electricity for periods of time longer than batteries can handle, there are plenty of ideas being tossed around. Pump water uphill to a reservoir! Fill underground caverns with compressed air! Push a train up an incline! Stack a tower of concrete blocks!
All very neat but they won’t be built in New England for a long while, if ever. So I was excited to hear about another idea that’s also neat but we might actually see fairly soon: Freeze a lot of air so that it compresses, then let it thaw and have it spin a turbine as it expands.
“Yes, that’s about it,” said Salvatore Minopoli, vice president of U.S. operations for Highview Power. This British firm wants to build the country’s first “air liquefaction” storage site by 2022 in northern Vermont, where grid congestion sometimes occurs when wind farms are over-producing.
Although the technology sounds straightforward, it has taken Highview more than a decade to turn storage via freezing air into utility-scale reality. The company has a plant in the U.K. operating since 2018 that can generate five megawatts of electricity for three hours and says they’re ready to move up an order of magnitude.
Their Vermont proposal could store enough energy in liquid air to generate 50 MW for eight hours, according to Highview. Fifty megawatts isn’t a game-changer – Seabrook Station can generate 1,200 MW – but it isn’t trivial. It’s equal to almost all the large hydro dams in New Hampshire, combined.
More important is that eight hours of storage is longer than any utility-sized battery storage system around. Most batteries are lucky to hit four hours, not really enough to cover shortfalls that intermittent wind and solar can create.
In summer, for example, eight hours could cover a long, hot, night when air conditioning is at full blast but solar power is quiescent. Very useful, indeed.
“New England needs longer duration. Sub-4-hours just isn’t enough,” said Minopoli.
The company is still working on the all-important business model for the project, talking to utilities and others. Highview might try to insert it into the regulated grid or might keep it out on the open market, Minopoli said.
Most non-battery ideas for long-term storage use excess electricity to move things, increasing their potential energy. When power is needed they turn the potential energy into a form of kinetic energy that creates electricity – usually via gravity, such as by letting the train roll back downhill and turn a generator.
Highview’s system, called CryoBattery (as in “cryogenic,” the science of very low temperatures) liquifies air through something called the Claude cycle, which involves repeated expansion and compression. That technology was new to me but it’s worth reading about, if only because you encounter excellent words like “isenthalpic.”
Highview says using frozen air has several advantages over the other large-scale energy storage systems.
It’s expensive but not prohibitively so. Construction is estimated at $1,000 or so per kilowatt, which would be at least $50 million for the Vermont site, and the firm claims electricity “can be delivered at a cost of approximately half of the current cost of traditional lithium-ion batteries.”
Highview says the system takes relatively little land, about two acres, and uses existing technologies from natural gas and power plant industries, minimizing operational surprises. The plant looks sort of like a very small refinery, with storage tanks like big farm silos.
Most importantly, said Minopoli: “We don’t have any geographical limitations – we don’t need water, don’t need high elevation, don’t need deep caverns.” If there’s a business case, they can set up shop.
So what brings them to a non-obvious location like northern Vermont?
“We can be in many places: southeast, Texas, California, the desert West. … There’s no strategic reason why Vermont now, it just happens to have moved the fastest,” said Minopoli.
For almost a decade, officials have recognized that the existing transmission system in northern Vermont can’t always handle the power produced by some large wind farms and a big gas plant. The area, known as the Sheffield-Highgate Export Interface, is so constrained that a 500-kilowatt solar farm was rejected last year by solar-loving Vermont regulators because it might block less-expensive power from using the transmission system.
Storing excess power and releasing it when lines are freed up could be worth a lot of money under that circumstance.
There’s a big environmental benefit, too. If you can store power from wind farms and release it on schedule, there’s less need to have gas plants or other baseload systems, particularly those known as “peaker plants” that can turn on and off quickly for short periods during extreme need. Renewables and batteries are starting to cut into the business of gas-fired peaker plants. Longer-term storage will hasten their demise.