Storing electricity underwater in the form of compressed air is a tantalizing notion that could, if it works, help solve the intermittency problem of wind, solar, and other renewable sources. That “if” is a big one, though, because there are many details engineers have yet to nail down for underwater compressed-air energy storage (UW-CAES). One company that’s been trying to nail down those details is the Canadian start-up Hydrostor. I recently wrote about its plans to deploy the world’s first commercial UW-CAES system in Lake Ontario.
The Hydrostor system will use electricity from the Toronto Hydro power grid to run a compressor; the compressed air will then be stored in flexible energy bags submerged at a depth of about 80 meters. Later, the air will be run through a turbine when the energy is needed.
For all that effort, the system will be able to supply just a megawatt of electricity for up to three hours. Eventually, the company is aiming for a capacity of 20 to 30 megawatts that can be discharged over 10 to 20 hours. But a big wind or solar farm would require a lot more storage than that. How much? Well, the offshore wind farm known as the London Array has 175 turbines and an installed capacity of 630 megawatts. To compensate for a one-day lull would require up to 812,000 cubic meters of compressed air, according to an analysis by Maxim de Jong. He’s the design engineer and CEO of Thin Red Line Aerospace, which makes energy bags and also inflatable space structures.
That figure assumes an average output of 30 percent, or 189 MW, which means you'd need 4.54 GWh of storage. De Jong also assumed the bags are at a depth of 500 meters and have an energy density of at least 5.59 kWh/m3. An isothermal compressed-air storage system, in which the air temperature is kept constant as the air is being compressed, would have that kind of energy density, according to research by de Jong together with Andrew J. Pimm and Seamus Garvey, both of the University of Nottingham. The three collaborated on an underwater storage experiment deployed off the coast of Scotland in 2012. More efficient than isothermal would be an adiabatic compressed-air storage system, in which the heat from the compressed air is removed, stored, and then used to heat the air when it’s expanded; that would yield an energy density of around 10.32 kWh/m3.
So how many bags do you need to store 812,000 cubic meters of compressed air? Size matters, of course, so if your bags are, say, 41 meters in diameter—comparable to NASA’s Echo II balloon satellite launched in 1964—you’d need 23 of them, de Jong says. If, on the other hand, you were using much smaller bags, like the 5-m-diameter ones used in the Scottish trial, you’d need more than 27,500, de Jong says. To restrain and anchor the buoyant vessels, you’d need plenty of strong rope, made from Dyneema, Spectra, or something comparable, and plenty of ballast. (De Jong’s paper is available here [PDF].)
That’s just for the London Array. In June the UK approved another giant offshore wind farm, the East Anglia ONE, which will be twice the size of the London Array. Worldwide, offshore wind capacity grew nearly 40 percent last year. So commercializing energy bags on a wide scale won’t be trivial. “That’s why I wrote this paper,” says de Jong. “So people will get an idea just how much compressed air we’re talking about.”
Despite these caveats, he’s eager to see the technology move forward. Thin Red Line started out developing fabric components for helicopter rescues, which led to developing flexible inflatable structures for space. The company designed and manufactured the inflatable pressure shells for Bigelow Aerospace’s Genesis I and II spacecraft, among other projects.
“As thrilled as I am that 95 percent of our revenue comes from NASA, I decided a few years ago that I also wanted to do something that would have a more tangible immediate effect,” de Jong says. Energy bags, it turns out, “exactly matched the architecture of the structures we were developing for space.”
De Jong was supposed to present his findings at the Offshore Energy and Storage Symposium, held last week in Windsor, Ontario, but a scheduling conflict prevented him from attending. According to the symposium’s chair, Rupp Carriveau of the University of Windsor, it was the world’s first gathering of its sort, and the chief goal was to address concerns like de Jong’s and to lay out what will be needed to move the technology forward.
“For instance, the cost of installation right now is staggering, and that has to change,” Carriveau says. “Fortunately, people are still willing to foot the cost because they recognize the resource could be really valuable.”
Jean Kumagai is the Executive Editor at IEEE Spectrum. She holds a bachelor's degree in science, technology, and society from Stanford University and a master's in journalism from Columbia University.