Stashing Energy in Underwater Bags
Submerged bags of air could turn wind and solar power into round-the-clock resources
With the worldwide proliferation of wind- and solar-generated power, the fickleness of these renewable sources is a problem crying out for a good solution. A Canadian start-up called Hydrostor thinks it has an answer: air-filled bags.
In August, the Toronto company plans to sink several large balloonlike bags into Lake Ontario, and then, using electricity from Toronto Hydro’s grid to run a compressor, it will fill the bags with air. Later, when the utility needs electricity, the air will be emptied from the bags and run through a turboexpander, which uses the expanding air to drive a turbine. The result will be the world’s first commercial facility for underwater compressed-air energy storage. This animation from Hydrostor explains how its system works:
Using compressed air to store energy is not a new idea. The first such systems emerged back in the 1870s, and these days compressed air is stored in underground caverns, in pipes, and even in small tanks for powering cars and locomotives. Variants of the underwater storage idea have also been floated, so to speak, since at least the 1980s, says Seamus Garvey, a professor of dynamics at the University of Nottingham, in England. Garvey, who’s not affiliated with Hydrostor, designed an underwater storage system using Thin Red Line Aerospace’s bags and deployed a prototype off Scotland’s Orkney Islands, in 2012. “The idea is to put the storage where it matters most, which is where the intermittent energy is being generated from offshore wind,” Garvey says.
Hydrostor CEO Curtis VanWalleghem says his company began looking at the technology four years ago as a side project to a wind farm it wanted to develop. At first, the company planned to use pumped hydro storage, in which water is pumped uphill and then released later to reclaim the electricity. Pumped hydro can have efficiencies of 80 percent or higher, but it works only in certain geographies and isn’t economical on a small scale. “So we thought, if lifting a cubic meter of water into the air is the best way to store energy, maybe the reverse would also work—submerging air below water,” says VanWalleghem.
The concept is simple enough: When the energy bag is anchored underwater—at least 25 meters deep and ideally 100 meters or more—the weight of the water naturally pressurizes the air, allowing more air, and thus energy, to be stored in a given volume. (The pressure increases roughly 1 atmosphere, or about 100,000 pascals, every 10 meters.) At depths greater than 500 meters, says Garvey, “the cost of the containment becomes negligible compared with the costs of the power-conversion machinery.”
In the Toronto system, the bags (or “flexible accumulators,” as Hydrostor calls them) will be deployed at a depth of 80 meters, and they should be able to supply about a megawatt of electricity for 3 hours or so. The company will also be testing fixed-wall accumulators, in which the compressed air will displace water inside the vessel. “This is the smallest size we would contemplate,” says VanWalleghem. A more typical capacity, he says, would be 20 to 30 megawatts that can be discharged over 10 to 20 hours. Eventually, the company will aim for an efficiency of about 60 to 70 percent. The technology easily scales up, he adds. “We just make the air cavity bigger, so there really is no upper limit.” By year’s end, the company plans to build a bigger and deeper underwater energy storage facility in Aruba.
One key challenge Hydrostor faced was how to capture the heat given off when air is compressed and then use it later to warm the air as it cools during expansion. “The air can reach temperatures of 650 °C during compression, so if you’re not judicious in capturing that waste heat, you lose efficiency,” explains Rupp Carriveau, an associate professor at the University of Windsor, in Ontario, Canada, who advised Hydrostor early on. The solution they settled on was an off-the-shelf heat exchanger coupled with an insulated water bath.
In fact, Hydrostor has tried to use existing components wherever possible. VanWalleghem explains why: “Reliability is very important for utilities. You need them to be comfortable with the technology.” Off-the-shelf equipment already has rated life spans in the field, which Hydrostor’s partners and investors found reassuring. “The downside is that you have to live with what’s available,” VanWalleghem says. “But it’s worth it in terms of speed to market and not having to design and build everything from scratch.” Hydrostor wouldn’t disclose the exact cost of the Toronto system, but VanWalleghem says it’s “in the numerous millions of dollars.”
Although the technology is still new, the need for this kind of energy storage is obvious, says Carriveau. Much of the world’s population lives near a coast, he notes. “So that’s your load. And because of the losses you get during transmission, it follows that you want to keep your energy generation and your storage as close as possible to your load.”
Garvey sees the underwater storage as part of a holistic system. “An offshore wind farm should not simply be a subsystem that produces electricity when the wind blows. It should be a system which takes energy from the wind and does whatever is needed to deliver energy to shore as that [energy] is needed.”
The energy bags, he says, “are one very possible step toward that utopian view.”