Forget Dams: Let's Store Extra Energy in Rocks on Trains and Underwater Balloons

The problem with renewable energy isn't getting it—it's having it around when we need it.

Renewables are abundant, but they tend to make themselves available at inconvenient times—the wind often blows strongest at night, when the lights are off—and in out-of-the-way places, like sunlight in the desert. In order to get more from renewables, we need a better storage solution than even the most advanced batteries, which aren't likely to store anywhere near what we need or what renewables produce.

So far, the main large-scale storage option is even more inconvenient than the renewables themselves.

Pumped hydro storage is an ingenious, 100-year-old solution that uses no fancy science, only physics and engineering: falling water turns turbines to run a generator that produces electricity. All you need to do is pump water to the top when you have excess energy, and then, when you're running low, let it fall down again.

The big hitch in this scheme is that you generally have to build a dam to make it work—and building dams means making lakes, and making lakes means covering lots of stuff with water. For someone trying to create a viable grid-scale energy storage solution, the chance of finding a suitable valley that's available for flooding is basically pretty much zero. "It's incredibly difficult to permit the civil works that are necessary for large-scale hydro," said James Kelly, CEO of California-based Advanced Rail Energy Storage (ARES).

Rather, ARES and a Canadian start-up called Hydrostor have developed large-scale systems based on the simple physics of pumped hydro storage (i.e. gravity), but without all the extra lakes.

Instead of trying to build new pumped hydro facilities, the founders of ARES—William Peitzke, Matt Brown and John Robinson—asked themselves, "How can we do pumped storage hydro-electric, but without any water?" The answer they found was basically the opposite of water: rocks. Or more specifically, rocks on trains.

"We realized the solution was right in front of us," said Kelly. "The railroad industry had developed an incredibly efficient way to move mass." One ARES engineer determined that the coefficient friction of steel wheels on railroad track is lower than the coefficient friction of ice skates on ice.

The ARES system uses excess energy from the grid to pull 140-ton railcars up hills (total train weight: 1,350 tons). When the grid needs that power back, they simply let gravity take the weighted cars back down. Regenerative braking—similar to what you find in a Toyota Prius, or in Japanese subways—captures the energy the trains produce along the way

ARES built a test facility in California to prove the concept, and now they're in the final stages of building a 50 megawatt facility in Nevada, which will come online in 2016. For comparison, this facility alone will add more energy storage than was built across the entire US in 2013 (44.2 megawatts), according to a recent recent report by US Energy Storage Monitor. The same report suggests that 220 megawatts will be deployed in 2015, twice the capacity of the previous two years combined. ARES is part of a rapidly growing sector—and it's not the only one.

Hydrostor is a Toronto-based startup that also expects to play a big role in grid-scale energy storage. Rather than pumping water to a higher altitude and letting it fall down again, they decided to pump air underwater instead.

The company compresses the air into giant balloons—the kind used to lift sunken ships and downed planes—located several kilometers offshore and deep below the surface. Each plant is also scalable by depth: the deeper the balloons are, the greater the water pressure, and the more efficient their output. If the company doubles their depth, they can get the same amount of energy from half the number of balloons.

That's a bigger cost-saving than it seems, because those balloons are really big: 9 metres high by 5 metres in diameter. The upward lift on one balloon is 100 tons, so for Hydrostor, the greatest expense is installing structures with enough ballast to hold the bags down.

The underwater real-estate market isn't exactly competitive, which makes Hydrostor's installations well-suited for islands and waterfront cities without a lot of extra space for big power facilities. Like ARES, they built a test facility to show it works, and now they're preparing to start construction on their first project in Aruba, with six more in the works.

Both ARES and Hydrostor pride themselves on offering off-the-shelf mechanical solutions. Apart from their patented computer technology, both companies buy all their parts out of catalogues, and so they scale up or down very easily. They each could make a 5mw or 1,000mw facility, as needed, and they don't need to plug up any rivers to do it.

"With the renewables that we can harvest cost-effectively, the potential is enormous," says Kelly. "The problem will not be supply." The problem, rather, is storing all that surplus energy. For ARES, that means having access to a few good rocks. For Hydrostor, some airbags will do the trick. Either way, it beats making lakes.