Starting Wednesday, a tiny bit of Toronto’s excess energy is going to be stored underwater in giant balloons.

Calling it “the world’s first-ever underwater compressed air energy storage system,” local energy firm Hydrostor and Toronto Hydro are to officially unveil the latest addition to the city’s electricity grid Wednesday morning.

Throughout the two-year pilot project, the Hydrostor system is expected to improve power quality for island residents while engineers monitor and test its performance.

Cost-effective and clean energy storage is a key step toward a future renewable-energy grid, according to Hydrostor Inc. chief executive officer Curtis VanWalleghem.

“Most of the world is saying we have to get off fossil fuels,” he said. “To do that, you need lots of energy storage.”

In the same way an everyday battery banks energy using chemicals, Hydrostor relies on compressed, bottled air. And because it produces zero emissions, the system can help Toronto adjust to a healthier low-carbon diet.

It works by stockpiling surplus energy generated during off-peak periods and converting it into compressed air. This air is then sent underwater, stored in balloons and saved, literally in some cases, for a rainy day.

“The wind doesn’t always blow, the sun doesn’t always shine and certainly not at the right amount when you need it,” said VanWalleghem.

So when demand for energy rises and the grid needs a boost, the weight of the water forces the air back to the surface through a large pipe. At this point, an expander converts it back into electricity.

At peak output, the storage unit is capable of powering about 330 homes. Depending on how much power is drawn, the system can run for a little over an hour.

The underwater structure is composed of six three-storey-tall balloons anchored roughly 60 metres below the surface of Lake Ontario. They’re made of coated nylon fabric, the same material used by marine salvagers to lift shipwrecks and planes off the sea floor.

“We bought them as big as they come,” said VanWalleghem, adding that each balloon is held in place thanks to 30 heavy-duty, industrial-sized straps. Together, they would fill a basketball court.

Jack Simpson, Toronto Hydro’s director of generation and capacity planning, said the underwater project appeared to be a good fit for an urban area as densely populated as Toronto.

“What we see is that land is a premium space in the city,” he said. “So the other positive virtue of this technology is that it is pretty benign and doesn’t have that big of a footprint.”

The technology also fulfils Toronto Hydro’s goal of modernizing its own electricity distribution system.

“We’re in the middle of a massive capital program to renew and upgrade our grid, so we’re actively looking for new ways to extend the life of our equipment, which is where energy storage comes in,” said Toronto Hydro president and CEO Anthony Haines. “Now that the system is ready, we’ll be looking at how it performs.

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“If it works well during this pilot phase, we’ll be looking at a possible expansion.”

Beyond southern Ontario, Hydrostor sees potential to expand to metropolitan coastal cities such as Los Angeles or Tokyo. Mining operations past their prime could also be recycled to house the system, VanWalleghem suggested, by placing the balloons at the bottom of a pit and filling it with water.