Energy Storage in Underwater Balloons

Grid-level energy storage takes many forms, including flow batteries, Li-ion batteries, pumped hydro, compressed air in underground caverns, and even flywheels. Toronto’s Hydrostor just added another tool to the arsenal: underwater compressed air energy storage (UCAES). Hydrostor recently activated a pilot UCAES plant – the first of its kind – that will provide grid-level storage for the city of Toronto. In addition to supplying the city with cost-effective energy storage, the system will allow engineers to study its behavior and optimize the design. Can UCAES become a viable energy storage technology?


The idea has been around for many years: when supply exceeds demand, use the excess energy to run an air compressor and store the air in an underwater balloon. When power is needed, open a valve and let the compressed air run a turbine to generate electricity. The principle is simple, but the economic feasibility has yet to be demonstrated. Hydrostor hopes that their facility becomes the proving ground.


Here’s a simplified version of how it works:


According to Hydrostor’s CEO, Curtis VanWalleghem, the “balloons” are made of urethane coated nylon.  They come with a 10 year warranty, but they hope to extend it to 20 years. The balloons will be anchored to the floor of Lake Ontario, about 2 km from the shore at a depth of 60 meters, where the pressure is 95 psi.




All electrical equipment is on land. During the charging stage, air is compressed to 95 psi and pumped into the balloons. Compressing air is highly inefficient, since much of the electrical energy is converted to heat, so Hydrostor includes a heat exchanger to recover that heat and store it in an insulated tank. During the discharge cycle, water pressure squeezes the balloons, forcing the compressed air back to the shore. Stored heat is added back to the system as the air decompresses and spins the turbine of a generator.


Hydrostor expects a round trip efficiency of 60% to 80%, comparable to battery storage. The pilot system will use a 660 kW generator, but the turbine is capable of driving a generator nearly twice that size. Hydrostor wouldn’t give specifics about the storage capacity, only saying that it’s scalable.



Images and video courtesy of Hydrostor


Since the system is scalable – one compressor/generator can handle multiple balloons – the anticipated cost is $200 to $300 per kWh. Depending on who you ask, that’s comparable to the current cost of Li-ion batteries and flow batteries. However, batteries can only be charged and discharged a limited number of times, and that number decreases with frequent deep discharges. UCAES balloons offer virtually unlimited charge/discharge cycles with no degradation caused by deep discharging.


I’ve read a few technical articles about UCAES and the idea is controversial, to say the least. From my standpoint, I’m concerned that this new technology comes in at roughly the same cost as mature technologies like Vanadium and Li-ion, whose prices are continuing to drop. Will the “unlimited” charging cycles be enough to make UCAES competitive?


You can only argue about theoretical quantities and hypothetical situations for so long; eventually you have to build it to see how it performs. I’m glad that someone is finally implementing the idea. At best, it proves to be a feasible, cost-effective, and clean way to store energy. At worst, we eliminate it from the list of possibilities and move on.