About two years ago (August 2012), this magazine reported on an unusual underwater compressed-air-storage idea that uses a compressor powered by wind-generated electricity to pump up bladders secured at over a 50-m depth. When the grid needs power, the compressed air would be fed through an expander and a gas turbine to drive a generator.
“Since 2012, we have conducted a pilot study with the company Hydrostor to demonstrate the viability of the idea,” says Rupp Carriveau, Professor of Civil and Environmental Engineering at University of Windsor, Ontario, Canada. “We selected a marine salvage bag for the accumulator and anchored it at 35 m to the floor of Lake Ontario. Then with a truck-mounted compressor and heat exchanger onshore, we pumped air into the bag to record pressure and flow profiles as we filled and depleted the accumulator. The setup gives a fairly constant pressure profile. In a rigid container, pressure drops as air leaves the container.”
Another characteristic to examine is how hard the compressor works to inflate the bag. “Although that information is pretty well known, investors can be dubious until they see things at a full scale,” says Carriveau. “The pilot project recorded sufficient data to acquire additional government and private funding. Soon Toronto Hydro, the biggest urban utility in Canada, became part of the collaboration. Their interest is in buffering the instability of intermittent feeds like wind that we get around Toronto. Response time for the system is expected to be half or better than that of a combined cycle gas plant.”
As Carriveau understands it, the utility weighed several options for this particular storage mission. “The underwater option came up quickly as a leading choice thanks to its use of well-tested, off-the-shelf components,” says Carriveau. The pieces will just be connected for a different purpose. This builds confidence and represents an edge over newer battery technologies when forecasting service lives out past 20 years.
As of this writing, Hydrostor and Toronto Hydro are building a grid-connected demonstration facility about 5 km off of Toronto Island in Lake Ontario, near the island airport. Facility operation startup is planned for late Summer. This technology has also been of great interest to island nations that boast much deeper coastal depths on the order of 400 to 600 m. The greater depth means you can put more air in the same size volume. The lake test is small but it is something that Hydrostor will scale up for these island communities to get away from the diesel back-ups, he adds.
Carriveau anticipates efficiencies in the 60 to 65% range. “Even if it falls short of the initial targeted range on commissioning, it can be improved on. The critical thing is to get it connected and going and then we can fine tune it.”
Carriveau also commented on the growing activity in offshore energy and storage sector in general. “As North America nears deployment of its first offshore commercial wind facilities, Germany will be quadrupling its capacity over the next year. Offshore and near shore storage is a great dispatchable system solution for these generators”. In response to the massive interest in this growing area, Carriveau’s Group along with energy storage experts, the Seamus Garvey Group from UNottingham, UK are holding an international Offshore Energy and Storage Symposium and Connector Event July 10 and 11 in Windsor Ontario, Canada. WPE
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