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
Filed Under: Energy storage, News
Ajay Goyal says
Just convert the 1TW Open Loop Hydro Plants to 10TW Pumped Storage with just 120TWhrs of Lower Reservoir (a small percentage of the large Open Loop Reservoir with 4-5000TWhrs (??) of Storage) and using a 12/12 hr Pump/Generate Cycle generate 120TWhr/DAY providing 40,000TWhrs/yr of S2S (Sunset To Sunrise) Energy Storage to support a 180,000TWhrs/yr ZERO POLLUTION ENERGY FUTURE FOR THE WORLD.. using ONLY AgriVoltaics (AV) on just 7% of the Global Farmland today (while still growing food below.. that AV enables)…
100% SUPPLY OF ENERGY (180,000TWhrs/yr) BY AV.. 100% S2S Energy Storage by 10TW Pumped Storage… Supply side “problems” .
Now challenge Users of Energy ……(Industry, Transportation, Commericial, Transpirtation etc..) to convert to AV-PV Electricity… and you can have a ZERO POLLUTION EARTH as we inherited over 200 Years back..
Fred Fastiggi says
Really interested in how the pilot develops. Have done work in the Caribbean where electricity costs are $0.50+ per delivered kwh, primarily due to a lack of local fuel resources and the subsequent need to import fuel to generate electricity, this has great promise.
The concept of using renewable resources such as wind or solar PV to make electricity and then converting that electricity to compressed air which is stored underwater until needed, when coupled with a few other technologies like thermal storage for large resort hotels, and solar thermal for domestic hot water, would transform these economies.
Depending upon the island nation, with a reasonably consistent build up of capacity, I believe this approach could have the ability to cut overall electric costs in half for island residents within a 4 to 5 year period while still providing reasonable returns to developers. The fact that it is simple, easily understood and commercially proven with efforts such as this is a big plus for financial institutions who would provide project funding.
Robert R. Bullard says
Mike Bartlett is correct. Compressible fluid (gas) energy storage obeys the Ideal Gas Law (pV=nRT), with thermal loss issues on the compression and expansion side. Unless there is some form of capture of these losses in usable work, the losses for a compressed gas storage system will be significantly greater than those for “uncompressible” (e. g., water) systems such as gravity pumped storage closed loop systems in which the liquid “frictional” losses (heating) of the water are in the range of a few percent. Including electro-machinery losses a well-designed pump storage system will be at 75 to 80 per cent round trip efficiency, the lower side of the range being determined by the distance between pools, range of head difference and the extent to which pool charging water must be initially pumped from somewhere or to replenish pool leakage and evaporation.
Walt Atwood says
You have a few typos in the last paragraph. Interesting article, though.
Mike Bartlett says
This is interesting. Global NRG has a energy storage division Global NRG Storage Ltd. We have a number of established energy storage technologies either in the market or in final stages of research.
We have developed a similar system, but we use a hydraulic fluid as the medium for storing the energy. We originally used a system of 300 tonne weights to maintain the high pressure in the accumulators, but we have also been experimenting with bladders as accumulators anchored under deep fluid to retain a constant pressure on the bladder.
As many wind farms are located inland we have developed a method of using old mine shafts or open pit mines to maintain the pressure via water.
More recently we have dug special deep shafts into which we anchor the accumulators and the shafts are then filled with a special slurry which has a higher density value than water, but a good viscosity. The top part of the shaft wall is is lined and a weighted plunger is inserted that can move up and down as the hydraulic fluid is forced out to incease the kinetic energy to generate the electricity.
The hydraulic fluid is used to drive special hydraulic motors that in turn are coupled to generators. These hydraulic motor/generators are owned by Mitsubishi and are currently used in 5 MW wind turbines.
There is in theory no limit to the amount of wind or solar energy we can store, as it is simply a matter of adding additional shafts/accumulators. Hydraulic power is more efficient that air and less costly.