Thin Red Line Aerospace completes first Undersea Energy Storage Structure
Canadian firm Thin Red Line Aerospace has completed the first structure specifically designed and built for undersea compressed air energy storage (CAES). The structure, also referred to as an “Energy Bag”, is to be anchored to the seabed off the coast of Scotland next month as part of a major renewable energy research project conceived and led by Professor Seamus Garvey of the University of Nottingham and supported by European renewable energy leader E.ON. The project is the first to investigate large scale offshore storage of wind, tidal and wave power as compressed air.
Compressed Air Energy Storage (CAES) is a way to store energy generated at one time for use at another time. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods.
Conceptual representation of the compressed-air energy storage concept. Off-peak (low-cost) electrical power compresses air into an underground air-storage “vessel” (the Norton mine), and later the air feeds a gas-fired turbine generator complex to generate electricity during on-peak (high-price) times. A similar concept  uses wind powered air compressors.
Compression of air generates a lot of heat. The air is warmer after compression. Decompression requires heat. If no extra heat is added, the air will be much colder after decompression. If the heat generated during compression can be stored and used again during decompression, the efficiency of the storage improves considerably.
There are three ways in which a CAES system can deal with the heat. Air storage can be adiabatic, diabatic, or isothermic
’The overall process for adiabatic compression sounds simple, but the difficulty is how you handle the heat when you compress the air to 60 and 70 times atmospheric pressure,’ said Edward Barbour, a researcher in CAES at Edinburgh University. ’There are two main problems. One is that the heat is generated very quickly and that’s difficult for the machinery to handle, and the second is that your energy density suddenly becomes very low.’
At these pressures, the heat from compressed air can reach temperatures of 650°C. Seamus Garvey, a professor of dynamics at Nottingham University, believes he has come up with a solution that will allow for cost-effective heat storage. Garvey’s idea is to compress air in containments called Energy Bags held down on the sea bed in deep ocean water. ’Underwater bags are an attractive option because the sea acts as the pressure vessel,’ he said. ’You don’t have to pay for the full structure, just the structural material required to hold the bag down. No matter how full or empty your container is, the pressure stays the same, and that’s lovely for the machinery at the sea surface.’
Designed and developed for Garvey’s project by Canadian firm Thin Red Line Aerospace, the bags use a butyl rubber bladder and a polyester-reinforced fabric for the outer surface. Special coated steel or aramid straps provide the main structural strength. At depths of around 600m, there will be enough pressure in one 20m-diameter bag to store around 70MW hours of energy. That’s around the same as 14 hours of energy generation from the largest offshore turbines currently in operation.
As momentum picks up in CAES research, Garvey’s concept is gaining attention. It remains to be seen whether adiabatic compressed air energy storage will be viable, and whether Energy Bags are the right way forward. But without someone thinking outside the box, the concept of AA-CAES is likely to remain firmly on the drawing board.