Design and testing of Energy Bags for underwater compressed air energy storage

An Energy Bag is a cable-reinforced fabric vessel that is anchored to the sea (or lake) bed at significant depths to be used for underwater compressed air energy storage. In 2011 and 2012, three prototype sub-scale Energy Bags have been tested underwater in the first such tests of their kind. In the first test, two 1.8m diameter Energy Bags were submerged in a tank of fresh water and submitted to over 400 complete inflation/deflation cycles. The Energy Bags generally performed as expected despite minor air leakage which allowed water to accumulate in the bag's pneumatic fill/exhaust line which was initially connected to the base. In the second test, a 5m diameter Energy Bag was submerged at 25m depth in seawater at the European Marine Energy Centre (EMEC) in Orkney. Damage incurred by the Energy Bag upon initial deployment necessitated repair, emphasising the need for itemised handling and deployment protocol, and correspondingly robust bag materials. The Energy Bag was re-deployed and cycled several times, performing well after several months at sea. Backed up by computational modelling, these tests indicate that Energy Bags potentially offer cost-effective storage and supply of high-pressure air for offshore and shore-based compressed air energy storage plants.