Finding an effective way to store and tap into large amounts of excess wind-generated power would have the attention of many utilities and other power users. New companies are working on a range of batteries for just that purpose. Another idea – and with a 10 year success record – is Compressed Air Energy Storage or CAES.
In general, CAES systems are comprised of an air compressor, a storage vessel, and a turbo-expander. In a nutshell, electricity is converted to and generated from compressed air. In conventional CAES plants, rigid vessels or reservoirs, such as unused mines, underground caverns, and aquifers, store the compressed air.
An update of the idea comes from a collaboration between industrial partner Hydrostor Inc. and Brian Cheung, Rupp Carriveau, and David Ting, all affiliated with the University of Windsor, Ontario. Their idea is to use conventional thermodynamic concepts such as isentropic air compression, gas expansion, and heat transfer, in an UW (underwater) CAES design for long-term, high capacity, energy storage. “By going underwater, compressed air can be stored in flexible air accumulators near the bottom of lakes and oceans. Hydrostatic pressure would drive the air out when needed,” says Carriveau. “This could let electricity producers maximize use of existing assets as opposed to adding more generation and transmission infrastructure to meet peak demand.
In UWCAES, flexible air accumulators are placed near the bottom of a large body of water. As air is pumped in, the accumulators expand, like a balloon. The design air pressure within the accumulator equals the hydrostatic pressure exerted by the surrounding water. It keeps the air inside the accumulators compressed, even when partially filled. This is a key advantage over rigid containers that experience pressure reductions as their stored volume decreases.
During discharge, the rapidly expanding air can cool the turbine to a point of freezing it. So, heat must be added to prevent the freezing. To accomplish this, the UWCAES system uses the heat of compression captured during the operation’s charging phase. The heat is then stored at a nearby platform for up to eight hours before it’s applied to the discharge phase of the plant cycle.
In addition to the geographic advantage, UWCAES systems are scalable. The primary limitation in the use and performance of CAES is the fixed capacity of the reservoir. In UWCAES, simply adding air accumulators to the system increases the total reservoir capacity. The flexible nature of the system potentially allows for UWCAES deployments at scales that can be even larger than some underground systems. “Thermodynamically speaking, the compressed air in the UWCAES system has a constant pressure regardless of the filled volume,” says Carrivaeau.
Operators can count on near constant energy input and output exhibited during system charge and discharge phases. This also affords greater opportunity for improving system roundtrip efficiency because the appropriate turbomachinery can be selected to operate optimally at or near the system’s near-constant flow and pressure.
Several utility-scale CAES plants are already working. One, a 110 MW, 26-hr plant built in 1991 in McIntosh, Alabama, has an availability and starting reliability of 91.2% and 92.1%, as well as a 99.5% average running reliability for more than 10 years. WPE