Advanced Adiabatic Compressed Air Energy Storage (AA-CAES)
Advanced-adiabatic compressed air energy storage (AA-CAES) is an evolution of traditional CAES, designed to deliver higher efficiencies via a zero-carbon process. Operation is similar to traditional CAES in that energy is stored by compressing air with turbomachinery and storing in an underground cavern. The difference lies in the treatment of the heat of compression.
Since it is not possible to process and store compressed air at the very high temperatures reached during compression, the heat must be removed prior to storage. Traditional CAES essentially dumps the heat into the atmosphere, therefore requiring a second injection of heat prior to re-expansion. AA-CAES instead aims to remove the heat and store it separately, then re-inject the heat at the expansion stage. This has potential to substantially increase the round-trip efficiency of the process.
One possible means is to heat-exchange the compressed air with oil, which can be stored in an insulated above-ground vat. If the compression process takes place in several stages (e.g., using multiple compressors) with heat-exchange occurring between each stage, the oil could be kept to a safe temperature for storage (e.g., below 600K).
A variation of this scheme is to modify the design of wind turbines so as to compress air directly using a mechanical link. In theory, skipping the conversion to and from electricity should improve efficiency and lower costs. A further variation is to store the compressed air in inflatable underwater ‘bags’. This could provide an alternative to the requirement for a storage cavern and would allow air to be stored and retrieved at constant pressure, which could improve operating characteristics.
A number of AA-CAES schemes have been proposed, but none have yet made it past the design stage; however, this may be about to change. In 2010, an international consortium headed by German electrical utility RWE announced plans to construct a new AA-CAES plant in Germany. They aim to achieve efficiencies in excess of 70% – the demonstration plant is due to go on stream in the middle of the decade.