Compressed Air Energy Storage (CAES) is the current second choice for bulk energy storage behind pumped-hydro storage. There are two commercial CAES plants; Huntorf, Germany (commissioned in 1978) and McIntosh, Alabama (commissioned in 1991) which represent about 440 MW of power and in excess of 3 GWh of energy storage. These two plants, however, are not purely electricity storage, as they also use natural gas. Accordingly variations of the CAES process which do not use natural gas have been proposed, often called Adiabatic CAES (A-CAES), Advanced Adiabatic CAES or fuelless CAES. While so far a successful demonstration has yet to be achieved, these concepts appear to offer a reasonable-cost long-duration energy storage mechanism which is entirely mechanical in nature and thus does not require expensive, toxic or exotic materials. CAES in naturally occurring caverns has the lowest energy storage cost per kWh of equivalent electrical energy stored of any electrical energy storage technology.
While traditional CAES uses several compressions in series and dumps the heat generated by the compression, A-CAES stores the heat of compression as well as the cool high pressure air and uses it to re-heat the air during the expansion process. This replaces the fossil fuel combustion used for the reheat stage in traditional CAES with a fuel-free heat-storage-recovery process. My research into A-CAES systems started in my PhD studies at Edinburgh, when I was investigating an energy storage method that could be coupled with Tidal Current Energy Conversion. I have performed rigorous thermodynamic analyses on a range of A-CAES designs, investigating the effect of the number of compression and expansion stages, system pressures and temperatures, and heat exchanger designs.
Noting that a system with a sliding storage pressure must preserve the temperature range encountered in the compression process – i.e. the thermal energy storage must remain stratified – in order to operate efficiently, I have analysed the performance of a theoretical A-CAES system which uses packed beds to store the compression heat. My work suggests that this type of system may offer a better performance than a system based around shell and tube exchangers and a separate thermal fluid. This is due to the higher heat transfer rates available and simpler construction, and even though larger heat exchanger units with high pressure tolerances will be required, our cost analysis suggested that these should be less expensive than indirect-contact exchangers. A paper describing this analysis has been published in the journal Applied Energy. A video of the simulation is shown below.
I have also designed, built and tested an experimental system which mimics an expansion stage of a A-CAES system (photo shown below). This work illustrated how the pressure variation that occurs while charging and discharging the system can lead to low compressor and expander efficiencies.
I am currently working on a concept to minimise the pressure variation in the high pressure air store. I believe that, if feasible, a system which maintains a constant pressure in the high pressure air store would facilitate a step-change in the performance of CAES systems. It would allow the compressors and expanders to work at their design-point operation and would also reduce exergy losses associated with the mixing of heat at different temperatures. My design exploits liquid-vapour phase-change to achieve isobaric high pressure air storage. Although this work is currently conceptual and modelling-based, I hope to have an option to test it experimentally.