Adiabatic Compressed Air Energy Storage

For bulk electricity storage, Compressed Air Energy Storage (CAES) is the current second choice behind pumped-hydro storage. There are two commercial grid-scale CAES plants currently in existence; Huntorf, Germany (commissioned in 1978 -- see Figure 1) 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, running a process very similar to an open cycle gas turbine except that the air is pre-compressed, rather than operating the compressor and the turbine simultaneously. To overcome the need for fossil fuel, variations of the CAES concept which do not use natural gas have been proposed. In Adiabatic CAES (ACAES), heat generated during the compression is stored and then returned to the air prior to expansion, replacing the use of fossil fuel. However so far - despite much promising research - a successful prototype ACAES plant has yet to be demonstrated. If it proves practical, it is widely believed that ACAES has the potential for a reasonable-cost long-duration energy storage method which is entirely mechanical in nature and thus does not require expensive, toxic or exotic materials.

Huntorf CAES plant
Fig.1 - Huntorf CAES plant, Germany.

Research in the ESES lab includes rigorous thermodynamic analyses on a range of ACAES designs (and other thermomechanical storage systems). We employ both numerical simulations and experimental methods and all our work is strongly grounded in engineering thermodynamics. Previous work from the group has investigated the potential for using packed bed heat exchangers in ACAES, which are particularly appealing due to the exceptionally high heat transfer rates available, simple construction required and low-cost thermal storage materials (i.e. crushed rocks or ceramics). Our paper describing this analysis has been published in the journal Applied Energy. A video of the simulation is shown below.

Figure 2 illustrates a previous experimental setup for a testing CAES discharge on a micro-level. We used a reciprocating expander and measure the power and the total work output from the air tank discharge.

Tank discharge
Fig.2 - micro CAES system discharge experimental setup.

Isobaric CAES

Currently, we are investigating isobaric (constant pressure) air storage as a means of improving the performance of ACAES systems. We are particularly interested in the potential for constant high-pressure air storage by exploiting the phase change between liquid and vapour CO2 as a pressure buffer.