Large scale energy storage for power generation is moving up the power industry agenda, particularly as we contemplate more renewables on the grid. But if you don’t happen to be blessed with the right terrain for pumped storage, and are not inclined to create it (eg by construction of artificial islands, as proposed by KEMA) current options are rather limited. In fact if you want power levels above about 100 MW and capacities of 10 000 MWh or more, some form of compressed air system employing an underground cavern seems to be the only concept currently available, and experience thus far with it is very limited. Indeed, as far as I know, despite much talk about the need for energy storage in general and compressed air systems in particular there are only two large compressed air energy storage schemes currently actually in operation, Huntorf, rated at 290 MW, which started up as long ago as 1978, and McIntosh, a 110 MW plant, which entered operation in 1991.

The Norton compressed air project in Ohio, envisaged to have an eventual power rating of 2000 MW, has been under development for a good few years, by Haddington, but nothing has been built and the rights to the project have recently been sold to FirstEnergy. A compressed air storage project has been under planning in Iowa for some time (the Iowa Stored Energy Park) but not much seems to be happening there, while EnBW’s project (announced in March 2006) to build a compressed air storage facility in Lower Saxony has been abandoned.

On the positive side it is noteworthy that General Compression in the USA, which envisages arrays of modular compressed air storage units, “from 2 MW to 1000s of MW”, reports it has obtained funding commitments from investors (including Duke Energy) that will “support development of a first commercial scale unit.”

It is also very good to see that the RWE/GE led feasibility study on adiabatic compressed air energy storage, initiated a couple of years ago, has moved to the next phase, with the launch of the ADELE project and the signing of a co-operation agreement between RWE, GE, Züblin and DLR (Germany’s National Research Centre for Aeronautics and Space), see pp 10-11.

There are of course some formidable challenges ahead, not least in the area of turbomachinery, with the compressor discharging at 600°C and cycling daily. There is also the question of scale. In both Huntorf and McIntosh compressed air extracted from the cavern is mixed with natural gas and combusted in a gas turbine, the main difference being that McIntosh has recuperation, with heat recovered from the gas turbine exhaust gas used to preheat the air, and is classified as “second generation.” ADELE is “third generation”, having no combustion turbines only an air turbine and a compressor, with a large thermal energy store employed to capture heat generated during compression rather than allowing it to be lost (hence the term adiabatic). This stored heat is subsequently used to preheat the air prior to its expansion in the turbine.

The compressor-only power ratings of Huntorf and McIntosh are in fact a mere 60 MW and 49 MW, respectively, whereas ADELE is aiming for a power of 200 MW and a capacity of 2000 MWh – a considerable scaling up. And then there are the heat storage aspects, which also look daunting and promise to take that technology into new territory.

The German government has very wisely expressed willingness to provide support for ADELE, in addition to the investments to be made by the industrial partners. This is undoubtedly the kind of development project where government support is warranted, indeed essential, to address the risks and get things moving. And the returns could be colossal. As RWE says, “storing energy efficiently, safely and in large quantities – this is certainly one of the key challenges of future power supply.”