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Developing bulk energy storage from compressed air and hydrogen

Ewan Murray | 05 May 2016 | Comments

In January, I wrote about how compressed air energy storage (CAES) might be a solution to keeping the lights on. Since then, we’ve been exploring in a lot more detail how salt caverns can be converted into suitable compressed air storage; a technique which is increasingly being accepted as a significant contributor to solving the supply/ demand challenges faced by the UK and international energy power systems.

We’ve also discussed previously why it’s important that quick response energy storage is available (in short, more variable generation from renewables means there has to be quick response storage available for any peaks in demand).

To date, only CAES and pumped-hydro have been proven capable of producing grid-scale, bulk energy storage with good response times.

Conversion of existing natural gas storage assets to compressed air is potentially an effective means of deploying CAES as the cost and time to develop a suitable cavern is significantly improved, and can also present an opportunity to add significant value to an existing asset whose revenue potential has been adversely affected by market conditions. Conversion of existing caverns presents a unique challenge but one which Atkins is well placed to support.

There are a number of aspects to consider, including:

  • The storage and generation capacity

        The performance criteria to consider are the compression/generating capacity (MW) and storage capacity (MWh), which will determine how long the plant can generate at the rated capacity. Downhole and surface equipment will need to be selected to suit the cavern to ensure that the plant performance is optimised.

  • Suitability of existing caverns for CAES

        Not all existing caverns will be capable of withstanding the anticipated thermal loading conditions that are expected to occur during CAES operations. The geomechanical stability of salt caverns must be determined when considering them for conversion and development of CAES.

Consideration should be given not only to the effect of the external geostatic loading and of the loading due to the pressure of the stored air, but also to the temperature effects which can be significant for geological materials, such as halite (a type of salt).

  • Impact on remnant plant

        It is desirable to re-purpose as much of the existing infrastructure as possible in order to minimise the capital cost of the conversion by adapting rather than replacing existing plant. The existing equipment would need to be assessed in detail for suitability of operation in a CAES application (e.g. consideration of materials suitability).

  • Location and layout of CAES plant and consideration of grid connection

        Gas storage facilities typically have minimal surface footprints. Development of a CAES facility requires a sizeable footprint for both the plant systems and construction. Furthermore, it may not be possible to construct the plant directly above the existing caverns. A suitable electrical grid connection would also need to be considered.

Hydrogen – identified as a key enabling technology for advances in stationary and portable power, as well as uses in transportation and grid stability – is another candidate for bulk energy storage in converted gas caverns.

Hydrogen has to be stored at high pressure to ensure high energy mass, which means that it typically has to be kept in a very large container, which is tricky when it has the smallest molecule of the chemical elements and can even migrate through steel vessels.

A salt cavern provides a safe, low cost, reliable solution. The production of hydrogen is a mature technology and there are many proven means of manufacture (e.g. H₂ through gasification from coal, steam reformer through methane or by electrolysis). It is the economics of producing H₂ that has limited the technological advances, but with a high renewable energy mix and ‘excess generation’ market now around the corner, hydrogen becomes an attractive means of bulk storage.

We’re really working at the leading edge of what it is possible to do in storing energy. Exciting times are ahead.