Ewan Murray

UK & Europe

Ewan is a mechanical engineer in Atkins’ energy storage business where he has been involved in a variety of disciplines from rotating plant monitoring to feasibility studies, and technical authority work at two of the UK’s largest onshore gas storage sites.

Ewan is also actively involved in business development and provides a link to the futures energy business; which is currently looking at wider thermal, mechanical and electrical energy storage.

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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.

UK & Europe, North America, Middle East, Asia Pacific,

Energy supply in the UK faces a problem. It has been discussed at length by leading energy experts, yet I feel it can be summed up simply:

“With diminishing baseload generation in the UK, combined with the impact of the rapid growth in intermittent generation (i.e. offshore wind), the household peak demands cannot be met with our current power infrastructure.”

There are revolutionary techniques being developed which will completely change the way in which energy is stored; something which is becoming increasingly important in securing supply through intermittent generation.

Unfortunately, most low carbon energy storage technologies currently lack the investment needed to make them commercially viable on a large scale, and would benefit greatly from a similar level of support that has been given to wind power in the last few years. But despite the lack of investment, some of these low carbon breakthrough technologies are almost ready.

Compressed air energy storage (CAES) is a technique used to store energy using compressed air for later use.

The first of its kind – and still the world’s largest – E.ON’s Huntorf power station in Germany was commissioned in 1978. The 321megawatt (MW) plant uses two 150,000m³ salt caverns to store compressed air. Two projects in development, Project ADELE in Germany and another in Larne, Northern Ireland, will also use salt caverns as a means of storage. RWE’s ADELE will have a capacity of 90MW and is due for completion in 2019. Larne will be larger, at approx. 300MW, which is being developed by Gaelectric.

The CAES process:

  • Atmospheric air pressure at sea level is defined as just over one bar. For storage, air is generally pressurised to around 45-70 bar (although higher pressures can be achieved depending on the structural integrity and depth of the storage volume) and is typically stored in underground caverns
  • These underground caverns are usually solution mined in the salt layer using freshwater under a blanket of N2 gas to extract a brine solution. We discussed salt caverns in a recent piece on carbon capture and storage
  • When required, the compressed air is heated and expanded in a turbine which drives a generator to produce electricity.

On a larger scale the heat energy associated with compressing air must be managed properly. There are examples where this has been done on a small scale (such as in transport) and there are a number of other ways of doing this in mediums that can store heat (such as molten salts, gravel pits, concrete, oil, or water). The challenge at a larger scale is to store the heat safely with minimal losses.

Atkins is currently working alongside leading salt storage specialists DEEP on several gas storage projects in the UK. We see standalone energy storage and grid integrated energy storage – where primary energy goes through energy storage before being converted to electricity for supply to the national grid – as a crucial part of the future global energy mix.

With the recent Paris (COP21) commitment from 195 countries to meet a global temperature rise of less than 2˚C it is very likely that CAES will play a role in maximising renewable energy efficiencies and meeting peak energy demands.

In addition, different divisions within Atkins is currently working on first-of-a-kind offshore floating wind platforms. It is possible that an element of energy storage could be built into these structures to improve individual wind turbine generator capacity factors. One solution is for direct conversion of the energy generated by the wind turbine to compressed air which is then stored under the sea in “energy bags” which could provide some element of ballast to the structure. Trials of this technique are already being carried out in Toronto, Canada at a 5MW capacity.

It is clear there is no front runner in energy storage, and a combination of innovative solutions (including thermal, electrical and mechanical) will help solve the instability in the grid. This makes it a fascinating time to be involved in such an evolving energy market and the Atkins energy storage team intends to be at the forefront of the latest energy storage projects.

UK & Europe, North America, Middle East, Asia Pacific,