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Going Underground | Compressed air energy storage

Cost effective large-scale energy storage is the Holy Grail of renewable energy enthusiasts. One UK company believes it has the answer – based on proven off the shelf technology.

Intermittency is the Achilles Heel of almost all alternative energy sources. Wind, solar, wave and even tidal power are never available 100% of the time and rarely match patterns of demand. Inevitably there are embarrassing surpluses – at night, during the summer – and equally embarrassing shortfalls. Yet the UK, like many other countries, is set to increase its dependency on renewables, with the current target being 30% of electricity coming from renewables by 2020.

“The basic problem is that the current grid system was never designed for renewables,” says UK-based Storelectric chief executive officer Jeff Draper. “Without the capacity to store massive amounts of surplus renewable energy and with more and more ageing base load power stations being closed, the UK’s security of supply will be under severe threat.”

Storelectric’s solution is a development of a technology that has already been tried in several other countries, notably Germany and the United States. Compressed air energy storage (CAES) is, in principle, akin to pumped storage hydroelectric plants, such as the one at Dinorwig in Wales. 

Dong Energy, offshore wind farm

Dong Energy wind farm with Siemens turbines

Source: Dong Energy

Intermittency is the Achilles heel of renewable energy

When cheap, surplus electricity is available, air is compressed and pumped into vast underground salt caverns, aquifers or depleted oil and gas wells where it is stored at around 60bar pressure, depending on the depth.

At times of peak demand when base load power stations are struggling to cope, the compressed air is released to drive generators. This extra power is available at short notice, the ideal situation.

The reality is more complex, as Storelectric commercial manager Tallat Azad explains.

energy storage

energy storage

“Compressing the air raises its temperature to around 500°C. It has to go through a heat exchanger to bring it down to ambient temperature before entering the store. 

“Conversely, expansion would reduce the stored air’s temperature to – 150°C or even lower. This is too low for the turbine to function, so the air has to be heated to 200°C before it expands. So far this heating has come from methane.

“In effect, the storage replaces the compressor stage of a conventional gas turbine.”

Current CAES installations have an overall energy recovery efficiency of little more than 50% at best. Storelectric aims to boost this to as much as 70% through two crucial innovations.

Energy harvesting

Harvesting the thermal energy from the compression phase rather than letting it go to waste is the first step towards greater efficiency. The proposal is to use pressurised water thermal stores, a mature technology, where the heat of compression will be stored in water at 220°C and 20bar pressure.

A more radical proposal is to utilise surplus renewable electricity to do more than just compress air. Hydrogen is a viable alternative to methane, and the electrolysis of water to yield hydrogen gas is another mature technology, available virtually off the shelf.  

The storage replaces the compressor stage of a conventional gas turbine

Tallat Azad


Perhaps the most significant advantage of CAES is its potential to provide large grid-scale storage of up to 500MW within the near future. There are suitable salt caverns and aquifers conveniently located throughout the UK – and most of the rest of the world.

Pilot projects

Currently Storelectric has three pilot projects on the drawing board, one in Cheshire utilising one of the many salt caverns in the county, one in Scotland using aquifer storage, and the third in Northern Ireland, which will use above-ground storage.

“The project in Fife is in collaboration with the University of St Andrews,” Azad reports. “They take alternative energy very seriously. They’ve already commissioned a combined heat and power (CHP) plant and plan to open a wind farm nearby.

“An aquifer storage facility will be sited in an old paper mill, using hydrogen from an electrolyser. Like the other two pilot projects it will have a 5MW to 10MW capacity.”

These should be operational within 18 months, he adds. 

Grid scale project

Also in hand are plans for the first commercial grid-scale project in Cheshire, where there are five suitable caverns ready for adaptation. Almost certainly fitted with a thermal store, this will be capable of generating 20MW for 50 hours and could be ready within two years.

A 500MW plant, again in Cheshire, is also at the advanced state of planning, and Storelectric hopes to have this up and running in five to six years. It will store at least 2.5GWh of off peak electricity, with the potential of 4GWh.

Draper says he sees demand in the UK for up to 40, 500MW units costing around £16bn. “Current government calculations predict an overall energy storage demand of about 30GW by 2050. Pumped hydro storage and the various battery proposals are not seen as being able to provide much more than 4GW. There is no obvious alternative technology likely to be available in time.”

Global demand

Worldwide the potential demand for CAES is huge, of the order of 1,500GW or more. This could be met by some 3,000 CAES plants.

 torelectric now has a number of big name backers, including Siemens, Alstom and Balfour Beatty. Six UK universities are also on board.

Along the way the company has collected a string of awards. Most importantly Storelectric’s potential was recently recognised by the European Union. The company was granted technical approval as a project of common interest, implying the EU views Storelectric as an important infrastructure project at a continental scale.

What Azad describes as a “multi-faceted grid scale response system” would seem to have a promising future, provided it can attract the necessary financial support.

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