Pumped-storage hydroelectric plants play the biggest role in the European grid. In Germany alone, the installed capacity is now seven gigawatts. The alpine countries, Austria and Switzerland, with their mountainous landscape, can meet their short-term energy needs with hydroelectric power.

However, their storage potential cannot be increased any further and the Alps cannot play the role of a 'battery' for all of Europe.

Norway and Sweden have many possible locations for new pumped-storage hydroelectric facilities, but the grid connections to central European consumers would be costly and have yet to be built.

"Flat coastal areas, where it is predicted that the largest demand for energy storage will be in the future, are unsuited to this technology," says Stefan Zunft from the DLR Institute of Technical Thermodynamics in Stuttgart.

Pumped-storage hydroelectric plants in disused opencast mines
German engineer Matthias Popp from Wunsiedel has proposed a solution to this problem. He has taken out a patent for a ring-wall facility, which would be suited for use as a pumped-storage hydroelectric facility in flat terrain.

Short term excess capacity on the grid could be used to pump water into a basin elevated 10 to 30 100 to 400 metres above ground and enclosed by a ring wall which would then drive turbines when released into the surrounding lake.

Disused opencast mines could be a good location for such facilities. Today, these are being turned into large lakes, but they could be supplemented with ring-wall islands in the future.

Another option for flat areas would be underground compressed air storage, a concept Stefan Zunft's team at DLR is working on. "The technology has enormous potential throughout north-western Europe," says Zunft. One option would be to force air into underground salt caverns and store it there under pressure.

If an energy shortfall occurs, the air warmed by another heat storage device could be used to drive generators. Efficiencies of up to 70 percent are possible, almost as high as is attainable with pumped-storage hydroelectric plants. The ADELE pilot project will be launched in 2013, with a storage capacity of up to one gigawatt-hour.

Flywheels the renaissance of an old technology
Another option with unexploited potential is flywheel storage. The system is simple electric motors set heavy flywheels spinning. The use of almost frictionless bearings allows modern flywheels to turn at 80,000 revolutions per second.

When power is needed, the flywheel is coupled to a generator, which converts the kinetic energy back into electricity. This technology has an extremely high efficiency up to 95 percent. In principle, it can make megawatts of energy available on demand for short periods.

An intermediate storage in the form of chemical energy is also an option for the future. The best candidate for this is hydrogen, with its high energy content. Excess capacity could be used to generate hydrogen by means of electrolysis, which could then be used in fuel cells for use when required. However, these conversions reduce the overall efficiency, although work is being done to increase it.

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