Energy | 18. October 2010 | posted by Jan Oliver Löfken

Energy question of the week: What techniques are available for storing energy?

Batteries are certainly the most familiar energy storage devices – reliable, available everywhere and convenient. Rechargeable batteries are ideal for cell phones and electric cars, but for large amounts of energy, to overcome shortages in the power grid, they are not the best solution. What other options are available to us today?

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.

Die Grafik zeigt die wesentlichen Elemente eines Ringwallspeichers. Das dargestellte Ringwallspeicher-Hybridkraftwerk bedeckt mit seinen 11,4 Kilometern Durchmesser eine Fläche, die 25% größer wäre, als der Chiemsee. Der 215 Meter hohe Ringwall umschließt das Oberbecken mit einem Durchmesser von sechs Kilometern. Bild: Dr. Matthias Popp

The graphic shows the basic components of a ring-wall storage facility. This ring-wall hybrid power station is around 25 percent bigger than the Chiemsee in Bavaria. The 215-metre high ring wall encloses the upper reservoir, which has a diameter of six kilometres. Credit: Dr Matthias Popp.

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 [updated following discussions in the German-language comments about this blog post see below †] 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.

The DLR Energy question of the week in 'The future of energy' Year of Science

The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.

†German-language comments related to the update above:

From Matthias Popp on 18 October 2010
The height of the ring walls should be an order of magnitude greater than indicated in the text. We are thinking of 100 to 400 metres. The doubling of the diameter, height and level differences in upper and lower reservoirs increases the energy content by a factor of 16. Therefore, the area required per kilowatt-hour of capacity is drastically reduced and a higher-capacity plant can be built. For further details, see (in German).

From Jan Oliver Löfken on 19 October 2010
Dear Mr Popp, many thanks for the additional numbers. These make the potential of ring-wall storage even clearer. Best regards, Jan Oliver Löfken


About the author

Energy journalist Jan Oliver Löfken writes among other things, for the Technologie Review, Wissenschaft aktuell, Tagesspiegel, Berliner Zeitung and P.M. Magazin on issues involving energy research and industry. For DLR, he answered the Energy question of the week during the Year of Energy 2010. to authorpage