As part of its 20-20-20 goals, the European Union aims to achieve a share of power generation from renewable sources of 20 percent by 2020. With this growing share of renewable power in the energy mix comes a challenge, however: as opposed to traditional power plants, renewables do not continuously generate power, causing fluctuations in the power grid. Highly flexible back up power generation solutions like GE’s FlexEfficiency 50 combined cycle plant are one way of balancing out the grid. Another one must be the vast expansion of energy storage.

Energy storage technology has traditionally been used as a means for load balancing in power grids, i.e. to partly balance out the regular and reasonably predictable variation in energy demand between day and night time. Most European countries today rely on pumped storage plants in mountainous regions. These use excess power, e.g. at night, when demand is low, to pump water from a lake into a reservoir at a higher elevation. When demand picks up during peak consumption times, this water is released through turbines back into the lower lake, generating additional electricity for the grid.

With a growing share of renewables, most countries in Europe will need more high volume energy storage like this, which will act as a gigantic buffer between the intermittent power supply from renewables and power consumers on the grid. However, suitable locations for traditional pumped storage plants are limited and, even if there were more of them, usually quite far from the location of the renewable power source. In Germany, for example, the vast majority of wind power is generated in the North of the country, close to the coast, where winds are particularly strong, but the pumped storage plants are mostly in the mountainous south. This in turn raises the issue of how to transport vast quantities of energy from the point of generation at the coast to the proposed storage in the mountains.

A very promising solution to the issues of availability and location is compressed air storage. It uses excess power to compress ambient air to above 50 bar, which is stored underground and at peak times is released to drive a gas turbine, generating electricity. So far, however, this approach has had a major drawback: It was not as energy efficient as pumped storage plants. The gas turbines had to be heated using natural gas, as the decompressing stored air generated a wind chill so cold that it would freeze the turbines. Naturally, the need for additional heat impacted the overall energy efficiency of the plant negatively.

Next generation of air pressure storage plants

In Germany, utility RWE, technology company General Electric, construction company Züblin and the German Aerospace Centre (DLR) cooperate in building the next generation of air pressure storage plants, called ADELE (adiabatic air pressure storage for electricity generation). Their approach is as simple as it is ingenious.

As most cyclists will know from their bike pumps, air heats up significantly during compression – and acompressed air storage plant is no exception, generating temperatures of above 550 degrees Celsius during compression. So far, however, this heat is lost. ADELE will capture this heat and store it in ceramic or natural stone thermal storage. During decompression, the air flows through this thermal storage, which preheats it before it hits the turbines. This measure cuts out the additional heating of the turbines and increases the efficiency of air pressure storage from previously around 50 to around 70 percent, bringing it much closer to the efficiency of pumped storage plants, which range around 80 percent.

ADELE utilises large underground salt caverns to store the compressed air, and conveniently, these are located close to where the majority of German wind turbines are installed. RWE, the leading project partner for ADELE, has already identified 20 suitable regions for air pressure storage in and around Germany.

Starting in 2013, the first ADELE storage plant will be built in Staßfurt in eastern Germany, with a capacity of up to 360 Megawatt hours and an electrical output of up to 90 Megawatt, equivalent to the generation of 30 modern wind turbines in four hours. The plant is expected to start operation in 2018.

Advanced large scale energy storage technologies like ADELE are key for an energy future which will feature a much larger share of intermittent power sources like wind and solar. Air pressure storage can be built very close to where renewable power is generated and there are many geologically suitable sites available that can be developed into this clean form of energy storage.

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• Energy storage
• europe
• Renewables
• smart grid