SIEMENS

The wind blows when and where it will, and it rarely heeds our wishes. These days, that can have a serious impact on our power supply, to which wind energy is now making an increasingly important contribution. In 2007, wind power accounted for 6.4 % or 39.7 TWh of gross power consumption in Germany, and this proportion, according to a projection by the German Renewable Energy Federation (BEE), could rise to as much as 25 % (149 TWh) by the year 2020. By then, Germany should have wind farms with a total output of 55 GW, compared to 22 GW at the end of 2007.
Germany already accounts for approximately 20 % of the world’s total wind power generating capacity. Until recently, it was the pacesetter, but has now been pushed into second place in this particular world ranking by the U.S.
Although this is all excellent news as far as the climate is concerned, it presents the power companies with a problem. Wind power isn’t always generated exactly when consumers need it. As a rule, wind generators produce more power at night, and that’s exactly when demand bottoms out. With conventional power plants, output can be adjusted in line with consumption, merely by burning more or less fuel. With fluctuating sources of energy, however, this is only possible to a limited degree. And that goes for both wind and photovoltaic power, which, according to the BEE, will together account for 7 % of gross power consumption in Germany by the year 2020.
The ideal solution is to cache the surplus electricity and feed it back into the grid as required. The power network itself is unable to assume this function, since it is a finely balanced system in which supply and demand have to be carefully matched. If not, the frequency at which alternating current is transmitted deviates from the stipulated 50 Hz, falling in the case of excess demand, or rising in the case of oversupply.
Both scenarios must be avoided, as there would otherwise be a danger of damage to connected devices such as motors, electrical appliances, computers and generators. For this reason, power plants are immediately taken offline whenever an overload pushes the grid frequency below 47.5 Hz.
Oversupply can likewise pose problems. Germany’s Renewable Energy Act stipulates that German network operators must give preference to power from renewable sources. But an abundance of wind power means that conventional power plants have to be ramped down. This applies particularly to gas- and coal-fired plants, which are responsible for providing the intermediate load — in other words, for buffering periodic fluctuations in demand. For the power plants assigned to provide the base load — primarily nuclear power and lignite-fired plants — ramping up and down is relatively complicated and costly.
On windy days, this can have bizarre consequences. For example, it may be necessary to sell surplus power at a giveaway price on the European Energy Exchange (EEX) in Leipzig. In fact, the price of electricity may even fall below zero. Such negative prices actually became a reality on May 3, 2009, when 1 MWh was briefly traded at minus €152. In other words, the operator of a conventional power plant chose to pay someone to take the power rather than to temporarily reduce output.

Storing Power with Water. By far the best solution is to cache the surplus electricity and then feed it back into the grid whenever the wind drops or skies are cloudy. Here, a proven method is to use pumped-storage power plants. Whenever demand for electricity falls, the surplus power is used to pump water up to a reservoir. As soon as demand increases, the water is allowed to flow back down to a lower reservoir — generating electricity in the process by means of water turbines. It’s a beautifully simple and efficient idea. Indeed, pumped-storage power plants have an efficiency of around 80 %, reflecting the proportion of energy generated in relation to the energy used in pumping the water to the top reservoir. At present, no other type of storage facility is capable of supplying power in the GW range over a period of several hours. In fact, more than 99 % of the energy-storage systems in use worldwide are pumped-storage power plants.
Germany’s largest pumped-storage power plant is in Goldisthal, about 350 km southwest of Berlin. The facility has an output of 1,060 MW and could, in an extreme situation, supply the entire state of Thuringia with power for eight hours. In all, 33 pumped-storage facilities operate in Germany, providing a combined output of 6,700 MW and a capacity of 40 GWh. Each year, they supply around 7,500 GWh of so-called balancing power, which covers heightened demand at peak times — in the evenings, for example, when people switch on electric appliances and lights. The energy held in reserve by pumped-storage power plants can be called up within a matter of minutes.
In Germany, however, simply increasing the number of pumped-storage power plants isn’t such a simple option. There is a lack of suitable locations, and such projects often trigger protests. As a result, Germany’s power plant operators coordinate their activities with their counterparts in neighboring countries. Energie Baden-Württemberg (EnBW) in Karlsruhe, for example, uses pumped-storage facilities not only in Germany, but also in the Vorarlberg region of Austria. Norway, too, which has a long history of hydropower, is now looking to market its potential for electricity storage. However, the capital expenditure for doing so would be substantial. Such a pro-ject would involve more than just laying a long cable to Norway. The grid capacity at the point of entry in both countries would also have to be increased in order to avoid bottlenecks in transmission capability. "Such a step would be necessary because electricity always looks for the path of least resistance and will take another route when it encounters an obstruction," explains Dirk Ommeln from EnBW.