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.

Batteries and Compressed Air. Other major industrialized countries such as the U.S. and China also make significant use of pumped-storage power plants. In addition, major efforts are being made to find alternative methods worldwide. The best-known of all electricity storage devices is the rechargeable battery, which can be found in every mobile phone and digital camera. Although the amounts of energy involved here are tiny by comparison, this has not stopped some countries from using batteries as a cache facility for the power network. "In Japan, for example, this method is used practically throughout the country," says Dr. Manfred Waidhas from Siemens Corporate Technology (CT). "Batteries the size of a shipping container can store about 5 MWh of electrical energy and are installed in the grid close to the consumer." They are used as an emergency power supply, as a reserve at times of peak load, and as a buffer to balance out fluctuations from renewable sources of energy. Sodium-sulfur batteries, which have an efficiency of as much as 70 to 80 %, are used for this purpose.

Similarly, in a method known as V2G (vehicle to grid), electric vehicles could also serve as local cache facilities for electricity in the future, provided they are connected to the grid via a power cable. Although their battery capacity is small in comparison with the amounts of energy required in the grid, the sheer number of such vehicles and the relatively high powers involved—e.g. 40 kW per vehicle—could make up for this. "As few as 200,000 vehicles connected to the grid would produce 8 GW. And that’s enough balancing energy to improve grid stability," says Prof. Gernot Spiegelberg from Siemens CT.

"On the other hand, we need to remember that such batteries will be relatively expensive due to their compactness, safety specifications, and low weight," warns Dr. Christian Dötsch from the Fraunhofer Institute for Environmental, Safety and Energy Technology (UMSICHT) in Oberhausen, Germany. "What’s more, their service life—the number of times they can be recharged—is still very limited. At present, the extra recharging and discharging for the purposes of load balancing would seriously reduce battery life." (Electromobility)

Another concept is to warehouse potential kinetic energy underground by a technique known as compressed air energy storage (CAES). This involves pumping air, which has been pressurized to as much as 100 bar, into underground cavities such as exhausted salt domes with a volume of between 100,000 and 1 mill. m³. "This compressed air can be used in a gas turbine," says Waidhas. "You still need a fossil fuel such as natural gas, but energy is saved because the compressed air for combustion is already available."

There are two CAES pilot projects worldwide: the first went into operation in Huntorf, Germany, in 1978; the second in McIntosh, Alabama, in 1991. The basic idea behind CAES is simple, but there are drawbacks. "In both projects, the gas turbines are custom made, and that kind of special development costs money," says Waidhas. "CAES only gives you storage capacity of around 3 GWh."

Hydrogen: Ideal Storage Medium? An interesting alternative to the methods already mentioned is hydrogen storage. Here, surplus electricity is used to produce hydrogen by means of electrolysis. The gas is then stored in underground caverns at a pressure of between 100 and 350 bar, where, according to Erik Wolf from Siemens Energy Sector in Erlangen, Germany, leakage is not a problem. "Typically, each year, less than 0.01 % is lost," he say. "This is because the rock-salt walls of such caverns behave like a liquid, and any leaks seal up automatically." For this reason, says Wolf, any of the caverns already used for the short-term storage of natural gas would also be suitable for hydrogen.

Around 60 caverns are now under construction in Germany. "If we were to use only 30 of these for hydrogen storage, we would be able to cache around 4,200 GWh of electrical energy," Wolf points out. Hydrogen has such a high energy density that as much as 350 kWh can be squeezed into every cubic meter of available storage space. This significantly exceeds CAES (2.7 kWh/m³) and is only matched by lithium-ion batteries.

Whenever the demand for electricity rises, hydrogen is removed and used to power a gas turbine or a fuel cell. "At present, underground hydrogen storage is unmatched by any other energy-storage system," says Wolf. "Each cavern is capable of providing more than 500 MW for up to a week in base-load operation. That’s the equivalent of 140 GWh. By way of comparison, all the pumped-storage power plants in Germany only have a combined capacity of 40 GWh." What’s more, underground hydrogen storage facilities can supply power quickly to the grid and are as flexible as a combined-cycle power plant.

Hydrogen also compares well in terms of costs. According to a study by the German Association for Electrical, Electronic & Information Technologies (VDE), the costs of long-term storage—to compensate for unfavorable weather situations and seasonal fluctuations—will be under €0.10 per kWh. In contrast, the cost of CAES is estimated to be around €0.20 per kWh. At the same time, underground hydrogen storage facilities can help cover short-term peaks in demand and therefore boost the existing capacity provided by pumped-storage power plants.

Siemens has been conducting research into this technology for the last four years, and most of the components required, including the electrolyzers and gas turbines, are now available—as are safe caverns for hydrogen storage. Engineers from Siemens Corporate Technology are currently working on higher performance electrolyzers and gas turbines that are specially modified for use with hydrogen (Combustion Simulation). "The first patent applications have already been filed, and a larger-scale pilot plant could be up and running within three to five years," says Wolf.

Hydrogen has other advantages too. Apart from storing energy for generating power or heat, it can also be mixed with syngas (synthesis gas)—from, for example, biomass plants—to produce fuel in a biomass-to-liquid (BtL) process. "Hydrogen gives us a whole range of options, and significant progress has been made here in recent years," says Stephan Werthschulte, an energy expert from management consultants Accenture.

By way of example, he points to an exciting pilot project in Brandenburg, Germany. In April of this year, Enertrag, a company specializing in wind-power generators, laid the foundation stone for a new test facility in Prenzlau. This will be the world’s first hydrogen-wind-biogas hybrid power plant capable of producing hydrogen from surplus wind power. The hydrogen will be used to power hydrogen vehicles or mixed with biogas to produce electricity and heat in two block-type cogeneration plants with a total output of 700 kW. The facility is scheduled to enter service in mid-2010.
                                                                                                          Christian Buck