Wind energy

Author: Prof. Dr. J. Fricke (as of June 2018)

The use of wind energy for power generation in our country continues to progress. At the end of 2017, due to a 5 GW increase, on-shore wind energy plants (WTGs) with a capacity of about 50 GW were connected to the grid. They generated about 88 TWh of electricity. The number of full-load hours, which is important for the amount of electricity generated annually, was 1750 h for on-shore WTGs installed in Germany in 2017, compared to only 1400 h in 2016 . (This parameter is obtained by dividing the electrical energy generated in 2017 by the installed capacity; it is also an indicator of how much the wind supply fluctuates from year to year.)

The installed capacity of off-shore plants was about 5 GW, 18 TWh were fed into the grid. Since on the sea the wind blows stronger and more durable than on land, the full load hours there are 3,000 to 4,000 h. On the other hand, the installation costs on the sea are with about 3 - 5 €/Watt still clearly higher than on the land with 1 to 2 €/Watt. WTGs in Germany now contribute a good 13% to electricity generation. Worldwide, WTGs with a capacity of about 540 GW were installed in 2017 - 50 GW more than in 2016. The electrical energy generated amounted to about 1,000 TWh.

After 92 WTGs with a capacity of 260 MW were installed in 2017, there are now a total of 1,153 WTGs in Bavaria with a capacity of about 2.5 GW. They provide a share of around 4% of electricity generation in Bavaria. The expansion target by 2021 would have been an electricity share of 6 to 10%. This is opposed by the 10H regulation passed by the Bavarian state parliament in November 2014. This regulation is to be evaluated at the end of 2018.

Types of wind turbines

Today's installed WTGs for Bavaria with 2 to 3 MW output have towers with heights of up to 200 meters. Hybrid towers are often installed; these consist, for example, in the lower part of concrete rings braced with steel cables, and in the upper part of beam tubes. They stand on a solid concrete foundation. Approximately 400 tons of steel and 4,000 tons of concrete are required for such tower installations.

Steel lattice towers for WTGs in Germany are rare. Wooden towers are under development. Off-shore WTGs have steel towers and steel foundations, either tripods or monopiles. These are driven into the seabed, which is associated with extreme sound emission. To reduce the sound emission, a bubble curtain is placed around the pile driving site. Recently, there are also floating foundations that are anchored to the seabed with steel cables.

The rotors rotate clockwise and are positioned "on-wind," i.e., in front of the tower. This is accomplished by "yaw control." The wind current tube impinging on the rotor is decelerated by the extraction of flow energy, expands to maintain mass flow, and rotates counterclockwise due to conservation of angular momentum. The disturbed current tube is also highly turbulent and forms the wake.On the relatively smooth sea, the wake length is much greater than on land with a rough bottom. Therefore, off-shore WTs in particular must not be positioned behind each other with respect to the main wind direction, and larger distances must be chosen between WTs in a wind farm so that they do not interfere with each other. Tracking phenomena are modeled by computer at the TU Munich, for example, and simulated in the wind tunnel. The rotors consist quite predominantly of 3 aerodynamically shaped blades or vanes, only in off-shore WTs are 2-bladed rotors occasionally found. The former rotate more slowly and therefore generate less noise. Typically, they rotate at 12 revolutions per minute, which corresponds to blade tip speeds of approx. 70 m/s for a blade length of 60 m. If this speed is put in relation to the prevailing wind speed at the time, e.g. 10 m/s, the result is a typical "high-speed number" of 7.

Although more than a hundred thousand WTGs have already been installed worldwide, the blades are still manufactured by hand. They consist of a fiber composite material (GRP), at the particularly heavily loaded blade root also CFRP). The blades are reinforced on the inside by glued-on webs and straps. Due to the extremely frequent load changes over a period of 20 years, very careful laying and bonding of the fiber webs is required. During operation, the integrity of the blades is checked by visual inspection and by mechanical tapping. In the future, sensors and actuators will be integrated into "smart" wings, which can detect even the smallest changes in the composite material. This process is called "condition monitoring," which is also applied to the tower, bearings, gearbox and generator. Heatable wing leading edges provide "anti-icing," which helps prevent imbalance and reduces downtime.

Another objective is to transition from manual manufacturing to industrial production. At the "Blademaker" demonstration center in Bremerhaven, which Fraunhofer IWES operates together with 15 partners, the first step is to optimize the surface treatment of the blades. In the process, robots clean the edges of the blade shells, grind the surface and apply a color protective coating.

Calculation of wind power

The power PWind contained in the wind is proportional to the third power of the wind speed v, i.e. PWind ~ v³. Optimal for the utilization of wind energy is a deceleration by the rotor to v/3. This was already shown by Albert Betz in 1919 and he also calculated the maximum theoretical efficiency Nmax = P/PWind for the conversion of the wind power PWind into the mechanical power P of the rotor. According to Betz Nmax = 0.59 is valid, whereby the so-called transverse force on a wing profile flowing around is used. If only the drag force is used, as in the case of the Savonius rotor (or before-the-wind sailing), only a maximum efficiency of 15% can be achieved.

Typical switch-on wind speeds for the WTGs installed in our country with a rated power PNenn of 2 to 3 MW are 3 m/s. Here, however, the turbine is not yet operating efficiently; the efficiencies for converting wind power into rotor power are around 20%. At wind speeds between about 5 and 10 m/s, the WT power increases steeply and the WTs work optimally, with about 45% efficiency. 

At wind speeds of about 12 m/s, the nominal power is reached. This remains constant until the shutdown speed at about 25 m/s. Since the power in the wind increases about eightfold in this interval, above 12 m/s an increasingly smaller fraction is extracted from the wind power as the wind speed increases. Efficiency drops continuously to about 3% at 25 m/s. In the range of nominal power, wind turbines are therefore quite inefficient converters. But wind energy is, after all, free.

The blade orientation must be adapted to the prevailing wind speed. The required blade twist is achieved by "pitch control". At start-up, the chords of the blades are almost perpendicular to the rotor plane. (The chord is the straight connection between the leading and trailing edges of the blades). When rated power is reached, the chords are almost in the plane of rotation. Above 25 m/s, the blades are then placed in "vane" position, i.e., perpendicular to the rotor plane, to prevent damage to the turbine.

The blades are attached to the hub, which rotates with them. This is held in position by a bearing located in the nacelle at the head of the tower. From the hub, the rotational energy is transferred to the gearbox, which translates the slow rotation of the rotor into the fast rotation of the generator required for grid feed-in. The electrical energy generated is stepped up to 20 kV by means of a transformer at the base of the tower and fed into the distribution grid at 50 Hz. However, many WTGs are also realized without a gearbox and are equipped with a synchronous generator. The current with the "wrong" frequency is then converted by an inverter into current with 50 Hz.

The Bavarian Wind Atlas

In conclusion, it should be noted that the Bavarian Wind Atlas, published by the Bavarian Ministry of Economic Affairs, offers first aid, as it were, when data on wind conditions in Bavaria are needed. It graphically presents the average wind speeds at heights of 100, 130 and 160 m; it also presents the possible energy yields via full load hour values at these heights. For this reason, we would like to emphasize that it is not sufficient to know only the wind speed averaged over the year in order to evaluate a site. Rather, the temporal distribution of the speeds is important. A simple example may illuminate this: We choose a mean velocity v = 6 m/s. On the one hand, the whole year is constant in time v = 6 m/s; on the other hand, half of the year is v = 4 m/s and the other half of the year is v = 8 m/s. Because of the v³ dependence of the wind power, the annual energy yields behave as 6³:(4³/2 + 8³/2) = 1:1.33. Considering that a few percentage points can determine the economic success or failure of an investment in a WT, it becomes clear that very detailed wind data, resolved in time and height, must be available for the chosen WT location.