Wind power plants

(Transcript of the video commentary.)

Every day on earth, air masses move in what we call the wind. This phenomenon is powered by the energy of the sun. The sun's rays heat the earth's surface. The air above becomes warmer, lighter and rises upwards. Cooler, heavier air from other regions takes its place. A wind is formed.

It can take on a variety of speeds, from a gentle breeze to a violent gale.

The wind speed is measured with an anemometer. It consists of several bowl-shaped blades on a vertical shaft. The faster the wind blows, the faster the anemometer spins.

Even if we don't have an anemometer available, we can estimate the wind speed by the Beaufort scale. It was introduced in 1806 for naval use. Describes wind speed by its effect on the environment. So we know that if the twigs are only gently rustling, the wind speed will be about 10 km/h, while if the wind is so strong that we struggle to walk against it, it will be over 50 km/h.

This scale helped sailors to determine how much sail they should lower in order to be able to sail safely in a given wind. After all, sails are one of the first means by which man harnessed the power of the wind. Sail-powered ships enabled shipping, trade and the exploration of new territories.

On land, wind pushes on the blades of windmills. The oldest known windmill is from Mesopotamia, dating back 3,700 years. Both vertical and horizontal axis windmills were used. Wind energy ground grain, pumped water into irrigation canals, and powered saws and hammers.

Nowadays, wind energy is mainly used to power wind turbines. The wind transfers part of its energy to the turbine blade and spins the rotor, which turns a generator that produces electricity.

The more energy the wind could transfer to the blade, the more efficient the turbine would be. But a wind turbine can never reach 100% efficiency - the wind would have to stop behind the blade! And because some air must always pass through the blades to the other side, the highest theoretical efficiency of wind turbines is 59.26%.

The second limit in the use of wind energy is the wind speed. It might seem that the faster the wind, the faster it will spin the turbine and produce more electricity, but too high speeds can damage the turbine. That's why modern wind turbines perform best at wind speeds of around 15 m/s and must be shut down for safety reasons if the wind is gusting faster than 25 m/s.

Even in the best locations, the wind does not blow at suitable speeds all year round, so the average annual utilisation of wind power plants is only 20%.

There are many types of turbines that use wind energy. Basically, they can be divided into drag and lift principle turbines, and turbines with vertical and horizontal axis.

The oldest method of harnessing wind energy is the drag principle. A solid blade is placed in the path of the flowing air and the wind pushes against it. This is how historic windmills were turned.

Among the current devices, the Savonius turbine, which consists of two or more semi-circular parts, works on the drag principle. The wind is always leaning on one of them, which makes the whole system spin. It is used in roof vents or an anemometer.

They are not very suitable for electricity generation because the maximum efficiency of drag based wind turbines is only around 15%.

Lift turbines have a blade shaped like an aircraft wing. When properly oriented to the wind, the blade will experience lift, and because it is attached to the rotor, it will spin it. Lift turbines have a much higher efficiency, around 40 to 50%, which is why almost all modern wind turbines are based on the lift principle.

The turbine shaft can be positioned vertically or horizontally.

With a vertical axis, the generator and other equipment can be placed on the ground, which simplifies construction and maintenance. In addition, the power plant does not need to be rotated with the wind, it will spin in any orientation. A typical vertical axis turbine is the Darrieus turbine. It usually consists of two or three aerofoil blades that rotate around a vertical axis creating a cylindrical, spherical or parabolic surface.

Some types of these turbines may have a dead angle at which the wind will not turn them. They have to be started by an external motor. There is also a risk of pulsation, where the plant spins faster the moment its blade is facing the wind. Both can be prevented by twisting the blades so that the wind is pushing them in the same way.

Most contemporary wind turbines have a horizontal axis. This allows the wind to hit all the blades at once, making the device work more efficiently. However, all the necessary machinery has to be up near the rotor, which means lifting a multi-tonne nacelle up to 200 metres.

Inside the nacelle is an electric generator. There is also a nacelle turning mechanism. A device for blade pitch control. A small weather station that informs the power plant about the current wind speed and direction, and gears. The blades of the power plant spin quite slowly, around 20 revolutions per minute. At this speed, the generator could not reasonably generate electricity, so the speed of rotation is increased by gears.

The bigger the power plant, the slower the rotor spins. What looks like slow speed turns into respectable speed at the ends of the long blades. The tips of the blades can cut through the air at velocities of up to 200 km/h.

Blades made of fibreglass or carbon fibre already exceed 100 metres in length, which increases the requirements for their manufacture and transport.

The choice of a site for a wind farm is essential. It only makes sense to build it where the wind blows at optimum speed for most of the year. In the chosen location, long-term wind speed measurements are often taken before construction, ideally at the height where the rotor will eventually be located. Extremes in the weather must also be taken into account. Frequent hurricanes or prolonged freezing periods can also endanger the plant.

The wind farm should stand away from human settlements. However, at the same time, there must be the possibility to efficiently discharge the power output. As transport and installation of the components is difficult, the selected location must be sufficiently accessible.

Wind turbines are built "on-shore", often in higher, sparsely populated areas where there is a lot of wind throughout the year.

The second option is the currently massively expanded "off-shore" power plants, i.e. built in coastal areas. Here, there is no problem with human settlement, the wind blows most of the year, and many power stations can be built side by side. But they have to be properly anchored in the seabed and the electricity has to be led out through special underwater cables.

Like all sources, wind power has some negatives. They can be disturbing because of their appearance, possible noise or strobe effect. A single turbine has a capacity of only a few megawatts, so many need to be built. Their rotors have a negative effect on birds, insects and bats, which are not always able to avoid the fast-moving blades.

Wind is available for free and therefore it is a clean and renewable resource. Since the wind also blows at night, it is a great complement to photovoltaic power plants.