(Transcript of the lesson commentary.)

Energy of the wind

The phenomenon of wind 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. In other areas, the air is cooler and therefore denser and heavier. To compensate for the difference in pressure, the cooler air moves into the place where the warmer air was. 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. A cup anemometer consists of several bowl-shaped blades on a vertical shaft. The fan anemometer has a rotor with a horizontal axis that is spun by the wind. The faster the wind blows, the faster the anemometer rotates.

If we don’t have an anemometer available, we can estimate the wind speed by the Beaufort scale. It was introduced in 1806 by the British admiral Francis Beaufort for naval use. Because it estimated the strength of the wind by its effect on the surroundings, it helped sailors to determine how much sail they should lower in order to be able to sail safely in a given wind. Similarly, the scale describes the effects of wind on land. Its original twelve levels have been expanded to include five more for describing hurricanes.

Air molecules carried by the wind have kinetic energy that is greater the faster the wind moves. We can harness this energy by transferring it to a device that will do useful work or generate electricity. The energy of a moving air mass is directly proportional to the area through which the air is flowing times its speed squared. The power flowing through a unit area is directly proportional to the air density and the wind speed to the third power.

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. Variations in geographical conditions cause different areas of the world to have different average wind speeds. They also vary in the number of days that the wind blows at a usable speed. Therefore, only some are suitable for the construction of wind power plants. 

History of wind utilization

Sails are one of the first means by which man harnessed the power of the wind. Historical evidence of their first use dates back to 5,000 BC. Sail-powered ships enabled shipping, trade and the exploration of new territories.

A sail, most often made of cloth, captures the power of the wind and utilizes it to propel the ship. It uses both drag and lift principles. When the wind blows directly against the sail, the drag principle is used. If the leading edge of the sail is aligned with the wind, the sail can work as an airfoil and benefit from the lift principle.

On land, the wind pushes on the blades of windmills. The oldest known windmill is from Mesopotamia, dating back 3,700 years ago. In China, as well as in Persia, vertical axis windmills were used. This design has very low efficiency, yet it has been used quite extensively. Probably for ease of construction and maintenance.

Vertical axis windmill uses flat sails to catch the wind. These are either placed inside the structure so that the wind only ever hits half of the blades, or they have to be rotated by a special mechanism so that the blades going upwind do not slow down the system. In Europe, in approximately the twelfth century, horizontal axis design became more popular. The construction of such mills was more challenging because the entire top, carrying blades several metres long, had to be able to rotate with the wind. The power of a mill depended on the size and number of blades on the wheel and on the area of the blades covered by sails. Windmills were used to grind grain, to supply irrigation canals, to drive saws or trip hammers.

The first structure to generate electric power from wind was the 1887 original wind mill designed by the Scottish engineer James Blythe. It was used to charge accumulators for lighting his cottage. Several months later, the first automatically operated wind power plant was built in Ohio, America, by Charles Francis Brush. The wind wheel drove a 12 kW direct current generator charging the accumulators in the first electrified house in Cleveland. A direct forerunner of today’s horizontal axis wind turbines was the turbine built in 1931 in Yalta. With an output of 100 kW, the turbine was mounted on a 30-meter mast and supplied energy to the local grid.

Types of wind turbines

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.

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 pushing on one of them, which makes the whole system spin. It is used in roof vents or an anemometer. The drag principle turbines are not very suitable for electricity generation because the maximum efficiency is only around 15%.

Lift turbines have a blade shaped like an aircraft wing. The air flowing around the aerofoil flows faster along the upper side than the lower side. As a result, there is lower pressure on the upper side and the whole structure is pulled upwards by lift force. When the blade of a wind turbine is turned into the wind, it will experience lift, and because it is attached to the rotor, it was 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. They can have a vertical or horizontal axis.

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. 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 the 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.

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. It has several different designs: classical Darrieus turbine, Giromill and Gorlov helical turbine.

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. The many tonnes heavy nacelle has to also include a device that turns it together with the rotor upwind.

There are two types of horizontal axis lift turbines. The most common type is called upwind. The rotor is positioned in front of the support tower so that its operation is not affected by the turbulence behind the tower. However, it places high demands on the strength of the blades, which must not bend so much that they come into contact with the column, even in strong winds. A less common version has the rotor positioned downwind of the support tower. This eliminates the risk of unintentional contact between the blades and the column, but the turbine operation is in turn affected by turbulence behind the tower.

Modern wind turbines

The most commonly used wind plants today are horizontal axis lift turbines. They consist of a rotor and a nacelle on a tall column. The rotor most often consists of three blades. They are made of fibreglass or carbon fibre and exceed 100 metres in length in the most powerful plants. The leading edge of the blade can be rotated by the blade pitch mechanism against the wind to best convert its force into lift. If the wind is too strong, the blades are adjusted upwind so that the wind does not rotate them at all. This is gentler than using a brake. The rotation of the rotor is transmitted to a generator in the nacelle, which produces electricity. 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. Some power plants address the slow rotation of the rotor by a multipole generator.

A small weather station on the top of the nacelle informs the power plant about the current wind speed and direction. Accordingly, the turning mechanism rotates the entire rotor against the wind. It also contains a brake which can be used to fix the power plant in position. The propeller blades are rotated to the optimum angle of attack based on the weather station data.

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. The choice of a site for a wind farm is essential. In the chosen location the wind should blow at optimum speed for most of the year. 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. 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%.

Like all sources, wind power plants have some negatives. They can be disturbing because of their appearance, possible noise or strobe effect. A single turbine has the 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.