Development of Wind Energy from antiquity until today

Since time immemorial, mankind has been using wind energy; it is thus not a new idea. For centuries, windmills and watermills were the only source of motive power for a number of mechanical applications, some of which are in use even today. Humans have been using wind energy for some 4,000 years. Sails revolutionized seafaring, which no longer had to rely on muscle power. In 1700 B.C., King Hammurabi of Babylon used wind powered scoops to irrigate Mesopotamia.

Apart from pumps for irrigation or drainage, windmills were mostly used to ground grain. This is why the name "windmills" today, even when we are talking about machines that do not actually grind, such as sawmills and hammer mills. Historically Dutch windmill, © Bundesverband WindEnergie e.V. American windmill used for water pumping, © Bundesverband WindEnergie e.V. But the wind turbines that generate electricity today are new and innovative. Their success story began with a few technical innovations, such as the use of synthetics to make rotor blades. Developments in the field of aerodynamics, mechanical/electrical engineering, control technology, and electronics provide the technical basis for wind turbines commonly used today.

Development of Modern Wind turbines since 1900

The major success story - is wind turbines that generate electricity and feed it directly to the grid. They usually have two or three rotor blades, a horizontal axis, a nacelle with a rotor hub, gears, and a generator, all of which can be turned into and out of the wind. The rotor is positioned in front of the tower in the direction the wind is blowing (windward as opposed to leeward).In 1920 and 1926, Albert Betz calculated the maximum wind turbine performance, now called the "Betz limit", and the optimal geometry of rotor blades.

In 1950, Professor Ulrich Hütter applied modern aerodynamics and modern fibre optics technology to the construction of rotor blades on the wind turbines in his experimental system. Poul la Cour of Denmark developed a wind turbine that generated direct current. In 1958, one of his pupils named Johannes Juul developed the "Danish Concept," which allowed alternating current to be fed to the grid for the first time. This concept very quickly won over. Today, almost half of all wind turbines operate according to this principle.

In the 1980s, the Danes developed small turbines with a nominal output of 20 kW to 100 kW. Thanks to state subsidies, these turbines were set up on farms and on the coast to provide distributed power, with the excess power not consumed locally being fed to the power grid.

The physics of wind energy

Power is available from the kinetic energy of the mass of air moving in wind. The amount of energy that the wind carries increases by a factor of two as its speed increases and is proportional to the mass of air that passes through the plane of the area swept by the rotors. As power is the product of energy (work) within a given time frame, the power of wind increases by a factor of three as the speed of wind increases. Because of the low density of air (Pair=1.25 kg / m3), the power density of wind is much lower than that of water power (Pwater=1000 kg / m3), for instance. The power that can be harvested from wind is calculated in terms of the swept area -- for a horizontal axis wind turbine (HAWT), the area through which the rotor blades pass. As a result, if the diameter of the rotor blades is doubled, the power increases by a factor of four. If the wind speed then doubles, power increases by a factor of eight.

In 1920, Albert Betz demonstrated in his theory of the closed stream tube that a wind turbine can only convert a maximum of 16/27 or 59% of the energy in wind into electricity. This optimum performance cP is attained when wind turbines rotors slow the wind down by one third.

Current wind turbines convert up to 50% of energy in wind into electricity, thus coming very close to the theoretical limit.

Comparison of resistance and lift

Like some of these simple turbines with small output (up to 2 kW), historic windmills operate according to the principle of resistance. Here, a rotor with a vertical axis resists the wind, thus reducing wind speed. The maximum performance of such wind turbines is 12%. The performance of wind turbines based on the principle of lift is much greater at around 50% due to the relatively high lift-to-drag ratio.

The power coefficient (performance) of a wind turbine can be improved by optimizing the tip speed ratio (lambda), i.e. the ratio of wind velocity to the velocity of the tip of the rotor blade. If the tip speed ratio = 1, the rotor has many blades, generates great torque, and runs at slow speeds. If the tip speed ratio is higher, the rotor has few blades, generates less torque, and runs at higher velocity.

Current wind turbines convert up to 50% of energy in wind into electricity, thus coming very close to the theoretical limit.

The Aerodynamics of Wind Turbines

The power coefficient of a wind turbines rotor blade is calculated according to the laws of airfoil theory. As with the wing of an airplane, air passing over a rotor blade creates an aerodynamic profile with low pressure above the wing, pulling the wing up, and overpressure below, pushing it up.

The difference in pressures exerts a lift on the wing vertical to the direction in which the wind is blowing and creates resistance in the direction of the wind (incident flow). For a wind turbines rotor blade rotating around the rotor axis, the incident flow is the result of the geometric addition of wind velocity v and the circumferential speed u, which increases in linear fashion the longer the blade is. In other words, the lift exerted on the rotor blade is not only the result of wind velocity, but mostly out of the blade's own rotation. Speeds at the tip of the blade are thus very great. Current wind turbines have rotor tips travelling at velocities six times faster than the speed of the wind. The tip speed ratio is thus lambda = 6. The rotor tip can then be travelling at velocities of 60 m/s to 80 m/s.

The energy that the rotor harvests is equivalent to the lifting force in the swept area minus the resistance force in the swept area. The forces applied in the direction of the axis drive the rotor, which then not only harvests the energy of the wind, but also exerts a load on the tower and the foundation.

Types of Wind Turbines

The position of the axis (horizontal or vertical) is obvious. Horizontal axis wind turbines (HAWTs) can be further divided into those with rotors rotating in front of the tower (windward) and those rotating behind the tower (leeward) vis- à-vis the direction of the wind. The tip speed ratio and the number of blades determine the response of the drive, and hence how the wind turbine can be used.

In modern wind turbines that generate electricity, there are different types of nacelles that turn on top of the tower to face the wind. There are turbines with gearboxes and without and nacelles whose components (bearings, gears, and generator) are positioned separately or have multiple functions integrated in one component (bedding of rotor shaft in the gearbox).

The energy that the rotor harvests is equivalent to the lifting force in the swept area minus the resistance force in the swept area. The forces applied in the direction of the axis drive the rotor, which then not only harvests the energy of the wind, but also exerts a load on the tower and the foundation.