2.5 Types of wind power installations
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Historically, the first stationary installation that used the energy of wind was the windmill, for which wind orientation was performed manually. The windmill’s main operating component was a multi-blade wheel with a horizontal rotation axis, which was oriented along the current wind direction. Such wind engines were widely used in the Middle Ages and henceforth for grain milling, water pumping and water delivery, as well as to supply energy for other production uses. Large industrial-type windmills could develop capacities of up to 60 kW at high wind speeds. In the 19th century, the number of windmills on Russian territory exceeded 200,000, and their total capacity was around 1.3 million kW. By 1930, the USSR already had over 800,000 wind power converters. Today, the windmill has evolved into a whole range of different types of wind energy converters. Wind-driven energy converters with impeller-fitted wind wheels and horizontal rotation axes (Fig.2.8) have received wide recognition. Most developed among these are two-blade and three-blade wind wheels. The torque of the wind wheel is produced by a lifting force that is created when air flow streams around the profile of the blade. As a result, the kinetic energy of the air flow within the area swept by the blades is converted into the mechanical energy of the wind wheel rotation.
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Fig. 2.8. Impeller wind wheels
1- multi-blade, 2 - three-blade,
3 - two-blade, 4 - single-blade with counterbalance.
Power capacity developed on the wind wheel axis is proportional to the square of its diameter value and the wind speed to the power of three. According to the classical theory by Nikolai Zhukovsky, the ideal wind wheel will be defined by the wind energy conversion coefficient ξ equaling 0.593. In other words, an ideal wind wheel – where the number of wheel blades is indefinite – will retrieve an output of 59.3% out of total energy passing through its cross section. In real life, however, best high-speed wheels can achieve a maximum wind energy conversion coefficient of between 0.45 and 0.48; while for low-speed wheels this coefficient only reaches between 0.36 and 0.38.
Specific wheel speed Z is an important parameter for the wind wheel. It is the ratio of the blade tip rotation speed to the speed of wind flow and is also called “tip speed ratio”: The tip of the blade usually moves along the plane of the wind wheel at a speed several times exceeding that of the wind. The optimal values for the tip speed ratios of various rotors are: 5 to 7 for a two-blade, 4 to 5 for a three-blade, and 2.5 to 3.5 for a six-blade.
Among design features that can impact a wind wheel’s capacity, the wheel’s diameter and the shape and profile of the blades have most significance. The number of blades is not too essential for the wheel’s power output. The wind wheel’s rotation frequency is proportional to its tip speed ratio and the speed of wind, and is inversely proportional to its diameter. The height of the wheel axis location also plays a role in how big a capacity value the wheel can achieve, since wind speeds correlate with wind elevations.
Wind energy converter capacity, as was noted above, is proportional to the wind speed to the power of three. With wind speeds equaling the design-based speed, or exceeding it, the WEC operates at a nominal capacity. When wind speeds fall below the design-based speed, the WEC’s capacity is limited to only 20% to 30% of the nominal value, if that. Such operation conditions lead to big energy losses in the generators due to their low efficiency rates at low loads. Furthermore, in the asynchronous generators, such conditions result in big reactive currents, which have to be compensated for. To eliminate this drawback, certain wind energy converters are outfitted with two generators: one with a nominal capacity of 100% and another operating at 20% to 30% of the nominal capacity of the WEC. Under breeze conditions, the smaller generator is switched on, while the 100%-capacity generator is switched off. In some wind energy converters, the smaller generator allows for the installation’s operation at low wind speeds and at reduced rotation speeds, while the wind energy conversion coefficient remains high.
The wind-facing orientation of the wind wheel – or when the wheel’s rotation plane is perpendicular to the wind direction – is done in installations with extreme low capacities with the help of a tail, or tail fins; in medium-capacity converters, the orientation is changed by a wind rose mechanism; and in large up-to-date installations, special orientation systems do the task as they receive the driving impulse from a wind direction sensor, or wind vane, fixed on top of the wind turbine’s gondola. The wind rose mechanism is one or two medium-sized wind wheels, which rotate in a plane that is perpendicular to the plane of rotation of the main wind wheel and drive the worm gear, which turns the platform of the windmill headpiece, or gondola, until the wind roses are set in the plane parallel to the wind direction.
Horizontal-axis rotors can be installed either in front or behind the tower. If the latter is the case, the blade is subject to consistent and repeated impact of variable forces when moving through the shadow of the tower, which at the same time increases considerably the levels of noise. To regulate capacity and limit the wind wheel rotation frequency, a number of methods are applied, including spinning the blade or its part around its roll axis, as well as blade flaps, blade valves and other means.
The main advantage of horizontal-axis wind rotors is the consistent condition of a streamline motion of air flow around the blades, which does not change with the wind wheel’s rotation, but depends only on the speed of the wind. Because of this, as well as the considerably high coefficient of wind energy conversion, horizontal-axis types of wind energy converters have now become most popular.
The Savonius rotor (Fig. 2.9.) is another type of wind wheel. Torque is obtained through airflow created by the difference between resistances of the convex and concave parts of the rotor. The rotor’s advantage is its simplicity, but it has a very low coefficient of wind energy conversion: only 0.1 to 0.15.
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Fig. 2.9. The Savonius rotor
а) - two-bladed, b) - four-bladed
In recent years, studies in certain countries, specifically Canada, have been focused on the development of a wind engine employing the so-called Darrieus rotor, which was proposed in France in 1920. This is a vertical-axis rotor, which consists of two to four curved blades (Fig. 2.10). These airfoils form a spatial framework, which rotates from the impulse of lifting forces created at the blade by the flow of wind. The Darrieus rotor is capable of reaching a wind energy conversion efficiency of 0.30 to 0.35. Lately, designers have been working on developing a straight-blade (Gyromill design) Darrieus rotor (Fig. 2.10. b, c). The main advantage of Darrieus wind turbines is that they do not need a wind-orienting mechanism. In such wind energy converters, the generator and other mechanisms are located at an insignificant height, close to the installation’s base. This simplifies the construction greatly. However, a serious inherent disadvantage of such engines is that airflow conditions can vary considerably within one rotation period, and this variation is cyclically repeated under operation. This can cause a fatigue effect and result in destruction of the rotor’s components and serious accidents, which have to be taken into consideration when the rotor is mounted (especially when large WEC capacities are concerned). Furthermore, the rotor needs to have a spin-up to enter into operation mode.
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Fig. 2.10. Wind energy converters with the Darrieus vertical-axis turbine: a - Egg-beater (troposkein) design, b - Helical blades, c - Gyromill (H-bar) design.
1 - tower, 2 - rotor, 3 - braces, 4 - frame, 5 - torque
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Fig. 2.11. Rotor-type-specific dependence of wind energy conversion coefficient ξ
on tip speed ratio Ζ
1 - ideal blade wind rotor;
2,3 and 4 - two-, three- and multi-blade rotors;
5 - the Darrieus rotor; 6 – the Savonius rotor;
7 - the four-blade rotor of the Danish mill.
Correlations between wind energy conversion efficiency x and tip speed ratio Ζ for different rotor types are demonstrated in Fig. 2.11. It is evident from the diagram that two- and three-blade rotors with a horizontal rotation axis have the highest ξ value. For these rotors, ξ remains high within a wide range of tip speed ratio Ζ. The latter is especially important since wind energy converters have to operate under widely variable wind speeds. This is why installations of this type have received global recognition in recent years.
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