Text iiia. Wind Turbine

A wind turbine, windmill or wind generator is a device for convert­ing wind power to mechanical rotation with a low velocity turbine de­signed for compressible fluids (air). It is a device for producing renew­able energy in the form of electric power and is a component of one of the newest forms of power plant to be put into operation.

A wind turbine strongly resembles a propeller, but has subtle differ­ences. The turbine is perpendicular to the wind, mounted on a tower. With small wind generators the tower height is usually at least twenty meters. In the case of large generators, the tower height is about twice as great as the propeller radius. Power output from a wind generator is proportional to the cube of the wind speed. As wind speed doubles, the capacity of wind generators increases eightfold . There is usually a means of stalling the turbine's blades to reduce its wind resistance when the wind is extremely strong. For a given survivable wind speed, the mass of a turbine is approximately proportional to the cube of its blade-length. The maximum blade-length of a turbine is limited by the strength and stiffness of its material.

Labor and maintenance costs increase only with increasing turbine size, so given all these factors to minimize costs, wind farm turbines are basically limited by the strength of materials, and siting. One of the best construction materials is graphite-fiber in epoxy. Graphite composites enable turbines of sixty meters to be built, enough to trap a few mega­watts of power. Smaller turbines can be made of lightweight fiberglass, aluminum, or sometimes laminated wood. Smaller machines are pointed into the wind by a vane. Large machines have a wind-sensor driving a servomotor. When it turns to face the wind, the turbine acts like a gyroscope. When a turbine pivots to face the wind, precession tries to twist the turbine into a forward somersault. For each blade on

eightfold - в восемь раз.

a wind generator's turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cycling twisting can quickly fatigue and crack the blade roots, hub, and axle of the turbine. To reduce the precessive stresses, modern tur­bines have three blades, only one of which is in a maximum stress posi­tion (vertical) at a time. The major historical design defect is to have an even number of blades, so that two blades are vertical at the same time. Two-bladed turbines have the highest cyclic stresses.

Home-made wind turbines often have two blades. Two-bladed tur­bines thus often avoid the need for using a hub with linkages to indi­vidual blades. Three-bladed turbines, which are much more efficient, and more quiet, must usually be assembled onsite.

There are a number of vibrations that decrease in peak intensity as the number of blades increases. Some of the vibrations, besides operate at a higher Reynolds number of blades, so the optimum number of blades turns out to be three.

Since a tower produces turbulence behind it, the turbine is usually placed in front.

The turbine has to be placed a considerable distance in front and sometimes tilted up a small amount to ensure that the lower blade doesn't need an additional pointing device and in high winds the blades can be allowed to bend which reduces their wind resistance.

Sails were originally used on early windmills. Unfortunately, they have a short service life. Also, they have a relatively high drag for the force they capture. They turn the generator slowly, waste much of the available wind power and have a large wind resistance for their power output, requiring a strong wind tower. For these reasons they were suspended with solid airfoils.

When the turbine is spun by the wind, it adds a rotation to the wind, increasing the apparent wind on the blade. Since blades are really de­signed to work like an airplane wing, this increases the toque produced by the turbine. But this also increases the force in the wind direction on the blade and therefore on the tower. The mechanical stress is signifi­cantly higher when the turbine rotates. That's why wind turbines are stopped during high wind.

Wind has been used to grind grain, pump water, heat water and produce electricity. In modern times, almost all turbines either pump water or generate electricity.

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A wind generator usually consists of an aerodynamic mechanism for converting the movement of air into a mechanical motion, which is then converted with a generator into electrical power.

Wind generators are impractical in many areas as the available power grows as the cube of the average wind speed. A site with prevail­ing winds of 30 km/h is eight times as valuable as a site with only 15 km/h. As a general rule, wind generators are practical where the av­erage wind speed is greater than 20 km/h. Another lesser-known factor is the prevailing temperature - the lower the temperature, the greater the density of the air and thus, the greater the energy contained for the same given wind.

Wind is powered by a temperature differential. It is slowed by ob­structions and is generally stronger at high altitudes. Plains have high winds because they have few obstructions. Mountain passes have high winds mostly because they funnel high-altitude winds. Some passes have winds powered by a temperature differential between the sides of the ridges. Coastal areas have high winds because water has few obstructions and because of the temperature difference between the land and the sea. Off-shore also generally has high winds for the same reason.

In urban locations, where it is difficult to obtain large amounts of wind energy, smaller systems may still be used to run low power equipment. Distributed power from rooftop mounted wind turbines can also alleviate power distribution problems, as well as provide resilience to power failures. Important equipment such as wireless internet gate­ways may be powered by a wind turbine that charges a small battery.

2. Ask 10 questions to text HI A in written form.

3. Describe a wind turbine design in brief.

4. Say a few words about the wind turbine application.

5. Translate the sentences into Russian paying attention to the verb "to cause".

1. The wind causes the propeller to turn, which then turns the at­tached shaft to generate electricity. 2. It's not so much that we use fossil fuels for energy that is problematic, but it's the side effects of using them that causes all of the problems. 3. In the Arctic and An­tarctica, warmer temperatures are causing the ice to melt which will increase sea level and change the composition of the surrounding

sea water. 4. Wind energy will never be subject to embargoes or price shocks caused by international conflicts, and, unlike oil energy, clean wind power is renewable. 5. Fossil fuel mining and oil production can and has caused irreparable damage to our environ­ment.