Wind Turbines

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Wind Mills

Abstract

Windmill is a machine that converts wind into useful energy . This energy is derived from the force of wind acting on oblique blades or sails that radiate from a shaft . The turning shaft may be connected to machinery used to perform such work as milling grain , pumping water , or generating electricity . When the shaft is connected to a load , such as a pump , the device is typically called a windmill . When it is used to generate electricity , it is known as

a wind turbine generator

This finds out the first windmills and its uses and improvements It also tackles on the modern wind turbines

I . Introduction

Wind-driven mills are of ancient origin . Simple windmills may have been used in Persia (now Iran ) as early as the 7th century ad . They were used for irrigation and milling grain . The wheel bearing the wind sails of the earliest windmills was horizontal and supported by a vertical shaft These machines were relatively inefficient . Nevertheless , this type of windmill spread to China and throughout the Middle East

The earliest European windmills appeared in France and England in the 12th century and became widely used throughout Europe . These early wood structures , called post mills , were rotated by hand around a central post to bring the sails into the wind

The tower mill was developed in France during the 14th century . It consisted of a stone tower topped by a rotatable wooden cap that supported the windshaft and the upper portion of the mill gearing (see Wind Mills . Grolier Encyclopedia of Knowledge , pp . 678-681 . vol 18

Early windmills all share certain features . A horizontal shaft protrudes from the cap , or upper portion of the mill building . Four to eight wind sails , each about 3 to 9 m (10 to 30 ft ) in length , radiate from the shaft . The wood frames of the sails are either covered with canvas or fitted with wood shutters . The power of the turning shaft is transmitted through a system of gears and shafts down to the mill machinery at the base of the building

II . Discussion

A . Uses and Improvements of Windmills

Besides milling grain and irrigating farmland , windmills built in the period from the 15th century to the 19th century were adapted to a variety of tasks , including pumping seawater from land below sea level sawing wood , making , pressing oil from seeds , and grinding many different materials . By the 19th century the Dutch had built about 9 ,000 windmills

Of the major improvements on the windmill , the most important was the fantail , a mechanism invented in 1745 that automatically rotates the sails into the wind . In 1772 the spring sail was developed . This type of sail consists of wood shutters , the openings of which can be controlled either manually or automatically to maintain a constant sail speed in winds of varying speeds . Other improvements include air brakes to stop the sails from rotating and the use of propeller-like airfoils in place of sails , which increases the usefulness of mills in light winds (see Windmill : Uses and Improvements . New Standard Encyclopedia , pp 645-668 , vol . 19

The application of wind turbines for generating electricity was pioneered in Denmark late in the 1890s and is in widespread use there today . Water-pumping windmills were widely employed during the settlement of the arid areas of the western United States . Small wind turbine generators supplied electricity to many rural communities until the 1930s , when power lines were extended across America large wind turbines were also built during this time

B . Modern Wind Turbines

Modern wind turbines are propelled by one of two effects : drag , by which wind pushes the blades and lift , by which the blades are moved in the same way an aircraft wing rises on an air current . Turbines operated by lift turn more rapidly and are inherently more efficient . Wind turbines can be classified as horizontal-axis machines , with their main shafts parallel to the ground , or vertical-axis machines , with shafts perpendicular to the ground . Horizontal-axis turbines used for generating electricity have one to three blades those used for pumping may have many more (see Windmill : Uses and Improvements . New Standard Encyclopedia , pp . 645-668 , vol . 19 . The most common vertical-axis machines , named after their designers , are the Savonius used primarily for pumping , and the Darrieus , a higher-speed machine resembling an eggbeater

Water Pumps

The water pumper is a high-torque , low-speed windmill common in rural areas of the United States . Water pumpers are used mainly to draw water from underground . These machines use a rotor , usually from 2 to 5 m (6 to 16 ft ) in diameter , with a number of oblique blades radiating from a horizontal shaft . The rotor is mounted on a tower high enough to catch wind . A large , rudderlike vane directs the wheel into the wind . The wheel turns gears that operate a piston pump . When wind velocities become excessive , safety devices automatically turn the rotor out of the wind to prevent damage to the mechanism (see Windmill : Uses and Improvements . New Standard Encyclopedia , pp . 645-668 , vol . 19

Electricity Generators

Scientists have estimated that as much as 10 per cent of the world 's electricity could be provided by wind generators by the middle of the 21st century . Wind turbine generators consist of a variety of components . The rotor converts the power of the wind to the rotating power of the shaft a gearbox increases speed and a generator converts the shaft power into electrical power (see Wind Mills . Grolier Encyclopedia of Knowledge , pp . 678-681 . vol .18 . In some horizontal-axis machines , the pitch of the blades can be adjusted to regulate the speed during normal operation and to shut down the machine when wind speeds are excessive . Others use stall , an aerodynamic phenomenon that naturally limits the power at high wind speeds . Modern machines usually start operating when wind speeds reach about 19 km /h (12 mph , achieve their rated power at about 40 to 48 km /h (25 to 30 mph , and shut down in wind speeds of about 100 km /h (60 mph . The best sites for turbine generators have annual average wind speeds of at least 21 km /h (13 mph . See also Electricity Generation

Wind energy , which contributes very little pollution and few greenhouse gases to the environment , is a valuable alternative to nonrenewable fuel , such as oil . The most successful wind turbine generators for large-scale power generation have been of medium size (from 50 to 100 ft in diameter , with power ratings of 100 to 400 kw . These are sometimes installed in groups or arrays , known as wind farms . Some of the world 's largest wind farms are in California , where wind turbines can generate power up to about 1 ,120 MW (a typical nuclear plant has a rating of about 1 ,100 MW . The cost to produce wind power in such applications is competitive with many other forms of power generation (see Windmill The Columbia Encyclopedia , Sixth Edition , 2004

Denmark now obtains more than 2 per cent of all its electricity from wind turbines , which are also used to increase the power supply to communities on islands or in other remote locations . In Britain , one of the world 's windiest countries , wind turbine projects in mainly Wales and north-west England generate a small proportion of electricity from renewable energy sources . These provide around 6 per cent of primary energy production in European Union countries

B . Wind Power

Wind Power , energy present in a moving mass of air (wind , harnessed and converted into a useful form of power , usually electricity

As with all renewable energy sources (except tidal and geothermal , the energy in the wind comes from the Sun . One or two per cent of the energy absorbed by the Earth from the Sun is converted into wind via the mechanisms of convection and Coriolis forces . There are extensive sites around the world where the prevailing wind conditions could provide significant wind energy

There are currently about 10 gigawatts (GW , or billion watts ) of installed wind power extracting devices in the world , with Germany and Denmark leading the way in Europe , and the United States accounting for 1 .7 GW . By far the most common wind turbines are horizontal axis machines that look much like a traditional windmill , but there have been many novel designs of both horizontal and vertical axis machines . Wind turbine blades operate in a similar way to an aeroplane wing and use a lift rather than a drag mechanism , which means they are much more efficient than their windmill predecessors . Efficiencies of up to 40 per cent are quoted , but there is a theoretical maximum efficiency of 67 per cent , as proved by the German physicist Albert Betz in 1919 . In addition , a turbine may , on average , only generate 30 per cent of the time in a given location , due to seasonal and daily wind variations (see Windmill . The Columbia Encyclopedia , Sixth Edition , 2004

Wind turbines today are typically rated at between 750 kilowatts (kW to 1 megawatt (MW , with 2 MW machines now in production . The main components of a wind turbine are a rotor that drives a gearbox . The gearbox increases the rotational speed by approximately 50 times before driving an electrical generator . The highly variable nature of the wind means that , depending on the application , the power generated may be rectified and smoothed before being sold to the grid , or used in a nearby facility . The wind turbine also has sophisticated hydraulic and blade-feathering systems to ensure optimum and safe operation

Designing a device to withstand all wind conditions is a complex engineering problem . Wind turbines are by necessity large structures and are therefore subject to large forces . The power available in the wind is proportional to the cube of the wind speed . This means that as the wind speed doubles , the energy in the wind goes up 23 , or 8 times . In addition , the rotor area increases by the square of the radius , so doubling the diameter increases the available power by 4 (see Windmill . The Columbia Encyclopedia , Sixth Edition , 2004 Unfortunately , these cube and square laws mean that the output of a wind turbine is extremely sensitive to wind speed , and subject to very large forces during periods of unusually high winds

III . Conclusion

Governments are increasingly looking towards wind power and other renewable energy sources to combat increasing greenhouse gases and global warming . During its operating life , a typical wind turbine can produce 80 times the energy used to build , operate , and dismantle it Public opinion is broadly in favor of wind power , but the environmental impact of wind farms could be a barrier to future developments

References

Wind Mills . Grolier Encyclopedia of Knowledge , pp . 678-681 . vol . 18

Windmill : Uses and Improvements . New Standard Encyclopedia , pp 645-668 , vol . 19

Windmill . The Columbia Encyclopedia , Sixth Edition , 2004 . Columbia University Press

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