Wind Energy

I

Introduction

Wind Energy, energy contained in the force of the winds blowing across the earth’s surface. When harnessed, wind energy can be converted into mechanical energy for performing work such as pumping water, grinding grain, and milling lumber. By connecting a spinning rotor (an assembly of blades attached to a hub) to an electric generator, modern wind turbines convert wind energy, which turns the rotor, into electrical energy.

Wind is created when air that has been warmed over sun-heated land rises, leaving a vacuum in the space it once occupied. Cooler surrounding air then rushes in to fill the vacuum. This movement of rushing air is what we know as wind.

Egyptians may have been the first to capture wind energy when they sailed boats up the Nile River beginning around the 4th century bc. For centuries afterward, wind-powered sailing vessels plied the world's seas and oceans, serving as the principal form of commercial transport. Wind energy has been harnessed on land since the first windmill was developed by the ancient Persians in the 7th century ad. Windmills have since been used to mill grain, pump water, saw timber, and provide other forms of mechanical energy.

Because wind is a clean and renewable source of energy, modern wind turbines had been installed in 26 countries by 2007, including such nations as Germany, Denmark, India, China, and the United States, to supplement more traditional sources of electric power, such as burning coal. Design improvements such as more efficient rotor blades combined with an increase in the numbers of wind turbines installed, have helped increase the world’s wind energy generating capacity by nearly 150 percent since 1990. In 2006 the United States became the world’s third largest producer of energy from wind power, generating more than 11.5 megawatts of electricity.

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II

Evolution of the Windmill

The oldest known windmills were crude, simple devices used in the 7th century by the Persians (a region now occupied by Iran). Europeans made extensive use of the windmill beginning in the 12th century, providing mechanical energy for pumping water, sawing lumber, and grinding grain. In the United States, the windmill was used to pump water on homesteads across the American frontier. In the late 20th century, windmills were developed to convert wind energy into electric power.

A

Ancient Persian Windmills

Early Persian windmills were crude devices consisting of a simple tower supporting an array of paddles made from bundled reeds. These paddles spun around a vertical axis, with a wall to protect the blades as they spun back into the direction of the wind. These early windmills were used for grinding grain.

B

Traditional European Windmills

Traditional European windmills have been used for centuries on the lowlands of northern Europe. In fact, the term windmill derives from using these machines to grind, or mill, grain. The first windmills to appear in Europe were built during the 12th century in northwestern France and southern England. Use of the windmill subsequently spread into northern Belgium, Germany, and north to Denmark during the late 12th and 13th centuries.

Only with wind energy could Jan Leegwater and the Dutch engineers that followed him drain the wetland areas of the Netherlands and make them habitable. European windmills were also used for sawing timber, shredding tobacco, manufacturing paper, pressing flaxseed for oil, and grinding stone for paint dye. The 700 windmills erected in the Zaan district north of Amsterdam formed the core of what evolved into the center of Dutch manufacturing—an area that eventually helped launch the Industrial Revolution.

Europeans, unlike their Persian counterparts, developed windmills with rotors that turned around a horizontal axis. Typical European windmills used four blades, although some used five, and occasionally even six. The earliest European windmills placed the tower holding the windmill rotor on a vertical post. This allowed the entire windmill to turn and face the wind. Many of these short postmills, as they are called, are still standing in northern Europe.

Toward the end of the 14th century, these postmills evolved into the traditional European tower windmills—some with towers up to three stories high. The rotors of these windmills are attached to a rotating tower cap, allowing the windmill operator to point the rotor blades into the wind by turning the cap. Many of these European tower windmills contain two or three interior levels where goods milled or manufactured inside the windmill—including grain, lumber, paint, and tobacco—could also be stored.

European windmill performance increased greatly over the next 500 years. The typical windmill evolved into a tower built of wood, stone, or brick that supported a rotor with four cloth-covered blades that acted like sails. This rotor, spanning a diameter of 25 meters (80 feet), was capable of delivering 25 to 30 kilowatts of mechanical power. Technical innovations to the European windmill included the multi-blade fantail protruding behind the rotor to automatically keep the rotor pointing into the wind; air brakes; automatically adjusting slats on the blades (instead of cloth); and blades with airfoil-shaped leading edges that anticipated modern aircraft wings. During the zenith of the European windmill (which ended in the late 19th century when the steam engine came into widespread use), some 1,500 megawatts of power were being produced, a level not reached again until 1988.