Wind
How does the sun create wind?
Wind is a result of solar energy. Wind is created when the sun unevenly heats up the atmosphere. The molecules in the warmer air move faster than the cooler air, causing the warmer air to rise away from the earth's surface. As the warmer air rises, the cooler air replaces it. In the process, energy from the sun is converted into kinetic energy (the energy of motion).
A cheap way to investigate this phenomenon is to observe a 
radiometer. A radiometer is a small, light bulb-shaped educational tool that can be used to show how light energy is transformed into mechanical energy. The opposing sides of each vane of the radiometer are alternately dark and light in color. As light (infrared radiation) hits the vanes, the lighter side reflects the light while the dark side absorbs it. As the dark side absorbs the radiant energy, a difference in temperature develops between the vanes. The freely moving air molecules bounce off the dark side with a great deal of energy. As the air molecules move away from the dark side of the vane, they form convection currents (wind) and momentum causes the vanes to spin.
Radiometers can be found at many science and educational stores.
Image: Courtesy of NASCO
What are the differences between WINDMILLS and WIND TURBINES?
Many people wonder what the difference is between a windmill and wind turbine. The differences are great and are explained in more detail here.
Windmills are much shorter than wind turbines, and usually have many blades (8-24). The blades catch more wind causing the windmill to be able to do more physical work. The propeller blades are connected to an axle with gears. The gears are connected to a vertical shaft that runs down the length of the tower and is connected to other mechanical equipment. Windmills do work such as pump water or grind grain, which is why they are a common site on farms where they are used in crop production. They are not built to produce electricity.
Photo by S. Lane (Spain windmill)
Wind Turbines harness the kinetic energy of the wind and convert it into electrical energy. This is accomplished by turning blades called aerofoils, which drive a shaft, which turns a motor (turbine), which is connected to a generator. Wind turbines need to be much taller than windmills and usually have 2-3 blades. Wind turbines can be used to provide electricity to single-family homes, especially in rural areas, businesses, or even many homes and businesses if owned by a utility company. The electricity produced can be stored in batteries for use when wind speeds are too low (less than 6 mph) to produce electricity or when high winds (greater than 55 mph) could damage the turbine (in this case, the wind turbine can be turned off to prevent the generator from overheating).
Midwest Renewable Energy Association's Renew the Earth Institute, Custer, WI
Electric utilities use larger wind turbines. Often the utility will place many wind turbines together in what is called a wind farm. Five of the 10 largest wind farms in the world are located in the United States.
Photos by M. Gransee-Bowman
How it Works
Wind turbines capture the wind's energy with two or three propeller-like blades, which are mounted on a rotor, to generate electricity. Depending on location and optimal wind speeds, commercial wind turbine blades can range from 116-148 feet long. The turbines sit high atop towers, taking advantage of the stronger and less turbulent wind at 280-600 feet or more above the ground. An average wind turbine rated at 2.5-3 megawatts can produce more than 6 million kWh every year.
A blade acts much like an airplane wing:
- When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade.
- The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift.
- The force of the lift is actually much stronger than the wind's force against the front side of the blade, which is called drag.
- The combination of lift and drag causes the rotor to spin like a propeller, and the turning shaft spins a generator to make electricity.
Wind turbines can be:
- connected to a utility power grid or
- combined with a photovoltaic (solar cell) system (to create electricity).
There are two types of wind turbines: Horizontal axis and vertical axis (eggbeaters). Horizontal axis wind turbines are most commonly used today.
1998 by the American Wind Energy Association
Wind energy, used by civilizations for thousands of years to grind grain and pump water, was reborn during the energy crisis of the 1970's when improvements in materials and technology made wind turbines more common. Today, wind-generated electricity is used in 40 states to help provide for U.S. electrical needs. In 2019, wind energy accounted for nearly 7.3 percent of the United States electricity supply, and this number is growing. Worldwide, wind power is used on a commercial basis in more than half of all the countries in the world generating more than 654 GW of electricity.
Locations
All markets for wind turbines require an estimate of how much wind energy is available at potential development sites. Correct estimation of the energy available in the wind can make or break the economics of a wind farm development. To provide the best information possible, National Renewable Energy Laboratory (NREL) researchers have been assembling data sets and refining modeling techniques for more than three decades. Check out their Wind Energy Maps and Data.
Wind resources are plentiful in the United States. With average, reliable wind speeds of 15 miles per hour or more, Iowa, Nebraska, Minnesota, and North and South Dakota possess nearly half of the nation's wind potential. To supply the U.S. with one-third of its electrical needs would require covering one percent of the nation's land area with wind machines.
The best sites for wind potential in Wisconsin are found along Lake Michigan and Superior, where average wind speeds may reach 14 miles per hour. In 2019, installed wind capacity in Wisconsin was 746 MW with 435 turbines operating in utilities. Wind electricity production accounted for three percent of all in-state electricity production, enough to power 172,000 Wisconsin homes. Utility electricity generated by wind in Wisconsin has increased nearly 97 percent since 2001.
The Kewaunee County sites lie along the naturally windy Niagara Escarpment. Only 5 percent of Wisconsin's land area has enough wind to economically support electricity generation (MG&E).
For more information on Wind Basics, please visit the American Wind Energy Association.
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