How Wind Works
Energy from Moving Air: How Uneven Heating of Water and Land Causes Wind
Source: National Energy Education Development Project
Wind is simply air in motion. It is caused by the uneven heating of the Earth’s surface by the sun. Because the Earth’s surface is made of very different types of land and water, it absorbs the sun’s heat at different rates. One example of this uneven heating can be found in the daily wind cycle.
The Daily Wind Cycle
During the day, the air above the land heats up more quickly than the air over water. The warm air over the land expands and rises, and the heavier, cooler air rushes in to take its place, creating wind. At night, the winds are reversed because the air cools more rapidly over land than over water.
In the same way, the atmospheric winds that circle the earth are created because the land near the Earth’s equator is heated more by the sun than the land near the North and South Poles.
Wind energy technologies use the energy in wind for practical purposes, such as generating electricity, charging batteries, pumping water, and grinding grain. Mechanical or electrical power is created through the kinetic energy of the wind. Wind power available is proportional to the cube of its speed, which means that the power available to a wind generator increases by a factor of eight if the wind speed doubles.
The turbine’s blades are similar to the propeller blades on an airplane. The hub of the turbine is rotated as the rotor blades generate lift from the passing wind. This rotating action then turns a generator, which creates electricity.
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 is what causes the rotor to spin.
Since the wind’s speed typically increases with height above ground (due to decreasing friction with the ground), wind turbines are mounted on a tower to capture more energy. At 100 feet (30 meters) or more above ground, they can take advantage of faster and less turbulent wind.
Wind turbines are mounted on a tower to capture the most energy. At 100 feet (30 meters) or more above ground, they can take advantage of faster and less turbulent wind.
For the best utilization of wind turbines, they should be placed where wind speeds reach 16-20 mph and are at a height of 50m. It is also important that utility-scale power plants are located near existing power lines and in the windiest sites available.
Wind energy technologies can be used as stand-alone applications, connected to a utility power grid, or even combined with a photovoltaic system. For utility-scale sources of wind energy, turbines are usually built close together to form a wind farm that provides bulk power. Several electricity providers use wind farms to supply power to their customers, including Xcel Energy, MidAmerican Energy, and Basin Electric.
Stand-alone turbines are typically used for water pumping or communications. However, homeowners and farmers in windy areas can also use small wind systems to generate electricity.
Wind Turbine Design
There are different styles and many different sizes of wind turbines to accommodate different needs. The most common style, large or small, is the “horizontal-axis design” (with the axis of the blades horizontal to the ground). On this turbine, two or three blades spin upwind of the tower.
Less common are the vertical-axis turbines: the Savonius and the Darrieus. The Darrieus turbine was invented in France in the 1920s and is often described as looking like an eggbeater. It has vertical blades that rotate into and out of the wind. The Savonius turbine is S-shaped if viewed from above. This drag-type turbine turns relatively slowly but yields a high torque. It is useful for grinding grain, pumping water, and many other tasks, but its slow rotational speeds are not optimal for generating electricity.
Small wind turbines are used for providing power off the grid, and range from very small, 250-watt turbines designed for charging batteries on a sailboat to 50-kilowatt turbines that power dairy farms and remote villages. Like old farm windmills, they have tail vanes that keep them oriented into the wind.
Large wind turbines, used by utilities or independent generators to provide power to a grid, range from 100 kilowatts up to the enormous multi-megawatt machines that are being tested in Europe. Large turbines sit on towers that are up to 100 meters tall and have blades that range from 30 to 60 meters long. Utility-scale turbines are usually placed in groups or rows to take advantage of prime windy spots. Wind farms like these can consist of a few or hundreds of turbines, providing enough power for whole towns.
The European Wind Energy Association hosts an interactive How a Wind Turbine Works educational tool. On the fourth page, you can change the wind speed via a slider to see how speeds affect capacity, electricity produced, homes supplied, CO2 avoided, and number of electric cars fueled.
National Public Radio recently produced a segment that investigated the myth of “wind power saturation,” a hypothetical situation in which so many turbines are installed that they begin losing power because they impede each other’s wind resource.