The Power of Wind

This page explains wind energy systems in more detail. More information is available at:

• American Wind Energy Association
• Energy Efficiency and Renewable Energy (DOE)
• National Renewable Energy Laboratory

Wind Resource

The wind has been harnessed for centuries in a variety of different ways. The wind that we hope will help alleviate some of the problems that result from the world's thirst for energy is actually the result of complex global interactions triggered by the sun's energy. As the sun's most direct rays are absorbed by the equatorial region there develops an unequal heating of the earth's surface. The warmer air of the equatorial region rises and moves toward the poles, leaving a low pressure area at the equator. The process of diffusion causes molecules to flow from areas of high pressure to areas of low pressure. Thereby, the solar energy that warms the earth unevenly gives rise to moving air, or wind. This general picture of wind determines the basic speeds and prevailing direction of global winds. There are other complexities in overall wind patterns that take into account the rotation of the earth and other factors, but that detail is addressed here.

The important point is that surface features and structures can have a dramatic effect on local wind patterns and can have a profound effect on wind turbine placement. The friction from the surface of the earth will slow down global wind as one gets closer to ground level. Depending on the landscape of the local area, the effects of this slow down could be large or small. Roughness gives rise to wind shear, or the gradient of wind speed as a function of distance above ground. These differential wind speeds have an important impact on the placement and the design of wind turbines. If there exists a steep wind sheer, a taller turbine assembly would take advantage of wind speeds above ground level.

Mountain valleys and canyons, daily coastal cooling effects, as well as seasonal weather changes can have great influence on the wind characteristics of particular locations. The meteorological tower that has been erected in Howard will help engineers decipher the local wind patterns and find the best location for wind turbine installation.

To further understand how the wind becomes electricity, the next section gives a basic technological description of wind turbines.

Wind Technologies

Basic Conversion

The wind turbine converts the kinetic energy of wind into the mechanical energy of a rotating turbine shaft. This transfer of energy is facilitated by the rotor blades. The rotor blades are connected to a hub, which is connected to a shaft. This shaft transports the torque (mechanical energy) into the nacelle, which contains all of the key equipment used for transforming the mechanical energy of the shaft into electrical power. The rotating shaft is connected to a generator that turns the mechanical torque into electrical power.

Generally, the turbine also consists of various control and safety devices. The electronic controller constantly monitors the wind characteristics. As the direction of the wind changes, the electronic controller will communicate with the yaw mechanism, which turns the nacelle and the rotors more directly into the wind. Also, in the case of turbine malfunction, the electronic controller will stop the turbine and notify the turbine operator.

Output from a wind turbine, or a wind energy plant, is always changing because the wind is never constant. Over the course of a year, wind power plants typically operate at an average of 30-40 percent of their rated capacity. Output depends on factors such as average wind speeds, the design of the turbine (i.e., blade shape), and operating characteristics such as cutout speed, reliability, and the efficiency of the drive train. A 100 megawatt (MW) wind power plant in New York State will produce 30-35 MW and could produce the amount of electricity that is used by approximately 44,000 households in one year.

Wind and blades

Certainly the most visible component to the wind turbine is the blades, or rotors. Properly designed, the shape of the rotor can maximize the efficiency with which the wind turbine captures wind energy.

The blades of the turbine act under the same principles as the wings of an airplane. The design of wings allows lift by forcing air above the wing to travel faster than air below. Thus, there is a low pressure area created above the wing and the lift is experienced when air moves from the higher pressure area below the wing to the lower pressure area above it. Lift is always perpendicular to the direction of the air.

An airplane will lose lift and experience stall if the air above the wing fails to run smoothly over the wing and becomes turbulent. This normally occurs as a result of improper wing design (the wing tapers off too rapidly) or improper angle of the wing into the wind.

Wind turbines are designed with a cut-in speed, or wind speed, at which it begins to produce power, and a cut-out speed, or the wind speed at which the turbine will be shut down to prevent the drive train from being damaged. Cut-in speeds are typically 7-9 miles per hour (mph). Maximum electric generation occurs at speeds of 30-35 mph. As wind speeds increase beyond 30-35 mph, the generator maintains its maximum electrical generation until wind speeds reach 55-65 mph and the turbine cuts-out. When the cut-out wind speed is reached, the turbine automatically stops production and the nacelle is rotated away from the wind to protect the drive train from mechanical damage. As high winds subside, the computer automatically rotates the nacelle back into the wind and the unit returns to operation.

Blade design and wind characteristics ultimately determine the energy output of the generator. Advances in manufacturing and design capabilities are constantly improving. Future research efforts will allow wind turbines to become more reliable, more efficient and have more capacities.

Glossary of Commonly Used Terms

• Anemometer: Measures the wind speed and transmits wind speed data to the controller.
• Blades: Most turbines have either two or three blades. Wind blowing over the blades causes the blades to "lift" and rotate.
• Brake: A disc brake, which can be applied mechanically, electrically, or hydraulically to stop the rotor in emergencies.
• Controller: The controller starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 65 mph. Turbines cannot operate at wind speeds above about 65 mph because their generators could overheat.
• Gear box: Gears connect the low-speed shaft to the high-speed shaft and increase the rotational speds from about 30 to 60 rotations per minute (rpm) to about 1200 to 1500 rpm, the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.
• Generator: Usually an off-the-shelf induction generator that produces 60-cycle AC electricity.
• High-speed shaft: Drives the generator.
• Low-speed shaft: The rotor turns the low-speed shaft at about 30 to 60 rotations per minute.
• Nacelle: The rotor attaches to the nacelle, which sits atop the tower and includes the gear box, low- and high-speed shafts, generator, controller, and brake. A cover protects the components inside the nacelle. Some nacelles are large enough for a technician to stand inside while working.
• Pitch: Blades are turned, or pitched, out of the wind to keep the rotor from turning in winds that are too high or too low to produce electricity.
• Rotor: The blades and the hub together are called the rotor.
• Tower: Towers are made from tubular steel (shown here) or steel lattice. Because wind speed increases with height, taller towers enable turbines to capture more energy and generate more electricity.
• Wind direction: This is an "upwind" turbine, so-called because it operates facing into the wind. Other turbines are designed to run "downwind", facing away from the wind.
• Wind vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.
• Yaw drive: Upwind turbines face into the wind; the yaw drive is used to keep the rotor facing into the wind as the wind direction changes. Downwind turbines don't require a yaw drive; the wind blows the rotor downwind.
• Yaw motor: Powers the yaw drive.