Wind Turbines

Wind mills are devices that convert the kinetic energy of wind into the mechanical energy of rotating blades. The more kinetic energy, the greater the potential for producing work and power. The total energy transferred depends on the wind speed, the size of the rotor, and the density of the air. In practical terms, there are limits to the wind velocities that wind turbines can utilize. If the wind velocity is too low (cut-in speed), the kinetic energy is not sufficient to overcome friction at the bearings. At velocities above about 50 mph (cut-out speed), wind power is strong enough to knock the blades off, rendering the turbine inoperable and creating a real danger to nearby buildings, traffic, and people.

Question: What is the difference in operation of an electric fan and a wind turbine?

Answer: A fan uses electricity to produce wind, whereas a wind turbine uses wind to make electricity.

In ancient windmills and other early vertical axis windmills, the wind pushed and rotated the sail with a force called a drag. These windmills were inherently less efficient than later horizontal axis wind turbines that have been put into operation in the past four hundred years.

Modern wind turbine operation is similar to that of the wings of an aircraft. Because of the shape and angle of the airfoil (blade) cross-section, the air changes direction as it approaches the airfoil. The change in airstream direction results in a change in its momentum. As we have learned from Newton’s second law, a change in momentum requires a force. The force of the airfoil on the air is counteracted with an equal and opposite force on the airfoil (remember action and reaction), which is called lift. In airplanes, lift causes the plane to become airborne, whereas in wind turbines, lift forces the blades’ rotation about the hub, which in turn drives a generator. Perpendicular to the lift force, a drag force impedes rotor rotation.

Unlike helicopter blades and aircraft wings, which are designed for the greatest lift, turbine blades are designed to reduce drag. High lift is essential in aircrafts to prevent stall. Stall refers to a condition at which the angle of attack is so steep that no lift is produced. Even a small dent in the blade or airfoil can trigger a stall. In wind turbine applications, some stalls are welcome in high wind conditions to slow the turbine down and prevent damage. In new designs, the blade angles can be adjusted to change the lift-to-drag ratio and to optimize the turbine’s energy output for different wind speeds and directions.

Figure 1: Principle of operation of a wind turbine

Question: Most rotor blades have twisted surfaces. Why?

Answer: To optimize the angle of attack and prevent stall as wind flows from the tip (with a very high rotational speed) toward the hub (with a low rotational speed).

The major components of a wind power generator are the hub (on which rotor blades are attached), a gearbox, an electrical generator, and a controller with associated cooling units. Except for the hub, the entire assembly is housed inside an enclosure called a nacelle that is mounted on top of a tower (Figure 1). In addition, modern wind generators have anemometers for measuring wind speed, vanes for determining direction, and a yaw mechanism that can tilt the rotor in the direction of the wind. The vane assures that the turbine continuously faces into the wind. The tower carries the weight and raises the turbine above trees, buildings and other nearby obstructions. Wind turbines are more efficient when installed on tall towers because they face faster winds. In addition, most turbines are equipped with automatic governors that protect rotors from spinning out of control in gusts and during high wind speeds.

Basic operation of wind power generation is as follows. Wind drives the blades mounted on a shaft rotating at about 10 to 50 revolutions per minute. The shaft is connected to a transmission or gearbox and changes the speed to the 3,000 rpm needed to generate utility grade electricity. An electrical generator converts the rotational speed of the shaft to electricity, at which point it is transmitted through an underground cable to a field transformer and, eventually, to a utility substation. Here, electricity is added to the grid and combines with electricity generated by a variety of other sources such as coal, nuclear, and hydroelectric power plants.

Not all kinetic energy can be converted to useful work. As incoming wind collides with the blades, some reflects off of the rotor surface, and some is lost as a result of friction at the generator, limiting the wind turbine’s efficiency to about 40% at best.( a )

Wind turbines are available in a variety of sizes. Machines vary in size from 0.6 m in diameter (rated at about 500 W) to 60 m in diameter (rated at about 3 MW). Rated power is the power that a wind turbine produces when running at its optimum, i.e. at the highest wind speed.

References

(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

(a) It can be shown that the theoretical maximum efficiency of wind turbines is 59%, known as the Betz limit.

Gipe P., Wind Energy Basics, —A comprehensive guide to modern small wind technology. AWEA (http://www.awea.org).

Elliott, D. et al., Wind Energy Resource Atlas of the United States, by American Wind Energy Association (http://rredc.nrel.gov/wind/pubs/atlas).

Khennas, S., Small wind systems for rural energy services, London: ITDG Pub., 2003.

Solar Energy, Direct Science Elsevier Publishing Company, the official journal of the International Solar Energy Society ®, is devoted to the science and technology of solar energy applications, and includes the indirect uses such as wind energy and biomass.

Home Power Magazine—bimonthly magazine for farm and home wind turbines (http://www.homepower.com).