Has come of age, again.
Late in the 19th century water pumping wind turbines were common in the US plains states. Many tower skeletons remain, but few are in operation. The push for rural electrification left those intermittent and mechanically troublesome machines in its wake.
Awakening awareness of this fragile planet and the oil embargo of the early 1970's rekindled interest in wind power. Many jumped at the promise of "free" electricity from the wind. But that optimism ended with the repeal of tax incentives and the very visible failure of some machine designs. Apparently to the person looking up at a still wind machine getting electricity from the wind didn't work. However advances in technology are making wind power work in the 1990s. One key advance is the variable speed generator coupled with power electronic devices.
NREL Photo, Warren Gretz
A wind turbine converts the kinetic energy of the wind into electricity. The wind pushes aerodynamically designed blades. The blades are attached to a rotating shaft. The shaft carries the mechanical energy into the generator housing. There the shaft turns through a magnetic field inducing current in the wire coils wound inside the generator.
The power available from wind is proportional to the swept area of the blades and the velocity of the wind cubed. The kinetic energy of an object is one-half its mass times its velocity squared. Consider how fast the wind's mass is moving past the wind turbine. The wind mass flow rate equals the air density, times its velocity, times the area swept by the blades. The area swept by the blades is the area of a circle made by the blade tips. Put all that into the kinetic energy equation and the result is an energy per time equation or a power equation.
The wind turbine can theoretically convert 60% of the power from the wind into electricity. Today's wind turbine models actually convert about 30%.
Examine these equations to see what is important in a wind energy conversion system. Note that a increase in wind speed from 4 to 5 meters per second (9 to 12 mph) results in nearly two and a half times more power. Also a doubling of the blade length quadruples the power. Another effect is the density of air. According to the Real Goods Alternative Energy Source Book, at 15 degrees C (60F) a change in elevation from sea level to 2,300 meters (7,500 feet) reduces the wind turbine's performance 25 percent.
Beyond a maximum wind speed, mechanical stresses on the turbine are too great. The machine could be damaged. Secondly, too much wind generates too much electric power, overloading the generator. Above a wind speed of 20 to 30 meters per second (40 to 60 mph) the turbine begins protecting itself and puts out a lower power.
AC generators are designed to be tied into the 60 cycle AC power grid. In early designs, the turbine spun at a set speed to generate the 60 cycle AC electrical frequency. When the wind gusted the turbine would resist changing speed, putting mechanical stress on the machine. Newer models are designed as variable speed generators. The blades turn in harmony with the wind speed creating a varying frequency power output. The varying frequency is converted to the standard 60 cycle AC power using recently developed semiconductor power electronic devices.
Spera, D.A. (ed.). (1994). Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering. New York: ASME Press.
2. Electric Design of Wind Electric Water Pumping.
The University of Massachusetts at Amherst has an active wind power program. I was fortunate to be able to help them bring the blades up Mount Tom for their working laboratory wind turbine.
More questions? Check the American Wind Energy Association.
My brother lives in the wrong Oregon university town to attend U of Oregon's Alternative Energy Course but
Here's a beautiful picture of an English Windmill of the last century.
Timothy John Cochran
Energy Resources, Geology 411, University of Wisconsin-Madison
December 7, 1995
cochran@cae.wisc.edu
