To a novice, high winds seem like a bonus. But to seasoned wind-energy users and turbine designers, once the wind turbine is already going flat out, stronger winds become a cause for concern. Doubling the wind speed makes eight times as much power available, and also increases the thrust force on the turbine and tower by a factor of four.
A turbine should be able to shed excess force, or it will overspeed, burn out, or self-destruct in some way. Protection from strong winds, called governing, can be accomplished either by furling or by blade pitch-control. The most common governing system is a furling tail that steers the rotor out of the wind. Some machines twist the angle (pitch) of their blades toward stalling, which decreases their efficiency and prevents overload. Both of these systems are passive, in the sense that they are driven by the force of the wind, or by the speed of the blade rotor. Very recently, electrical braking also has been introduced to automatically control a turbine’s speed.
In an effective wind generator design, all of these systems and components are carefully matched to each other. For example, the alternator must produce power at the best rotational speed (rpm) to get the most power from the blade rotor. Too slow and the rotor will stall; too fast and the machine will be noisy and ineffectual in low-speed winds.
Of course, a complete wind-electric system is much more than just the turbine. Other major components are the tower, rectifier, charge controller and dump load, battery bank (if needed), inverter, and metering. Several of these components can be more expensive than the turbine, and all of them need to be considered as you design a complete system.
The bottom line is to buy a turbine that is robust, simple, low speed, and reliable. Talk with experienced wind-energy users to find out which turbines stand the test of time. Heavier wind turbines with larger swept areas are usually more expensive, but more durable.
Focus on the energy the turbine makes, not its peak power. The energy will depend on the swept area and the average wind speed at the tower top. A small rotor on a short tower will not generate much energy. Remember that low and medium wind speeds are the most frequent, and therefore the most important. Do it right and you can share our glee in wind energy.
Ian Woofenden, PO Box 1001, Anacortes, WA 98221 • firstname.lastname@example.org
Major U.S. Turbine Manufacturers/Importers:
Abundant Renewable Energy • 503-538-8298 • www.abundantre.com
Bergey WindPower Co. • 405-364-4212 • www.bergey.com
DC Power Systems • 800-967-6917 • www.dcpower-systems.com
Pine Ridge Products LLC • 406-738-4283 • www.pineridgeproducts.com
Southwest Windpower • 866-807-9463 • www.windenergy.com
Wind Turbine Industries Corp. • 952-447-6064 • www.windturbine.net
“Apples & Oranges: Choosing a Home-Sized Wind Generator,” by Mick Sagrillo in HP90
“Wind Generator Tower Basics,” by Ian Woofenden in HP105
“Wind-Electric Systems, Simplified,” by Ian Woofenden in HP110
Wind Power: Renewable Energy for Home, Farm, and Business, by Paul Gipe, 2004, Paperback, 496 pages, ISBN 1-931498-14-1, $50 from Chelsea Green Publishing Co. • 800-639-4099 or 802-295-6300 • www.chelseagreen.com