Drop your lance, Don Quixote, for your enemy this time is sturdier, mightier, and way out of your reach.
Those aren't your legendary wheat-grinding windmills with wooden sails and brink walls straddling the globe. They're electricity-generating wind turbines that have steel towers taller than Big Ben, rotors spanning wider than Boeing-747 wings, and sometimes structures massive enough to land helicopters that transport maintenance crews.
In terms of capacity, some of these behemoths boast production of 3,000 kW. In Europe alone, where three-quarters of the wind energy worldwide is generated, wind power capacity has been growing by an annual average of 35% since the late-90s, according to the American Wind Energy Association (www.awea.org).
At the same time, wind energy costs are lower. Today, wind energy costs about 3 cents per kWh, compared to 10 cents per kWh in the 1980s, says Henrik Jorgensen, Senior Technical Manager at Denmark's Vestas Wind System A/S (www.vestas.dk), the leading wind-turbine manufacturer in the $6.5 billion global market. The wind industry as a whole is poised to grow and compete as an economical source of renewable power.
This year, Vestas is introducing its V90-3.0 MW model that features a 90m-diameter rotor, a significant expansion from the 15m-diameter rotors the company made 20 years ago.
In the U.S., GE Wind Energy (www.gepower.com/dhtml/wind/en_us), which accounts for more than 5,600 wind turbine installations worldwide, has also erected on the farmland southeast of Madrid, Spain, an operating prototype of its largest turbine ever—a 3.6-MW offshore high-wind machine with a 104m-diameter rotor and an optional helicopter hosting platform. The company is hoping to erect 130 3.6-MW turbines off Cape Cod, MA, for the proposed Cape Wind project (www.capewind.org), the first offshore wind farm in the country.
Critical to these massive creations are the rotors and blades, comments Ken Polnicky, engineering manager at Vestas American Wind Technology. A combination of blade optimization principles is often used in modern wind-turbine design, Jorgensen adds.
When the blades capture the wind, power is transferred to the rotor hub, which is attached to the shafts for the gearbox and electrical generator (see sidebar on page 35). The rotor of a wind turbine moves when wind passes over the aerofoil-shaped blades, creating an aerodynamic lift. A drag force perpendicular to the lift force, however, counters the rotation. So a prime objective in rotor design—no matter for what size—is to give the blades a relatively high lift-to-drag ratio. The ratio often varies along the length of the blades to optimize the turbine's energy output at various wind speeds.
Even the blade number matters. A rotor with an odd number of blades—and three is the predominant choice—is similar to a "solid" disk when considering the dynamic properties of the turbine. If the rotor has an even number of blades, it will have stability problems when the uppermost blade gets the maximum power from the wind and bends backward while the lowermost blade passes into the "wind shade" in front of the stiff tower, inducing a racking moment into the hub.
In addition, "the aerofoil shape is designed with a twist as you look along the blade to maintain the optimum angle of attack along the blade length," says Aaron Avagliano, who works as a mechanical engineer at GE Global Research.
And to achieve the optimal strength/weight ratio for blades that now easily weigh more than 6 metric tons, Avagliano adds, rotors are usually made of balsa wood, fiberglass, and epoxy resin, with the top and bottom half of the blades manufactured in a resin-infusion process.
In fact, Vestas considers blades and rotors such critical parts in wind turbine design that it has decided not to outsource their manufacturing. At the Vestas lab in Denmark, the design team uses a software called Vestas Turbine Simulator to optimize each rotor so that it will last 20 years, or 10 billion cycles, Jorgensen says. In addition, each Vestas blade is made in one single mold so that the company can eliminate joining errors, Jorgensen adds.
Also critical in windmill design is the controller, which monitors and controls the blade pitch—the turning of the blades around their longitudinal axis to optimize power output. On both the Vestas V90 and GE 3.6-MW models is a microprocessor-based controller for pitch control. Monitoring the wind turbine's power output at all times, these controllers can order the blades to turn slightly out of the wind when the output becomes too high, or turn back into the wind if the wind drops. On the latest Vestas and GE machines, blades can rotate in pitch at a few degrees independent of each other.
Pitch control also minimizes machine fatigue by slowing, sometimes even stalling, rotors in the case of strong wind, adds Christine Deal de Azua, spokeswoman of the AWEA. Such fatigue control is critical because most wind turbines receive maintenance only two times a year, comments Jim Lyons, GE's chief engineer of research.
Despite all these efforts, the wind industry still has room for improvement for sure. GE, for example, is looking into putting more carbon fiber into the blades to reduce the mass, Lyons says. That will lower costs, since simply shipping a 40m-diameter rotor can account for up to 10% of the turbine costs, Avagliano explains. Vestas has also asked its vendors to manufacture noiseless gearboxes by adopting helical gears to ease the noise concern about wind turbines, Jorgensen comments.
But the biggest growth hurdle of the wind industry—at least in the U.S.—lies outside the engineering arena, comments Deal de Azua. It's the costs to the consumers and utilities.
Today, wind produces less than 1% of electricity in the U.S., the AWEA says. But that number may reach 6% by 2020 if the federal government provides production incentives to the wind-energy industry as has been done in California. The state provides 50% cash rebate for small wind (10 kW or less) and other renewable energy systems. As of the end of 2002, California reported a wind power capacity of 1,822.3 MW, or 38.9% of the nationwide capacity.
The AWEA hopes that Congress will extend the wind energy production tax credit, enacted in 1992, that provides a 1.5-cent credit per kWh of electricity produced from wind technology. The existing legislation is expiring on Dec. 31, 2003.
"We can generate smoother and more growth if we can have more consistent support," says Kathy Belyeu of Strategic Communications at the AWEA.