Machine Design

Wind Power Is Getting Too Good To Resist

Better airfoils, constant-frequency power production, and zero pollution let wind energy rival traditional sources.

Modern wind turbines won't be a main power source for a developed country any time soon, but with the capability of generating electricity at about 6 cents/kWh without pollution, it's getting too good to ignore.

The map shows wind speeds for sites in the U.S. Areas with winds of 6 m/sec or greater have commercial possibilities.

Mechanics work in a 1.5-MW Enron nacelle. This is one unit of six being erected offshore in Sweden.

Electrical power generated by wind turbines accounts for a tiny fraction of the U.S.'s total usage, but the potential is enormous. In fact, the Pacific Northwest Laboratory says the wind blowing over some parts of the western and midwestern states could generate more power than the known oil reserves of Saudi Arabia. The country's estimated reserves at 261 billion barrels is enough for about 90 years at the current production of 8 million barrels/day. If burned to produce electricity, that oil would generate about 153 trillion kW-hr. The Lab estimates U.S. wind resources in the West and Midwest as capable of producing 10.8 trillion kW-hr annually. Thus, in 15 years, U.S. winds could generate more electricity than all of Saudi Arabia's oil, and without being depleted. So why isn't wind power more widely used?

One concern has been the intermittent nature of wind power. Although it cannot supply on demand, it's predictable and can contribute to an "energy mix." That means when it is producing energy, other sources can be scaled back. Cost has been another inhibitor. But today, say experts, wind power is cost competitive in many areas with natural gas at about 3 to 6 cents/kW-hr to produce.

Costs have declined thanks to the economies of scale associated with the larger wind turbines now being manufactured and to technological advancements. These include variable-speed and constant-frequency technology, dynamic voltage controls and advanced wind-turbine blades.

The trends
Manufacturers in the windpower industry cater to two types of clients, small and large. Small machines are usually rated below 100-kW capacity and are intended for use by villages, remote operations, and commercial establishments such as small factories. Larger units, 750 kW and up, are for electrical utilities and independent power produces who will install them at windy locations.

As the industry has grown, so have the turbines. "Greater generating capacity from a single turbine is certainly one trend," says James Johnson, an engineer with the National Renewable Energy Laboratory, Boulder, Colo.

A few years ago most wind turbines were of the 750-kW variety, but today, more megawatt-sized machines are being ordered. At least one 3.2-MW turbine is slated for installation in Europe. And Enron Wind, a wind-turbine manufacturer in Tehachapi, Calif., has a 3.6-MW unit in development. Engineering changes have been just about everywhere. Research at NREL has tuned the air foils in the blades so they now capture more energy than those from just a few years ago. "The designs also reduce operational maintenance by compensating for leading-edge roughness," says Johnson. "A lot of insects in warm climates hit the leading edge of the airfoil, decreasing its efficiency. But newer designs compensate for that roughness and don't have to be cleaned as often."

Despite their size (a 1.5-MW machine has a 70-m rotor diameter, larger than Boeing 747's wingspan) blades are made of fiberglass using advanced manufacturing techniques to reduce costs. The methods are a mix of aerospace and boat-building technology to keep tolerances accurate and costs low. Different manufacturers use different techniques, including hand lay-up, resin infusion, and the use of prepreg materials. Designers are aiming at 20-year blade life.

Another development has been to generate constant frequency from a variable input. Because the wind is not constant, their generators produce a variable voltage and frequency. "We call that wild power," says Steve Wilke, a spokesman for Bergey Windpower Inc., Norman, Okla. The company manufactures turbines with capacities up to 50 kw.

To get an early turbine up to speed, operators would draw power from the grid and run the generator as a motor. When the speed was synchronous with the grid, the system could be locked on and produced power. A controller would then monitor the situation and decide when to take the generator off-line. "Power electronics now change the wild ac power to clean dc voltage and then reinvert it to exactly 60 Hz," says NREL's Johnson. "This is the wave of the future."

Where to build
Although land-based wind turbines are most prominent, off-shore installations have several advantages. For example, sea-level air is denser than air at higher elevations and therefore produces more power for a given wind speed. The smooth water surface lets the wind flow with less turbulence than air around land-mounted units. The steady nature of sea breezes makes the power more dependable. And there are usually lots of people living near seashores, so transmission distances are minimal. Construction, however, is more difficult at sea.

The challenges
Over 90% of the world's electrical power will continue to come from coal, nuclear, and gas-fired plants for the foreseeable future. So the most promising roles for wind power will be to compliment that production with a process called peak shaving. For example, in some parts of Southern California, wind speeds increase in the afternoon, about when air-conditioning loads are highest. "Wind power can help knock those loads down," says Johnson. "The trick is finding locations with loads and wind. Unfortunately, it seems winds are highest in the Great Planes or mountains where there are not many people."

Another challenge for the U.S. wind industry, says Johnson, is getting more units into production. "Several U.S. companies are competing with good designs but they are doing so against Danish and German machines."

Germany and Denmark take wind power quite seriously. Denmark, for instance, gets 12% of its electrical power from the wind and has set a goal of generating 50% of its electrical power from the wind by about 2030. The U.S.-installed wind-generated capacity is about 2,500 MW as compared with more than 17,500 MW globally. The World Energy Council estimates that the new capacity worldwide will total 180,000 to 474,000 MW by about 2020.

For home and shops
Small wind turbines are models of simplicity and efficiency. The XL.50, a 50-kW turbine from Bergey Windpower Co., for example, has only three moving parts: the rotor, a yaw assembly that points the unit into the wind, and the tail pivot. There are other differences.

"The XL.50 turbine uses an air foil developed in house," says Bergey's Wilke. It's nontapered and nontwisted with a uniform airfoil from root to tip, and the product of extensive computer modeling. The company pultrudes the blades, pulling the fiberglass through a die to shape it and orient the fibers, giving them greater strength than manually laid fiber-glass blades. "The shape gives it similar performance characteristics as the tapered and twisted blade but with a lower manufacturing cost and greater strength," he adds.

The blades turn an alternator with a 48-in. diameter and neodymium permanent magnets that generate at 93% efficiency. This eliminates the need for a transmission or gearbox. The generator can produce 480 or 380 Vac. "The primary use will initially be for running agricultural operations. It might power a large irrigation pivot or a manufacturing plant with a line load that exceeds local capacity. The one we have powers our factory," says Wilke. "And it runs our electric meter backwards at times to reduce our electric bill."

Bergey units also use a controller that takes the wild ac (variable voltage and frequency) and turns it into utility-grade 60 Hz. Wilke says their converter, developed with assistance from ABB, works with 97% efficiency. Most converters work from 80 to 95% efficiency, he adds.

The challenges faced by owners of small turbines are just as daunting as that for utilities. "The permit processes are arduous," he says. "We could have three times the number of turbines in California if there weren't so many legal roadblocks. Individual counties consider wind turbines visual blight and will not allow their construction. They have an arbitrary 35-ft height restrictions, (towers start at 85 ft) and they aren't willing to negotiate.

Wind on the Web
The Web shows that the wind-power industry is active worldwide. Here are few detailed sites from national labs and commercial firms.

www.windpower.orgincludes several calculators that find, for example, the reclaimable wind energy at your site and the payback for a wind-powered system. from Enron Wind, a manufacturer of large turbines, 900 kW and up. The photo-library link shows wind farms in Pennsylvania, Iowa, Texas, California, and others. The newsroom holds press releases on additions at several wind farms. an online version of the WindStats Newsletter. The best technology discussions are found here. The latest issue, for instance, discusses segmented blades for offshore turbines in the 3 to 5-MW range. These giants have blades with diameters of 90 m and more, making them difficult to transport. Download the sample back issue (Volume 13, No. 1) for a discussion of augmented wind turbines. The idea is to surround the rotor with a diffuser that creates low pressure behind the blades, thereby pulling more wind over them and producing greater power than an unaugmented device. Vortec Energy Limited in New Zealand may have tests underway.

www.bergey.comis for the Bergey Windpower Co., a manufacturer of wind systems that range from 1.5 to 50 kW. One page shows what it would take to equip a house for self-sufficiency. a wide range of research conducted by the National Renewable Energy Laboratory. The wind-energy resources provide a detailed map of the U.S. and a detailed description of annual wind flow for each region. Sections in Wind Turbine Research include discussion of the next generation of wind turbines, and small wind turbines and hybrid power systems.


Forget high towers. Think jetstream

Three tethers hold the Gyromill in place at about a 40° angle to the ground.

A Gyromill hovers at a few meters in a recent test. The largest craft flown so far has twin 12-ft-diameter rotors capable of generating 5 kw.

At 4 km up, the wind blows strong and steady. Some estimate their power at 50 to 100 times that of wind energy available near the earth's surface. But 4,000 meters is a long way up.

One way to harness upper-level winds, says Bryan Roberts, a professor of mechanical automation engineering with the University of Western Sydney, would be to tether a rotor craft to the Earth, then lift it into those winds which would support it like a kite and carry it higher. Power would come down the tethering cables. Operators could tilt the rotor frame to the best angle to the oncoming wind.

Roberts has been developing the idea and rotor craft — a Gyromill — with the help of colleagues and students for several years. The Australian Civil Aviation Safety Authority has given approval for tests in a location of minimal aircraft activity. "If we find funds, the design and construction stage could be completed in about a year," says Roberts, "with a year of testing immediately thereafter. We've planned a 50-kW machine, plus a standby of the same capacity, as a system demonstrator to operate at around 2 km up."

An Enron 1.5-MW turbine rotor turns at 11 to 20 rpm. The three 34-m blades attach to the main shaft and a gearbox (speed increaser) with a 1:72 ratio. The generator operates between 800 to 1440 rpm, producing its rated capacity at 12 m/sec (27 mph). The machine shuts itself down at 25 m/sec (56 mph). Once the machine reaches rated power (1.5 MW), the blades are pitched to spill wind and keep output at rated maximum.

TAGS: Energy Archive
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