Better alternatives for wind power

May 10, 2012
Innovative new designs for wind turbines promise to cure problems ranging from noisy props to poor efficiency in moderate breezes

Authored by:
Leland Teschler
[email protected]

Key points:

• Gearboxes with high step-up ratios tend to give horizontal wind turbines poor efficiency when wind speeds are low.
• One way to eliminate wind-turbine bird kills: Locate turbines far out at sea.

Altaeros Energies

American Offshore Energy, vertical wind-turbine technology

Wind Sail Receptor Inc.

Wind turbines are drawing more flak these days than a squadron of B-17s over Düsseldorf. Critics claim, among other things, that the machines are noisy, inefficient, and destroy wildlife. Ammunition for this view comes from researchers who say conventional turbines generate about 95 dB(A) of sound power at frequencies that many people find annoying. And in 2009, the U. S. Fish and Wildlife Service estimated domestic wind turbines kill about 440,000 birds every year. This includes about 70Œgolden eagles, 2,400 raptors, and 7,500 other birds, nearly all protected by federal legislation, killed annually by turbines in Altamont Pass near Oakland, Calif.

Lack of performance in slight breezes constitutes an additional point of contention. Many large-scale wind turbines don’t start turning until wind speeds reach about 20Œmph. Worse, some brands need a running start when wind speeds are low. This “kick start” comes from using a starter motor to get the prop moving, thus consuming electricity for a time rather than generating it.

To address such issues, some promoters advocate siting turbines offshore in relatively windy areas away from most birds and humans. But offshore wind-farm construction can be pricey, partly because of the need for erecting beefy wind-turbine structural members on the sea bed.

These issues have been so well publicized that serious inventors have taken notice. Now, numerous innovative wind-turbine designs are taking shape that, at least on paper, overcome many of the difficulties associated with ordinary horizontal wind turbines. There are probably dozens if not hundreds of interesting designs in development somewhere between fanciful paper studies and physical prototypes. A few are nearing commercial production. So here are a handful of concepts representing current out-ofthe- box thinking on wind-turbine technology.

Quiet and efficient
A novel wind turbine developed by Wind Sail Receptor Inc., Boulder City, Nev., could have a big impact on the small wind-turbine market when it goes into production this month. The turbines sport four flared blades that make them look a little like pinwheels. The blades are of a special polyurethane material that inventor and WSRI Chairman Richard Steinke says is strong enough to withstand a round from an AK-47, important in developing countries where there is a big market for small turbines.

The shape of the blades is such that the turbines can capture more wind energy than traditional designs and, thus, can do useful work at much lower wind speeds partly because the blades are a relatively large percentage of the area they sweep out, compared to that of conventional turbines. The blade shape is the culmination of 52 different trials aimed at optimizing the turbine rotational speed versus the torque generated, says Steinke. “Existing wind turbines can run with winds as low as perhaps 7 or 8 mph. But our blade design lets us start generating power at 1.5mph and we don’t use a starting motor,” says Steinke.

Another benefit of the blade shape is superlow noise, below about 10 dB. “About the sound level of normal breathing,” says Steinke.

One reason Steinke’s turbines can do useful work at such low rpms is that they don’t need a gearbox between the turbine rotor and the generator, unlike conventional wind turbines. The gearbox normally has a high step-up ratio, necessary to get the rotational speed high enough to drive a generator at useful speeds. This step-up ratio, plus the friction and stiffness of their bearings, gives gearboxes a significant amount of inertia that must be overcome before the turbine rotor can start spinning.

Interestingly, Steinke’s turbines have generators sitting in the turbine base rather than in the nacelle. They connect to the rotor with a mechanical driveshaft. “I wanted something that won’t break down, so we went with a design having as few components as possible. Driveshafts don’t break down,” Steinke explains.

The mechanical connection is practical because the turbines are relatively short. The first production units have a 12-in. hub system and 2.5-ft-long blades. The hub is just 6 ft off the ground. This unit will generate as much as 7kW in a 35-mph wind. Another optimized generator now on the drawing board will bring that figure up to 9kW.

In a few months, Steinke expects to come out with a 12-ft-diameter turbine able to generate 30kW. This will be a game changer, he says, because similar conventional turbines this size typically generate only about 2.5kW.

Go fly a ... wind turbine
If you crossed the Goodyear Blimp with a wind turbine, the result might look something like the Airborne Wind Turbine (AWT) from Altaeros Energies. The Boston- based start-up has developed a prototype that, so far, has floated 350-ft (107-m) high in a demonstration that it can produce power.

There are multiple reasons for coming up with a scheme for wind turbines able to function hundreds of feet in the air: Such devices should be able to harness winds at and above 1,000 ft (305 m). Winds at that altitude are about five times stronger than surface winds, thus quickening the payback for a capital investment in the necessary equipment. Any wind-turbine noise will be too far away to annoy people on the ground. Moreover, only migratory birds fly at those altitudes, and the turbines will be highly visible objects easy for them to avoid. The AWT uses an inflatable shell filled with helium. Altaeros worked with Doyle Sailmakers of Salem, Mass., on the shell, which is assembled from the same sail material found on advanced racing boats. Tethers are designed to passively help align the craft with the wind, and there is some control authority for steering built into the winches on the ground feeding out the tethers.

The first application Altaeros has in mind for the flying turbines is in powering remote locations that now use diesel generators. Altaeros CTO Ben Glass says the goal is to field units initially able to generate up to about 200‘kW. These will be about twice the size of the recently flown prototype, and Altaeros calculates they will generate electricity costing about 35% of that from diesel gen sets. Eventually, the company wants to deploy utility-scale AWTs generating power in the megawatt range.

However, Altaeros has a way to go before it is ready for prime time. It doesn’t have a final design yet, only guiding concepts. The prototype used an off-the-shelf turbine just to prove the concept. Altaeros says it will work with turbine-development partners to produce optimized components made from lightweight materials. To keep weight and complexity down, it is looking at using permanent-magnet motors and a direct drive that eliminates the need for a gearbox.

Tests so far, however, show that electrical cables strung to an airborne turbine 1,000 ft off the ground won’t be a large component of the overall weight. The plan is to build power-conversion electronics into the turbine nacelle that will convert generated electricity to a high voltage before feeding it down to the ground. Conductors handling the high-voltage/low-current juice can have a relatively small diameter while still incurring low losses, Glass says.

Glass figures it will take about two years to finish the detailed design for the first commercial units. And these will probably undergo several months of testing before they are ready to start generating power.

Goodbye gearbox
One of the biggest sources of criticism in wind-turbine technology is the gearbox. Most horizontal wind-turbine nacelles contain a planetary gearbox that steps up the rotor speed from tens of revolutions per minute to the 1,800-rpm range for running the electrical generator. Problem is, these gearboxes have a reputation for being unreliable. Their manufacturers generally peg their expected life at better than a decade, but it is not unusual for major problems to emerge after only a few years.

The big step-up ratio typically also gives the gearbox appreciable inertia, which must be overcome when the turbine begins rotating. Particularly on utility-scale turbines, this inertia limits the power that can be harvested from moderate breezes.

No surprise, then, that such woes have fostered a lot of thinking about how to devise big wind turbines that don’t need all this gearing. One of the more-novel ideas comes from former Massachusetts Institute of Technology professor David Wilson. His concept: Eliminate the need for a gearbox by generating electricity at the tips of the turbine blades.

The resulting structure looks a little like a turbine with its blades running through a piece of a shroud at the bottom of their arc. The shroud holds what amounts to a stator of an electrical generator. Blade tips carry either magnets or a stack of laminations that travel through the stator as the wind blows, generating electricity directly.

The thinking behind Wilson’s scheme becomes clear from an analysis of blade-tip speed compared with windturbine rotor speed. The optimum blade-tip speed for a two or three-bladed turbine is about five times the wind speed. Wind turbines generally operate at up to a maximum wind speed of about 20 m/sec (44 mph). The blade speed at the tip is then around 100 m/sec regardless of size. A large turbine rotates slowly and a small turbine has a higher rotating speed even if the blade-tip speed is the same in the two cases.

Of course, the diameter of the rotor of a conventional wind turbine is perhaps one-tenth or one-twentieth that of the blade tips, hence the need for at least a 20:1 stepup gearbox. (Some wind-turbine gear transmissions have step-up ratios of over 100, Wilson points out.)

The turbine nacelle for this type of scheme would just contain a bearing for the rotor. It would see a continuous torque, exerted on each blade in turn. Wilson figures it may be desirable to have a tensioning wire connecting the three blade tips to avoid vibratory responses.

Wilson thinks the resulting wind turbines would be considerably more reliable, possibly more efficient (because of the elimination of the gear losses), less expensive to erect and to service, and possibly lower in first cost compared with the traditional approach. He also figures his new approach would generate less noise than ordinary turbines. “A lot of wind buffeting in conventional turbines is because of eddies coming off the back side of the circular tower form. But with this new design, the whole tower can be streamlined to minimize that effect because the nacelle just contains a rotor bearing instead of a heavy gearbox, generator, and supporting subsystems,” Wilson says.

However, don’t expect to see one of these turbines in a wind farm any time soon. Wilson, whose career has mostly concentrated on the engineering of gas turbines, says he holds provisional patents on the basic technology behind his design. But at least for now, the turbine remains a design study.

Off-shoring turbines
Ocean winds tend to be stronger and more consistent than those on shore. This is one of the arguments for siting wind turbines in coastal waters. Trouble is, the underwater infrastructure for ocean-based wind farms is expensive, and permission to use these sites is often fraught with political problems, as exemplified by the fight over the Cape Wind site off Cape Cod.

One solution to such controversy is to site wind harvesting more than 50 miles offshore where winds are even stronger, as the effects of the jet stream approach sea level, becoming trade winds. There is also less wildlife this far offshore and less recreational and commercial boating to boot. And wind turbines that far out would be over the horizon, out of sight from the shore.

But if it is expensive to mount wind-turbine foundations in shallow water, it is megaexpensive to do so farther out where the seabed drops away off the continental shelf. The fix: Eliminate the seafloor foundation and let the wind turbines float on the surface

The price of offshore wind plants has generated a lot of hard thinking about alternatives. The Dept. of Energy, for example, pegs the price of offshore wind even on shallow seabeds at 4 to 5 million dollars/megawatt, about twice what terrestrial turbines cost. The Energy Technology Institute (ETI) in the U.‰K. is spending millions of pounds trying to develop floating offshore wind turbines that halve the cost of offshore wind. There are similar efforts underway in the Netherlands, Norway, Spain, and Portugal.

One design they’ve tried uses vertical-axis wind turbines (VAWTs), where vertically oriented turbine blades are positioned around the perimeter of a vertically oriented rotor. These aren’t as efficient as conventional horizontal turbines, but they can be cheaper to build, more reliable, and have a low center of gravity and, thus, are more amenable to flotation.

One configuration of VAWT proposed by Drew Devitt, chief technical officer at New Way Air Bearings, Aston, Pa., uses near-frictionless radial-air bearings to support the rotor at the perimeter. The rotor is the only moving part

Devitt, an entrepreneur, created a company called American Offshore Energy, Aston,Pa., to develop the offshore VAWT design. He says the VAWT has less than half the efficiency of horizontal designs, but this deficiency is canceled out by the fact that the stronger winds offshore have twice the energy as breezes characterizing terrestrial sites, because power in the wind is a cubed function of its velocity. Importantly, the capacity factor is significantly better in offshore wind. Capacity factors are based on the power curve for the particular wind turbine combined with wind speed and duration data from the proposed site. Horizontal-axis wind turbines tend to be most efficient at higher wind speeds. But poor performance at low speeds tends to give them a low capacity factor, meaning these turbines will generate their rated capacity only a small fraction of the time.

Devitt says VAWTs avoid such difficulties. For one thing, VAWTs in an impulse configuration — where wind pushes on the blades to move the rotor — have a relatively high efficiency in lower wind speeds because their blades have cross sections that are an appreciable percentage of swept area. This design will make power most of the time the wind is blowing.

Large horizontal wind turbines also have a disadvantage associated with the speed of the outmost tips of their huge blades. Conventional horizontal-axis wind turbines are aerodynamic turbines — the blades are airfoils shaped to create low pressure that turns the rotor. A 100-m swept area has a 314-m circumference. In a wind speed of 14 m/sec, the tip speed can be six times that, or 84 m/sec. This equates to over 300 km/hr and is a fundamental limitation on the degree to which horizontal turbines can be scaled up. Eventual loss of blade strength due to fatigue becomes an issue on big blades, and the fact that the blades comprise a small percentage of the swept area also helps explain why the aerodynamic design needs a relatively high wind speed just to start spinning.

The VAWTs Devitt has in mind are supported on three bearing points, giving them stability analogous to that of a three legged stool. The three-point flotation provides spots for three mooring tethers to provide antirotation force. The VAWT will always wind up and tighten its tethers in the same direction. The bearings are fixed on gimbaled mounts so they self-align to the rotor. This simplifies assembly and avoids the requirements for rigid structures. Moment loads from the wind spread across the large base circumference of the VAWT, rather than on a narrow pole as would be the case for horizontal turbines.

Devitt foresees a direct-drive arrangement with the generator directly coupled to the rotor shaft and residing below the water line to give the VAWT a low center of gravity. Everything above the base ring would be built with lightweight materials. Fiberglass blades would support a top ring also made from fiberglass. Steel wires in a combination of sailboat mast and bicycle spoke technology would create a lightweight, but stiff cylindrical shape, he says.

Additionally, a steadier wind means less wear on turbine components. Undersea cables are also much-less expensive to permit and do not require high-tension towers. And 30 to 50 miles of undersea cable is significantly less expensive than the 1,500-mile run of high-tension lines currently contemplated for transmitting Midwest wind power to the East Coast, he says.

Undersea cables have another significant advantage in that they are insulated from summertime heat, Devitt notes. Higher temperatures reduce the conductivity of transmission cables, so just when the grid needs to work hard in the summertime heat, the heat reduces their transmission capacity.

Offshore VAWTs would have other advantages as well, Devitt argues. If necessary the turbine could be towed back to land in a day for service. A 200-ft turbine would also provide excellent horizontal radar reflection for maritime visibility with few vertical reflections. And such turbines would be well outside the 12-mile state control zones, this further minimizing theaters in which legal action may be taken against a proposed wind farm.

What about the occasional hurricanes and rogue storms that can wreak havoc with wind turbines? Because the VAWT would have no gearbox, it has no oil reservoir. All the components on the turbine would be waterproof and rustproof, so they could be easily sunk by remote control to ride out a storm safely beneath the ocean surface.

Still, don’t expect to see Devitt’s VAWT design floating in the sea anytime soon. He says he is working on a prototype but at this point, it is only a part-time effort.

© 2012 Penton Media, Inc.

About the Author

Leland Teschler

Lee Teschler served as Editor-in-Chief of Machine Design until 2014. He holds a B.S. Engineering from the University of Michigan; a B.S. Electrical Engineering from the University of Michigan; and an MBA from Cleveland State University. Prior to joining Penton, Lee worked as a Communications design engineer for the U.S. Government.

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