Manufacturers, Researchers Quest for a Workable, Large-Scale Vertical Axis Wind Turbine
February 12, 2013
We all have a mental image of wind mills, perhaps borne of romantic visions of the Netherlands, or being forced to read Don Quixote in high school. While modern wind turbines bear only cursory resemblance to the windmills of yore, they have a few things in common: they are tall and topped with shaped blades designed to catch the wind and turn, creating electricity (or, historically, grinding grain); and when one of the blades or part of the turbine mechanism became damaged, someone unlucky had to gather tools and start climbing.
Increasingly, the wind industry is becoming interested in what’s known as the vertical-axis wind turbine (VAWT), a unique looking piece of equipment on which the main rotor shaft is set vertically, with the blades wrapping around it, and the main components are located at the bottom, rather than the top, of the turbine.
(Technically, the design is called the Darrieus model, and it’s often described as looking like a giant eggbeater.)
This design has a distinct advantage when it comes to servicing the turbine: easier access to critical components. VAWTs have other advantages over traditional, horizontal axis wind turbines (HAWT), as well: because of the way they work, the turbines can be located closer together, allowing for greater density in wind fields. Horizontal turbines need to be set apart by a factor of 10 times their width so the wake stirred up by the turning blades doesn’t affect the aerodynamics of blades on other turbines.
In addition, unlike horizontal turbines, VAWTs are omnidirectional and do not need to be strategically pointed into the wind, which makes them less complicated from a mechanical perspective. They put less stress on the turbine support structure, and they don’t need as much wind to generate the same amount of power as a horizontal turbine. (They can actually generate electricity from wind speeds as low as 4.5 miles per hour.) They are quieter, which makes them less likely to annoy nearby residents or interfere with radar. Because they can cope better with the more turbulent wind located closer to the ground, they can be shorter, which makes for easier maintenance, and they are better suited to be located on roofs or in urban environments where the wind is very changeable. Finally, to the approval of environmentalists and animal lovers everywhere, they are less deadly to birds and bats.
Sounds great… so why on earth does anyone still install horizontal wind turbines?
VAWTs still have a number of drawbacks. There is more play in the blades as they turn, which makes them more prone to fatigue, flexing and cracking. They are more likely to stall in gusty wind, and as a result, aren’t suitable in locations with very high wind speeds. For these reasons, VAWTs have been largely confined to smaller installations at lower wind speeds, and no one has been able to solve the problems they present to build them to compete with horizontal turbines. Yet.
“VAWTs are a perfectly legitimate technology, but these are not toys or lawn ornaments,” Paul Gipe, a renewable energy advocate and analyst, told Scientific American. “They are supposed to produce electricity, and must compete with all the other machines that produce electricity. Right now, no one produces a VAWT that is more cost effective than a conventional wind turbine,” he said.
Not for lack of trying. Several VAWT start-ups opened their doors to great promise, only to fail at their quest and quietly declare bankruptcy. While wind advocates still believe that VAWTs have promise for the future, no one has found precisely the right design and the right materials to make them succeed.
Traditional horizontal-axis wind turbines maintain a steady torque in constant wind. VAWTs, on the other hand, have two “pulses” of torque and power for each blade, based on whether the blade is in the upwind or downwind position, according to CleanTechnica. This can lead to something called “torque ripple” that in turn can lead to drive train fatigue. Any improvements on VAWT technology will need to involve new rotor designs that can overcome torque oscillations without boosting costs significantly.
There’s an end goal in solving the problems inherent in vertical-axis turbines. Because they can be located closer together, it’s a way of achieving better wind efficiency without having to resort to either moving turbines offshore or making them taller, something that HAWT operators have had to do to boost the amount of power generated to make wind installations financially solvent. It’s not coincidental that both of these activities tend to irk neighbors and generate legal challenges by homeowners and municipalities. Doing so also greatly boosts infrastructure costs.
For this reason, VAWT technology remains something of a Holy Grail to wind advocates. The higher efficiency potential is just too attractive to ignore, since it would help solve a long-standing problem for wind power.
“What has been overlooked to date is that, not withstanding the tremendous advances in wind turbine technology, wind 'farms' are still rather inefficient when taken as a whole," John Dabiri, professor of Engineering and Applied Science and director of the Center for Bioinspired Engineering at Caltech, told ScienceDaily. "Because conventional, propeller-style wind turbines must be spaced far apart to avoid interfering with one another aerodynamically, much of the wind energy that enters a wind farm is never tapped. In effect, modern wind farms are the equivalent of 'sloppy eaters.' To compensate, they're built taller and larger to access better winds.”
To solve the mechanical problems, a number of companies and organizations are engaging in modeling with VAWTs in an attempt to find the right design out of the right materials in the right configuration to make it all work, and work efficiently.
Caltech is one of the organizations that continues to pursue VAWTs. The school’s Biological Propulsion Laboratory has an ongoing project called the Field Laboratory for Optimized Wind Energy (FLOWE), an array of 18 counter-rotating, vertical-axis wind turbines located in Northern Los Angeles County. Researchers, who can change the positions and locations of the turbines, study flow fields and turbine spacing, kinetic energy and power production, friction velocity, turbulence intensity and many other factors in an attempt to uncover just the right mix.
While the Caltech lab may not have anything ready for commercial launch, it has concluded that the benefits of VAWTs over horizontal turbines could be staggering. A study by the group published in the Journal of Renewable and Sustainable Energy in 2011 found that VAWT could increase wind power output by at least a factor of 10 compared to horizontal turbines.
The key is to be able to maximize a turbine’s energy-collecting efficiency lower to the ground, where the wind is more easily harvested. Noted Dabiri, “The global wind power available 30 feet off the ground is greater than the world's electricity usage, several times over.”
Sandia National Laboratories, which is also engaging in active VAWT research, has proposed that vertical axis wind turbines could be more efficient for offshore purposes, as well as onshore. A lower center of gravity on the vertical turbine means improved stability afloat and lower gravitational fatigue loads, which would reduce maintenance costs (which are already sky-high for traditional offshore installations). Sandia is in the process of developing advanced rotor technologies for VAWTs with the help of $4.1 million in investment from the U.S. Department of Energy. The lab hopes that advances in blade design and materials will help it solve some of the earlier problems of VAWTs.
The ultimate goal, of course, is to bring down the costs of wind turbines while at the same time boosting efficiency by harvesting more wind. Despite the difficulties the nascent VAWT industry has experienced thus far, the potential benefits are simply too great to ignore, and research will continue until someone gets it just right.