Small Turbines can Outperform Conventional
by Jeff McMahon
Decades ago, engineers largely dismissed vertical-axis wind turbines -- which spin around a central axis like a top -- because they could not match the efficiency of the propeller-like turbines common today.
But researchers have found recently that clusters of vertical-axis turbines, arranged to take advantage of each other's turbulence, can outperform conventional wind farms. When added to conventional farms, according to a study published this month, they can even increase the older turbines' output.
"We're able to get significantly higher performance," said Stanford Professor John Dabiri, "both due to the fact that you can put the turbines closer together, but also in putting the turbines together we improve the performance of the individual turbines."
Dabiri works with vertical-axis turbines that are 10 meters tall, compared to 100 meters for a conventional horizontal-axis turbine. He arranges the small turbines the way schooling fish arrange themselves, so that the turbulence created by each fish aids, rather than hinders, the progress of the fish beside it and behind it.
Even a dead fish, placed in such a school, will move forward, propelled by the turbulence of the group.
"They've been able to optimize their movement in interaction with their neighbors so that the group is a more efficient system flowing through the water," Dabiri said.
The hydrodynamics of schooling fish have been applied to wind energy before, and wind-farm operators like General Electric and Berkshire Hathaway Energy have used those insights to improve the placement of conventional turbines. But Dabiri and his team have shown that arrays of much smaller vertical-axis turbines can outperform these conventional wind farms because they respond to turbulence much better.
"I would note that they're still doing this using conventional horizontal wind turbines. Even in these systems you can see benefits. It's going to be something on the order of 5-20 percent, which is actually huge if you're talking about a 300 MW wind farm project, but the thing that we are excited about is that in the vertical-axis turbine approach to this problem it can be factors of 3 and 4 and 5."
Turbulence is bad for conventional turbines, not only reducing their efficiency, but also their lifespan. As a result, they are typically spaced far apart, and the wind that passes between them, according to Dabiri, is "wasted energy."
Small vertical-axis turbines placed in those open spaces can capture that wind and reduce turbulence for the conventional windmills around them.
In a study published this month in the journal Wind Energy, researchers from Stanford, Johns Hopkins, and the University of Delaware simulated the addition of small vertical-axis turbines to a conventional wind farm, and found the mix not only improves the performance of the farm, but also of the older turbines.
With the small turbines added, the mixed-turbine wind farm "produces up to 32 percent more power than the traditional one and the power extracted by the large turbines alone is increased by 10 percent, caused by faster wake recovery from enhanced turbulence due to the presence of the small turbines," according to the study.
Vertical-axis turbines also present less threat to wildlife.
"Of all the companies that are out there operating these, there have been no reported bird or bat strikes," Dabiri said. "This is for two reasons: one, of course, they're closer to the ground so they're not going to be necessarily in the flight corridor. But they also have this rotation around a vertical axis which has a very different visual signature than the very fast moving blades of a conventional wind turbine."
Dabiri presented his findings last week at Stanford University in a seminar sponsored by the Stanford Precourt Institute for Energy. One attendee asked whether the improved outlook for wildlife was merely theoretical.
"The data we have is that we've been running a field site in Southern California, we have about 25,000 hours of data now, and no bird strikes and no bat strikes. We've been doing monitoring at our site."
Windmills kill between 140,000 and 328,000 birds in the U.S. each year, according to a study published in 2013 in the journal Biological Conservation. For comparison, house cats kill 1.3 billion to 4 billion birds annually, according to the journal Nature Communications.
Vertical-axis wind turbines have 12-20 components, Dabiri said, compared to about 8,000 components in a conventional turbine, so they should have longer lifetimes and lower costs for operations and maintenance.
"Both of those are on paper -- and I emphasize on paper -- lower cost than the conventional system, but I don't have a system I can point to today and say, that costs a dollar a watt to install or 2 or 3 cents per kWh. On paper it should be less expensive, and the models say it should be, but I wouldn't believe it until I saw it."
The smaller turbines haven't caught on yet, Dabiri said, because the startup companies now producing them have not yet produced a design resilient enough to withstand the environment.
"There's certainly interest in installing these wind turbines, but the challenge has been delivering a product that's bug free. Most of the companies that sell these are startups that are still trying to work through the kinks of building something that will last 20 years," he said.
"There are a couple of companies that are close to having something that I would recommend to my parents to buy -- that's the metric that I use -- but they're not quite there yet. So there is still work to be done in terms of the reliability required, especially when you're making so many of them."
Vertical Wind Turbines from Center for Bioinspired Engineering at California Institute for Technology, 2016
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