Algorithms Help Turbines Share the Wind

Jeffrey Cuebas

“Logic clearly dictates that the needs of the many outweigh the needs of the few.” So declares Spock, Star Trek’s Vulcan hero, as he sacrifices himself to save the Starship Enterprise and its crew in the 1982 film Star Trek: The Wrath of Khan. Today Stanford University researchers presented the clearest proof to date that self-sacrifice can also benefit wind farms. In their demonstration at an Alberta wind farm, one turbine sacrifices a fifth of its generating potential to enable better performance by neighboring turbines, boosting the group’s collective output.

And while Spock’s heroics necessitated a major plot twist to revive his character for the next Star Trek sequel, teaching turbines to behave altruistically requires just a small (but intelligent) tweak to their control systems. What they learn is how to share the wind.

Like parasols casting shadows, spinning rotors in a wind farm cast an energy-depleted “wake” that can slow downstream turbines. The resulting lost energy can be 10 percent or more of a wind farm’s annual power generation. At Denmark’s Horns Rev offshore wind farm, wake losses cut annual energy production by a hefty 20 percent [photo above].

This week’s report in the Proceedings of the National Academy of Sciences proves out a coordinated control scheme to cut the losses. It is called “wake steering” because rotors are turned about their towers to point slightly away from the oncoming wind and thus deflect their wakes away from downstream turbines [see artist’s rendition].

Modeling and wind tunnel experiments have shown for more than a decade that wake steering should boost overall output, but it’s been hard to test at wind farms according to John Dabiri, the Stanford fluid mechanics expert behind the Alberta test. Wind producers are understandably reluctant to risk losing revenue during a test or, worse still, damaging multi-million-dollar turbines by placing them slightly off-kilter. “When you have technology that’s seen as mature, people just want to operate it as is,” says Dabiri.

Through a friend of a friend Dabiri found one company willing to give wake steering a try: Calgary-based wind operator TransAlta Renewables. Last October TransAlta made a row of six turbines at its Summerview 1 wind farm at Pincher Creek, Alberta available to Dabiri and his team for ten days. The plant’s turbines are laid out to face Pincher Creek’s strong southeast winds. But during the summer and fall winds can also blow from the northwest, flowing straight down its tightly-packed rows of turbines [see photo below]. The northwest wind is no bother for the rows’ lead turbines, but thanks to wind wakes each following turbine captures 30-40 percent less energy than its upstream neighbor.

To determine the best yaw angle for their experiment, the Stanford team fed five years of wind speed, wind direction and power generation data from the six test turbines to their proprietary optimization algorithm. Combining that data with a simple wind model, the algorithm projected that yawing each of the five upstream turbines about 20 degrees to the north would maximize the group’s generation from the northwest winds.

Next they had to find a way to command the turbines. All commercial turbines are programed to always turn or ‘yaw’ their rotors to face the wind. Teaching them a new angle would require just a few days of coding work on most contemporary turbines, says Dabiri, but that was not an option for the relatively inflexible control systems running at Pincher Creek. Dabiri’s team got it done with a manual work-around: repositioning the direction-tracking wind vanes atop the turbines’ nacelles during the 10-day test and thereby tricking the control system to turn 20 degrees off the wind.

The resulting power gains were significant. Power generation rose 13 percent under northwest winds blowing into the wind farm at 7-8 meters per second (mps)—average speeds for Pincher Creek. Steering had a still greater impact amidst slower northwest winds by reducing the times when the wind hitting turbines fell below the 5 mps—the threshold at which they automatically shut down. For 5-6 mps winds wake steering boosted generation by up to 47 percent.

Dabiri says in its commercial incarnation wake steering should yield even better results by adjusting turbines dynamically, based on a table of optimal yaw angles for each turbine under a range of wind conditions. He says the group is readying a workaround to make such dynamic tuning possible for turbines with older control systems: a small logic circuit to tweak the data feed from the wind vane, spoofing the control system. “We intercept and change that number,” says Dabiri.

Meanwhile they are planning with TransAlta for a larger wake steering run next year at an Ontario wind farm. Job one will be measuring mechanical loads to ensure that wake steering is not straining the turbines. Dabiri says there is reason to expect the opposite, since wakes often hit rotors unevenly, torquing their components.

The U.S. National Renewable Energy Lab (NREL) is simultaneously pushing to improve its own wake steering algorithms, and testing them at commercial wind farms. NREL struck first in 2017 in a collaboration with Shanghai-based Envision Energy. In that test steering one turbine at an offshore farm in China boosted output for several neighboring turbines. Two months ago NREL reported successful steering of two turbines at a Colorado wind farm operated by Juno Beach, FL-based NextEra Energy Resources.

The NREL team says its results suggest that wake steering can boost annual wind farm output by at least 1-2 percent, lifting revenues at a typical 300-megawatt U.S. wind farm by $1 million or more. Dabiri says his results suggest that wake steering could yield 7 percent more energy annually from those six Alberta turbines. Applied across the more than 600 gigawatts of wind power capacity installed worldwide, either estimate represents an impressive bolus of clean energy that can be seized with little extra investment.

Dabiri predicts that wake steering’s greatest impact will be on future installations. While wind farms installed today are among the cheapest power sources available, he says decarbonizing economies in the decades ahead requires a multiplication of global wind capacity that will lead to more densely-packed wind farms tapping lower-quality winds. “There’s a misconception that good winds are unlimited. That’s just not the case,” says Dabiri. “We’re going to need to do a better job of extracting wind energy.”

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