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Starting in 2012, Hong and other researchers tried placing spotlights downwind from the 2.5-megawatt (MW) wind turbine at the U.S. Department of Energy-funded EOLOS Wind Energy Research Field Station in Rosemount, Minnesota.
The research team was attempting to study turbulent airflow around a turbine in the field. “You need to find tracers to put in the airflow,” Hong said. “It is very easy to do in the lab, but very difficult—if not impossible—to do at the 50- or 100-meter scale.”
In a controlled situation, researchers add tracers and photograph the tracers’ flow through miniature turbines. Under those conditions, researchers can compare two consecutive images to get an idea of how air travels and impacts other turbines downwind.
But Hong wanted a way to measure the flow from a real turbine. That's when difficulties mount. For example, to be successful, researchers would have to figure out how to inject a large volume of tracers in a way that did not disturb the currents—furthermore, the tracers would need to be environmentally friendly.
“I started thinking about the weather,” Hong said. “Minnesota has a lot of snow. Maybe there would be a way to use natural snow.” Gradually, after a number of trials and errors, his team figured out a way to characterize airflow motion using snowflakes.
Then he encountered another problem: the uncertainty of weather forecasts. Sometimes a predicted snowfall never arrived as the team waited for up to 6 hours—or came in insufficient quantities to measure. Finally, early in the morning of February 22, 2013, a true blizzard roared in—and the researchers were ready. They had positioned a large searchlight with reflecting optics designed to create a light sheet reflecting off snow particles in an area that was 36 meters wide and 36 meters high.
As the wind drove the snow, researchers videotaped patterns created as the blades of the wind turbine rotated. The turbulent airflow measurements were synchronized with the turbine operational and loading information obtained from the sensors embedded in the EOLOS turbine. “We looked at the video of the snow’s motion, and correlated the frames, which showed how the snow was placed and moved in the air,” Hong said. “We were able to quantify the turbulence in unprecedented detail at such scale.” The team’s results were published in the scientific journal Nature Communications.
“These measurements are extremely important in our efforts to improve the efficiency of wind energy and develop and validate high-fidelity computational models for optimizing wind farms,” said Fotis Sotiropoulos, co-author of the study and director of the St. Anthony Falls Laboratory and the Eolos Wind Energy Research Center. “Who would have ever thought we’d use a Minnesota blizzard to help fight global warming?”
“This research is at a very early stage,” Hong said, adding that the team has repeated the study a half dozen times so far, most recently on April 3, 2014. He hopes this research idea can be implemented not only in Minnesota, where snow in May is common, but elsewhere where meteorological conditions are suitable. This method could produce a range of valuable data on turbulent air flows around utility-scale turbines and contribute to better layouts and control strategies of modern wind farms. Such data could also increase energy production and decrease turbine component fatigue from wakes.
As for waiting long hours in the cold only to be disappointed, Hong remains positive: “We're getting more and more experienced,” he said. And when the deep freeze returns, he'll be ready for another round of winter—and research.