Whirling Snowflakes Unmask Turbulent Air on Wind Farms
The way snow dances around wind turbines is shedding light on the mysteries of how air turbulence behaves, a new study reveals.
This research could help improve how efficient and reliable wind turbines are, and could yield insights on where to best place wind farms, scientists said.
Wind turbines harness the wind's energy to generate electricity, using massive rotors that typically measure 80 to 300 feet (24 to 91 meters) across. Wind is clean and inexhaustible, making wind power an attractive form of alternative energy worldwide — the amount of electricity from wind power has increased more than 16 times between 2000 and 2012. The United States gets 3.5 percent of its electricity from wind overall, but certain windier states get more — for instance, Iowa and South Dakota get more than 20 percent of their electricity from wind, according to the American Wind Energy Association. [Top 10 Craziest Environmental Ideas]
To improve wind turbine power production and structural reliability, scientists want to learn more about how these devices interact with the surrounding air. Turbulent airflow in the wake of a turbine can impact how much power it produces and increase the mechanical strain on its framework.
However, until now, there was no way to properly visualize air turbulence around full-scale turbines. At best, researchers had to rely on wind turbines 3 feet (1 meter) or so high in laboratory wind tunnels, but the problem with such work is that the way air flows can vary with scale, meaning that results from experiments with small turbines might not apply to larger ones. Prior research suggests this deficit in understanding how wind turbines work causes wind farms to perform less efficiently, with an average power loss of 10 to 20 percent.
Now, lead study author Jiarong Hong, a fluid dynamicist at the University of Minnesota in Minneapolis, and his colleagues have developed a way to model air turbulence around wind turbines — by analyzing snow as it whirls around wind turbines during snowstorms.
The concept occurred to Hong as he applied for his current job. "The idea of using snowflakes came naturally to me when I was thinking about the fact that I was going to start my career in Minnesota," Hong said. "When I started paying attention to snowflakes illuminated by a street lamp during a snowy night at Minnesota, I felt that I was onto something."
During field work, the researchers analyzed snow blowing past a 2.5-megawatt wind turbine about 260 feet (80 m) high with blades about 157 feet (48 m) long. Measurements were taken at night, and snow was illuminated with sheets of light from a powerful searchlight. Pictures and videos of the snow from high-resolution cameras later helped scientists trace air turbulence from the wind turbine in the region between 10 and 128 feet (3 and 39 m) above the ground. [Infographic: Earth's Atmosphere Top to Bottom]
"Initially, when I proposed using snowflakes to measure flow at large scales, people laughed — they didn't believe it, and thought I was crazy," Hong said. "They were skeptical we could illuminate an area the size of a building, and whether the cameras had a high enough resolution to capture individual snowflakes, and many other technical challenges."
Moreover, attempting to carry out research late at night during winter snowstorms in Minnesota was challenging, Hong recalled.
"We had to listen to weather forecasts, and went out to the site when forecasts predicted high probabilities of snow, but many times the forecasts were not accurate, and many deployments didn't work out," Hong said. "Also, with one big snowstorm, the snow was too heavy, and we got completely stuck — it took five or six hours to take out all the instruments, and that deployment was not successful, either. We didn't get our results on the first try — it really took a lot of trial and error."
The researchers successfully analyzed differences between lab turbines and real-size turbines. To start with, airflow past real-size turbines can be significantly more turbulent than with lab turbines.
"Quantifying turbulent air flows around modern-size wind turbines is a very significant yet challenging problem for the development of wind energy — it is crucial for not only optimizing wind-farm siting and power generation, but also for understanding the environmental impact of wind turbines," Hong said. "The most exciting part of the results for me is that with the help of Mother Nature, we are now able to provide a tool to address this challenging engineering problem."
In addition, "real-size wind turbines use different materials and have different structures than smaller turbines used in labs, and they respond differently to wind," Hong said. "At the large scales you see with real-size wind turbines, they are not really rigid — the turbine can distort, and the blade can deform."
Furthermore, "we saw real-scale atmospheric conditions with very turbulent flows," Hong said. "Those conditions are very difficult to reproduce in the lab."
The scientists noted potential associations between turbine operation, control and performance with patterns of wind turbulence surrounding a real-size turbine. Future research could modify the way turbines are built and operate to optimize their performance, the scientists said.
These findings could help improve the efficiency of many wind farms in cold regions. Although they may not directly apply to wind farms in other regions, "we can use the insights we get from snow to understand the general fundamental dynamics of turbines, improving numerical simulations to apply this research to wind farms in many other regimes," Hong said.
In the future, "we would like to upgrade our instruments to further extend the measurement range and improve the accuracy of our technique," Hong said. Moreover, the effect of weather conditions, snowflake size and other factors need further investigation.
The scientists detailed their findings online today (June 24) in the journal Nature Communications.
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