A lot of people try to encourage more wind turbines because wind power uses renewable energy. Whenever I play a game called “Cities: Skylines”, I always try to have more wind energy than the conventional ones. But I digress. Wind turbines are not superheroes, and if they are, they’ve got their kryptonite.
We’d think that winter is the perfect time and season for wind turbines because the wind is stronger. Moreover, air density increases as it becomes colder, there’d be more force to push on the blades. The thing is, winter can be so cold that it freezes the turbines.
Why is that a problem you may ask? Well, light icing in wind turbines can produce enough surface roughness on the blades. That reduces aerodynamic efficiency, which reduces the amount of power they produce.
And not just that. If there’s frequent icing, it can cut a wind farm’s annual energy production by over 20% and that costs a lot of money (we’re talking hundreds of million dollars here). The problem from icing isn’t just power loss either.
When uneven ice forms on blades, it creates imbalances and each turbine’s parts can wear out more quickly. There are also vibrations (from icing), causing them to shut down completely. Also, when there’s extreme icing, it’s not easy to restart turbines, taking hours or potentially days.
We can always warm things up and, you know, sort of thawing the blades. Or maybe put something inside the turbines so that ice won’t keep forming. And that’s true theoretically. However, most of the strategies for keeping ice off wind turbines blades come from aviation.
The thing is, airplane wings and wind turbines are different in build. Both operate under very different conditions.
According to Hui Hu, professor of aerospace engineering from Iowa State University, he and his colleagues have been studying wind turbine icing physics for 10 years. They’ve explored and tried better solutions for turbine icing protection.
Hu stated that ice is not all the same; it can come from precipitation, clouds or frost. Ice also freezes in different ways in different climates. Rime icing, for example, forms when tiny, supercooled water droplets hit the surface.
This type of ice usually occurs in areas with relative dry air and colder temperatures under –6.7C (20F). Hu said that this usually happens in Iowa and other Midwest states in the winter.
Glaze icing, for another example, usually occurs with much wetter air and warmer temperature, commonly seen on the Northeast coast. Hu stated that this is the worst type for wind turbine blades.
“It forms complicated ice shapes because of its wet nature, which results in more power loss. It’s also likely what formed in Texas in February 2021 when the cold air from the north collided with the moist air from the Gulf Coast,” he said.
The storm in Texas shut down power not only from natural gas, coal, and nuclear, but also wind turbines.
Using fly-drones with digital cameras for the study
Per the Iowa professor, making wind power operation that stays strong in icy conditions needs a good understanding of the underlying physics. Namely, how ice forms, and the performance degradation that results from ice building up on turbine blades.
The team use a special wind tunnel that can demonstrate how ice forms on samples of turbine blades and camera-equipped drones in order to explore those forces.
Then, using the Icing Research Tunnel at Iowa State University, the team’s replicated the complex 3-D shapes of ice forming on turbine blade models in different environments. The purpose is to study how they affect the wind and the blades.
Ice can create massive airflow separation. In airplanes, that’s bad because it can cause them to stall, hence the technology we have now for aircrafts. On the other hand, in wind turbines, there’s no such thing for now. Icing reduces their rotation speed and the amount of power they can produce.
Hu and his colleagues also study wind turbines currently operating around the USA because they also face some of their toughest conditions. Using drones with high-res digital cameras, the team can hover in front of 80-meter-high wind turbines and take photos of the ice right after it forms on the blades.
After, they pair the photos with the turbine production data, showing them how the ice influences power production.
They found out that while ice can form over the entire span of the blade, there’s more ice near the tips. After one 30-hour icing event the team found ice up to about 30cm (one foot) thick.
So, even though the wind’s strong and all, turbines with thick ice on them rotate much slower and, as mentioned, even shut down. The turbines produced only 20% of their normal power over that period.
Making the blades ice-free
Aircrafts have strategies and ways to keep ice off their wings, but not for wind turbine blades.
Why is that? One of the reasons is the materials. Airplane wings are typically made of metals like aluminum alloy. Utility-scale wind turbines, on the other hand, are made of polymer-based composites. Therefore, thermal-based systems that circulate heat are better for aircrafts.
Additionally, polymer-based turbine blades are also more likely to get covered by dust and insect collisions. That can change the smoothness of the blade surface and slow water running off the blade, promoting ice formation.
Also, wind turbines are more prone to encounters with freezing rain and other water such as ocean spray for offshore wind turbines.
Current wind turbines have anti-icing and de-icing technology that removes ice buildup through electric heating or blowing hot air inside. Unfortunately, it adds to the cost of the turbine, is inefficient and energy-consuming.
The reason being that it heats massive areas, which are many times larger than airplane wings.
Moreover, composite-based turbine blades are weak to overheating as well. Plus, another problem: water from melting ice may simply run back and refreeze elsewhere. Not exactly a problem solver.
There’s also another strategy in cold areas, which is using surface coatings that repel water or prevent ice from sticking. But, Hu stated that none of the coatings has been able to eliminate ice completely, especially in critical regions near the blades’ leading edges.
Better solution from the team
Professor Hu and his colleagues have been developing a novel method that sort of combines both technologies. They heat only the critical regions, particularly the blades’ leading edges using water, and utilize the ice-repelling coatings.
With the novel method, they were able to reduce the amount of heat needed and the risk of running back water to refreeze over the blade surfaces. The managed to prevent ice from forming on the entire surfaces of turbine blades.
According to Hu, compared to the conventional brute-force surface-heating methods, his strategy used much less power, totaling up to 80% energy savings. Since ice doesn’t slow them down, the turbines can produce more power in winter.
Hu concluded that there’s about 800 gigawatts of wind power all over the world, including over 110 gigawatts in the USA alone. “As the market quickly grows and wind power supplants higher-polluting energy sources, de-icing and ice-proofing strategies are becoming essential,” he said.