The Science Behind Frozen Wind Generators – And How To Flip Them By way of The Winter – Watts Up With That?
Hui Hu, Iowa State University
Winter is said to be the best season for wind power – the winds are stronger, and as the air density increases as the temperature drops, more force is exerted on the leaves. But winter also brings with it a problem: frosty weather.
Even slight icing can cause wind turbine blades to have sufficient surface roughness to reduce their aerodynamic efficiency, thereby reducing the amount of power they can generate, Texas learned in February.
Frequent heavy icing can reduce a wind farm’s annual energy production by over 20% and cost the industry hundreds of millions of dollars. Loss of power isn’t the only problem from icing, either. The uneven way ice forms on the blades can create imbalances, which can cause parts of a turbine to wear out faster. It can also cause vibrations that cause the turbines to shut down. In extreme icing conditions, it may not be possible to restart the turbines for hours or even days.
The solution is obvious: de-ice the blades or find a way to prevent ice from forming. So far, however, most of the strategies for keeping ice away from wind turbine blades have come from aviation. Airplane wings and wind turbines are built differently and work under very different conditions.
I am an aerospace and mechanical engineer. My colleagues and I have studied the physics of wind turbine icing for the past 10 years and have been looking for better solutions for protecting turbine icing.
Not every ice cream is created equal
Ice cream is not the same everywhere. It can come from precipitation, clouds or frost. It also freezes in different ways in different climates.
For example, hoarfrost glaze, which is created when tiny, supercooled drops of water hit the surface, usually occurs in regions with relatively dry air and colder temperatures below 20 ° F. This is what we typically see in Iowa and other Midwestern states during winter.
The comparison of hoarfrost and glazed ice shows how the texture of the blade changes. Gao, Liu and Hu, 2021, CC BY-ND
Glaze is associated with much more humid air and warmer temperatures and is common on the northeast coast. This is the worst type of ice for wind turbine blades. It forms intricate ice shapes due to its humid nature, resulting in greater loss of performance. It’s also likely what formed in Texas in February 2021, when the cold air from the north collided with the humid air from the Gulf Coast. While most of the electricity cut off by the storm came from natural gas, coal, or nuclear power, wind turbines also had problems.
Storms in a wind tunnel
Building a wind farm that can thrive in icy conditions requires a thorough understanding of the underlying physics, both ice formation and the performance degradation that results from ice formation on turbine blades.
To study these forces, we use a special wind tunnel that can show how ice forms on samples from turbine blades, and drones fly with a camera.
Using the Icing Research Tunnel at Iowa State University, my team replicated the complex 3D shapes of ice formation on turbine blade models in different environments to study how they affect wind and blades. Ice can cause massive airflow separation. In aircraft, this is a dangerous situation that can lead to a standstill. In wind turbines, this reduces their speed and the amount of electricity they can generate.
Ice formation changes the airflow around the turbine blade, which can slow it down. The photos above show the ice formation after 10 minutes at different temperatures in the wind research tunnel. The measurements below show airflow separation as ice accumulates. Iowa State University’s Icing Research Tunnel, CC BY-ND
We also study wind turbines that are in operation across the country as they face some of their harshest conditions.
With drones equipped with high-resolution digital cameras, we can hover in front of 80-meter-high wind turbines and photograph the ice on the blades as soon as it is formed. The combination with the production data of the turbine shows us how the ice affects the generation of electricity.
While ice can form over the entire span of the blade, there is much more ice near the tips. After a 30 hour icing event, we found ice up to a foot thick. Despite the strong wind, the icy turbines turned much more slowly and even switched off. The turbines only produced 20% of their normal output during this period.
How ice accumulates on the tips of the turbine blades. Gao, Liu and Hu, 2021, CC BY-ND
Keep ice away from the blades
There are several reasons why the strategies used to effectively keep ice away from airplane wings are not as effective for wind turbine blades.
One is the materials they are made of. While airplane wings are typically made of metals such as aluminum alloys, utility-scale wind turbines are made of polymer-based composites. Metal conducts heat more effectively, so thermal systems that circulate heat are more effective in aircraft wings. Also, polymer-based turbine blades are more likely to become covered in dust and insect collisions, which can alter the smoothness of the blade surface and slow down the water draining from the blade and promote ice formation.
Wind turbines are also more prone to encounters with freezing rain and other low-altitude environments with high water content, such as B. Ocean spray for offshore wind turbines.
Most of the common icing and deicing methods for wind turbines remove ice build-up by using electrical heating or blowing hot air inside. Heating these massive areas, which are many times larger than airplane wings, increases the cost of the turbine and is inefficient and energy consuming. Composite-based turbine blades are also easily damaged by overheating. And there is another problem: water from melting ice can simply run back and freeze again elsewhere.
Another strategy in cold weather regions is to use surface coatings that repel water or prevent ice from adhering. However, none of the coatings was able to completely remove ice, particularly in critical areas near the leading edges of the blades.
A better solution
My team developed a novel method that uses elements of both technologies. By heating only the critical areas – especially the leading edges of the blades – and using water and ice repellent coatings, we were able to reduce the amount of heat required and the risk of water running back to freeze again over the blade surfaces. The result effectively prevents ice from forming on the entire surfaces of the turbine blades.
Compared to traditional brute force surface heating methods, our hybrid strategy also used much less electricity, resulting in energy savings of up to 80%. Without ice to slow it down, the turbines can produce more electricity in winter.
Almost 800 gigawatts of wind power have been installed worldwide to date, over 110 gigawatts of which in the USA alone. As the market grows rapidly and wind power replaces more polluting energy sources, deicing and ice protection strategies are becoming increasingly important.
Hui Hu, professor of aerospace engineering, Iowa State University
This article is republished by The Conversation under a Creative Commons license. Read the original article.
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