Although last month’s widespread grid failure in Texas was caused by a number of factors, some lawmakers implied that frozen wind turbines were the main culprit. And while some of the state’s wind turbines did shut down due to the cold, the outage was largely caused by Texas utilities that neglected to winterize their natural gas supply and distribution network. Nonetheless, some people were left with the impression that wind turbines cannot be relied upon in cold climates when, in fact, colder places around the globe have wind farms that operate reliably throughout the winter, thanks to anti-icing and de-icing technologies.
Ice buildup on turbines can cause a number of problems, some of which can result in extensive damage to equipment. In order to obtain optimal energy production, turbines use anemometers and weather vanes to determine wind speed and direction. Icing on these instruments can cause significant measurement error, reducing the turbine’s electrical output. Ice accumulation on the blades can change the rotor’s aerodynamics and balance, affecting production and potentially causing permanent damage to the structure. And of course, chunks of ice flying off a blade present a safety issue. Fortunately, the aircraft industry has already laid the foundation for de-icing technology, as airplane wings and turbine blades are both airfoils with similar properties. Some engineers have adopted and modified those techniques, while others are designing solutions that are specific to the wind industry.
De-icing technology can be broken into two categories: passive and active. Passive systems don’t require power in order to function, while active systems consume electricity when operational. Each system has its pros and cons, so the best practice may be to attack the problem on multiple fronts by employing a combination of passive and active systems.
Passive De-Icing Systems
Many passive systems are preemptive; instead of removing ice, they prevent ice from accumulating in the first place. Taking a cue from nature, where bacterial biofilms prevent water from gathering on certain plant leaves, engineers have developed spray-on hydrophobic coatings that curtail water buildup. Many of these substances can also reduce the effects of insect collisions (preventing the bugs from sticking to the surface) and damage caused by fast-moving particles of sand and dust in the air—both of which affect the aerodynamic qualities of the turbine. Hydrophobic coatings are inexpensive and easy to apply but may require occasional reapplication. Although the coatings tend to perform well under mild icing conditions, they don’t handle severe icing.
Painting the turbine blades black, which is often used in conjunction with hydrophobic coatings, can help them to absorb more heat, reducing ice buildup in moderately cold conditions. On the other hand, the coatings can cause the blades to get too hot during the summer.
The aforementioned coatings are typically applied at the factory or can be retroactively applied to existing turbines in the field. In either case, reapplication may be necessary over time.
For wind farms that rarely experience icing, it may be more cost-effective to simply apply an antifreeze chemical to the turbines when weather forecasts predict severe cold. The most common way to do this is with helicopters. Yes, I’m aware of the irony of burning fossil fuels in a chopper to de-ice a piece of green technology, but the amount of greenhouse gas emitted by the helicopters is insignificant compared to the amount of pollutants saved by replacing fossil fuel power plants with wind farms. Plus, there are safer and more eco-friendly methods of airborne de-icing, such as using a heavy-lifting drone. Aerones developed a tethered drone whose 36 propellers can lift over 100 kg (220 lb) The drone carries a power cable and a hose that provide a virtually unlimited supply of energy and hot water/antifreeze, allowing the aircraft to run for as long as necessary. (Onboard batteries ensure a safe landing if the main power is disrupted.) The same system can be used for routine cleaning of the turbines, which is currently done by turbine-climbing technicians. The drones do the job for a fraction of the cost with no risk to personal safety. Here is Jonas Putrams, CEO of Aerones, explaining how it works:
Active De-Icing Systems
Passive systems may suffice in mild climates, but in locations where icing is heavier, the solution is to remove the ice with thermal or mechanical energy. This obviously consumes power, but it’s a small amount compared to the energy that a turbine produces. Nonetheless, an active system is only turned on when ice is detected, often using optical or ultrasonic sensors.
Many turbine manufacturers offer winterizing options that employ electric resistive heating along the leading edge of each blade. When ice is detected, the elements turn on to melt it. Nordex’s Advanced Anti-Icing System uses a resistive heating element that consumes no more than 125 kW per turbine. On a 4 MW unit, that’s about 3 percent of the turbine’s nominal production. Even with the turbine operating at 50 percent capacity factor, it’s still only 6 percent of the turbine’s available power. Not a bad trade-off, considering the heating elements only run occasionally and the potential power loss due to icing could be significantly higher.
If you think people who talk about wind turbine de-icing are full of hot air, you may be onto something. Borealis Wind offers a turbine winterization package that can be factory installed or retrofitted into turbines in the field. The system, which can be customized to fit various turbine models, includes a heating element, a blower and a duct to carry hot air. Borealis says the retrofit can be done without removing the turbine blades.
All components are small and light enough to be moved into the blade by technicians. The components are lifted to the nacelle using a service winch, a blade is pinned horizontally, and the components are mounted inside the blade. The process repeats for the remaining blades.
Borealis’s systems have been retrofitted on Senvion, GE and Siemens turbines currently operating in Canada. A research group at the Université Laval determined that the system reduces icing loss by at least 50 percent.
De-Icing Wind Turbines Using Solar Power
Researchers at MIT have developed a passive solar system that can be applied to a turbine blade or other surface. Made from three layers of inexpensive and readily available materials, the system absorbs sunlight, converts it to heat, and distributes it around the surface. No word yet on when the system will be ready for commercialization, but if it works as advertised, it could be a game changer.
Assessing the Need
Before utilities invest in de-icing technology, they must conduct a cost-benefit analysis to determine the likelihood of an icing scenario, the potential loss of power (and income) due to icing, and how much power loss can be prevented by a winterization package. At that point, they can determine which, if any, of the available systems are the most cost-effective given the climate and other factors.
Just as airplanes can stay aloft in frigid temperatures, wind turbines have proven that they can operate reliably in the coldest of climates, from the Arctic Circle to the South Pole. All you need is a little ingenuity … and a few lessons from the aerospace industry.