Wind turbines built with serrated trailing edges are not necessarily new, and as more data about them is available, they are getting more popular among wind turbine operators. But, how do they work? What do they do? And, what negative consequences might they have when considering lightning strike damage? In this article, we’ll discuss how trailing-edge serrations improve performance and what we can expect from the lightning protection system.
First: What is a Serrated Trailing Edge on a Wind Turbine?
Serrated trailing edge technology is relatively simple – it’s a sawtooth pattern that helps improve airflow over the wind turbine blade, resulting in less turbulence, better aerodynamics and reduced noise as the blade cuts through the air. Research proves that these serrations work, so naturally more and more wind farms are retrofitting their turbines with them.
Noise is a serious concern because wind farms must meet noise ordinances to prevent undue harm to local wildlife and people. Meeting noise statutes, however, can be a challenge– and a financial hardship – as larger turbines turning at higher blade tip speeds means more power generation, lower generation cost and therefore, more profit.
How Do The Serrations Work?
As air flows over the turbine blade to the trailing edge, high pressure air from one side mixes with the low pressure air flowing over the other. This turbulent collision of the two airflows causes noise. Serrations on the trailing edge help the two flows of air to mix more gently, essentially folding the flows together with less turbulence. This reduces the sound produced by the wind turbine, and provides a slight boost in aerodynamic efficiency.
Reducing noise is financially important for the industry (more on why noise matters a bit later), and so significant resources are being devoted to the problem. In efforts to reduce noise, engineers took a page out of mother nature’s book and studied birds–most notably, the owl.
But why the owl?
Owl Wings Are The Model for Silent Airflow
Owl wings have been honed over time through natural selection for silent flight. The trailing edges of an owl’s wing are rough with fringes, showing a similarly serrated look that scientists have discovered is part of the reason they can fly and hunt with such stealth. As air flows over their wings, it’s smoothed and quieted by a number of different factors, as explained in the excellent short video below.
Unsurprisingly, the aviation industry figured this out early and has applied it to another major noise problem: roaring jet engines.
A Serrated Trailing Edge Also Reduces the Noise of Jet Engines
Serrated trailing edges on jet engines, known as chevrons, help reduce the tremendous blast noise that’s potentially damaging to the ears of everyone involved in aviation – pilots, mechanics, runway workers, crew, passengers, local wildlife, and surrounding communities. Reducing noise is a major challenge at airports, and all new airport expansions are subject to environmental impact analyses that include noise projections and the effect on neighboring communities and wildlife.
The brilliant engineers at NASA were instrumental in helping to bring this design feature to the aircraft industry (spoiler alert: the same owl wing principle applies.)
Research on nozzle airflow mixing has been devoted to determining the optimal shape and size of chevrons to yield the highest sound reduction benefit. Some planes have serrations at the trailing edge of both the nacelle (shown above) and the nozzle (the nozzle shown above does not have serrations, however).
For a deeper dive on chevrons and the need to reduce jet engine noise, check out the video below.
It’s Not Just Owls, Either…Whales Have Inspired New Leading Edge Prototypes
This article about the Whalepower Corporation details how bumps on the leading edge (not the trailing edge, as we’ve been focused on thus far) of a wind turbine helps improve aerodynamics and efficiency. Beyond just the owl, other animals like the humpback whale are inspiring improved aerodynamics as scientists take a page out of mother nature’s book. Mother nature is quite smart, so this really shouldn’t surprise anyone that human engineers continue to learn from our animal friends.
The Whalepower corporation claims that their patented wind turbine leading edge design–inspired by the fins of the humpback whale–should become the standard for the wind industry. The bumpy ridges of their prototype blades improve flow and aerodynamics significantly over traditional wind turbine blades.
In the quest to increase aerodynamic and power efficiency, it seems fitting that these wind farms–that are planted within the environment themselves–should take significant inspiration from the nature. As of today, it’s unlikely that you’ll see Whalepower turbine blades spinning in a wind farm near you, however, they are one of several very promising designs in testing as engineers seek every advantage in the quest for cheaper, cleaner energy. As designs prove their effectiveness and reliability to the wind industry – and feasibility to manufacturers – they will make it to market, and the best will become standard equipment.
Turbines Often Have Speeds Throttled So They Meet Noise Standards…and Throttled Turbines Produce Less Power.
Wind turbine noise is considered by many to be a health hazard, especially in areas more densely populated by humans and wildlife. Thus, noise ordinances and regulations in the wind industry abound. Common ways to meet noise standards are by limiting the maximum speed a turbine can spin or limiting the daily number of hours it can operate. While both measures help meet noise regulations, they also hurt profitability and are thus not desirable outcomes for wind farm operators.
The best possible outcome would be a silent wind turbine, which would allow the turbine to be run 24/7 at speeds yielding maximum power production. Silent operation probably isn’t possible; some turbine noise likely will always be present. Noise reduction technology is focused on reducing sound output to acceptable levels so that the public is welcoming of new wind farm projects instead of viewing them as nuisances. The increased speed capability and improved aerodynamics are also important pieces of the puzzle.
But Does a Serrated Trailing Edge Affect Lightning Strike Damage?
Unfortunately, the answer in many cases is yes. These serrations change the way lightning interacts with the turbine blade.
The reason is a bit complex, but what happens is that the serrations, though they’re made of non-conductive materials, cause electrical charge to build up on them. Since these serrations span almost the entire length of the turbine blade and stop just before the blade tip, this means that electrical charge is forming away from the lightning receptor, which is obviously at the tip.
For a deep dive on lightning strikes and the threat they pose to blades outfitted with “dino tails” or serrated trailing edges, watch the video below with lightning protection expert, Allen Hall.
Want to Learn More About Wind Turbine Power Curve Upgrades?
Here at Weather Guard, we aim to be a great resource for lightning protection knowledge and other operations and maintenance topics in wind power. Check out more of our articles, videos and podcast content below.
- Leading Edge Erosion: a BIG Problem for Wind Farm Operators
- The Uptime Wind Energy Podcast
- Our YouTube Channel with lots of great videos on wind turbine tech
- More Great Articles are Found Here
Making Blade Repairs or Power Curve Upgrades? Upgrade Your Wind Turbine LPS at the Same Time
If you operate or repair wind turbines and have maintenance ahead of you, don’t forget about your lightning protection system (LPS). Our StrikeTape segmented lightning diverters are the answer to your LPS upgrade or repair, as our lightning protection will outlast anything mother nature can throw at it.
We build and test our StrikeTape diverters to ensure quality, durability and reliability, so trust us to help keep your wind farm running for years to come.
