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Do wakes, worn blades add to wind turbine noise?

Human impacts, News, Wind turbines Add comments

A couple of articles in this month’s Wind Power Engineering caught my eye.  What follows is more speculative than what you normally find on AEInews, but with that in mind, I encourage you to check the articles out yourself.

Both articles address basic issues regarding wind turbines: the wakes they create downwind, and normal wear and tear of turbine blades.  Neither one considers noise impacts at all, and I don’t have the engineering background that might allow me to make truly informed extrapolations.  But both certainly seem to be worth bearing in mind as project planners and managers address possible noise issues.  The main thing I wonder is whether either of these factors might make turbines slightly louder than expected, or than modeled under more ideal assumptions.

The first article covers a topic we’ve covered here at AEInews before: ongoing research into the turbulent wakes that stretch out downwind from wind turbines:

Today’s massive wind turbines reach into a complicated part of the atmosphere, Julie Lundquist expalins. “If we can understand how gusts and rapid changes in wind direction affect turbine operations and how turbine wakes behave, we can improve design standards, increase efficiency, and reduce the cost of energy.”

The second article focuses on routine maintenance of wind turbine blades, and explains some of the normal wear and tear that needs to be attended to.  It seems likely that at least some blade imperfections would add extra noise, which is of course largely caused by the airflow coming off the blades.

Traditionally, less attention has been paid to the repair and upkeep of turbine blades versus other components. Instead, preventive maintenance programs have focused on the internal mechanics of turbines due to the predictability of their maintenance requirements. Typical preventive maintenance plans for internal components fall into 3, 6, and 12-month work schedules. By nature, blade repairs are more difficult to plan. Blade damage can arise in manufacturing, transportation, and tower construction and erection. However, maintenance issues more often occur in the field from leading-edge erosion, weather, and other factors. A lack of predictability and historical data complicates preventive maintenance for blades.

Commercial turbines can have tip speeds of over 200 miles per hour. At these speeds, rain drops can take on the impact of small stones, and blowing sand has the erosion power of a plasma cutter. Studies have shown blade roughness and accumulated debris on the blades can reduce wind turbine performance by 5 to 30%. Blades that aren’t working efficiently can also create vibration that contributes to gearbox failures.

Joshua Crayton, a contractor who provides blade maintenance services, notes that regular inspections are especially important in windy seasons and following lightning storms. “Operators and owners are inheriting their wind farm assets and the responsibility of maintaining blades that are no longer covered by the (manufacturer) warranty,” he says. “Like any business, wind farm owners and operators typically run a lean staff and may not have an experienced maintenance technician in-house. Partnering with a service company can help them design a long-term, post warranty, preventive maintenance plan.”According to Crayton, a maintenance plan should be initiated before the warranty period expires. “A thorough internal and external blade inspection should be scheduled in the warranty period,” he says. “Once owners and operators take over care of a wind farm, these inspections should take place every two years. Personnel can conduct simple ground inspections while on-site, but there is no substitution for a close, visual examination performed uptower.”

PLUS: Two other short articles may also be of interest: one on design concepts for a 20MW turbine (today’s big ones are 2MW), and the other a very positive development, a 36MW battery designed to stablize output to the grid from wind farms.

 

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