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Lightning bolts love wind turbines a little too much  

Credit:  by Scott K. Johnson - Jan 31 2014, arstechnica.com ~~

The engineers and operators who run wind turbines are acutely aware that one can easily have too much of a good thing. Windy days mean lots of generation, but the turbines can be severely damaged if the wind velocity is too great, so they must be carefully managed. That’s not the only thing a wind turbine has to look out for, though. Being such tall objects, they stick their heads high above their surroundings, and in doing so they are likely targets for lightning strikes. Most of the time, that electrical energy is safely carried to the ground with the turbine no worse for the experience, but certain types of powerful lightning strikes can result in costly damage.

In fact, wind turbines seem to attract more than their fair share of lightning damage as compared to buildings and towers of a similar height. This has prompted research into why that might be and how turbines could be better protected. The distinguishing characteristic of a wind turbine as compared to, say, a cellular tower is obviously the giant, whirling blades. So what are the blades doing?

To see what was going on, Joan Montanyà and Oscar van der Velde of the Universitat Politècnica de Catalunya in Barcelona and Earle Williams of MIT set up an array of radio sensors spaced kilometers apart around an area with several wind farms in Spain. The system maps the location of lightning radio emissions in three dimensions.

The array caught several interesting phenomena in the act. On a few occasions, periodic flashes about three seconds apart were detected over wind turbines—in one case lasting for over an hour. These turned out to be very low-energy discharges that sparked upward each time a blade of the turbine swept past the high point.

Most of us are familiar with cloud-to-cloud and cloud-to-ground lightning, but the sensors picked up a fairly extraordinary ground-to-cloud-to-ground strike during one storm. For most cloud-to-ground strikes of tall objects, fingers of positive charge called “leaders” often travel upward from the object before reaching a region of negative charge in a cloud. Current then courses downward through the path of that leader, through the object being struck, and into the ground.

In this case, a negative leader reached upward from the turbine and into an area of positive charge about 5 kilometers up. This is the kind of strike that can damage turbines so badly. In this case, however, there were both positive and negative charges interacting in the clouds, and the downward strike lashed out laterally, striking the ground fully 20 to 25 kilometers from the turbine.

The researchers also captured some high-speed footage of lightning striking some nearby wind turbines, seen below. Of interest here is the fact that several turbines get involved in a single strike. Positive upward leaders jump from three different turbines at the same time, with one becoming the lucky recipient of the downward strike. Close examination also shows small, failed leaders extending from the tips of three other turbines. This shows that the turbines weren’t very well isolated from each other electrically.


High-speed video of a wind turbine getting a jolt.
Video by courtesy of Joan Montanyà and the American Geophysical Union.

So what are these turbine blades doing to deserve all this electrical abuse? For starters, the materials they’re made of are electrical insulators. Remember the glass rod your science teacher rubbed with a silk cloth to give it a static charge? Insulating materials can’t easily dissipate charge that builds up. The blades rub against air and precipitation while whooshing about their business, racking up charge, and more is picked up by induction when charged clouds are overhead.

Second, the researchers think the motion of the blades allows them to outrun their corona—the ionized air that surrounds a charged object. For a stationary object, that corona acts as a sort of buffer that dampens the electric field. By escaping the sheath of ionized air, moving objects become more likely to experience an electrical discharge. That’s part of what makes small rockets trigger lightning, a phenomenon that’s used for research work.

The researchers say this lays out some ideas for future studies to pin down. This work will hopefully lead to better ways of protecting wind turbines from lightning damage—through changes in construction or operation. For example, it could be that slowing or stopping the turbines during certain thunderstorm conditions would significantly lower the risk of damage. Unless Zeus just really hates renewable energy.

Journal of Geophysical Research: Atmospheres, 2014. DOI: 10.1002/2013JD020225.

Download original research paper: “Lightning discharges produced by wind turbines”

Source:  by Scott K. Johnson - Jan 31 2014, arstechnica.com

This article is the work of the source indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

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