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Wind Turbines: really green?

Editorial Staff

They were claimed to be the solution to fossil fuels. Acres of land and sea have been devoted to the alien structures that make even power-grid towers seem inconspicuous. Now about two decades into what was the future, has the promise of Wind Turbines turned into reality? Are there unintended consequences and are they good or bad?

In the mid-1990s, the tech was poorly designed and developed: the primary point of failure ( if one leaves out the fact that they need more wind than expected and are not particularly good at turning to maximise response to wind) has long been the gearbox. Put simply, they put in a boy to do a man's job. There were critical design issues with the first generation of gearboxes which were simply bought in items designed for generic industrial purposes. Their failure rate was prodigious, leaving windmills standing still or spinning idly. The second generation, with a design life of 20 years, rarely make it to ten years, according to the Wind Power Energy website. That analysis says "wind-turbine drive-trains undergo severe and variable transient loading during start-ups, shut-downs, emergency stops, and grid connections. A turbine’s loading depends on its location in the wind farm and the terrain. Load cases that result in torque reversals may be particularly damaging to bearings because rollers may skid during the sudden relocation of the loaded zone. Micro-pitting, a form of surface fatigue, is one example of damage in bearings that can affect its longevity." - in short, the life of the technology is largely unpredictable because the environment in which it operates is largely unpredictable. But the serious shock in that article is that "the probability of one or more bearings failing within seven years is 37%"

The bearings are critical components (as are most of the parts that make up the so-called drive-chain). Bearings are the bits that allow one shaft to rotate inside another. Think of your vacuum cleaner tubes: you put the small one inside the big one and its fits snuggly and doesn't spin around. But what if you wanted one of the two tubes to rotate relative to the other? You could grease the joint but that wears dry easily and then seizes up. Or you could insert a ring inside the big tube and insert a smaller internal tube. That ring can be low friction or, for heavier duty applications, it's device that is, itself, made up of two rings between which are steel balls. The balls are greased and, usually, there is a sealed unit to keep out dirt and moisture. There are other types of bearing, for example roller bearings, but they all serve the same function in a broadly similar way. You come across bearings most often in your mode of transport: they are the parts that allow the wheels to turn in cars, buses, etc.

An article in Machine Design magazine explains the principle causes of failure for bearings: "A bearing that has been damaged due to excessive force or shock loading during assembly, or is fitted too tight or loose may cause the device to perform below expectations...Assemblers should (inter alia) account for the effect of differential thermal expansion when establishing shaft or housing sizes. The partial table of recommended fits assumes stable operating conditions, so when thermal gradients are present, or dissimilar materials are being used, room temperature fits must be adjusted to attain the proper operating temperature."

That doesn't happen in a wind turbine because it must operate in a wide range of temperatures and humidity. It must also cope with shocks in high-wind conditions: bearings are designed to let tubes go round and round, not to act as shock-absorbers when the weather has a hissy-fit. And those hissy-fits are inevitable precisely because the windmills are put on sites that are, it is said, the places where the most wind will be caught. It seems, however, that part of the reliability problem is due to there being the wrong kind of wind.

 


 

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