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Sunday, July 18, 2010

Wind turbine Effects

Some of the detrimental effects reported for wind turbines are hugely exaggerated and misrepresented while some may have some truth in them. Lets have a look at a few of them.

Visual Pollution
I suspect there are more of us that like the look of wind turbines, both for their elegant shape and for what they represent than the contrary. Certainly a lot of people in the crowded areas of the world with air pollution, heat waves and a fear of their coastal cities being flooded by rising sea level, look with great favor on wind turbines. Here in New Zealand, we are somewhat spoiled. We sometimes seem to believe that we are immune from the effects of climate change and that overpopulation will never darken our shores.  Good luck to us.

Shadow Flicker

Shadow Flicker is seen when the sun, the wind turbine and the observer are in a direct line and the sun is interrupted as the blades turn. I'll use my own situation here in Waipara, New Zealand as an example. You can apply the principles to your own area. There are plans to build a multi-turbine wind farm on the ridge to the East of Waipara. The ridge is 5000 meters from the town. We are located at 45degrees south and, as the year advances, the sun rises over the ridge at different locations. In the summer it rises roughly 25degrees south of a direct line to the ridge and in the winter at about 25 degrees north of a direct line to the ridge. Throughout the year, then, there will be days when my house is exactly lined up with the sun and one of the wind turbines early in the morning when the sun rises. Here we come to the first point.

The sun 'moves' a quarter of a degree per day south  between the summer solstice and the winter solstice and a quarter of a degree North each day, between the winter solstice and the summer solstice. With a wing span of 90m at a range of 5000m, the wing span of the turbine, from where I see them, is just over 1 degree. If there is any shadow flicker it will occur for 4 days as the sun moves south and 4 days as the sun moves north. A total of 8 days per year for each wind turbine which is visible from my house.

As mentioned, the arc of a 3mw turbine is just over 1 degree. The sun rises 15 degrees per hour or 15arc minutes per minute of time. Since the sun travels 15minutes of arc per minute of time, the flicker will last, where I live, for just over 4 minutes starting a few minutes after my apparent dawn and will only occur on those days when I am perfectly lined up with the sun and the turbine in question. Sofar, we  have a possibility of shadow flicker for 4 minutes at dawn for 8 days of the year for each wind turbine I can see from my house.

Lastly, what about the intensity of the shadow. If the sun was a point source and ignoring refractive edge effects, there would be a hard shadow cast at my house. However, the sun subtends 32 arc minutes(just over half a degree). As you move away from an object that shades the sun, there is a cone of darkness behind the object (umbra). When you get to a distance such that the object in question subtends 32 arc minutes, the object will just cover the sun. Further away, you will be able to see more and more of the sun on either side of the object. Stated another way, light from the opposite sides of the sun fills in the shadow behind the object. If you are familiar with eclipses here we are talking about the umbra and preumbra. Doing the same calculation that we used above, it turns out that a 3.5m wide blade (width of a typical blade of a 3mw generator at its root) will just cover the sun at 364m. Using the same calculation to see over what distance the shadow of the blade will be spread at my house, it turns out to be 46m. A 3.5m shadow is dispersed over 46m. Doing a little more math, we see that the intensity of the sun will decrease and increase 7%.

In short, there will be a 7% change in light intensity which will occur on those 8 days during the year when I am perfectly lined up with a turbine. It will last for 4 minutes and will occur at my apparent dawn. lets see if the rest of the arguments against wind turbines have a little more substance.

Glint occurs when the light from the sun, which is behind or to the side of the observer, is reflected off a blade into the eyes of the observer. If glint actually occurs, it should be relatively easily solved by using matte paint of a dark color.

Noise is the only one of the objections that seems to have the possibility of some validity.

The two bladed Gebes Pass turbine was noisy initially. Apparently the blades were broadcasting generator noise like a loud speaker. The solution was to put a sound absorbing coupling in the drive shaft. Most of the noise disappeared . Tonal noises have also being observed in early wind turbines. Bias cut gears (taken from automotive gear shifts) and isolation of the generator from the nacel solved this problem.

Blades swish as they travel through the air. This problem has been addressed by making sure that the trailing edge of the blades is sharp and that wind turbines are built with the blades upwind of the tower. Further work will undoubtedly further reduce this source of noise. If a house is close enough to a wind turbine to hear air born noise, one solution which has been found to be effective is to double glaze windows. Such retrofits should, of course, be undertaken by the wind farm.  It has been noted that when the wind is blowing, the swish of the wind through the trees to a large extent masks any swish from the blades.

A further decrease in noise is connected with the number of blades. Three bladed turbines are quieter than 2 bladed turbines.

Low frequency noise is another question. Frequencies below the 20cps detectable by the human ear can be disruptive to animals and if intense enough, can disturb humans. Such frequencies are hard to exclude from a building as they are not absorbed by conventional sound deadening materials and they can propagate through the ground. For instance, it has recently been found that Elephants communicate over long distances using 'infra sound' which they detect with their feet. Sound waves in the ground travel in peculiar ways. For instance, an earthquake, which in simply high intensity low frequency sound, can leave a nearby town unscathed and destroy one further away from the epicentre. There exists the possibility, however remote, that a dwelling could be effected by vibration from a wind turbine. Fortunately the solution is not difficult.

The first stage is detection. Instruments exist that can detect sound below the auditory frequency range of humans. First a reading is taken on the foundation of each turbine as it is commissioned to see which, if any, frequencies it is generating. Then at the location where it is believed that these vibrations are occurring, similar readings are taken. If there is any indication that vibrations are being transmitted from the wind farm to the location, all the turbines can be feathered to see if this stops the vibrations. Then the turbines can be started up one by one, to detect the offending unit or units.

Assume for the sake of the argument, that one or more turbines are found to be sending vibrations to a house. There are at least two possible solutions. The first solution is to determine where on the offending supporting tower the inter-nodes are located. Dampers are than attached at this (these) locations.

Alternately, the same sort of devices which are used to isolate buildings from earth quake vibrations can be used to isolate a tower from the ground. These consist of lead filled rubber pads.

The bottom line of all this is that humanity in general and New Zealand in particular is desperately in need of cutting its dependence on fossil fuel to the absolute minimum. That doesn't mean that people must put up with annoyance from sources of renewable energy including wind turbines. If legitimate problems arise from the installation of wind turbines, they must be addressed and solved. However, they must be genuine problems and not nimbiism.

What is needed here is a thorough, honest research effort by government to examine complaints adjacent to existing wind farms. Let the chips fall where they may. This must not be a cover up but a genuine examination to see if there are legitimate reasons for complaint. If it is found that, for instance, infra sound is being transmitted to a nearby house, work needs to be done to determine how to mitigate the annoyance. Once the work has been done, it must be widely published and used by wind farm developers. If it has been found that wind turbines cause legitimate annoyance, they must take the measures which have been found to mitigate the annoyance.

The uptake of wind farms is far too important for the future well being of New Zealand to allow Nimbiism to halt their development.  They are far too important to allow genuine concerns go unsolved.

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