In the lemming-like world rush to build these uneconomic monstrosities (where only huge, often hidden, tax-payer subsidies make them viable) there are numerous reasons why they should not be built. This Blog concentrates only on the potential damage to peoples’ health.
Not all wind farms cause a problem with noise. Earlier examples which were built in remote parts rarely caused problems but as these sites became used developers looked more and more towards developed areas and thus came ever closer to human habitation.
There are two potential sources of noise: that from the turbine blades passing through the air at the speed of a light aircraft and that from the gearbox and generator in the nacelle (or housing). The aerofoil blade (usually three) the length of a jumbo jet’s wing whose tip travels at 150 miles per hour (241 km/h) and harvesting 0.6 MW or more of power inevitably makes a substantial sound. The air passing through the rotor is swept into turbulent wake vortices (which, incidentally, attract, trap and kill many bats in a process akin to the “bends” in divers) is the source of much of the sound. Within a few feet it encounters an obstruction in the form of the tower as a blade passes every one to two seconds, even at its slowest viable turning speed. This imposes a pulsating quality to the aerodynamic sound which many people find deeply disturbing.
Other periodic sounds arise as the blades sweep down into the region of wind shear so that the lowest blade position experiences both a different wind speed and varying turbulence. Some developers state categorically that, “Noise isn’t a problem” but they rely on ETSU-R-97 which is not a fit instrument to assess it.
Clusters of windfarms can produce further interaction of sound periodicity as the rotors of different machines go into and out of phase creating the periodic sounds (aerodynamic or amplitude modulation) generating the “whoomph, whoomph” at one or two second intervals rising and falling in loudness – an effect which disturbs some people and which can be likened to the base “woofer” speaker in a sound system.
Furthermore, as well as the normally audible sounds machines produce a low frequency sound from vibrations which ranges from the barely audible “sub-woofer” frequencies below 200 Hz down to wavelengths which cannot be heard but often sensed as bodily discomfort (below 20 Hz) and often referred to as infrasound which is difficult to measure instrumentally but which effects ALL animals, including horses, cattle and sheep.
Why ETSU-R-97 isn’t fit for purpose
Noise is measured in decibels (dB) which is a logarithmic measure of sound pressure levels in the air and shown as a ratio to a reference pressure. An increase of, say, 3dB doubles the noise level.
Environmental noise levels are usually expressed in dB(A) which includes a correction for the frequencies best heard by the human ear. Unfortunately this biases against the low frequency end of the spectrum and fails where low frequencies such as repetetive bass notes in music and the “whoomph” of turbine blades are important.
The C weighting curve [dB(C)] is better as it is less selective against low frequency but ETSU-R-97 mandates dB(A) rather than dB(C) – as do many other sound measuring conventions.
In the Whinash Enquiry (2005) acoustic consultant R. Bowdler stated, “ETSU-R-97 Why It Is Wrong”:
The conclusions of ETSU-R-97 are so badly argued as to be laughable in parts (the daytime standard is based on the principle that it does not matter if people cannot get to sleep on their patio so long as they can get to sleep in their bedrooms. [But what about night workers? —BJ] It is the only standard where the permissable night time level is higher than the permissable day time level.
He further comments, in Para. 1 of the Executive Summary:
This document describes a framework for the measurement of wind farm noise and gives indicative noise levels thought to offer a reasonable degree of protection to wind farm neighbours without placing unreasonable restrictions on wind farm development or adding unduly to the costs and administrative burdens on wind farm developers or local authorities.
Furthermore, in technical publications, writers such as G.P. Van den Berg’s investigation of a Dutch wind farm of variable speed turbines (10-20 rpm) found that complaints about noise, especially at night, extended to 1.9 km whereas the developer had claimed that there would be no problem over 0.5 km (2004, “Effects of the wind profile at night on wind turbine sound” – Journal of Sound and Vibration, 277, 955-970.)
He also showed that the night time wind speed at hub height was up to 2.6 times higher than expected from the conventional extrapolation from wind speed measured at 10m height. The higher wind speed causes faster rotation and up to 15 dB higher sound levels relative to the same reference wind speed during the day.
He concluded that day or night the background noise did not effectively mask the thumping sound of the blades passing the tower. More telling is the fact that the thumping is not perceptible close to the turbines but only at a distance.
Shadow Flicker – The Stroboscopic Effect
When the sun passes behind the hub of a turbine shadows of the rotating blades are repeatedly cast over nearby properties. The seasonal timing and duration of this flickering can be calculated from the size, relative alignment of the site and its geographical position.
The flicker rate from a typical three-bladed turbine is below three per second (at 60 rpm) so photosensitive epilepsy from any stroboscopic effect is unlikely. The problem arises when there are several such turbines in alignment with any habitation whereby the shadows of the blades can inter-react with one another causing a cumulative flicker of over 3 per second. Flicker rate is not affected by distance so any risk does not significantly decrease until it reaches about 100 times the hub height – 10 km approx. for a big turbine; (Harding et al, 2008)
Siting turbines so that they catch the early morning or late evening sun can compound the problem by silhouetting.
As recently as March 2011, under the head-line, “Flicker of Hope for wind-turbine victims” the UK national daily press reported (inter alia) that, “The misery of shadow flicker, which blights the lives of people living near some wind turbines, could soon be over.” It goes on to say that the report commissioned by the Department for Energy and Climate Change recommended that turbines should not be built any closer that 10 rotor diameters from the nearest homes. Translated this means that an 80m blade span (262′) should be at least 800m or half a mile away. Currently, in France for example, they are proposing and have built them within 400m of habitation.
The report recommended that buildings within a third of a mile should not suffer flicker for more than 30 minutes a day or 30 hours per year.
The paper reports one Lee Moroney, a wind energy expert with the Renewable Energy Foundation, as saying “that wind turbines should not be built within a mile of residential areas”.
Les turbines de vent peuvent être mauvaises pour votre santé
This article is the work of the author(s) indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.
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