6/3/09 When the Minnesota Department of Health took a closer look at wind turbine noise and shadow flicker, what did they find? PART ONE
In March of 2009, the Minnesota Department of Health began an evaluation of health impacts from wind turbine noise and low frequency vibration, which resulted in the May 22, 2009 report called
"Public Health Impacts of Wind Turbines"
[Download the entire report by clicking here]
Here's our summary of the report:
(presented in two parts)
What are current setbacks based on?
At present, common setback distances from homes appear to be based on setbacks needed to optimize wind resources, rather than studies which consider wind turbine impacts on human health, safety, and well-being.
From Page 3: "[Wisconsin Power and Light] is required to develop a site layout that optimizes wind resources.
Accordingly, project developers are required to control areas at least 5 rotor diameters in the prevailing (north-south) wind directions (between about 1300 and 1700 feet for the 1.5 to 2.5 MW turbines under consideration for the project) and 3 rotor diameters in the crosswind (east- west) directions (between about 800 and 1000 feet).
Thus, these are minimum setback distances from properties in the area for which easements have not been obtained.
Why is a minimum setback of half a mile better for people?
Page 6: "The National Research Council of the National Academies (NRC, 2007) has reviewed impacts of wind energy projects on human health and well-being. The NRC begins by observing that wind projects, just as other projects, create benefits and burdens, and that concern about impacts is natural when the source is near one’s home.
Further, the NRC notes that different people have different values and levels of sensitivity.
Impacts noted by the NRC that may have the most effect on health include noise and low frequency vibration, and shadow flicker.
While noise and vibration are the main focus of this paper, shadow flicker (casting of moving shadows on the ground as wind turbine blades rotate) will also be briefly discussed.
[...T]he NRC concludes that noise produced by wind turbines is generally not a major concern beyond a half mile. Issues raised by the NRC report and factors that may affect distances within which wind turbine noise may be problematic are discussed more extensively below.
What are the elementary characteristics of sensory systems affected by wind turbine noise?
On pages 6-7 is a concise, technical description of how human hearing works, followed by a description of the vestibular system.
What is the Vestibular System?
P. 7- "The vestibular system reacts to changes in head and body orientation in space, and is necessary for maintenance of equilibrium and postural reflexes, for performance of rapid and intricate body movements, and for stabilizing visual images (via the vestibulo-ocular reflex) as the direction of movement changes.
[....]While vestibular system activation is not directly felt, activation may give rise to a variety of sensations: vertigo, as the eye muscles make compensatory adjustments to rapid angular motion, and a variety of unpleasant sensations related to internal organs.
In fact, the vestibular system interacts extensively with the “autonomic” nervous system, which regulates internal body organs (Balaban and Yates, 2004). Sensations and effects correlated with intense vestibular activation include nausea and vomiting and cardiac arrhythmia, blood pressure changes and breathing changes.
What is Sound?
Pages 8-9: Concise, technical description of audible frequency sound, and sub-audible frequency sound.
What is Resonance and Modulation and what does it have to do with wind turbine noise?
Page 8- "Sound can be attenuated as it passes through a physical structure. However, because the wavelength of low frequency sound is very long (the wavelength of 40 Hz in air at sea level and room temperature is 8.6 meters or 28 ft), low frequencies are not effectively attenuated by walls and windows of most homes or vehicles.
(For example, one can typically hear the bass, low frequency music from a neighboring car at a stoplight, but not the higher frequencies.)
In fact, it is possible that there are rooms within buildings exposed to low frequency sound or noise where some frequencies may be amplified by resonance (e.g. 1⁄2 wavelength, 1⁄4 wavelength) within the structure.
In addition, low frequency sound can cause vibrations within a building at higher, more audible frequencies as well as throbbing or rumbling.
Sounds that we hear generally are a mixture of different frequencies. In most instances these frequencies are added together.
However, if the source of the sound is not constant, but changes over time, the effect can be re-occurring pulses of sound or low frequency modulation of sound.
This is the type of sound that occurs from a steam engine, a jack hammer, music and motor vehicle traffic.
Rhythmic, low frequency pulsing of higher frequency noise (like the sound of an amplified heart beat) is one type of sound that can be caused by wind turbine blades under some conditions.
What is the human response to low frequency stimulation?
P. 10- concise, technical description of response to low frequency stimulation.
How is sound measured?
P. 10-11 Concise, technical description of how sound is measured.
What kinds of noise do wind turbine make?
P.11-10 " 1. Mechanical noise: Mechanical noise from a wind turbine is sound that originates in the generator, gearbox, yaw motors (that intermittently turn the nacelle and blades to face the wind), tower
ventilation system and transformer.
Generally, these sounds are controlled in newer wind turbines so that they are a fraction of the aerodynamic noise.
Mechanical noise from the turbine or gearbox should only be heard above aerodynamic noise when they are not functioning properly.
2. Aerodynamic noise Aerodynamic noise is caused by wind passing over the blade of the wind turbine.
The tip of a 40-50 meter blade travels at speeds of over 140 miles per hour under normal operating conditions. As the wind passes over the moving blade, the blade interrupts the laminar flow of air, causing turbulence and noise.
Current blade designs minimize the amount of turbulence and noise caused by wind, but it is not possible to eliminate turbulence or noise.
Aerodynamic noise from a wind turbine may be underestimated during planning.
One source of error is that most meteorological wind speed measurements noted in wind farm literature are taken at 10 meters above the ground.
Wind speed above this elevation, in the area of the wind turbine rotor, is then calculated using established modeling relationships. In one study (van den Berg, 2004) it was determined that the wind speeds at the hub at night were up to 2.6 times higher than modeled.
Subsequently, it was found that noise levels were 15 dB higher than anticipated.
Why is wind turbine noise more annoying than other noise, why do they sometimes make a thumping sound and what can be done about it?
P.12-13 "Rhythmic modulation of noise, especially low frequency noise, has been found to be more annoying than steady noise (Bradley, 1994; Holmberg et al., 1997).
One form of rhythmic modulation of aerodynamic noise that can be noticeable very near to a wind turbine is a distance-to-blade effect.
To a receptor on the ground in front of the wind turbine, the detected blade noise is loudest as the blade passes, and quietest when the blade is at the top of its rotation. For a modern 3-blade turbine, this distance-to-blade effect can cause a pulsing of the blade noise at about once per second (1 Hz).
On the ground, about 500 feet directly downwind from the turbine, the distance-to-blade can cause a difference in sound pressure of about 2 dB between the tip of the blade at its farthest point and the tip of the blade at its nearest point (48 meter blades, 70 meter tower). Figure 5 demonstrates why the loudness of blade noise (aerodynamic noise) pulses as the distance-to-blade varies for individuals close to a turbine.
If the receptor is 500 feet from the turbine base, in line with the blade rotation or up to 60° off line, the difference in sound pressure from the tip of the blade at its farthest and nearest point can be about 4-5 dB, an audible difference.
The tip travels faster than the rest of the blade and is closer to (and then farther away from) the receptor than other parts of the blade. As a result, noise from other parts of the blade will be modulated less than noise from the tip.
Further, blade design can also affect the noise signature of a blade.
The distance-to-blade effect diminishes as receptor distance increases because the relative difference in distance from the receptor to the top or to the bottom of the blade becomes smaller. Thus, moving away from the tower, distance-to-blade noise gradually appears to be more steady.
Another source of rhythmic modulation may occur if the wind through the rotor is not uniform. Blade angle, or pitch, is adjusted for different wind speeds to maximize power and to minimize noise. A blade angle that is not properly tuned to the wind speed (or wind direction) will make more noise than a properly tuned blade.
Horizontal layers with different wind speeds or directions can form in the atmosphere. This wind condition is
called shear. If the winds at the top and bottom of the blade rotation are different, blade noise will vary between the top and bottom of blade rotation, causing modulation of aerodynamic noise.
This noise, associated with the blades passing through areas of different air-wind speeds, has been called aerodynamic modulation and is demonstrated in Figure 5. [above]
In some terrains and under some atmospheric conditions wind aloft, near the top of the wind turbine, can be moving faster than wind near the ground.
Wind turbulence or even wakes from adjacent turbines can create non-uniform wind conditions as well.
As a result of aerodynamic modulation a rhythmic noise pattern or pulsing will occur as each blade passes through areas with different wind speed.
Furthermore, additional noise, or thumping, may occur as each blade passes through the transition between different wind speed (or wind direction) areas.
What is wind shear, what does it have to do with turbine noise being louder at night, and why are levels of wind turbine noise often underestimated?
P.13- Concise, technical description of wind shear, turbine noise at night, and calculation of noise preditictions.
Why does the noise from multiple turbines seem louder and why does it sometimes seem to have a beat or pulse?
P.14- "The noise from multiple turbines similarly distant from a residence can be noticeably louder than a lone turbine simply through the addition of multiple noise sources.
Under steady wind conditions noise from a wind turbine farm may be greater than noise from the nearest turbine due to synchrony between noise from more than one turbine (van den Berg, 2005).
Furthermore, if the dominant frequencies (including aerodynamic modulation) of different turbines vary by small amounts, an audible beat or dissonance may be heard when wind conditions are stable."
What is wind turbine shadow flicker, how long can it last, and why is it a problem?
P.14 Rhythmic light flicker from the blades of a wind turbine casting intermittent shadows has
been reported to be annoying in many locations (NRC, 2007; Large Wind Turbine Citizens Committee, 2008). (Note: Flashing light at frequencies around 1 Hz is too slow to trigger an epileptic response.) [Note from the BPWI Research Nerd: Read more about epilepsy and wind turbine shadow flicker clicking here]
Modeling conducted by the Minnesota Department of Health suggests that a receptor 300 meters perpendicular to, and in the shadow of the blades of a wind turbine, can be in the flicker shadow of the rotating blade for almost 11⁄2 hour a day.
At this distance a blade may completely obscure the sun each time it passes between the receptor and the sun.
What minimum setback will prevent shadow flicker on a home?
With current wind turbine designs, flicker should not be an issue at distances over 10 rotational diameters (~1000 meters or 1 km (0.6 mi) for most current wind turbines).
This distance has been recommended by the Wind Energy Handbook (Burton et al., 2001) as a minimum setback distance in directions that flicker may occur, and has been noted in the Bent Tree Permit Application (WPL, 2008).
Shadow flicker is a potential issue in the mornings and evenings, when turbine noise may be masked by ambient sounds.
While low frequency noise is typically an issue indoors, shadow flicker can be an issue both indoors and outdoors when the sun is low in the sky. Therefore, shadow flicker may be an issue in locations other than the home.
Ireland recommends wind turbines setbacks of at least 300 meters from a road to decrease driver distraction (Michigan State University, 2004). The NRC (2007) recommends that shadow flicker is addressed during the preliminary planning stages of a wind turbine project.
[PART TWO TO COME]
Click on the image below to see what wind turbine shadow flicker looks like: