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Wind Shear and Wind Turbine Noise FAQ

by Chris Kapsambelis

Introduction

The wind industry relies on wind shear is an important factor in the generation of wind energy. It is the main reason why wind turbines are taller than the tallest building. The tall designs are able to capture the stronger winds aloft during extreme wind shear conditions. Extreme wind shear is also responsible for worst case noise emissions that are likely to be out of compliance with existing noise pollution regulations.

What is Wind Shear?

Wind shear is the difference in wind speed by height. The higher the wind shear, the higher the wind speeds aloft when winds are close to calm on the ground.

How Is Wind Shear Measured?

Anemometers are erected at two different heights above ground, typically at 10 meters and at 40 meters (or more) above ground. The 10-minute wind speed average is logged simultaneously at each height for as long as a year.  The formula   α= ln(V2/V1)/ln(H2/H1) has been developed to estimate the wind speed at hub height (80 to 100 meters) once the coefficient or exponent α has been determined from actual measurements at lower levels.

Why is Wind Shear important in relation to wind turbine noise?

Extreme wind shear conditions are when ambient is at its lowest while wind turbine noise is at maximum, and is the most likely condition for the noise to increase the broadband sound level more than 10 dB(A) above ambient, resulting  in violations of the MassDEP noise pollution regulation. The MassDEP regulation considers any new noise source  a violation when the new noise source increases the broadband sound level 10 dB(A) above ambient. The DEP defines ambient as the sound level exceeded 90% of the time. The MassCEC-funded WIND TURBINE RESEARCH STUDY  has identified wind as the largest contributor to ambient sound during their measurements. The higher the wind speed on the ground the higher will be the ambient sound level. The higher the ambient sound level the less likely it is that wind turbine noise will result in a violation. At times of extreme wind shear, the wind at hub height can be strong enough to emit maximum wind turbine noise, while on the ground there is little or no wind to increase the ambient sound level.

How is Extreme Wind Shear Defined?

There is no accepted definition of extreme wind shear. The typical wind shear condition results in a ratio of 1.5:1 between hub height wind speed and ground level wind speed. If the wind speed on the ground is 1 meter/second, the speed at hub level would be 1.5 meters/second. In extreme wind shear conditions that ratio grows to 8:1 or more, which indicates a hub level speed beyond 8 meters/second, while on the ground the speed remains below 1 meter/second. Wind turbines are designed to generate maximum power when the wind speed at hub height ranges from 8 to 12 meters/second; this is also understood to produce maximum sound power levels.

How Does Wind Shear Change Day to Night, Month to Month?

Wind Shear is generally more severe at night than in the daytime. The graph below shows nighttime wind shear collected in 50 meter height winds between 6 and 8 meters/second. This is the wind range where wind turbines generate maximum noise.

Extreme wind shear conditions are more common at night between late Spring and early Winter.

The source of the data is from the Savoy wind shear data from the RERL website:

http://www.ceere.org/rerl/publications/resource_data/Savoy/

How Often Do Extreme Wind Shear Conditions Occur?

Analysis of wind shear data collected by RERL in advance of siting the Fairhaven wind turbines yielded the following results:.

  • Number of days/year where ground wind is less than 1 m/s and Hub wind speed is more than 8 m/s = 163
  • Days/year  when extreme wind shear lasts longer than one hour = 112
  • Days/year  when extreme wind shear lasts longer than two hours = 77
  • Days/year  when extreme wind shear lasts longer than three hours = 42

How does the “MassCEC Acoustic Study Methodology for Wind Turbine Projects” Use Wind Shear?

MassCEC allows developers to estimate hub height wind speed using an appropriate wind shear coefficient for the site. Developers will choose a value that represents the average or typical value resulting in a hub height to ground wind speed ratio of less than 2:1. Since wind measurements are generally made at a height of 10 meters above ground, the projected  ambient sound value shows a false trend of increasing ambient sound with the projected increasing hub height wind speed. The effects of extreme wind shear, which can occur for prolonged periods particularly at night, are not considered.

How Would the Recognition of Extreme Wind Shear Change the MassCEC methodology?

In extreme wind shear conditions the wind turbine can emit maximum sound power while the ground level winds are less than 1 meter per second (essentially calm). Under those conditions the correlation between of projected hub height wind speed and ambient sound level increase  is irrelevant. During extreme wind shear at night, the only relevant ambient sound levels are those measured with ground level winds less than 1 meter per second, and the only relevant estimates of impact sound levels are those that occur while the wind turbine is emitting maximum noise at hub height wind speeds above  8 meters/second

How Does Extreme Wind Shear Affect Wind Turbine Noise Measurements?

MassDEP noise measurements are taken when hub level wind conditions are within the operating parameters of the wind turbine. The ambient is established by taking measurementsamples with the turbine off shortly before and/or after before wind conditions change. The effect of wind shear is left to chance. The likelihood of being able to measure under worst case conditions in extreme wind shear, when ambient sound levels are at their lowest and turbine noise is maximum, is very low.

What Is The Spread Range of Ambient Sound Measurements between Typical and Extreme Wind Shear Conditions?

The ambient sound level reading can vary as much as 20 dB(A). Given that the MassDEP limit for impact noise is 10 dB(A) above ambient, ignoring extreme wind shear, the current MassDEP testing protocol avoids the detection of a substantial number of violations.

What Would Happen If MassDEP Measured Noise in Extreme Wind Shear Conditions?

The number of violations would skyrocket.

How Can the MassDEP Change the protocol To Account for Extreme Wind Shear?

Since testing is based on hub level wind speeds, wind shear does not affect wind turbine sound power generation.  On the other hand, wind shear can change the value of ambient by 20 dB(A) or more. There is no valid reason for the turbine on/off measurements to be made at nearly the same time. Ambient should be established with ground winds less than 1 meter/second and turbine noise should be measured when the wind turbine is generating maximum noise.

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