Experts from Wardell Armstrong discuss the importance of allowing for wind direction when assessing the noise impacts of windfarm developments
Wind turbines emit noise as they rotate to generate power. This only occurs above the “cut-in” wind speed and below the “cut-out” speed. Below the cut-in wind speed there is insufficient strength in the wind to generate electricity efficiently and above the cut-out wind speed the turbine is automatically shut down to prevent any malfunctions from occurring. The cut-in wind speed at turbine hub height is normally 2.5–3.5mper second (m/s), and the cut out wind speed is normally around 22–28m/s.
The principal sources of noise are from the blades rotating in the air (aerodynamic noise) and from internal machinery (mechanical noise) – normally the gearbox and, to a lesser extent, the generator.
The blades are designed to minimise noise emission, while optimising power transfer from the wind. The hub at the top of the tower is insulated to minimise noise radiation from the gearbox, generator and other components, which are also isolated from the tower and the blade assembly to prevent structure-borne noise.
Many modern turbines are pitch regulated turbines, with a noise output that does not increase once their rated power is reached. Older stall-regulated turbines had a noise output which tended to continue to increase with increasing wind speed.
It is possible to manage the noise output of modern turbines with a sophisticated sector management system that allows the operational mode of the turbines to be changed for specific wind speed, direction conditions and time of day. This ensures that, should a noise issue arise, the turbines can be effectively managed to resolve the impact. The prediction of the effects of wind direction on the propagation of noise towards receptors can now be assessed.
Noise impact criteria
The acceptable limits for wind turbine operational noise are clearly defined in ETSU-R-97, The assessment and rating of noise from wind turbines. Consequently, the test applied to operational noise is whether the calculated windfarm noise emission levels at nearby noise sensitive properties lie below the noise limits derived in accordance with ETSU-R-97.
Noise predictions
Assessment of noise levels generated at sensitive receptors, as a result of a windfarm development, and any cumulative effects that may exist, are generally performed according to the methodology in ISO 9613-2, Attenuation of sound during propagation outdoors, which is the preferred method of predicting wind turbine emission levels.
The general approach for the assessment is as follows:
- Agree an appropriate reference sound power level and appropriate wind shear factors for the proposed turbine type.
- Predict received noise levels at receptors, corrected for tonal noise emission.
- Compare predicted levels with agreed noise limits.
The ISO 9613-2 model for predicting noise propagation includes attenuation for atmospheric absorption, ground effects, geometric spreading and barriers which include terrain.
Noise predictions take a received height to be 4m, across mixed ground types (G=0.5) and have an air absorption based on 70% humidity and temperature of 10°C. All receptors are considered to be downwind of the turbine for calculation purposes.
However, it is unlikely that all receptors will, in reality, be downwind of all the turbines at all times, therefore a correction for turbine directivity should be applied.
The effect of wind direction on the contribution from each turbine will vary depending on the location of the receptor in relation to the turbine. The directional function is determined from the location of the receptor in relation to the wind direction, which could be downwind, crosswind or upwind.
To calculate the effects due to the wind direction, a model has been derived which evaluates the potential reduction in noise levels owing to the relationship between the noise source, the receptor and the wind direction. The reduction in noise can be as much as 10dB if the location is upwind of the turbine. Calculations are carried out for a wind speed of 7m/s as this is generally accepted to be where noise emissions are closest to the noise limits set in accordance with ETSU-R-97.
When the available noise-level reductions are applied to turbines that are downwind of receptors it is possible, in many circumstances, to allow turbine development to take place closer to receptors than previously considered without breaching accepted noise limits.
This article was written as a contribution to the EIA Quality Mark’s commitment to improving EIA practice.
