Windenergie 2 - Environmental Impacts

19 November 2025, Po Wen Cheng

Questions at the beginning

• Which wake model assumes uniform velocity deficit in the rotor area?
  • Jensen Model
• Which effects are included in the DWM model?
  • Velocity deficit
  • Meandering of the wake
  • The wake added turbulence
  • It does not include wind evolution (changes in wind over time and distance)
• Which is the shape of the wake behind a wind turbine measured at the distance of 2D?
  • Double gaussian shape
• What is the effect of wind turbine/wind farm induction?
  • Reduced velocity in front of the turbine (opposite of wake)
• Which turbine in a wind warm suffers the worst wake loss?
  • The one behind the first turbine, no free flow there
• What happens when the wind turbine is in partial wake compared to full wake?
  • The power production goes up
  • The blade fatigue goes up (more fluctuation)
  • The thrust goes up
• Wake deflection is more effective (turning the turbine up to 30 degrees out of the wind)
  • Low turbulence, stable atmospheric conditions, which constrain the movement of the wake
  • Load on turbine is increased at an angle

General Principle of Acoustics

Sound may be defined as any pressure variation that the human ear can detect. A wave motion is set off when an element sets the nearest particle of air into motion. This motion gradually spreads to adjacent air particles further away from the source.

• Frequency $f$ [Hz]
• Wavelength $\lambda$ [m]
• Speed of sound $c$ [$\frac{m}{s}$]

$$ c = f \cdot \lambda $$

In most cases sounds contain a mixture of all frequencies and are complex. For simplification sounds can be represented in a series of intervals called frequency bandwidth
Bandwidth of a sound is the difference between its highest and lowest frequency components

Sound

A form of vibrational energy transmitted through the air. Frequencies in the approximate range of 20 to 20,000 Hertz are detected by the ear and converted into electrical signals.

Noise

An unwanted sound as defined by WHO guidelines (e.g. road traffic noise, industrial noise)

Our hearing is less sensitive to very low and very high frequencies. In order to account for this, weighing filters can be applied when measuring sound. The curve is derived experimentally, which means it is an average and does not apply to everyone.

• dB(A) (Most used)
• dB(B) (No longer in common use)
• dB(C)

Sound Power Level / Sound Pressure Level

• Sound Power Level
  • Characterization of the strength of the sound source
  • Independent of the environmental conduítions
• Sound Pressure Level
  • Sound intensity at the site of perception
  • Dependent of the environment

Doubeling the sound energy corresponds to an increase of 3 dB

Wind tubrine noise during operation/construction

Slide 30: Definition of sound pressure level at an immission point due to the presence of a wind turbine.

Sources of wind turbine noise

• Aerodynamic noise (dominant source)
  • Biggest portion is from the blade tip
  • Trailing edge noise
• Mechanical noise

Larger wind turbines are not much louder than smaller ones, because the blade tip speed is about the same (Bigger but smaller RPM)
Noise can also always be reduced by reducing the power by reducing the speed of the turbine