Unit 9 Flashcards
wind turbine types
There are both vertical and horizontal axis turbines
power generation
power generated by a wind turbine is proportional to the total blade area and to the cube of a wind speed,
the larger the blades the more is the power generated.
- a doubling of wind speed increases the power generated by a factor of eight.
increasing the turbine height means the rotor will operate in faster and more stable air flow.
- On shore turbines are best placed in locations away from buildings, for example, on tops of hills in rural areas.
two types of noise from turbine
The noise generated and emitted from various parts of a wind turbine can be grouped into two broad types:aerodynamic and mechanical.
Aerodynamic noise sources include:
- Self-noise due to the interaction of turbulent boundary layer with the blade trailing edge
- Noise due to wind-induced turbulence interacting with the blades
- Rotor tip noise
- Tones produced by edge thickness, and
- Noise generated by instabilities in the laminar boundary flow over the blades.
Mechanical source noises include:
Discrete frequency noise generated by gearbox and tooth mesh
Generator noise caused by coils and cooling fans
Control equipment noise.
The Assessment & Rating of Noise from Wind Farms” prepared for DTI in 1997 (ETSU-R-97)
recommends setting relative noise limits using the LA90,10min index outside the nearest noise sensitive properties.
- Noise levels are limited to 5 dB above the background levels for both day- and night-time.
- In low noise environments, the day-time levels have an absolute limit of between 35-40 dB(A).
- For the night period, a fixed lower limit of 43 dB is recommended
- Of course, if the background level is greater than 38 dB, the back ground plus 5 dB rule still applies.
- A penalty of up to 5 dB is added to the measured noise levels if there is a measurable tonal component in the turbine noise.
Wind turbine sound power levels are measured in accordance with the second edition of IEC 61400-1188.
The wind speed is measured at the turbine hub height or derived from the power output of the turbine.
The corresponding wind speeds are then referenced to a standardised 10 m height using a groundroughness length of 0.05 m.
Consequently, the IEC 61400-11 test data provides data that, on its own, does not take account of the site-specific wind shear of a potential wind farm site.
what can mismtach between wind speed cause
different Relationships between
wind speed at different heights above the ground,
hub height and wind speed
actual wind speed that would be measured at 10 metres above ground level, varies from site to site.
This mismatch can lead to significant errors when comparisons are made between measured background noise levels and the wind turbine emission levels.
Brief summary of IoA Good Practice Guide on Wind Farms
Background noise surveys may be carried out at any time of the year provided seasonal effects leading to raised noise levels can be excluded.
-Periods of rainfall must be excluded,andalso ‘non-typical’ noise.
Noise measurements should be made in amenity areas between 3.5 and 20 m from a dwelling.
-The measurement of background noise should be indicative of the area
Noise measurements should be correlated with values of standardised 10 m wind speed calculated from hub height wind speed.
The ‘correction’ to the 10 m reference height should be made using a standard ground roughness length (Z0) of 0.05 metres.
The correlation is determined by fitting a polynomial (from linear up to 4th order) to the measurements with separate charts for ‘amenity hours’ and ‘night-time hours’.
The noise levels should not be extrapolated beyond the measured wind speeds.
what does best practice suggest re wind speed measurements
best practice suggests that measured background noise levels are correlated with derived (and not measured) 10m height wind speeds.
This procedure references both measured background noise levels and turbine immission levels to the wind speed at the turbine hub height
Predictions of turbine noise levels made using the ISO 9613-2 scheme,
tips and tricks
𝐿A90 levels should be determined from calculated 𝐿Aeq levels by subtracting 2 dB;
Predictions should be based on octave band frequency data, when available
Topographical or other screening should be limited to no more than 2 dB [Abar
Atmospheric conditions should be assumed to be 10C and 70 % relative humidity to represent are asonably low level of air absorption
Downwind propagation should be assumed with zero directivity correction.
Amplitude Modulation
The aerodynamic noise normally associated with wind turbines, commonly referred to as ‘swish’, is the modulation of noise produced at blade passing frequency.
A report researching low frequency noise from wind turbines noted that Amplitude Modulation (AM) was occurring in ways not anticipated by ETSU-R-97.
WSP effects of AM report concluded that there is….
- sufficient robust evidence that excessive AM leads to increased annoyance from wind turbine noise
- should be controlled using suitable planning conditions
