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NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy operated by the Alliance for Sustainable Energy, LLC
Wind Turbine Spacing in Wind Farms and Sound
BLM WEATS Workshop
September 1, 2010
Patrick Moriarty, NREL
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Why is noise an issue?
Objectionable noise is an impediment to deployment– Complaints by residents threaten permitting– Projects must comply with established community noise
standards (40 – 45 dBA is typical)Noise reduced operation (NRO)
– Energy also lost
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Sound Pressure LevelMost common noise measurement (in Decibels – dB)
r = observer distanceDoubling distance = -3 dB in SPLDoubling intensity = +3 dB in SPLHuman ear detects around 3 dB change+10 dB = subjective doubling of perceived loudness
Sound Power Level
=
=
refref pp
IISPL 1010 log20log10
21r
I ∝r
p 1∝
=
010log10
WWLW
21010 log (2 )WSPL L rπ≈ −
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A-weighting
dB vs. dB(A)Mimic behavior of human earCertain frequencies appear louder
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Noise and SPL
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Noise Trends for Large Wind Turbines
sound power level = 22 LOG(D) + 72
sound power level = 11 LOG (D) + 82
70
80
90
100
110
120
130
1 10 100 1000
Diameter [m]
soun
d po
wer
leve
l dB
[A]
80's90'sMulti MWSWT'sLog. (80's)Log. (90's)
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Types of wind turbine noise
Mechanical– Mostly tones– Gearbox– Generator– Tower resonance– Blade movement
Aerodynamic– Blades & tips
• Proportional to Vtip5
• Higher frequency and broadband
– Tower wake• Rotational (low
frequency)• 1-3 per rev
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Relative noise contributions
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Typical noise spectra
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Noise Regimes
3 different regimes
• Human perception
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Propagation
HumidityWind direction and speedWind ShearTurbulenceTerrain
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Noise Ordinances
Varies by place and use
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Background/Ambient Noise
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Human Response
Not as predictable as everything else
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Noise descriptions
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Low Frequency Noise and Infrasound
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Noise reduction
Move turbines farther away– Offshore– Low frequencies travel farther
Mechanical– Isolation– Insulation
Aerodynamic– Lower tip speed (Noise Reduced Operation)– Modify Blade Shape
• Sharp trailing edges
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Observer location influence
Noise Footprint - Directivity Rotor Plane – Doppler Amplification
AOC 15/50
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Wind Plant Noise Footprint
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Wind Farm Layout Issues
Optimal spacing/layout– Power– O&M
Design Constraints– Turbine interactions– Atmospheric
conditions– Terrain– Electrical Interconnect
& substation– Roads– Environmental impacts– Land Rights– Wind Rights
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Why is layout design important?
Optimal power production– Example
• 200 - 2 MW wind turbines• 1.26 x 109 kWh/year (36% capacity factor)• 5 ¢/kWh• 1% change in efficiency OR• ~0.1 m/s change in annual wind speed • = $630k/year = $12.6 million/farm lifetime
– 10% underproduction for existing farms is common ($$!)
O&M costsNear-term forecasting
– Pricing– Load matching
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Wind Farm Wakes
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Spacing Distance
Mean wind speed
Turbulence intensity
Near wake Far wake
Power Losses
O&M Costs
Rule of thumb - 10D downwind, 3D crosswind
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Dominant Wind Direction
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Power Losses
Relative production
0
0.2
0.4
0.6
0.8
1
1.2
60 70 80 90 100 110 120 130Wind Direction (deg)
Pi/P
1
av P2/P1av P3/P1av P4/P1av P5/P1
Spacing = 3.8DBiggest loss between row 1 & 2
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Complex terrain
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Atmospheric Stability
Diurnal variationInfluences
– Turbulence– Wake propagation– Most important at
• greater hub heights > 50 m• low wind speeds < 10m/s• low surface roughness
Starting to be considered important for wind farm models
-30 -20 -10 0 10 20 30
Time Record 566
60
80
100
-30 -20 -10 0 10 20 30
Time Record 2500
60
80
100
-30 -20 -10 0 10 20 30
Time Record 4000
60
80
100
z (m
)
-30 -20 -10 0 10 20 30
Time Record 5000
60
80
100
Ti R d 6000
t = 0 s
t = 91.5 s
t = 166.2 s
t = 217.7 s
rotorplane
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Large Scale Impacts
Wind Rights
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Wind Farm Modeling Tools
Engineering Models– Linear or Semi-Linear – WAsP,WindFarmer, WindPro, etc
Computational Fluid Dynamics (CFD)
– Reynolds Averaged Navier Stokes (RANS)• Turbulence modeled• MetoDyn, Ventos, WindSim,
RaptorNL – Mainly resource assesment
• Fast– Detached Eddy Simulation (DES)
• Hybrid RANS/LES– Large Eddy Simulation (LES)
• Most turbulence calculated• Expensive
– Direct Numerical Simulation (DNS)
Industry
Research
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Questions?
