The Impact of Ice Formation on Wind Turbine Performance and Aerodynamics S. Barber, Y. Wang, S. Jafari, N. Chokani and R.S. Abhari [email protected] European Wind Energy Conference, Warsaw 21st April 2010
Jan 03, 2016
The Impact of Ice Formation on Wind Turbine Performance
and Aerodynamics
S. Barber, Y. Wang, S. Jafari, N. Chokani and R.S. Abhari
European Wind Energy Conference, Warsaw21st April 2010
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Overview
• Motivation• Research objectives• Experimental approach• Results and discussion
– Experiment (performance)– CFD (aerodynamics)
• Conclusions
• Wind energy is world’s fastest growing source of electricity production− 160 GW installed wind capacity reached in 2009
• Wind-rich sites must be effectively taken advantage of– Many wind-rich sites are in cold, wet regions
Icing a Global Challenge for Wind Energy
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Northern USA & Canada
Scandinavia & Russia
ChinaAlps
Decreasing temperatureIncreasing humidity
Icing Dependent on Altitude• Ice formation dependent on many factors,
including:– Air humidity– Air density– Air temperature– Wind velocity– Object size on which ice formed– Cloud water droplet concentration
• Rate of ice formation therefore highly altitude-dependent:– Altitude 800-1,500m: high risk of ice formation– Altitude > 1,500m: lower risk of ice formation
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Pow
er (
kW)
Velocity (m/s)
• Results from Alpine Test Site Gütsch, Switzerland: 2,300 m altitude– 10-min average power and velocity measurements over a year (Meteotest)*– Corrected for density and hub height
• Measured Annual Energy Production 20% less than predicted• Possible reasons:
– Icing: investigated here– Gusts and turbulence in complex terrain: being investigated in ETH sub-scale test facility
Measured Energy Yield 20% Less Than Predicted
21.04.10 5*Barber et al, “Assessment of wind turbine performance in alpine environments,” submitted to J. Wind Eng. Ind. Aero
Power curve
Annual average of measurements
Research Objectives
• Quantify performance of wind turbines with specified icing on rotor blades in a systematic, parametric study
• Detail impact of icing on aerodynamics
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Specification of Simulated Icing
2D profile 2D ice accretion code (LEWICE), atmospheric
conditions at Gütsch
Span-wise distribution1000s of photographs from Alpine Test Site
Gütsch
r/R = 0.90 = 8.8o
Vrel = 31.6 m/s
r/R = 0.63 = 8.8o
Vrel = 22.2 m/s
r/R = 0.30 = 6.9o
Vrel = 11.2 m/s
2D profile + spanwise distribution ≅ simulated icing
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Specified Ice Shapes
high-altitude, Gütsch conditions = non-“extreme”
low altitude, Bern Jura conditions = “extreme”
5% chord 5% chord 5% chord 5% chord 5% chord 10% chord
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ETH Sub-Scale Model Wind Turbine Test Facility
• Velocity and acceleration of turbine can be precisely specified: arbitrary velocity profiles• Turbulence intensity can be controlled with grids• Systematic and parametric studies can be carried out: not possible in field
Salient characteristics of facility• For given model & flow velocity, advantage in Reynolds number of factor 15 gained using water as test medium, compared to air• Free-stream turbulence intensity is zero: reliable baseline conditions• Controlled test conditions: accurate assessment of performance due to ice shapes.
Summary of test conditions
Tip speed ratio = 3 - 8
Re0.75 = 1.4 x 105
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Model and InstrumentationRotor geometry:• Blade geometry matches NREL
S809• Interchangeable hub, 2 or 3
bladed
Instrumentation:• Torque measured with in-line
torquemeter• Torquemeter installed between
motor & shaft• Series of tare measurements
undertaken to remove drive & seal resistances
• Power coefficient:
CP Trotor
0.5u3Arotor
Max. relative errors3.0% in CP
1.1% in tip speed ratio
Turbulent skin friction:
Reynolds number correction:
ETH Sub-Scale Model Matches NREL
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corrected uncorrected
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Effect of Ice on Performance
• Ice on outboard 5% of span has most significant effect on performance
• Ice removal / prevention systems can be substantially more efficient if their effectiveness is tailored to outboard 5% span of blades
No ice
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Effect of Ice on Performance
• Sawtooth shapes do not have significantly different effect on CP compared to smooth shapes
• No power generated for Case F (“extreme”) at tip speed ratio ≥ 6
No ice
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“Extreme” Icing Has Large Impact on Annual Energy Production
Gütsch conditions / non-“extreme”
Bern Jura conditions / “extreme” − Predicted loss is in good agreement with Gütsch data− Non-”extreme” icing has small impact− “Extreme” icing has large (15% loss) impact
Annual Energy Production (AEP) • Estimated using IEC standard bins method • Optimal tip speed ratio• Measured wind speeds & atmospheric conditions at Gütsch; icing in 2 months per year
Gütsch measurements
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CFD ModelANSYS CFX• Commercial, implicit flow solver• One blade, periodic boundaries, k- turbulence model with scalable
wall function• Computational grid: 4 million cells
Blade surface
Periodic boundary Periodic boundary
4R
4R
R = rotor radius
x
y
z
CFD Results Match Experiments
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Tip speed ratio = 6
Cp
,wit
hou
t ic
e –
CP
, w
ith
ice
(C
P)
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“Extreme” Ice Causes Extensive Flow Separation
• Flow separation limited to root for non-“extreme” ice
• No separation on blade
Clean Non-“extreme” “Extreme”
• Flow separation over ¾ of blade for “extreme” ice
3.0
2.0
1.0
0.0
Total Velocity (m/s) z-y plane, x = -0.1R
Blade rotation
Incidence ≈ 15o
Incidence ≈ 5o
Incidence ≈ 5o
Incidence ≈ 15o
Incidence ≈ 5o
Incidence ≈ 5o
Incidence ≈ 30o
Incidence ≈ 15o
Incidence ≈ 15o
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Conclusions
• For icing at high altitudes > 1,500 m: non-”extreme” ice on outboard 5% of the blade has most significant impact on performance → tailor removal systems for outboard 5% of blade
• For icing at lower altitudes, 800 – 1,500 m: Annual Energy Production can be reduced up to 15% due to “extreme” ice
• At the Alpine Test Site Gütsch, icing does not explain the losses of 20% in Annual Energy Production
• Gusts and turbulence are being investigated in the sub-scale model wind turbine test facility at ETH Zurich, which allows testing of dynamically scaled models at near full-scale non-dimensional parameters
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Acknowledgements• Financial support: Swiss Federal Office of Energy (BFE)• LEC workshop: H. Suter, T. Künzle, C. Troller and C.
Reshef