Global Wind Power ConferenceIcing Characteristics & Mitigation Strategies for Wind Turbines in Cold Climates

Andrea G. KrajAndrea G. Kraj11 and Eric L. Bibeauand Eric L. BibeauDepartment of Mechanical and Manufacturing Engineering Department of Mechanical and Manufacturing Engineering University of ManitobaUniversity of ManitobaWinnipeg, Manitoba, CANADAWinnipeg, Manitoba, CANADA11umkrajag@cc.umanitoba.caumkrajag@cc.umanitoba.ca

September 21, 2006Adelaide, Australia

OUTLINE• Introduction• Icing Mitigation Strategy• Experiment Description• Experimental Analysis• Results & Discussion

• Ice Profile Changes• Adhesion Force • Accumulation

• Wind Turbine Performance Measures• Conclusion

INTRODUCTION• Wind

– Energy Resource – Canadian & International regions– Optimize wind turbine technology for enhanced

performance & power generation• Icing

– Negative Influence on power generation– Cold Climates– Rime & Glaze or Mixed

WIND + ICE + TURBINE =

Glaze & Rime Icing Regimes

White & Feathery

Clear & Uneven

APPEARANCE

LOWHIGHLWC

LOWHIGHDENSITY< -100 to -6TEMP [C]

RIMEGLAZEICE TYPE

GLAZE

RIME

Need For Icing Mitigation• Ice removal from blades is critical to safe

& efficient power generation• Private sector, public utilities, land owners• St. Leon, Manitoba, Canada: 99.9 MW

wind farm

Literature Review• Aerospace applications

– Chemicals, surface deformation, thermal melting– Hot compressor/exhaust gas; hot oil; electrical energy

• Complex thermo-fluid dynamic system– Ice shapes and rate of ice accumulation vary with time

• Reducing Ice-Adhesion Force– Adhesion Force ~ linear with temperature (8.5 psi @ -1 C)

– High contact angles enhance ability of droplets to shed• Coatings

– Wear resistance, durability, low adhesion, safety• Thermal

– Traditional wires heaters vs. SOTA technology

ICING MITIGATION STRATEGY•Objective:

–Obtain icing characteristics on the turbine blades–Reduce the negative icing characteristics on the blade surface

•Profile shape changes, adhesion force, accumulation amount

–Prevent and delay ice accretion

•Limitations:–Environmental

• hot oil, chemicals and their derivatives are not viable strategies.

–Electrical• contradict the purpose of generating electricity, and must be cautiously implemented.

ICING MITIGATION STRATEGY

* BOTH* cycling of power * integration with surfacestrategies to optimize

• HEATER• finely dispersed metal coated carbon fibre• heat is generated at the point of use• thin film of water is formed, ice is swept away by aerodynamic forces.• cycling power: control & reduce energy consumption

• COATINGS• Icephobic: icing• Hydrophobic: water

• more controlled passive deicing• combined effectiveness

• less-passive •highly controllable,•Lightweight electro-thermal ice protection system

• passive• improve resistance of ice accretion and adhesion to the aerofoil surface

THERMFACETHERMALSURFACE

Stage 3Stage 2Stage 1

Surface Mitigation•Selected hydrophobic and ice-phobic coatings to scaled-down models of wind turbine blades

•Desired properties•non-stick ability, good wear properties, excellent adhesion and water resistance, weatherability, low surface energy

•Wearlon Super-Icephobic (black)•Wearlon Super-Hydrophobic (white)•Uncoated (yellow)

Thermal Mitigation•Heaters

• easily integrated into composite or polymer material structures• ideally suited for leading edge blade ice protection applications due to their lightweight and uniform heat distribution capability, contrary to traditional wire heaters

•Cycling of the heater power

•Anti-icing & De-icing regimes• 20 min with full heat• last 10 min with heat

Thermface Mitigation• More controlled-passive deicing

• Combined effectiveness

• Optimize mitigation efforts

• Many possible combinations

EXPERIMENT DESCRIPTION• Experiments performed in the University of

Manitoba Icing Tunnel Facility, (UMITF)– Simulated climatological conditions of severe

icing events• Specimens

– Stationary Aerofoils at Zero A.O.A. downstream• Icing

– Cooled airflows + H20 droplets– Spray-bar

• Variables– Glaze & Rime ice conditions – Plain, Icephobic, & Hydrophobic– De-icing & Anti-icing thermal regimes– Combinations

•Contracting duct-work

•Constant air speed: 18-151 kph @ -30 C

•Variable cross-sectional test area

•Max droplet flow rate: 75 L per hour

Icing Tunnel Facility

Spray System•Variable droplet size, air velocity & temperature

•Air atomizing nozzles produce water mean droplet diameters of 10e-3 m to 10e-5 m.

•NACA 634421 aerofoil

•Al frame + fiberglass substrate

•Coatings applied using specialized paint equipment

•Planform Dimensions: 0.30m x 0.64m

Test Specimen

Experimental Parameters

50

7.37

-5

9.0

Glaze

9.0Water Flow [GPH]

50Air-line pressure [psi]

7.97Wind Speed [m/s]

-15Tunnel Temp. [C]

Rime

•UMITF experimental conditions for the icing regimes:

•Wind tunnel cooling•Injection water system cooling•Thermocouple in inner duct: temperature verification•Spray system activated•Data recording

–High speed camera and DAQ system–Photographs of LE profile–Acquisition period: 20 mins

•Visual Observation– Ice profile shape changes

•Measured–Profile Shape Extension–Ice Adhesion Force–Accumulation Amounts

Test Procedure

EXPERIMENTAL ANALYSIS

• Collected image data was processed using graphics software to determine changes in ice profile shape – ice shape, symmetry and consistency observed– LE shape extension measured

• Shear-adhesion force test– New test apparatus designed to measure ice adhesion on

curved surface– Performed on the LE curved surface to determine the

required force to remove the ice from the surface– LE ice specimen geometry measured

• Accumulation Amount– Specimens were melted for a liquid volume measurement

due to variance in solid shapes between tests

RESULTS & DISCUSSION

Ice Profile ChangesOne example: varying amounts of ice accumulation and profile shapes between surfaces in glaze icing conditions

• LE aerofoil has greatest ice accretion

• Overall ice accretion: Plain > Hydrophobic > Icephobic

• Icephobic most controlled & symmetric ice profile

Profile Shape ExtensionGlaze vs. Rime•uncoated plain surface,

-glaze ice 8% less shape extension than rime ice.

Surface Mitigation:•Glaze conditions

-Hydrophobic coating reduces shape extension by 25%,-Icephobic coating reduces by 16%

•Rime conditions-Surface mitigation is not found to be effective-Rime icing subdues the efforts for shape extension reduction

Thermal mitigation:•Glaze conditions

-Anti-icing effectively reduces shape extension by 29%, whereas de-icing is not effective-Glaze responds more effectively than rime

Thermface mitigation:•Glaze conditions

-Hydrophobic coating with the de-icing regime, which reduces shape extension by 70%; the next best option is the icephobic coating with the de-icing regime.

•Rime conditions-Icephobic coating with the anti-icing thermal regime, which has an overall effectiveness of reducing shape extension by 12%.

Expt List

ice specimen

anti-icing

de-icing

1 min 10 mins. 20 mins.

glaz

ew

hite

n/a

n/a

rime

yello

wn/

an/

a

glaz

eye

llow

yes -

rime

yello

wye

s -

glaz

ew

hite - yes

rime

blac

kye

s -

Best Performers at Reducing Profile Shape Extension

Ther

mfa

ceTh

erm

alSu

rfac

e

LE Shape Profile IntervalsTest Description

Adhesion Force Glaze vs. Rime• uncoated plain surface,

– glaze ice 40% less adhesive than rime ice.

Surface Mitigation:• Glaze conditions

– Hydrophobic coating which reduces adhesion force by 97%. • Rime conditions

– Ice phobic coating is more effective, reducing adhesion by 15%.• Apply the proper coating in its designed-for conditions.

Thermal mitigation:• Anti-icing is more effective than the deicing regime, in both glaze and rime icing conditions, reducing

adhesion 89% and 46% respectively.• Glaze responds more effectively than rime

Thermface mitigation:• In both icing conditions

– anti-icing regime with the hydrophobic coating which reduces adhesion by 78% in glaze and 50% in rime, is more effective than the icephobic coating.

– deicing regime, the icephobic coating is more effective, reducing adhesion by 83% in glaze and 46% in rime.• Overall, the deicing regime is more effective and consumes less energy, which is ideal for energy

production systems; and is best paired with the icephobic coating.• Interesting finding:

– isolating the surface variable, for glaze icing, in both anti-icing and deicing regimes• there is no apparent adhesion reduction for either coating, suggesting that the material characteristic, k, of the coatings influences

the thermal gradient such that ice removal is impeded.Result• In both glaze and rime conditions the most effective thermface mitigation strategy is the icephobic

coating in the deicing regime.

Accumulation AmountLess ice accumulates in glaze then in rime icing

Surface Mitigation• Most effective: icephobic coating which reduces accumulation by 5% in glaze and 23% in rime.• Since results are also dependent on their climatological designed-for performance characteristics for the

given icing conditions, the icephobic coating is expected to, and does, outperform the hydrophobic coating.

Thermal Mitigation• Plain uncoated surface

– glaze conditions, is the anti-icing regime, reducing accumulation by 37%– rime conditions, accumulation is not significantly reduced

• the amount of accumulation appears to supersede the effectiveness of the mitigation technique, suggesting that it does not have the capacity to be effective in the given conditions.

Thermface Mitigation• Glaze conditions

– de-icing regime with the icephobic coating, which reduces accumulation 83%• Rime conditions

– icephobic coating is effective with either anti-icing or deicing regime, reducing accumulation 78% and 19% respectively.

– the reduced effectiveness of this mitigation strategy with the deicing regime in rime conditions indicates that the thermal technique has nearly reached its max capacity of effectiveness, yet the combination with the surface mitigation remains more effective than the thermal technique alone.

Results• deicing regime utilizes less energy and is effective in either icing condition, suggesting that it is the preferred

regime.• combined mitigation of the thermface strategy provides the most substantial reduction in accumulation.• For reducing accumulation either glaze or rime conditions : implementation of an icephobic coating in

combination with a deicing regime.

WIND TURBINE PERFORMANCE MEASURES• Aerodynamics principle: Optimized performance is

function of the lift to drag ratio of the blades

• Ice accumulation changes the aerodynamic profile shape of the blade– directly affects the lift– creates trigger points for non-laminar flow over the blade

surface– results in less-efficient energy capture– adds weight to the aerofoil– extends chord length, not designed for

• With a less-effective lift-to drag ratio the turbine is not capable of efficient wind energy production

• An effective mitigation strategy, must not consume more energy than the system produces

• Balance these performance measures

CONCLUSION• The presented mitigation strategies:

– significantly reduce the profile shape extension, adhesion forceand accumulation amount of ice on a blade surface

– influence the formation of a more symmetric LE ice profile– enhance wind energy production in adverse cold climates

• Glaze or Rime conditions:– For reducing adhesion force & accumulation amount

• Both icing conditions: thermface strategy employing the de-icing thermal regime with the icephobic surface coating

– For reducing shape extension• glaze conditions: de-icing regime with the hydrophobic coating• rime conditions: icephobic coating with the anti-icing regime.

• The combination of mitigation strategies must be carefully designed:– employed in the climatological conditions best suited for their use

and within their scope of capability– do not adversely affect the capabilities of one another

• coating material characteristic, k, influencing the thermal gradient such that ice removal is more difficult.

ACKNOWLEDGEMENTS

• Funding from the Manitoba Hydro NSERC Alternative Energy Chair and NRCAN is gratefully acknowledged.

• St. Leon Wind Farm Personnel

QUESTIONS?

APPENDICES

Icing Accumulation: Full Blade

Rime

Glaze

View along Leading Edge (root to tip)Top View