Separated Pitch Control at Tip (SePCaT): Innovative Blade Designs and Associated Controls

Ranjeet Agarwala

Instructor: East Carolina University , Technology Systems PhD Student: North Carolina State University, Mechanical Engineering

Dr. Paul I. Ro

Advisor Professor and Associate Head, North Carolina State University

Mechanical and Aerospace Engineering

Growth of Wind Turbine Sizes

Growth in GW-

Courtesy DOE[2]

Growth in the size of wind turbines since 1985-Courtesy UpWind [1]

Pitch Mechanisms of a Large wind Turbine

Pitch Mechanisms of a Large wind Turbine –Courtesy Emerson and Liebherr [5,6]

Power Curve of A Wind Turbine

Power Curve of the representative Wind Turbine. Courtesy NREL [3]

Problems with Traditional Control

• Actuations of pitch angle are inhibited by large blade mass

• Power required for pitching for large blades are high thereby undermining power generation

• Mechanisms for pitching are large, complex, and expensive requiring higher manufacturing and maintenance costs

• Independent or collective full length pitch control become increasingly cumbersome

Examples of Alternate Control Strategy

Coutesy-Imraan et

al. (2013) [8]

Trailing-Edge Mid and End- Flap Control Surface Courtesy Agarwala and Ro [7]

Alternatives to Full Length Blade Pitch Control

• Trailing edge-flaps, transitional tabs, active flaps etc.

• Partial span-wise trailing-edge 3D control surfaces (Mid-Flap and End-Flap) • Telescopic Blades

• Rotor complexity, increase manufacturing, maintenance, and deployment costs

Blade Construction

and Transportation

Courtesy Modular

Wind Energy,

www.modwind.com

[16]

Coutesy-Siemens [9]

SePCaT-Continued

• Investigation of a novel and simple separated pitch control strategy at blade tip

(SePCaT) for a large MW wind turbine

• 3D control surfaces are deployed as separated sections of partial blade length

• 5, 10, 15, 20, 25, and 30 percentages of total blade length at the tip

• SePCaT5, SePCaT10, SePCaT15, SePCaT20, SePCaT25, SePCaT30

Sections of the wind turbine blade

Courtesy Agarwala and Ro [7]

Courtesy NREL [3]

(a) SePCaT30 (b) SePCaT25

(c)SePCaT20

(d) SePCaT15 (e) SePCaT10 (f) SePCaT5

Separated Pitch Control at Tip-SePCaT

Blade Configuration as Partial Blade Lengths.

SePCaT-Continued

• The main blade and SePCaT angles were varied in both counter-clockwise ( Feather direction) and clockise direction (Stall direction) broadly first by 60 degrees in 10degree increments.followed by precise variations of 2 degrees.

• SePCaT results are then compared traditional control by varying blade pitch angle by 20 degrees in 1 degree increments In both feather and stall directions.

• The findings of previous research studies depicted the onset of stall around 22 degrees versus 10 to 15 degrees in contrast to 2D studies.

• Flow simulation results of the study revealed negligible lift around 0 to 3 degrees.

SePCaT control angle configurations

Blade Cross-Section

Courtesy-Wright and Cooper , Dowell, O’Neil and Strganac, Theodorsen [12,13,14,15]

Traditional control angle configurations

SePCaT-Continued

• Power abatement factors versus feather angles in response to increasing wind speed are obtained from NREL studies

• The data for power reduction versus pitch angle curve is obtained from NREL

pitch angle setting recommendations to achieve set power factor reductions in response to velocity increments in region 3

• As per the International Electro-Technical Commission (IEC), extreme wind speed (EWM) which are defined by 50-year and 1-year extreme wind recurrence probabilities are 40 and 30 percent higher than reference wind speed.

• Hence for the purposes of this study low, moderate, high, and extreme wind speeds increments were treated as being 10, 20, 30, and 40 percent more than the reference speed.

Fig. 14: SePCaT versus traditional: rotor power abatement comparisons

at low and moderate wind speed increments

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

Pitch to Feather

SePCaT5-LF

SePCaT10-LF

SePCaT15-LF

SePCaT20-LF

SePCaT25-LF

SePCaT30-LF

Trad-LF

NREL-Power Factor

NREL-Velocity Factor

1.2 U

1.1 U

1.1 U

1.2 U

Low and

Medium Wind

Speed Variation

SePCaT-Continued

Power abatement strategies at low to moderate wind speeds

• As the wind speed increases by a factor of 1.1U (10 percent), the rotor power increases to around 6.75MW warranting its reduction to a factor of .74 approximately

• To achieve this, the blade is feathered by around 6 degrees for traditional control. This is also achieved by approximately feathering SePCaT30 by 14, SePCaT25 by 16 , SePCaT20 by 26, and SePCaT15 by 30 degrees respectively.

• If wind speed increases by a factor of 1.2U (20 percent), the rotor power increases to around 8.33MW warranting its reduction to a factor of .6 approximately

• To achieve this, the blade is feathered by around by around 9 degrees for traditional. This is also achieved by approximately feathering SePCaT30 by 18, SePCaT25 by 26 and SePCaT20 by 30 degrees respectively.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

Pitch to Feather

SePCaT5-LF

SePCaT10-LF

SePCaT15-LF

SePCaT20-LF

SePCaT25-LF

SePCaT30-LF

Trad-LF

NREL-Power Factor

NREL-Velocity Factor

1.3 U

1.4 U

1.3 U

1.4 U

SePCaT versus traditional: rotor power abatement

comparisons at high and extreme wind

speed increments

High and Extreme

Wind Speed Variation

SePCaT-Continued

Power abatement strategies at high to extreme wind speeds

• As the wind speed increases by a factor of 1.3U (30 percent), the rotor power increases to around 11.75 MW warranting its reduction to a factor of .43 approximately.

• To achieve this, the blade is feathered by around 12 degrees for traditional control. This is also achieved by approximately feathering SePCaT30 by 26 degrees.

• If wind speed increases by a factor of 1.4U (40 percent), the rotor power increases to around 14.30MW warranting its reduction to a factor of .35 approximately

• To achieve this, the blade is pitched to feather by around 14 degrees for traditional control. This is also be achieved by approximately feathering SePCaT30 by 32 degrees.

Generalized Analysis

Proposed Steps

1. Start with optimized aerodynamic design for the modified blade that compares to the original blade

2. Construct the blade along the lines with structural layups and modeling information as per previous research

3. Subject the model to the Aerodynamic forces imported from CFD Analysis

4. Obtain numerical results and check to see if any structural test criteria fails.

5. Address the failure criterion/criteria by changing the structural configuration through a iterative process until all the criteria are satisfied.

6. Subject the modified structural design to Aerodynamic tests and repeat the steps until both Aerodynamic and Structural Conditions are satisfied .

Design Changes

Iterative Aerodynamic Design- Gap and Size

Optimization

produced the best lift forces when compared to other setups.

Iterative Structural Design Criterion

Criterion 1-Tip Deflection Criterion 2: Maximum Material Strains Criterion 3: Resonance conditions

Modeling

Blade Structure

Blade Redesign

Blade Redesign

SePCaT-Continued

Disclaimer

The results and deductions are proprietary to the wind turbine setup depicted in this paper and hence the community is encouraged to conduct comprehensive analysis and testing before full functional deployment. Broader modeling, simulations, analysis, and field testing is recommended for wind turbines tailored to specific environmental conditions and power capacities.

References

1. UpWInd (2011), Design Limits and Solutions for Very Large Wind Turbines, EWEA, Brussels. 2. U.S Department of Energy. http://www1.eere.energy.gov/wind 3. U.S Department of Energy- National Renewable Energy Laboratory (NREL)- http://www.nrel.gov/ 4. GWEC (2012), Global Wind Report: Annual Market Update 2011, GWEC, Brussels. 5. Large Components for Offshore-Wind Turbines -Liebherr-Components Biberach GmbH -www.liebherr.com 6. Emerson-SSB Wind Systems GmbH & Co. KG-www.emerson.com 7. Agarwala, R. and Ro, P.I. (2013), “3D analysis of lift and moment adaptation via control surface deployments on a 5MW wind turbine

blade”, Wind Engineering, 37(5),447-468. 8. Imraan, M., Sharma, R.N. and Flay, R.G.J. (2013), “Wind tunnel testing of a wind turbine with telescopic blades: The influence of blade extension”, Energy, 53,22-32 9. Siemens AG Energy Sector- www.siemens.com. 10 .Laks, Pao, Wright (2009) ; Control of Wind Turbines: Past, Present, and Future; American Control Conference. 11. Hau; Wind Turbines: Fundamentals, Technologies , Applications, Economics; 2nd Edition Springer. 12. Wright, J.R. and Cooper, E.C. (2007), Introduction to Aircraft Aero Elasticity and Loads, John Wiley and Sons Ltd., Chichester, West Sussex, England. 13. Dowell, E. H., Curtiss, H. C., Jr., Scanlan, R. H., and Sisto,F. (1989), “A Modern Course in Aeroelasticity, 2nd ed.”,Kluwer Academic, Norwell,

MA, USA. 14. O’Neil, T. and Strganac, T. W. (1998); “Aeroelastic Response of a Rigid Wing Supported by Nonlinear Springs”, Journal of Aircraft, 35(4), 1-7. 15. Theodorsen, T. (1935), “General Theory of Aerodynamic Instability and the Mechanism of Flutter”, NACA Report 496, USA. 16. Modular Wind Energy: www.modwind.com