Control of Rotor Geometry and Aerodynamics: Retractable Blades and Advanced Concepts

Transcription

1 WIND ENGINEERING VOLUME 32, NO. 1, 2008 PP Control of Rotor Geometry and Aerodynamics: Retractable Blades and Advanced Concepts Timothy J. McCoy and Dayton A. Griffin Global Energy Concepts, LLC, th Avenue, Suite 900, Seattle, WA 98101, (206) s: ; ABSTRACT This research focuses on the combination of two areas: aerodynamics and controls for wind turbine load reduction. The goal of the study is to develop baseline estimates of the cost benefits available from the use of advanced control of wind turbine rotors. The first category includes devices or methods that can be used to actively alter the local aerodynamic properties of the rotor blade. These devices have response times at least as fast as a full-span, variable-pitch system. The second category is geometry control, based on extendable length blades. These technologies are applied to a virtual turbine design with a 90-m diameter and a 2.5 MW rating, variable-speed, and variable-pitch. The approach will include simulations using MSC-ADAMS and detailed cost modeling based on the simulated loads. The control algorithms will employ linear state space methods that include individual blade pitch and multi-input, multi-output control of the selected aerodynamic devices. 1. INTRODUCTION Global Energy Concepts, LLC (GEC) has conducted research on using active aerodynamic control for commercial wind turbine load reduction. This development was performed under contract with the National Renewable Energy Laboratory (NREL). The overall objective of this research was to support the Low Wind Speed Turbine (LWST) project goal of identifying means for reducing the cost of energy for wind energy projects in low wind speed areas. The concepts evaluated in this study fall into two distinct categories. The first is devices or methods that can be used to actively alter the local aerodynamic properties of the rotor blade. These devices would typically have response times on the order of, or faster than, a full-span, variable-pitch system. Cost of energy (COE) reductions are realized by cost savings through reduced system loads. The second concept to be studied is an actively controlled retractable blade rotor (RBR). For this concept, increased energy capture is the primary advantage, and mitigating cost increases due to increased loading and added mechanical systems is the engineering challenge. In contrast to the active aerodynamic control, the response and actuation time for varying diameter is expected to be slow relative to pitch control. Substantial research has been done on active aerodynamic modification, and to a lesser extent, on retractable blade rotors. However, no systematic study of the performance gains and loads is available in the public domain. GEC has conducted a study intended to provide detailed performance and loads for a range of parameters for active aerodynamic and geometry control at a rating consistent with the current market for utility-scale turbines, and incorporating the most current technology in controls, materials, and mechanisms. The performance and load

2 14 CONTROL OF ROTOR GEOMETRY AND AERODYNAMICS: RETRACTABLE BLADES AND ADVANCED CONCEPTS results are used with cost of energy models to determine how much such systems could add to the cost of a wind turbine without increasing the cost of energy. These results will permit researchers and turbine designers to efficiently determine the degree to which a selected approach to implementing these devices is likely to reduce the cost of energy. 2. SCOPE AND APPROACH The scope of this research has been to conduct analytical studies of the COE benefits of active aerodynamic control of wind turbine rotors. The analysis typically includes the following: Blade structural and aerodynamic properties Performance and load predictions Cost predictions based on loads and material costs Because this is a concept study, many assumptions were made including the following: Detailed engineering of the devices was not considered Sensors and actuators required for control are assumed to exist The approach has been to use the MSC-ADAMS general purpose, commercially available simulation code combined with the Aerodyn aerodynamic force calculation subroutines to calculate turbine loads. These loads are then used to size the major turbine structural components, material costs are applied, and a turbine capital cost is supplied to the COE calculation. 3. BASELINE TURBINE 3.1. Description A baseline turbine design was chosen in order to make comparisons. The design was initially based on the work done in the WindPACT rotor study [1]. Whereas the rotor study had its primary baseline rated at 1.5 MW, for this study a larger turbine was chosen since the technologies being examined are more likely to show benefit at a larger size. The following are the primary architectural details of the virtual design used for the baseline: 3-blade upwind, 90-m diameter, 2.5 MW, variable speed, pitch to feather 80-m hub height tube tower, fiberglass reinforced polyester (FRP) blades, 3-stage gearbox with doubly fed generator Controls: torque speed curve to follow optimum tip speed ratio (TSR) in low wind and a proportional-integral (PI) controller from rotor speed to collective pitch in high wind. The baseline blade aerodynamic design was developed using the PROPID inverse-design code [2], employing the S818/S825/S826 family of airfoils [3]. Following the methodology in Griffin [4], the blade structural design was developed using the ANSYS finite element code with NuMAD pre-processor [5] and the EBEAM code [6] for determining blade local mass and stiffness properties. The blade spar was dimensioned at several spanwise locations, based on simulations of IEC load cases, and design properties for the FRP assumed for the baseline blade structure. Considerations for the blade structural design included static and fatigue strength, as well as tip-tower clearance.

3 WIND ENGINEERING VOLUME 32, NO. 1, Sea Level Power Curve Net Electrical Power, kw Wind Speed, m/s Figure 1. Baseline turbine power curve at sea level 3.2. Controls The controls for the baseline turbine include a PI loop from rotor speed to blade pitch for revolutions per minute (RPM) regulation in high winds and a torque speed curve for tracking optimal TSR in low winds. In addition, feedback of tower fore-aft acceleration was used to dampen tower vibrations via a low-pass filter. The blades were pitched collectively. The pitch actuator is modeled as a fast servo (16 Hz bandwidth) but the pitch demand is filtered with a first order filter with a 1 Hz cutoff frequency. No rate or acceleration limits are applied to the pitch motion; however, the filter typically limits the pitch rate to about 10 /s and the acceleration to about 50 /s/s Performance Performance calculations were made using the WTPerf code once the blade aerodynamic design was finalized. Annual Energy Production (AEP) calculations were made using the steady power curve as predicted by WTPerf. No turbulence or control losses have been included. The power curve is shown in Figure 1 for sea level air density Load Calculations Turbine structural loads were calculated using ADAMS for a selected set of IEC load cases [7]. 4. RETRACTABLE BLADE ROTOR 4.1. Performance Trade Studies An aerodynamic trade study was conducted to determine an appropriate diameter range for the RBR rotor. The study assumed that the fixed inboard portion of the blade will have the same profile as the baseline blade. The extendable outboard section of the blade will be a constant chord, constant twist section sized to fit within the inboard section at the cut-off radius. The performance calculations assumed that the maximum generator torque would be maintained at the same level as the baseline but that the rotor RPM could increase as the diameter decreases below nominal in high winds. This approach maintains the same

4 16 CONTROL OF ROTOR GEOMETRY AND AERODYNAMICS: RETRACTABLE BLADES AND ADVANCED CONCEPTS Table 1. Candidate RBR configurations. Config Tip Tip Min Max Max Wind Wind Cut-off Chord Radius Radius Power Class AEP Class AEP Radius Fraction** 4 AEP 6 AEP # (%R*) (%) (m) (m) (kw) (MWh) (%) (MWh) (%) 0 N/A N/A , % 10, % , % 11, % , % 12, % , % 11, % , % 12, % , % 11, % , % 12, % , % 11, % , % 12, % , % 12, % , % 12, % , % 12, % , % 12, % , % 12, % *Percentage of baseline rotor radius **Percentage of local blade chord at cut line maximum tip speed but allows the rated power to increase. Performance and annual energy results were calculated for several cut-off radii and several maximum lengths as indicated in Table 1. Based on the amount of predicted energy capture, and on assumptions regarding the feasibility of accommodating different blade tip lengths within the blade, Configuration 6 was chosen as the first candidate for simulation in ADAMS. Figure 2 depicts the chord schedule for Configuration 12. The steady power curve for Configuration 12 is shown in Figure ADAMS Model The ADAMS model of the RBR is a modification of the model for the baseline turbine. The blade has been modified to incorporate changes to the structural properties. The tip section is attached to the inboard blade sections via a linear translational joint Controls The standard turbine controls were modified to include a torque speed curve that is dependent on both rotor radius and RPM and a PI control for speed regulation with blade pitch. The blade tip extension is dependent on measured wind speed. The tip actuation is modeled as a servo drive with a rate limit of 0.02 m/sec. An example of operation in moderate winds is shown in Figure Loads and COE Results Using the above described model, a selected set of IEC load cases was run in ADAMS as described above for the baseline turbine. These cases included load cases 1a and 1b for normal operation in turbulence, extreme 50-year return wind speed with turbulence, and most of the extreme deterministic gust cases. Specifically for the RBR, many of the extreme gust cases were run at a range of wind speeds to accommodate the range of diameters at which the turbine can operate. A comparison of the loads to the baseline is shown in Figure 5

5 WIND ENGINEERING VOLUME 32, NO. 1, Chord (m) Baseline RBR at 42.75m RBR at 50m RBR at 55m 90%R Tip R(m) Figure 2. RBR blade profile for configuration Sea Level Power Curves Power (kw) RBR Baseline Radius Radius (m) Wind Speed (m/s) Figure 3. RBR power curve for configuration 12 and Figure 6. The acronyms used in these figures are defined in Table 3. The loads were input to the COE model and the results are shown in Table 2. Losses applied to the gross energy include blade soiling, wakes, controls and turbulence, and availability. For the RBR larger controls, turbulence, and availability losses were assumed. 5. ACTIVE AERODYNAMIC DEVICES 5.1. Generic Aerodynamic Properties The devices for active aerodynamic control fall into two broad categories. The first is those that delay the onset of stall such as vortex generators, boundary layer suction/blowing, and plasma actuators. The second category is devices that change the effective camber of the airfoil. This could include conventional flaps, gurney flaps, micro-tabs, and morphing trailing-edge shapes [7 17]. After consideration of the possible aerodynamic modifications, it was determined that camber-changing devices held the greatest potential for realizing load reduction benefits on the baseline wind turbine blade application. Since a broad variety of camber-changing devices have very similar effects on the lift curves, it was decided to model the aerodynamic effects generically. After considerable literature review, a set of lift and drag curves was developed manually to represent the camber modification class of devices. An example of these lift and drag curves is shown in Figure 7. The minimum drag coefficients for the airfoil shown in Figure 7 are , , and for the zero deployment, full negative deployment, and full positive deployment, respectively.

6 18 CONTROL OF ROTOR GEOMETRY AND AERODYNAMICS: RETRACTABLE BLADES AND ADVANCED CONCEPTS Wind Speed (m/s) Wind Speed 45 Radius Time (sec) Radius (m) Figure 4. RBR operation in moderate winds Table 2. Cost of energy results for RBR. Configuration Mechanism O&M Cost COE, $/kwh % Change from Cost (Total) Increase Class 4 Class 6 Baseline Baseline $0 0% RBR # 12 $300,000 10% Table 3. Turbine load acronyms. Acronym Description B1 Blade 1 Rt, 25, 50, 95 Root, 25%, 50%, 95% span respectively LSS Low Speed Shaft (Main shaft) Yaw Yaw Bearing TwrBase Tower Base Mx Edgewise moment in blade, shaft torque, tower lateral moment My Flapwise moment in blade, shaft bending, tower fore-aft moment Mz Shaft bending moment, yaw moment 5.2. Controls General Approach The use of many independently controllable aerodynamic devices for turbine control means that a multi-input multi-output control design approach must be employed. Modern state space control design methodologies have been used for this research to design controller gains that meet the objectives of load reduction. Specifically, the linear quadratic regulator (LQR) method available in MATLAB has been used extensively. The LQR methodology is both powerful and convenient; however, there are limitations and assumptions that have to be made to employ this method. An LQR controller requires a linear model of the system to be controlled. It also requires knowledge of the values of all

7 WIND ENGINEERING VOLUME 32, NO. 1, % Load / Baseline Load % 200% 150% 100% 50% 0% 50% B1RtMx B1RtMy B125Mx B125My B150Mx B150My B195Mx B195My LSSMx LSSMy LSSMz yawmy yawmz twrbasemx Figure 5. Change in peak loads relative to baseline for RBR 500% 14 Load / Baseline Load % 300% 200% 100% 0% SN Curve Slope % B1RtMx B1RtMy B125Mx B125My B150Mx B150My B195Mx B195My LSSMx LSSMy LSSMz yawmy yawmz twrbasemx twrbasemy 0 Figure 6. Change in fatigue loads relative to baseline for RBR states in the model at all times. In practice the latter requirement is very restrictive. For the purposes of this research the ADAMS simulation can provide the state values necessary for the LQR controller. No state estimation has been used State Space Model State space models to be used for control design are derived from the ADAMS model through a linearization procedure [18]. This procedure is supplied with a linearization of the aerodynamic effects that result from changes in wind speed, rotor RPM, blade pitch, and aerodynamic device deployment. The resulting large state space model is reduced to the essential fundamental modes listed in Table 4. Additional states are appended for disturbances, and error integrals Load and COE Results Independent Blade Pitch One of the near term control approaches that will be used for wind turbine load reduction is independent blade pitch (IBP). It is likely that any advanced control strategy will include independent blade pitch as a component, even on a rotor that includes advanced aerodynamic control devices. Most of the aerodynamic control devices addressed in this study have a limited range of authority, while blade pitch, although likely slower, has a wide range of control authority. So it is assumed for this study that the primary control configuration will include IBP in addition to aerodynamic devices controlled independently for each blade. The first step then for this study will be to design a controller that uses IBP only as a comparison to the primary control configuration that includes both IBP and aerodynamic devices.

8 20 CONTROL OF ROTOR GEOMETRY AND AERODYNAMICS: RETRACTABLE BLADES AND ADVANCED CONCEPTS Zero Deployment Full + Deployment Full Deployment Lift Coefficient Angle of Attack, deg Zero Deployment Full + Deployment Full Deployment Drag Coefficient Angle of Attack, deg Figure 7. Lift and drag for the S825 airfoil with generic camber modification Active Aerodynamic Configurations Inclusion of both active aerodynamic devices and independent blade pitch is the primary control configuration for this study. Several assumptions have been made regarding the aerodynamic devices: Instantaneous aerodynamic response, although this is tempered somewhat by the use of the dynamic inflow option of Aerodyn. Continuous linear behavior within the defined function range of the devices. Specifically any position can be achieved between the positive and negative limits. Grouped into three regions of each blade, each region controlled as one unit. The regions for the primary configuration are: inboard (12.25 m to m), mid-span (22.50 m to m), and outboard (33.75 m to 45.0 m). The following combinations and configurations were studied: 1. Independent blade pitch only; no active aerodynamic devices. 2. Active aero only: This configuration uses collective blade pitch plus aerodynamic devices only. No IBP is used. The devices are deployed into three separate regions consisting of one third of the blade span outboard of maximum chord (at about 25% span) each. Within each region the devices are controlled as one.

9 WIND ENGINEERING VOLUME 32, NO. 1, Table 4. States for linear control design model. Plant States Eigenvalue at 14 m/s Description Tower ± 2.014i Tower top fore-aft position and velocity Rotor out of plane ± 5.321i Rotor asymmetric out-of-plane position and velocity in the fixed-frame multiblade coordinates Rotor out of plane ± 6.215i Rotor symmetric out-of-plane position and velocity in the fixed-frame multiblade coordinates Rotor out of plane ± i Rotor asymmetric out of plane position and velocity in the fixed-frame multiblade coordinates Rotor in plane i Rotor symmetric in-plane position and velocity Rotor Rotor + drive train rigid body Blade pitch 6.25 Pitch system response Controller States Wind speed 2.0 Three filtered wind speed, mean, vertical shear, horizontal shear (in multi-blade coordinates) RPM error 0.0 Integral Vertical and horizontal rotor out of plane 0.0 Integral displacement Aero device position 0.0 Integral 3. Active aero on the entire portion of the blade outboard of the maximum chord plus independent blade pitch (essentially Configurations 1 and 2). 4. Configuration 3, but the aerodynamic controls have reduced authority: In this configuration the range of authority of the active aerodynamic devices is reduced to half of the primary configuration. 5. Configuration 3, but the aerodynamic controls have time delay: In this configuration the response of the active aerodynamic devices is lagged by using a first order lowpass filter with a time constant of 0.08 seconds (2 Hz). 6. Configuration 3, but the aerodynamic controls on the innermost portion of the blade are eliminated. In this configuration the inboard active aerodynamic devices are not used. The mid-span and outboard devices are used in addition to the IBP. 7. Configuration 6 but the only the outermost aerodynamic controls are active. In this configuration only the aerodynamic devices on the outer third of the blade are used in addition to the IBP Loads and COE Summary For each of the described control configurations simulations of the IEC load cases were carried out in ADAMS. The changes in peak and fatigue loads are compared to the baseline loads in Figure 8 and Figure 9. The acronyms used in these figures are defined in Table 3. Depending on the loading location, different values for the fatigue slope are used to evaluate the fatigue equivalent loads. The slopes used are indicated in Figure 9.

10 22 CONTROL OF ROTOR GEOMETRY AND AERODYNAMICS: RETRACTABLE BLADES AND ADVANCED CONCEPTS Figure 8. Change in peak loads relative to baseline for IBP and active aero (AA) control Table 5. Cost of energy results for active aero control. Configuration Active Aero O&M COE, $/kwh % Change System Cost Cost Increase Class 4 Class 6 From Baseline Baseline $0 0% IBP only $10,000 0% AA only $40,000 5% IBP and full blade AA $40,000 5% IBP and full blade - reduced authority AA $40,000 5% IBP and full blade AA with time lag $40,000 5% IBP and mid to tip blade AA $33,000 5% IBP and tip AA $26,000 5% These load results are used to modify the size and mass of turbine structural components, which when combined with material costs are used to calculate the turbine capital. This capital cost is then input to the cost of energy calculation. Table 5 shows the resulting cost of energy for the different control configurations. 6. DISCUSSION AND CONCLUSIONS 6.1. Retractable Blade Rotor There are numerous challenges to be faced in implementing the RBR technology. A primary issue is achieving the required stiffness within the limits of cost-effective materials and manufacturing processes. Using the baseline blade outer mold line as the available structural envelope, the structural modeling conducted in this study indicates that achieving the needed stiffness is feasible, but optimization for structural and manufacturing efficiency will present difficulties.

11 WIND ENGINEERING VOLUME 32, NO. 1, Figure 9. Change in fatigue loads relative to baseline for IBP and active aero (AA) control The laminate properties used for structural modeling of Configuration 12 assume a carbon spar fiber volume fraction that is characteristic of many aerospace applications, but higher than typical for current commercial wind turbines. While this is not an insurmountable issue, it does imply that some advancement in blade manufacturing processes may be required to facilitate the cost-effective manufacture of blade structure with very stiff laminate. The scaling and fit of the retractable tip is another issue to consider. Figure 10 illustrates that self-similar scaling of the airfoil at the cut-line does not result in a favorable shape to nest with the inner blade. GEC investigated this issue by using CAD modeling to create an airfoil shape with a near-constant surface normal offset. The results indicate two significant changes in the aerodynamic shapes. First, there is a noticeable reduction in the reflex at the trailing edge, which will reduce the effective camber of the tip airfoil. Second, there is an approximate 1.8 rotation of the chord line toward lower angle of attack (AOA). De-cambering and reduced AOA would have a compounding effect as each would tend to reduce lift. The bearing and actuation mechanisms also present major challenges for successful implementation of RBR technology. Integration of these mechanisms includes the design of the hardware, hard-points for attaching, and consideration of the load transfer between parts. The bearing/sliding mechanism would need to operate without binding under man-varied load conditions. The internal structure of the inner blade part would need to accommodate the tip section when it is fully retracted and provide adequate support for the tip when fully extended. The interface between the two parts would also need to be smooth to minimize associated noise. Closely related to the issue of bearing design is addressing the bending compliance of the two nested parts. The tip and the inner blade will have differing magnitude and spanwise variation of their bending stiffness. In general, the inner blade would be substantially stiffer than the retractable tip and would have a different shape for given bending loads. When the RBR tip is retracted, bending of the main blade structure will tend to also bend the nested tip. The extent of this bending and shape under load will depend on the design of the bearing/reaction points. This mismatch in bending compliance and associated load transfer will need to be accommodated at every position in the tip extension/retraction range.

12 24 CONTROL OF ROTOR GEOMETRY AND AERODYNAMICS: RETRACTABLE BLADES AND ADVANCED CONCEPTS Outer Blade Shell (90% Span) Y (mm) 0 RBR Tip 250 Carbon Spar in ADAMS Model ,000 1,250 1,500 X (mm) Figure 10. Comparison of RBR tip profile with inner-blade spar for configuration 12 The modeling of Configuration 12 indicates that the RBR technology holds some promise for enabling meaningful COE reductions. However, this conclusion is highly dependent on the development and demonstration of the associated structure, mechanisms, and control strategies. The COE benefits of this technology would be strongly dependent on the impact of O&M costs and turbine availability. While GEC expects that the 10% O&M increase used in the current modeling would be an achievable number, the complexity of this system introduces risk for higher penalties. Modeling indicated that achieving sufficient stiffness while allowing for the integration of the RBR tip and main structure presents significant technical challenges. Realizing a commercially optimal RBR would likely be facilitated by a design approach that integrates aerodynamic, structural, and mechanism design considerations from the start rather than trying to retrofit an RBR into an existing blade profile. A fully-optimized RBR might require the development of new purpose-designed airfoils to facilitate structural nesting while mitigating performance losses Active Aerodynamic Devices The use of active aerodynamic control devices for wind turbine load reduction presented several challenges and required many assumptions. It has been assumed for the purposes of this study that these devices can be categorized into two primary groups: boundary layer control and camber modification. The former approach was reviewed briefly and it was concluded that, for the variable-speed, variable-pitch baseline turbine design, boundary layer control did not offer much advantage. This is due to the fact that they operate on the upper portion of the lift curve near and beyond stall, a region where the baseline turbine does not typically operate. For the camber modification devices this study created a representative set of lift and drag curves. These curves were developed based on the more optimistic levels of authority that these devices might provide as found in the literature. It was further assumed that the devices could be operated in a linear continuous fashion, although the maximum authority was limited. This assumption allowed for a significant simplification of control design, namely allowing linear state space control design techniques.

13 WIND ENGINEERING VOLUME 32, NO. 1, The results of this study indicate that there is significant potential to reduce rotor and turbine structural loads using active aerodynamic control devices deployed on the blades. The load reductions will depend on the assumptions made about the device behavior, specifically the authority of the devices and their time domain responsiveness. It should be kept in mind that the use of Blade Element Momentum (BEM) theory to make the aerodynamic calculations in this study introduces a significant amount of uncertainty. BEM assumes 2-dimensional flow conditions while it is well understood that 3-dimensional flow conditions exist. Also many of the results assumed that the aerodynamic forces developed instantaneously upon activation of the devices. Beyond development and characterization of the devices themselves, the primary challenge for the development of controls that utilize these devices is their nonlinear behavior. If devices can be fashioned that have a large linear range of authority, the control design will be more straightforward and robust. However if the devices exhibit significant nonlinear behavior, the control challenge increases considerably. While the cost of energy improvements that result from use of these devices appear favorable, it must be emphasized that these results are highly sensitive to assumptions about initial capital cost and O&M costs. Relatively small increases in O&M costs in particular can wipe out the benefits of the load reductions. For this study, the costs of the devices and their maintenance are estimated roughly but calculated to provide a realistic target for designers and researchers to shoot for. REFERENCES 1. Malcolm, D.J. and Hansen, A.C., WindPACT Turbine Rotor Design Study. NREL/SR Golden, CO: National Renewable Energy Laboratory. August Selig, M.S., and Tangler, J.L., A Multipoint Inverse Design Method for Horizontal Axis Wind Turbines, Presented at the AWEA Windpower 94 Conference, Minneapolis, MN, May Tangler, J.L., and Somers, D.M., NREL Airfoil Families for HAWTs, Presented at the American Wind Energy Association Windpower 95 Conference, Washington, DC, March Griffin, D.A., Blade System Design Studies Volume II: Preliminary Blade Designs and Recommended Test Matrix, SAND , Sandia National Laboratories, June Laird, D.L., 2001: A Numerical Manufacturing and Design Tool Odyssey, Proceedings of AIAA/ASME Wind Energy Symposium. Reno, Nevada, January Malcolm, D.J. and Laird, D.L., Modeling of Blades as Equivalent Beams for Aeroelastic Analysis, AIAA , American Institute of Aeronautics and Astronautics, Reno, Nevada, January International Electrotechnical Commission. IEC : Wind Turbine Generator Systems Part 1: Safety Requirements, 2nd Edition. International Standard Rueger, M.L. and Gregorek, G., An Experimental Investigation of the Effect of Vortex Generators on the Aerodynamic Characteristics of a NACA 0021 Airfoil Undergoing Large Amplitude Pitch Oscillations. Sandia National Laboratories, SAND , April Stuart, J., Wright, A. and Butterfield, C., Considerations for and Integrated Wind Turbine Controls Capability at the NWTC: an Aileron Control Case Study for Power Regulation and Load Mitigation. Proceedings of WindPower 96, American Wind Energy Association, Washington DC. June 1996.

14 26 CONTROL OF ROTOR GEOMETRY AND AERODYNAMICS: RETRACTABLE BLADES AND ADVANCED CONCEPTS 10. Griffin, D., Performance Augmentation with Vortex Generators: Design and Testing for Stall-Regulated AWT-26 Turbine. Proceedings of WindPower 96, American Wind Energy Association, Washington DC. June Perivoralis, Y.G. and Voutsinas, S.G., A CFD Performance Analysis of Vortex Generators used for Boundary Layer Control on Wind Turbine Blades. Proceedings of the European Wind Energy Conference, Copenhagen, Denmark, July Timmer, W. and van Rooij, R.P.J.O.M., Summary of the Delft University Wind Turbine Dedicated Airfoils. Proceedings of the 2003 ASME Wind Energy Symposium (AIAA ), Reno, Nevada, January Janiszewska, J., Gregorek, G. and Lee, J., Aerodynamic Characteristics of the LS(1)- 0417MOD Airfoil Model. Proceedings of the 2003 ASME Wind Energy Symposium (AIAA ), Reno, Nevada, January Windward Engineering, Aerodyn User s Guide, Version 12.50, December 24, Yen, D., van Dam, C.P., Smith, R.L., and Collins, S.D., Active Load Control for Wind Turbine Blades Using MEM Translational Tabs, Proceedings of the 2001 ASME Wind Energy Symposium (AIAA ), Reno, Nevada, January Yen Nakafuji, D.T. van Dam, C.P., Michel, J., and Morrison, P., Load Control for Turbine Blades: A Non-Traditional Microtab Approach. Proceedings of the 2002 ASME Wind Energy Symposium (AIAA ), Reno, Nevada, January vandam, C.P et. al., Computational and Experimental Investigation into the Effectiveness of a Microtab Aerodynamic Load Control System, Sandia National Laboratories, August, 2004, DRAFT 18. McCoy, T.J., Wind Turbine ADAMS Model Linearization Including Rotational and Aerodynamic Effects, American Institute of Aeronautics and Astronautics, Reno, Nevada January, 2004.