Active Stall Control of Horizontal Axis Wind Turbines

Abstract

The contribution of sustainable Wind Energy (WE) to the global energy scenario has been steadily increasing over the past decades. In the process, Horizontal Axis Wind Turbines (HAWT) became the most widespread and largest WE harvesting machines. Nevertheless, significant challenges still lie ahead of further expansion of HAWT, namely concerning systemrobustness and cost-of-energy(COE) competitiveness. This dissertation studies aHAWT design concept termed modern Active Stall Control (ASC). With this concept HAWT power regulation is achieved using flowcontrol actuators to trim the aerodynamic loads across the operational envelope. The underpinning idea is that as the aerodynamic loads are trimmed by flowcontrol actuatorswithout pitching the blades, the pitch system may be mitigated. In turn, this might lead to decreased failure-rates and down-time, and thus eventually present a more cost-effective solution than state-of-the art HAWTs. Going specifically into ASC, if aerodynamic load trimming is performed it is necessary to employ a flow control actuator. From different flow control actuator types, since the aim is to reduce the maintenance and operational costs of ASC machines, actuators with few mechanical parts become more interesting. As such the present research also focuses on the Alternating Current Dielectric Barrier Discharge (AC-DBD) plasma actuator, owing among other things to its absence of moving parts, negligible mass and virtually unlimited bandwidth of actuation. A preliminary study on the feasibility of active stall control to regulate HAWT power production in replacement of the pitch system is conducted. By taking the National Renewable Energy Laboratory 5MWturbine as reference, a simple blade element momentum code is used to assess the required actuation authority. Considering half of the blade span is equipped with actuators, the required change in the lift coefficient to regulate power is estimated in ¢Cl Æ 0.7. Concerning actuation technologies, three flow control devices are investigated, namely Boundary Layer Transpiration, Trailing Edge Jets and Dielectric Barrier Discharge plasma actuators. Results indicate the authority of the actuators considered is not sufficient to regulate power, since the change in the lift coefficient is not large enough. Especially if a pitch-controlledmachine is used as baseline case. Active stall control of Horizontal AxisWind Turbines appears feasible only if the rotor is re-designed from the start to incorporate active-stall devices. Regarding AC-DBD plasma actuators, three specific topics are investigated. The different studies aim at DBD performance characterization, namely at the influence of external flow on DBD plasma momentum transfer and on the frequency response of actuator flow region characteristic of DBD pulse operation. Both these topics are important to bridge the gap between academic-laboratory employment of DBD and large-size industrial applications. Finally regarding DBD plasma actuator modeling, a method is developed to describe plasma actuation effects in integral boundary layer formulation, and coupled to a viscous-inviscid panel code (similar to XFOIL), while an experimental campaign is carried to validate the predictions. The three DBD plasma studies are further described below. Addressing cross-talk effects between DBD plasma actuators and external flow, a study is carried out in which an actuator is positioned in a boundary layer operated in a range of free stream velocities from 0 to 60m/s, and tested both in counter-flow and co-flow forcing configurations. Electrical measurements and a CCD camera are used to characterize the DBD performance at different external flow speeds, while the actuator thrust is measured using a sensitive load cell. Results show the power consumption is constant for different flow velocities and actuator configurations, while the plasma light emission is constant for co-flow forcing but increases with counter-flow forcing for increasing free stream velocities. The measured force is constant for free stream velocities larger than 20m/s, with same magnitude and opposite direction for the counter-flow and co-flow configurations. In quiescent conditions the measured force is smaller due to the change in wall shear force by the induced wall-jet. In addition to the experimental study, an analytical model is presented to estimate the influence of external flow on the actuator force. It is based on conservation of momentum through the ion-neutral collisional process while including the contribution of the wall shear force. Model results compare well with experimental data at different external flow velocities, while extrapolation to larger velocities shows variation in actuator thrust of at least 10% for external speedU Æ 200m/s. Concerning the response of DBD actuator region flow to pulsed operation, a methodology is provided to derive the local frequency response of flow under actuation, in terms of the magnitude of actuator induced velocity perturbations. The method is applied to an AC- DBD plasma actuator but can be extended to other kinds of pulsed actuation. For the derivation, the actuator body force termis introduced in the Navier-Stokes equations, from which the flow is locally approximated with a linear-time-invariant (LTI) system. The proposed semi-phenomenologicalmodel includes the effect of both viscosity and external flow velocity, while providing a system response in the frequency domain. Experimental data is compared with analytical results for a typical DBD plasma actuator operating in quiescent flow and in a laminar boundary layer. Good agreement is obtained between analytical and experimental results for cases below the model validity threshold frequency. These results demonstrate an efficient yet simple approach towards prediction of the response of a convective flow to pulsed actuation. Future application of the methodology might include actuation scheduling design and optimization for different flow control scenarios. The third study specifically addressing DBD plasma actuators presents a methodology to model the effect of DBD plasma actuators on airfoil performance within the framework of a viscous-inviscid airfoil analysis code. The approach is valid for incompressible, turbulent flow applications. The effect of (plasma) body forces in the boundary layer is analyzed with a generalized form of the von Kármán integral boundary layer equations. The additional terms appearing in the von Kármán equations give rise to a new closure relation. The model is implemented in a viscous-inviscid airfoil analysis code and validated by carrying out an experimental study. PIV measurements are performed on an airfoil equipped with DBD plasma actuators over a range of Reynolds number and angle of attack combinations. Balance measurements are also collected to evaluate the lift and drag coefficients. Results show the proposed model captures the magnitude of the variation in IBL parameters from DBD actuation. Additionally the magnitude of the lift coefficient variations (¢Cl ) induced by plasma actuation is reasonably estimated. As such, this approach enables the design of airfoils specifically tailored for DBD plasma flow control. Transitioning into ASCrotor design, and building on the previously presented, a methodology is introduced for designing airfoils suitable to employ actuation in a wind energy environment. The novel airfoil sections are baptized WAP (Wind Energy Actuated Profiles). A genetic algorithm based multiobjective airfoil optimizer is formulated by setting two cost functions, one for wind energy performance and the other representing actuation suitability. The wind energy cost function considers ’reference’ wind energy airfoils while using a probabilistic approach to include the effects of turbulence and wind shear. The actuation suitability cost function is developed for HAWT active stall control, including two different control strategies designated by ’enhanced’ and ’decreased’ performance. Two different actuation types are considered, namely boundary layer transpiration and DBD plasma. Results show that using WAP airfoils provides much higher control efficiency than adding actuation on reference wind energy airfoils, without detrimental effects in non-actuated operation. The WAP sections yield an actuator employment efficiency that is 2 to 4 times larger than obtained with reference wind energy airfoils. Regarding geometry, WAP sections for decreased performance display an upper surface concave aft-region compared to typical wind energy ’reference’ airfoils,while retaining common sharp nose and S-tail (characteristic aft-loading) features. Results show there is much to gain in designing airfoils from the beginning to include actuation effects, especially compared to employing actuation on already existing airfoils, which ultimately might pave theway for novelHAWT control strategies. Finally addressing the complete rotor planform design, an optimization study tailors a HAWT rotor to ASC operation, in a aero-structural-servo formulation. The study considers the aerodynamic and structural loads are in static equilibrium, and as such no unsteady effects are taken into account. The optimization includes planformgeometry design but also actuation scheduling, rated rotational speed and spanwise laminate skin thickness. Results show that, compared to variable-pitch turbines, ASC planform displays increased chord at inboard stations with decreased twist angle towards the tip, resulting in increased AOA. Actuation is employed to trim the (static) loads across the operational wind speed envelope and hence reduce load overshoots and associated costs. Comparing with state-of-the-art pitch machines, the expected COE of the ASC rotor does not indicate a significant decrease, but appears to be at least competitive with pitch-controlled HAWTs if the pitch system is effectively mitigated. Additionally, and though not explicitly considered in the present work, it is foreseen ASC might become interesting if the actuation system allows for further OM cost reduction via fatigue load-alleviation, since the actuation trimming load system is anyhow included in an ASC machine.