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Stall

Public summary - Passive stall control

Although the number of, installed, wind turbines in offshore wind farms is increasing rapidly, there are still many challenges ahead for making the cost of generating energy from wind competitive with other energy sources. One method for making the cost of energy from wind market competitive is to reduce the operational and maintenance cost of wind turbines, which is more substantial for offshore wind farms compared to their onshore counterparts. The operational and maintenance cost of wind turbines may be reduced by eliminating, as much as possible, rotating components of the turbine which are prone to wear and tear. Among the rotating components of a wind turbine, the control mechanism for regulating power and load is of interest in this project.

With recent advances in composite technology for tailoring the structural response of composite structures, it may be possible to apply the composite optimization to the conventional passive stall control scheme. Particularly, the use of twist coupling for regulating, passively, the angle of attack thus also the torque and power of the wind turbine, shows a promise to design adaptive blades for stall regulated wind turbines, with improved performance.

The aim of the project is to investigate the possibility of replacing the pitch system with stall control system to regulate the load/power of a 5mW wind turbine in order to reduce the cost of far-offshore wind energy.

The project is divided as follows:

• Write an aeroelastic tool for wind turbine analysis suitable for multipurpose optimization(shape and sizing optimization). This includes: suitable parametrization method for the blade geometry and material properties to facilitate shape and sizing optimization; create a nonlinear beam model including sensitivity of the structural responses; create an aerodynamic tool, based on BEM, capable of incorporating effect of flexible and/or non-straight blades on the aerodynamic loads;  couple the structural and aerodynamic modules for aeroelastic simulation including sensitivity analysis of the aeroelastic loads.
• Setup a variable fidelity analysis tool that combines high-fidelity structural analysis tool and the aeroelastic tool, with the correct sensitivity. In addition, formulate an approximation method of the responses to be used for the sizing (material design variables) optimization routine.
• Formulate a methodology to combine different type of optimization. This means to find a method to combine shape optimization routine with sizing optimization, in a staggered approach.  This opens up the possibility of investigating the optimum blade design in terms of shape and material simultaneously.
• Perform optimization on the Cost Of Energy (COE) using the two power regulation techniques (pitch and stall) and compare the relative difference of the (minimum) COE between the two power regulation techniques.  

 

Results show a significant increase of the blade twist towards stall (up-to 2 degrees of induced twist towards stall for unbalanced glass fibre laminates). The final relative cost of energy is reduced to 1.06 times the COE of the NREL5MW pitch regulated machines, assuming the same O&M cost for stall regulated machines. This makes the potential reduction in COE conservative and further reduction is possible provided that a better cost model for large scale stall regulated wind turbines is formulated. Furthermore, it is assumed that by reducing the use of the pitch system by adapting  passive stall control scheme, the failure rates of HAWT will reduce, and hence decrease the risks involved for the different stakeholders. Finally, it is observed that stall regulated wind turbines with down-wind configuration exhibit better performance, in reducing COE, compared to wind turbines with up-wind configuration. Therefore, stall regulated wind turbines with down-wind configuration and twist coupled blades show promise to replace pitch regulated machines in offshore wind farms. However, further research is needed to realize this goal. 

The results of this project are:

Multi-Fidelity Optimization Framework
An optimization software is developed to design the next generation composite wind turbine blades. The optimization tool is gradient based, meaning the derivatives of the responses is used to find the minimum of the objective that satisfies all the constraints. Furthermore, a novel methodology is implemented for optimizing composite materials resulting in an efficient procedure to find the optimum composite layout for the wind turbine blade that yields the desired performance of the blade. The tool is also capable of performing shape optimization, meaning both the planform (chord and twist) and the beam axis (e.g. straight or curved blades) can be used as design variables during the optimization process.

New blade design for 5MW stall regulated wind turbine
A new 61.5m blade design is presented for 5MW stall regulated wind turbine, to be used in offshore wind farms. The composite material, along the blade span, is optimized to significantly increase the blade twist towards stall (up-to 2 degrees of  induced  twist towards stall  for glass fibres), thus stalling the blade further improving the effectiveness of the stall control methodology for large-scale wind turbines.

Cost of energy for stall regulated wind turbines
The reduction of cost of energy is assessed based on COE for the NREL 5MW pitch-regulated wind turbine. It is estimated the cost of energy for stall regulated turbines can be reduced to 1.06 times the COE of the NREL5MW pitch regulated machine. From the study, it is concluded that it is possible to replace the pitch system for power regulation. However, the pitch mechanism is still needed for emergency cases. Since the operational demand for the pitch mechanism is decreased substantially, when switched to stall control for power regulation, the availability of the turbine increases, decreasing the cost of energy for the new stall regulated wind turbine.

In summary the estimated COE reduction potential is resulting in about 1.0%.

Public summary - Active stall control

Active Stall Control of HAWTs was employed in the early days of commercial wind energy development for small to medium-sized machines. Current HAWTs mostly employ variable-speed and variable-pitch to perform power regulation across the operational wind speed envelope. However, with the recent tendency for increasing HAWT size and the trend of installing wind farms further offshore, a renewed interest is placed in Active Stall Control for providing a power regulation technology for future HAWTs.

Modern ASC is a concept in which flow control actuators are used to trim the aerodynamic loads across the operational wind speed envelope of  HAWTs. The main idea is that the pitch system could be mitigated by employing flow control actuators which actively promote stalling of the blades. The flow control actuators thus regulate the aerodynamic loads experienced by the turbine across the operational envelope without using the pitch system. As such, it would be possible to mitigate the pitch system which may result in decreased failure rates and replacement and overhaul costs. 

The aim of this project is to investigate the concept of ASC for large offshore HAWT. Emphasis is put on characterization of flow control devices, tailored airfoil design and ASC rotor planform and control strategy design, within the framework of FLOW Theme 4: Far-Offshore Turbine Development – R&D line 4.4: Blade Development. The main results of the project are:

Preliminary Study ASC Concept for large Off-shore HAWT
A preliminary study showed a large portion of the blades must be actuated, and the actuators must be effective especially at large angles of attack. Different actuation technologies have advantages and drawbacks. DBD plasma actuators become attractive since they have no moving parts, but transfer a limited amount of momentum to the air and it is thus not clear whether they provoke separation at the Reynolds numbers being considered. Active stall control of HAWT is feasible only if the blade and the airfoils are designed from the beginning to be active stall controlled.

DBD Plasma Actuators Characterization and Modelling
An experimental study assessed the influence of the external flow velocity on the momentum transfer capability of DBD plasma actuators, for both counter-flow and co-flow configurations. A simple analytical model is proposed to estimate the influence of the external flow velocity on the momentum transfer of DBD actuators. Satisfactory agreement between modelled results and experimental data was obtained for both co-flow and counter-flow DBD configurations at different external flow speeds. Extrapolation of the analytical model to larger flow speeds indicates the DBD force may vary by 15% if the external flow speed reaches 200m/s .

A methodology is also provided to estimate the local frequency response of flow under actuation corresponding to DBD plasma actuators operating in pulse mode. The analytical model includes the effect of external flow velocity and viscosity. The model is compared with experimental data for a typical DBD plasma actuator operating in quiescent flow and in a laminar boundary layer. Reasonable agreement is obtained between analytical and experimental data, and thus results demonstrate an efficient and simple approach towards prediction of the response of a convective flow to pulsed actuation. The proposed model may be used to design and optimize DBD actuation in flow control applications.

Still regarding DBD actuator modelling, a method is proposed to include the influence of DBD plasma actuators in viscous-inviscid codes used for airfoil design, suited to incompressible, turbulent flows. An experimental study of PIV measurements on an airfoil equipped with DBD plasma actuators was used for validation. The airfoil was tested at different operational Reynolds number and angles of attack. Results show the proposed model captures the magnitude of the variation in IBL parameters  and lift coefficient brought upon by the plasma actuators at different operating conditions. Ultimately the approach enables the design of airfoils specifically tailored for flow control through DBD employment.

ASC Rotor Design
Addressing the design of ASC rotor, a specific study presented novel airfoil sections suitable to employ actuation in a WE environment, named WAP (Wind Energy Actuated Profiles). The airfoils are designed with a genetic algorithm multi-objective optimizer considering two cost functions, representing wind energy performance and actuator employment suitability. Results show WAP airfoils provide control efficiency 2 to 4 times larger than obtained with reference WE airfoils, at equivalent WE performance. Regarding geometry, and compared to typical WE airfoils, WAP sections for decreased performance display an upper surface concave aft-region, while for increased performance a convex upper surface aft-region is obtained. It is clear that designing wind energy and actuation tailored airfoils paves the way for new HAWT control strategies to become seriously considered, namely active stall control.

Finally, the design of ASC rotor is carried out in an aero-structural-servo optimization, including planform design but also actuation scheduling, rated rotational speed and spanwise laminate skin thickness. Results show that, compared to variable-pitch turbines, ASC planform shows increased chord at inboard stations with decreased twist angle towards the tip, resulting in increased AOA. Actuation is employed to trim the loads and hence reduce load overshoots. Comparing with state-of-the-art pitch machines, the ASC rotor has decreased AEP, decreased blade mass and increased ICC. As for the expected COE of the ASC rotor, the results obtained do not indicate a significant decrease compared to pitch-controlled machines, but ASC appears to be at least competitive with state-of-the-art pitch-controlled HAWTs. Additionally, and even though not explicitly considered in the present study, ASC may become very interesting if the actuation system allows for further OM cost reduction via fatigue load-alleviation.

Regarding risk reduction of far-offshore WE, it is foreseen that mitigating the pitch system through ASC could reduce failure rates of HAWT, and hence reduce the risks involved for the different stake-holders. Additionally, it is expected ASC technology might contribute to accelerate the deployment of far-offshore WE within few years, though industrial and logistical constraints may not allow for immediate large scale installation of ASC turbines.

In summary the estimated COE reduction potential is resulting in about 1.1%.