Small Wind Turbine Starting Behaviour

Small wind turbines that operate in low-wind environments are prone to suffer performance degradation as they often fail to accelerate to a steady, power-producing condition. The behaviour during this process is called “starting behaviour” and it is the subject of this present work. This thesis evaluates potential benefits that can be obtained from the improvement of starting behaviour, investigates, in particular, small wind turbine starting behaviour (both horizontal- and vertical-axis), and presents aerofoil performance characteristics (both steady and unsteady) needed for the analysis. All of the investigations were conducted using a new set of aerodynamic performance data of six aerofoils (NACA0012, SG6043, SD7062, DU06-W-200, S1223, and S1223B). All of the data were obtained at flow conditions that small wind turbine blades have to operate with during the startup - low Reynolds number (from 65000 to 150000), high angle of attack (through 360◦), and high reduced frequency (from 0.05 to 0.20). In order to obtain accurate aerodynamic data at high incidences, a series of CFD simulations were undertaken to illustrate effects of wall proximity and to determine test section sizes that offer minimum proximity effects. A study was carried out on the entire horizontal-axis wind turbine generation system to understand its starting characteristics and to estimate potential benefits of improved starting. Comparisons of three different blade configurations reveal that the use of mixed-aerofoil blades leads to a significant increase in starting capability. The improved starting capability effectively reduces the time that the turbine takes to reach its power-extraction period and, hence, an increase in overall energy yield. The increase can be as high as 40%. Investigations into H-Darriues turbine self-starting capability were made through the analogy between the aerofoil in Darrieus motion and flapping-wing flow mechanisms. The investigations reveal that the unsteadiness associated with the rotor is key to predicting its starting behaviour and the accurate prediction can be made when this transient aerofoil behaviour is correctly modelled. The investigations based upon the analogy also indicate that the unsteadiness can be exploited to promote the turbine ability to self-start. Aerodynamically, this exploitation is related to the rotor geometry itself

Performance modelling of the Darrieus wind turbine

Three-dimensional numerical investigation of the Darrieus wind turbines equipped with different aerofoils is presented in this paper. In the modelling, the computational domain was divided into three different domains and they are blade, rotor, and tunnel domains. A cylindrical domain was created to cover the blade area so that a fine mesh can be applied. The Computational Fluid Dynamics (CFD) was employed to solve and analyze the flow field around the turbine. The Menter Shear Stress turbulence model was chosen in this investigation. Boundary conditions applied were velocity at the inlet, pressure opening at the outlet, and symmetry on other sides. Comparison of simulation results and experiments showed good agreement. The investigation of the effects of the rotor solidity and the aerofoil shape was performed. The simulation results reveal that the aerofoil shape has a significant impact on the turbine performance. For the rotor solidity of 0.7, the change from the NACA section to the S1046 leads to a reduction of power at low tip speed ratios but the performance improvement is observed when the tip speed ratio is greater than 1.5. With the lower solidity of 0.375, the effects of the aerofoil change is less pronounced at low tip speed ratios. Nevertheless, the maximum power coefficient increases for both cases. Further analysis shows that the S1046 is less sensitive to the wind speed change and is promising in the urban application where the wind speed is relatively low

The Physics of H-Darrieus Turbines Self-Starting Capability: Flapping-Wing Perspective

It has been widely reported that Darrieus turbines cannot self-start and that they require external assistance to accelerate to their operating tip speed ratios. However, recent experiments have shown conclusively that H-Darrieus rotors with fixed-pitch blades that employ a symmetrical aerofoil can reliably self-start in steady controlled environments. Previous attempts have also been made to model the starting characteristics but there still exists a significant discrepancy between the experimental data and model prediction, suggesting that our understanding of this starting characteristic remains weak. The investigation and explanation of the starting characteristics is the focus of this paper. The investigation was made through a careful analysis of aerofoils that undergo Darrieus motion, giving some insights on how the blade experiences different flow conditions and how driving force is developed over the flight path. The analysis reveals that the aerofoil in Darrieus motion is analogous to flapping wing mechanism; the mechanism that fish and birds employ to generate propulsion. The explanation of flow physics and torque development can then be made through a simple pitch-heave concept. The investigation using this concept together with observations of flapping creatures suggests that the key feature that promotes driving torque generation and the ability to self-start is the unsteadiness associated with the rotor. This unsteadiness is related to chord-to-diameter ratio. This, together with blade aspect ratio, and number of blades, is the reason why H-Darrieus turbines that employ a symmetrical aerofoil can self-start.</jats:p

Effects of Wind Turbine Starting Capability on Energy Yield

The purpose of this study was to investigate the effect of turbine starting capability on overall energy-production capacity. The investigation was performed through the development and validation of matlab /Simulink models of turbines. A novel aspect of this paper is that the effects of load types, namely resistive heating, battery charging, and grid connection were also investigated. It was shown that major contributors to improved starting performance are aerodynamic improvements, reduction of inertia, and simply changing the pitch angle of the blades. The first two contributors can be attained from an exploitation of a “mixed-airfoil” blade.The results indicate that starting ability has a direct effect on the duration that the turbine can operate and consequently its overall energy output. The overall behavior of the wind turbine system depends on the load type, these impose different torque characteristics for the turbine to overcome and lead to different power production characteristics.When a “mixed-airfoil” blade is used the annual energy production of the wind systems increases with the exception of resistive heating loads. Net changes in annual energy production were range of −4% to 40% depending on the load types and sites considered. The significant improvement in energy production strongly suggests that both the starting performance and load types should be considered together in the design process

Unsteady Surface Pressures and Airload of a Pitching Airfoil

This paper presents unsteady surface pressure variations of a symmetrical NACA0012 section commonly used on Vertical-axis wind turbine blades. The section was tested at a Reynolds number of 90,000, mean incidence angles of 15 and -15, and a reduced frequency of 0.20. The experimental results show that the airfoil motion had significant effects on surface pressure distribution over the airfoil, leading to a formation and shedding of an energetic leading edge vortex which strongly affects the overall airload over the incidence range

The Physics of H-Darrieus Turbine Starting Behavior

This paper provides a resolution to the contradictory accounts of whether or not the Darrieus turbine can self-start. The paper builds on previous work proposing an analogy between the aerofoil in Darrieus motion and flapping-wing flow mechanisms. This analogy suggests that unsteadiness could be exploited to generate additional thrust and that this unsteady thrust generation is governed by rotor geometry. Rotors which do not exploit this unsteadiness will not self-start. To confirm the hypothesis, unsteady effects were measured and then incorporated into a time-stepping rotor analysis and compared to experimental data for self-starting wind turbines. When unsteady effects were included, the model was able to predict the correct starting behavior. The fundamental physics of starting were also studied and parameters that govern the generation of unsteady thrust were explored, namely, chord-to-diameter and blade aspect ratios (ARs). Further simulation showed that the Darrieus rotor is prone to be locked in a deadband where the thrust is not continuous around a blade rotation. This discrete thrust is caused by the large variation in incidence angle during startup, making the Darrieus blade ineffective during part of the rotation. The results show that unsteady thrust can be promoted through an appropriate selection of blade aspect and chord-to-diameter ratios, therefore self-starting rotors may be designed. A new definition of self-starting is also proposed

The physics of H-Darrieus turbine starting behavior.

This paper provides a resolution to the contradictory accounts of whether or not the Darrieus turbine can self-start. The paper builds on previous work proposing an analogy between the aerofoil in Darrieus motion and flapping-wing flow mechanisms. This analogy suggests that unsteadiness could be exploited to generate additional thrust and that this unsteady thrust generation is governed by rotor geometry. Rotors which do not exploit this unsteadiness will not self-start. To confirm the hypothesis, unsteady effects were measured and then incorporated into a time-stepping rotor analysis and compared to experimental data for self-starting wind turbines. When unsteady effects were included, the model was able to predict the correct starting behavior. The fundamental physics of starting were also studied and parameters that govern the generation of unsteady thrust were explored, namely, chord-to-diameter and blade aspect ratios (ARs). Further simulation showed that the Darrieus rotor is prone to be locked in a deadband where the thrust is not continuous around a blade rotation. This discrete thrust is caused by the large variation in incidence angle during startup, making the Darrieus blade ineffective during part of the rotation. The results show that unsteady thrust can be promoted through an appropriate selection of blade aspect and chord-to-diameter ratios, therefore self-starting rotors may be designed. A new definition of self-starting is also proposed