This article describes an experimental, aerodynamic investigation of four aerofoils intended for small wind turbine applications. The aerofoils of these small machines (both horizontal and vertical axes) normally experience conditions that are quite different from large-scale machines due to smaller chord length and lower wind speed, resulting in significantly lower Reynolds numbers. They also operate with an unusually wide range of incidence angles (0° to 90° for horizontal axis and 0° to 360° for vertical axis). Four appropriate aerofoils were chosen for testing at three Reynolds numbers (65 000, 90 000, and 150 000) through 360° incidence to cover almost all possible conditions that might be encountered by both types of turbines. The investigations were conducted in terms of lift, drag, and surface static pressure coefficients. The experimental results show that both geometry and Reynolds number had significant effects on aerodynamic lift, not only when unstalled but particularly in the post-stall region from 20° to 50° incidence. These effects were also seen at other incidences but to a lesser extent. By contrast, the drag characteristics were similar for all blade geometries. Static pressure measurement revealed that, at these low Reynolds numbers, separation bubbles always form near the leading edge of the suction surface at moderate incident angles and increase in size with decreasing Reynolds number. Comparisons of force and static pressure measurements showed that the aerofoil stalling behaviour is closely related to the presence of a separation bubble at the leading edge of the suction surface. Discrepancies between the experiments and predictions using the AERODAS model confirm the continued need for accurate wind tunnel testing.
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.
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 behaviour.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. 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 of incidence angle during start-up 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. List of SymbolsAR Aspect ratio B Number of blades c Aerofoil chordDistance between blades * Address all correspondence to this author. Aerofoil operating modes INTRODUCTIONAs energy demand grows, the use of small wind turbines has become increasingly attractive. Small wind turbines can be broadly categorised as horizontal-axis and vertical-axis, each having its own niche applications. In general, the latter is better suited to the urban environment where wind direction rapidly changes as it is insensitive to wind direction. However, Darrieus (vertical-axis) turbines are commonly believed to be non self-starting [1,2].Despite this perception, it has been increasingly reported that the turbine can self-start in even its simplest configuration of fixed-pitch, straight blades with a symmetrical aerofoil section [3][4][5][6][7]. These conflicting perceptions and observations indicates that there may be a physical aerodynamic mechanism which is not well understood.Advances continue to be made in understanding of the physics of torque generation and starting behaviour. Hill et al [6] investigated the starting performance of a three-bladed H-rotor with NACA0018 blades, experimentally and numerically. Experimental wind-tunnel testing had demonstrated unaided startup in steady wind conditions. According to Hill et al, there are four main processes taking place during startup (Fig. 1b). The first process is an acceleration in which the turbine rotational speed linearly increases with time (1st acceleration). The turbine then enters its idling period when the rotor speed rema...
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.
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