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

Aerodynamic investigation of the start-up process of H-type vertical axis wind turbines using CFD

In this study, a CFD start-up model has been built after conducting the sensitivity studies to evaluate the self-starting behaviour of the H-type vertical axis wind turbines (VAWTs). The self-starting behaviour of a well-investigated VAWT is used for the model validation, and then the details of aerodynamics of the start-up process have been examined. Finally, the effect of the moment of inertia and the blade number on the aerodynamic behaviour of the self-starting and power performance of the H-type VAWT are analysed. It has been found that in the critical region, where TSR<1, the contribution of the drag to the torque generation plays a significant role in the second and third quarters of the rotor revolution, where the azimuthal position varies between 100° and 253°. The results also show that increasing the turbine inertia did not show a noticeable effect on the start-up behaviour of the turbine and final rotational speed. However, an increase in the instantaneous turbine power during the start-up process after the optimum TSR is observed with decreasing the turbine inertia. The current findings also show that an increase in the blade number makes the turbine easier to start-up; however, this may reduce the turbine power coefficient

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

Identifying knowledge transfer requirement in global organisational contexts

This article studies the inter-organisational (IO) knowledge transfer in global supply chains. A representative case was selected that consists of a number of laboratory analysis equipment dealer companies in Thailand and their customer companies in different industries in various parts of the world. Using the theories and frameworks in knowledge transfer in inter-organisational relationships, a survey questionnaire instrument was designed and data were collected from 73 employees of dealer companies and 87 employees of their customer companies (users of the laboratory equipment) scattered in different industries worldwide. Results indicate that the intra-organisation characteristics of the dealer companies which are expected to affect transfer of the necessary knowledge to the customers has no significant relationship with the nature of knowledge that is transferred or with the inter-organisation management. Moreover, there is no statistical significance in the intra-organisation relationships within the client companies perhaps because the majority of laboratory analysis equipment dealer companies import a great part of their products from overseas companies and they do not possess the technology themselves. Based on the findings, appropriate guidelines were provided for the enhancement of the effectiveness of the knowledge transfer process within the wide case study of this research. © 2012 Inderscience Enterprises Ltd

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