Two-bladed Turbine Research Articles

Symmetry Considerations When Using Proper Orthogonal Decomposition for Predicting Wind Turbine Yaw Loads

In an earlier study, the authors discussed the efficiency of low-dimensional representations of inflow turbulence random fields in predicting statistics of wind turbine loads that included blade and tower bending moments. Both root-mean-square and 10-min extreme statistics for these loads were approximated very well when time-domain simulations were carried out on a 600kW two-bladed turbine and only a limited number of inflow “modes” were employed using proper orthogonal decomposition (POD). Here, turbine yaw loads are considered and the conventional ordering of POD modes is seen to be not as efficient in predicting full-field load statistics for the same turbine. Based on symmetry arguments, reasons for a different treatment of yaw loads are presented and reasons for observed deviation from the expected monotonic convergence to full-field load statistics with increasing POD mode number are illustrated.

An assessment of governmental wind power programmes in Sweden—using a systems approach

The purpose of this paper is to assess the effects, the cost efficiency, and the goal achievement of policy instruments aimed for the development and deployment of wind power in Sweden during the period 1975 to 2000. The paper presents an empirical example of a socio-technical system-based approach for impact assessment, in which changes in the wind power system are described and analysed with respect to technology development, cost development and actors’ involvement. The results show that the policy instruments were not designed to have a broad, system-oriented perspective but targeted and included restricted technology concepts—i.e. large, two-bladed turbines—and limited involvement of actors. The assessment shows that early inflexible steering of technology and market development, together with a lack of comprehensive, long-term strategy, lack of continuity in policy interventions and weak combinations of policy programmes and measures have contributed to a very limited wind power development in Sweden. A rough cost efficiency analysis reveals that Sweden has much less wind power installed in relation to expenditures on various support than Germany and Spain. The Swedish policy instruments did, however, achieve the Government's goals to increase wind energy production.

Numerical Simulation for Flow Fields of Darrieus Turbine.

Flow around a rotating Darrieus turbine was simulated by solving 2-d Navier-Stokes equations. The equations were discretized by the Finite volume method for space and the Fractional step method for time. The convection terms were specially discretized by an upwinding scheme for an unstructured grid. In this report, single-blade and two-blade turbine without the duct were simulated. Values of the parameters were as follows : blade section, NACA 0018 ; and Reynolds number, 3×105. The results showed good agreement with experimental data in the following point : optimum tip speed ratio decreases inversely with solidity. Simulated flows were visualized by stream lines, vorticity, pressure distribution, and streak lines. The visualized flow fields can explain the torque-generating mechanism.

An experimental investigation of blade element theory for wind turbines. Part 1. Mean flow results

This paper describes an investigation of blade element theory, which is commonly used to describe the flow through wind turbine blades. A two-bladed model turbine was shrouded by a constant-diameter pipe. The tip clearance was sufficiently small to minimise and localise the tip clearance losses. By passively heating the wake of one of the turbine supports, it was found that supports did not produce any significant circumferential non-uniformity in the mean axial and circumferential velocities. These velocities were measured using a conventional three-hole yawmeter and an X-probe hot-wire anemometer. The agreement between the two methods was good. The turbine's power output was also determined by two independent methods; again with good agreement. The major finding was that blade element theory underestimated the power contribution as a function of radius wherever the local angle of attack exceeded the angle that gives the maximum lift/drag in two-dimensional flow. Several possible causes of the discrepancy are discussed.

Some effects of finite solidity on the aerodynamics of horizontal-axis wind turbines

This paper describes an exploratory, computational study of the effects of local solidity on the aerodynamics of a horizontal-axis wind turbine. The parameters governing the path of the trailing vorticity in the crude model employed were chosen to match the circulation measured behind a two-bladed model turbine by previous workers. Traditional blade element theory, which assumes two-dimensional aerofoil characteristics, significantly underestimated the measured circulation at high angles of incidence. The calculations suggest that the major departure from two-dimensional behaviour is a reduction in the surface vorticity near the leading edge due to the trailing vorticity of a blade. This reduction, which is a consequence of finite local solidity, appears sufficient to prevent boundary layer separation at angles at which this would occur for two-dimensional flow. The magnitude of the reduction decreases as the angle of incidence is reduced. The results suggest that the optimization of wind turbine design will require fully three-dimensional modelling.

Optimum Design Method of Horizontal-axis Wind Turbine Blades Based on Lifting-line Theory

A design method of horizotal-axis wind turbines is presented, which is based on the lifting-line theory. †The design theory has two themes ; one is to obtain the circulation distribution of the minimum energy lost for a given output power, the other is to determine the expanded profiles and the pitch curve of a wind turbine blade for a given circulation distribution. The Goldstein function can be applied to the former theme. Kramer's diagram for two-bladed, three-bladed and four-bladed wind turbines are represented by means of the Goldstein function. Applying the present method, wind turbine blades are designed and their profiles are investigated. Furthermore, the performance of a two-bladed wind turbine is obtained by the experiments and compared with the numerical result calculated by the lifting-line theory considering viscous effects. It is concluded that the present design method is applicable to horizontal-axis turbines.