van Gjw Gerard Bussel

The science of making more torque from wind: Diffuser experiments and theory revisited.

History of the development of DAWTs stretches a period of more than 50 years. So far without any commercial success. In the initial years of development the conversion process was not understood very well. Experimentalists strived at maximising the pressure drop over the rotor disk, but lacked theoretical insight into optimising the performance. Increasing the diffuser area as well as the negative back pressure at the diffuser exit was found profitable in the experiments. Claims were made that performance augmentations with a factor of 4 or more were feasible, but these claims were not confirmed experimentally. With a simple momentum theory, developed along the lines of momentum theory for bare windturbines, it was shown that power augmentation is proportional to the mass flow increase generated at the nozzle of the DAWT. Such mass flow augmentation can be achieved through two basic principles: increase in the diffuser exit ratio and/or by decreasing the negative back pressure at the exit. The theory predicts an optimal pressure drop of 8/9 equal to the pressure drop for bare windturbines independent from the mass flow augmentation obtained. The maximum amount of energy that can be extracted per unit of volume with a DAWT is also the same as for a bare wind turbine. Performance predictions with this theory show good agreement with a CFD calculation. Comparison with a large amount of experimental data found in literature shows that in practice power augmentation factors above 3 have never been achieved. Referred to rotor power coefficients values of CP,rotor =2.5 might be achievable according to theory, but to the cost of fairly large diffuser area ratios, typically values of β >4.5.

Simulating Dynamic Stall in a 2D VAWT: Modeling strategy, verification and validation with Particle Image Velocimetry data

The implementation of wind energy conversion systems in the built environment renewed the interest and the research on Vertical Axis Wind Turbines (VAWT), which in this application present several advantages over Horizontal Axis Wind Turbines (HAWT). The VAWT has an inherent unsteady aerodynamic behavior due to the variation of angle of attack with the angle of rotation, perceived velocity and consequentially Reynolds number. The phenomenon of dynamic stall is then an intrinsic effect of the operation of a Vertical Axis Wind Turbine at low tip speed ratios, having a significant impact in both loads and power. The complexity of the unsteady aerodynamics of the VAWT makes it extremely attractive to be analyzed using Computational Fluid Dynamics (CFD) models, where an approximation of the continuity and momentum equations of the Navier-Stokes equations set is solved. The complexity of the problem and the need for new design approaches for VAWT for the built environment has driven the authors of this work to focus the research of CFD modeling of VAWT on: •comparing the results between commonly used turbulence models: URANS (Spalart-Allmaras and k-) and large eddy models (Large Eddy Simulation and Detached Eddy Simulation) •verifying the sensitivity of the model to its grid refinement (space and time), •evaluating the suitability of using Particle Image Velocimetry (PIV) experimental data for model validation. The 2D model created represents the middle section of a single bladed VAWT with infinite aspect ratio. The model simulates the experimental work of flow field measurement using Particle Image Velocimetry by Simao Ferreira et al for a single bladed VAWT. The results show the suitability of the PIV data for the validation of the model, the need for accurate simulation of the large eddies and the sensitivity of the model to grid refinement.

The DOWEC Offshore Reference Windfarm: Analysis of Transportation for Operation and Maintenance

The Dutch Offshore Wind Energy Converter project (DOW EC) aims to design large wind turbines for application in large-scale offshore farms. A baseline windfarm has been considered 43 km off the Dutch coast, with 80 wind turbines, each of 6 MW capacity. In this paper, access methods for O&M crew have been investigated. The methods include: (i) inflatable boats (zodiacs), (ii) special offshore access systems and (iii) helicopters. For the methods considered, operational limits have been established, expressed as ‘maximum-mean’ wind speed and as significant wave height. On the basis of the NEXT database the statistics of the corresponding weather-windows have been determined. These statistics, together with the failure statistics and the regular maintenance dem and of the wind turbines and the other offshore windfarm components determine its overall-availability at the specific site. Estimates of this overall-availability of the offshore windfarm have been obtained with a sophisticated Monte Carlo simulation model of the operations within the windfarm. The simulations show that use should be made of access methods with which an accessibility of at least 82% of time can be achieved. This rules out standard options, e.g. direct inflatable boat access, in order to maintain a windfarm availability greater than 90%.

Combined technical and economic evaluation of the Northern European offshore wind resource

A model encapsulating the major technical and economic features of bottom-mounted offshore wind farms is described. Intended as a preliminary design tool, the model has been used for studies of the relationship between farm design parameters and the resulting cost of energy. When coupled with a Geographical Information System, the model facilitates evaluation of the offshore wind resource across wide regions. Starting with a limited quantity of environmental information, contour maps showing areas suitable for offshore wind farm construction, and the cost of the energy that would result can be produced. The system has been used to assess the Northern European offshore resource, allowing conclusions to be drawn regarding the influence of turbine size on the cost of energy.

Experimentally observed effects of yaw misalignment on the inflow in the rotor plane

Near-wake measurements, focussed on yawed flow conditions, are conducted on a wind turbine rotor model in an open jet wind tunnel. Tip vortex center locations and phase-locked average flow velocity distributions are recorded. Experimental conditions at the blade are not measured and hence are estimated from a measurement analysis tool, named inverse vortex wake model. The unknown bound circulation is derived by the vortex wake type method without the need for an airfoil model. The analysis of the effects yawed operating conditions have on the blade flow conditions concentrates on the individual contributions of tip, root, trailed and shed vortices to the inflow in the rotor plane.

Bayesian Analysis Applied to Statistical Uncertainties of Extreme Response Distributions of Offshore Wind Turbines

Extreme response is an important design variable for wind turbines. The statistical uncertainties concerning the extreme response distribution are simulated here with data concerning physical characteristics obtained from measurements. The extreme responses are the flap moment at the blade root and the overturning moment of the support structure of an offshore wind turbine situated in the North Sea. The statistical uncertainties concern the choice of the distribution model and uncertainties concerning the distribution parameters. The uncertainties are treated with Bayesian analysis. The inclusion of the uncertainties has only marginal effect for the calculated long-term estimates of extreme responses when non-informative priors for the distribution parameters are used. The inclusion of uncertainties may have larger effect if data concerning the moments are obtained from measurements.

The use of the asymptotic accelertion potential method for horizontal axis windturbine rotor aerodynamics

The acceleration potential method is introduced as a powerful approach to develop computational tools for aerodynamic calculations on horizontal axis windturbine rotors. The basic equations are given as well as a general analytical first-order asymptotic solution. Three computer codes are described in some detail as well as their application. In its simplest appearance the method is equivalent to a lifting line method with a wake relaxing in axial direction. The code in which this model is implemented, PREDICHAT, has been used extensively for calculations of performance and (stationary) blade loads. A more elaborated code, VIAX, has been developed specifically for the calculation of axial velocities in the near wake. Finally an approach is presented for the computation of loads under dynamic inflow conditions. Some results of the various codes are presented.