0


EDITORIAL

J. Sol. Energy Eng. 2004;126(4):969-970. doi:10.1115/1.1792673.
FREE TO VIEW
Commentary by Dr. Valentin Fuster

SOLAR SCENERY

J. Sol. Energy Eng. 2004;126(4):971-972. doi:10.1115/1.1793210.

 CRES Wind Farm

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):973. doi:10.1115/1.1794711.

 WMC tests blades for European, Asian, and U.S. manufacturers (Photo source: WMC Archives)

Commentary by Dr. Valentin Fuster

TECHNICAL PAPERS

J. Sol. Energy Eng. 2004;126(4):974-985. doi:10.1115/1.1790535.

Aeroacoustic tests of seven airfoils were performed in an open jet anechoic wind tunnel. Six of the airfoils are candidates for use on small wind turbines operating at low Reynolds numbers. One airfoil was tested for comparison to benchmark data. Tests were conducted with and without boundary layer tripping. In some cases, a turbulence grid was placed upstream in the test section to investigate inflow turbulence noise. An array of 48 microphones was used to locate noise sources and separate airfoil noise from extraneous tunnel noise. Trailing-edge noise was dominant for all airfoils in clean tunnel flow. With the boundary layer untripped, several airfoils exhibited pure tones that disappeared after proper tripping was applied. In the presence of inflow turbulence, leading-edge noise was dominant for all airfoils.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):986-1001. doi:10.1115/1.1793208.

This paper presents detailed wind tunnel tests data taken on six airfoils having application to small wind turbines. In particular, lift, drag and moment measurements were taken at Reynolds numbers of 100,000, 200,000, 350,000 and 500,000 for both clean and rough conditions. In some cases, data were also taken at a Reynolds number of 150,000. The airfoils included the E387, FX 63-137, S822, S834, SD2030, and SH3055. Prior to carrying out the tests, wind tunnel flow quality measurements were taken to document the low Reynolds number test environment. Oil flow visualization data and performance data taken on the E387 compare favorably with measurements taken at NASA Langley in the Low Turbulence Pressure Tunnel. Highlights of the performance characteristics of the other five airfoils are presented.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1002-1010. doi:10.1115/1.1766024.

This paper presents the design and experimental verification of the Risø-B1 airfoil family for MW-size wind turbines with variable speed and pitch control. Seven airfoils were designed with thickness-to-chord ratios between 15% and 53% to cover the entire span of a wind turbine blade. The airfoils were designed to have high maximum lift and high design lift to allow a slender flexible blade while maintaining high aerodynamic efficiency. The design was carried out with a Risø in-house multi disciplinary optimization tool. Wind tunnel testing was done for Risø-B1-18 and Risø-B1-24 in the VELUX wind tunnel, Denmark, at a Reynolds number of 1.6×106. For both airfoils the predicted target characteristics were met. Results for Risø-B1-18 showed a maximum lift coefficient of 1.64. A standard case of zigzag tape leading edge roughness caused a drop in maximum lift of only 3.7%. Cases of more severe roughness caused reductions in maximum lift between 12% and 27%. Results for the Risø-B1-24 airfoil showed a maximum lift coefficient of 1.62. The standard case leading edge roughness caused a drop in maximum lift of 7.4%. Vortex generators and Gurney flaps in combination could increase maximum lift up to 2.2 (32%).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1011-1016. doi:10.1115/1.1807854.

In the present paper it is first demonstrated that state of the art 3D CFD codes are capable of predicting the correct dependency of the integrated drag of a flat plate placed perpendicular to the flow. This is in strong contrast to previous 2D investigations of infinite plates, where computations are known to severely overpredict drag. We then demonstrate that the computed drag distribution along the plate span deviate from the general expectation of 2D behavior at the central part of the plate, an important finding in connection with the theoretical estimation of drag behavior on wind turbine blades. The computations additionally indicate that a “tip effect” is present that produces increased drag near the end of the plate, which is opposite of the assumptions generally used in drag estimation for blades. Following this several wind turbine blades are analyzed, ranging from older blades of approximately 10 meter length (LM 8.2) over more recent blades (LM 19.1) around 20 meters to two modern blades suited for megawatt size turbines. Due to the geometrical difference between the four blades, the simple dependency on aspect ratio observed for the plates are not recovered in this analysis. The turbine blades behave qualitatively very similar to the flat plates and the spanwise drag distributions show similar “tip effects.” For the turbine blades this effect is even more pronounced, because the tapering of the blades makes the tip effect spread to a larger part of the blades. The findings are supported by visualizations of the wake patterns behind the blades.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1017-1024. doi:10.1115/1.1800551.

The purpose of this research is to investigate the physical mechanisms associated with broadband tip vortex noise caused by rotating wind turbines. The flow and acoustic field around a wind turbine blade is simulated using compressible large-eddy simulation and direct noise simulation, with emphasis on the blade tip region. The far field aerodynamic noise is modeled using acoustic analogy. Aerodynamic performance and acoustic emissions are predicted for the actual tip shape and an ogee type tip shape. For the ogee type tip shape the sound pressure level decreases by 5 dB for frequencies above 4 kHz.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1025-1033. doi:10.1115/1.1793209.

Under zero yaw conditions, rotational effects substantially and routinely augment HAWT blade aerodynamic response. To better comprehend the fluid dynamic mechanisms underlying this phenomenon, time dependent blade surface pressure data were acquired from the National Renewable Energy Laboratory (NREL) Unsteady Aerodynamics Experiment (UAE), a full-scale HAWT tested in the NASA Ames 80 ft×120 ft wind tunnel. These surface pressure data were processed to obtain normal force and flow field topology data. Further analyses were carried out in a manner that allowed tip speed ratio effects to be isolated from other confounding influences. Results showed clear correlations between normal forces, flow field topologies, and tip speed ratios. These relationships changed significantly at different blade radial locations, pointing to the complex three-dimensional flow physics present on rotating HAWT blades.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1034-1040. doi:10.1115/1.1765683.

This paper presents results from a wind tunnel based examination of the response of a wind turbine blade to tower shadow in head-on flow. In the experiment, one of the blades of a small-scale, two-bladed, downwind turbine was instrumented with miniature pressure transducers to allow recording of the blade surface pressure response through tower shadow. The surface pressures were then integrated to provide the normal force coefficient responses presented in this paper. It is shown that it is possible to reproduce the measured responses using an indicially formulated unsteady aerodynamic model applied to a cosine wake velocity deficit. It is also shown that agreement between the model and the measured data can be improved by careful consideration of the velocity deficit geometry.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1041-1049. doi:10.1115/1.1785160.

In many analyses of composite wind turbine blades, the effects of mean stress on the determination of damage are either ignored completely or they are characterized inadequately. An updated Goodman diagram for the fiberglass materials that are typically used in wind turbine blades has been released recently. This diagram, which is based on the MSU/DOE Fatigue Database, contains detailed information at thirteen R-values. This diagram is the most detailed to date, and it includes several loading conditions that have been poorly represented in earlier studies. This formulation allows the effects of mean stress on damage calculations to be evaluated. The evaluation presented here uses four formulations for the S-N behavior of the fiberglass. In the first analysis, the S-N curve for the composite is assumed to be independent of mean stress and to have a constant slope. The second is a linear Goodman diagram, the third is a bi-linear Goodman diagram and the fourth is the full Goodman diagram. Two sets of load spectra, obtained by the LIST (Long term Inflow and Structural Test) program, are used for this evaluation. The results of the analyses, equivalent fatigue loads and damage predictions, are compared to one another. These results illustrate a significant overestimation of the equivalent fatigue loads when the mean stress is not considered in the calculation. And, the results from the updated Goodman diagram illustrate that there are significant differences in accumulated damage when the Goodman diagram includes information on the transition between compressive and tensile failure modes.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1050-1059. doi:10.1115/1.1800533.

This paper examines the importance of load phase angle variations on fatigue damage and evaluates the potential effects of varying the load phase angle during dual-axis constant amplitude fatigue testing. The scope of this paper is limited to results from simulated wind and dynamic loads. The operating loads on a generic three bladed up-wind 1.5 MW wind turbine blade were analyzed over a range of operating conditions, and an aggregate probability distribution for the actual phase angles between the peak in-plane (lead-lag) and peak out-of-plane (flap) loads was determined. Using a finite element model (FEM) of the 1.5 MW blade and Miner’s Rule [Miner, A., 1945, “Cumulative Damage in Fatigue,” Trans. ASME, 67 ], the accumulated theoretical fatigue damage (based on axial strains) resulting from a fatigue test with variable phase angles using the aggregate distribution was compared to the damage resulting from a fatigue test with a constant phase angle. The FEM nodal damage distribution at specific blade cross sections were compared for the constant and variable phase angle cases. Single-node stress concentrations were distributed arbitrarily around one cross section to simulate material defects in a blade undergoing testing. Results show that the variable phase angle case results in higher damage on the critical nodes. In addition, the probability of discovering a material defect during a test was substantially increased when variable phase loading was used. The effect of phase angle sequence on the damage accumulation was also considered. For this analysis, the finite element results were processed using a nonlinear damage accumulation model. Results show that the sequence of the phase angle can have a large effect on the fatigue damage, and multiple, shorter length sequences produce higher damage than a single, long term sequence.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1060-1068. doi:10.1115/1.1796971.

The influence of turbulence conditions on the design loads for wind turbines is investigated by using inverse reliability techniques. Alternative modeling assumptions for randomness in the gross wind environment and in the extreme response given wind conditions to establish nominal design loads are studied. Accuracy in design load predictions based on use of the inverse first-order reliability method (that assumes a linearized limit state surface) is also investigated. An example is presented where three alternative nominal load definitions are used to estimate extreme flapwise bending loads at a blade root for a 600 kW three-bladed, stall-regulated horizontal-axis wind turbine located at onshore and offshore sites that were assumed to experience the same mean wind speed but different turbulence intensities. It is found that second-order (curvature-type) corrections to the linearized limit state function assumption inherent in the inverse first-order reliability approach are insignificant. Thus, we suggest that the inverse first-order reliability method is an efficient and accurate technique of predicting extreme loads. Design loads derived from a full random characterization of wind conditions as well as short-term maximum response (given wind conditions) may be approximated reasonably well by simpler models that include only the randomness in the wind environment but account for response variability by employing appropriately derived “higher-than-median” fractiles of the extreme bending loads conditional on specified inflow parameters. In the various results discussed, it is found that the higher relative turbulence at the onshore site leads to larger blade bending design loads there than at the offshore site. Also, for both onshore and offshore environments accounting for response variability is found to be slightly more important at longer return periods (i.e., safer designs).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1069-1082. doi:10.1115/1.1797978.

The Long-term Inflow and Structural Test (LIST) program, managed by Sandia National Laboratories, Albuquerque, NM, is gathering inflow and structural response data on a modified version of the Micon 65/13 wind turbine at a site near Bushland, Texas. With the objective of establishing correlations between structural response and inflow, previous studies have employed regression and other dependency analyses to attempt to relate loads to various inflow parameters. With these inflow parameters that may be thought of as single-point-in-space statistics that ignore the spatial nature of the inflow, no significant correlation was identified between load levels and any single inflow parameter or even any set of such parameters, beyond the mean and standard deviation of the hub-height horizontal wind speed. Accordingly, here, we examine spatial statistics in the measured inflow of the LIST turbine by estimating the coherence for the three turbulence components (along-wind, across-wind, and vertical). We examine coherence spectra for both lateral and vertical separations and use the available ten-minute time series of the three components at several locations. The data obtained from spatial arrays on three main towers located upwind from the test turbine as well as on two additional towers on either side of the main towers consist of 291 ten-minute records. Details regarding estimation of the coherence functions from limited data are discussed. Comparisons with standard coherence models available in the literature and provided in the International Electrotechnical Commission (IEC) guidelines are also discussed. It is found that the Davenport exponential coherence model may not be appropriate especially for modeling the coherence of the vertical turbulence component since it fails to account for reductions in coherence at low frequencies and over large separations. Results also show that the Mann uniform shear turbulence model predicts coherence spectra for all turbulence components and for different lateral separations better than the isotropic von Kármán model. Finally, on studying the cross-coherence among pairs of turbulence components based on field data, it is found that the coherence observed between along-wind and vertical turbulence components is not predicted by the isotropic von Kármán model while the Mann model appears to overestimate this cross-coherence.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1083-1091. doi:10.1115/1.1792654.

The wind industry seeks to design wind turbines to maximize energy production and increase fatigue life. To achieve this goal, we must design wind turbines to extract maximum energy and reduce component and system loads. This paper applies modern state-space control design methods to a two-bladed teetering-hub upwind machine located at the National Wind Technology Center. The design objective is to regulate turbine speed in region 3 (above rated wind speed) and enhance damping in several low-damped flexible modes of the turbine. The controls approach is based on the Disturbance Accommodating Control method and provides accountability for wind-speed disturbances. First, controls are designed with the single control input rotor collective pitch to stabilize the first drive-train torsion as well as the tower first fore-aft bending modes. Generator torque is then incorporated as an additional control input. This reduces some of the demand placed on the rotor collective pitch control system and enhances first drive train torsion mode damping. Individual blade pitch control is then used to attenuate wind disturbances having spatial variation over the rotor and effectively reduces blade flap deflections caused by wind shear.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1092-1100. doi:10.1115/1.1792653.

The standard region 2 control scheme for a variable-speed wind turbine, τc=Kω2, has several shortcomings that can result in significant power loss. The first of these is that there is no accurate way to determine the gain K; modeling programs are not accurate enough to represent all of the complex aerodynamics, and these aerodynamics change over time. Furthermore, it is not certain whether the value of K used in the standard control even provides for the maximum energy capture under real-world turbulent conditions. We introduce new control methods to address these issues. First, we show in simulation that using smaller values of K than the standard can result in increased energy capture. Second, we give simulation results showing that an optimally tracking rotor control scheme can improve upon the standard scheme by assisting the rotor speed in tracking wind-speed fluctuations more rapidly. Finally, we propose an adaptive control scheme that allows for maximum power capture despite parameter uncertainty.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2004;126(4):1101-1104. doi:10.1115/1.1800534.

This work presents a new way of minimize the losses on a flat plate solar energy collector. A wind barrier is added along the collector perimeter in order to modify the flow pattern over it. This barrier creates a region of recirculating separated flow on top of the collector. This is an initial phase of the work and only winds aligned with the solar collector are investigated. In other words no influence of lateral winds is accounted for. The first experimental results proved very encouraging. It was observed a 12% heat-loss reduction in comparison with the traditional double glazing solution.

Commentary by Dr. Valentin Fuster

DISCUSSION

J. Sol. Energy Eng. 2004;126(4):1105-1109. doi:10.1115/1.1792671.
FREE TO VIEW

Hall,  C., Tharakan,  P., Hallock,  J., Cleveland,  C., and Jefferson,  M., 2003, “ Hydrocarbons and the evolution of human culture, Insight commentary,” Nature (London), 426 .Shimon, A., 2003, “Determining the real cost—Why renewable is more cost-competitive than previously believed,” Renewable Energy World, March-April 2003.Wind Force 12, A blueprint to achieve 12% of the world’s electricity from wind power by 2020, European Wind Energy Association & Greenpeace, May 2003.

Commentary by Dr. Valentin Fuster

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In