Acoustic Emission Monitoring of Small Wind Turbine Blades

Wind turbine blade certification tests, comprising a static test, a fatigue test, and finally a residual strength test, often involve sudden audible cracking sounds from somewhere within the blade, without the operators being able to locate the noise source, or to determine whether damage (minor or major) has occurred. A current EC-funded research project is looking at the possibility of using acoustic emission (AE) monitoring during testing of fibre composite blades to detect such events and assess the blade condition. AE can both locate and characterise damage processes in blades, starting with non-audible signals occurring due to damage propagation at relatively low loads. The test methodology is discussed in the context of the blade certification procedure and results are presented from a series of static and fatigue blade tests to failure in the laboratory. Inferences are drawn about small differences in the manufacture of the nominally identical blades and conclusions are presented for the application of the methodology.

The NREL Large-Scale Turbine Inflow and Response Experiment: Preliminary Results

The accurate numerical dynamic simulation of new large-scale wind turbine designs operating over a wide range of inflow environments is critical because it is usually impractical to test prototypes in a variety of locations. Large turbines operate in a region of the atmospheric boundary layer that currently may not be adequately simulated by present turbulence codes. In this paper, we discuss the development and use of a 42-m (137-ft) planar array of five, high-resolution sonic anemometers upwind of a 600-kW wind turbine at the National Wind Technology Center (NWTC). The objective of this experiment is to obtain simultaneously collected turbulence information from the inflow array and the corresponding structural response of the turbine. The turbulence information will be used for comparison with that predicted by currently available codes and establish any systematic differences. These results will be used to improve the performance of the turbulence simulations. The sensitivities of key elements of the turbine aeroelastic and structural response to a range of turbulence-scaling parameters will be established for comparisons with other turbines and operating environments. In this paper, we present an overview of the experiment, and offer examples of two observed cases of inflow characteristics and turbine response collected under daytime and nighttime conditions, and compare their turbulence properties with predictions.

Predicting the Long Term Distribution of Extreme Loads From Limited Duration Data: Comparing Full Integration and Approximate Methods

In this paper we present a methodology for proceeding from the short-term observations of extreme loads to the long-run load distribution of these extreme events, for both flap and edge loading in both operating and parked wind turbine conditions. First a general approach utilizing full integration, where numerical routines are used to directly integrate the conditional short-term load distribution over the annual occurrence of wind speeds and turbulence intensities, is presented. Then starting from this general approach, a qualitative analysis is undertaken to explore the extent of the contribution of each of the three variables, in the governing equation, to the variability in the long-term extreme load distribution. From this analysis, lower order models are considered, where instead of using the entire distribution of the variables, a constant fractile of the short-term extreme load distribution, turbulence intensity distribution, or both are used. Finally recommendations are given to guide the analyst to decide when simpler, yet robust, methods which account for sufficient variability in extreme load event may be employed with confidence.

Challenges in Modeling the Unsteady Aerodynamics of Wind Turbines

Many of the aerodynamic phenomena contributing to the observed effects on wind turbines are now known, but the details of the flow are still poorly understood and are challenging to predict accurately, issues discussed herein include the modeling of the induced velocity field produced by the vortical wake behind the turbine, the various unsteady aerodynamic issues associated with the blade sections, and the intricacies of dynamic stall. Fundamental limits exist in the capabilities of all models, and misunderstandings or ambiguities can also arise in how these models should be properly applied. A challenge for analysts is to use complementary experimental measurements and modeling techniques to better understand the aerodynamic problems found on wind turbines, and to develop more rigorous models with wider ranges of application.

Alternative Materials, Manufacturing Processes, and Structural Designs for Large Wind Turbine Blades

As part of the U.S. Department of Energy’s Wind Partnerships for Advanced Component Technologies program, Global Energy Concepts LLC (GEC) has performed a study concerning innovations in materials, processes and structural configurations for application to wind turbine blades in the multi-megawatt range. Constraints to cost-effective scaling-up of the current commercial blade designs and manufacturing methods are identified, including self-gravity loads, transportation, and environmental considerations. A trade-off study is performed to evaluate the incremental changes in blade cost, weight, and stiffness for a wide range of composite materials, fabric types, and manufacturing processes. Fiberglass/carbon hybrid blades are identified as having a promising combination of cost, weight, stiffness and fatigue resistance. Vacuum-assisted resin transfer molding, resin film infusion, and pre-impregnated materials are identified as having benefits in reduced volatile emissions, higher fiber content, and improved laminate quality relative to the baseline wet lay-up process. Alternative structural designs are identified, including jointed configurations to facilitate transportation. Based on the study results, recommendations are developed for further evaluation and testing to verify the predicted material and structural performance.

Field Tests of a Wind-Electric Controller for Parallel Stock Water Pumping and Heating

Many of the improvements in wind-electric stock water pumping systems are attributable to advanced controller strategies and hardware that maximize performance over a range of wind speeds. The cost of the early and more complex controllers was of the order of one quarter of the whole system. Sophisticated yet inexpensive-programmable micro-controllers are now being introduced that enhance both performance and reliability. This study utilized a micro-programmable logic controller (PLC) to place a variable auxiliary load in the form of a stock water heater in parallel with the pump motor. This improves the system’s economic viability on the Northern High Plains by mitigating stock tank freezing to help extend the grazing season. For the variable auxiliary load, the PLC uses long period pulse width modulation to drive a 3-phase solid-state relay. This continuously variable load strategy was designed to both increase the power factor when the pump is operating, and to extract resistive heating power in wind regimes not suitable for operating the pump. This paper reports on the preliminary but encouraging field studies directed toward optimizing the low wind speed water heating performance of this multi-tasking controller when the pump motor is inoperable.

Conclusions From the Comparisons of Numerous 2D Airfoil Computations With Experiments

The aim of this work is two-fold. Firstly, 28 sets of airfoil (widely used for wind turbine applications) measurements were compared with numerical results from a 2D Navier-Stokes solver and a panel method code. These results have been collected into an airfoil catalogue that has been separately published. Secondly, based on the previous results, criterions for evaluating the airfoils are derived. Thereby, the performance of the Navier-Stokes solver is evaluated. Further analysis of the results determines geometrical and flow properties that may cause problems when computing airfoil flows with a Navier-Stokes solver, and some recommendations are given.

Vibrations of a Three-Bladed Wind Turbine Rotor Due to Classical Flutter

The aeroelastic stability of a three-bladed wind turbine is considered with respect to classical flutter. Previous studies have shown that the risk of stall-induced vibrations of turbine blades is related to the dynamics of the complete turbine, for example does the aerodynamic damping of a rotor whirling mode depend highly on the tower stiffness. The results of this paper indicate that the turbine dynamics also affect the risk of flutter. The study is based on an eigenvalue analysis of a linear aeroelastic turbine model. In an example of a MW sized turbine, the critical frequency of the first torsional blade mode is determined for which flutter can occur under normal operation conditions. It is shown that this critical torsional frequency is higher when the blades are interacting through the hub with the remaining turbine, than when all blades are rigidly clamped at the root. Thus, the dynamics of the turbine has increased the risk of flutter.

Cogging Torque Reduction in a Permanent Magnet Wind Turbine Generator

Most small wind turbines use permanent magnet (PM) generators. The generators are usually direct-drive (i.e., no gearbox is required). Direct-drive PM generators are characterized by low maintenance and high efficiency. Small wind turbines are usually self-starting and require very simple controls. Cogging torque is an inherent characteristic of PM generators and is caused by the geometry of the generator. Cogging torque affects self-start ability and produces noise and mechanical vibration. Thus, minimizing cogging torque is important in improving the operation of small wind turbines. In this paper, we investigate three design options to minimize cogging torque: uniformity of air gap, pole width, and skewing. Although the design improvement is intended for small wind turbines, it is also applicable to larger wind turbines.

Modal Response of 3-Bladed Wind Turbines

A set of linear equations describing the motion of an operating 3-bladed HAWT are obtained from the dynamic characteristics of the stationary turbine by adding rotating frame effects. The approach makes use of the Coleman multi-blade transformation to present all results relative to the fixed frame. The formulation is in terms of a selected number of stationary, real, mode shapes. The formulation is applied to the expression of both the aerodynamic loading and the displacement response in terms of the operating mode shapes. This technique is applied to the conditions of vertical wind shear and off-yaw operation of a hypothetical 46-m wind turbine. The principal objective of the paper is to enable the characteristic of the inflow to be related to the nature of the response. A second objective is to illustrate a method of extracting linearized models from general aeroelastic codes such as ADAMS™.