People have been using the wind as an almost-free source of power to drive electric generators since the late 1800s, and have sought to improve the technology since then through better understanding of the wind itself and how wind-power machinery interacts with it. Some of the recently reported investigations sponsored by the U. S. Department of Energy have addressed:
As we may see from the reports described below, two major means of conducting this (but not the only ones) are experiments with real wind turbines and computer simulations of turbines.
Much of this work is published by the Energy Department’s National Renewable Energy Laboratory (NREL); other long-term and short-term recipients of Departmental support include Sandia National Laboratories, Oak Ridge National Laboratory, Savannah River National Laboratory, The Ohio State University , AlphaSTAR Corporation , Western Ecosystems Technology Inc. , and Wyoming Wildlife Consultants LLC .
The DOE-GE 1.5-MW wind turbine is 80 meters tall and has a 77-meter rotor diameter. Photo by Dennis Schroeder, NREL/PIX 19083. (From fact sheet “NREL Establishes a 1.5-MW Wind Turbine Test Platform for Research Partnerships”.)
An important experimental resource of the National Renewable Energy Laboratory is its National Wind Technology Center in Boulder, Colorado. The Center’s newest facility, described in a recent fact sheet,[Information Bridge] is a 1.5-megawatt wind turbine used as a test platform. This turbine is fitted with an extensive set of instruments; testing it not only determines its power performance, but its vibration characteristics and fatigue loads as well. The new turbine can be used to test wind-turbine control systems and components and to validate mathematical models of wind-turbine behavior.
Like many wind turbines, this new NREL turbine is installed on land. But many other wind turbines are installed offshore, where they offer some advantages over onshore turbines although they present design problems of their own. One type of offshore turbine under development would rest on a floating platform. In actual use, such a turbine would be connected to the onshore power grid by an underwater cable, but for testing, it’s only necessary to connect the turbine to a load that simulates the grid and interacts with it appropriately; and it may be more cost-effective to have this load on the platform itself. The design of such a power grid simulator is described in the report “Grid Simulator for Testing a Wind Turbine on Offshore Floating Platform”.[Information Bridge]
Improving wind-turbine reliability requires finding out what limits their reliability now. How to go about learning this is described in the Sandia National Laboratories report “Wind Energy Computerized Maintenance Management System (CMMS) : Data Collection Recommendations for Reliability Analysis”.[Information Bridge] Fixing component failures, as well as maintaining and operating a wind plant in general, all cost money. How much each of these things costs under various assumptions is addressed in the report “Data Collection for Current U.S. Wind Energy Projects: Component Costs, Financing, Operations, and Maintenance”. [Information Bridge]
The power output of a wind turbine can only be as steady as the wind driving its blades, which is not steady at all. We can get a clearer idea of what performance to expect if we observe any patterns in the wind’s variations or in the resulting power output. One recent study[Information Bridge] of a set of wind-power plants in Minnesota confirms the major daily and seasonal variations one might expect from ordinary experience, but it also shows a sizeable variation from year to year during the plants’ history, with one plant producing almost 40% more power in its most productive year than in its least productive year.
The National Wind Technology Center, whose new 1.5-megawatt turbine is described above, also measures features of the wind that affect the performance of this turbine and others. Since wind speed, turbulence intensity, and dissipation all vary with height across the disk-shaped area swept by a turbine’s rotor, they are measured at multiple heights, as described in the recent poster “Characterizing Inflow Conditions Across the Rotor Disk of a Utility-Scale Wind Turbine”[Information Bridge] and the technical report “Turbine Inflow Characterization at the National Wind Technology Center”.[Information Bridge] A detailed example of how certain data is taken, analyzed, and used at the Center is given in the technical report “Inflow Characterization and Aerodynamic Measurements on a SWT-2.3-101 Wind Turbine”.[Information Bridge]
The view to the northwest across the NWTC in May 2011. Three utility-scale turbines are in the foreground of the picture to the left and right. The 38-m hub height Advanced Research Turbines are slightly set back. Photo by Dennis Schroeder, NREL/PIX 19018. (From “Turbine Inflow Characterization at the National Wind Technology Center”.)
The biggest wind power plants use multiple large wind turbines. These plants of course produce many times more power than a single turbine would, but not necessarily as many times more as one might expect. The wind that goes through the rotor disk slows down as it drives the rotor blades and transfers some of its energy to them. The wind that flows around the disk instead of going through it maintains its higher speed, and forms a shear layer with the slower air stream that it overtakes just beyond the disk. This shear layer thickens and becomes more turbulent downstream. If this turbulent wake flows into the disk of another turbine beyond the first, it will drive the second turbine with less force, giving it less energy. Thus in a wind farm, the turbines at the front that are exposed to less turbulence often produce more energy than the ones behind them that catch more turbulence.
The extent of this effect is the subject of both analytical studies and observations of real turbines. The Lawrence Livermore National Laboratory report “Synergistic Effects of Turbine Wakes and Atmospheric Stability on Power Production at an Onshore Wind Farm”[Information Bridge] compares observations and computer simulations of a wind farm in western North America. Findings from computer simulation of a wind farm off the coast of Sweden are described in the National Renewable Energy Laboratory technical report “A Large-Eddy Simulation of Wind-Plant Aerodynamics”[Information Bridge] and the fact sheets “NREL Develops Simulations for Wind Plant Power and Turbine Loads”[Information Bridge] and “NREL Studies Wind Farm Aerodynamics to Improve Siting”[Information Bridge]; the latter fact sheet also describes the use of a high-resolution Doppler laser system for measuring wind-turbine wakes. Understanding the size and effect of one turbine’s wake on another can lead to turbine arrangements optimized for more power output and lower maintenance costs.
Contours taken in a horizontal plane at the rotor hub height of instantaneous streamwise resolved velocity normalized by hub height wind speed. The black bars indicate the turbine rotors. (From “A Large-Eddy Simulation of Wind-Plant Aerodynamics”.)
Beyond calculation and experiment with full-sized systems, a third way to study the fluid dynamics of wind turbines is with scale models, as is done with ships and airplanes. But small-scale models present a problem. The different forces that act on a full-sized system are of different strengths, some affecting the system more than others. In a scale model, the same forces may act, but their strengths may be in different proportions. For example, viscosity is more important in relation to gravity for a small-scale fluid system than for the full-size version. Gravitational effects may not be that significant for land-based turbines, but gravity has a significant effect in the water waves that affect a floating platform. So the response of a scale-model turbine on a floating platform to the flow of wind and water won’t be similar to the response of the full-size turbine: the smaller the model is, the greater effect viscosity will have relative to gravity. Implications of this fact for scale-model studies of offshore floating wind turbines are discussed in the NREL report “FAST Code Verification of Scaling Laws for DeepCwind Floating Wind System Tests”.[Information Bridge]
A wind turbine’s usefulness depends not only on how windy its environment is, but on how well the turbine’s components hold up to the way the wind stresses them. Predicting these effects is the purpose of several computer programs and experimental facilities.
While we can’t see the wind directly, we can determine its force from what we see it doing. This is the principle behind a computer program that takes measurements of how a turbine’s support structure responds to the wind, and calculates how forcefully the wind is driving the turbine’s rotor. As described in another NREL technical report (“Inverse Load Calculation of Wind Turbine Support Structures – A Numerical Verification Using the Comprehensive Simulation Code FAST”),[Information Bridge] the accuracy of the program’s force-from-response calculations for a 5-megawatt onshore turbine was tested by comparing them with corresponding response-from-force deductions of the previously mentioned simulation code FAST. The program’s estimates of the rotor thrust are described as “very good”.
Simulations can sometimes demonstrate a system’s behavior more quickly than experiments with the actual system. But even supercomputers can’t calculate fast enough to account for every single feature of a complex system and still outrun an experiment. This isn’t a problem if only some features matter enough to significantly affect the system’s behavior; as long as a simulation accounts for all significant details, it offers an advantage. But different simulation programs that account for different details may predict different behaviors. Are the differences big enough to matter? That’s the subject of the NREL report “Offshore Code Comparison Collaboration Continuation (OC4), Phase I – Results of Coupled Simulation of an Offshore Wind Turbine with Jacket Support Structure”.[Information Bridge] The report discusses sources of the differences between different simulations of wind turbines and their support structures, depicts the importance of a support structure’s local dynamics, and ends with recommendations about how to simulate the structures.
Wind affects turbine support structures, but its effect on turbines themselves is why turbines are built. Yet the wind doesn’t just rotate turbine blades—it also flexes and wears the blades, so the blades have to be designed not to fail as a result. The soundness of blade designs can be checked by actual experiment or by a mathematical analysis of all factors that affect their soundness. The Wind Technology Testing Center in Boston, Massachusetts offers the first option: there, turbine blades up to 90 meters long can be put through static deflection and strain tests as well as multimillion-cycle fatigue tests.[Information Bridge] New computer software designed to certify blades by mathematical analysis is described in the AlphaSTAR Corporation report “Advanced Composite Wind Turbine Blade Design Based on Durability and Damage Tolerance”.[Information Bridge]
NREL verified the functionality of the WTTC facilities and test systems by testing a blade from one of Clipper Windpower’s 2.5-megawatt wind turbines. Photo by Derek Berry, NREL/PIX 20067. (From fact sheet “Building State-of-the-Art Wind Technology Testing Facilities”.)
While the wind’s adverse effects on turbines’ blades and support structures is significant, the most crippling and expensive failures in wind turbines occur in their gearboxes. With some wind farms having been in service for many years now, turbine gearboxes are found not to have lasted as long as expected. Apparently this is not due to manufacturing problems, but because design analyses haven’t sufficiently accounted for important effects on the gears and bearings. Several recent reports describe efforts to discover some of these effects.
One such report[Information Bridge] from The Ohio State University describes experiments with micro-pitting—a progressive wear of gearbox-component surfaces in rolling contact due to severe localized stress concentrations—and efforts to accurately model it mathematically. Another report, from a seminar sponsored in November 2011 by the Energy Department, summarizes many other investigations of damage and failure of components in contact. The report is entitled “Wind Turbine Tribology Seminar – A Recap”[Information Bridge]; tribology (from the Greek tribein, to rub) is the science and engineering of surfaces in relative motion—a common condition in gearboxes and other machinery.
Another report[Information Bridge] from Savannah River National Laboratory describes equipment designed for the Wind Turbine Drivetrain Test Facility, a state-of-the-art laboratory for making actual measurements of turbine drivetrains. And three additional reports from NREL deal specifically with the work of a group called the Gearbox Reliability Collaborative: one report[Information Bridge] describes the Collaborative itself and its plan to understand the causes of wind turbine gearbox failures, while the other two[Information Bridge], [Information Bridge] describe specific experiments with gearboxes and their results.
Exploded view of GRC gearbox components. (From “Determining Wind Turbine Gearbox Model Complexity Using Measurement Validation and Cost Comparison”.)
As newer wind turbines are designed to be larger and their components lighter and more flexible, controlling the way they move and interact under the wind’s influence becomes more important to prevent damage and possible failure. Algorithms for active computer control of the newer turbines must account for more complex motions. Facilities for testing new control systems at the National Wind Technology Center are described in the fact sheet “Advanced Wind Turbine Controls Reduce Loads”.[Information Bridge]
Control of wind turbines can affect not only turbines’ own structural soundness, but the power and frequency of current in the utility grid. These physical effects and their economic implications become increasingly important as individual wind turbines and wind farms are added to the power grid, particularly their role in balancing the power load and supplying power during peak demand periods. These points are addressed in the fact sheet “NWTC Controllable Grid Interface”[Information Bridge] and the technical report “Tutorial of Wind Turbine Control for Supporting Grid Frequency through Active Power Control”.[Information Bridge]
Four other recent reports deal with the role of wind in relation to other means of producing electric power. One Sandia National Laboratories report deals with reactive power interconnection requirements for wind and solar plants[Information Bridge]; another NREL report discusses methods for estimating the capacity value of wind and solar power.[Information Bridge] The third report, also from NREL, analyzes the variance of wind and natural gas generation under different market structures,[Information Bridge] and the fourth, from Oak Ridge National Laboratory, examines how wind power might be distributed across the country so utilities with scarce wind resources could meet proposed standards for renewable-energy use.[Information Bridge]
These last two reports consider economic aspects of wind power. A different sort of economic effect, in the current market for small sub-utility scale wind turbines, is examined in the report “Certification for Small Wind Turbine Installers: What’s the Hang Up?”[Information Bridge] While there is a market for small wind turbines, there is little customer demand as yet for an installer certification standard; the report examines why.
A Skystream turbine in front of the U.S. Capitol. Photo from Southwest Windpower. NREL/PIX 19410. (From “Certification for Small Wind Turbine Installers: What’s the Hang Up?”.)
Like any other structures, the presence of wind turbines affects their environment in various ways, some better understood than others. One of the more recent reports on wildlife near wind turbines, [Information Bridge] a study by Western EcoSystems Technology, Inc. and Wyoming Wildlife Consultants, LLC of sage-grouse populations near an existing wind farm and at some distance from it, provides preliminary data from observations conducted over a two-year period—not enough to ascertain long-term trends, but a starting point. A quite different environmental effect is described in the Sandia National Laboratories report “Radar-Cross-Section Reduction of Wind Turbines (Part 1)”.[Information Bridge] Radar is an important tool in air traffic control, air defense, and weather sensing. Present-day wind turbines interfere with radar because of their size, and their rotating blades cause Doppler shifts in the reflected waves that make it difficult to identify and track moving targets. The report describes how these problems might be lessened through stealth technology.
National Renewable Energy Laboratory (NREL)
National Wind Technology Center
Sandia National Laboratories
Oak Ridge National Laboratory
Savannah River National Laboratory
The Ohio State University
Western Ecosystems Technology, Inc.
Wyoming Wildlife Consultants, LLC
Reports Available through OSTI’s Information Bridge
Testing and Data in General
Wind and Turbine Dynamics
Control, the Power Grid, and the Grid’s Economics
Prepared by Dr. William N. Watson, Physicist
DOE Office of Scientific and Technical Information