Validation of the Eppler Airfoil Design and Analysis Code has been a goal of several NREL-sponsored, two-dimensional investigations in the low-turbulence wind tunnel of the

The development of special-purpose airfoils for horizontal-axis wind turbines (HA WT�) began in 1984 as a joint effort between the National Renewable Energy Laboratory (NREL), formerly the Solar Energy Research Institute (SERI), and Airfoils, Incorporated. Since that time seven airfoil families have been designed for various size rotors using the Eppler Airfoil Design and Analysis Code. A general performance requirement of the new airfoil families is that they exhibit a maximum 1ift coefficient ('1,rnaJ which is relatively insensitive to roughness effects. The airfoil families address the needs of stall-regulated, variable-pitch, and variable-rpm wind turbines. For stall-regulated rotors, bett er peak-power control is achieved through the design of tip airfoils that restrain the maximum 1ift coefficient. Restrained maximum 1ift coefficient all ows the use of more swept disc area for a given generator size. Also, for stall-regulated rotors, tip airfoils with high thickness are used to accommodate overspeed control devices. For variable-pitch and variable-rpm rotors, tip airfoils having a high maximum lift coefficient lend themselves to lightweight blades with low solidity. Tip airfoils having low thickness result in less drag for blades having full-span pitch control Annual energy improvements from the NREL airfoil families are projected to be 23% to 35% for stall regulated turbines, 8% to 20% for variable-pitch turbines, and 8% to 10% for variable-rpm turbines. The improvement for stall-regulated turbines has been verified in field tests.

The U.S. Department of Energy (DOE), working through its National Renewable Energy Laboratory (NREL), is engaged in a comprehensive research effort to improve the understanding of wind turbine aeroacoustics. Motivation for this effort is the desire to exploit the large expanse of low wind speed sites that tend to be closer to U.S. load centers. Quiet wind turbines are an inducement to widespread deployment, so the goal of NREL's aeroacoustic research is to develop tools for use by U.S. industry in developing and deploying highly efficient, quiet wind turbines at these low wind speed sites. NREL's National Wind Technology Center (NWTC) is implementing a multifaceted approach that includes wind tunnel tests, field tests, and theoretical analyses in direct support of low wind speed turbine development by its industry partners. NWTC researchers are working hand in hand with industry engineers to ensure that research findings are available to support ongoing design decisions. The work described in the present report focuses on the theoretical aerodynamic analysis of airfoils that are candidates for use on small wind turbines. Admittedly, the connection between this aerodynamic work and NREL's aeroacoustic research is somewhat vague. But without a knowledge of both aerodynamic and aeroacoustic performance of airfoils, engineers are frustrated in making decisions on new blade designs. This is particularly true for small wind turbines, which operate at low Reynolds numbers at which airfoil aerodynamic characteristics are both sensitive and difficult to predict. Thus, the present work needs to be considered in the context of the broader research effort on aeroacoustics for small wind turbines. Wind tunnel aerodynamic tests and aeroacoustic tests have been performed on six airfoils that are candidates for use on small wind turbines. Results are documented in the following NREL reports:

A family of airfoils, the S825 and S826, for 20-to 40-meter, variable-speed and variable-pitch (toward feather), horizontal-axis wind turbines has been designed and analyzed theoretically. The two primary objectives of high maximum lift, insensitive to roughness, and low profile drag have been achieved. The constraints on the pitching moments and the airfoil thicknesses have been satisfied. The airfoils should exhibit docile stalls.

This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness,, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.