Blade Design Trade-Offs Using Low-Lift Airfoils for Stall-Regulated HAWTs Page: 3 of 14
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BLADE DESIGN TRADE-OFFS USING LOW-LIFT
AIRFOILS FOR STALL-REGULATED HAWTS
Philippe Gigubre* and Michael S. Seligt
University of Illinois at Urbana-Champaign
Urbana, IL 61801
James L. Tangler$
National Renewable Energy Laboratory
Golden, CO 80401
Abstract
A systematic blade design study was conducted to explore the trade-offs in using low-lift airfoils for a
750-kilowatt stall-regulated wind turbine. Tip-region airfoils having a maximum lift coefficient ranging
from 0.7-1.2 were considered in this study, with the main objective of identifying the practical lower
limit for the maximum lift coefficient. Blades were optimized for both maximum annual energy
production and minimum cost of energy using a method that takes into account aerodynamic and
structural considerations. The results indicate that reducing the maximum lift coefficient below the upper
limit considered in this study increases the cost of energy independently of the wind regime. As a
consequence, higher maximum lift coefficient airfoils for the tip-region of the blade become more
desirable as machine size increases, as long as they provide gentle stall characteristics. The conclusions
are applicable to large wind turbines that use passive or active stall to regulate peak power.
1. Introduction
The use of aerodynamic stall is a common means of regulating peak power for horizontal axis wind
turbines (HAWTs) operating at constant speed. Stall regulation can be performed either actively (pitch
control) or passively (fixed pitch), the latter being the more popular approach. For both of these
approaches, the stall characteristics of the airfoils used over the tip region of the blades are important
because they strongly affect the dynamics of the rotor, and thus the structural design of the blades.
Reducing blade dynamic excitations, such as stall-induced vibrations, can be achieved using airfoils
having a gentle stall, which typically have a low maximum lift coefficient (cimax), i.e., low-lift airfoils. In
contrast, high-lift airfoils often have an abrupt or hard stall that is characterized by a rather large loss in
lift and negative lift-curve slope. As a result, high-lift airfoils increase blade dynamic excitations as
compared with low-lift airfoils. The use of high-lift airfoils, however, is beneficial for minimizing blade
solidity and enhancing starting torque. Also, for a given amount of laminar flow, high-lift airfoils are
more efficient (higher lift-to-drag ratio) than airfoils with a low c/max. Therefore, there are design trade-
offs in selecting the lift range of the airfoils for a particular rotor.
The trade-offs between low-lift and high-lift airfoils for stall-regulated HAWTs have been recognized for
some time now. Since 1984, the National Renewable Energy Laboratory (NREL), in collaboration with
Airfoils Inc., have developed over 10 airfoil families specifically for wind turbines. Most tip-region
airfoils have a low cImax, in the range of 0.9-1.2, while the root airfoils have a higher cImax.1-3 Another
important characteristic of the NREL airfoils is that their cImax is less sensitive to roughness effects as
* Graduate Research Assistant, Aeronautical and Astronautical Engineering Department.
t Associate Professor, Aeronautical and Astronautical Engineering Department.
$ Senior Scientist, National Wind Technology Center.
The results of this paper were presented at the ASME Wind Energy Symposium, Reno, NV, January 11-14, 1999.1
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Giguere, P.; Selig, M. S. (University of Illinois at Urbana-Champaign) & Tangler, J. L. (National Renewable Energy Laboratory). Blade Design Trade-Offs Using Low-Lift Airfoils for Stall-Regulated HAWTs, article, April 8, 1999; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc711944/m1/3/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.