Evolution of Modern Wind Turbines Part A: 1940 to 1994 - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 130

Sandia/DOE Experimental VAWTs
The Darrieus Wind Turbine Rotor
An elegant rotor design had been invented in France in the 1920s by G. J. M. Darrieus
[Darrieus 1931]. It utilized a vertical axis around which rotated curved airfoil blades in the
shape of a hoop, somewhat resembling an egg-beater. This curved-blade rotor was re-
invented in the late 1960s by engineers at the National Research Council in Canada (Fig.
3-8), where it was the subject of extensive study and development for two decades [ e.g.
South and Rangi 1972].
The Darrieus rotor has long been recognized as containing the seeds of a highly
eficient and intrinsically simple VAWT. It has its gearbox and generator located at the
base of the rotor for ease of maintenance. It will accept the wind from any direction
without a yaw mechanism. Although Darrieus blades are about three times as long as the
blades of a HAWT with the same swept area, they are generally so slender that the actual
rotor solidities ( i.e. , planform area per unit of swept area) are similar. However, a cost-
effective Darrieus VAWT required considerable development to overcome several disadvan-
tages. Guy cables required to support the top rotor bearing make it dificult to mount the
rotor very far above ground level to take advantage of higher wind speeds. Also, a Darrieus
rotor produces higher gearbox torques, both steady and cyclic, than a HAWT of the same
power and requires more material.
Sandia/DOE 17-m Experimental VAWT
Darrieus research was undertaken briely at the NASA Langley Research Center in the
early '70s [Muraca et al. 1975, Muraca and Guillote 1976]. However, the Sandia National
Laboratories in Albuquerque became the center of VAWT research and development in the
U.S., building on technology from the NRC in Canada. After initial analysis, laboratory
research, and small-scale ield tests, a 17-m VAWT was designed and built at Sandia (Fig.
3-20). The main purpose of this turbine was to determine what design and manufacturing
improvements would be required to make Darrieus VAWTs competitive with HAWTs.
Economic studies supporting the early research tests on the 17-m VAWT suggested
several improvements: First, two blades would be more cost-effective than the original
three on this turbine. Second, the long struts used to strengthen the rotor should be
eliminated. These struts were adding drag that consumed signiicant power, and their cost
was of the same order of magnitude as that of the working blades. Third, the blade airfoil
shape should be changed to one speciically tailored for the VAWT application. It is worth
noting that all three of these indings also applied to HAWTs. Strut drag losses, for
example, were found to be signiicant on the Gedser HAWT (Fig. 3-1) during tests in the
late 1970s, and few (if any) modern HAWT rotors have struts.
VAWT Blade Airfoils
Each time the blade of a VAWT makes a 360-deg rotation around its central column,
its angle of attack changes rapidly and over a wide range. This presents VAWT designers
with a complicated aerodynamic problem not present in aircraft and only to a small degree
in HAWT rotors. Airfoils tailored for VAWT use must have not only enhanced lift at low
angles of attack, both plus and minus, but must also be shaped to control stall behavior
during normal operation. In high winds, stall moderates lift so that constant power is