Environmental Engineering Reference
In-Depth Information
Future Developments of VAWT Aerodynamic Theory
Continued comparison of theoretical and experimental VAWT performance has focused
current research on the following:
-- improving the accuracy of predicting peak power output;
-- understanding dynamic stall at high wind conditions;
-- including stochastic wind models in order to predict fatigue loads on blades;
-- correcting the tendency to over-predict power output for operations between maxi-
mum rotor power coefficient and the onset of dynamic stall;
-- introducing interactions between adjacent flow regions;
-- eliminating assumptions that are known to be mathematically incorrect.
Local circulation methods [Azuma
et al.
1983, Massé 1984] are candidates for improved
models of the performance of VAWTs.
Aerodynamic Behavior of VAWTs in Operation
More than 600 VAWTs were placed in commercial service, mostly in California, with
generator capacities that range from 120 to 300 kW. All were two-bladed machines. De-
spite their lack of wind direction sensitivity, yaw control equipment, aerodynamic control
surfaces, and teetered hubs, VAWTs still have several operational characteristics that are
unique, such as rotor blade shape and torque ripple. Other characteristics, such as starting
and stopping, the role of transient aerodynamics, the use of vortex generators, and the effects
of rotor solidity are subjects that VAWTs have in common with HAWTs.
Rotor Blade Shape
The blade shape of a Darrieus blade is patterned after the
troposkien
(“spinning rope”),
a configuration in which every section of an ideal blade is locally in tension under action of
the centrifugal acceleration (including a small correction for gravity loads), without bend-
ing stresses. Practical Darrieus blade shapes are composed of mixtures of circular arcs and
straight sections that approximate the ideal shape, in order to reduce bending stresses and yet
have a shape that can be easily manufactured. The
Sandia/DOE 17-m
research VAWT (as
well as several commercial VAWTs that are derivatives of it) uses the blade configuration
shown in Figure 5-40. It consists of a circular arc,
AB
, and a straight section,
BC
, instead of
the continuously changing curvature of a true troposkien. The ratio of height to diameter,
H
/
2R
, is very close to unity in this rotor. U.S. commercial VAWTs have employed
aspect
ratios
between 1.2 and 1.3, and the Canadian
Etolé
VAWT (Fig. 3-36) had a ratio of 1.5.
On the circular arc or
equatorial section
of the blade, local angles of attack are found to
decrease as the radius
r
decreases. By contrast, angles of attack on the straight blade section
increase from the junction with the arc toward the axis of rotation. Some of the operational
characteristics of a VAWT rotor may be inferred by an examination of these variations in the
local angle of attack.
First, consider the performance of a two-bladed Darrieus rotor in high winds (lower
tip-speed ratios), during which power output is produced almost exclusively by the circular
arc sections of the rotor. Since angle of attack decreases along the circular arc, and since a
decrease in angle of attack reduces the amount of stall caused by high winds, the power out-
put of the rotor will increase as the central angle Q increases (holding blade chord constant).
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