Environmental Engineering Reference
In-Depth Information
As a result of using time-averaged data, performance measured by field testing lacks the
potential accuracy of wind tunnel or tow testing. At the same time, it contains the response
of the turbine to turbulence, wind shear, and terrain features. The
reference wind speed
for
performance testing is the wind speed measured at the elevation of the center of the swept
area of the rotor. Because a rotor retards the wind flowing through it, the reference wind
speed must be measured at a distance upwind, usually a minimum of about 1.5 to 2.0 rotor
diameters. Performance test data based on a single reference wind speed measurement is
the current industry standard, but two anemometers are usually used for redundancy [ASME
1988].
Examples of Field Tests of HAWT Performance
Numerous field tests have been conducted on three generations of medium- and large-
scale prototype HAWTs, from the 100-kW 38-m diameter Mod-0 to the 3.2-MW 97.5-m
diameter Mod-5B (see Chapter 3). Performance test data from three of these evolutionary
turbines will be presented here and compared with performance predictions made in accor-
dance with modified strip theory. The specific examples are these:
-- The Mod-0 HAWT (38-m diameter; 100-kW rating) operating at
fixed pitch
. Under
high wind conditions the flow over the rotor blades was well into the stall region.
-- The MOD-1 HAWT (61-m diameter; 2-MW rating), which operated in the
vortex
ring
/
windmill brake
state under low wind conditions.
-- The MOD-2 HAWT (91.4-m diameter; 2.5-MW rating) using
partial-span pitch
control as well as
teetering
.
As discussed in the introduction to this chapter, performance data taken during a field
test are stored and plotted using the
method of bins
[Akins 1978], producing a graph of out-
put power versus wind speed or a
power curve
. Comparisons with theory will be made on
the basis of the mean value of the test data in each bin. The
standard deviation
will also be
shown when available.
Mod-0 HAWT Performance
This rotor had tapered and twisted blades with airfoil sections from the NACA 23000
series. Airfoil thickness-to-chord ratios varied ranging from 0.12 at the tip to more than 0.40
at the blade root, with a rotor solidity of 2.9percent. During this series of tests, the MOD-0
wind turbine was operated at fixed pitch, with a tip speed of 55 m/s [Viterna and Corrigan
1981]. Results from these tests are shown in Figure 5-26 and are typical of the performance
of fixed-pitch HAWTs. Power increases with increasing wind speed, holds constant, and
then decreases somewhat. This process is referred to as
stall regulation
. Strip theory pre-
dictions of power agree with the test results over the entire range of wind speeds, including
post-stall operations for which an empirical model was developed by the investigators.
Mod-1 HAWT Performance
The Mod-1 rotor had tapered and twisted blades with a solidity of 4.2 percent, half
again as much as the Mod-0 rotor. The higher solidity of the Mod-1 turbine resulted in the
development of large axial inductions at low wind speeds, so wake expansion effects were
considerable. The NACA 4400 series of airfoils was employed for the two rotor blades, with
thickness-to-chord ratios varying from 0.09 at the tip to 0.33 at the root. Performance test
data [Spera and Janetzke 1981] were measured at a rotor speed of 3.63 rad/s, which produced
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