Environmental Engineering Reference
In-Depth Information
Table 6-3. Sample Comparative Annual Energy Ratios
Test Rotor
Configuration
Gross Annual
Energy Output
1
(MWh/y)
Annual Energy
Ratio
Original: Clean
194
1.00
Original: Dirty
139
0.72
SERI: Clean
225
1.16
SERI: Dirty
184
0.95
1
7.54 m/s mean wind speed at hub elevation, with a
Weibull wind speed distribution
The relative energy productions of the two rotors will depend on site conditions, such
as the rate of blade soiling by dirt and insects and the frequency of washing. For example,
is we assume that the clean condition is present 40 percent of the time and the dirty condition
60 percent, a rotor with SERI blades can be expected to produce about 24 percent more
energy under the specified wind regime. This is calculated using the annual energy ratios in
Table 6-3, as follows:
Original
-
Rotor Energy
=
0.40 ´ 1.16 + 0.60 ´ 0.95
SERI
-
Rotor Energy
0.40 ´ 1.00 + 0.60 ´ 0.72
= 1.24
(6-6)
Actual field test comparisons in Tehachapi and San Gorgonio wind power stations in
California have demonstrated improvements in annual energy output from 25 percent to 30
percent [Tangler 1993]. In addition to the improvements calculated in accordance with the
annual energy ratios in Table 6-3, the SERI airfoils produce more energy because of easier
startup of the turbine and longer run times in low winds. These are additional benefits of the
lower sensitivity of the SERI airfoils to roughness. At a site with a low annual average wind
speed, improvements in annual energy output as high as 40 percent have been projected.
Tip Shapes of HAWT Blades
One of the many distinctions between HAWT and VAWT blades is that the former
have a tip whose effects on lift and drag forces must be taken into account. Two
fundamental fluid-dynamic effects occur here. One is the purely
inviscid
effect of the
termination of the lifting
vorticity.
This results in a shedding of vorticity near the tip which
causes a streamwise vorticity sheet of high intensity in the vicinity of the tip. This inviscid
effect modifies the induced
downwash field
in this region. The second effect is a viscous
one associated with the highly three-dimensional flow around the tip. Normally, the
inviscid and viscous effects interact, with the trailing vortex system inducing flow around
the tip while flow separation near the tip (resulting from blade separation) may change the
tip air loading and hence affect the trailing vorticity.
These effects in planar wings have been studied extensively. Many wing-tip shapes
have been tested in attempts to improve aerodynamic performance, and advantages have
been claimed for a number of these. Studies of tip shapes for HAWT rotors have also been
made, dating back at least to those that resulted in the semi-circular tapered tips on the
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