Wind Turbine Airfoils and Rotor Wakes - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 389

Comparative Power Performance

As an example of a field test conducted to obtain comparative power curves, let us exam-
ine the side-by-side performance tests conducted on two stall-controlled, three-bladed rotors
installed on identical medium-scale turbines [Tangler et al. 1990]. The wind turbines in these
tests were Micon 65/13 HAWTs, rated at 65 kW. One turbine had its original-equipment
Aerostar blades, with NACA 4415-24 airfoils (Fig. 6-6) and the planform shown in Figure
6-10(a). Candidate replacement blades on the second turbine [Jackson and Migliore 1987]
utilize the SERI airfoils in Figure 6-7, and their planform is shown in Figure 6-10(b). Table
6-2 lists comparative data for the two sets of blades. The restrained C L,max in the tip region of
the SERI blade permits the length extension and allows the use of 14 percent more swept area
for the same generator rating.
With respect to the prevailing wind at the site, the test turbines had a crosswind spacing of
2.2 diameters, and a meteorological tower with an anemometer at hub height was located 2.0
diameters upwind. Based on the relative positions of the anemometer and the test rotors, data
were collected at a sampling rate of 1 Hz, and then averaged for 30 sec, in compliance with
standard procedures [AWEA 1988]. Both clean and “dirty” blades were tested. Dirty blade
roughness was simulated by strips of tape on upper and lower surfaces near the leading edge.
Over 100 hours of operational data were collected for each cleanliness condition, which is a
typical duration for a reliable performance test.
The results of these comparative power performance tests are shown graphically by the
power curves in Figures 6-11 and 6-12. The significant performance improvements exhibited
by the SERI blades compared to the Aerostar blades are attributed to the following three fac-
tors, in the order of their relative contributions [Tangler et al. 1990]:

-- larger swept area of the SERI blades allowed by their restrained C L,max ;

-- less sensitivity of C L,max to leading-edge roughness in the outboard SERI

airfoils;

-- improved aerodynamic performance at low and moderate wind speeds of the

inboard SERI airfoils.

Figure 6-10. Planforms of two blade designs used in comparative field performance
tests on Micon65/13 HAWTs. (a) Original-equipment Aerostar blade with NACA 4415-24
airfoils (b) Candidate replacement blade with SERI airfoils [Tangler et al. 1990]