The Effect of Blade Pitch on Small Multi-Bladed
Horizontal-Axis Wind Turbines
M. Curtis Rector1 and Dr. Kenneth Visser2
Department of Mechanical and Aeronautical Engineering
The general goal in optimizing the aerodynamic performance of wind turbines is to increase
power extraction with a minimal increase to the overall cost of the system. For this reason, utility scale
wind turbines are optimized for least cost, since they are equipped with large, very expensive blades. It is
believed, however, that the idea of aerodynamic optimization could play a larger role in the efficiency of
small wind turbines, as these extreme costs do not apply on horizontal-axis wind turbines (HAWT’s)
more suited for smaller utilities such as home use. The goal of this study is to optimize the performance
of small HAWT’s by examining the effect of blade pitch on turbines with more than three blades.
The optimization of a wind turbine’s aerodynamic performance relies on many factors and
variables, including blade number (B); rotor solidity (σ), the ratio of blade area to swept area; blade pitch
(β), the angle between the chord line and the plane of rotation; and tip speed ratio (λ), the ratio of
peripheral tip speed to wind speed. Aerodynamic performance of small HAWT’s has been an active
research topic at Clarkson over the past few years. Initial numerical studies investigated the implications
of solidity and blade number and have since been tested experimentally in the Clarkson wind tunnel and
at a full-scale level at the Clarkson University Wind Turbine Test Site, located at the Potsdam Airport.
The full scale experiment, comparing a standard three-blade Bergey XL.1 with an experimental prototype
six-blade turbine, shows an increase in power extraction at lower wind speeds for increased blade number
and solidity. The prototype has also shown that its greater torque results in lowered operational tip speed
ratio, a decreased tip speed ratio at maximum power coefficient (Cp), the ratio of extracted power to the
power in the wind, and reduced cut-in wind speeds.I
The impact of blade pitch angle is a critical parameter for the aerodynamic optimization of
untwisted blades. The numerical studies performed earlier for multi-bladed turbines include this
parameter, as well, indicating a turbine with a 10° blade pitch angle resulting in a 20% greater Cp than one
at 20°. They also suggest an increased tip speed ratio at maximum Cp for a reduced blade pitch angle,
greatly affecting the tip speed ratio range of operation.II This thesis examines the effect of blade pitch on
a six-bladed turbine in an effort to optimize its aerodynamic performance so as to extract the greatest
1
2
Class of 2005, Mechanical Engineering, Clarkson University, Honors Program, oral presentation
Advisor, Dr. Kenneth Visser, Aeronautical Engineering, Clarkson University
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power and energy production for given wind conditions. Since this thesis is intended to complement the
full-scale research regarding small HAWT’s, data is being acquired at the Clarkson University Wind
Turbine Test Site. The site includes a silo, a meteorological tower and two fully operation 1kW wind
turbines. A meteorological tower, located approximately 20 meters upwind of the two test turbines, is
equipped with three anemometers to measure wind speed at heights of 6, 12 and 18 meters. Pressure,
temperature and humidity are measured at 6 meters and corrected to the hub height of both turbines.
The first of the two turbines, pictured in Figure 1a, is that of an unmodified Bergey XL.1, and is
used as the control for the experiment. This 3 bladed, horizontal-axis, upwind style wind turbine has a
6% solidity, blade diameter of 2.5 m, and estimated blade pitch angle of 10°. It is positioned at a height
of 18 m, approximately 5 blade diameters away from the prototype, perpendicular to the prevailing winds.
The experimental prototype, pictured in Figure 1b, is also a Bergey XL.1, modified to accept multiple
blades at various pitch angles. In its current modified state, this turbine is a 6 bladed, horizontal axis,
upwind style wind turbine having 12% solidity and blade diameter of 2.5 m. A hub attachment was
designed to enable the 6 blade configuration, which will remain throughout these studies, and variable
pitch angles that are manually adjusted through the use of metal shims. All other aspects of the prototype
turbine have remained unchanged from the as-bought condition in an effort to maintain uniformity with
the control. Both turbines have been installed as separate systems, each consisting of identical resistive
load banks, length and grade of wire and safety equipment.
a)
b)
Figure 1: Installed turbines at Clarkson University Wind Turbine Test Site: a) Control – Bergey
XL.1, b) Experimental prototype
Data from the turbines and meteorological tower is collected every second and stored in a
computer housed inside the test silo. The data acquisition system, within, uses two National Instruments
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PCI-6024e DAQ boards with a BNC-2120 interface in conjunction with LABVIEW 6.0i. Current and
voltage transducers located at the resistive load bank are used to determine power, and RPM is calculated
from the frequency in the voltage signal. The resistive load bank, replacing what would be a battery
charging unit during normal use, allows manual adjustment of the load between 1 and 25 ohms, for a
more controlled study. Analysis procedures follow the NERL performance test plans for a wind turbine
very similar to the Bergey XL.1 used on site, along with the test procedures developed by Advanced
Wind Turbines Incorporated. The acquired data is sorted according to wind speed into bins 0.5 m/s wide,
with bin centers located at half-integer multiples. From these averaged bins, power curves are then
generated for each resistance value tested. Performance curves of power coefficient versus tip speed ratio
and total energy are also created to compare and analyze the overall performance of the turbines.
The present study is investigating the impact of 10° and 20° blade pitch angles, as data has
previously been obtained for a 15° pitch angle. Since resistive loads directly impact the power output of
the wind turbines, resistances ranging between 1 and 8 ohms were tested to represent a range of high-tolow load conditions. As can be seen in Figure 2, the prototype having greater torque is able to operate at
a higher load for a given wind speed, so a comparison of identical loads does not reflect optimum
conditions for either turbine. Figure 2 compares the 2 ohm and 6 ohm cases for a 15° blade pitch angle.
As the load increases from 6 ohms to 2 ohms, the rotor speed decreases on the Control, closing the
difference in energy extraction between the two turbines.
Figure 2: Comparison of the power curves for the control and prototype turbines
under both high and low load conditions
Figure 3 displays the overall power curve for each turbine, representing the optimum power
output at a given wind speed. The results show the prototype turbine able to produce a greater power
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output than the control at lower wind speeds, under 3.5 m/s, while the reverse is true for wind speeds
greater than 3.5 m/s. The energy extraction of each turbine for a given month, June 2004, is illustrated in
Figure 4. The improved power of the prototype at low wind speeds results in a greater extraction of
energy than the control turbine at these speeds. Although it is not clearly evident from the figure, the
wind speeds averaged 3.5 m/s and below for more than 60% of the month. Comparing the total energy
extraction for the given month, the as-bought Bergey XL.1 produced 9.7 KW·hr more than the prototype.
a)
b)
Figure 3: Optimal Power Curve for control and prototype turbines per given wind speeds: a) Complete wind
distribution, b) Same distribution, zoomed in for a closer look between wind speeds of 1.5 and 4.5 m/s
Figure 4: Total energy extraction of the control and prototype turbines per given
wind speeds for June 2004
Humiston, Christopher and Visser, Kenneth. “Full Scale Aerodynamic Effects of Solidity and Blade Number on
Small Horizontal Axis Wind Turbines.” Clarkson University, presentation at World Wind Energy Conference,
South Africa, 23-26 November 2003.
II
Duquette, Mathew M., Swanson, Jessica, and Visser, Kenneth D. “Solidity and Blade Number Effects on a Fixed
Pitch, 50W Horizontal Axis Wind Turbine.” reprinted from Wind Engineering. Volume 27, No. 4, 2003.
I
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