Honors Thesis Proposal
Studying the Effect of Blade Pitch on Small Multi-Bladed
Horizontal-Axis Wind Turbines
By
Mark Curtis Rector
Advisor: Dr. Kenneth D. Visser
Abstract
During the past couple years, a study on the effects of solidity and blade number on small
horizontal-axis wind turbines has been underway at Clarkson University. This thesis will
study the impact of blade pitch angle on power extraction for the given wind conditions,
in an effort to maximize the energy that can be produced. While similar studies have
been conducted numerically and experimentally on scaled-down models in the on-site
wind tunnel, the new data will be acquired from the Clarkson University Wind Turbine
Test Site at the Potsdam airport.
Introduction
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 turbines are often 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 (HAWTs)
more suited for the smaller utilities such as home use.
The optimization of a wind turbine’s aerodynamic performance relies on many factors
and variables, including blade number; rotor solidity, the ratio of blade area to swept
area; blade pitch, the angle between the chord line and the plane of rotation; and tip speed
ratio, the ratio of peripheral tip speed to wind speed. Past studies have shown that
increasing blade number increases power output; however, most small wind turbines
continue to utilize 3 blades. It is thought that this is because the smaller turbines are still
based on the economic philosophy carried down from the larger utility scale turbines.
Nybroe’s Windflower design, an increased blade-number concept developed in 1999,
supports this theory [9]. This 12-bladed turbine with a 3.8 meter rotor diameter, seen in
Figure 1, was found to operate at a competitive power coefficient, ratio of power
extracted per the power in the wind (Cp), while at a much lower tip speed ratio, λ, than its
counterparts.
Figure 1: Windflower DK 4 kW [9]
Figure 2 displays Johnson’s performance comparison of power coefficient versus tip
speed ratio for some of the most common wind turbine designs [7].
Figure 2: Wind turbine Cp-λ comparison [7]
Though the figure shows the two bladed rotor with tip speed ratios ranging between 4 and
7 producing the greatest power output, ideas such as the Windflower, producing a
reported Cp = 0.47, have motivated the interests of further research here at Clarkson [8].
Previous Studies at Clarkson
Aerodynamic performance of small horizontal axis wind turbines has been an active
research topic at Clarkson over the past few years. Initial numerical studies investigated
the implications of solidity, σ, and blade number, B, which were later tested
experimentally in the Clarkson wind tunnel [1]. The results suggest that the greatest
power coefficients result from increased blade number and greater rotor solidity, both of
which contribute to the added torque that improves cut-in wind speed. Figure 3 displays
the numerical results of the Expanding Wake Method (EWM), a performance analysis
process based partly on the method of Gould and Fiddes with lifting line theory [5]. As
shown, the theoretical results predict a 30% increase in Cp going from a 3 bladed rotor to
12, at equal solidities of 0.27. Even at σ = 0.14, an increase from 3 to 6 blades provides
10% greater Cp. These predictions are dynamic in suggesting the opportunity for
significantly greater power extraction than comes from the two or three blade turbines
most commonly in use.
Figure 3: Numerical EWM results portraying the effect of blade number and
solidity on Cp-λ characteristics [1]
Figure 4 displays the experimental results for the performance analysis of varying blade
number and solidity. Although the wind tunnel experiments failed to show the same
results in regards to increased blade number, the experiments did coincide with the
numerical results regarding solidity. This data suggests a much greater Cp at an increased
solidity, when comparing the 3 bladed rotors with solidities of 14% and 27%. In
conjunction with this increased power coefficient, is a reduction in tip speed ratio at
maximum Cp. Though slight, this too, coincides with the results of the EWM theoretical
study performed.
Figure 4: Wind Tunnel experiment results portraying the effect of blade number
and solidity on Cp-λ characteristics [1]
More recently, full-scale experiments have been initiated to examine blade number and
solidity at the Clarkson University Wind Turbine Test Site by comparing a Bergey XL.1
with a prototype six-blade turbine. Initial results agreed with the numerical study,
suggesting the presence of increased power extraction at lower wind speeds, for increased
blade number. The prototype has also shown that its greater torque results in lowered
operational tip speed ratio, a decreased tip speed ratio at maximum Cp and reduced cut-in
speeds [6].
Though it hasn’t been studied on a full-scale model until now, blade pitch angle, β, has
been observed to be an important factor in both the numerical analysis and wind tunnel
experiments, as it dramatically changes the tip speed ratio range of operation. While the
two studies showed agreement that maximum Cp moved to smaller tip speed ratio, λ, with
increasing pitch angle, they disagreed as to which pitch angle was optimum. Figure 5
displays the numerical Blade Element-Momentum (BEM) analysis results of Cp-λ
characteristics for a 12 bladed rotor with solidity equal to 0.27, while Figure 6 displays
that of the experimental case.
Figure 5: Numerical BEM analysis of Cp-λ characteristics based on varying blade
pitch with B = 12, σ = 0.27 [1]
Figure 6: Wind tunnel experiment analysis of Cp-λ characteristics based on
varying blade pitch with B = 12, σ = 0.27 [1]
Both studies conducted analysis at 10°, 15° and 20°, however, the theoretical study
indicated that β = 10° achieves the maximum power coefficient, while the wind tunnel
experiment indicated 15° as optimum.
Proposed Research
Upon further analysis of the previous work performed at Clarkson, this thesis will
examine the effect of blade pitch on a multi-bladed turbine to determine the optimum
power and energy production for the given wind conditions. As a continuation of prior
research regarding the aerodynamic performance of small horizontal-axis wind turbines,
this study will be conducted on a full-scale level with the turbines already existing at the
Clarkson University Wind Turbine Test Site. Data will be acquired at varying load and
rotor speeds for multiple pitch angles to offer a complete comparison between the two
turbines.
Objective
The ultimate goal in optimizing the aerodynamic performance of small horizontal-axis
wind turbines is to work toward developing a machine more effective at reducing the
costs of electricity and making wind a more affordable alternative to the pollutants
currently used in energy production. Through the use of additional blades, increased
solidity and optimized blade pitch, the intent is to improve the power coefficient at a
lower tip speed ratio in an effort to make small HAWTs a more viable option for energy
production. Of course, when evaluating the increased performance, it must be kept in
mind that blades cost money, so an increase in blade number equates to a similar increase
in turbine cost.
Methodology
Since this thesis is intended to complement the full scale research regarding the
aerodynamic performance of small horizontal-axis wind turbines, it is important to keep
the set-up and measurements consistent with studies done previously. Most of the
necessary tools are already in place at the Clarkson University Wind Turbine Test Site, as
the studies surrounding blade number and rotor solidity have been underway for almost a
year. In fact, the data currently being recorded, at a blade angle of 15 degrees, will be
used as the basis for which other blade angles will later be compared.
The Clarkson University Wind Turbine Test Site is located at the Potsdam airport. The
site includes a test silo, a meteorological tower and two fully operational 1 kW wind
turbines. The meteorological tower, located approximately 20 meters (8 blade diameters)
upwind of the two test turbines, uses 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 [6]
The first of the two turbines, pictured in Figure 7a, is that of an unmodified Bergey XL.1,
and is used as the control for the experiment. This 3 blade, horizontal-axis, upwind style
wind turbine has a 6% solidity, blade diameter of 2.5 m, and approximate 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. This turbine was chosen for study due
to its simple design, small size and relatively inexpensive cost.
The prototype, pictured in Figure 7b, is also a Bergey XL.1, purchased and 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, blade
diameter of 2.5 m, and blade pitch equal to 15°. A hub attachment was designed to
enable the 6 blade configuration, which will remain throughout these studies, and
variable pitch angles. Upon lowering the rotor to the ground, made-to-order metal shims
are used to adjust the pitch. 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 7: Installed turbines at Clarkson University Wind Turbine Test Site [6] a) Control –
Bergey XL.1, b) Prototype – 6 bladed rotor, 12% solidity
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 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 allows manual adjustment of the load from anywhere
between 1 and 25 ohms, for a more controlled study. At midnight each day, the data
stored in the test silo is transferred, via the internet, to a campus computer where it is
stored for analysis.
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 [6]. The acquired data for each resistance is
sorted according to wind speed into bins 1 m/s wide, with bin centers located at integer
multiples of 1 m/s. From these averaged bins, power curves are then generated at each
resistance value. Performance curves of power coefficient versus tip speed ratio and total
energy can be created to compare and analyze the overall performance of the turbines.
Schedule
The following is a tentative schedule, outlining prospective dates for my thesis
accomplishment:
April 2004
Review previous data collected with blade pitch
angle at 15 degrees
May – November 2004
Run, collect and analyze data based on blade pitch
angles of 10 and 20 degrees
Early December 2004
Compile research into rough draft thesis form
End February 2004
Submit Thesis final draft
Mid March 2004
Present final Thesis
References
[1] Duquette, Mathew M. “The Effect of Solidity and Blade Number on the
Aerodynamic Performance of Small Horizontal Axis Wind Turbines,” Master’s
Thesis June 2002, Clarkson University, Potsdam, NY.
[2] 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.
[3] Duquette, Matthew M. and Visser, Kenneth D. “Numerical Implications of Solidity
and Blade Number on Rotor Performace of Horizontal-Axis Wind Turbines.” Journal
of Solar Energy Engineering Vol. 125. November 2003: 425-432.
[4] Gipe, Paul. Wind Energy Basics: A Guide to Small and Micro Wind Systems.
Vermont: Chelsea Green Publishing Co., 1999.
[5] Gould, J. and Fiddes, S.P. “Conputational Methods for the Performace Prediction of
HAWTs.” J. Wind Eng Indust. Aerodyn., 39: 61-72.
[6] 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.
[7] Johnson, Gary L. Wind Energy Systems: Electronic Edition. Manhattan, KS, 10
December 2001.
[8] Manwell, J.F., McGowan, J.G., and Rogers, A.L. Wind Energy Explained: Theory,
Design and Application. England: John Wiley & Sons, Ltd, 2002.
[9] Nybroe, Klaus. “Windmission of Denmark.” 18 May 2002.
<http://www.windmission.dk>.