Ain Shams Engineering Journal (2010) 1, 85–95

Ain Shams University

Ain Shams Engineering Journal

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MECHANICAL ENGINEERING

Effect of some design parameters on the performance

of a Giromill vertical axis wind turbine
M. El-Samanoudy, A.A.E. Ghorab, Sh.Z. Youssef *

Mechanical Power Engineering Department, Ain Shams University, Cairo 11517, Egypt

Received 15 June 2010; accepted 5 August 2010

Available online 5 November 2010

KEYWORDS Abstract This paper describes the effect of some design parameters on the performance of a Giro-
Wind turbine; mill vertical axis wind turbine. A Giromill wind turbine has been designed, manufactured and
VAWT; tested. The turbine performance has been investigated with varying the design parameters such
Vertical axis wind turbine; as, pitch angle, number of blades, airfoil type, turbine radius and its chord length. Then, the results
Giromill; were used for the comparison between the performance achieved while changing the design param-
Turbine performance; eters.
Power coefficient Vast number of experiments have been performed with changing the above mentioned parame-
ters. The effect of each parameter on the power coefficient and torque coefficient has been studied
and explanation of the results was also discussed. It has been found that the pitch angle, turbine
radius and chord length have a significant effect on turbine power coefficient.
The maximum power coefficient obtained in this research was 25% using turbine radius of 40 cm,
chord length 15 cm, pitch angle of 10, airfoil type NACA 0024, and four blades (which is found to
be the best configuration in this study). For the effect of pitch angle, the obtained maximum power
coefficient is decreasing, this decrease in performance was due to increasing in the pitch angle above
10 and also due to decreasing it below this value showing the high effect of pitch angle. It was also
noticed that, when decreasing the turbine radius to 20 cm at 0 pitch angle the maximum power
coefficient is much decreased. Moreover, decreasing the chord length to 12 cm at 10 pitch angle

* Corresponding author.
E-mail addresses: melsamanoudy@yahoo.com (M. El-Samanoudy),
ashrafghorap@hotmail.com (A.A.E. Ghorab), sherif_z_y@yahoo.
com (Sh.Z. Youssef).

2090-4479  2010 Ain Shams University. Production and hosting by

Elsevier B.V. All rights reserved.

Peer review under responsibility of Ain Shams University.

doi:10.1016/j.asej.2010.09.012

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86 M. El-Samanoudy et al.

decreases the maximum power coefficient significantly, which again show the high effect of turbine
radius and chord length. In order to compare the effect of airfoil type; the blades with NACA 4420
were used compared to NACA 0024 at the same above parameters of turbine radius 40 cm, chord
length 15 cm, pitch angle of 10 and four blades. The maximum power coefficient obtained was
15%. Finally, the effect of the number of blades have been investigated using two, three and four
blades at 0 pitch angle and the same other above parameters of turbine radius 40 cm, chord length
15 cm and airfoil type NACA 0024. The obtained maximum power coefficients were decreased sig-
nificantly when decreasing the number of blades from four to two blades.
 2010 Ain Shams University. Production and hosting by Elsevier B.V.
All rights reserved.

1. Introduction the comparison of the different model results such as, the
Momentum model, the Double-Multiple stream tube model,
The vertical axis wind turbines have many advantages over the Stochastic model, Viscous model and Dynamic-Stall model
horizontal wind turbines such as, the rotor shaft is placed ver- with experimental data.
tically and can be located near the ground. The generator and An experimental and theoretical study of a prototype was
the gearbox are placed near the ground. There is no need for a performed for a 2.5 kW turbine using wind tunnel experimen-
tower. Also, the turbine does not need to be pointed into the tal data and CFD analysis by Tullis et al. [7], in which power
wind. This makes maintenance of the wind turbine quite easy. curves have been developed at different wind speeds. The max-
Also, the vertical axis wind turbine is quite cost effective. imum power coefficient obtained was 0.28 for this prototype.
They can be placed on hilltops, on ridgelines and on the top The performance of a Giromill with active blade control
of buildings and in any areas where the force of the wind is was previously measured and the control of pitch angle to im-
more near the ground. Since they are placed lower, they can prove the power coefficient was also studied by Hwang et al.
be used where tall devices are not allowed by the law. The main [8]. It was found that the maximum power generated was at
advantage of a vertical axis wind turbine, however, is that it pitch angle 80 and phase angle 0.
turns in any direction with the wind; so, they do not need Paraschivoiu et al. [9] also studied the optimal variation of
the yaw mechanism that is required in the horizontal axis de- the blades’ pitch angle of an H-Darrieus wind turbine that
sign. As a result, the use of the vertical axis wind turbine maximizes its torque at given operational conditions. They
may be efficient; although of having some disadvantages such found that the optimized variable pitch leads to an improve-
as, they cannot cover a large area of wind. They are not very ment in Cp compared to 0 pitch angle of about 21% at wind
efficient with regards to extraction of energy because they speed of 7.3 m/s and turbine speed 125 rpm. At higher wind
operate near the ground where the air flow is turbulent. speeds (above 9 m/s) it results in a decrease of the turbine
An example of the design of Giromill wind turbine was pre- power. Three modifications for variable pitch control to im-
viously carried out and the analyses of some design parameters prove the performance of the turbine were studied by Staelens
was explained by Solum et al. [1]. The designed wind turbine et al. [10]. They examined the performance of a VAWT when
was a three bladed 12 kW H-rotor with tapered NACA 0018 the local angle of attack is kept just below the stall value
wing sections. It is connected to the rotating shaft through airf- throughout its cycle of rotation. This is accomplished by
oiled struts with a Cp of about 0.35. Also, the experimental re- adjusting the local geometric angle of attack of the blade. This
sults for this turbine were introduced and studied by Deglaire modification results in a very significant increase in the power
et al. [2], the turbine performance was investigated in highly output for wind speeds above 10 m/s. However, this modifica-
turbulent wind conditions and it was found that it is reacting tion requires sharp changes (jumps) in the local angle of attack
fairly well with respect to these conditions. making it physically and mechanically impossible to realize.
Theoretical analysis and computational modeling was pre- They replaced the local geometric angle of attack by the local
viously studied by Gyulai and Bej [3] and by Cooper and Ken- profile stall angle, obtained in the same manner as before.
nedy [4] to predict the relation between the tip speed ratio and As a consequence, this modification eliminates the two
the power coefficient of the turbine. They developed a mathe- jumps in the local effective angle of attack curve but at the cost
matical model for calculation of the performance and com- of a slight decrease in power output. Moreover, it also renders
pared the results with the experimental results. Gyulai and the angle of attack correction function which may be practi-
Bej used some experimental results to adjust their model. Coo- cally difficult to implement and result in an early fatigue. A
per and Kennedy, upon comparing the experimental results remedy for this limitation is to introduce a smooth and contin-
with multiple stream tube analysis, they found a fair agree- uous variation in the local angle of attack correction. Finally,
ment. Many other studies were carried out to facilitate the the- he overcomes the limitation of the second modification by
oretical analysis and the performance predications of the ensuring a continuous variation in the local angle of attack
vertical wind turbine such as, the study of the post-stall airfoil correction during the rotation cycle through the use of a sinu-
performance characteristics by Tangler and David Kocurek soidal function. The amplitude of the sinusoidal function was
[5]. set equal to the maximum difference between the local geomet-
An excellent agreement was found between predicted rotor ric angle of attack and the blade static-stall angle. Although
power and the measured ones. A study of the accuracy of dif- the power output obtained by using this modification is less
ferent mathematical models which predict the performance of than the two preceding modifications, it has the inherent
the turbine was carried out by Paraschivoiu et al. [6] through advantage of being practically feasible.
Effect of some design parameters on the performance of a Giromill vertical axis wind turbine 87

Nomenclature

A turbine swept area, m2 Cl lift coefficient

c chord length, m Cp power coefficient = P/(0.5 · q · A · v3)
F force, N Ct torque coefficient = T/(0.5 · q · r · A · v2)
l blades length, m
P power, W Greek letters
r turbine radius, m q density, kg/m3
T torque, N m x rotational speed, rad/s
v wind velocity, m/s k tip speed ratio
vrel blade relative velocity, m/s c pitch angle, degrees
a airfoil type h azimuth angle, degrees
n number of blades a angle of attack, degrees
Cd drag coefficient N.B. refer to Fig. 2 for an explanation of the above angles.

The effect of some design parameters such as, pitch angle

and airfoil type has been previously introduced with some
experimental data for comparison and analysis. Kinloch Kirke
[11] studied ways for self starting of the turbine, performance
prediction and the different parameters of airfoil which affect
the performance.
No studies were carried out to determine the effect of differ-
ent parameters variation such as, number of blades, pitch an-
gle, turbine radius, chord length and airfoil type, and to
determine the optimum variation. Accordingly, this paper
aims at studying the effect of changing the design parameters
on the performance of the Giromill vertical axis wind turbine
with fixed pitch angle variation (coefficient of performance,
tip speed ratio and torque coefficient). Also, to determine the
variation which will result in the best performance based on
the different performance parameters.

2. Experimental setup and procedure

Figure 1a c = 0.
2.1. Wind turbine design

The used Giromill wind turbine model has the following com- the shaft by means of friction. They were fixed to the shaft
ponents and parameters: as shown in Fig. 3b.
Shaft: The shaft is a solid steel shaft with a diameter of Rings: Used to protect Artelon disks from being worn out
30 mm as shown in Figs. 3a and b. as shown in Fig. 3b.
Blades: Three different NACA airfoil types were used. The Glands: Two Aluminum glands were keyed to the shaft by
material used was ‘‘Ayos’’ wood. The types are as follows: means of a (8 mm) diameter clearance bolt as shown in Fig. 3b.
NACA 0024, NACA 4420 and NACA 4520 with chord lengths Pulley: A grooved Aluminum pulley was used to transmit
(8 cm, 12 cm, 15 cm) for each airfoil type with span length of the exerted force on the rope which is in contact with the pulley
70 cm. as shown in Fig. 3b. The exerted force on the rope here acts as
Caps: They were used to fix the blade from its upper and the applied load on the turbine.
lower ends to the main links that are connected in turn to As the load increases the turbine slows down according to
the disk. They were made from sheet metals taking the shape the following equations: P = T · x, T = (F1  F2) · r, where
of the corresponding blades as shown in Fig. 3b. 1 donates for one rod and 2 for the other.
Main links: Steel link were mainly used to transmit power Flanged bearing: Two square flanged ball bearings were
from the blades to the disk and consequently to the shaft. Five used to fix the turbine to the test rig with 20 mm inner
holes were drilled in the link responsible for specifying the diameter.
pitch angle as shown in Figs. 3a and b.
Auxiliary links: Steel link was used to change the pitch an- 2.2. Experimental setup
gle, this link has two holes one to fix it on the cap and the other
matched with one of the five holes for the main link to change The used wind tunnel in this experiment consists of the follow-
the pitch angle, as shown in Figs. 1a, b and 2. ing components:
Disks: Two Artelon – 10 mm thick and 20 cm diameter – (a) Blower: Double suction centrifugal blower was used
disks were used to transmit the power from the main links to with a power of 40 HP.
88 M. El-Samanoudy et al.

(80 cm · 80 cm) of length 100 cm each. The second section is

increasing the cross sectional area from 6400 cm2 to
22,500 cm2 through a diffuser section. A tunnel with 205 cm
length is the third section which acts as a damper for the flow
to decrease the amount of vibrations on the tunnel. The fourth
section is a (1 m · 1 m) cross sectional area which is the outlet
to the turbine under test.
Test section: A steel frame was used to fix the turbine in
front of the wind tunnel inlet as shown in Fig. 4.
Air speed: Air speed used in this experiment was 8 m/s.

2.3. Instrumentation

Digital tachometer: To measure the turbine speed (rpm) with

an accuracy of ±1%.
Vane anemometer: The vane anemometer is an instrument
that is used to measure wind speed (air speed), it can also mea-
sure wind direction. The outlet of the wind tunnel outlet was
divided into nine sections in order to measure the wind speed
in each section to obtain the average wind speed at the tunnel
outlet with an accuracy of ±1.5%.
Force gauges: It measured the tension in the rope attached
to the pulley with an accuracy of ±1.5%.

Figure 1b c = 40. 3. Results and discussions

The following curves describe the comparisons of the perfor-

mance of some variations resulting from changing the design
parameters (airfoil type, chord length, turbine diameter, num-
ber of blades and pitch angle).

3.1. Effect of pitch angle on Cp, Ct and k

As shown in Figs. 6–9, these curves were obtained from chang-

ing the pitch angle (0, 10, 20, 30, 40, 50, 60 and -10)
with different variations of number of blades, turbine radii,
chord lengths and airfoil types.
We can see from Figs. 6–8 that the pitch angle of 10 gives
the highest performance regarding Cp, Ct and k (Cp = 25% at
Figure 2 Schematic diagram showing c, a and h angles. k = 1.3 and Ct  0.2 at k = 1). It can be concluded from the
below figures that the performance decrease significantly (re-
sults in lower Cp and Ct both at a lower k) with increasing
(b) Wind tunnel sections: A 985 cm length tunnel made from the pitch angle above 10. The performance also decreases
metal sheets delivering air flow from the blower to the when decreasing the pitch angle below 10, which shows that
turbine under test as shown in Fig. 4. pitch angle has a significant effect on the performance. The
same comparison of the effect of pitch angle is also shown in
The tunnel is divided into four main sections: the first sec- Figs. 7 and 9 but using a different airfoil type and we get the
tion consists of successive square shaped cross sectional area same above conclusions.

Figure 3a Turbine and test rig photo.

Effect of some design parameters on the performance of a Giromill vertical axis wind turbine 89

Figure 3b Turbine diagram.

Figure 4 Test rig and wind tunnel arrangement.


1 1 1
Cp ¼ Ct qclv2rel rxn qAv3 ¼ Ct cv2rel kn=rv2 ð1Þ
2 2 2

Ct ¼ Cl sin a  Cd cos a ð2Þ

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
vrel ¼ ððv sin hÞ2 þ ðxr þ v cos hÞ2 Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
¼ v sin2 h þ ðk þ cos hÞ2 Þ ð3Þ

Cl and Cd are functions of a, Re (Reynolds number) and the

Figure 5 (Ct · vrel) vs. c for a four symmetrical airfoil blades at particular airfoil section:
k = 1.4 using approximation for Cl and Cd values.
a ¼ tan1 ðvsinh=ðxr þ vcoshÞÞ  c
This can be explained through the following formulas
which describes the changes of Cp with a, c, Cl, Cd and k ¼ tan1 ðsinh=ðk þ coshÞÞ  c ð4Þ
90 M. El-Samanoudy et al.

Figure 8 Ct vs. k at different pitch angles using four blades,

Figure 6 Cp vs. k at different pitch angles using four blades,
turbine radius 40 cm, chord length 15 cm and airfoil type NACA
turbine radius 40 cm, chord length 15 cm and airfoil type NACA
0024.
0024.

Figure 7 Cp vs. k at different pitch angles using four blades, Figure 9 Ct vs. k at different pitch angles using four blades,
turbine radius 40 cm, chord length 15 cm and airfoil type NACA turbine radius 40 cm, chord length 15 cm and airfoil type NACA
4420. 4420.

Cl ¼ ð1 þ 0:05 ð% camberÞÞsin2a ðas an approximation ½11Þ

ð5Þ values of k using the above formulas we will find that the max-
imum value of ðCt  v2rel Þ and accordingly Cp occurs at about
Cd ¼ ð0:9 þ 0:025 ð% camberÞÞ  ð1:5sin3 a þ 0:5sina 10 as shown in Fig. 5. There are deviations in the actual val-
þ 0:05 ð% camberÞÞ ðas an approximation ½11Þ ð6Þ ues from the calculated values because of the approximation of
Cl and Cd values and due to changes in Re number and the un-
We can see from Eq (1) that the relation between Cp and k steady effects of changing a rapidly which have an effect on
depends on the term ðCt  v2rel Þ. By plotting the relation increasing the stall angle and maximum values of Cl as ob-
between ðCt  v2rel Þ (for the upwind blades) and c at different served by Klimas.
Effect of some design parameters on the performance of a Giromill vertical axis wind turbine 91

Figure 10 Cp vs. k using different number of blades at pitch Figure 12 Ct vs. k using different number of blades at pitch angle
angle 10, turbine radius 40 cm, chord length 15 cm and airfoil 10, turbine radius 40 cm, chord length 15 cm and airfoil type
type NACA 0024. NACA 0024.

Figure 11 Cp vs. k using different number of blades at pitch

Figure 13 Ct vs. k using different number of blades at pitch angle
angle 0, turbine radius 40 cm, chord length 15 cm and airfoil type
0, turbine radius 40 cm, chord length 15 cm and airfoil type
NACA 0024.
NACA 0024.

3.2. Effect of number of blades on Cp, Ct and k

mance was much decreased. It can be seen from Figs. 11 and
As shown in Figs. 10–13, these curves were obtained from 13 that when decreasing the number of blades to three blades
changing the number of blades (2, 3 and 4 blades) with differ- there is a small decrease in performance. So, we can conclude
ent variations of pitch angles, turbine radii, chord lengths and that increase in the number of blades from two to four blades
airfoil types. has a significant effect in increasing the performance while
We can see from Figs. 10 and 12 that using four blades increasing the number of blades from three to four has a small
results in the highest performance regarding Cp, Ct and k increase in performance.
(Cp = 25% at k = 1.3 and Ct = 0.205 at k = 1) and when This is expected as the higher number of blades results in
decreasing the number of blades to two blades the perfor- higher torque at the same speed.
92 M. El-Samanoudy et al.

Figure 14 Cp vs. k using different turbine radius at pitch angle

0, four blades, chord length 15 cm and airfoil type NACA 0024. Figure 16 Cp vs. k using different airfoil types at pitch angle 10,
using four blades, chord length 15 cm and turbine radius 40 cm.

Figure 15 Ct vs. k using different turbine radius at pitch angle 0,

Figure 17 Cp vs. k using different airfoil types at pitch angle 0,
four blades, chord length 15 cm and airfoil type NACA 0024.
using four blades, chord length 15 cm and turbine radius 40 cm.

3.3. Effect of turbine radius on Cp, Ct and k 3.4. Effect of airfoil type on Cp, Ct and k

As shown in Figs. 14 and 15, these curves were obtained from As shown in Figs. 16–19, these curves were obtained from
changing the turbine radius (20 cm and 40 cm) with different changing the airfoil type (NACA 0024, NACA 4420 and
variations of number of blades, pitch angles, chord lengths NACA 4520) with different variations of number of blades,
and airfoil types. turbine radii, chord lengths and pitch angles.
We can see from Figs. 14 and 15 that; when we decrease the As shown in Figs. 16 and 18, the symmetrical airfoil NACA
turbine radius from 40 cm to 20 cm, the performance was 0024 results in higher performance regarding Cp, Ct and k; the
much decreased showing the significant effect of changing airfoil NACA 0024 gives higher Cp compared to NACA 4420
the turbine radius. and NACA 4520. There is a significant improvement in
The lower performance at the lower radius is due to the performance results from using NACA 0024 airfoil instead
effect of the shaft and the downwind blades on the flow past of NACA 4520, while there is a much lower improvement in
the blades which decrease the velocity of the wind past the performance results from using NACA 4420 airfoil instead
upwind blades. of NACA 4520.
Effect of some design parameters on the performance of a Giromill vertical axis wind turbine 93

Figure 20 Cp vs. k at different chord lengths at pitch angle 10,

Figure 18 Ct vs. k using different airfoil types at pitch angle 10, using four blades, airfoil type NACA 0024 and turbine radius
using four blades, chord length 15 cm and turbine radius 40 cm. 40 cm.

Figure 19 Ct vs. k using different airfoil types at pitch angle 0, Figure 21 Cp vs. k at different chord lengths at pitch angle 0,
using four blades, chord length 15 cm and turbine radius 40 cm. using four blades, airfoil type NACA 0024 and turbine radius
40 cm.

The higher performance of the symmetrical airfoil is the

result of the difference of the characteristics of the change of Figs. 20 and 22 show that increasing the chord length has a
Cl and Cd with a (for example the cambered airfoils has posi- significant effect in increasing performance, while Figs. 21 and
tive values of Cl for certain range of negative a, while this is not 23 give the same conclusion but at different pitch angle.
the case with symmetrical airfoil which has negative Cl for all This can be explained due to the higher projected area
negative values of a. Also, cambered airfoils have higher Cd which increases the aerodynamic force on the blades and due
values compared with that of symmetrical airfoils at the same to the higher Re number which results in higher Cl values with
angle of attack). increasing the chord length.

3.5. Effect of chord length on Cp, Ct and k 4. Comparison between experimental data and previously
published works
As shown in Figs. 20–23, these curves were obtained from
changing the chord length (8 cm, 12 cm and 15 cm) with differ- A theoretical analysis was carried out by Whitten [12] by
ent variations of number of blades, turbine radii, pitch angles using approximation of lift and drag data of NACA 0012
and airfoil types. blades [13], and a comparison of this theoretical analysis
94 M. El-Samanoudy et al.

Figure 24 Comparison between present experimental results and

previous results [8].

Figure 22 Ct vs. k at different chord lengths at pitch angle 10, A comparison between the results obtained in the previ-
using four blades, airfoil type NACA 0024 and turbine radius ously experimental results [8] and the results obtained in this
40 cm. research is shown in Fig. 24.
The parameters of the present and previous experiments in
the above comparison are as follow:

Parameters Present exp. Previous exp. Previous

(8 m/s) (13 m/s) [8] exp. (4 m/s)
[8]
Number of blades 4 4 4
Airfoil type NACA 0024 NACA 0018 NACA 0018
Radius of rotor (m) 0.4 1.6 1.6
Span length of blades (m) 0.7 2 2
Chord length of blades (m) 0.15 0.45 0.45
Pitch angle () 10 8 8
Wind speed (m/s) 8 13 4

A fair agreement was found between previous experiments

and the results of this research.

5. Conclusions

From the above results we can conclude the following:

(1) The pitch angle is an important parameter on the Cp, Ct

and k (the maximum values obtained at about 10).
Figure 23 Ct vs. k at different chord lengths at pitch angle 0, (2) The chord length has a significant effect on the Cp, Ct
using four blades, airfoil type NACA 0024 and turbine radius and k (increasing the chord length increasing these
40 cm. values).
(3) The turbine radius has a remarkable effect on the Cp, Ct
and previous experimental data was also performed by Cooper
and k (increasing turbine radius increases these values in
and Kennedy [4].
the range of this study).
The theoretical analysis and the results of the experimental
(4) There is noticeable increase in Cp, Ct and k resulting
data previously obtained by Cooper and Kennedy [4]; showed
from increasing the number of blades from two to four
that the maximum power coefficient is about 0.25 for the mod-
blades. Less increase in these values results from the
el used.
increase in the number of blades from three to four
It has been showed by Hwang et al. [8] that the best perfor-
blades.
mance is achieved at pitch angle about 8 which is confirmed
(5) Symmetrical airfoil (NACA 0024) results in higher Cp,
by the results obtained in this research.
Ct and k, and there is a little increase in these values
It is also showed by Hwang et al. [8] that the improvement
resulting in changing airfoil type from NACA 4520 to
achieved by active pitch control over fixed pitch turbine is
NACA 4420.
more with higher wind speeds.
Effect of some design parameters on the performance of a Giromill vertical axis wind turbine 95

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