on the performance analysis of savonius rotor with twisted blades (1)
TRANSCRIPT
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Renewable Energy 31 (2006) 17761788
www.elsevi er.com/locate/renene
On the performance analysis of Savonius rotor withtwisted blades
U.K. Saha
, M. Jaya Rajkumar
Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati-781 039, India
Received 1 March 2004; accepted 6 August 2005
Available online 21 October2005
Abstract
The present investigation is aimed at exploring the feasibility of twisted bladed Savonius rotor for
power generation. The twisted blade in a three-bladed rotor system hasbeen tested in a low speed
wind tunnel, and its performance has been compared with conventional semicircular blades (with
twist angle of01). Performance analysis has been made on the basis of starting characteristics, static
torque and rotational speed. Experimental evidence shows the potential of the twisted bladed rotor in
terms of smooth running, higher efficiency and self-starting capability as compared to that of the
conventional bladed rotor. Further experiments have been conducted in the same setup to optimize
the twist angle.
r2005 Elsevier Ltd. All rights reserved.
Keywords: Savonius rotor; Twisted blade; Starting characteristics; Static torque; Coefficient ofperforman ce
1. Introduction
Savonius rotor is a unique fluid-mechanical device that has been studied by numerous
investigators since 1920s. Applications for the Savonius rotor have included pumping
water, driving an electrical generator, providing ventilation, and agitating water to keep
stock ponds ice-free during the winter [14]. Savonius rotor has a high starting torque and
a reasonable peak power output per given rotor size, weight and cost, thereby making it
less efficient [5]; the coefficient of performance is of the order of 15% [6,7]. From the
point of aerodynamic efficiency, it cannot compete with high-speed propeller and the
Corresponding author. Tel.: +91 361 2691085; fax: +91 361 2690762.E-mail address: [email protected] r net.in (U.K. Saha).0960-1481/$ - see front matter r2005 Elsevier Ltd. All rights
reserved. doi:10.1016/j.renene .2005.08.030
http://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenehttp://www.elsevier.com/locate/renenemailto:[email protected]:[email protected]://www.elsevier.com/locate/renene -
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Nomenclature
A projected area of rotor, m2
AR aspect ratio, H/dCp coefficient of performance, P1/(1/2rAU
3)
d blade chord (2r), mm
H blade height, mm
N rotational speed of rotor, RPM
P1 shaft power (2pNTB/60), W
R tip radius of semicircular bladed rotor, mm
R1 top tip radius of twisted bladed rotor, mm
R2 bottom tip radius of twisted bladed rotor, mm
r blade arc radius, radius of brake wheel, mm
S gap width, mm
TB brake torque, Nm
U mean stream velocity in x-direction, m/s
r density of atmospheric air, kg/m3
a twist angle (deg)
y Orientation angle (deg)
Z efficiency, P1/(0.593 1/2rAU3)
Darrieus-type wind turbines. Various types of blades like semicircular, bach type [810],
Lebost type [11,12] have been used in vertical axis wind turbine to extract energy from the
air, however, no attempt so far has been made to reduce the negative torque, and increase
the starting characteristics and efficiency with the changes in the air direction. The use of
deflecting plates [8,13] and shielding to increase the efficiency has not only made the system
structurally complex, but also dependent of air direction. In view of this, a distinct blade
shape with a twist for the Savonius rotor has been designed, developed and tested in the
labora tory [14,15]. Preliminary investigation has shown good starting characteristics of the
twisted blades.
2. Brief overview of past work
Numerous investigations have been carried out in the past to study theperformance
characteristics of two and three bucket Savonius rotor. These investigations included wind
tunnel tests, field experiments and numerical studies. Blade configurations were studied in
wind tunnels to evaluate the effect of aspect ratio, blades overlap and gap, effect ofadding
end extensions, end plates and shielding [8,10,1618]. Vishawakarma [4] attempts todiscover an alternate energy option for water pumping, which can be cost-efficient,
environment friendly and sustainable. Two types of installations viz., low-speed wind
turbines operating piston pumps, and high speed wind turbines driving rotary pumps have
been studied. Kumar and Grover [6 ] have investigated a case study of a Savonius rotor forwind power generation. Mojola has investigated field tests of Savonius rotor where data
were collected for speed, torque, and power of the rotor at a large numbers of wind speeds
at different overlap ratio [12]. Detailed experiments have been conducted by some
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investigators to increase the output of a Savonius rotor by using a flow deflecting plate
[13,20]. The aerodynamic performance was also studied by Fujisaw and Gotoh [19] from
the blade surface pressure distribu tions at various rotor angles and tip-speed ratios.
Fujisaw and Gotoh [21] studied the power mechanism of Savonius rotor by pressure
measurements on the blade surface and by flow visualization experiments. Modi
and Fernando [18] have described a mathematical model based on the discrete vortex
method to predict the performance of a stationary and a rototary Savonius configuration.
Table 1 shows the details of the experiments carried out with varying tunnel dimensio ns,
Reynolds number and tip speed ratio. The data obtained from the recent investigations
[14,15] have been included in the table along with the data available in the published
literature [13].
3. The present study
In the present investigation, the twist angle of the blade was varied from a 01 to 251
and the performance of the rotor was studied in a low speed wind tunnel to find the
optimum twist angle. It is worth mentioning here that the blade with a twist of a 01
corresponds a semicircular blade. All the tests were carried out in a three-bladed system
with blade aspect ratio of 1.83. Performance studies of the rotor system have been made on
the basis of starting characteristics, static torque, rotational speed and coefficient of
performance.
3.1. Blade manufacture
The blades of Savonius rotor fabricated from galvanized iron sheets are attached to a
central shaft held between the two bearings in framework. The schematic diagram of
developed blades is shown in Fig 1. In either case, the blades are having an aspect ratio (H/d) of 1.83, where H and d are the height and the blade chord, respectively. The maingeometric parameters are the blade chord ( 120 mm), blade height ( 220 mm) and the
twist angle (a). The semicircular (a 01) shape of the blade has been made on a rollingmachine. The radius of the rotation R is measured from axis of rotation to the outer edge
of the blades. Twisted blade (a 1012251) under present investigation has a tip radius R1
measured from the tip of the blade to the axis of rotation, whereas root radius R2 ismeasured from the root of the twisted blade (Fig. 2). Each blade has a mass of 126.5 g.
4. Experimental setup
The experiments were carried out in an open circuit wind tunnel (Fig. 3) with the exit
section of 0.375 m 0.375 m in cross section [15,28,29]. The air speed at the tunnel exit
(wind speed) could be varied from 6 to 12 m/s. A single block dynamometer was used to
measure the static torque, while a digital tachometer (with an accuracy of 71RPM)measured the rotational speed (RPM) of the rotor. A thermal velocity probe anemometer
(with an accuracy of70.1 m/s) was used to measure the air velocity. The rotor consisted of blades rolled from sheet metal and attached to a central vertical shaft held between two
bearings in the framework. The rotor axis was kept at a distance of 200 mm from the
tunnel exit (Fig. 3).
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Table 1
Performance of Savonius /S-shaped rotor
Authors Yearof
study
Typeof
rotor
Rotor dia
(m)
Rotor
height
Wind tunnel
dimensions
Free stream
velocity(m/s)
Reynolds
number 105
Tipspeed
ratio
Correctedmax.
Cp (%)
(m) (m m)
Sheldahl et al. [16] (two-
bladed rotor)
1978 Savonius 1.000 1.500 4.9 6.1 closed
sec
14 9.3 0.85 19.5
Sheldahl et al. [16]
(three-bladed rotor)
1978 Savonius 1.000 1.500 4.9 6.1 closed
sec
14 8.67 0.65 15 including
frictional power
Alexander and 1978 Savonius 0.383 0.460 Closed sec 69 1.532.32 0.49 12.5
Holownia [17]
Baird and Pender[23] 1980 Savonius 0.076 0.060 0.305 0.305
closed sec
29.224.6 1.041.25 0.78 18.118.5
Bergless and
Athanassiadis [24]
1982 Savonius 0.700 1.400 3.5 2.5 closed
sec
8 2.83.7 0.70 12.512.8
Sivasegaram and
Sivapalan [25]
1983 0.120 0.150 0.46 0.46 open
jet
18 1.44 0.75 20
Bowden andMc-Aleese 1984 Savonius 0.164 0.162 0.76 m dia open 10 0.871.09 0.680.72 1415
[26] jet
Ogawa and Yoshida
[27] withoutdeflector
1986 S-shaped 0.175 0.300 0.8 0.6 openjet 7 0.81 0.86 17
Ogawa and Yoshida
[27] with deflector
1986 Savonius 0.175 0.300 0.8 0.6 openjet 7 0.81 0.86 21.2
Huda et al. [13] without 1992 S-shaped 0.185 0.320 0.5 m dia open jet 6.512.25 0.081.5 0.680.71 15.217.5
deflector
Huda et al. [13] with 1992 S-shaped 0.185 0.320 0.5 dia open jet 12.25 1.5 0.650.72 1721
deflector
Grinspan [15] (twistof
101)
2002 Twisted
Savonius
0.280 0.22 0.375 0.375
open sec
8.22 1.327 0.669 11.59 excluding
frictional power
Raj Kumar [22] (twist
of 12.51)
2004 Twisted
Savonius
0.250 0.220 0.375 0.375
open sec
8.23 1.327 0.6523 13.99 excluding
frictional power
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Height(H)
=220
mm
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R1
R
R2
120
r
S S
Top view of semicircular
bladed rotorTop view of twisted bladed
rotor
Fig. 1. Schematic diagram of the developed blades.
Y-axis
yaxis
=10.28
x x
=10.28
60 mm
Chord = 120 mm
Section at XX
x x
zaxis
xaxis
60mm
Chord = 120mm
Section atX-X
Z-axis
X-axis
Fig. 2. Schematic diagrams of semicircular and twisted blades.
5. Results and discussion
A series of experiments have been carried out with semicircular and twisted types of
Savonius wind turbine rotor in a three-bladed system. All the tests were conducted at a
room temperatu re of 25 1C. Performance studies of the rotor system in both the cases have
been made on the basis of starting characteristics, No load speeds, static torque, torque
coefficient, coefficient of performance and efficiency. The difference of experimental
condition such as the tunnel blockage effect, the Reynolds number, the rotor conditionsand experimental uncertainty makes difficult to compare quantitat ively all the researchers
works. Frictional losses should be taken into account as they may affect performance of
small models substantially. Hence, series of experiments have been conducted in the set upto compare the results of semicircular and twisted blades.
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508mm
RPM
8H
H
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 17761788 1781
450 mm 769 mm 750 mm
Coarse screenHoney comb
2.42H
Fan section
Diffuser Fine screenSetting chamber
Contraction cone (8:1) 240 mm
Bearing
Housing
Twisted bladed
Savonius Rotor
Fan
A.C. Motor20-deg
3500 mm920 mm
Fig. 3. Schematic diagram of the wind tunnel with Savonius rotor.
500
450
400
350
300
250
200
150
100
50
0 deg
10 deg
12.5 deg
15 deg20 deg
25 deg
00 5 10 15 20 25
Time - Sec
Fig. 4. Starting characteristics at wind speed, U 10 m=s.
5.1. Starting characteri stics
The starting characteristics of the twisted bladed rotor at various twist angles (a) at awind speed of U 10 m=s is shown in Fig. 4. The rotor with semicircular blade (a 01)
attains RPM of N 232 in 5 s, while all other twisted bladed rotor goes beyond 350 RPM,thereby indicating a better starting characteristics of twisted bladed rotor. The rotor with
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RPM
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a 12:51 shows a maximum value of N 365 in 5 s. It can also be seen from the plot that
after 20 s, the difference in RPM between the twisted bladed and semicircular bladed rotors
is more than 20. Thus, at a wind speed of U 10 m=s, twist angle of 12.51 is preferable. It
stands to reason that for the semicircular blade, the maximum force acts centrally
(curvature center) and vertically. Whereas for the twisted blade, the maximum force moves
towards to the tip of the blade because of the twist in the blade. Due to these changes, a
twisted blade gets a longer moment arm, and hence a higher value of net positive torque.
Moreover, with the increase of twist angles, the energy capture in the lower part of the
blade reduces drastically as compared to the upper part, and hence the net positive torque
reduces.
When tested at a wind speed of U 8 m=s, blades with a 12:51 and 151 show similar
starting characteristics over the entire range of time (Fig. 5), and thus found to be superiorthan the semicircular bladed rotor. The starting characteristics at a wind speed of U
7 m=s shows an optimal twist angle of151 as seen from Fig. 6. The effect of twist angleat
various airspeeds can be studied from Fig. 7. It has been observed that higher twist angle
captures more energy at lower airspeeds and vice versa. Furthermore, the starting
characteristics are better at higher airspeeds than at lower airspeeds for all the twist angles.Three-bladed semicircular Savonius rotor is well known for its self-starting character-
istics and it has been improved by providing a twist to these blades. Semicircular blades are
taken as zero angle of twist, and by increasing the angle, the performance of the Savonius
rotor is increased in its starting characteristics and static toque.
5.2. No-load speeds
Variation of no-load RPM with the wind speed is shown in Fig. 8. There is a sharp rise
in speed at U 6:528 m=s. Blade with a 151 shows maximum rise in RPM than a
450
400
350
300
250
200
150
100
50
0 deg
10 deg
12.5 deg
15 deg
20 deg
25 deg
00 5 10 15 20 25
Time - Sec
Fig. 5. Starting characteristics at wind speed, U 8 m=s.
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RPM
RPM
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300
250
200
150
100
50
0 deg
10 deg
12.5 deg
15 deg
20 deg
25 deg
0
0 5 10 15 20 25
Time - Sec
Fig. 6. Starting characteristics at wind speed, U 7 m=s.
500
450
400
350
300
250
200
150
100
50
10 m/s
8 m/s
7 m/s
0
0 5 10 15 20 25Time - sec
Fig. 7. Starting characteristics at wind speed, U 7; 8; 10 m=s.
12:51 in the range of U 6:528 m=s. However, a 12:51 gives a better performance thana 151 in the range of U 8210 m=s. It is evident that larger twist angle is preferable in
the lower range of wind speed for producing maximum power and better starting
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0 deg 10 deg 12.5deg
15 deg 20 deg 25 deg
RPM
Torque
Nm
040
80
120
160
200
240
280
320
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600
500
400
300
200
100
0
6 6.5 7 7.5 8 8.5 9 9.5 10 10.5
Wind Speed, m/s
Fig. 8. Variation of RPM with velocity for twisted bladed rotor at various twistangles.
0340
320
300
280
260
240
2040
60
80
100
120
0.07
0.06
0.05
0.04
0.03
0.02
12.5 deg 0 deg
220200
180
140160
0.01
0
12.5 deg 0deg
Angle deg
Fig. 9. Static torque vs. orientation angle diagram at U 10:17 m=s.
characteristics. Thus, from starting acceleration and maximum no load speed character-
istics, a 151 becomes the optimal angle at low velocity of 6.5 m/s. Further, with the
increase of twist angles (from a 151 to 251), the energy capture in the lower part of the
blade reduces drastically.
5.3. Static torque diagram comparisons
The static torque of the rotors has been measured at 201 intervals for one complete
revolution as shown in Fig. 9. The area under T y diagram for twisted blade shows a
larger area as compared to the semicircular bladed rotor. The static torque coefficient of
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Co-effofTo
rque
Co-effofTorque
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 17761788 1785
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0 20 40 60 80 100 120 140
Angle,deg
0 deg Twist 10 deg Twist 12.5 deg Twist 15 deg Twist
Fig. 10. Static torque coefficient for various twisted bladed Savonius rotor at U 10 m=s.
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
75 80 85 90 95 100 105 110 115 120
Angle, deg
0 deg 10 deg 12.5 deg 15 deg
Fig. 11. Shipment of stall angle for various twisted bladed rotor at wind speed U 10 m=s.
semicircular and twisted blades (a 02151) in a three-bladed rotor system is shown for
1201orientat ion (Fig. 10). The stalling angle of twisted blade is found to be shifted by 251
with the increase in angle of twist from a 0 to 12.51 (Fig. 9). It can also been seen from
Fig. 11 that with the increase of twist angles, the stalling angle shifts further. Moreover, the
twisted blade shows a maximum peak torque and a lesser falling slope, and hence a greater
area than the semicircular blades (Fig. 9). It is clear that the Savonius rotor is not self-
starting at three specific positions. Due to friction, these models are not developing
sufficient powers to start rotation. However, by measuring frictional tare torque with an
air motor, it is possible that at every angle oforientation the rotor will develop some static
torque as observed by Sheldahl et al. [16]. This stalling problem can be avoided by making
two stages of rotor one above the other with a stagger of 601. Due to this, the starting
capability would be higher, thus giving a higher torque and efficiency as compared to thesemicircular bladed rotor. There is a wide variation of static torque coefficient with angular
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Cp
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0.16
0.12
0.08
0 deg
10 deg
12.5 deg15 deg
0.04
0
0 2 4 6 8 10 12
Wind Speed,m/s
Fig. 12. Variation of coefficient ofperformance with velocity for various twisted bladed rotors.
position of rotor. Thus, to initiate rotation, the aerodynamic torque must exceed combined
load and friction torques for a rotor from any angular position. This implies that the
minimum value of static torque coefficient may be the deciding factor controlling the size
and stacks of the Savonius rotor [30].
5.4. Coefficient of performance comparison
Fig. 12 compares the performance of the Savonius rotor with different twist angles at
various airspeeds. From the performance viewpoint, a 151 is superior at lower windvelocities, whereas a 12:51 is suitable at higher velocities. Maximum coefficient of
performance, Cp 13:99 is found at tip speed ratio of l 0:65 (U 8:23 m=s) and
forsemicircular bladed rotor is giving Cp 11:04 at the same velocity.
6. Conclusions
In summary, wind tunnel studies show the potential of the Savonius rotor with twisted
blades in terms of smooth running, higher efficiency and self-starting capability as
compared to that of the semicircular bladed rotor. The principal observations of the
present findings can be briefly stated as under:
For the twisted blade, the maximum force moves towards to the tip of the blade
because of the twist in the blade. Due to these changes, a twisted blade gets
a longer moment arm, and hence a higher value of net positive torque. Moreover,
with the increase of twist angles, the energy capture in the lower part of the
blade reduces drastically as compared to the upper part, and hence the netpositive
torque reduces.
Three-bladed semicircular Savonius rotor is well known for its self-star ting
characteristics and it has been improved by providing a twist to these blades.Semicircular blades are taken as zero angle of twist, and by increasing the angle, the
performance of the Savonius rotor is increased in itsperformance.
Larger twist angle is preferable in the lower wind velocity for producing maximum
power and better starting characteristics. The twist angle a 151 gives optimum
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U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 17761788 1787
performance at low airspeeds of U 6:5 m=s in terms of starting acceleration and
maximum no load speed.
The stalling angle of twisted blade is found to be shifted by 251 with the increase in angle
oftwist from a 01 to 12.51, and it has been found that the stalling angle shifts further
with the increase of twist angle.
This stalling problem can be avoided by making two stages (stacking) of rotor one
above the other with a stagger of 601. Due to this, the starting capability would be
higher, and hence a higher torque and efficiency as compared to the semicircularbladed
rotor.
Twisted blade with a 151 shows a maximum of Cp 13:99 and Z 23:6 at tip speed
ratio ofl 0:65 (i.e., at U 8:23 m=s), whereas the semicircular blade (a 01) shows aCp 11:04 and Z 18:67 at the airspeed. This significant raise ofCp and efficiency are
inevitable to further proceeding in this area.
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