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Renewable Energy 31 (2006) 1776–1788
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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 October 2005
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 has been tested in a low speed
wind tunnel, and its performance has been compared with conventional semicircular blades (with
twist angle of 01). 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.
r 2005 Elsevier Ltd. All rights reserved.
Keywords: Savonius rotor; Twisted blade; Starting characteristics; Static torque; Coefficient of performance
1. Introduction
Savonius rotor is a unique fluid-mechanical device that has been studied by numerousinvestigators since 1920s. Applications for the Savonius rotor have included pumpingwater, driving an electrical generator, providing ventilation, and agitating water to keepstock ponds ice-free during the winter [1–4]. Savonius rotor has a high starting torque anda reasonable peak power output per given rotor size, weight and cost, thereby making itless efficient [5]; the coefficient of performance is of the order of 15% [6,7]. From thepoint of aerodynamic efficiency, it cannot compete with high-speed propeller and the
see front matter r 2005 Elsevier Ltd. All rights reserved.
.renene.2005.08.030
nding author. Tel.: +91 361 2691085; fax: +91 361 2690762.
dress: [email protected] (U.K. Saha).
ARTICLE IN PRESS
Nomenclature
A projected area of rotor, m2
AR aspect ratio, H/dCp coefficient of performance, P1/(1/2rAU3)d blade chord (2r), mmH blade height, mmN rotational speed of rotor, RPMP1 shaft power (2pNTB/60), WR tip radius of semicircular bladed rotor, mmR1 top tip radius of twisted bladed rotor, mmR2 bottom tip radius of twisted bladed rotor, mmr blade arc radius, radius of brake wheel, mmS gap width, mmTB brake torque, NmU mean stream velocity in x-direction, m/sr density of atmospheric air, kg/m3
a twist angle (deg)y Orientation angle (deg)Z efficiency, P1/(0.593� 1/2rAU3)
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–1788 1777
Darrieus-type wind turbines. Various types of blades like semicircular, bach type [8–10],Lebost type [11,12] have been used in vertical axis wind turbine to extract energy from theair, however, no attempt so far has been made to reduce the negative torque, and increasethe starting characteristics and efficiency with the changes in the air direction. The use ofdeflecting plates [8,13] and shielding to increase the efficiency has not only made the systemstructurally complex, but also dependent of air direction. In view of this, a distinct bladeshape with a twist for the Savonius rotor has been designed, developed and tested in thelaboratory [14,15]. Preliminary investigation has shown good starting characteristics of thetwisted blades.
2. Brief overview of past work
Numerous investigations have been carried out in the past to study the performancecharacteristics of two and three bucket Savonius rotor. These investigations included windtunnel tests, field experiments and numerical studies. Blade configurations were studied inwind tunnels to evaluate the effect of aspect ratio, blades overlap and gap, effect of addingend extensions, end plates and shielding [8,10,16–18]. 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 windturbines operating piston pumps, and high speed wind turbines driving rotary pumps havebeen 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 datawere collected for speed, torque, and power of the rotor at a large numbers of wind speedsat different overlap ratio [12]. Detailed experiments have been conducted by some
ARTICLE IN PRESSU.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–17881778
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] fromthe blade surface pressure distributions at various rotor angles and tip-speed ratios.Fujisaw and Gotoh [21] studied the power mechanism of Savonius rotor by pressuremeasurements on the blade surface and by flow visualization experiments. Modiand Fernando [18] have described a mathematical model based on the discrete vortexmethod 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 dimensions,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 publishedliterature [13].
3. The present study
In the present investigation, the twist angle of the blade was varied from a ¼ 01 to 251and the performance of the rotor was studied in a low speed wind tunnel to find theoptimum twist angle. It is worth mentioning here that the blade with a twist of a ¼ 01corresponds a semicircular blade. All the tests were carried out in a three-bladed systemwith blade aspect ratio of 1.83. Performance studies of the rotor system have been made onthe basis of starting characteristics, static torque, rotational speed and coefficient ofperformance.
3.1. Blade manufacture
The blades of Savonius rotor fabricated from galvanized iron sheets are attached to acentral shaft held between the two bearings in framework. The schematic diagram ofdeveloped 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 (¼ 120mm), blade height (¼ 220mm) and thetwist angle (a). The semicircular (a ¼ 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 edgeof the blades. Twisted blade (a ¼ 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 exitsection of 0.375m� 0.375m in cross section [15,28,29]. The air speed at the tunnel exit(wind speed) could be varied from 6 to 12m/s. A single block dynamometer was used tomeasure 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 of70.1m/s) was used to measure the air velocity. The rotor consisted ofblades rolled from sheet metal and attached to a central vertical shaft held between twobearings in the framework. The rotor axis was kept at a distance of 200mm from thetunnel exit (Fig. 3).
ARTICLE IN PRESS
Table
1
Perform
ance
ofSavonius/S-shaped
rotor
Authors
Yearof
study
Typeof
rotor
Rotordia
(m)
Rotor
height
(m)
Windtunnel
dim
ensions
(m�m)
Freestream
velocity
(m/s)
Reynolds
number�105
Tip
speed
ratio
Correctedmax.
Cp(%
)
Sheldahlet
al.[16](two-
bladed
rotor)
1978
Savonius
1.000
1.500
4.9�6.1
closed
sec
14
9.3
0.85
19.5
Sheldahlet
al.[16]
(three-bladed
rotor)
1978
Savonius
1.000
1.500
4.9�6.1
closed
sec
14
8.67
0.65
15including
frictionalpower
Alexander
and
Holownia
[17]
1978
Savonius
0.383
0.460
Closedsec
6–9
1.53–2.32
0.49
12.5
BairdandPender
[23]
1980
Savonius
0.076
0.060
0.305�0.305
closedsec
29.2–24.6
1.04–1.25
0.78
18.1–18.5
Bergless
and
Athanassiadis[24]
1982
Savonius
0.700
1.400
3.5�2.5
closed
sec
82.8–3.7
0.70
12.5–12.8
Sivasegaram
and
Sivapalan[25]
1983
—0.120
0.150
0.46�0.46open
jet
18
1.44
0.75
20
Bowden
andMc-Aleese
[26]
1984
Savonius
0.164
0.162
0.76m
dia
open
jet
10
0.87–1.09
0.68–0.72
14–15
OgawaandYoshida
[27]withoutdeflector
1986
S-shaped
0.175
0.300
0.8�0.6
open
jet
70.81
0.86
17
OgawaandYoshida
[27]withdeflector
1986
Savonius
0.175
0.300
0.8�0.6
open
jet
70.81
0.86
21.2
Hudaet
al.[13]without
deflector
1992
S-shaped
0.185
0.320
0.5m
dia
open
jet
6.5–12.25
0.08–1.5
0.68–0.71
15.2–17.5
Hudaet
al.[13]with
deflector
1992
S-shaped
0.185
0.320
0.5
dia
open
jet
12.25
1.5
0.65–0.72
17–21
Grinspan[15](twistof
101)
2002
Twisted
Savonius
0.280
0.22
0.375�0.375
open
sec
8.22
1.327
0.669
11.59excluding
frictionalpower
RajKumar[22](twist
of12.51)
2004
Twisted
Savonius
0.250
0.220
0.375�0.375
open
sec
8.23
1.327
0.6523
13.99excluding
frictionalpower
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–1788 1779
ARTICLE IN PRESS
Top view of semicircularbladed rotor
Top view of twisted bladed rotor
R
S
r
120°
R1
R2
S
Fig. 1. Schematic diagram of the developed blades.
60 mm
60 mmChord = 120 mm
Chord = 120 mm
Section at X−X
Section at X-X
y − axis
Y - axis
Z - axis
X - axisx − axis
z − axis
x x
x x
Hei
ght (
H)
= 2
20 m
m
Hei
ght (
H)
= 22
0 m
m
α =10.28˚
α =10.28˚
Fig. 2. Schematic diagrams of semicircular and twisted blades.
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–17881780
5. Results and discussion
A series of experiments have been carried out with semicircular and twisted types ofSavonius wind turbine rotor in a three-bladed system. All the tests were conducted at aroom temperature of 25 1C. Performance studies of the rotor system in both the cases havebeen made on the basis of starting characteristics, No load speeds, static torque, torquecoefficient, coefficient of performance and efficiency. The difference of experimentalcondition such as the tunnel blockage effect, the Reynolds number, the rotor conditionsand experimental uncertainty makes difficult to compare quantitatively all the researcher’sworks. Frictional losses should be taken into account as they may affect performance ofsmall models substantially. Hence, series of experiments have been conducted in the set upto compare the results of semicircular and twisted blades.
ARTICLE IN PRESS
0
50
100
150
200
250
300
350
400
450
500
0 5 10 15 20 25
RP
M
0 deg 10 deg12.5 deg15 deg20 deg25 deg
Time - Sec
Fig. 4. Starting characteristics at wind speed, U ¼ 10m=s.
450 mm50
8 m
m 8H
H
769 mm 750 mm
240 mm
920 mm3500 mm
990
mm
2.42H
BearingHousing
Twisted bladedSavonius Rotor
FanA.C. Motor 20-deg
Fan section
Diffuser
Coarse screenHoney comb
Fine screenSetting chamber
Contraction cone (8:1)
Fig. 3. Schematic diagram of the wind tunnel with Savonius rotor.
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–1788 1781
5.1. Starting characteristics
The starting characteristics of the twisted bladed rotor at various twist angles (a) at awind speed of U ¼ 10m=s is shown in Fig. 4. The rotor with semicircular blade (a ¼ 01)attains RPM of N ¼ 232 in 5 s, while all other twisted bladed rotor goes beyond 350RPM,thereby indicating a better starting characteristics of twisted bladed rotor. The rotor with
ARTICLE IN PRESSU.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–17881782
a ¼ 12:51 shows a maximum value of N ¼ 365 in 5 s. It can also be seen from the plot thatafter 20 s, the difference in RPM between the twisted bladed and semicircular bladed rotorsis more than 20. Thus, at a wind speed of U ¼ 10m=s, twist angle of 12.51 is preferable. Itstands to reason that for the semicircular blade, the maximum force acts centrally(curvature center) and vertically. Whereas for the twisted blade, the maximum force movestowards to the tip of the blade because of the twist in the blade. Due to these changes, atwisted 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 theblade reduces drastically as compared to the upper part, and hence the net positive torquereduces.When tested at a wind speed of U ¼ 8m=s, blades with a ¼ 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 ¼
7m=s shows an optimal twist angle of 151 as seen from Fig. 6. The effect of twist angle atvarious airspeeds can be studied from Fig. 7. It has been observed that higher twist anglecaptures more energy at lower airspeeds and vice versa. Furthermore, the startingcharacteristics 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 aretaken as zero angle of twist, and by increasing the angle, the performance of the Savoniusrotor 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 risein speed at U ¼ 6:528m=s. Blade with a ¼ 151 shows maximum rise in RPM than a ¼
0
50
100
150
200
250
300
350
400
450
0 5 10 15 20 25
RP
M
0 deg10 deg12.5 deg15 deg20 deg25 deg
Time - Sec
Fig. 5. Starting characteristics at wind speed, U ¼ 8m=s.
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500
450
400
350
300
250
200
150
100
50
00 5 10 15 20 25
Time - sec
RPM
10 m/s
8 m/s
7 m/s
Fig. 7. Starting characteristics at wind speed, U ¼ 7; 8; 10m=s.
0
50
100
150
200
250
300
0 5 10 15 20 25
RP
M
0 deg10 deg12.5 deg15 deg20 deg25 deg
Time - Sec
Fig. 6. Starting characteristics at wind speed, U ¼ 7m=s.
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–1788 1783
12:51 in the range of U ¼ 6:528m=s. However, a ¼ 12:51 gives a better performance thana ¼ 151 in the range of U ¼ 8210m=s. It is evident that larger twist angle is preferable inthe lower range of wind speed for producing maximum power and better starting
ARTICLE IN PRESS
020
4060
80
100
120140
160180
200220
240
260
280
300320
340
12.5 deg 0 deg
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0 40 80 120
160
200
240
280
320
To
rqu
e N
m
12.5 deg 0 deg
Angle deg
Fig. 9. Static torque vs. orientation angle diagram at U ¼ 10:17m=s.
0
100
200
300
400
500
600
6 6.5 7 7.5 8 8.5 9 9.5 10 10.5
RP
M
0 deg 10 deg 12.5 deg
15 deg 20 deg 25 deg
Wind Speed, m/s
Fig. 8. Variation of RPM with velocity for twisted bladed rotor at various twist angles.
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–17881784
characteristics. Thus, from starting acceleration and maximum no load speed character-istics, a ¼ 151 becomes the optimal angle at low velocity of 6.5m/s. Further, with theincrease of twist angles (from a ¼ 151 to 251), the energy capture in the lower part of theblade reduces drastically.
5.3. Static torque diagram comparisons
The static torque of the rotors has been measured at 201 intervals for one completerevolution as shown in Fig. 9. The area under T– y diagram for twisted blade shows alarger area as compared to the semicircular bladed rotor. The static torque coefficient of
ARTICLE IN PRESS
0
0.05
0.1
0.15
0.20.25
0.3
0.35
0.4
0.45
0 20 40 60 80 100 120 140
Co
-eff
of
To
rqu
e
0 deg Twist 10 deg Twist 12.5 deg Twist 15 deg Twist
Angle, deg
Fig. 10. Static torque coefficient for various twisted bladed Savonius rotor at U ¼ 10m=s.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
75 80 85 90 95 100 105 110 115 120
Angle, deg
Co
-eff
of
To
rqu
e
0 deg 10 deg 12.5 deg 15 deg
Fig. 11. Shipment of stall angle for various twisted bladed rotor at wind speed U ¼ 10m=s.
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–1788 1785
semicircular and twisted blades (a ¼ 02151) in a three-bladed rotor system is shown for1201 orientation (Fig. 10). The stalling angle of twisted blade is found to be shifted by 251with the increase in angle of twist from a ¼ 0 to 12.51 (Fig. 9). It can also been seen fromFig. 11 that with the increase of twist angles, the stalling angle shifts further. Moreover, thetwisted blade shows a maximum peak torque and a lesser falling slope, and hence a greaterarea 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 developingsufficient powers to start rotation. However, by measuring frictional tare torque with anair motor, it is possible that at every angle of orientation the rotor will develop some statictorque as observed by Sheldahl et al. [16]. This stalling problem can be avoided by makingtwo stages of rotor one above the other with a stagger of 601. Due to this, the startingcapability 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
ARTICLE IN PRESS
0
0.04
0.08
0.12
0.16
0 2 4 6 8 10 12
Wind Speed, m/s
Cp
0 deg
10 deg
12.5 deg
15 deg
Fig. 12. Variation of coefficient of performance with velocity for various twisted bladed rotors.
U.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–17881786
position of rotor. Thus, to initiate rotation, the aerodynamic torque must exceed combinedload and friction torques for a rotor from any angular position. This implies that theminimum value of static torque coefficient may be the deciding factor controlling the sizeand 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 atvarious airspeeds. From the performance viewpoint, a ¼ 151 is superior at lower windvelocities, whereas a ¼ 12:51 is suitable at higher velocities. Maximum coefficient ofperformance, Cp ¼ 13:99 is found at tip speed ratio of l ¼ 0:65 (U ¼ 8:23m=s) and forsemicircular bladed rotor is giving Cp ¼ 11:04 at the same velocity.
6. Conclusions
In summary, wind tunnel studies show the potential of the Savonius rotor with twistedblades in terms of smooth running, higher efficiency and self-starting capability ascompared to that of the semicircular bladed rotor. The principal observations of thepresent findings can be briefly stated as under:
�
For the twisted blade, the maximum force moves towards to the tip of the bladebecause of the twist in the blade. Due to these changes, a twisted blade getsa 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 theblade reduces drastically as compared to the upper part, and hence the net positivetorque reduces. � Three-bladed semicircular Savonius rotor is well known for its self-startingcharacteristics 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, theperformance of the Savonius rotor is increased in its performance.
� Larger twist angle is preferable in the lower wind velocity for producing maximumpower and better starting characteristics. The twist angle a ¼ 151 gives optimum
ARTICLE IN PRESSU.K. Saha, M.J. Rajkumar / Renewable Energy 31 (2006) 1776–1788 1787
performance at low airspeeds of U ¼ 6:5m=s in terms of starting acceleration andmaximum no load speed.
� The stalling angle of twisted blade is found to be shifted by 251 with the increase in angleof twist from a ¼ 01 to 12.51, and it has been found that the stalling angle shifts furtherwith the increase of twist angle.
� This stalling problem can be avoided by making two stages (stacking) of rotor oneabove the other with a stagger of 601. Due to this, the starting capability would behigher, and hence a higher torque and efficiency as compared to the semicircular bladedrotor.
� Twisted blade with a ¼ 151 shows a maximum of Cp ¼ 13:99 and Z ¼ 23:6 at tip speedratio of l ¼ 0:65 (i.e., at U ¼ 8:23m=s), whereas the semicircular blade (a ¼ 01) shows aCp ¼ 11:04 and Z ¼ 18:67 at the airspeed. This significant raise of Cp and efficiency areinevitable to further proceeding in this area.
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