on the performance analysis of savonius rotor with twisted blades

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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

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dress: [email protected] (U.K. Saha).

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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


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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.

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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.


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


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


ARTICLE IN PRESS

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



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

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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.


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

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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.


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

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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.


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-starting
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, theperformance of the Savonius rotor is increased in its performance.
� Larger twist angle is preferable in the lower wind velocity for producing maximum
power 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 angle
of 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 one
above 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 speed
ratio 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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