DESIGN AND ANALYSIS OF 5 KW SAVONIUS ROTOR BLADE.
GLO BAL EN GIN EERS & TECH N O LO GISTS REVIEW
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DESIGN AND ANALYSIS OF 5 KW SAVONIUS ROTOR BLADE
WIDODO1, W.S., CHIN2, A.C., HAERYIP SIHOMBING3, and YUHAZRI 4, M.Y.
1, 2, 3, 4
Faculty of Manufacturing Engineering
Universiti Teknikal Malaysia Melaka
Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, MALAYSIA
1
[email protected]
3
iphaery@ utem.edu.my
4
[email protected]
A BSTRA CT
This paper presents the design and analysis of the Savonius rotor blade to generate 5 kW power
output. The relevant design parameters and theories were studied in this paper and used to
determine related design geometry and requirements of the Savonius rotor blade. The Savonius rotor
was designed with the rotor diameter of 3.5 m and the rotor height of 7 m. The 3D model of Savonius
rotor blade was created by using SolidW orks software. Computational Fluid Dynamics (CFD)
analysis and structural Finite Element Analysis (FEA) are presented in this paper. CFD analysis
was performed to obtain the pressure difference between concave and convex region of the blade
while FEA was done to obtain the structural response of the blade due to the wind load applied in
term of stresses and its displacements.
Keyw ords: Savonius, Rotor Blade, CFD, FEA.
1.0
I NTRODUCTION
The developments of r enew able ener gy especially wind ener gy become w idely since 1973 due to the oil cr isis
issues. This view has been suppor ted by Peter et al., (2008) w ho states that the oil pr ice is for ecasted to be
r aised in the futur e and thus w ind ener gy is an alter native ener gy sour ces. The w ind tur bine is a device that
utilizes w ind ener gy to gener ate mechanical or electr ical pow er . Accor ding to Manw ell et al., (2009), ther e ar e
tw o types of wind tur bine: Hor izontal Axis Wind Tur bine (HAWT) and Ver tical Axis Wind Tur bine (VAWT).
HAWT ar e most commonly know n type of wind tur bine and it oper ates par allel to the dir ection of the w ind
w her eas VAWT r otor is oper ated per pendicular to the direction of wind. The tw o most common design pr inciple
of VAWT is Savonius type and Dar r ieus type.
The paper focuses on the Savonius type r otor blades design and analysis. Savonius type r otor blade is a
simple w ind tur bine that oper ates based on dr ag concept. Accor ding to Akw a et al., (2011) Savonius r otor can be
designed w ith tw o or thr ee blades, in single stage or multi-stages. The w or king pr inciple of Savonius r otor is
r esembled to a cup anemometer . The low efficiency of VAWT limited its use in lar ge pow er pr oduction.
How ever , VAWT has sever al advantages over HAWT that make it w idely use in another sector such as water
pumping system. The most appar ent advantage of VAWT is it can oper ate in all w ind dir ection and thus ar e built
w ithout using any yaw mechanism (Halsey, 2011). Other advantages included low noise and simplicity.
In this pr oject, a VAWT is designed to pr oduce 5 kW pow er output. As the per for mance of VAWT is
r elatively low , it becomes necessar y to explor e the effect of number of blades and sizing of the r otor blades in
the design stage. The Savonius wind tur bine is a dr ag type VAWT w her e utilizes the drag for ce for its oper ation.
D’Alessandr o et al., (2009) highlighted that the aer odynamic theor ies developed for lift type w ind tur bine
(HAWT and Dar r ieus wind tur bine) cannot be applied for Savonius r otor . Accor ding to Islam et al., (2005) the
flow patter n ar ound the Savonius r otor blade is char acter ized by flow phenomena that pr oduce pr essure
differ ences betw een the concave and convex sur faces of the blades w hich w ill induce to aer odynamic for ce and
tor que. Islam et al., (2005) fur ther explained that the featur e of flow phenomena including high tur bulence,
unsteadiness and flow separ ation. Figur e 1 show s the nor mal dr ag for ce, FN acts per pendicular on the blade
sur face w her eas tangential dr ag for ce, FT acts along tangential dir ection on each blade. Both FN and FT equations
have been stated by Islam et al., (2005) as follow :
FN = ∆ PS sin ∅
(1)
FT = ∆ PS cos ∅
(2)
Wher e the △P r epr esents the pr essur e differ ence betw een the concave and convex sur faces of the blade
and the S r epr esent the chor d length.
-
G. L. O. B. A. L E. N. G. I . N. E. E. R. S. . & . . T . E. C. H. N. O. L. O. G. I. S. T . S R. E. V. I. E. W
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Global Engineers & Technologist s Review , Vol.2 No.8
(2012)
Figure 1 : For ces acti ng on a bl ade of tw o blades Savonius r otor , (Islam et al., 2005).
2.0
T HEORY AND DESIGN
Wind pow er , Pw is defined as the multiplication of mass flow r ate, ρAV and the kinetic ener gy per unit mass, ½ V2
(Musgr ove, 2010). The w ind pow er is denoted by the equation of:
Pw = ½ ρAV2
(3)
Hayashi, et al . (2004) found that the sw ept ar ea for Savonius Wind Tur bine is calculated by multiplication
of r otor diameter , D and the r otor height, H. The lar ger the sw ept ar ea, the lar ger the pow er gener ated.
(4)
A = D.H
The w ind pow er in Equation (3) r epr esents the ideal pow er of a wind tur bine, as in case of no
aer odynamic or other losses dur ing the ener gy conver sion pr ocesses. How ever , as stated by Manw ell et al.,
(2009) ther e is not possible for all ener gy being conver ted into useful ener gy. The ideal efficiency of a wind
tur bine is know n as Betz limit. Accor ding to the Betz limit, as suppor ted by Musgr ove (2010) ther e is at most
only 59.3 % of the wind pow er can be conver ted into useful pow er . Some of the ener gy may lose in gear box,
bear ings, gener ator , transmission and other s (Jain, 2011). The maximum pow er coefficient, Cp for Savonius r otor
is 0.30. Hence, the Cp value used in this pr oject is 0.30 and the pow er output, P w ith consider ing the pow er
efficiency is:
P = 0.15 ρAV2
(5)
Wind speed is the major element that affects the pow er output. The thr ee w ind speed par ameter s involve
in this pr oject is cut-in speed, r ated w ind speed and cut-out speed. Jain (2011) stated that the thr ee w ind speed
par ameter s r elated to the pow er per for mance ar e as follow :
Vcut-in = 0.5 Vavg
(6)
VRated = 1.5 Vavg
(7)
Vcut-out = 3.0 Vavg
(8)
All these par ameter s depend on the value of aver age w ind speed. The aver age w ind speed, Vavg w as found
at 7 m/ s. Table 1 summar izes the value of these thr ee w ind speed par ameter s.
Table 1 : Value of cut-i n speed, r ated w ind speed, and cut-out speed.
Wind Speed Parameter
Cut-in speed, Vcut-in
Rated w i nd speed, VRated
Cut-out speed, Vcut-out
Equation
Vcut-in = 0.5 Vavg
VRated = 1.5 Vavg
Vcut-out = 3 Vavg
Calculation
3.5 m/ s
10.5 m/ s
21 m/ s
Aspect r atio is a cr ucial criter ion to evaluate the aer odynamic per for mance of Savonius r otor . Johnson
(1998) suggests the Savonius r otor is designed with r otor height tw ice of r otor diameter and this lead to better
stability w ith pr oper efficiencies.
(9)
AR = H/ D
Tip speed r atio, λ is defined as the r atio of the linear speed of r otor blade ω.R to the undistur bed w ind
speed, V (Solanki, 2009). ω is the angular velocity and R r epr esent the r adius r evolving par t of the tur bine. The
maximum tip speed r atio that Savonius r otor can r each is 1. Manw ell et al ., (2009) w rite that high tip speed r atio
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Global Engineers & Technologist s Review , Vol.2 No.8
(2012)
impr oves the per for mance of w ind tur bine and this could be obtained by incr easing the r otational rate of the
r otor .
λ = ω.R /V
(10)
Accor ding to Manw ell et al ., (2009) solidity is r elated to ti p speed r atio. A high tip speed r atio will r esult in
a low solidity. Musgr ove (2010) defines solidity as the r atio of blade ar ea to the tur bine r otor sw ept ar ea. For
VAWT, the solidity is defined as
σ = nd /R
(11)
Wher e n is the number of blades, d is the chor d length or can be defined as the diameter of each half
cylinder , and R is r adius of w ind tur bine. Many r esear cher s have pr oved that the higher the number of blades,
the higher the per for mance of most wind tur bine. How ever , Saha et al., (2008) and Zhao et al., (2009) found that
the tw o-bladed Savonius r otor has higher per for mance than thr ee-bladed Savonius r otor . Refer r ing to Saha then
the tw o-blade r otor has been chosen for this pr oject. Table 2 show s the design par ameter s used in this r esear ch.
Table 2: Summar y of design par ameter s.
Parameter
Value
Pow er Gener ated
5 kW
Sw ept Ar ea
23.5 m 2
Rated Wi nd Speed
10.5 m/ sec
Aspect Rat io
2
Tip Speed Ratio
1.0
Solidi ty
2.114
Diameter – Height
3.5 m – 7 m
Number of Blade
2
The design of the Savonius r otor blade is show n in Figure 2, w hile Table 3 show n the detail dimension of
the Savonius r otor blade. The mater ial pr oposed for the Savonius r otor blade in this paper is E-glass fiber . Since
the manufacturing pr ocess and mater ial cost ar e not consider ed in this paper , the mater ial is chosen based on
the mater ial pr oper ties. Table 3 summar izes the gener al pr oper ties of the E-glass fiber and also r otor blade for
this r esear ch.
Figure 2 : Design of Savoni us r otor blade.
Table 3: Summar i es of r otor blade design and t he mat er ial pr oper t ies of E-glass fiber .
Parameter
Sw ept Ar ea, A
Rotor Diamet er , D
Value
23.5 m 2
3500 mm
Rotor Height, H
7000 mm
End Plate Diameter , Df
3850 mm
Chor d Length, d
1850 mm
Over lap Distance, e
200 mm
Blade Thickness, t
10 mm
End Plate Thickness, t f
50 mm
Densi ty
Young’s modulus
Poisson’s r atio
1.7e3 –
27.2
2e3 kg/ m 3
–
0.07
39.4 GPa
–
0.11
Tensile str ength
217
–
520 Mpa
Compr essive str ength
276
–
460 MPa
© 2012 GETview Limit ed. All right s reserved
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Global Engineers & Technologist s Review , Vol.2 No.8
3.0
(2012)
SIMULATION AND ANALYSIS
In this paper , tw o kinds of simulation and analysis w er e done i.e. Computational Fluid Dynamics (CFD) Analysis
and Str uctur al Analysis by using SolidWor ks Flow Simulation and SolidWor ks Str uctur al Simulation/ Cosmo.
3.1
Computational Fluid Dynamics (CFD) Analysis
The pur pose of this simulation is to obtain the pr essur e differ ent betw een the concave and convex
sur face. The pr essur e differ ence betw een the concave and convex blade sur face of the Savonius r otor
induced dr ag force that tur ns the blade. The pr essur e difference w as obtained by implementing
Computational Fluid Dynamics (CFD) analysis on SolidWor ks Flow Simulation. The tw o flow types in this
paper w er e ext er nal flow and inter nal flow analysis. Both analyses w er e static analysis.
The engineer ing goals for the inter nal analysis and the exter nal analysis ar e tw o Sur face Goals and
four Global Goals. The tw o Sur face Goals ar e dealing with total pr essur e, for both concave and convex
sur face. The four Global Goals ar e deal w ith total pr essur e, velocity, nor mal for ce and for ce.
3.1.1 External flow analysis
The flow type of Savonius r otor blade is consider ed as exter nal flow since it involves a solid model which
is fully sur r ounded by the flow . The fluid flow is not bounded by outer sur face, but bounded only by the
Computational Domain boundar ies. The Computational Domain is fir stly defined to 7m x 7m x 9 m, w hich
means that the Savonius r otor is enclosed by this r egion and the volume is fixed w ithin this fluid flow field
as show n in Figur e 3. The Computational Domain can be set in differ ent size, how ever , the bigger the
Computational Domain, the longer the meshing and solution time.
Figure 3 : Comput ational domai n for Savoni us r otor in exter nal flow analysis.
After the input data is r eady, the model then is enter i ng the meshing pr ocess. The meshing is
view ed thr ough a cut plot as show n in Figur e 4 (a). The fluid is exper ienced separ ation w hen it passes
thr ough the blade and this r egion is consider ed as high-gr adient flow r egion. The mesh contr ol is set to be
finer in this r egion to obtain better solution accur acy.
(a)
(b)
Figure 4 : Ext er nal flow di str ibution (a) Meshing and (b) pr essur e.
The pr essur e distr ibution ar ound the Savonius r otor is view ed by a contour cut plot fr om the top
view . The contour cut plot display the higher pr essur e r egion and low er pr essur e r egion as r ed and blue
color r espectively. Fr om Figur e 4(b), the pr essur e is high near the concave sur face and is low near the
convex sur face. The maximum and minimum pr essur es ar e 101.469 kPa and 101.206 kPa r espectively.
The flow patter n is view ed by a flow trajector y is show n in Figur e 5.
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Global Engineers & Technologist s Review , Vol.2 No.8
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Figure 5 : Flow patter n of exter nal flow (ISO vi ew and top vi ew ).
3.1.2 I nternal flow analysis
In this analysis, the w ind enter s the w ind tunnel w ith the size of 10m x 25m x 8m, flow thr ough the
Savonius r otor blade, and then exits thr ough the outlet that is set to envir onmental conditions. The
Savonius r otor blade is placed in the middle of w ind tunnel. In inter nal analysis, the Computational
Domain is automatically enveloped the model w all, w hich is the wind tunnel size for this paper . The lids
ar e used to apply boundar y conditions w hich intr oduce to inlet velocity and outlet condition as show n in
Figur e 6. The lid thickness for an inter nal analysis is usually not impor tant for the analysis. How ever , the
lid should not be so thick until the flow patter n is affected dow nstr eam in some w ay. If the lid is created to
be too thin, this w ill make the number of cells to be ver y high. For most cases the lid thickness could be
the same thickness used to cr eate the neighbor ing w all (Dassault System, 2011).
Figure 6 : Setup boundar y condition for i nter nal flow analysis.
Figur e 7 shows the meshing of the model and pr essur e distr ibution along the w ind tunnel,
obser ved fr om the top. The maximum pr essur e is indicated by the r ed color r egion w her eas the minimum
pr essur e is in the blue color r egion. Figur e 7 also show s that the concave sur face exper ience higher
pr essur e than convex sur face. The maximum and minimum pr essur e found in this analysis ar e 101.511
kPa and 101.189 kPa r espectively. The flow trajector y patter n of the inter nal flow analysis is show n in the
Figur e 8.
Figure 7 : Meshi ng and pr essur e distr ibution r esul t of t he inter nal flow .
Figure 8 : Flow patter n of i nt er nal flow (ISO view and top view ) .
© 2012 GETview Limit ed. All right s reserved
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Global Engineers & Technologist s Review , Vol.2 No.8
(2012)
3.1.3 Comparison of external and internal flow analysis
Both exter nal and inter nal flow analysis show s differ ent r esults. Fr om the view of pr essur e distr ibution,
both analyses have show n that the pr essur e is higher on the concave sur face and low er on the convex
sur face. This has ver ified the r esearch statement by Sar golzaei and Kianifar (2007). How ever , the
pr essur e differ ences of both analyses ar e not same. Inter nal flow analysis has higher pr essur e differ ences
than exter nal flow analysis, w hich ar e 322.15 Pa and 262.65 Pa r espectively. This explained by the
velocity vector in Figur e 4 and Figur e 7.
Since the Savonius r otor w as set at static position, w hen the w ind r eached on the blade, it w as not
tur n as in r eal w or ld condition and hence blocked the w ind flow . Fr om Figur e 4, the velocity vector plot
show s that the wind w hich blocked by the Savonius r otor blade ar e tend to flow in the dir ection that bias
fr om the w ind flow in the exter nal flow analysis. The w ind was not completely flow thr ough the r otor
blades. How ever , the w ind w as completely passed thr ough the Savonius r otor blades as show n in Figur e
7. This is because the Savonius r otor is bounded by the w ind tunnel w all. The air w hich blocked by the
blade is r eflected t o the w ind tunnel w all and then passes thr ough the Savonius r otor blades. Hence, in the
static analysis, the r esult of inter nal flow analysis is mor e accur ate and pr ecise. Table 4 show s the
comparison summar y of the exter nal and inter nal flow analysis.
Table 4: Compar ison of ext er nal and int er nal flow analysi s.
Aspect
Pr essur e Differ ences
High Pr essur e Region
Low Pr essur e Region
External Flow Analysis
262.15 Pa
Concave sur face
Convex sur face
I nternal Flow Analysis
322.15 Pa
Concave sur face
Convex sur face
The w ind flow is blocked by t he Savoni us r otor
blades and bias fr om t he w i nd dir ection. Si nce t her e
is no w all boundar y, the w i nd flow out fr om the
Comput ational Domai n. The w ind i s not completely
flow thr ough the r otor blades.
The w ind flow is blocked by the Savonius
r otor blades and bias. How ever , the Savonius
r otor is bounded by the w all of w i nd tunnel.
Hence, t he w i nd is compl etely flow thr ough
the r otor blades.
Vector Plot
Flow Patter n
3.2
Structural Analysis
The str uctur e of the Savonius r otor blade is analyzed using FEA static method by SolidWor ks
Simulation/ Cosmo softwar e. Since the tw o Savonius r otor blades ar e symmetr y, the analysis is per for med
only on one blade. The FEA r esult is inter pr et ed in thr ee cr iter ia: str ess, defor mation, and factor of safety.
Fir st step of FEA analysis is assigned mater ial to the blade model w her e E-glass fiber w as the mater ial
chosen. Then the fixed constr aints/ fixtur es ar e applied on the top, bottom and center of the shaft, as
show n in Figur e 9. The fixtur es constr ained all tr anslational and all r otational degr ees of fr eedom.
Ther efor e, the blade is stay in a static and fixed position. The load for this analysis is force with 595.98 N
obtained fr om the aer odynamic analysis and equally distributed on the concave blade sur face.
Figure 9 : Boundar y condi tion of the r otor blade.
Figur e 10 show s the meshing of the model by using tetr ahedr al shape mesh elements and also
depicts the FEA r esult of the model w hich pr esents the st r ess distr ibution over the r otor blade str uctur e.
© 2012 GETview Limit ed. All right s reserved
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Global Engineers & Technologist s Review , Vol.2 No.8
(2012)
Figure 10: Meshi ng and str ess distr ibution of t he r otor blade.
The maximum and minimum Von Mises str ess for the Savonius r otor blade ar e 6.6052 x 105 Pa and
5.0534 x 10 -2 Pa r espectively. The r esult is acceptable because the maximum Von Mises str ess is much
low er than the yield str ength of the mater ial and pr ovide the minimum Factor of Safety as 787.26. Figur e
11 show s the defor mation of the model under the given load, the maximum displacement is 2.94 mm at
the edge of the blade. The defor mation is acceptable because it is small in r elation to the over all size of the
blade str uctur e.
Figure 11: Defor mation of the r otor blade.
4.0 CONCLUSION
The paper has investigated the elements that contr ibute to the design and analysis Savonius r otor blade. The
blade w as design by using SolidWor ks softwar e. CFD analysis was per for med in or der to obtain the pr essur e
differ ent betw een the concave and convex sur face of the r otor blade. The for ce induced to the blade w as
calculated fr om aer odynamic analysis. The str uctural feasibility was analyzed by Finite Element Analysis
method to obtained the maximum defor mation and str ess exper ienced by the r otor blade.
From the CFD analysis, it is found that the concave blade region experience high pressure while the convex
blade region experience low pressure for two blades Savonius rotor. The maximum pressure and minimum pressure
from internal flow analysis were 101.511 kPa and 101.189 kPa respectively. The high pressure region produces 595.98
N of drag force that spinning the Savonius wind turbine.
The maximum deformation of the Savonius rotor blade was 2.94 mm. This deformation is acceptable because it
is relatively small compared to the whole blade model. The maximum Von Mises stress obtained from the FEA was
6.6052 x 105 Pa with the minimum Factor of Safety was 787.26. This concluded that the Savonius rotor blade was safe
enough to withstand the aerodynamic force on the turbine.
The pr esented study w as limited by the softw ar e and computer capability. In r eal w or ld condition, w hen
the air flows thr ough the blade, it will induce a for ce to tur n the r otor blade. How ever , the SolidWor ks softw ar e
is unable to per for m the CFD analysis w hile the blades ar e tur ning (dynamic condition). Ther efor e, only the
static CFD analysis was per for med in this paper , for both exter nal and inter nal flow analysis.
ACKNOWLEDMENT
Author s w ould like to thanks to the Univer siti Teknikal Malaysia Melaka for its financial suppor t of the Ver tical
Axis Wind Tur bine Pr oject under shor t ter m gr ant PJP/ 2011/ FKP (7A)/ S00874.
© 2012 GETview Limit ed. All right s reserved
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Global Engineers & Technologist s Review , Vol.2 No.8
(2012)
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Confer ence on Mechanical Engineer ing 2005, 28-30 December 2005, Dhaka, Bangladesh.
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November 2011] .
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© 2012 GETview Limit ed. All right s reserved
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DESIGN AND ANALYSIS OF 5 KW SAVONIUS ROTOR BLADE
WIDODO1, W.S., CHIN2, A.C., HAERYIP SIHOMBING3, and YUHAZRI 4, M.Y.
1, 2, 3, 4
Faculty of Manufacturing Engineering
Universiti Teknikal Malaysia Melaka
Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, MALAYSIA
1
[email protected]
3
iphaery@ utem.edu.my
4
[email protected]
A BSTRA CT
This paper presents the design and analysis of the Savonius rotor blade to generate 5 kW power
output. The relevant design parameters and theories were studied in this paper and used to
determine related design geometry and requirements of the Savonius rotor blade. The Savonius rotor
was designed with the rotor diameter of 3.5 m and the rotor height of 7 m. The 3D model of Savonius
rotor blade was created by using SolidW orks software. Computational Fluid Dynamics (CFD)
analysis and structural Finite Element Analysis (FEA) are presented in this paper. CFD analysis
was performed to obtain the pressure difference between concave and convex region of the blade
while FEA was done to obtain the structural response of the blade due to the wind load applied in
term of stresses and its displacements.
Keyw ords: Savonius, Rotor Blade, CFD, FEA.
1.0
I NTRODUCTION
The developments of r enew able ener gy especially wind ener gy become w idely since 1973 due to the oil cr isis
issues. This view has been suppor ted by Peter et al., (2008) w ho states that the oil pr ice is for ecasted to be
r aised in the futur e and thus w ind ener gy is an alter native ener gy sour ces. The w ind tur bine is a device that
utilizes w ind ener gy to gener ate mechanical or electr ical pow er . Accor ding to Manw ell et al., (2009), ther e ar e
tw o types of wind tur bine: Hor izontal Axis Wind Tur bine (HAWT) and Ver tical Axis Wind Tur bine (VAWT).
HAWT ar e most commonly know n type of wind tur bine and it oper ates par allel to the dir ection of the w ind
w her eas VAWT r otor is oper ated per pendicular to the direction of wind. The tw o most common design pr inciple
of VAWT is Savonius type and Dar r ieus type.
The paper focuses on the Savonius type r otor blades design and analysis. Savonius type r otor blade is a
simple w ind tur bine that oper ates based on dr ag concept. Accor ding to Akw a et al., (2011) Savonius r otor can be
designed w ith tw o or thr ee blades, in single stage or multi-stages. The w or king pr inciple of Savonius r otor is
r esembled to a cup anemometer . The low efficiency of VAWT limited its use in lar ge pow er pr oduction.
How ever , VAWT has sever al advantages over HAWT that make it w idely use in another sector such as water
pumping system. The most appar ent advantage of VAWT is it can oper ate in all w ind dir ection and thus ar e built
w ithout using any yaw mechanism (Halsey, 2011). Other advantages included low noise and simplicity.
In this pr oject, a VAWT is designed to pr oduce 5 kW pow er output. As the per for mance of VAWT is
r elatively low , it becomes necessar y to explor e the effect of number of blades and sizing of the r otor blades in
the design stage. The Savonius wind tur bine is a dr ag type VAWT w her e utilizes the drag for ce for its oper ation.
D’Alessandr o et al., (2009) highlighted that the aer odynamic theor ies developed for lift type w ind tur bine
(HAWT and Dar r ieus wind tur bine) cannot be applied for Savonius r otor . Accor ding to Islam et al., (2005) the
flow patter n ar ound the Savonius r otor blade is char acter ized by flow phenomena that pr oduce pr essure
differ ences betw een the concave and convex sur faces of the blades w hich w ill induce to aer odynamic for ce and
tor que. Islam et al., (2005) fur ther explained that the featur e of flow phenomena including high tur bulence,
unsteadiness and flow separ ation. Figur e 1 show s the nor mal dr ag for ce, FN acts per pendicular on the blade
sur face w her eas tangential dr ag for ce, FT acts along tangential dir ection on each blade. Both FN and FT equations
have been stated by Islam et al., (2005) as follow :
FN = ∆ PS sin ∅
(1)
FT = ∆ PS cos ∅
(2)
Wher e the △P r epr esents the pr essur e differ ence betw een the concave and convex sur faces of the blade
and the S r epr esent the chor d length.
-
G. L. O. B. A. L E. N. G. I . N. E. E. R. S. . & . . T . E. C. H. N. O. L. O. G. I. S. T . S R. E. V. I. E. W
1
Global Engineers & Technologist s Review , Vol.2 No.8
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Figure 1 : For ces acti ng on a bl ade of tw o blades Savonius r otor , (Islam et al., 2005).
2.0
T HEORY AND DESIGN
Wind pow er , Pw is defined as the multiplication of mass flow r ate, ρAV and the kinetic ener gy per unit mass, ½ V2
(Musgr ove, 2010). The w ind pow er is denoted by the equation of:
Pw = ½ ρAV2
(3)
Hayashi, et al . (2004) found that the sw ept ar ea for Savonius Wind Tur bine is calculated by multiplication
of r otor diameter , D and the r otor height, H. The lar ger the sw ept ar ea, the lar ger the pow er gener ated.
(4)
A = D.H
The w ind pow er in Equation (3) r epr esents the ideal pow er of a wind tur bine, as in case of no
aer odynamic or other losses dur ing the ener gy conver sion pr ocesses. How ever , as stated by Manw ell et al.,
(2009) ther e is not possible for all ener gy being conver ted into useful ener gy. The ideal efficiency of a wind
tur bine is know n as Betz limit. Accor ding to the Betz limit, as suppor ted by Musgr ove (2010) ther e is at most
only 59.3 % of the wind pow er can be conver ted into useful pow er . Some of the ener gy may lose in gear box,
bear ings, gener ator , transmission and other s (Jain, 2011). The maximum pow er coefficient, Cp for Savonius r otor
is 0.30. Hence, the Cp value used in this pr oject is 0.30 and the pow er output, P w ith consider ing the pow er
efficiency is:
P = 0.15 ρAV2
(5)
Wind speed is the major element that affects the pow er output. The thr ee w ind speed par ameter s involve
in this pr oject is cut-in speed, r ated w ind speed and cut-out speed. Jain (2011) stated that the thr ee w ind speed
par ameter s r elated to the pow er per for mance ar e as follow :
Vcut-in = 0.5 Vavg
(6)
VRated = 1.5 Vavg
(7)
Vcut-out = 3.0 Vavg
(8)
All these par ameter s depend on the value of aver age w ind speed. The aver age w ind speed, Vavg w as found
at 7 m/ s. Table 1 summar izes the value of these thr ee w ind speed par ameter s.
Table 1 : Value of cut-i n speed, r ated w ind speed, and cut-out speed.
Wind Speed Parameter
Cut-in speed, Vcut-in
Rated w i nd speed, VRated
Cut-out speed, Vcut-out
Equation
Vcut-in = 0.5 Vavg
VRated = 1.5 Vavg
Vcut-out = 3 Vavg
Calculation
3.5 m/ s
10.5 m/ s
21 m/ s
Aspect r atio is a cr ucial criter ion to evaluate the aer odynamic per for mance of Savonius r otor . Johnson
(1998) suggests the Savonius r otor is designed with r otor height tw ice of r otor diameter and this lead to better
stability w ith pr oper efficiencies.
(9)
AR = H/ D
Tip speed r atio, λ is defined as the r atio of the linear speed of r otor blade ω.R to the undistur bed w ind
speed, V (Solanki, 2009). ω is the angular velocity and R r epr esent the r adius r evolving par t of the tur bine. The
maximum tip speed r atio that Savonius r otor can r each is 1. Manw ell et al ., (2009) w rite that high tip speed r atio
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Global Engineers & Technologist s Review , Vol.2 No.8
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impr oves the per for mance of w ind tur bine and this could be obtained by incr easing the r otational rate of the
r otor .
λ = ω.R /V
(10)
Accor ding to Manw ell et al ., (2009) solidity is r elated to ti p speed r atio. A high tip speed r atio will r esult in
a low solidity. Musgr ove (2010) defines solidity as the r atio of blade ar ea to the tur bine r otor sw ept ar ea. For
VAWT, the solidity is defined as
σ = nd /R
(11)
Wher e n is the number of blades, d is the chor d length or can be defined as the diameter of each half
cylinder , and R is r adius of w ind tur bine. Many r esear cher s have pr oved that the higher the number of blades,
the higher the per for mance of most wind tur bine. How ever , Saha et al., (2008) and Zhao et al., (2009) found that
the tw o-bladed Savonius r otor has higher per for mance than thr ee-bladed Savonius r otor . Refer r ing to Saha then
the tw o-blade r otor has been chosen for this pr oject. Table 2 show s the design par ameter s used in this r esear ch.
Table 2: Summar y of design par ameter s.
Parameter
Value
Pow er Gener ated
5 kW
Sw ept Ar ea
23.5 m 2
Rated Wi nd Speed
10.5 m/ sec
Aspect Rat io
2
Tip Speed Ratio
1.0
Solidi ty
2.114
Diameter – Height
3.5 m – 7 m
Number of Blade
2
The design of the Savonius r otor blade is show n in Figure 2, w hile Table 3 show n the detail dimension of
the Savonius r otor blade. The mater ial pr oposed for the Savonius r otor blade in this paper is E-glass fiber . Since
the manufacturing pr ocess and mater ial cost ar e not consider ed in this paper , the mater ial is chosen based on
the mater ial pr oper ties. Table 3 summar izes the gener al pr oper ties of the E-glass fiber and also r otor blade for
this r esear ch.
Figure 2 : Design of Savoni us r otor blade.
Table 3: Summar i es of r otor blade design and t he mat er ial pr oper t ies of E-glass fiber .
Parameter
Sw ept Ar ea, A
Rotor Diamet er , D
Value
23.5 m 2
3500 mm
Rotor Height, H
7000 mm
End Plate Diameter , Df
3850 mm
Chor d Length, d
1850 mm
Over lap Distance, e
200 mm
Blade Thickness, t
10 mm
End Plate Thickness, t f
50 mm
Densi ty
Young’s modulus
Poisson’s r atio
1.7e3 –
27.2
2e3 kg/ m 3
–
0.07
39.4 GPa
–
0.11
Tensile str ength
217
–
520 Mpa
Compr essive str ength
276
–
460 MPa
© 2012 GETview Limit ed. All right s reserved
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Global Engineers & Technologist s Review , Vol.2 No.8
3.0
(2012)
SIMULATION AND ANALYSIS
In this paper , tw o kinds of simulation and analysis w er e done i.e. Computational Fluid Dynamics (CFD) Analysis
and Str uctur al Analysis by using SolidWor ks Flow Simulation and SolidWor ks Str uctur al Simulation/ Cosmo.
3.1
Computational Fluid Dynamics (CFD) Analysis
The pur pose of this simulation is to obtain the pr essur e differ ent betw een the concave and convex
sur face. The pr essur e differ ence betw een the concave and convex blade sur face of the Savonius r otor
induced dr ag force that tur ns the blade. The pr essur e difference w as obtained by implementing
Computational Fluid Dynamics (CFD) analysis on SolidWor ks Flow Simulation. The tw o flow types in this
paper w er e ext er nal flow and inter nal flow analysis. Both analyses w er e static analysis.
The engineer ing goals for the inter nal analysis and the exter nal analysis ar e tw o Sur face Goals and
four Global Goals. The tw o Sur face Goals ar e dealing with total pr essur e, for both concave and convex
sur face. The four Global Goals ar e deal w ith total pr essur e, velocity, nor mal for ce and for ce.
3.1.1 External flow analysis
The flow type of Savonius r otor blade is consider ed as exter nal flow since it involves a solid model which
is fully sur r ounded by the flow . The fluid flow is not bounded by outer sur face, but bounded only by the
Computational Domain boundar ies. The Computational Domain is fir stly defined to 7m x 7m x 9 m, w hich
means that the Savonius r otor is enclosed by this r egion and the volume is fixed w ithin this fluid flow field
as show n in Figur e 3. The Computational Domain can be set in differ ent size, how ever , the bigger the
Computational Domain, the longer the meshing and solution time.
Figure 3 : Comput ational domai n for Savoni us r otor in exter nal flow analysis.
After the input data is r eady, the model then is enter i ng the meshing pr ocess. The meshing is
view ed thr ough a cut plot as show n in Figur e 4 (a). The fluid is exper ienced separ ation w hen it passes
thr ough the blade and this r egion is consider ed as high-gr adient flow r egion. The mesh contr ol is set to be
finer in this r egion to obtain better solution accur acy.
(a)
(b)
Figure 4 : Ext er nal flow di str ibution (a) Meshing and (b) pr essur e.
The pr essur e distr ibution ar ound the Savonius r otor is view ed by a contour cut plot fr om the top
view . The contour cut plot display the higher pr essur e r egion and low er pr essur e r egion as r ed and blue
color r espectively. Fr om Figur e 4(b), the pr essur e is high near the concave sur face and is low near the
convex sur face. The maximum and minimum pr essur es ar e 101.469 kPa and 101.206 kPa r espectively.
The flow patter n is view ed by a flow trajector y is show n in Figur e 5.
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Global Engineers & Technologist s Review , Vol.2 No.8
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Figure 5 : Flow patter n of exter nal flow (ISO vi ew and top vi ew ).
3.1.2 I nternal flow analysis
In this analysis, the w ind enter s the w ind tunnel w ith the size of 10m x 25m x 8m, flow thr ough the
Savonius r otor blade, and then exits thr ough the outlet that is set to envir onmental conditions. The
Savonius r otor blade is placed in the middle of w ind tunnel. In inter nal analysis, the Computational
Domain is automatically enveloped the model w all, w hich is the wind tunnel size for this paper . The lids
ar e used to apply boundar y conditions w hich intr oduce to inlet velocity and outlet condition as show n in
Figur e 6. The lid thickness for an inter nal analysis is usually not impor tant for the analysis. How ever , the
lid should not be so thick until the flow patter n is affected dow nstr eam in some w ay. If the lid is created to
be too thin, this w ill make the number of cells to be ver y high. For most cases the lid thickness could be
the same thickness used to cr eate the neighbor ing w all (Dassault System, 2011).
Figure 6 : Setup boundar y condition for i nter nal flow analysis.
Figur e 7 shows the meshing of the model and pr essur e distr ibution along the w ind tunnel,
obser ved fr om the top. The maximum pr essur e is indicated by the r ed color r egion w her eas the minimum
pr essur e is in the blue color r egion. Figur e 7 also show s that the concave sur face exper ience higher
pr essur e than convex sur face. The maximum and minimum pr essur e found in this analysis ar e 101.511
kPa and 101.189 kPa r espectively. The flow trajector y patter n of the inter nal flow analysis is show n in the
Figur e 8.
Figure 7 : Meshi ng and pr essur e distr ibution r esul t of t he inter nal flow .
Figure 8 : Flow patter n of i nt er nal flow (ISO view and top view ) .
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Global Engineers & Technologist s Review , Vol.2 No.8
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3.1.3 Comparison of external and internal flow analysis
Both exter nal and inter nal flow analysis show s differ ent r esults. Fr om the view of pr essur e distr ibution,
both analyses have show n that the pr essur e is higher on the concave sur face and low er on the convex
sur face. This has ver ified the r esearch statement by Sar golzaei and Kianifar (2007). How ever , the
pr essur e differ ences of both analyses ar e not same. Inter nal flow analysis has higher pr essur e differ ences
than exter nal flow analysis, w hich ar e 322.15 Pa and 262.65 Pa r espectively. This explained by the
velocity vector in Figur e 4 and Figur e 7.
Since the Savonius r otor w as set at static position, w hen the w ind r eached on the blade, it w as not
tur n as in r eal w or ld condition and hence blocked the w ind flow . Fr om Figur e 4, the velocity vector plot
show s that the wind w hich blocked by the Savonius r otor blade ar e tend to flow in the dir ection that bias
fr om the w ind flow in the exter nal flow analysis. The w ind was not completely flow thr ough the r otor
blades. How ever , the w ind w as completely passed thr ough the Savonius r otor blades as show n in Figur e
7. This is because the Savonius r otor is bounded by the w ind tunnel w all. The air w hich blocked by the
blade is r eflected t o the w ind tunnel w all and then passes thr ough the Savonius r otor blades. Hence, in the
static analysis, the r esult of inter nal flow analysis is mor e accur ate and pr ecise. Table 4 show s the
comparison summar y of the exter nal and inter nal flow analysis.
Table 4: Compar ison of ext er nal and int er nal flow analysi s.
Aspect
Pr essur e Differ ences
High Pr essur e Region
Low Pr essur e Region
External Flow Analysis
262.15 Pa
Concave sur face
Convex sur face
I nternal Flow Analysis
322.15 Pa
Concave sur face
Convex sur face
The w ind flow is blocked by t he Savoni us r otor
blades and bias fr om t he w i nd dir ection. Si nce t her e
is no w all boundar y, the w i nd flow out fr om the
Comput ational Domai n. The w ind i s not completely
flow thr ough the r otor blades.
The w ind flow is blocked by the Savonius
r otor blades and bias. How ever , the Savonius
r otor is bounded by the w all of w i nd tunnel.
Hence, t he w i nd is compl etely flow thr ough
the r otor blades.
Vector Plot
Flow Patter n
3.2
Structural Analysis
The str uctur e of the Savonius r otor blade is analyzed using FEA static method by SolidWor ks
Simulation/ Cosmo softwar e. Since the tw o Savonius r otor blades ar e symmetr y, the analysis is per for med
only on one blade. The FEA r esult is inter pr et ed in thr ee cr iter ia: str ess, defor mation, and factor of safety.
Fir st step of FEA analysis is assigned mater ial to the blade model w her e E-glass fiber w as the mater ial
chosen. Then the fixed constr aints/ fixtur es ar e applied on the top, bottom and center of the shaft, as
show n in Figur e 9. The fixtur es constr ained all tr anslational and all r otational degr ees of fr eedom.
Ther efor e, the blade is stay in a static and fixed position. The load for this analysis is force with 595.98 N
obtained fr om the aer odynamic analysis and equally distributed on the concave blade sur face.
Figure 9 : Boundar y condi tion of the r otor blade.
Figur e 10 show s the meshing of the model by using tetr ahedr al shape mesh elements and also
depicts the FEA r esult of the model w hich pr esents the st r ess distr ibution over the r otor blade str uctur e.
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Global Engineers & Technologist s Review , Vol.2 No.8
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Figure 10: Meshi ng and str ess distr ibution of t he r otor blade.
The maximum and minimum Von Mises str ess for the Savonius r otor blade ar e 6.6052 x 105 Pa and
5.0534 x 10 -2 Pa r espectively. The r esult is acceptable because the maximum Von Mises str ess is much
low er than the yield str ength of the mater ial and pr ovide the minimum Factor of Safety as 787.26. Figur e
11 show s the defor mation of the model under the given load, the maximum displacement is 2.94 mm at
the edge of the blade. The defor mation is acceptable because it is small in r elation to the over all size of the
blade str uctur e.
Figure 11: Defor mation of the r otor blade.
4.0 CONCLUSION
The paper has investigated the elements that contr ibute to the design and analysis Savonius r otor blade. The
blade w as design by using SolidWor ks softwar e. CFD analysis was per for med in or der to obtain the pr essur e
differ ent betw een the concave and convex sur face of the r otor blade. The for ce induced to the blade w as
calculated fr om aer odynamic analysis. The str uctural feasibility was analyzed by Finite Element Analysis
method to obtained the maximum defor mation and str ess exper ienced by the r otor blade.
From the CFD analysis, it is found that the concave blade region experience high pressure while the convex
blade region experience low pressure for two blades Savonius rotor. The maximum pressure and minimum pressure
from internal flow analysis were 101.511 kPa and 101.189 kPa respectively. The high pressure region produces 595.98
N of drag force that spinning the Savonius wind turbine.
The maximum deformation of the Savonius rotor blade was 2.94 mm. This deformation is acceptable because it
is relatively small compared to the whole blade model. The maximum Von Mises stress obtained from the FEA was
6.6052 x 105 Pa with the minimum Factor of Safety was 787.26. This concluded that the Savonius rotor blade was safe
enough to withstand the aerodynamic force on the turbine.
The pr esented study w as limited by the softw ar e and computer capability. In r eal w or ld condition, w hen
the air flows thr ough the blade, it will induce a for ce to tur n the r otor blade. How ever , the SolidWor ks softw ar e
is unable to per for m the CFD analysis w hile the blades ar e tur ning (dynamic condition). Ther efor e, only the
static CFD analysis was per for med in this paper , for both exter nal and inter nal flow analysis.
ACKNOWLEDMENT
Author s w ould like to thanks to the Univer siti Teknikal Malaysia Melaka for its financial suppor t of the Ver tical
Axis Wind Tur bine Pr oject under shor t ter m gr ant PJP/ 2011/ FKP (7A)/ S00874.
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Global Engineers & Technologist s Review , Vol.2 No.8
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© 2012 GETview Limit ed. All right s reserved
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