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Figur e 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
D
ESIGN
Wind pow er , P
w
is defined as the multiplication of mass flow r ate,
ρAV
and the kinetic ener gy per unit mass, ½ V
2
Musgr ove, 2010. The w ind pow er is denoted by the equation of: P
w
= ½ ρAV
2
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. A = D.H 4
The w ind pow er in Equation 3 r epr esents the ideal pow er of a w ind 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 w ind 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 w ind 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, C
p
for Savonius r otor is 0.30. Hence, the C
p
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
ρAV
2
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 :
V
cut -in
= 0.5 V
avg
6 V
Rat ed
= 1.5 V
avg
7 V
cut -out
= 3.0 V
avg
8 All these par ameter s depend on the value of aver age w ind speed. The aver age w ind speed, V
avg
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 Par ameter
Equation Calculation
Cut-in speed, V
cut-in
V
cut-i n
= 0.5 V
avg
3.5 m s Rated w i nd speed, V
Rated
V
Rat ed
= 1.5 V
avg
10.5 m s Cut-out speed, V
cut-out
V
cut-out
= 3 V
avg
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 w ith r otor height tw ice of r otor diameter and this lead to better
stability w ith pr oper efficiencies. AR = H D
9 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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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 w ill 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 w ind 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 .
Par ameter 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.
Figur e 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 .
Par ameter Value
Sw ept Ar ea, A 23.5 m
2
Rotor Diamet er , D 3500 mm
Rotor Height, H 7000 mm
End Plate Diameter , D
f
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
1.7e
3
– 2e
3
kg m
3
Young’s modulus 27.2 – 39.4 GPa
Poisson’s r atio 0.07 – 0.11
Tensile str ength 217 – 520 Mpa
Compr essive str ength 276 – 460 MPa
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3.0 S
