Simulations of Small Scale Straight Blade Savonius Wind Turbine

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International Journal of Advance Foundation and Research in Science & Engineering (IJAFRSE)
Volume 1, Issue 6, October 2014. Impact Factor: 1.036, Science Central Value: 10.33
Simulations of Small Scale Straight Blade Savonius Wind
Turbine Using Latest CAE Techniques to get Optimum
Power Output.
Hiren Tala (B.E.Mechanical),
Hiren Patel (Lecturer in Mechanical department in Government Polytechnic College Valsad)
Rahulkumar R. Sapra (B.E.MEchanical) Surat,India
Jayesh R. Gharte (B.E.Mechanical) Surat,India
[email protected] , [email protected] , [email protected] ,
[email protected]
ABSTRACT
We are going to design & simulate small scale Savonius windmill by using CAE techniques. After
referring previous literatures, we conclude that there is no any research has been done to
optimize aspect ratio of vertical axis savonius wind turbine. We had optimized aspect ratio by
using CAD & CFD software to get maximum efficiency at particular aspect ratio with the same
frontal area. Our concluding work is out of 1,2,3,4,5 aspect ratios of wind turbine that we had
chosen, Maximum efficiency has been shown by Aspect Ratio 1.
I.
INTRODUCTION
Wind energy is an indirect form of solar energy. It has been estimated that total solar power received by
the earth is approximately 1.8 x 1011 MW per years. Of this solar input, only 2% (3.6 x 109 MW) is
converted into wind energy and about 35% is dissipated within 1000m of the earth’s surface. Since wind
is introduced mainly by the uneven heating of the earth’s crust by the sun. [1] The Savonius rotor is a
vertical axis machine driven predominantly by drag force that has been studied by numerous
investigators since the 1920’s. Savonius rotors operate at low tip speed ratios and their efficiency is low.
Savonius rotors have not been widely developed; they were first applied only in such limited areas as
ventilation. As the utilization of natural energy has become more important in recent years, Savonius
rotors have been finding an application in providing a complementary contribution in both
technologically advanced and developing countries.
II. LITERATURE REVIEW
Some studies have been carried out in wind tunnels, using controlled conditions. Other considers freespace experiments. Generally, the global performance of a rotor, identical to or derived from the
conventional Savonius rotor, is presented in such studies but without realizing any detailed, quantitative
parametric study. Sometimes, some visualizations of the flow in and around the rotor are proposed, In
particular for Savonius turbines, the resulting flow conditions are highly unsteady. Furthermore,
boundary layer separation is an essential aspect for the efficiency of the system. As a consequence,
detailed aerodynamic studies are rare and often do not allow the prediction of the energetic behavior of
the rotor. However, some publications [3,4] are of higher quality and give a precise description of the
aerodynamics of the conventional Savonius rotor, mainly obtained by pressure measurements on the
paddles.
37 | © 2014, IJAFRSE All Rights Reserved
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Further articles describe an extensive experimental study in a wind-tunnel
wind tunnel to evaluate the importance of
geometrical parameters on the together with the flow stagnation region on the front side of the rotor,
contribute to the powerr producing mechanism of the Savonius rotor [8,9]
III. DESIGN PROCEDURE
Wind machine design is a process of trial and retrial. Once wind mill type has selected, overall system
efficiency should estimated and rotor size has calculated. Then design of all the components
com
of the
machine would carry out. In next step proper analysis of whole system is to be done and if it found
satisfactory then it bring to practical realization. The wind machine design process consists of two major
task aerodynamic design and structural
structural design. If the load is a mechanical device i.e. water pump-high
pump
starting torque-from
from the rotor will be needed. If the load is an electrical generator, low starting torque
but high rpm will be required.
A. Drag Translator
Perhaps the simplest type of wind
d power machine is the device that lies in a straight line under action of
the wind. Figure 1 illustrates the action of the elementary drag device.
Figure
ure 1. Schematic of translating drag device.
The drag results from the relative velocity between the wind and the device, and the power that is
generated by the device may be expressed as the product of the drag force and the translation or blade
velocity.
P
1
ρ U
2
V C CHV 1
The translation (or blade) velocity of the device must always be less than the wind velocity or no drag is
generated and no power is produced.
According to Betz’s Theory [2], the kinetic energy of an air mass m moving at a velocity U can be
expressed as:
E
mU
2
2
Considering a certain cross-sectional
sectional area A, through which the air passes at velocity U, the volume flow
rate Q (m s ) flowing through during a time unit, the so-called
so called volume flow rate, is:
Q
AU 3
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The mass Flow rate with the air density ρ is:
m = ρAU[4]
The equations expressing the kinetic energy of the moving air and the mass flow yield the amount of
energy passing through cross-section A per unit time. This energy is physically identical to the power P in
(W):
P=
ρAU
[5]
2
Mechanical energy can only be extracted at the cost of the kinetic energy contained in the wind stream;
this means that, with an unchanged mass flow, the flow velocity behind the wind energy converter must
decrease.
Here, U1 is the un-delayed free-stream velocity,
The wind velocity before it reaches the converter, whereas U2 is the flow velocity behind the converter.
Neglecting any losses, the mechanical energy, which the disk-shaped converter extracts from the airflow,
corresponds to the power difference of the air stream before and after the converter:
P=
ρAU − ρAU
[6]
2
Figure 2. Pressure and velocity variation V/S distance of wind turbine
Maintaining the mass Flow (continuity equation) requires that:
ρA U = ρA U [7]
Thus,
P=
m(U − U )
[8]
2
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From this equation it follows that, in purely formal terms, power would have to be at its maximum when
U2 is zero, and namely when the air is brought to a complete standstill by the converter.
However, this result does not make sense physically. If the out Flow velocity U2 behind the converter is
zero, then the in Flow velocity before the converter must also become zero, implying that there would be
no more Flow through the converter at all. As could be expected, a physically meaningful result consists
in a certain numerical ratio of U2=U1 where the extractable power reaches its maximum.
This requires another equation expressing the mechanical power of the converter. Using the law of
conservation of momentum, the force which the air exerts on the converter can be expressed as:
F = m(U − U )[9]
According to the principle of "action equals reaction", this force, the thrust, must be counteracted by an
equal force exerted by the converter on the airflow. The thrust, so to speak, pushes the air mass at air
velocity U0, present in the plane of Flow of the converter. The power required for this is:
P = FU = m(U − U )[10]
Thus, the mechanical power extracted from the airflow can be derived from the energy or power
difference before and after the converter, on the one hand, and, on the other hand, from the thrust and
the Flow velocity. Equating these two expressions yields the relationship for the Flow velocity U0:
m(U − U )
= m(U − U )U [11]
2
Thus, the Flow velocity in the converter plane is equal to the arithmetic mean of U1 and U2.
U =
U +U
[12]
2
The mass Flow thus becomes,
m = ρAU =
ρA(U + U )
[13]
2
The mechanical power output of the converter can be expressed as,
P# =
ρA(U − U )(U + U )
[14]
4
In order to provide a reference for this power output, it is compared with the power of the free-air
stream which Flows through the same cross-sectional area A, without mechanical power being extracted
from it. This power was given by,
P=
ρAU
[15]
2
The ratio between the mechanical power extracted by the converter and that of the undisturbed air
stream is called the "power coefficient (Cp)”,
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C$ =
C$ =
P#
[16]
P
ρA(U − U )(U + U )
2
×
4
ρAU
1
U
U
C$ = &1 − '(1 + )[17]
U
2
U
The dependence of power coefficient on tip speed ratio is typically shown in fig. 2 for various types of
turbine rotors. Following points can be observed from the graph.
Figure 3. Coefficient of Performance V, Tip Speed To Wind Speed Ratio
Cp is lowest for Savonius and Dutch type of blades. Ideal value of Cp is about 0.59, which is also known as
Betz’s limit (2), representing the maximum theoretical efficiency of a wind turbine. All wind machines
have Cp lower than the maximum value.
B. Rotor Size Calculation
There are two principal ways to determine frontal area of wind turbine: either approximate size of rotor
has to be decided and power generation to be calculated or first power requirements are identified &
then frontal area is to be calculated.
The swept area of Savonius VAWT can be calculated from following equation.
A = 2R × H[18]
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Figure 4. Schematic Diagrams for Savonius Rotor
Windmill blades are designed to move in response to wind force, and it can extract a substantial portion
of the energy and power available. The wind energy available in a unit volume of air depends only upon
the air density ρ and the instantaneous wind speed U.
This “kinetic energy” of the air in motion is given by the following formula.
Kineticenergy
1
= ρU [19]
unitvolume
2
The volume of air that passes through an imaginary surface-say the disk swept out by a horizontal-axis
windmill-oriented at right angles to the wind direction is equal to:
Volume = A × V × t[20]
Thus, the wind energy that flows through the surface during time t is just,
1
Availableenergy = ρAU t[21]
2
Wind power is the amount of energy which flows through the surface per unit time, and is calculated the
wind energy by the elapsed time (t),
1
Power = ρAU [22]
2
By putting value of area from Equation (16)
Power = ρRHU [23]
C. Aerodynamic Design
From an aerodynamic point of view, most of the rotation of the Savonius turbine is caused due to drag
forces. The turbine consists of two blades, the advancing blade and the retreating blade, in the case of a
two-bladed Savonius. The blades of a Savonius rotate on a horizontal plane whose axis is at right angle to
the wind direction. The aerodynamic angle of attack of the blades varies constantly during the rotation.
So during rotation, one blade moves on the downwind side and the other on the upwind side. The torque
extraction from the blade that moves on the downwind side is less compared to the energy extracted by
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the upwind blades. This describes the characteristic of the torque. Hence, power generation is less in the
downwind sector of rotation. The torque caused by drag forces is generally lesser than that produced by
lift forces. In one revolution, a single rotor blade generates a mean positive torque but there are also
short sections with negative torque.
Figure 5. Top view of savonius rotor
Aerodynamic design of the Savonius rotor is mostly a matter of drawing something that looks like it will
work (10). Due to complex flow around the Savonius rotor, prediction of its aerodynamic behavior by any
analytical model is merely not possible. Thus one has to depend on experimental analysis of Savonius
rotor, which involves plenty of experiments for every combination of geometrical and wind parameters.
This is very expensive and tedious job. Many researchers had performed these experiments for various
configurations. So, in this work the optimum/best parameters based on referred documents have
adopted.
D. Geometrical Parameters
This project is limited to Single Step conventional (semicircular bladed) Savonius rotor. Selection &
calculations of other parameters are discussed in following topics.
No. of Blade :
It is evident that with the increase of the number of blade there is no significant change in the value of
drag coefficient, and thus of torque coefficient, rather the starting torque increases slightly and is
obtained at smaller value of rotor angle [4] Optimum number of blade is two for the Savonius rotor
whether it is single, two or three-stage [5,11,12]
Overlap ratio:
Overlap ratio can be defined as following equation.
S = e⁄C [24]
suggested through extensive wind tunnel experiments that a dimensionless overlap ratio = 0.1-0.15
appears to yield optimum performance rate for two bucket Savonius rotor without central shaft. The
similar kind of study done by [8] concluded that the torque and the power performance of the rotating
rotor reach a maximum at an overlap ratio of 0.15.
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Aspect ratio:
The aspect ratio (AR) plays an important role in the aerodynamic performances of a Savonius rotor.
Aspect ratio can be defined as following equation.
AR = 2R⁄H [25]
Performance of Savonius rotor increases slightly with increasing aspect ratio [5,12]. For a given frontal
area, higher aspect ratio rotors will run at higher rpm and lower torque than those with a low aspect
ratio [10]. Values of AR around 4.0 seem to lead to the best power coefficient for a conventional Savonius
rotor, but this value has no proper validation. Thus optimum value of AR has to found out.
End plate :
The rotor with end plates gives higher efficiency than those without end plates [12]. The influence of
radius Ro of these end plates relatively to the diameter R of the rotor has been experimentally studied
[8]. The higher value of the power coefficient Cp is obtained for a value of Ro around 10% more than R,
whatever is the velocity coefficient.
R = = 1.1R[26]
Torque and Power coefficients
Calculations of torque and power coefficient of 2-blade rotor are as following:
B
C−e
Tθ = @ FA (
)[27]
2
BC
For 2-blade rotor,
C−e
Tθ = DFAE + FAF G (
)[28]
2
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Figure 6: Force on s-rotor blade
Here FAE and FAF can be found out by following equation,
F AE
F AF
x cos θ
y sin θ 29
x cos θ " y sin θ 30
Torque produced on rotor is,
torque
z % FAE " z % FAF 31
torque
z % DFAE " FAF G 32
Mean torque for 2-bladed rotor,
T#
∑θLMC
TθL
i
33
Torque co-efficient,
C#
T#
34
ρR HU
N
C
P#
35
ρRHU
Power co-efficient,
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E. Preliminary Design Parameters
Summary of Preliminary design parameters are given in following table.
Table.1. Preliminary design parameters
Parameter
No. of stages
Shape of blade
No. of blade
Aspect ratio
Frontal area
Overlap ratio
Value
1
Semi-circular
2
1 to 5
360000 mm2
0.15
F. Preliminary Power Estimation
Wind power is the amount of energy which flows through the surface per unit time. The preliminary
power estimation can be calculated by taking wind speed (U) is equal to 4 m/s and Density of Air=1.225
kg/m3
Maximum power can be extract from equation 20,
1
Power = ρAU [36]
2
=
1
× 1.225 × 0.36 × 4 [37]
2
= 14.112Watt[38]
According to betz's theory [2] maximum power can be extracted by using any kind of wind turbine is
59.29%.
Power = 14.112 × 0.5929[39]
= 8.3670Watt[40]
According to the graph fig. 2, maximum power can be extracted by savonius wind turbine is 30%.
= 8.3670 × 0.30[41]
= 2.5101Watt[42]
G. Numerical Analysis
For this project ANSYS Workbench-14.0 is used for CFD analysis. Fluid flow (fluent) Analysis is carried
out on straight blade savonius rotor.
Workbench fluent analysis is 5 steps procedure, in 1st step geometry is defined which is made in CREO
and exported to ANSYS. The geometry has two domains called fluid domain and rotor. Which are shown
in (fig 6). The fluid domain is square prism in shape and approximately 10 times bigger than rotor, i.e.
10m × 10m × 13m long. This fluid region mimics environment. The reason behind bigger size of fluid
domain compared to rotor size is that effect of external wall can be minimize, so that flow of air will be
same as environmental conditions. The rotor volume is subtracted from the fluid region
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Figure 7. Fluid Domains in ANSYS
In fluid domain name selection are defined in order to apply load as shown in (Figure 8) Surface A
indicates "INLET", Surface B indicates "OUTLET" Surface C indicates "WING 1" Surface & D indicates
"WING 2".
Figure 8. Name selection of surfaces
In mesh step element type & its size is defined and meshing of body is performed (Fig 9) for meshing
tetrahedral elements are used and refined meshing is applied near rotor in 5 layers. The minimum size of
elements is 0.1 mm.
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Figure 9. Mesh model ( Savonius rotor) with fluid domain
Figure 10. Enlarged meshed region showing rotor
In setup step boundary condition and load are applied. Steady air of 4 m/s is applied normal to "INLET
(A)" face and these exits from “OUTLET (B)” face.
In solution setup stage Momentum, turbulent kinetic energy & Turbulent Dissipation Rate has been set as
second order upwind. Hybrid initialization has been selected. Solution for 500 iterations is applied which
converge almost in 190 iterations for different analyses & results are
are obtained in final step.
In result stage, various Forces and pressure are acting on rotor which is reported in form of numerical
data as well graphical representation.
Total 20 numbers of CFD analysis were preformed for different aspect ratio , i.e. aspect
aspec ratio 1,2 ,3 ,4 and
5,for that different azimuth position.
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Pressure gradient and velocity gradient around the rotor at 45° azimuth for different aspect ratio (1, 2, 3,
4 and 5) are show in subsequent figure. For aspect ratio 1 Pressure gradient and velocity gradient around
the rotor from this graphs flow behavior around the rotor can be studied.
Figure 11. Pressure gradients for Aspect Ratio 1 and Azimuth Angle 450
Figure 12. Velocity gradients for Aspect Ratio 1 and Azimuth Angle 450
Figure 13. Pressure gradients for Aspect Ratio 2 and Azimuth Angle 45°
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Figure 14. Velocity gradient for Aspect Ratio 2 and Azimuth Angle 450
Figure 15. pressure gradient for Aspect ratio 3 and Azimuth Angle 450
Figure 16. Velocity gradients for Aspect Ratio 3 and Azimuth Angle 450
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Figure 17.Pressure gradients for Aspect Ratio 4 and Azimuth Angle 45°
Figure 18.Velocity gradients for Aspect Ratio 4 and Azimuth Angle 45°
Figure 19. Pressure gradients for Aspect Ratio 5 and Azimuth Angle 45°
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Figure 20. Velocity gradients for Aspect Ratio 5 and Azimuth Angle 45°
Figure 21. Average torque (Nm) VS Aspect ratios
IV. CONCLUSION
From the CFD analysis, forces acting on buckets of rotor are derived and using equations 32 mean torque
has been calculated. It has been found out that for same frontal area Aspect ratio 1 will give maximum
torque. In this case for frontal area 360000 mm2 and aspect ratio 1, mean torque will be 167.3 Nm. So
our final conclusion is, if we will keep on increasing the aspect ratio the resultant torque of turbine will
continue to decline. We can only get maximum torque at aspect Ratio 1.
V. REFERENCES
[1]
Wei tong, (2010), “WIT Press publishes leading books in Science and Technology”, southampton,
Boston, ISBN: 978-1-84564-205-1
[2]
Betz's Theory, (1966) Introduction to the Theory of Flow Machines. Oxord: Pergamon Press.
[3]
Chauvin, A. and Benghrib, D. (1989), "Drag and lift coefficients evolution of a Savonius rotor",
Experiments in Fluids, Volume 8, Issue 1-2, pp 118-120
[4]
Islam, M. D., Hasan M. N. and Saha, S. (2005), "Experimental Investigation of Aerodynamic
Characteristics of Two, Three and Four Bladed S-Shaped Stationary Savonius Rotors.",
www.ijafrse.org
Proceedings of the International Conference on Mechanical Engineering (ICME2005), 28- 30
December, Dhaka, Bangladesh
[5]
Blackwell, B. F., Sheldahl, R. E. and Feltz, L. V. (1977), "Wind Tunnel Performance Data for TWOand Three-Bucket Savonius Rotors", SAN D76-0131, Sandia Laboratories, operated for the United
States Energy Research & Development
[6]
Hayashi, T., Li, Y. and Hara, Y. (2005), "Wind tunnel tests on a different phase three-stage
Savonius rotor", JSME International Journal, Series B: Fluids and Thermal Engineering, 48(1):916.
[7]
Kamoji, M.A., Kedare, S.B. and Prabhu, S.V. (2009), "Experimental investigations on single stage
modified Savonius rotor", Applied Energy Volume 86, Issues 7–8, July–August 2009, Pages 1064–
1073
[8]
Fujisawa N. and Gotoh F. (1992) ,"Visualization study of the flow in and around a Savonius rotor" ,
Experiments in Fluids 12, pp.407-412
[9]
Driss, Z., Abid, M. S. (2012), "Numerical and experimental study of an open circuit tunnel:
aerodynamic characteristics" , Science Academy Transactions on Renewable Energy Systems
Engineering and Technology, United kingdom Vol. 2, N. 1, pp. 196-204.
[10]
Park J. (1981),"The wind power book", United States of America, ISBN 0-917352-05-X
[11]
Saha U. K. and Rajkumar, M. J. (2006), “on the performance analysis of Savonius rotor with
twisted blades”, Renewable Energy, pp. 960-1481
[12]
Mahmoud, N. H., et. al., (2012), “An Experimental study on improvement of savonius rotor
performance”, Alexandria Engineering Journal 51, pp. 19 –25
[13]
Altan, B. D. and Atylgan, M. (2008), “The use of a curtain design to increase the performance level
of Savonius wind rotors”, Renewable Energy, 35(4):821-829
[14]
Huda M. D., Selim M. A., Islam A. K. M. S, and Islam M. Q. (1992), “Performance of an S-shaped
Savonius rotor with a deflecting plate”, RERIC International Energy Journal, 14(1):25-32
[15]
Kailash Golecha, Eldho T.I. And Prabhu S.V. (2011), “Influence of the deflector plate on the
performance of modified Savonius water turbine Applied Energy”, Volume 88, Issue Pages 3207–
3217
[16]
Menet, J., Valdes, L. and Menart B (2001), “A comparative calculation of the wind turbines
capacities on the basis of the L-ϭ criterion”, Renewable Energy, 22(4):491-506
[17]
Savonius, S. J. (1929), “Rotor Adapted to be Driven by Wind or Flowing Water”, United states
patent office, Patent No. 1,697,574
[18]
Singhal, B.L. (2011), “Tech-max Publications”, pune-41, India, ISBN: 978-81-8492-052-2
www.ijafrse.org

Simulations of Small Scale Straight Blade Savonius Wind Turbine

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