◎논문 DOI: http://dx.doi.org/10.5293/kfma.2016.19.4.
043
◎ Original Paper ISSN (Print): 2287-9706
A Study on the Performance of a 5 kW Scale VAWT with
Omni-Directional Guide Vanes
In-Ho Ryu*⋅Anh Dung Hoang**⋅Jun-Ho Kim***†⋅Ki-Seon Lee****⋅Chang-Jo Yang***** 1)
Key Words : CFD(전산유체역학), Omni-Directional Turbine(방향키터빈), VAWT(수직축 터빈), Wind Energy(풍력)
ABSTRACT
Vertical axis wind turbine, despite of its limit in power efficiency, the simplicity in structure and maintenance is a
competitive factor that keeps this type of turbine in the game until nowadays. Continuous solutions have been given to handle
its major weakness and the use of omni-directional guide vane is an considerable one. In this paper, a 5kkW scale
Savonius-based wind turbine enhanced with such guide vane system was design and studied. Together with reasonable blade
design, the wind turbine shows promising performance compared with basic design while maintaining its original advantages.
1. Introduction two groups. First group, the so-called “positive
component”, consists of part of the blades which move
For the current situation of our modern world, it is in respect of wind direction, the second one consists of
obviously seen that mankind has been continuously negative blade elements which move against the wind
seeking for energy solutions to meets the not only the direction, consequently resist or limit the overall
growing global demand, but also the environment power output.
conservation. Wind turbine, which has a significant For decades, there are several methods invented for
long history of development, is still interested as a the reduction of negative effect in VAWTs. This paper
simple and reliable solution for such demands. introduce the design of a 5kW scale VAWT enhanced
Of all known designs, vertical axis wind turbine with stationary guide vanes which help orient the wind
(VAWT) has a notable feature, that is the advantage of direction in a positive way to increase power output.
being capable of catching the wind from all directions The use of guide vane is not an innovative idea,
(1)
without a need to orient the blades. Besides, another however, with the combination of blade shape the
advantage is that the blade usually take the shape of VAWT design in this study shows promising impoved
simple geometry. Thus, the turbine operates in almost performance while maintains the economic fator in
pure tension, it becomes relatively light and inexpensive terms of structure and construction.
to construct. The major disadvantage of VAWT is the
relatively low power efficiency compared to horizontal 2. Design
designs. This is due to the fact that vertical
orientation unwillingly divides the rotor blades into The VAWT presented here is of Savonius-based style
* Graduate School Division of Marine Engineering, National Maritime University
** DEA A Energy CO., Ltd
*** Division of Marine Mechatronics, National Maritime University
**** Korea Southern Power CO., Ltd
***** Division of Marine Engineering, National Maritime University
† 교신저자(Corresponding Author), E-mail : junho.kim@mmu.ac.kr
2015 한국유체기계학회 동계 학술대회 발표 논문, 2015년 12월 2-4일, 제주도
The KSFM Journal of Fluid Machinery: Vol. 19, No. 4, August, 2016, pp.43~47(Received 05 Nov. 2015; revised 29 Apr. 2016; accepted for publication 29 Apr. 2016)
한국유체기계학회 논문집: 제19권, 제4호, pp.43~47, 2016(논문접수일자: 2015.11.05, 논문수정일자: 2016.04.29, 심사완료일자: 2016.04.29) 43
In-Ho Ryu⋅Anh Dung Hoang⋅Jun-Ho Kim⋅Ki-Seon Lee⋅Chang-Jo Yang
Table 1 Design parameters
Design Power 5 kW
Rotor Diameter 9 m
Full Diameter 11 m
Number of Blade 24
a) Sectional View Rotor Height 6 m
Number of Guide Vane 42
Blade Type V1, V2, V3
Guide Vane Type Plain Rectangular
o
Design Tilted Angle 45
Design Wind Speed 10 m/s
TSR Range 0.2-0.5
b) Full Model View
Fig. 1 Design of Omni-Directional VAWT
with several modifications (Fig. 1). Firsly, the blades a) V1 Blade
are mounted at the periphery of the rotor, leaving a
big empty space at the center (traditional design has
the rotor interior mostly filled). Secondly, a large
number of blades is used, and for this scale 24 blades
are fitted. The purpose of this design is due to the fact b) V2 Blade
that the most effective region for generating torque
usually lies at far distance from the center. Thus, such
arrangement is more beneficial for power generating
while reduces the effect of resistance and blockage.
The final modification is the use of omni-directional c) V3 Blade
guide vane. As mentioned before, the main purpose of Fig. 2 Blade Designs
these vanes is to prevent turbine’s rotor from
generating negative effect. Moreover, the installation speed and rotor revolution shall decide the combined
of guide vanes also enable seft-starting ability for the velocity vector of an air particle passing through the
VAWT. In this design, 42 guide vanes are fitted. turbine. The 3 blade designs are proposed aiming at
For 5kW scale, device’s sizing was calculated and effectively taking the air and absorbing its kinetic
given in Table 1. Since the guide vanes play flow energy based on pre-described principle. Thus,
orientation role, the blades are designed to match the flanging is applied for all three types but with
direction of the air stream entering rotor. This is the different methods. Computational simulation is carried
most importance which directly influences the power out for each design in order to understand the flow
output. In this work, three types of blade are patterns and find out the most effective one.
analyzed. Fig. 2 shows the shapes of these blade The application of guide vane and flanging blade
designs, in turn named “V1”, “V2” and “V3”. also appeared in other’s work, i.e. Chong et al. in
Due to the orientation of guide vanes, the air is 2014, where a similar VAWT was introduced. The
directed into the rotating rotor with a tilted angle in authors uses guide vane to form a so-called power-
most of the blades. Thus, the wind flow takes part in augmented shroud and state that such design can
two aerodynamic motions expressed as direct inflow increase the coefficient of torque and the coefficient of
and rotating components. The magnitudes of wind power.(2,3) However, those researchers use traditional
44 한국유체기계학회 논문집: 제19권, 제4호, 2016
A Study on the Performance of a 5kW Scale VAWT with Omni-Directional Guide Vanes
Table 2 Simulation Cases
TSR 0.2 0.3 0.4 0.5
Corresponding RPM 4.244 6.366 8.488 10.61
Fig. 3 Meshing and Mesh Quality of Computational Domain
airfoil style blade and applied flanging for the guide
vanes in stead.
3. Computational Simulation
Fig. 4 Flow Visualizaton for V2 Design at TSR 0.2
Computational Fluid Dynamics (CFD) has been being
a reliable solution to solve fluid flow problem,
maintained. The coresponding rotor revolutions for
especially for wind turbines. The solver used in this
different TSR conditions are given in Table 2.
work is ANSYS CFX commercial code which is able to
generate accurate simulation results for correctly
4. Results and Discussions
defined problems. In order to do this, the turbine was
modeled carefully using CAD tools and precisely
4.1 Flow Patterns
meshed. The computational mesh is done with hexa
only approach to assure the robustness of the domain.
Fig. 4 illustrates an example vizualization of air
The mesh has high quality, dimensionless wall funtion
flow passed through the turbine, the model in the
value is within reliable range where maximum yplus is
figure is V2 design operating at TSR 0.2. The figure
under 7.67 for the whole computational domain, while
presents the cut plane of relative water velocity
at near wall (blade, guide vane) region, this value is
(velocity in rotating frame) which distributes over the
only about 0.05 (Fig. 3).
guide vanes, blades and hub. It is clearly seen that the
The computational domain is setup with one inlet
contrast in color expains the effect resulted from the
and one outlet in rectangular shape in order to
application of omni-directional guide vane. The rotor
simulate the far field flow direction. Turbulence flow
is set to turn counter-clockwise, so the flow tends to
is solved by Shear Stress Transport (SST) model which
move in a spline-like pattern. This is due to the
is stable and has low rate of encountering
guiding effect of the vanes, also it means the air is
computational errors. Most simulations are done in
oriented to pass the blades in a more effective way.
steady state analysis but some transient cases are also
Detailed wake patterns for each blade designs are
carried out to confirm the results’ accuracy. To
show in Fig. 5. The illustrations present these their
estimate turbine’s performance at different conditions,
simulation results in the same condition (at TSR 0.3).
the dimensionless speed called Tip-Speed Ratio (TRS,
According to these patterns, there is a significant
λ) is given as an input parameter.
difference between V2 design and the rest. The
difference is that the streamlines have smoother shape
and are well developed over the field. These
streamlines are also tend to move along with the
For these simulations, TSR is changed by changing rotating direction (from left to right) in counter-
rotor’s rotational speed and free stream wind speed is clockwise, means that the flow was guided effectively.
한국유체기계학회 논문집: 제19권, 제4호, 2016 45
In-Ho Ryu⋅Anh Dung Hoang⋅Jun-Ho Kim⋅Ki-Seon Lee⋅Chang-Jo Yang
a) V1 Blade
Fig. 6 Power Efficiency vs TSR Curves
b) V2 Blade
Fig. 7 Power Output vs TSR Curves
for each design and summarized in Fig. 6. where CP is
expressed as a function of TSR. The similar expression
is shown in Fig. 7 but the ouput is put in dimensional
value.
c) V3 Blade As seen in flow analysis section, the V2 design
Fig. 5 Wake Patterns at TSR 0.3 shows the highest power efficiency of all, and
throughout the TSR range. Moreover, the curve’s trend
For V1 design, the streamline patterns are also is in increasing state, thus within the scope of this
resonable but seems to be separated into two regions work, the performance of V2 design is not yet
(upper and lower components) as in traditional VAWT estimated fully. Unexpected, The poorest one is V1
(negative and positive components). For V3 design, design, V3 blade’s performance is acceptable but has
such effect also presents. Besides, there are many no positive trending. However, all of three designs
local vortexes and collisions inside the rotor region. have CP abobe 0.1 value, which is often the limit of
This may considerably influence the device’s Savonius-based turbine. Recently, Kadam et al. did a
aerodynamic performance. thorough work to review the performance of this type
of turbine but most of his tests results in maximum CP
4.2 Power Efficiency around 0.1.(5)
According to the results, the V1 and V3 designs have
Power performances of the designed VAWTs are optimum operating points at TSR 0.3, while V2 design
evaluated by power coefficient CP which is calculated can work efficiently at high TSR up to 0.5 or even
46 한국유체기계학회 논문집: 제19권, 제4호, 2016
A Study on the Performance of a 5kW Scale VAWT with Omni-Directional Guide Vanes
higher. At this point, the device’s efficiency reaches 3) The sizing and structure of the analyzed VAWT
almost 25% which is considered high so far. satisfied the power requirement while maintaining
For given initial conditions, the required power the advantage of simplicity in both construction,
output is 5kW. Except the V1 design, the other two are design and maintenance..
both able to capture that amount of energy. While V3 The VAWTin this paper is computational model and
design can get the required energy at high TSR only, its capability has not been fully comprehended yet
V2 design satisfies the requirement at all TSRs. since various factors were excluded, i.e. number of
The major reason for the efficiency difference is the blades, number of guide vanes, tilted angle, blade
flanging style. For V2 design, there is a clear open for angle of attack, sizing.... These problems should be
air to get out after contacting with the blade and investigated and taken into account in future work to
continue to pass through the hub, then get in the blades discover the optimal design for Savonius-based VAWT.
at the rear. Hence, air energy is transferred perfectly The results of this study help contributing to the
without or with least resistance. Despite the fact that present development of wind turbine as the potential
V1 design only differs from V2 design in blade’s angle of method of tidal energy conversion to serve the future
attack, such positioning creates a blockage on the flow demand of renewable energy.
and consequently there are energy losses. V3 design has
the same flaw but the flanging part is shorter. All in References
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