Smart Grid and Renewable Energy, 2015, 6, 141-147
Published Online June 2015 in SciRes.
How to cite this paper: Tabassum, S., Rahaman, M., Bashar, M.S., Islam, S., Sharmin, A., Imam, A.Y., Hoque, A., Mahbub, N.,
Khatun, S. and Khanam, M. (2015) Design and Analysis of Different Types of Rotors for Pico-Turbine. Smart Grid and Re-
newable Energy, 6, 141-147.
Design and Analysis of Different Types of
Rotors for Pico-Turbine
Samia Tabassum, Mashudur Rahaman, Muhammad Shahriar Bashar, Saidul Islam,
Afrina Sharmin, Abdullah Yousuf Imam, Azizul Hoque, Nahid Mahbub, Sayeda Khatun,
Mahfuza Khanam
Institute of Fuel Research and Development, Bangladesh Council of Scientific and Industrial Research, Dhaka,
Received 12 April 2015; accepted 6 June 2015; published 9 June 2015
Copyright © 2015 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
Small-scale vertical axis wind turbine (VAWT) rotor is developed for use in areas lacking adequate
energy infrastructure. The materials and methods of construction are selected to minimize cost as
much as possible. The paper describes the design of different kinds of vertical axis wind turbine
rotors having different number of blades and twist angle. The aim of the work is to study the in-
fluence of the different designs on rotational speed and power of rotor in different wind speed.
Vertic al Axis Wind Turbine, Savonius Rotor, Wind Energy, Power Co-Effic ien t
1. Introduction
The rising cost of fossil fuel and effects of climate change forces to enhance the demand of renewable energy
sources in different sector. Like as other renewable energy sources, wind power has a key role in reducing
greenhouse gas emissions [1]. Wind energy is a clean renewable energy source cheaper to maintain, saves fuel
and can give decentralized energy [2]. A wind turbine is a device that taps the renewable kinetic energy of
blowing wind, which converts it to useable mechanical, electrical or thermal energy. The main idea of wind
power is to produce mechanical or electrical power economically without air pollution by using exhaustible nat-
ural resources. The function of a wind electric generator is to convert the kinetic energy of the wind into most
useful form of energy i.e. electricity. The basic design requires the conversion done in the most efficient way
and at lowest cost. This is done in few steps such as kinetic energy of the wind, converted through blades to
mecha nical e nerg y of a ro tati ng sha ft and t hen tra nsmit ted thr ough a ge arbo x to a ge nera tor, whic h con verts t he
S. Tabassum et al.
mechanical energy into electrical energy [3]. Today, the most commonly used wind turbine is the Horizontal
Axis Wind Turb ine (HAWT), where the axis of rotatio n is parallel to the ground. Ho wever, Vertical Axis Wind
Turbine (VAWT), where, the axis of rotation is longitudinal to the ground is also an impe rative issue in power
generation sector. These devices can operate in flows coming from any direction, and take up much less space
than a traditional HAWT [4], and VAWTs are definitely a credible source of energy for the future. VAWTs
have a number of advantages over HAWTs, such as simple construction, low cost and can accept wind from any
direction. In Bangladesh, wind speed is not so high and direction is highly variable [5]. That’s the reason why
vertical axis wind blade can be more effective for this country [6]. In thi s co ntext, differe n t t ypes of vertical a xis
wind blade have been developed and analyzed [7]. A brief design description, details of the experiments con-
ducted and the conclusions drawn are the prominent features of the present paper.
1.1. Basic Principle of Savonius Wind Turbine
Savon ious wind t urbine is ver tical axis wind tur bine (VAW T), whic h conve rts the for ce of t he wind into to rque
on a r ota ting sha ft. I t is in vent ed b y the Fi nnis h eng ineer Si gurd J oha nnes S avo nius in 19 22. The Savoni us ro tor
consists of two (look like “S” shape in cross section) or three scoops. Because of the curvature, vertical axis
wind turbines allo w winds from any direction equally and force to rotate [8]. T he savonius wind turbine works
due to the difference in forces apply on each blade. The concave half part of the blade receives air and forces the
blade to rotate around its central vertical shaft. At the same time, the convex half part of rotor hits the blade and
causes the air to be deflected sideway around it, which is shown in Figure 1.
By placing two rotors on the same shaft, one designed to run clockwise and another one designed to run
counter clockwise and exposing them to a flow, a significantly better performing rotor can be made. Both rotors
should tur n in the direction o f the one with better startin g characteristic s. This type o f vertical axis r otor is very
rob ust and dur able i f built co rrect ly, is rela tivel y slow tur ning a nd can be e asil y built at home, witho ut the has-
sles of aerofoil blade design and other problems associated with horizontal axis propellertype turbines.
1.2. Design and Theo r y
In this work, we have developed three different types of blade for investigating various properties. The first
model (Blade-1), sho wn in Figure 2, is a twisted savonius with two helical blades arranged at 180˚ around the
shaft. GI wire s (15 swg) were placed on shaft (aluminum pipe with diameter of 0.0127 m) as frame of blade,
whi ch was covered by paper. Blade -2 is a drag-type savonius (Figure 3) made by plastic material. It has two
sections. Each two-scoop would like an “S” shape in cross section. The two scoops in the top section of the rotor
are rotated at 90˚ to the bottom ones. This ensures that there is always at least one scoop in a position to catch
the wind and as a result, the turbine is self-starting. The third one (Blade-3) is also a drag type savonius with
three blades (Figure 4). Here, the blades are arranged at 60˚ around the shaft. Each blade is triangular in shape
and curved a bit with a support. GI sheet s and aluminum pipe were used for the blades and shaft, respectively.
Figure 1. The effects of wind on savonius blade.
S. Tabassum et al.
Figure 2. Twisted vertical axis blade (Blade-1).
Figure 3. (a) Two steps savoni us bla de (Blade-2); (b) One step savonius blade.
Figure 4. Pictur e of different designed blades.
S. Tabassum et al.
The amount of energy produced by a wind turbine primarily depends on the rotor area, also referred to as
cross-sectional area, swept area, or intercept area. The swept area for savonius wind turbine can be calculated
from the dimens ions of the ro t or
Swept area*
A HD= =
where, H = rotor height; D = rotor diameter.
The tip sp e e d ratio, λ is the ratio o f the linear speed of r otor blade ω. R to t he undisturbed wind speed; V. Here,
ω is the angular velocity and R represents the radius revolving part of the turbine. High tip speed ratio improves
the performance of wind turbine and this could be obtained by increasing the rotational rate of the rotor.
= ⋅
The power coefficient (Cp) represents the amount of energy that can be harvested from the wind to convert it
into mechanical energy. The power coefficient reaches its maximum for a unique tip sp eed r atio. It is the ratio o f
maximum power obtained from the wi nd to t he tot al p owe r a vaila ble in t he wind. T his h ypo thesis sho ws t he r e-
lationship between the power coefficient (Cp) and the wind speed (V), which expresses the basic theory of the
Savon ius wind turbine. Princi pally the po wer that the savo nius ro tor can extract from the wind (Pw) is less than
the actual available from the wind power (Pa). The available power (Pa), whic h is also the kinetic energy (KE)
of the wind, can be defined as:
P mV=
where, ma (wind mas s flow rate striki ng the swept area of the wind turbine ( kg/sec))
ρ is density of air, A is the rotor area, V is the speed of wind.
The power coefficient (Cp) is given by:
Extracted power from the wind.
Available power of the wind
= =
The extracted power from t he wind of a wi nd turbine blade is deter mined by the follo wing e quation:
0.5 .
The fund ament al l aws o f c o ns er vat io n o f ma ss a nd ene rgy all o wed no more than 59 % of t he k inetic ene r g y o f
the wind to be captured which is well-known as Betzlaw. Practically, wind turbines cannot operate at this
maxi mum limit. The Cp value is unique to each turbine typ e and is a function of wind speed that the turbine is
operating in. In practice, values of obtainable power coefficients are in the range of 0.35 - 0. 45 co mmon e ven in
the best designed wind turbines. This value below the theoretical limit is caused by the inefficiencies and losses
attributed to different configurations, rotor blades and turbine designs. The maximum power coefficient, Cp for
savonius rotor is 0.30. Hence, the Cp value used in this work i s 0.30 [9]. Ho wever, density of air varies accord-
ing to ele vatio n, temperature and weather fronts. Density of air for 30˚C is 1.16 kg/m3 [10].
Wind speed was measured by cup anemometer (Lutron ABH-4224) and rotor speed in rotation per minute
(RPM) was measured by using laser type tachometer (Lutron DT-1236L). The data was taken at ro o m te mp er a-
ture. Swept area and obtained wind power is also important aspects for this purpose and it is calculated as well.
The wind tunnel was built to have interchangeable parts to test different combinations of design parameters.
Experiments were conducted in this wind tunnel. The schematic diagram of wind tunnel made by GI sheet is
sho wn in Fig ure 5. The wind tunnel is 172 cm long, which consists of contraction section, test section and dif-
fuser section. T he first par t is contraction zone to prod uce a uniform wind velocit y distrib utio n to the test section.
Rotors are placed in front of anemometer in the test section, where, wind speed data and rotational speed of rotor
is measure d. The e xhaust fan is d riven to flow the wind in t unnel in t he diffuser section.
2. Results and Discussion
The aim of this experimental study is to obtain the aerod ynamic performance of studied wind turbine rotor and
also the detailed information about velocity field around the rotor. Different types of blade is developed and
placed in wind tunnel, where, rotational speed is measured in variation of wind speeds. Table 1 sho ws t he su m-
mery of design parameters of blades.
S. Tabassum et al.
Figure 5. Schematic diagram of wind tunnel.
Table 1. Summary of desi gn parameter s of blades.
Blade Height (m) Diam eter (m ) Aspect ratio (H/D) Weight (kg) Swept area A (m2) Tip speed ratio at 4 m/s
Blade 1 0.46 0.46 1.00 0.34 0.217 0.88
Bla de 2 0.46 0.35 1.31 1.40 0.161 1.03
Bla de 3 0.39 0.29 1.34 0.44 0.113 0.81
Figure 6 shows rotational speed of three blades in different wind speed. During the test, the wind turbine
blades rotational speed varies from 72 rpm to 241 rpm and the upstream flow velocity varies from 2 m/s to 4.4
m/s. This figure state that the system has better performance for Blade-2 and 3. On the other hand, blade 1
showed low rotational speed. This may be due to the cause of slim rotor with small diameter which can get
higher rotatio nal speed b ut lower torque, and vice versa rotor with bigger rotatio nal diameter produces a b igger
torque but a lower rotational speed.
Figure 7 compares the theoretically calculated extracted power produced by the three different savonius ro-
tors alone in different wind speed. Figure shows that the extracted power from the wind of the wind turbine
blade mainly starts from 3 m/sec. Before that, power generation is very small. It largely increases when it turns
from 3 to 4 m/sec. It also showed the variation of extracted po wer with available wind speed in air. This differ-
ence is due to the variation of swept area which has been already explained in Equation (3). Be side this, weight
of turbine can play the role to change the extracted power with wind speed.
3. Conclusion
The average wind speed in coastal area of Bangladesh is not that much high (ab out 4 m/s). Hence, it can contri-
bute effectively on our energy sectors depending on perfect designing and optimization of wind turbine. An a d-
vantage of VAWTs is that they can catch the wind from all directions and also can withstand much harsher en-
vironments. The paper presents the different designs of rotor and reveals the influence of design parameters on
the mechanical performances of the rotor of wind turbines which can be used for small scale power generation.
The experiments were carried out in wind tunnel using three different model turbines and performance was
measured. The swept area and weight of the rotor can play an important role on the performance of rotor. The
rotor with s mall dia meter c a n get higher r otatio nal sp eed b ut lower torque and lower power, and vice versa rotor
with bigger rotational diameter produces a bigger torque and also extracts high power, but a lower rotational
speed. Moreover, further accurate assembly and proper choosing of blade material will help to achieve better
resul ts. However, blades profile can be improved to have more aerodynamic rotation with an airfoil d esign and
S. Tabassum et al.
Figure 6 . Data of rotational speed (rpm) for variation of wind speeds (v).
Figure 7 . Power vs. wind speed curve.
providing a load can ensure a smoother result. Experimental results can be affected a bit while applying in
coastal area with natural wind source and varying temperature as well.
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