Rachman et al. International Journal of Energy and Environmental Engineering 2013,
2013,: 4:32
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ORIGINAL RESEARCH
Open Access
Simulation on the effect of the blade number
on the rotational characteristic on a horizontal
axis river current turbine
Aditya Rachman*, Ridway Balaka, Jenny Delly and Yuspian Gunawan
Abstract
The power system based on the horizontal axis turbines has been extensively employed to harness the kinetic
energy of the moving fluid to generate the clean renewable energy in several domains, such as wind, river, and
sea. The characteristic of the rotational speed is one of the prominent aspects in designing the power based on the
horizontal axis turbine. This is the characteristic that can determine the quantity of the electricity production in the
generator on the power technology system. The number of blades is another important aspect in designing the
technology. It is believed that the number of blades not only determines the aerodynamic performance, the
construction, and the production cost but also the turbine rotational speed characteristics. This study investigates
the effect of the blade number on the rotational speed characteristic of a horizontal axis river current turbine. The
methodology of generating the investigation is a parametric study using the blade element momentum theory.
The result shows that the turbines with low blade number have high rotational speed characteristics while the
turbines with high blade number have low rotational speed characteristics. The very low attack angle which yields
into a low lift coefficient is one of the factors responsible for the inability of the turbines with high blade number
to operate at high rotation. A further discussion on the results on this investigation relating to the hydrodynamic
behavior is explored in this study. It is recommended that when a turbine is decided to have a high blade number,
the transmission ratio should be high to obtain a high rotation in the generator.
Keywords: River turbine; Design; Blade number; Rotational operation
Background
The role of energy in the economic development in all
nations is so indispensible [1,2]. The industry, transportation, trade, and communication will be posing massive
challenges without the support of energy. Unfortunately,
many regions, especially in developing nations, still seem
to pose the problem of energy access. The topography
and geography characteristic and the condition of a dispersed population on a region are some main obstacles
for an effort on the distribution of energy [3,4].
The power generation based on the river current turbine
is one of the clean decentralized renewable technology potential to supply the energy to the regions where the conditions of the topography, geography, and population become
the obstacles for the distribution of energy. The working
* Correspondence: aditya_rchmn@yahoo.com
Department of Mechanical Engineering of Haluoleo University, Anduonohu
Kendari 93232 Indonesia
principle of this technology is based on the conversion of
the kinetic energy of the river flow into electricity. This
technology requires no level difference (head) and less civil
works and causes less detrimental environmental impacts
than the conventional hydroelectric power [5].
There are two main classes of the turbine configurations: horizontal axis and vertical axis turbines. Some
papers [6-10] show the success of several projects in
generating the electricity from water flow, such as rivers,
tides, and ocean currents using the horizontal and vertical axis turbines. The horizontal turbines have several
beneficial features than the vertical turbines. Some of
these features are higher efficiency, easier start-up, less
torque fluctuations, and higher rotation speeds [11]. A
study [11] reports a survey on research, development,
and demonstration on the water current turbine technology which is conducted presently or had been
dismissed from the stage of the proof concept to the
© 2013 Rachman et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly cited.
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commercialization. The results on this survey show that
the horizontal axis turbines are more frequently
employed than the vertical turbines.
In the mechanism of energy conversion in a modern
horizontal turbine, the fluid kinetic energy is intercepted
by the rotor blades to produce lift, as shown in Figure 1.
This force is utilized to generate a torque, which is used to
drive the generator shaft to produce electricity [12,13].
The mechanism of the energy production in the generator is based on the production of the electromotive
force (EMF). Faraday's law states that the EMF will increase when a permanent magnet is moved relatively to
a conductor, or vice versa, to give the effect of the flux
change. The amount of EMF is proportional to the rate
of the flux change.
The rotational speed characteristic of the generator
shaft plays an essential role in determining the rate of
the flux change. This is because the shaft drives one of
the two components inside the generator, the conductor
or the magnet, to generate the relative movement in
order to produce the flux change. The role of the rotational speed characteristic of the turbine is also salient
in determining the rate of the flux change. This is because generally the shaft of the generator is connected
to the shaft of the turbine via a transmission system.
When the turbine rotational speed is high, the rotational
speed on the generator shaft is possibly high, making the
rate on the flux change to be high and vice versa. Thus,
in the design stage of the power technology based on
the horizontal axis river current, the turbine rotational
speed characteristics should conform to that of the generator rotation speed; otherwise, the rate on the flux
change will fail to meet its requirement, and thus, the
energy production in the generator is highly possibly unsatisfactory, resulting in an inefficient power technology.
Page 2 of 8
Instead of the aerodynamic performance, the manufacturing cost, and the construction, the design parameter
of the blade number seems to have a connection to the
rotational speed characteristics of the turbine. Studies
[14-18] indicate the role of the rotational speed on the
effect of the blade number on the performance of horizontal axis wind turbines. These studies show that the
turbines with high blade number perform better at low
rotation speeds, while those with lower number obtain
high performance at high rotation speeds.
In the water current domain, several papers [6-10,19-21]
show some projects on the power technology based on
horizontal axis turbines, in some fields such as tidal
streams, ocean currents, and river flows, conducted by
several companies and universities. Most of these papers
provide the detailed information of the number of blades
utilized in the projects. However, it seems that the explanation on the connection between the blade number and
the turbine rotational speed cannot be found in these papers. Some of the papers seem to focus on the effect of the
blade number on the hydrodynamic performance.
A study [8] shows a series of pilot projects on horizontal axis river current turbines by the Northern Territory
University, Australia. The projects show that the change
in the blade number is always accompanied by the
change on the transmission ratio. Turbines with higher
blade number require a higher transmission ratio than
that of the turbines with lower blade number. This may
indicate that the change on the transmission ratio is
conducted to maintain the generator rotational speed
when the number of blades is varied. A reference [13]
explains the relation between the tip speed ratio (TSR), a
ratio of the turbine rotational speed and the ambient
flow velocity, and the solidity of rotor, a ratio of the area
of the blades and the rotor swept area. A rotor with a
Figure 1 The mechanism of the energy conversion in a modern horizontal turbine. The lift force and the velocity in a blade. Illustrated by
Rachman adopted in [12,13].
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Page 3 of 8
Coefficient of Performance (CP)
0.5
0.4
0.3
0.2
BEM Model
0.1
Batten et al (2006)
0
0
2
4
6
8
10
12
Tip Speed Ratio (TSR)
Figure 2 The results in the validation of the BEM model to the experimental results in [21].
high speed will have less blade area than the rotor of a
slower machine. Another reference [12] explains that a
low solidity rotor is intended to obtain a high rotational
speed in order to reduce the gearbox ratio. These explanations can assert the indication in the previous references regarding the connection of the rotational speed
and the blade number. As the lower bladed turbine will
obviously have a less blade area than that of the turbine
with high blade number, the rotational speed of the
lower bladed turbine will be higher than that of the turbine with high bladed number. Although some of these
references can indicate more relationship between the
turbine rotational speed and the number of blades, a
comprehensive discussion of the phenomenon seems to
be less explored.
The knowledge on the behavior on the connection between the blade number and the turbine rotational speed
can assist the designer in obtaining an appropriate
decision in the design stage, especially in selecting the
specification of the generator and the transmission ratio
when the blade number is decided, in order to contribute in obtaining a technology with a desirable performance outcome. This study aims to investigate the effect
of the blade number on the turbine rotational speed
characteristic of the horizontal axis river current turbine.
This study also attempts to explore more comprehensively the reasons behind the results of the investigation.
Methods
This investigation is conducted by a parametric study
using the mathematical model of the Blade Element
Momentum (BEM).
Blade element momentum model
In wind domain, there are many approaches, theories,
and models to evaluate the performance of the horizontal axis turbine. The model of BEM is one of the oldest
methods and remains to be the most widely used
Figure 3 Coefficient of performance versus tip speed ratio in the variation of blade number.
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Page 4 of 8
Figure 4 Coefficient of torque versus tip speed ratio in the variation of blade number.
method [22]. This model allows the user to evaluate the
force (lift) acting on the blades, the torque in the shaft,
and the power at the blade with different wind speeds [23].
It is possible to apply this model into the water domain.
One of the reasons is the similarity in the working
principle of the horizontal wind turbine and that of the
water current turbine [24]. The mechanism of the water
current turbine is to extract the kinetic energy of the flow
of water, similar to the mechanism in the wind turbine.
According to [25], the mathematical equation of the water
current turbines is analogous to the wind turbines.
The development of BEM model is based on the combination of the momentum theory and the blade element
theory [12,13]. The momentum theory is developed form
the Bernoulli equation and the momentum balance
equation. Both equations can be applied into not only
air but also water. The blade element theory is based on
the equation of lift and drag forces on the cross section
of a blade. In water domain, the lift and drag force
mechanism has been applied in several applications such
as the boat propeller and the wing on the boat hull.
Validation of BEM model
To validate the BEM model, before being utilized in the investigation, a result from a parametric study using the
BEM model will be compared to the experimental result
obtained from a study in [21]. In this parametric study, the
blade number is set to be three with the diameter of 0.8 m.
The blade angle is 25° at the hub and 5° at the tip (twisted).
The hydrofoil type is NACA 638xx. The water velocity is
1.54 m/s. These data are similar to those in [21].
It is shown that the data obtained from the BEM
model and those of the experimental result in [21] show
a good agreement (see Figure 2). It can be seen that the
curve resulted from the BEM model is close to some of
the data in the experimental result.
Figure 5 Lift forces versus radius (> 0.3 m) in different blade numbers in TSR 11.
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Page 5 of 8
Figure 6 Attack angles versus radius (> 0.3 m) in different blade numbers in TSR 11.
Data of investigation
In the investigation on the effect of the blade number to
the rotational speed characteristic of the river turbine
employing BEM model, the input parameter of the turbine radius is 0.3 m and the hub radius is 0.07 m. The
hydrofoil type is NACA 63415. The data of the drag and
lift coefficients of this hydrofoil are obtained from [26].
The blade width is 0.14 m at the hub and 0.02 m at the
tip (tapered). The blade angle is 23° at the hub and 0° at
the tip (twisted). The water velocity is 2 m/s. The rotational speed of the turbine is varied from 0 to 400 RPM.
The number of blades is varied by two, three, and four.
The result of this parametric study is presented in a
graph illustrating the variation of the rotational speed of
the turbine blade, represented by TSR and the blade
number on the coefficient of performance generated. To
validate the result of this study, it compares to the computational results obtained in [20]. To obtain additional
information that can be used to explain the reasons of
the investigation results, this study presents the data of
Figure 7 The coefficients of (a) drag (b) lift of NACA 63415.
the lift force, the axial rotor flow velocity, and the attack
angle. These additional data are obtained during the calculation of the coefficient of performance using the
BEM model.
MATLAB program is employed to calculate the coefficient of performance and the additional data as required
by the iteration process in some of stages of the BEM
calculation.
Results and discussion
The results on the investigation using BEM model show a
good agreement to the computational results in [20]. Even
the amount of the coefficient of performance is different;
both numerical results show a similar tendency of the higher
rotational speed range of the turbine with lower blade number than that of the turbine with higher blade number. This
is because the power is still possible to be generated at the
high rotational speed for the low-bladed turbine.
The turbine with two blades possibly operates at TSR
of more than 11, while the turbine with four blades
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Axial velocity at rotor (m/s)
2
1.6
1.2
2 B (BEM)
0.8
3 B (BEM)
4 B (BEM)
0.4
0
0.3
0.34
0.38
0.42
0.46
Radius (m)
Figure 8 Axial velocities versus radius (> 0.3 m) in different blade numbers in TSR 11.
seems impossible to operate at TSR of more than 11
(Figure 3). The turbine with two blades still possibly produces the power at TSR of more than 11 as the torque can
still be generated at a corresponding rotation condition
(Figure 4). In contrary, the torque generated for the turbine with four blades at TSR of more than 11 seems to be
highly impossible; thus, the turbine seems to be impossible
to generate satisfactorily the power beyond TSR of 11.
At a high rotational speed (TSR 11), the lift forces in
high radius for the turbine with four blades tend to be
very low (see Figure 5). This results in the low torque
generated of the turbine at the high rotational speed. For
the lower bladed turbines, the lift forces in high radius
are still relatively high at the high rotational speed, making the turbines maintain the torque at corresponding
rotational speed.
Figure 9 The condition of attack angle and axial velocity at a high rotational speed. (a) High-bladed turbine. (b) Low-bladed turbine.
Rachman et al. International Journal of Energy and Environmental Engineering , 2013,
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At corresponding rotational speed condition, the attack angles of the turbine with four blades in high radius
are very low (negative) (see Figure 6). This results in the
low lift coefficient. This is due to the value of the lift coefficient for the NACA 63415 that would be low, even
negative, at the very low attack angle (see Figure 7b). As
the lift coefficient is low or even negative, the lift generated will be low or possibly be negative. For the turbines
with lower blade number, the attack angles for the high
radius are relatively not too low. This possibly makes the
lift still possible to exist, thus making the turbines to still
generate the torque at the high rotational speed.
At TSR of 11, the axial velocities at rotor in the high
radius for the turbine with four blades are much lower
than those of the turbines with less blade number (see
Figure 8). This low axial velocity may cause the low attack angle. The following paragraph explains the relation
of the low axial velocity at rotor and the low attack angle
for the high-bladed turbine at the high rotational speed.
In the BEM model, the amount and the direction of
the wind acting on the turbine blade depend on the sum
vector of the axial velocity and the blade rotational
speed. As both velocities (the axial velocity and the rotational speed) are perpendicular with each other, in the
condition of the very low axial velocity and the high rotational speed, the position of the velocity acting on the turbine blade tends to be closely parallel to the rotational
speed. This causes a low attack angle (see Figure 9).
The condition of the low axial velocity at the high rotational speed for the turbine with high blade number
could be the blockade effect created by the number of
blade area interacted with fluid. For the turbines with
lower blade number, the blockade effect is relatively less.
This makes the lower bladed turbines possible to obtain
a relatively higher attack angle at the high rotational
speed than that of the high-bladed turbine.
Conclusion
This study has investigated the effect of the blade number on the rotational speed characteristic of a horizontal
axis river current turbine. Following are some conclusions that can be drawn.
The small number of blades allows the turbine to operate at the high rotational speed; in contrast, it is impossible for the turbine with high blade number to
operate at the high rotation. One of the factors responsible is the inability of the high-bladed turbine to generate the power at the high rotational speed. This is
because the attack angle at corresponding rotational
speed is very low, resulting in low lift coefficient. In contrary, for the turbine with less blade number, it is possible to obtain a relatively more moderate attack angle at
the high rotational speed, to allow the turbine obtaining
the high lift coefficient at the high rotation speed.
Page 7 of 8
The low rotational speed characteristic for the highbladed turbine requires a high gearing ratio for the
mechanical transmission system. As a result, it is possible to obtain high looses as more friction surfaces can
exist caused by the requirement of the multi-gearing system. This may, thus, reduce the overall river power performance. In addition, the high ratio may add the
dimension of the transmission on the power based on
river current; thus, it may increase the weight which in
turn influences the construction consideration.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
AR developed a mathematical code for BEM Calculation in MATLAB. AR and
RB conducted the parametric study, collected the references and the
informations, and drafted the manuscript. JD and YG corrected the draft
manuscript. All authors read and approved the final manuscript.
Acknowledgements
AR developed a mathematical code for BEM Calculation in MATLAB. AR and
RB conducted the parametric study, collected the references and the
informations, and drafted the manuscript. JD and YG corrected the draft
manuscript. All authors read and approved the final manuscript. In addition:
Thank to the head of computer laboratory of Mechanical Engineering
Department of Haluoleo University due to MATLAB program facility.
Received: 25 March 2013 Accepted: 9 July 2013
Published: 14 Aug 2013
References
1. Oyedepo, S.O.: Efficient energy utilization as a tool for sustainable
development in Nigeria. Int J Energy Environ Eng 3, 11 (2012)
2. Kaundinya, D.P., Balachandra, P., Ravindranath, N.H.: Grid-connected versus
stand-alone energy systems for decentralized power. Renew Sustain Energy
Rev power rev literature 13(8), 2041–2050 (2009)
3. Lal, S., Raturi, A.: Techno-economic analysis of a hybrid mini-grid system for
Fiji islands. Int J Energy Environ Eng 3, 10 (2010)
4. Dasuki, A.S., Djamin, M., Lubis, A.Y.: The strategy of photovoltaic technology
development in Indonesia. Renew Energy 22(1–3), 321–326 (2001)
5. Khan, M.J., Iqbal, M.T., Quaicoe, J.E.: River current energy conversion systems:
Progress, prospects and challenges. Renew Sustain Energy Rev
12, 2177–2193 (2007)
6. Guney, M.S., Kaygusuz, K.: Hydrokinetic energy conversion systems: a
technology status review. Renew Sustain Energy Rev 14, 2996–3004 (2010)
7. Sornes, K.: Small-scale water current turbines for river applications. (2010).
http://www.zero.no/publikasjoner/small-scale-water-current-turbines-forriver-applications.pdf. Accessed 1 January 2012
8. Anyi, M., Kirke, B.: Evaluation of small axial flow hydrokinetic turbines for
remote communities. Energy Sustain Dev 14, 110–116 (2010)
9. Li, D., Wang, S., Yuan, P.: An overview of development of tidal current in
China: energy resource, conversion technology and opportunities. Renew
Sustain Energy Rev 14, 2896–2905 (2010)
10. Rourke, F.O., Boyle, F., Reynolds, A.: Marine current energy devices: current
status and possible future applications in Ireland. Renew Sustain Energy Rev
14, 1026–1036 (2010)
11. Khan, M.J., Bhuyan, G., Iqbal, M.T., Quaicoe, J.E.: Hydrokinetic energy
conversion systems and assessment of horizontal and vertical axis turbines
for river and tidal applications: a technology status review. Appl Energy
86, 1823–1835 (2009)
12. Burton, T., Sharpe, D., Jenkins, N., Bossanyi, E.: Wind Energy Handbook. Wiley,
Chichester (2001)
13. Manwell, J.F., McGowan, J.G., Rogers, A.L.: Wind Energy Explained: Theory,
Design and Application, University of Massachusetts, Amherst, John Wiley &
Son Ltd. (2002)
14. Nishizawa, Y.: An experimental study of the shapes of rotor for horizontal-axis
small wind turbines. In: Ibrahim, A.-B. (ed.) Wind Turbines, vol. 978-953-307-221-0.
Rachman et al. International Journal of Energy and Environmental Engineering , 2013,
:
4:32
http://www.journal-ijeee.com/content///
http://www.journal-ijeee.com/content/4/1/32
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
Page 8 of 8
InTech (2011). http://www.intechopen.com/books/wind-turbines/an-experimentalstudy-of-the-shapes-of-rotor-for-horizontal-axis-small-wind-turbines.
Accessed 1 February 2012
Rachman, A.: Parametric study on a horizontal axis ducted wind turbine.
University of Wollongong, Dissertation (2010)
Atmadi, S., Fitroh, A.J.: Analisis and Optimasi Sudu SKEA 5 KW untuk
Pemompaan. J Teknologi Dirgantara 8, 108–115 (2010)
Wang, S.H., Cheng, S.H.: Blade number effect for a ducted wind turbine.
J Mechanic Sci Technol 221, 1984–1992 (2008)
Swanson, J., Visser, K.: Improving the efficiency of small wind turbines by
optimizing blade number and solidity. (2002). http://www.clarkson.edu/reu/
archives/2002/abstracts/Swanson.pdf
Rachman, A., Balaka, R., Delly, J.: A mathematical model for predicting the
performance of a horizontal axis river current turbine with consideration of
the blade parameters. Proceeding International Conference and Exhibition
Sustainable Energy and Advanced Materials (ICESEAM 2011), Solo, 3–4
October 2011. UNS Indonesia 2011, Solo Indonesia (2011). ISBN 2088–
8864:66
Wu, H., Chen, L., Yu, M., Li, W., Chen, B.: On design and performance
prediction of the horizontal-axis water turbine. Ocean Eng
50(15), 23–30 (2012)
Batten, W.M.J., Bahaj, A.S., Molland, A.F., Chaplin, J.R.: Experimentally
validated numerical, method for the hydrodynamic design of horizontal axis
tidal turbines. Ocean Eng 34, 1013–1020 (2006)
Liu, S., Janajreh, I.: Development and application of an improved blade
element momentum method model on horizontal axis wind turbines.
Int J Energy Environ Eng 3, 30 (2012)
Lanzafame, R., Messina, M.: Horizontal axis wind turbine working at
maximum power coefficient continuously. J Renew Energy
35, 301–306 (2010)
Clark, R.H.: Elements of Tidal-Electric Engineering. Wiley, Pistcataway (2007)
Li, Y., Sander, S.M.: Numerical analysis of the characteristics of vertical axis
tidal current turbines. J Renew Energy 35, 435–442 (2010)
Aerofoil Tool: NACA 63-415 AIRFOIL (n63415-il). (2013). http://airfoiltools.
com/airfoil/details?airfoil=n63415-il
10.1186/2251-6832-4-32
doi:
Cite this article as: Rachman et al.: Simulation on the effect of the blade
number on the rotational characteristic on a horizontal axis river
current turbine. International Journal of Energy and Environmental
Engineering
2013
:. 4:32
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