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Experimental characterization and comparison of performance parameters of S-rotors for standalone wind power system
Energy 138 (2017) 752e763
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Experimental characterization and comparison of performance parameters of S-rotors for standalone wind power system Bilawal A. Bhayo a, Hussain H. Al-Kayiem b, * a b
Mehran University of Engineering and Technology, Shaheed Zulfiqar Ali Bhutto Campus Khairpur Mir's, 66020, Sindh, Pakistan Mechanical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2017 Received in revised form 21 June 2017 Accepted 19 July 2017 Available online 24 July 2017
An open wind flow test of seven Savonius models was conducted to explore the influence of design parameters on the performance and starting characteristics. The seven rotors are coded as Model 1 to Model 7, with various blade designs, number of blades, and number of stages. Results demonstrated that Model 1, a modified two-blade single-stage conventional S-rotor, has the highest power coefficient of 0.26. Model 1 also exhibited an improved maximum power coefficient of about 47% than the previously reported in the literature due to increasing of the aspect ratio from 0.77 to 2.0. The power coefficient of double-stage Savonius-rotors was found to be lower by about 11%e20% than the identical design singlestage S-rotors. The static torque assessment exhibited that double-stage rotors and rotor with more than three blades do not have any negative torque angle position. Nonetheless, the single-stage Savoniusrotor, even with three blades, has few negative torque angles. The study suggests that Model 2, modified double-stage rotor and Model 5, conventional double stage rotor with overlap ratio of 0.2, are most suitable rotors for stand-alone wind power systems, where they have not shown any negative torque angle, in spite that the power coefficient can be sacrificed for the sake of improved starting ability. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Savonius rotor S-rotor Wind turbine Wind energy Vertical axis wind turbine
1. Introduction Today, about 1.3 billion people lack access to electricity [1]. Most of these people are living in remote rural areas, where access to the electric grid is not achievable. Standalone power systems can be one of the options to make the electricity achievable by the poor communities in the rural areas. Wind as a green and tremendous energy source can be exploited for applications to remote rural areas [2]. To generate electric power in rural remote areas by exploiting wind energy, an S-shaped wind rotor, also known as the Savonius type wind rotor, can be a promising solution, especially for the lowwind speed regimes. Ragheb [3] described that the vertical axis wind turbines are being promoted, as they can accept wind from the all directions and do not need yaw mechanisms, rudders or downwind coning. Their electrical generators can be positioned at the ground, and hence easily accessible for the maintenance. A disadvantage is that some designs are not self-starting. Menet and Bourabaa [4] and Akwa et al. [5] have reported the
* Corresponding author. E-mail address: [email protected] (H.H. Al-Kayiem). http://dx.doi.org/10.1016/j.energy.2017.07.128 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
prominent characteristics of the S-rotor in contrast to other modern wind turbines, are as follows: they are simple to be constructed, affordable to construct, independent of wind direction and have low cut in speed due to high starting torque. Also, there is no requirement for a tower supporting structure. However, the particular challenge of the S-rotor is that it has a poor power coefficient and requiring comparatively a larger size to generate electric power equal to that of other modern wind turbines [4,5]. The first ever drag based S-rotor was developed in 1925 by the S. J. Savonius [6]. The inventor claimed that the S-rotor had power coefficient, Cp, of 0.31. Subsequent to Savonius, Bach [7] carried out investigations on S-rotors in 1964 and reported the highest maximum Cp of 0.24. Moreover, the study similar to S. J Savonius [6] was conducted by Blackwell et al. [8], the authors reported the highest Cp of the conventional S-rotor of about 0.24. However, Roy et al. [9] reported that the conventional single stage rotor had maximum Cp of about 0.18. Thus, it can be noticed that the claimed efficiency by S.J. Savonius [6] has not achieved by any of the ensuing researchers. Previous works on improving the performance of Srotors are summarized to understand the possible ways of improving S-rotor performance. Akwa et al. [5] reported that the Cp of an S-rotor is affected by
B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763
Nomenclature As Cp CTd CTs D Do e H N Pelect Protor Re
Rotor swept area (m2) Power Coefficient Dynamic torque coefficient Static torque coefficient Diameter of the rotor (m) End plate diameter (m) Overlap distance of blades Height of the rotor (m) Revolution per minute (rpm) Electrical power output (Watt) Wind rotor power (Watt) Reynolds number
testing conditions, tip speed ratio and geometrical parameters. Their study shows the two-blade single-stage wind rotor with an end plate ratio of 1.1 and an aspect ratio of 2 can result in an improved Cp. Nevertheless, the overlap ratio and the blade shape profile of S-rotors still require attention for further improvement to maximize its exploitation of wind energy. Al-Kayiem and Ming [10] experimentally investigated the Cp of three different S-rotors in open, partially bounded, and fully bounded wind flow. Under open wind flow conditions, Rotor 1, a conventional two-stage S-rotor, exhibited a higher Cp than Rotor 2, a modified conventional S-rotor, and Rotor 3, a newly proposed eight-blade wind rotor. To modify the blades design of simple S-rotor and to propose optimum geometry, Kamoji et al. [11] conducted an experimental study. The maximum average power coefficient of optimum suggested S-rotor without a central shaft was 0.21. The suggested optimum geometrical parameters at the corresponding Reynolds number of 150000 are: a blade arc angle of 124 , blade shape factor of 0.2, overlap ratio of 0, and aspect ratio of 0.77. Static torque analysis showed that S-rotors have negative torque angle positions at which they were unable to start rotating. To minimize negative torque angle positions, one approach is to increase the number of stages at a specified phase shift angle, as shown by Hayashi et al. [12]. They reported the S-rotor with three symmetrical stages has average and less torque variations throughout 360 than the single-stage wind rotor. Increasing the number of stages resulted in a decrease of Cp. The Cp of the threestage S-rotor was 0.75 times to that of the single-stage S-rotor. Their findings are similar to results reported by Blackwell et al. [8], who had conducted a study to minimize negative torque angle positions by increasing the number of blades from two to three. It was found that the two-blade S-rotors with an aspect ratio of 1.5 and an overlap ratio of 0.15 exhibit a maximum Cp of 0.24 at a corresponding free stream Re of 8.64 105. Moreover, the threeblade wind rotor exhibits better starting ability than the twoblade wind rotor, but comparatively at the decrease of Cp of about 50%. A study on the optimum overlap ratio found that rotors with zero overlap have a higher torque, with the disadvantage of poor performance at higher tip speed ratios. Roy and Saha [9] presented an experimental study to improve the blade profile of S-rotors. The investigation reported that the newly developed wind rotor has the highest Cp of about 34.8%, 19.2%, 6.9%, and 3.3% compared with simple semi-circular rotor, semi-elliptic, Benesh, and modified Bach rotors, respectively. It is also reported that all rotors are experiencing negative torque angle positions at which they are unable to self-start. In addition, the static torque variation of the newly developed wind rotor has been
R Ts Td V D/Do e/d H/D
a r m l hg u
753
Rotor radius (m) Static torque (N˖m) Dynamic torque (N˖m) Wind speed (m/s) End plate ratio Overlap ratio Aspect ratio Aspect ratio Density (kg/m3) Dynamic viscosity (kg/m˖s) Tip speed ratio Electric generator efficiency Angular velocity (rad/s) (2pN/60)
found improved. S-rotor for an off-grid system was designed and developed by Menet and Bourabaa [4]. The wind rotor was manufactured from lightweight PVC material. The Cp of the double stage simple S-rotor was 0.29 at an aspect ratio of 2, overlap ratio of 0.26 for each stage, and end plate ratio of 1.1. Summarizing the above discussion, S-rotor can be a promising solution to rural electrification because of its prominent characteristics as a standalone wind-harvesting device. However, generally, the best reported power coefficient of S-rotors is even less than half of the Betz limits. The S-rotors have also negative torque angles because of their convex blades that face the wind, which in turn make starting the wind rotor at specific angles difficult. Thus, researchers can improve S-rotor performance by improving its starting characteristics. Adding the contributions of Alexander and Holownia [13] and the comparison study of Muscolo and Molfino [14] to the above mentioned researches, it could be concluded that the performance and starting characteristics of S-rotors can be improved by improving geometrical parameters, such as an aspect ratio, number of stages, and number of blades, and by improving the blade shape design for aerodynamic performance. To explore the influence of the design parameters on the performance and starting characteristics of the S-rotor, the present experimental study has been conducted. Seven different models of S-rotors have been designed and fabricated, with similar height and diameter so that they could fairly be compared. Performance characteristics of the seven models of the wind S-rotors have been assessed in free stream wind flow. Experiments were designed to allow measurements of flow and operational variables to determine the power coefficients, dynamic torque coefficients, and static torque coefficients. Based on the analysis of the results, the most suitable S-rotors are suggested for stand-alone power systems. 2. Experimental methodology An experimental study was conducted to test real wind rotor models under real operating conditions. The purpose of this experimental approach is to determine the real-time performance and discuss the static and dynamic characteristics of S-rotors. 2.1. Design concept of the models The front and top view of a conventional two-blade S-rotor is presented in Fig. 1. Its dimensions are the same as those of all the seven wind turbine models considered in this study, as shown in Table 1. The wind rotor models are named Model 1, 2, 3, 4, 5, 6, and 7. For a fair comparative study of the considered wind models, the
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B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763 Table 2 Main geometry and basic concept of Model referred to. Model code
NBa
NSb
e/D
Blades Design Concept From
Model Model Model Model Model Model Model
2 2 3 5 2 2 2
1 2 1 1 2 1 2
0 0 0 0 0.16 0.16 0.16
Kamoji et al. [11] Kamoji et al. [11] Vanderhye et al. [15] Al-Kayiem and Ming [10] Menet and Bourabaa [4] New New
a b
(a)
1 2 3 4 5 6 7
NB, Number of Blades. NS, Number of Stages.
(b) Top View
Front View
Fig. 1. Schematic showing the dimensions of a conventional S-rotor (a) with overlap (b) without overlap.
following basic dimensions of all the models are kept identical: aspect ratio of 2 [10,15], end plate ratio of 1.1 [5], and swept area of 0.187 m2. Improvement of the starting characteristics of the models are also investigated by changing the number of stages, following the recommendations of [12,16]. The categorization of the models according to number of blades, number of stages, and overlap ratio with referred authors for blades design concept is presented in Table 2. Model 1 As Fig. 2 shows, Model 1 is a two-blade, single-stage rotor with zero overlap ratio. The basic concept of the blade shape of the model is as suggested by the authors in Ref. [11]. A blade arc angle, j of 124 and a blade shape factor (p/q) of 0.2 are considered for higher curvature. Where, p is the straight part to blade and q is the blade arc radius as shown in Fig. 2. Model 1 has been mainly modified from the rotor tested and characterized by Ref. [11] by changing the aspect ratio from previous 0.77 to new 2.0. It was expected that the aspect ratio of 2 might improve the Cp of previously referred model [4,17]. The total height of Model 1, including its two end plates, each with a thickness of 8 mm, is 628 mm.
Fig. 2. Schematic of isometric and top view of Model 1.
thickness 8 mm and diameter 330 mm. Each stage of the rotor has height 314 mm and the complete rotor was formed by putting one stage over the other with a phase shift angle of 90 . The overall aspect ratio of the Model 2 is same as that of Model 1. However, Model 2 only varies to that of Model 1 in terms of number of stages. Model 3
Model 2 As Fig. 3 shows, the blade shape design of Model 2 is the same as that of Model 1. The model has three Perspex plates of each
Table 1 Basic dimensions of the wind turbines models. Geometry
Value (m)
Geometry
Value (m)
H Do H
0.628 0.330 0.150
D e d
0.300 0 or 0.05 0.01
Fig. 3. Schematic of isometric and top view of Model 2.
B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763
As Fig. 4 shows, Model 3 is a three-blade single-stage wind rotor as recommended by Ref. [15]. This model is tested because of the author's claim that its Cp is twice that of the three-blade conventional S-rotor. The model optimum blade profile geometry suggested by Ref. [15] is as follows: the blade curvature (R/Y), the ratio of the rotor radius to the maximum depth distance is set to 2.5 and the skew factor (X/R), the ratio of the maximum depth location from the centre to the radius of the rotor is set to 0.75. The model aspect ratio and end plate ratio are fixed, like those in other models. Model 4
n ¼ 1 has highest power coefficient and that is greater of about 11% than the conventional semi-circular blade design. Therefore, in the present study, n has been taken 1.0 with little modification in the X/ Y. The X/Y has been slightly increased to introduce the overlap ratio of 0.16, which produced the best performance in terms of overlapping, according to [8]. Other geometrical parameters of this model are similar to other models. Model 6 design is predicted to have minimum drag on the convex side and higher drag on the concave side of the blade. The equation of the profile is given as: y ¼ 1.3845x2 þ 27.69x 33.928
As Fig. 5 shows, Model 4 in this study is a continuation of the work of [10]. The previous model's design was attempted to further improve by varying the number of blades from eight to five and by changing the ratio of the blade offset distance to the blade height from 2.86 to 0.38. Nevertheless, decrease of the number of blades with decrease of the offset distance to blade height ratio has increased the solidity of the rotor by about 20%. The blade shape factor (p/q) is also varied from 15, which is almost straight, to a curvature of 0.2. The total height and diameter of the wind rotor are set to 628 and 300 mm, respectively. The angular shift between the blades is 72 . To mount the blades helically on a shaft, six special pentagon nuts were designed as depicted in Fig. 5d. Each pentagon nut was provided five identical sides of 12 mm width and height 15 mm. The internal diameter of a nut was 10 mm and external diameter was about 20 mm. Model 5 As Fig. 6 shows, Model 5 is a two-blade, double-stage simple Srotor. The model has an overlap of 0.166 to ensure a higher power coefficient with improved starting characteristics at a higher tip speed ratio [8]. Model 6 Model 6, shown in Fig. 7, is new developed design, based on the concept from the numerical studies reported by Refs. [18,19]. The curvature of the Model 6 blades is based on the predicted body drag using Myring Equation [20], where n has been defined as the curvature. When, X/Y is 1.0 and curvature of n ¼ 2, gives the semicircular shape. The authors of [18] predicted that the blades with
755
(1)
where, x is 1.3 x 18.7 and y is 0 y 109 Model 7 As Fig. 8 shows, the blade shape design of Model 7 is the same as that of Model 6. Model 7 is modified from Model 6 only by varying the number of stages from one to two. 2.2. Fabrication procedure of the models The blades of the S-rotor models were fabricated from 1 mm aluminium sheets due to good manufacturability. The Perspex end plates with a radius of 165 mm and a thickness of 8 mm were also used, in which two slots of 2 mm were made for a specific blade profile design by a CNC machine. The sheets were cut as per circumferential distance of the blade design and height as presented in Table 3. To bend the aluminium sheets to the design profile, set of three wooden patterns for Models 3, 6 and 7 was made using a five-axis CNC machine. The aluminium sheets for Model 3, 6 and 7 were first bent to create an impression at the maximum identified depth by a bending machine. Then, hammering to bent sheets was carried out on a set of wooden patterns to mould a sheet to a pattern design. Lastly, finishing to the blades was provided by the manual rolling machines. However, blades of Model 1, 2, 4 and 5 were shaped mainly by manual rolling machines that have rollers of diameter 60 mm and 90 mm. The assembling of the rotor models was carried out by using super glue and bonding epoxies. An end plate was placed on a surface bench and the bended sheets for a rotor were embedded into the blade profile slots. The adhesives were applied to join the end plate and blades together. The procedure of joining blades and end plates was repeated for a complete rotor. The models were also provided machined protruding end shafts of mild steel with a diameter of 10 mm and a height of 150 mm. 2.3. Experimental setup
Fig. 4. Schematic of isometric and top view of Model 3.
An open-wind jet tunnel is used to test the wind rotor models to minimize the blockage effect and to create an environment similar to that of situ conditions, as shown in Fig. 9. The setup consists of two sections: Sections 1 and 2. Section 1 is an open wind tunnel with an air exit area of 0.8 m 0.8 m, where a large propeller fan, of 1.0 m diameter, operated at variable rpms by a 750-watt electric motor to blow the wind up to 32000 m3/hr. Section 1 has a honeycomb of total area of 1 m 1 m, with each cell dimension of 0.2 m 0.2 m for reducing the swirl in airflow caused due to fan operation. By contrast, Section 2 is a test rig and is used to vertically hold the wind models for testing and data acquisition. The test rig is placed at the downstream of the wind tunnel exit. The distance from the wind tunnel exit to wind model is decided as 0.8 m after
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(a) Model Design
(b) Model Top view
(c) Model Front View (d) Pentagon nut
Fig. 5. Schematic of isometric and top view of Model 4 with model front view and pentagonal nut.
Fig. 6. Schematic of isometric and top view of Model 5.
the trial and error method for acquiring a uniform airflow. At this position, the minimum wind velocity variation is obtained as ± 0.2 m/s. Moreover, to make the wind rotor to rotate smoothly in the test rig, the wind model end shafts were embedded into petrol washed ball bearings. The turbulence intensity has been measured, using portable hot-wire anemometer, VT 100 (accuracy of ±3%) from KIMO Instruments, at five locations [ Top Left, Top Right, Bottom Left, Bottom Right, and Center as shown in Fig] just in front of the turbine model. The percentage of determined TI were 4.7%, 5.9%, 7.4%, 5.5% and 4.5% at the corresponding position of Top Left, Top Right, Bottom Left, Bottom Right and Center, respectively. The setup also had equipped with the following measuring instruments: vane anemometer, static torque meter, laser tachometer, and multimeters. A VT 200 vane anemometer from KIMO with a ±1% accuracy rate was used for wind velocity measurement. It consisted of 70 mm diameter vane with a holding vane probe. A TQ8800 static torque meter with accuracy of ±1.5% was clamped with a chuck on the upper protruding wind rotor shaft to measure the static torque. The rpm of the rotor was recorded using a laser tachometer with an accuracy of (±0.05% þ 1 digit). A brushed 24 V DC generator was mounted with the setup to measure the dynamic torque and electric power output. The DC
Fig. 7. Schematic of isometric and top view of Model 6.
Fig. 8. Schematic of isometric and top view of Model 7.
generator was selected based on minimum rated torque required and that should be less than the expected starting torque of the
B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763 Table 3 Blade design circumferential distance and height with the number of end plates required for an S-rotor model fabrication. Rotor# Model Model Model Model Model Model Model
1 2 3 4 5 6 7
Number of end plates
Width of a blade (mm)
Height of a blade (mm)
No of Blades
02 03 02 02 03 02 03
190 190 197 190 275 289 289
628 318 628 176 318 628 318
02 04 03 05 04 02 04
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for plotting a smooth-trend graph. The CTs at each of 15 interval through 360 was also obtained by measuring the static torque at an angle position at a constant Re ¼ 1.2 105 and 1.5 105. The formulas given below illustrate how the power coefficient, tip speed ratio, dynamic torque coefficient, and static torque coefficient were calculated [21]. Power Coefficient,
Cp ¼
Protor 1 2
r V 3 As
Protor ¼ u Td
(2)
(3)
Dynamic Torque,
Td ¼ Pelect: = uhg
(4)
Dynamic Torque Coefficient,
Td
CTd ¼ 1
2 2 r V As R
(5)
Static Torque Coefficient,
CTs ¼
1r 2
Ts V 2 As R
(6)
Tip Speed Ratio, Fig. 9. Schematic of experimental setup (dimensions in cm).
wind models. The speed and torque of the wind models were transmitted to the DC generator using a timing belt and pulley system with a speed ratio of 3.75. The output current and voltages from the generator were measured using multi-ammeters having DC voltage and current sensor accuracy of ±0.1% and ±0.5%, respectively. To measure the electric current, an 18-ohm minimum resistor was connected for the maximum current output. 2.4. Experimental procedure and data reduction The experimental setup, as shown in Fig. 9, was used to test the wind rotor models. The free stream wind velocity was measured using the VT200 vane anemometer. The free stream wind velocity was recorded at nine different points before the wind rotor position at a distance of about 0.2 m, and an average was taken for further discussion. However, the wind velocity at each point was an average of 10 s of recorded data. The rotating speed of the rotors at specific wind speeds was measured using a laser tachometer. For every reading, the tachometer was held for about 30 s, and the maximum recorded rpm were noted. A DC generator was used for dynamic torque and electric power output. Moreover, to measure the static torque variations at each 15 along 360 , the static torque meter TQ-8800 was used. For every reading, the meter was made to record the maximum and minimum torque for about 90 s. The static torque variation measurement procedure was followed for two different wind velocities. The methodology mentioned here was repeated four times, and an average was taken for the data plot to confirm the repeatability of the data for every model. 3. Results and discussion To obtain the Cp and CTd of the seven wind rotor models, parameters, dynamic torques, and rpm were determined by varying the Re. The readings for Cp against tip speed ratio were taken at various pre-defined wind speeds from cut in to maximum available
TSR ¼ Rotor Tip Speed=Wind Speed
(7)
or
l ¼ uR= V
(8)
Reynolds Number,
Re ¼ rV2R=m
(9)
where m is the air dynamic viscosity in kg/m-s (¼ 1.825 105 at 20 C)
3.1. Power coefficient Fig. 10 shows the comparative power coefficient variations for all the seven S-rotor models. The average maximum Cp of Model 1 is superior to those of all the wind rotor models. The increase in the average maximum Cp of Model 1 is about 13%, 56%, 400%, 13%, 41%, and 75% higher than those of Models 2, 3, 4, 5, 6 and 7, respectively. The coefficients, Cp and CTd at the minimum, optimum, and maximum tip speed ratio for each model are shown in Table 4, as achieved from the experimental measurements. Model 1 The minimum Cp of the model was 0.23 at a minimum tip speed ratio of 0.28. An increase in Cp corresponded to an increase in tip speed ratio, and the maximum Cp was 0.26 at a tip speed ratio of 0.42. A further increase in the tip speed ratio caused a decrease in Cp, and the Cp at the maximum tip speed ratio of 0.62 was 0.2. For the similar Re of 105,000, Model 1 had a larger average maximum Cp by about 47% than the previous referred model in Ref. [11], which could be attributed to an increase in the aspect ratio from 0.77 to 2, as reported by Ref. [5]. Model 2
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B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763
possibly because the construction was not followed as suggested. In this regard, further study is suggested to test the design of Model 3 and the conventional three-blade S-rotors under the same conditions. Model 4
Fig. 10. Power coefficient variation of all the seven S-rotor wind models.
Table 4 Values of the Cp and CTd, obtained at the corresponding obtained, (a) minimum TSR, (b), optimum TSR and (c) maximum TSR. Model# Model 1
Model 2
Model 3
Model 4 Model 5
Model 6
Model 7
a b c a b c a b c a c a b c a b c a b c
TSR (l)
Cp
0.28 0.42 0.62 0.43 0.52 0.62 0.21 0.39 0.47 0.1 0.17 0.21 0.55 0.64 0.14 0.46 0.56 0.15 0.36 0.45
0.231 0.260 0.197 0.217 0.230 0.184 0.118 0.167 0.141 0.037 0.052 0.118 0.229 0.206 0.079 0.184 0.165 0.073 0.148 0.141
max
CTd 0.81 0.62 0.32 0.5 0.45 0.3 0.56 0.43 0.3 0.38 0.31 0.56 0.42 0.32 0.58 0.4 0.3 0.48 0.41 0.31
The minimum Cp of the model was 0.22 at a minimum tip speed ratio of 0.43. The maximum Cp obtained for the model was 0.23 at a tip speed ratio of 0.52. A further increase in the tip speed ratio also caused air deflection, causing Cp to decrease. The Cp at the maximum tip speed ratio of 0.62 was 0.18. The two-stage wind rotor had a lower Cp of about 12% than Model 1, a single-stage wind rotor. The decrease in the Cp with the increase in the number of stages could be a result of the fact that the angle positions in only one of the two stages generated torque, and some of the torque generated was lost in making the other stage face the wind. The decrease in Cp caused by the increase in the number of stages was also reported by Ref. [12].
The model achieved a minimum Cp of 0.037 at a minimum tip speed ratio of 0.1. The maximum Cp at the maximum tip speed ratio of 0.17 was 0.052. The full trend for model Cp variation was not achieved because the wind rotor started at a higher wind velocity because of its low solidity. The maximum Cp in this study was about 41% greater than the maximum Cp of the previously referred model [10]. This increase in Cp could be a result of the decrease in the number of blades from eight to five and the change in the design profile of the blades. Model 5 The average maximum Cp of the model was 0.23 at a tip speed ratio of 0.55. However, the minimum Cp at the minimum tip speed ratio was 0.12. The minimum Cp at the maximum tip speed ratio of 0.64 was 0.21. The maximum Cp of this model was almost identical to Model 2, another double-stage rotor. Model 6 A minimum Cp of 0.079 was achieved at the minimum tip speed ratio of 0.14. The average maximum Cp at a tip speed ratio of 0.46 was 0.184. The Cp at the maximum tip speed ratio of 0.56 was 0.165. The model had a decrease in maximum Cp by about 29% less than the two-blade wind Model 1. This decrease in the maximum Cp of Model 6 is a result mainly of the blade shape design. Model 7 A minimum Cp of 0.073 was achieved at a minimum tip speed ratio of 0.15. The average maximum Cp was 0.15 at a tip speed ratio of 0.36. The Cp at the maximum tip speed ratio of 0.45 was 0.14. Model 7, the double-stage rotor, had a decrease in Cp by about 20% less than the same design of the single-stage rotor Model 6. A comparison of the performances of the two-stage models shows that Model 7 had a decrease in its average maximum Cp by about 35% relative to Models 2 and 5, respectively. However, to compare the results obtained from the current study with the experimental findings from other researchers, Cpmax results of Model 1 and Model 6, which are both two-bladed, singlestage rotors have been compared with the published results in the [23e25]. Nevertheless, Table 5 presents the Cpmax and the corresponding TSR of each research work.
3.2. Dynamic torque coefficient Model 3 The trend variation of Cp for the model was similar to those in the other models. In this model, Cp also increased from a minimum of 0.12 to a maximum of 0.17 with an increase of the tip speed ratio from 0.21 to 0.39, respectively. The Cp at the maximum tip speed ratio of 0.47 was 0.14. The Cp of the model was expected to be twice that of the three-blade conventional S-rotor [15]. The Cp of threeblade conventional S-rotor with an aspect ratio of 1.6 was 0.15 [22]. Thus, the Cp of Model 3 was better by about 11% than that of the conventional S-rotor. However, the Cp of the model designed in this study could not achieve the expected value by about 0.3,
Fig. 11 represents the dynamic torque coefficient, CTd variation with the variation of tip speed ratio, l for all the seven S-rotor models. The trend variation of the dynamic torque is almost linear. At the minimum tip speed ratio, the models had maximum CTd. However, at the maximum tip speed ratio, the models had a minimum CTd. Table 4 shows the minimum and maximum CTd of S-rotor models at the corresponding tip speed ratio. The maximum CTd of Model 1 was higher by about 62%, 45%, 113%, 45%, 40%, and 69% than Models 2, 3, 4, 5, 6, and 7, respectively. The CTd of the singlestage model was found higher than that of the double-stage wind rotor model of the same design.
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3.3. Static torque coefficient To analyse the static torque characteristics of the rotor models, the static torque coefficients, CTs were measured at Re of 1.2 105 and 1.5 105 to observe the effect of the Reynolds number on CTs. The static torque was measured at 15 intervals as per the top view diagram of the models shown in Figs. 2e8 for every model. Results of peak, minimum and average static torque coefficients of the seven investigated models are shown in Table 6. The maximum peak torque coefficient was gained by S-rotor Model 7, as 0.72 at low Re. However, the average CTs value, 0.34, of Model 5 was superior to those of all the other wind models, where it is 0.34 at low Re and 0.5 at high Re. The CTs results are demonstrating that all the single-stage two-blade and three-blade models have possessed negative torque at certain angle positions. However, the results of the double-stage and five-blade models did not show any negative torque angle positions. The starting characteristics of the doublestage wind rotors and a five-blade rotor seemed to have improved relative to those of the other models. In spite that the static torque results have been presented in dimensionless form, as CTs, but the results demonstrate that the higher CTs values were achieved at higher Re value. The average values of CTs have been increased by 37.0%, 37.8%, 35.0%, 33.3%, 32%, 41.7% and 33.3% for the models 1 to 7, respectively as Re increased from 1.2 105 to 1.5 105. In general, as the Re increases by around 20%, the mean static torque coefficient of the S-rotors increases by around 35%. The CTs results have been presented in Fig. 12a for Model 1 and Model 2 and in Fig. 12b for Model 6 and Model 7. Arrangement of the results in this manner is meant to allow comparison between single and double stage wind rotors that have similar blade profiles. In case of two stage, as Model 2 and Model 7, the negative torque coefficients have been observed eliminated. Model 1 had peak torque coefficient of 0.7 at angles of 45 and 225 . Bumps of negative torque were found between 150 to 180 and 315 to 360 . At these negative torque angle positions, the rotor became idle, especially at a lower Re. However, at a higher Re, it started rotating after some time because of the vibration effect which boosts the starting to rotate. The average static torque of Model 1 at a higher Re and lower Re test was 0.27 and 0.17, respectively. Given that the Model 2 has two stages, four maximum static torque angle positions existed at 45 , 135 , 225 and 315 and four minimum static torques coefficients exist at angle positions 75 , 165 , 255 , and 345 . Compared with Model 1, which is a singlestage with same blade profile, this model demonstrated no negative torque coefficient. It showed fewer torque variations and a lower peak CTs value of about 26% relative to Model 1. However, the average static torque of Model 2 is higher by about 37% than that of Model 1. Torque characteristics of models 6 and 7 are shown in Fig. 12b. The peak static torque value of Model 6 was 0.6, and the model had a negative torque coefficient from 150 to 180 and from 330 to 360 . The peak negative torque coefficient of the model was 0.22,
Fig. 11. Dynamic torque coefficient variation of all the seven S-rotor wind models.
which was almost the same for both considered Re tests. The average static torque of the model was 0.36 and 0.21 at higher and lower Re tests, respectively. Model 7 had a highest peak CTs value of 0.72 at a low Re than the other wind models. The trend variations of the static torque of the model was almost symmetrical at the higher and lower Re tests, expect for the shift in the peak angle torque position from 45 to 60 and 225 to 240 at 1.2 105 and 1.5 105, respectively. In addition, the peak torque values of the model were about the same. The average static torque values of the model were 0.42 and 0.28 at higher and lower Re tests, respectively. As demonstrated by the two stages Model 2, the two stages Model 7 also eliminated the negative CTs. Torque characteristics of Model 3 are shown in Fig. 13. The static torque variation shows that each blade of the model had an almost similar trend of torque variations. However, each blade of the model had almost similar maximum torque coefficient values, because Model 3 had three identical blades at each 120 . In addition, the model had three bumps of negative torque between 75 to 105 , 195 to 225 , and 315 to 345 . The average static torque of the model at a higher and lower Re test was 0.24 and 0.17, respectively. Fig. 14 shows the torque characteristics of model 4. The model had fewer torque variations than all the other wind rotor models. The model peak torque values at higher Re were higher than those of Model 3, a three-blade wind rotor. However, lower Re tests showed that the trend variation of the CTs is almost similar to that of the tests at higher Re, except for the sudden increase in the CTs at 90 and 270 . In addition, each blade of the five-blade wind rotors was supported from only one end, with the other end free, thereby causing uncontrollable vibration. The unwanted vibration might have caused the increase in static torque value. The average static torque was 0.36 and 0.24 from the tests at Re of 1.5 105 and 1.2 105, respectively. Model 5 torque characteristics are shown in Fig. 15. The model had less torque variations by virtue of being a double-stage wind rotor. The trends of the static torque variations of the model from the higher and lower wind velocity tests were almost similar,
Table 5 Comparison with other experimental researches in terms of maximum power coefficient and the corresponding tip speed ratio. All models are single-stage and two-bladed. Investigator (s)
Cpmax
Corresp. TSR
Remarks
Rahamathullah et al. [23] Wenehenubun et al. [24] Sanusi et al. [25] Current work Model 1 Current work Model 6
0.16 0.24 0.24 0.26 0.18
0.46 0.50 0.70 0.42 0.46
Conventional Savonius with zero overlapping Conventional Savonius with e/d ¼ 0.15 overlapping S-Rotor with e/d ¼ 0.15 overlapping and spline blades Conventional Savonius with zero overlapping S-Rotor with e/d ¼ 0.16 overlapping and parabolic profiled blades
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Table 6 Peak, minimum and average static torque coefficients of the models. Model#
Model Model Model Model Model Model Model
1 2 3 4 5 6 7
Re ¼ 1.2 105
Re ¼ 1.5 105
Percentage of increase in CTs (%)
Max. CTS
Mini. CTS
Average CTS
Max. CTS
Mini. CTS
Average CTs
0.7 0.46 0.47 0.47 0.71 0.60 0.72
0.34 0.24 0.21 0.31 0.47 0.22 0.19
0.17 0.23 0.15 0.24 0.34 0.21 0.28
0.68 0.5 0.42 0.46 0.65 0.61 0.71
0.28 0.24 0.2 0.32 0.45 0.16 0.26
0.27 0.37 0.23 0.36 0.50 0.36 0.42
Model 1 Single stage 2 blades
Model 2 Double stages 2 blades (a) Models 1 & 2
Model 7 Double stages 2 blades
Model 6 Single stage 2 blades (b) Models 6 & 7
Fig. 12. Static torque coefficient at Re 1.2 105 and1.5 105; (a) Models 1 and 2; (b) Models 6 and 7.
37.0 37.8 35.0 33.3 32.0 41.7 33.3
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except at 90 and 225 . The increase in the static torque from the lower wind velocity tests at these particular angles can be justified due to the leakage flow caused by the overlap. The average static torques of the model are 0.5 and 0.34 from tests at Re of 1.5 105 and 1.2 105, respectively. 4. Uncertainty in experimental measurements The uncertainty is generally defined by the absolute error dx. The value of a quantity and its error are often expressed as an interval x ± dx. In the current study, the uncertainty analysis was carried out in two steps as per given procedure by Ref. [26]: - To define the uncertainty associated with an individual measurement. - To define the uncertainty propagated in the arithmetic calculation.
s, the population standard deviation for a finite number of measurements, N, was determined from: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N u1 X s¼ t ðx xÞ2 N i¼1 i
(10)
where, xi was the result of the ith measurement and x was the arithmetic mean of the N repeated measurements. u, absolute standard uncertainty of an individual measurement was determined from:
s
u ¼ pffiffiffiffi N
Fig. 14. Static torque coefficient in polar format of Model 4; 5 short blades.
(11)
To determine the uncertainty of the function, let's say R by accumulating the standard uncertainty in the measured variables, x1, x2, x3 … xn, on which a function based the following mathematical model, was used:
R ¼ f ðx1 ; x2 ; x3 ; …; xn Þ
(12)
Fig. 15. Static torque coefficient in polar format of Model 5; two blades two stages with e/d ¼ 0.166, overlap.
The absolute error dR in the function R is because of the absolute uncertainty dxi in xi and that is expressed in statistical form as:
dR ¼
n X i¼1
Fig. 13. Static torque coefficient in polar format of Model 3; 3 blades, single stage srotor.
dxi
vxi vR
In the present experimental study:
(13)
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B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763
performance and starting ability of seven different S-rotor models in an open wind flow. The study also proposes the suitable configuration and design of S-rotor models for stand-alone power systems. The salient findings of the study are: Model 1, which is modified conventional two-blade and singlestage rotor, possesses the highest power coefficient, of about 0.26. For the same model, it has been demonstrated that increase of the aspect ratio of the blade from 0.77 to 2.0 has resulted in enhanced power coefficient of about 47%. The starting ability of all double-stage S-rotors has shown improved, when compared to single-stage rotors. However, the double-stage rotors have drawback of drop in power coefficient of about 11%e20% compared to the identical single-stage wind model. To mitigate the wobbly rotation of the two blades-single stage, double staging has been proven to be appropriate solution. The increase of the number of blades is less effective than increasing the number of stages. The double-stage wind models have eliminated the negative torque angles. Model 4, helically arranged five-blade rotor has been found to have improved power coefficient of about 41% than the previous referred model from literature due to improving of the blade curvature and increase of the solidity of the rotor to about 20%. In the newly developed double stage wind rotor, Model 7, blade profile has been found to provide an improved average static torque by further minimizing the torque on convex blade and maximizing the torque on concave blade.
Fig. 16. The error associated with the measured power coefficient.
Acknowledgment Fig. 17. The error associated with the static torque coefficient (at Re ¼ 1.5 105).
The authors acknowledge Universiti Teknologi PETRONAS (UTP) for supporting this research financially and technically under the International research agreement with University of Stavanger e Norway, grant URIF 22/2013. References
Fig. 18. The error associated with the static torque coefficient (at Re ¼ 1.5 105).
Cp ¼ f ðTd ; N; VÞ
(14)
CTs ¼ f ðTs ; VÞ
(15)
The error in the Cp can be observed by the indicated error bar. The maximum percentage error in Cp is at the minimum tip speed ratio of about 4.7% as shown in Fig. 16. However, the errors in the CTs are shown in Figs. 17 and 18. It can be observed that the maximum percentage errors in the CTs at higher Re and at lower Re is of about ±7.6% and ±3.9% at the corresponding angles of 135 and 120 , respectively.
5. Conclusion The present experimental study is carried out to assess the
[1] Action P. Prelims poor People's energy outlook 2014. UK: Practical Action Publishing; 2014. p. 32e5. [2] Goudarzi N, Zhu WD. A review on the development of wind turbine generators across the world. Int J Dyn Control 2013;1:192e202. [3] Ragheb M. Vertical axis wind turbines. University of Illinois at UrbanaChampaign; 2011. p. 1. [4] Menet JL. Bourabaa N. Increase in the Savonius rotors efficiency via a parametric investigation. in European wind energy conference. London 2004. [5] Akwa JV, Vielmo HA, Petry AP. A review on the performance of Savonius wind turbines. Renew Sustain Energy Rev 2012;16:3054e64. [6] Savonius SJ. The S-rotor and its applications. Mech Eng 1931;53:333e8. [7] Bach G. Institute BR. Investigations concerning Savonius rotors and related bec: Brace Research Institute, Faculty of machines Ste. Anne de Bellevue. Que Engineering, Macdonald College of McGill University; 1964. [8] Blackwell BF, Sheldahl RF, Feltz LV. Wind tunnel performance data for twoand three-bucket Savonius rotors. United States of America Sandia Laboratories; 1977. [9] Roy S, Saha UK. Wind tunnel experiments of a newly developed two-bladed Savonius-style wind turbine. Appl Energy 2015;137:117e25. [10] Al-Kayiem HH, Ming GJ. Experimental investigation of S-rotors in open and bounded flows. World acade Sci, Eng Tech 2011;60:144e9. [11] Kamoji MA, Kedare SB, Prabhu S. Experimental investigations on single stage modified Savonius rotor. Appl Energy 2009;86:1064e73. [12] Hayashi T, Li Y, Hara Y. Wind tunnel tests on a different phase three-stage Savonius rotor. JSME Int J Ser B 2005;48:9e16. [13] Alexander A, Holownia B. Wind tunnel tests on a Savonius rotor. J Wind Eng Ind Aerodyn 1978;3:343e51. [14] Muscolo GG, Molfino R. From Savonius to Bronzinus: a comparison among vertical wind turbines. Energy Procedia 2014;50:10e8. [15] Vanderhye RA. Dexter MH. Aldrich AL. Rotsky BA. Hascup JR. Three bladed Savonius rotor. United States of America Patent US7314346 B2 2008. [16] Al-Kayiem HH, Bhayo BA, Assadi M. Comparative critique on the design parameters and their effect on the performance of S-rotors. Renew Energy 2016;99:1306e17. [17] Nakajima M, Iio S, Ikeda T. Performance of Savonius rotor for environmentally
B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763 friendly hydraulic turbine. J Fluid Sci Technol 2008;3:420e9. [18] Tian W, Song B, VanZwieten JH, Pyakurel P. Computational fluid dynamics prediction of a modified Savonius wind turbine with novel blade shapes. Energies 2015;8:7915e29. ^do ARL, de Amorim Junior WF, de Lima AGB. Numerical [19] de Sousa JVN, de Mace analysis of turbulent fluid flow and drag coefficient for optimizing the AUV hull design. Open J Fluid Dyn 2014;4:263. [20] Myring D. A theoretical study of body drag in subcritical axisymmetric flow. Aeronaut Q 1976;27:186e94. [21] Percival M, Leung P, Datta P. The development of a vertical turbine for domestic electricity generation. In: European wind energy conference; 2004. p. 22e5. London. [22] Saha U, Thotla S, Maity D. Optimum design configuration of Savonius rotor
[23]
[24]
[25] [26]
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through wind tunnel experiments. J Wind Eng Ind Aerodyn 2008;96: 1359e75. Rahamathullah I. Experimental investigations on the performance characteristics of a modified four bladed Savonius hydro-kinetic turbine. Int J Renew Energy Res (IJRER) 2016;6:1530e6. Wenehenubun F, Saputra A, Sutanto H. An experimental study on the performance of Savonius wind turbines related with the number of blades. Energy Procedia 2015;68:297e304. Sanusi A, Soeparman S, Wahyudi S, Yuliati L. Experimental study of combined blade Savonius wind turbine. Int J Renew Energy Res (IJRER) 2016;6:614e9. Wheeler AJ, Ganji AR, Krishnan VV, Thurow BS. Introduction to engineering experimentation. Prentice Hall New Jersey; 1996.
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Experimental characterization and comparison of performance parameters of S-rotors for standalone wind power system Bilawal A. Bhayo a, Hussain H. Al-Kayiem b, * a b
Mehran University of Engineering and Technology, Shaheed Zulfiqar Ali Bhutto Campus Khairpur Mir's, 66020, Sindh, Pakistan Mechanical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610, Perak, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 April 2017 Received in revised form 21 June 2017 Accepted 19 July 2017 Available online 24 July 2017
An open wind flow test of seven Savonius models was conducted to explore the influence of design parameters on the performance and starting characteristics. The seven rotors are coded as Model 1 to Model 7, with various blade designs, number of blades, and number of stages. Results demonstrated that Model 1, a modified two-blade single-stage conventional S-rotor, has the highest power coefficient of 0.26. Model 1 also exhibited an improved maximum power coefficient of about 47% than the previously reported in the literature due to increasing of the aspect ratio from 0.77 to 2.0. The power coefficient of double-stage Savonius-rotors was found to be lower by about 11%e20% than the identical design singlestage S-rotors. The static torque assessment exhibited that double-stage rotors and rotor with more than three blades do not have any negative torque angle position. Nonetheless, the single-stage Savoniusrotor, even with three blades, has few negative torque angles. The study suggests that Model 2, modified double-stage rotor and Model 5, conventional double stage rotor with overlap ratio of 0.2, are most suitable rotors for stand-alone wind power systems, where they have not shown any negative torque angle, in spite that the power coefficient can be sacrificed for the sake of improved starting ability. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Savonius rotor S-rotor Wind turbine Wind energy Vertical axis wind turbine
1. Introduction Today, about 1.3 billion people lack access to electricity [1]. Most of these people are living in remote rural areas, where access to the electric grid is not achievable. Standalone power systems can be one of the options to make the electricity achievable by the poor communities in the rural areas. Wind as a green and tremendous energy source can be exploited for applications to remote rural areas [2]. To generate electric power in rural remote areas by exploiting wind energy, an S-shaped wind rotor, also known as the Savonius type wind rotor, can be a promising solution, especially for the lowwind speed regimes. Ragheb [3] described that the vertical axis wind turbines are being promoted, as they can accept wind from the all directions and do not need yaw mechanisms, rudders or downwind coning. Their electrical generators can be positioned at the ground, and hence easily accessible for the maintenance. A disadvantage is that some designs are not self-starting. Menet and Bourabaa [4] and Akwa et al. [5] have reported the
* Corresponding author. E-mail address: [email protected] (H.H. Al-Kayiem). http://dx.doi.org/10.1016/j.energy.2017.07.128 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
prominent characteristics of the S-rotor in contrast to other modern wind turbines, are as follows: they are simple to be constructed, affordable to construct, independent of wind direction and have low cut in speed due to high starting torque. Also, there is no requirement for a tower supporting structure. However, the particular challenge of the S-rotor is that it has a poor power coefficient and requiring comparatively a larger size to generate electric power equal to that of other modern wind turbines [4,5]. The first ever drag based S-rotor was developed in 1925 by the S. J. Savonius [6]. The inventor claimed that the S-rotor had power coefficient, Cp, of 0.31. Subsequent to Savonius, Bach [7] carried out investigations on S-rotors in 1964 and reported the highest maximum Cp of 0.24. Moreover, the study similar to S. J Savonius [6] was conducted by Blackwell et al. [8], the authors reported the highest Cp of the conventional S-rotor of about 0.24. However, Roy et al. [9] reported that the conventional single stage rotor had maximum Cp of about 0.18. Thus, it can be noticed that the claimed efficiency by S.J. Savonius [6] has not achieved by any of the ensuing researchers. Previous works on improving the performance of Srotors are summarized to understand the possible ways of improving S-rotor performance. Akwa et al. [5] reported that the Cp of an S-rotor is affected by
B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763
Nomenclature As Cp CTd CTs D Do e H N Pelect Protor Re
Rotor swept area (m2) Power Coefficient Dynamic torque coefficient Static torque coefficient Diameter of the rotor (m) End plate diameter (m) Overlap distance of blades Height of the rotor (m) Revolution per minute (rpm) Electrical power output (Watt) Wind rotor power (Watt) Reynolds number
testing conditions, tip speed ratio and geometrical parameters. Their study shows the two-blade single-stage wind rotor with an end plate ratio of 1.1 and an aspect ratio of 2 can result in an improved Cp. Nevertheless, the overlap ratio and the blade shape profile of S-rotors still require attention for further improvement to maximize its exploitation of wind energy. Al-Kayiem and Ming [10] experimentally investigated the Cp of three different S-rotors in open, partially bounded, and fully bounded wind flow. Under open wind flow conditions, Rotor 1, a conventional two-stage S-rotor, exhibited a higher Cp than Rotor 2, a modified conventional S-rotor, and Rotor 3, a newly proposed eight-blade wind rotor. To modify the blades design of simple S-rotor and to propose optimum geometry, Kamoji et al. [11] conducted an experimental study. The maximum average power coefficient of optimum suggested S-rotor without a central shaft was 0.21. The suggested optimum geometrical parameters at the corresponding Reynolds number of 150000 are: a blade arc angle of 124 , blade shape factor of 0.2, overlap ratio of 0, and aspect ratio of 0.77. Static torque analysis showed that S-rotors have negative torque angle positions at which they were unable to start rotating. To minimize negative torque angle positions, one approach is to increase the number of stages at a specified phase shift angle, as shown by Hayashi et al. [12]. They reported the S-rotor with three symmetrical stages has average and less torque variations throughout 360 than the single-stage wind rotor. Increasing the number of stages resulted in a decrease of Cp. The Cp of the threestage S-rotor was 0.75 times to that of the single-stage S-rotor. Their findings are similar to results reported by Blackwell et al. [8], who had conducted a study to minimize negative torque angle positions by increasing the number of blades from two to three. It was found that the two-blade S-rotors with an aspect ratio of 1.5 and an overlap ratio of 0.15 exhibit a maximum Cp of 0.24 at a corresponding free stream Re of 8.64 105. Moreover, the threeblade wind rotor exhibits better starting ability than the twoblade wind rotor, but comparatively at the decrease of Cp of about 50%. A study on the optimum overlap ratio found that rotors with zero overlap have a higher torque, with the disadvantage of poor performance at higher tip speed ratios. Roy and Saha [9] presented an experimental study to improve the blade profile of S-rotors. The investigation reported that the newly developed wind rotor has the highest Cp of about 34.8%, 19.2%, 6.9%, and 3.3% compared with simple semi-circular rotor, semi-elliptic, Benesh, and modified Bach rotors, respectively. It is also reported that all rotors are experiencing negative torque angle positions at which they are unable to self-start. In addition, the static torque variation of the newly developed wind rotor has been
R Ts Td V D/Do e/d H/D
a r m l hg u
753
Rotor radius (m) Static torque (N˖m) Dynamic torque (N˖m) Wind speed (m/s) End plate ratio Overlap ratio Aspect ratio Aspect ratio Density (kg/m3) Dynamic viscosity (kg/m˖s) Tip speed ratio Electric generator efficiency Angular velocity (rad/s) (2pN/60)
found improved. S-rotor for an off-grid system was designed and developed by Menet and Bourabaa [4]. The wind rotor was manufactured from lightweight PVC material. The Cp of the double stage simple S-rotor was 0.29 at an aspect ratio of 2, overlap ratio of 0.26 for each stage, and end plate ratio of 1.1. Summarizing the above discussion, S-rotor can be a promising solution to rural electrification because of its prominent characteristics as a standalone wind-harvesting device. However, generally, the best reported power coefficient of S-rotors is even less than half of the Betz limits. The S-rotors have also negative torque angles because of their convex blades that face the wind, which in turn make starting the wind rotor at specific angles difficult. Thus, researchers can improve S-rotor performance by improving its starting characteristics. Adding the contributions of Alexander and Holownia [13] and the comparison study of Muscolo and Molfino [14] to the above mentioned researches, it could be concluded that the performance and starting characteristics of S-rotors can be improved by improving geometrical parameters, such as an aspect ratio, number of stages, and number of blades, and by improving the blade shape design for aerodynamic performance. To explore the influence of the design parameters on the performance and starting characteristics of the S-rotor, the present experimental study has been conducted. Seven different models of S-rotors have been designed and fabricated, with similar height and diameter so that they could fairly be compared. Performance characteristics of the seven models of the wind S-rotors have been assessed in free stream wind flow. Experiments were designed to allow measurements of flow and operational variables to determine the power coefficients, dynamic torque coefficients, and static torque coefficients. Based on the analysis of the results, the most suitable S-rotors are suggested for stand-alone power systems. 2. Experimental methodology An experimental study was conducted to test real wind rotor models under real operating conditions. The purpose of this experimental approach is to determine the real-time performance and discuss the static and dynamic characteristics of S-rotors. 2.1. Design concept of the models The front and top view of a conventional two-blade S-rotor is presented in Fig. 1. Its dimensions are the same as those of all the seven wind turbine models considered in this study, as shown in Table 1. The wind rotor models are named Model 1, 2, 3, 4, 5, 6, and 7. For a fair comparative study of the considered wind models, the
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B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763 Table 2 Main geometry and basic concept of Model referred to. Model code
NBa
NSb
e/D
Blades Design Concept From
Model Model Model Model Model Model Model
2 2 3 5 2 2 2
1 2 1 1 2 1 2
0 0 0 0 0.16 0.16 0.16
Kamoji et al. [11] Kamoji et al. [11] Vanderhye et al. [15] Al-Kayiem and Ming [10] Menet and Bourabaa [4] New New
a b
(a)
1 2 3 4 5 6 7
NB, Number of Blades. NS, Number of Stages.
(b) Top View
Front View
Fig. 1. Schematic showing the dimensions of a conventional S-rotor (a) with overlap (b) without overlap.
following basic dimensions of all the models are kept identical: aspect ratio of 2 [10,15], end plate ratio of 1.1 [5], and swept area of 0.187 m2. Improvement of the starting characteristics of the models are also investigated by changing the number of stages, following the recommendations of [12,16]. The categorization of the models according to number of blades, number of stages, and overlap ratio with referred authors for blades design concept is presented in Table 2. Model 1 As Fig. 2 shows, Model 1 is a two-blade, single-stage rotor with zero overlap ratio. The basic concept of the blade shape of the model is as suggested by the authors in Ref. [11]. A blade arc angle, j of 124 and a blade shape factor (p/q) of 0.2 are considered for higher curvature. Where, p is the straight part to blade and q is the blade arc radius as shown in Fig. 2. Model 1 has been mainly modified from the rotor tested and characterized by Ref. [11] by changing the aspect ratio from previous 0.77 to new 2.0. It was expected that the aspect ratio of 2 might improve the Cp of previously referred model [4,17]. The total height of Model 1, including its two end plates, each with a thickness of 8 mm, is 628 mm.
Fig. 2. Schematic of isometric and top view of Model 1.
thickness 8 mm and diameter 330 mm. Each stage of the rotor has height 314 mm and the complete rotor was formed by putting one stage over the other with a phase shift angle of 90 . The overall aspect ratio of the Model 2 is same as that of Model 1. However, Model 2 only varies to that of Model 1 in terms of number of stages. Model 3
Model 2 As Fig. 3 shows, the blade shape design of Model 2 is the same as that of Model 1. The model has three Perspex plates of each
Table 1 Basic dimensions of the wind turbines models. Geometry
Value (m)
Geometry
Value (m)
H Do H
0.628 0.330 0.150
D e d
0.300 0 or 0.05 0.01
Fig. 3. Schematic of isometric and top view of Model 2.
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As Fig. 4 shows, Model 3 is a three-blade single-stage wind rotor as recommended by Ref. [15]. This model is tested because of the author's claim that its Cp is twice that of the three-blade conventional S-rotor. The model optimum blade profile geometry suggested by Ref. [15] is as follows: the blade curvature (R/Y), the ratio of the rotor radius to the maximum depth distance is set to 2.5 and the skew factor (X/R), the ratio of the maximum depth location from the centre to the radius of the rotor is set to 0.75. The model aspect ratio and end plate ratio are fixed, like those in other models. Model 4
n ¼ 1 has highest power coefficient and that is greater of about 11% than the conventional semi-circular blade design. Therefore, in the present study, n has been taken 1.0 with little modification in the X/ Y. The X/Y has been slightly increased to introduce the overlap ratio of 0.16, which produced the best performance in terms of overlapping, according to [8]. Other geometrical parameters of this model are similar to other models. Model 6 design is predicted to have minimum drag on the convex side and higher drag on the concave side of the blade. The equation of the profile is given as: y ¼ 1.3845x2 þ 27.69x 33.928
As Fig. 5 shows, Model 4 in this study is a continuation of the work of [10]. The previous model's design was attempted to further improve by varying the number of blades from eight to five and by changing the ratio of the blade offset distance to the blade height from 2.86 to 0.38. Nevertheless, decrease of the number of blades with decrease of the offset distance to blade height ratio has increased the solidity of the rotor by about 20%. The blade shape factor (p/q) is also varied from 15, which is almost straight, to a curvature of 0.2. The total height and diameter of the wind rotor are set to 628 and 300 mm, respectively. The angular shift between the blades is 72 . To mount the blades helically on a shaft, six special pentagon nuts were designed as depicted in Fig. 5d. Each pentagon nut was provided five identical sides of 12 mm width and height 15 mm. The internal diameter of a nut was 10 mm and external diameter was about 20 mm. Model 5 As Fig. 6 shows, Model 5 is a two-blade, double-stage simple Srotor. The model has an overlap of 0.166 to ensure a higher power coefficient with improved starting characteristics at a higher tip speed ratio [8]. Model 6 Model 6, shown in Fig. 7, is new developed design, based on the concept from the numerical studies reported by Refs. [18,19]. The curvature of the Model 6 blades is based on the predicted body drag using Myring Equation [20], where n has been defined as the curvature. When, X/Y is 1.0 and curvature of n ¼ 2, gives the semicircular shape. The authors of [18] predicted that the blades with
755
(1)
where, x is 1.3 x 18.7 and y is 0 y 109 Model 7 As Fig. 8 shows, the blade shape design of Model 7 is the same as that of Model 6. Model 7 is modified from Model 6 only by varying the number of stages from one to two. 2.2. Fabrication procedure of the models The blades of the S-rotor models were fabricated from 1 mm aluminium sheets due to good manufacturability. The Perspex end plates with a radius of 165 mm and a thickness of 8 mm were also used, in which two slots of 2 mm were made for a specific blade profile design by a CNC machine. The sheets were cut as per circumferential distance of the blade design and height as presented in Table 3. To bend the aluminium sheets to the design profile, set of three wooden patterns for Models 3, 6 and 7 was made using a five-axis CNC machine. The aluminium sheets for Model 3, 6 and 7 were first bent to create an impression at the maximum identified depth by a bending machine. Then, hammering to bent sheets was carried out on a set of wooden patterns to mould a sheet to a pattern design. Lastly, finishing to the blades was provided by the manual rolling machines. However, blades of Model 1, 2, 4 and 5 were shaped mainly by manual rolling machines that have rollers of diameter 60 mm and 90 mm. The assembling of the rotor models was carried out by using super glue and bonding epoxies. An end plate was placed on a surface bench and the bended sheets for a rotor were embedded into the blade profile slots. The adhesives were applied to join the end plate and blades together. The procedure of joining blades and end plates was repeated for a complete rotor. The models were also provided machined protruding end shafts of mild steel with a diameter of 10 mm and a height of 150 mm. 2.3. Experimental setup
Fig. 4. Schematic of isometric and top view of Model 3.
An open-wind jet tunnel is used to test the wind rotor models to minimize the blockage effect and to create an environment similar to that of situ conditions, as shown in Fig. 9. The setup consists of two sections: Sections 1 and 2. Section 1 is an open wind tunnel with an air exit area of 0.8 m 0.8 m, where a large propeller fan, of 1.0 m diameter, operated at variable rpms by a 750-watt electric motor to blow the wind up to 32000 m3/hr. Section 1 has a honeycomb of total area of 1 m 1 m, with each cell dimension of 0.2 m 0.2 m for reducing the swirl in airflow caused due to fan operation. By contrast, Section 2 is a test rig and is used to vertically hold the wind models for testing and data acquisition. The test rig is placed at the downstream of the wind tunnel exit. The distance from the wind tunnel exit to wind model is decided as 0.8 m after
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(a) Model Design
(b) Model Top view
(c) Model Front View (d) Pentagon nut
Fig. 5. Schematic of isometric and top view of Model 4 with model front view and pentagonal nut.
Fig. 6. Schematic of isometric and top view of Model 5.
the trial and error method for acquiring a uniform airflow. At this position, the minimum wind velocity variation is obtained as ± 0.2 m/s. Moreover, to make the wind rotor to rotate smoothly in the test rig, the wind model end shafts were embedded into petrol washed ball bearings. The turbulence intensity has been measured, using portable hot-wire anemometer, VT 100 (accuracy of ±3%) from KIMO Instruments, at five locations [ Top Left, Top Right, Bottom Left, Bottom Right, and Center as shown in Fig] just in front of the turbine model. The percentage of determined TI were 4.7%, 5.9%, 7.4%, 5.5% and 4.5% at the corresponding position of Top Left, Top Right, Bottom Left, Bottom Right and Center, respectively. The setup also had equipped with the following measuring instruments: vane anemometer, static torque meter, laser tachometer, and multimeters. A VT 200 vane anemometer from KIMO with a ±1% accuracy rate was used for wind velocity measurement. It consisted of 70 mm diameter vane with a holding vane probe. A TQ8800 static torque meter with accuracy of ±1.5% was clamped with a chuck on the upper protruding wind rotor shaft to measure the static torque. The rpm of the rotor was recorded using a laser tachometer with an accuracy of (±0.05% þ 1 digit). A brushed 24 V DC generator was mounted with the setup to measure the dynamic torque and electric power output. The DC
Fig. 7. Schematic of isometric and top view of Model 6.
Fig. 8. Schematic of isometric and top view of Model 7.
generator was selected based on minimum rated torque required and that should be less than the expected starting torque of the
B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763 Table 3 Blade design circumferential distance and height with the number of end plates required for an S-rotor model fabrication. Rotor# Model Model Model Model Model Model Model
1 2 3 4 5 6 7
Number of end plates
Width of a blade (mm)
Height of a blade (mm)
No of Blades
02 03 02 02 03 02 03
190 190 197 190 275 289 289
628 318 628 176 318 628 318
02 04 03 05 04 02 04
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for plotting a smooth-trend graph. The CTs at each of 15 interval through 360 was also obtained by measuring the static torque at an angle position at a constant Re ¼ 1.2 105 and 1.5 105. The formulas given below illustrate how the power coefficient, tip speed ratio, dynamic torque coefficient, and static torque coefficient were calculated [21]. Power Coefficient,
Cp ¼
Protor 1 2
r V 3 As
Protor ¼ u Td
(2)
(3)
Dynamic Torque,
Td ¼ Pelect: = uhg
(4)
Dynamic Torque Coefficient,
Td
CTd ¼ 1
2 2 r V As R
(5)
Static Torque Coefficient,
CTs ¼
1r 2
Ts V 2 As R
(6)
Tip Speed Ratio, Fig. 9. Schematic of experimental setup (dimensions in cm).
wind models. The speed and torque of the wind models were transmitted to the DC generator using a timing belt and pulley system with a speed ratio of 3.75. The output current and voltages from the generator were measured using multi-ammeters having DC voltage and current sensor accuracy of ±0.1% and ±0.5%, respectively. To measure the electric current, an 18-ohm minimum resistor was connected for the maximum current output. 2.4. Experimental procedure and data reduction The experimental setup, as shown in Fig. 9, was used to test the wind rotor models. The free stream wind velocity was measured using the VT200 vane anemometer. The free stream wind velocity was recorded at nine different points before the wind rotor position at a distance of about 0.2 m, and an average was taken for further discussion. However, the wind velocity at each point was an average of 10 s of recorded data. The rotating speed of the rotors at specific wind speeds was measured using a laser tachometer. For every reading, the tachometer was held for about 30 s, and the maximum recorded rpm were noted. A DC generator was used for dynamic torque and electric power output. Moreover, to measure the static torque variations at each 15 along 360 , the static torque meter TQ-8800 was used. For every reading, the meter was made to record the maximum and minimum torque for about 90 s. The static torque variation measurement procedure was followed for two different wind velocities. The methodology mentioned here was repeated four times, and an average was taken for the data plot to confirm the repeatability of the data for every model. 3. Results and discussion To obtain the Cp and CTd of the seven wind rotor models, parameters, dynamic torques, and rpm were determined by varying the Re. The readings for Cp against tip speed ratio were taken at various pre-defined wind speeds from cut in to maximum available
TSR ¼ Rotor Tip Speed=Wind Speed
(7)
or
l ¼ uR= V
(8)
Reynolds Number,
Re ¼ rV2R=m
(9)
where m is the air dynamic viscosity in kg/m-s (¼ 1.825 105 at 20 C)
3.1. Power coefficient Fig. 10 shows the comparative power coefficient variations for all the seven S-rotor models. The average maximum Cp of Model 1 is superior to those of all the wind rotor models. The increase in the average maximum Cp of Model 1 is about 13%, 56%, 400%, 13%, 41%, and 75% higher than those of Models 2, 3, 4, 5, 6 and 7, respectively. The coefficients, Cp and CTd at the minimum, optimum, and maximum tip speed ratio for each model are shown in Table 4, as achieved from the experimental measurements. Model 1 The minimum Cp of the model was 0.23 at a minimum tip speed ratio of 0.28. An increase in Cp corresponded to an increase in tip speed ratio, and the maximum Cp was 0.26 at a tip speed ratio of 0.42. A further increase in the tip speed ratio caused a decrease in Cp, and the Cp at the maximum tip speed ratio of 0.62 was 0.2. For the similar Re of 105,000, Model 1 had a larger average maximum Cp by about 47% than the previous referred model in Ref. [11], which could be attributed to an increase in the aspect ratio from 0.77 to 2, as reported by Ref. [5]. Model 2
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possibly because the construction was not followed as suggested. In this regard, further study is suggested to test the design of Model 3 and the conventional three-blade S-rotors under the same conditions. Model 4
Fig. 10. Power coefficient variation of all the seven S-rotor wind models.
Table 4 Values of the Cp and CTd, obtained at the corresponding obtained, (a) minimum TSR, (b), optimum TSR and (c) maximum TSR. Model# Model 1
Model 2
Model 3
Model 4 Model 5
Model 6
Model 7
a b c a b c a b c a c a b c a b c a b c
TSR (l)
Cp
0.28 0.42 0.62 0.43 0.52 0.62 0.21 0.39 0.47 0.1 0.17 0.21 0.55 0.64 0.14 0.46 0.56 0.15 0.36 0.45
0.231 0.260 0.197 0.217 0.230 0.184 0.118 0.167 0.141 0.037 0.052 0.118 0.229 0.206 0.079 0.184 0.165 0.073 0.148 0.141
max
CTd 0.81 0.62 0.32 0.5 0.45 0.3 0.56 0.43 0.3 0.38 0.31 0.56 0.42 0.32 0.58 0.4 0.3 0.48 0.41 0.31
The minimum Cp of the model was 0.22 at a minimum tip speed ratio of 0.43. The maximum Cp obtained for the model was 0.23 at a tip speed ratio of 0.52. A further increase in the tip speed ratio also caused air deflection, causing Cp to decrease. The Cp at the maximum tip speed ratio of 0.62 was 0.18. The two-stage wind rotor had a lower Cp of about 12% than Model 1, a single-stage wind rotor. The decrease in the Cp with the increase in the number of stages could be a result of the fact that the angle positions in only one of the two stages generated torque, and some of the torque generated was lost in making the other stage face the wind. The decrease in Cp caused by the increase in the number of stages was also reported by Ref. [12].
The model achieved a minimum Cp of 0.037 at a minimum tip speed ratio of 0.1. The maximum Cp at the maximum tip speed ratio of 0.17 was 0.052. The full trend for model Cp variation was not achieved because the wind rotor started at a higher wind velocity because of its low solidity. The maximum Cp in this study was about 41% greater than the maximum Cp of the previously referred model [10]. This increase in Cp could be a result of the decrease in the number of blades from eight to five and the change in the design profile of the blades. Model 5 The average maximum Cp of the model was 0.23 at a tip speed ratio of 0.55. However, the minimum Cp at the minimum tip speed ratio was 0.12. The minimum Cp at the maximum tip speed ratio of 0.64 was 0.21. The maximum Cp of this model was almost identical to Model 2, another double-stage rotor. Model 6 A minimum Cp of 0.079 was achieved at the minimum tip speed ratio of 0.14. The average maximum Cp at a tip speed ratio of 0.46 was 0.184. The Cp at the maximum tip speed ratio of 0.56 was 0.165. The model had a decrease in maximum Cp by about 29% less than the two-blade wind Model 1. This decrease in the maximum Cp of Model 6 is a result mainly of the blade shape design. Model 7 A minimum Cp of 0.073 was achieved at a minimum tip speed ratio of 0.15. The average maximum Cp was 0.15 at a tip speed ratio of 0.36. The Cp at the maximum tip speed ratio of 0.45 was 0.14. Model 7, the double-stage rotor, had a decrease in Cp by about 20% less than the same design of the single-stage rotor Model 6. A comparison of the performances of the two-stage models shows that Model 7 had a decrease in its average maximum Cp by about 35% relative to Models 2 and 5, respectively. However, to compare the results obtained from the current study with the experimental findings from other researchers, Cpmax results of Model 1 and Model 6, which are both two-bladed, singlestage rotors have been compared with the published results in the [23e25]. Nevertheless, Table 5 presents the Cpmax and the corresponding TSR of each research work.
3.2. Dynamic torque coefficient Model 3 The trend variation of Cp for the model was similar to those in the other models. In this model, Cp also increased from a minimum of 0.12 to a maximum of 0.17 with an increase of the tip speed ratio from 0.21 to 0.39, respectively. The Cp at the maximum tip speed ratio of 0.47 was 0.14. The Cp of the model was expected to be twice that of the three-blade conventional S-rotor [15]. The Cp of threeblade conventional S-rotor with an aspect ratio of 1.6 was 0.15 [22]. Thus, the Cp of Model 3 was better by about 11% than that of the conventional S-rotor. However, the Cp of the model designed in this study could not achieve the expected value by about 0.3,
Fig. 11 represents the dynamic torque coefficient, CTd variation with the variation of tip speed ratio, l for all the seven S-rotor models. The trend variation of the dynamic torque is almost linear. At the minimum tip speed ratio, the models had maximum CTd. However, at the maximum tip speed ratio, the models had a minimum CTd. Table 4 shows the minimum and maximum CTd of S-rotor models at the corresponding tip speed ratio. The maximum CTd of Model 1 was higher by about 62%, 45%, 113%, 45%, 40%, and 69% than Models 2, 3, 4, 5, 6, and 7, respectively. The CTd of the singlestage model was found higher than that of the double-stage wind rotor model of the same design.
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3.3. Static torque coefficient To analyse the static torque characteristics of the rotor models, the static torque coefficients, CTs were measured at Re of 1.2 105 and 1.5 105 to observe the effect of the Reynolds number on CTs. The static torque was measured at 15 intervals as per the top view diagram of the models shown in Figs. 2e8 for every model. Results of peak, minimum and average static torque coefficients of the seven investigated models are shown in Table 6. The maximum peak torque coefficient was gained by S-rotor Model 7, as 0.72 at low Re. However, the average CTs value, 0.34, of Model 5 was superior to those of all the other wind models, where it is 0.34 at low Re and 0.5 at high Re. The CTs results are demonstrating that all the single-stage two-blade and three-blade models have possessed negative torque at certain angle positions. However, the results of the double-stage and five-blade models did not show any negative torque angle positions. The starting characteristics of the doublestage wind rotors and a five-blade rotor seemed to have improved relative to those of the other models. In spite that the static torque results have been presented in dimensionless form, as CTs, but the results demonstrate that the higher CTs values were achieved at higher Re value. The average values of CTs have been increased by 37.0%, 37.8%, 35.0%, 33.3%, 32%, 41.7% and 33.3% for the models 1 to 7, respectively as Re increased from 1.2 105 to 1.5 105. In general, as the Re increases by around 20%, the mean static torque coefficient of the S-rotors increases by around 35%. The CTs results have been presented in Fig. 12a for Model 1 and Model 2 and in Fig. 12b for Model 6 and Model 7. Arrangement of the results in this manner is meant to allow comparison between single and double stage wind rotors that have similar blade profiles. In case of two stage, as Model 2 and Model 7, the negative torque coefficients have been observed eliminated. Model 1 had peak torque coefficient of 0.7 at angles of 45 and 225 . Bumps of negative torque were found between 150 to 180 and 315 to 360 . At these negative torque angle positions, the rotor became idle, especially at a lower Re. However, at a higher Re, it started rotating after some time because of the vibration effect which boosts the starting to rotate. The average static torque of Model 1 at a higher Re and lower Re test was 0.27 and 0.17, respectively. Given that the Model 2 has two stages, four maximum static torque angle positions existed at 45 , 135 , 225 and 315 and four minimum static torques coefficients exist at angle positions 75 , 165 , 255 , and 345 . Compared with Model 1, which is a singlestage with same blade profile, this model demonstrated no negative torque coefficient. It showed fewer torque variations and a lower peak CTs value of about 26% relative to Model 1. However, the average static torque of Model 2 is higher by about 37% than that of Model 1. Torque characteristics of models 6 and 7 are shown in Fig. 12b. The peak static torque value of Model 6 was 0.6, and the model had a negative torque coefficient from 150 to 180 and from 330 to 360 . The peak negative torque coefficient of the model was 0.22,
Fig. 11. Dynamic torque coefficient variation of all the seven S-rotor wind models.
which was almost the same for both considered Re tests. The average static torque of the model was 0.36 and 0.21 at higher and lower Re tests, respectively. Model 7 had a highest peak CTs value of 0.72 at a low Re than the other wind models. The trend variations of the static torque of the model was almost symmetrical at the higher and lower Re tests, expect for the shift in the peak angle torque position from 45 to 60 and 225 to 240 at 1.2 105 and 1.5 105, respectively. In addition, the peak torque values of the model were about the same. The average static torque values of the model were 0.42 and 0.28 at higher and lower Re tests, respectively. As demonstrated by the two stages Model 2, the two stages Model 7 also eliminated the negative CTs. Torque characteristics of Model 3 are shown in Fig. 13. The static torque variation shows that each blade of the model had an almost similar trend of torque variations. However, each blade of the model had almost similar maximum torque coefficient values, because Model 3 had three identical blades at each 120 . In addition, the model had three bumps of negative torque between 75 to 105 , 195 to 225 , and 315 to 345 . The average static torque of the model at a higher and lower Re test was 0.24 and 0.17, respectively. Fig. 14 shows the torque characteristics of model 4. The model had fewer torque variations than all the other wind rotor models. The model peak torque values at higher Re were higher than those of Model 3, a three-blade wind rotor. However, lower Re tests showed that the trend variation of the CTs is almost similar to that of the tests at higher Re, except for the sudden increase in the CTs at 90 and 270 . In addition, each blade of the five-blade wind rotors was supported from only one end, with the other end free, thereby causing uncontrollable vibration. The unwanted vibration might have caused the increase in static torque value. The average static torque was 0.36 and 0.24 from the tests at Re of 1.5 105 and 1.2 105, respectively. Model 5 torque characteristics are shown in Fig. 15. The model had less torque variations by virtue of being a double-stage wind rotor. The trends of the static torque variations of the model from the higher and lower wind velocity tests were almost similar,
Table 5 Comparison with other experimental researches in terms of maximum power coefficient and the corresponding tip speed ratio. All models are single-stage and two-bladed. Investigator (s)
Cpmax
Corresp. TSR
Remarks
Rahamathullah et al. [23] Wenehenubun et al. [24] Sanusi et al. [25] Current work Model 1 Current work Model 6
0.16 0.24 0.24 0.26 0.18
0.46 0.50 0.70 0.42 0.46
Conventional Savonius with zero overlapping Conventional Savonius with e/d ¼ 0.15 overlapping S-Rotor with e/d ¼ 0.15 overlapping and spline blades Conventional Savonius with zero overlapping S-Rotor with e/d ¼ 0.16 overlapping and parabolic profiled blades
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Table 6 Peak, minimum and average static torque coefficients of the models. Model#
Model Model Model Model Model Model Model
1 2 3 4 5 6 7
Re ¼ 1.2 105
Re ¼ 1.5 105
Percentage of increase in CTs (%)
Max. CTS
Mini. CTS
Average CTS
Max. CTS
Mini. CTS
Average CTs
0.7 0.46 0.47 0.47 0.71 0.60 0.72
0.34 0.24 0.21 0.31 0.47 0.22 0.19
0.17 0.23 0.15 0.24 0.34 0.21 0.28
0.68 0.5 0.42 0.46 0.65 0.61 0.71
0.28 0.24 0.2 0.32 0.45 0.16 0.26
0.27 0.37 0.23 0.36 0.50 0.36 0.42
Model 1 Single stage 2 blades
Model 2 Double stages 2 blades (a) Models 1 & 2
Model 7 Double stages 2 blades
Model 6 Single stage 2 blades (b) Models 6 & 7
Fig. 12. Static torque coefficient at Re 1.2 105 and1.5 105; (a) Models 1 and 2; (b) Models 6 and 7.
37.0 37.8 35.0 33.3 32.0 41.7 33.3
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except at 90 and 225 . The increase in the static torque from the lower wind velocity tests at these particular angles can be justified due to the leakage flow caused by the overlap. The average static torques of the model are 0.5 and 0.34 from tests at Re of 1.5 105 and 1.2 105, respectively. 4. Uncertainty in experimental measurements The uncertainty is generally defined by the absolute error dx. The value of a quantity and its error are often expressed as an interval x ± dx. In the current study, the uncertainty analysis was carried out in two steps as per given procedure by Ref. [26]: - To define the uncertainty associated with an individual measurement. - To define the uncertainty propagated in the arithmetic calculation.
s, the population standard deviation for a finite number of measurements, N, was determined from: vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N u1 X s¼ t ðx xÞ2 N i¼1 i
(10)
where, xi was the result of the ith measurement and x was the arithmetic mean of the N repeated measurements. u, absolute standard uncertainty of an individual measurement was determined from:
s
u ¼ pffiffiffiffi N
Fig. 14. Static torque coefficient in polar format of Model 4; 5 short blades.
(11)
To determine the uncertainty of the function, let's say R by accumulating the standard uncertainty in the measured variables, x1, x2, x3 … xn, on which a function based the following mathematical model, was used:
R ¼ f ðx1 ; x2 ; x3 ; …; xn Þ
(12)
Fig. 15. Static torque coefficient in polar format of Model 5; two blades two stages with e/d ¼ 0.166, overlap.
The absolute error dR in the function R is because of the absolute uncertainty dxi in xi and that is expressed in statistical form as:
dR ¼
n X i¼1
Fig. 13. Static torque coefficient in polar format of Model 3; 3 blades, single stage srotor.
dxi
vxi vR
In the present experimental study:
(13)
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performance and starting ability of seven different S-rotor models in an open wind flow. The study also proposes the suitable configuration and design of S-rotor models for stand-alone power systems. The salient findings of the study are: Model 1, which is modified conventional two-blade and singlestage rotor, possesses the highest power coefficient, of about 0.26. For the same model, it has been demonstrated that increase of the aspect ratio of the blade from 0.77 to 2.0 has resulted in enhanced power coefficient of about 47%. The starting ability of all double-stage S-rotors has shown improved, when compared to single-stage rotors. However, the double-stage rotors have drawback of drop in power coefficient of about 11%e20% compared to the identical single-stage wind model. To mitigate the wobbly rotation of the two blades-single stage, double staging has been proven to be appropriate solution. The increase of the number of blades is less effective than increasing the number of stages. The double-stage wind models have eliminated the negative torque angles. Model 4, helically arranged five-blade rotor has been found to have improved power coefficient of about 41% than the previous referred model from literature due to improving of the blade curvature and increase of the solidity of the rotor to about 20%. In the newly developed double stage wind rotor, Model 7, blade profile has been found to provide an improved average static torque by further minimizing the torque on convex blade and maximizing the torque on concave blade.
Fig. 16. The error associated with the measured power coefficient.
Acknowledgment Fig. 17. The error associated with the static torque coefficient (at Re ¼ 1.5 105).
The authors acknowledge Universiti Teknologi PETRONAS (UTP) for supporting this research financially and technically under the International research agreement with University of Stavanger e Norway, grant URIF 22/2013. References
Fig. 18. The error associated with the static torque coefficient (at Re ¼ 1.5 105).
Cp ¼ f ðTd ; N; VÞ
(14)
CTs ¼ f ðTs ; VÞ
(15)
The error in the Cp can be observed by the indicated error bar. The maximum percentage error in Cp is at the minimum tip speed ratio of about 4.7% as shown in Fig. 16. However, the errors in the CTs are shown in Figs. 17 and 18. It can be observed that the maximum percentage errors in the CTs at higher Re and at lower Re is of about ±7.6% and ±3.9% at the corresponding angles of 135 and 120 , respectively.
5. Conclusion The present experimental study is carried out to assess the
[1] Action P. Prelims poor People's energy outlook 2014. UK: Practical Action Publishing; 2014. p. 32e5. [2] Goudarzi N, Zhu WD. A review on the development of wind turbine generators across the world. Int J Dyn Control 2013;1:192e202. [3] Ragheb M. Vertical axis wind turbines. University of Illinois at UrbanaChampaign; 2011. p. 1. [4] Menet JL. Bourabaa N. Increase in the Savonius rotors efficiency via a parametric investigation. in European wind energy conference. London 2004. [5] Akwa JV, Vielmo HA, Petry AP. A review on the performance of Savonius wind turbines. Renew Sustain Energy Rev 2012;16:3054e64. [6] Savonius SJ. The S-rotor and its applications. Mech Eng 1931;53:333e8. [7] Bach G. Institute BR. Investigations concerning Savonius rotors and related bec: Brace Research Institute, Faculty of machines Ste. Anne de Bellevue. Que Engineering, Macdonald College of McGill University; 1964. [8] Blackwell BF, Sheldahl RF, Feltz LV. Wind tunnel performance data for twoand three-bucket Savonius rotors. United States of America Sandia Laboratories; 1977. [9] Roy S, Saha UK. Wind tunnel experiments of a newly developed two-bladed Savonius-style wind turbine. Appl Energy 2015;137:117e25. [10] Al-Kayiem HH, Ming GJ. Experimental investigation of S-rotors in open and bounded flows. World acade Sci, Eng Tech 2011;60:144e9. [11] Kamoji MA, Kedare SB, Prabhu S. Experimental investigations on single stage modified Savonius rotor. Appl Energy 2009;86:1064e73. [12] Hayashi T, Li Y, Hara Y. Wind tunnel tests on a different phase three-stage Savonius rotor. JSME Int J Ser B 2005;48:9e16. [13] Alexander A, Holownia B. Wind tunnel tests on a Savonius rotor. J Wind Eng Ind Aerodyn 1978;3:343e51. [14] Muscolo GG, Molfino R. From Savonius to Bronzinus: a comparison among vertical wind turbines. Energy Procedia 2014;50:10e8. [15] Vanderhye RA. Dexter MH. Aldrich AL. Rotsky BA. Hascup JR. Three bladed Savonius rotor. United States of America Patent US7314346 B2 2008. [16] Al-Kayiem HH, Bhayo BA, Assadi M. Comparative critique on the design parameters and their effect on the performance of S-rotors. Renew Energy 2016;99:1306e17. [17] Nakajima M, Iio S, Ikeda T. Performance of Savonius rotor for environmentally
B.A. Bhayo, H.H. Al-Kayiem / Energy 138 (2017) 752e763 friendly hydraulic turbine. J Fluid Sci Technol 2008;3:420e9. [18] Tian W, Song B, VanZwieten JH, Pyakurel P. Computational fluid dynamics prediction of a modified Savonius wind turbine with novel blade shapes. Energies 2015;8:7915e29. ^do ARL, de Amorim Junior WF, de Lima AGB. Numerical [19] de Sousa JVN, de Mace analysis of turbulent fluid flow and drag coefficient for optimizing the AUV hull design. Open J Fluid Dyn 2014;4:263. [20] Myring D. A theoretical study of body drag in subcritical axisymmetric flow. Aeronaut Q 1976;27:186e94. [21] Percival M, Leung P, Datta P. The development of a vertical turbine for domestic electricity generation. In: European wind energy conference; 2004. p. 22e5. London. [22] Saha U, Thotla S, Maity D. Optimum design configuration of Savonius rotor
[23]
[24]
[25] [26]
763
through wind tunnel experiments. J Wind Eng Ind Aerodyn 2008;96: 1359e75. Rahamathullah I. Experimental investigations on the performance characteristics of a modified four bladed Savonius hydro-kinetic turbine. Int J Renew Energy Res (IJRER) 2016;6:1530e6. Wenehenubun F, Saputra A, Sutanto H. An experimental study on the performance of Savonius wind turbines related with the number of blades. Energy Procedia 2015;68:297e304. Sanusi A, Soeparman S, Wahyudi S, Yuliati L. Experimental study of combined blade Savonius wind turbine. Int J Renew Energy Res (IJRER) 2016;6:614e9. Wheeler AJ, Ganji AR, Krishnan VV, Thurow BS. Introduction to engineering experimentation. Prentice Hall New Jersey; 1996.