TWISTED BAMBOO BLADED ROTOR FOR SAVONIUS WIND TURBINES

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1 TWISTED BAMBOO BLADED ROTOR FOR SAVONIUS WIND TURBINES U. K. Saha, P. Mahanta, A. S. Grinspan, P. Suresh Kumar and P. Goswami Department of Mechanical Engineering, Indian Institute of Technology, Guwahati , India. Department of Mechanical Engineering, National Institute of Technology, Rourkela , India. ABSTRACT Due to low rotational speed and low power production, Savonius rotors are lagging in terms of technology from horizontal axis wind turbines. It is, however, believed that with some design modification of the blades, the Savonius types of machines may be quite useful for small-scale power requirement. Preliminary investigation in this direction led the development of a new blade shape with a twist for the Savonius rotor (Grinspan et al., 2001). The twisted blade fabricated from sheet metals has shown its potential as compared to the other conventional blades. In this investigation, twisted blades fabricated from bamboo strips have been tested to find its operational feasibility. Experiments with bamboo bladed rotor show a slightly lower rotational speed as compared to the earlier tested twisted metallic blades. However, the low cost and the ease of fabrication could make this type of design useful for small-scale power generation in rural areas. INTRODUCTION Savonius rotor is a unique fluid machine that has been studied by numerous investigators since 1920s. It can develop a relatively high torque at low rotational speeds and is cheap to build, but it harnesses only a small fraction of the wind energy incident upon it. It is simple to assemble but requires a lot of material in its construction (Reupke and Probert, 1991). Applications of Savonius rotor, in general, includes pumping water, driving an electrical generator, providing ventilation, and agitating water to keep stock ponds ice-free during the winter (Modi and Fernando, 1989; Ogawa et al. 1989; Sadrul Islam, 1993; Spera, 1994; Mennet, 2004). It is also commonly used as a meter to measure the speed of ocean currents. Various types of blades like semicircular, batch type (Ushiyama et al. 1982; Modi and Roth, 1982), lebost type (Mojola, 1985) etc. have been used in vertical axis wind turbine to extract energy from the air, however, no attempt has so far been made in the vertical axis wind turbine systems to reduce the negative torque, and to increase the starting characteristics, and efficiency with the changes in the air direction (Grinspan et al. 2003; Saha and Rajkumar 2004). The use of deflecting plates (Ushiyama et al. 1982, Huda et al. 1992) and shielding to increase the efficiency has not only made the system structurally complex, but also made it dependent on the air direction. Numerous investigations have been undertaken in the past to study the performance characteristics of two and three bucket rotor. These included wind tunnel experiments, field 1

2 experiments and numerical studies. Various types of blade configurations were studied in wind tunnels to evaluate the effect of aspect ratio, blades overlap and gap, effect of adding end extensions, end plates and shielding (Sheldahl et al. 1978; Alexander and Holownia, 1978; Ushiyama et al., 1982; Sayers, 1985; Modi and Farnando, 1989). The aerodynamic performance was also studied by Fujisaw and Gotoh (1994) from the pressure distributions on the blade surfaces at various rotor angles and tip-speed ratios. Mojola (1985) has investigated the performance of Savonius rotor under field conditions. Test data were collected for speed, torque, and power of the rotor at a large numbers of the wind speeds at different overlap ratio, and design criteria was established from the performance data. Detailed experiments have been conducted by Ogawa et al. (1989) to increase the output of a Savonius rotor by using a flow deflecting plate. Fujisaw and Gotoh (1992) studied the power mechanism of Savonius rotor by pressure measurements on the blade surface and by a flow visualization experiments. Kumar and Grover (1993) have investigated a case study of a Savonius rotor for wind power generation. Vishawakarma (1999) attempts to discover an alternate energy option for water pumping, which can be cost-efficient, environment friendly and sustainable. Two types of installations viz., low-speed wind turbines operating piston pumps, and high speed wind turbines driving rotary pumps have been studied. Fernando and Modi (1989) have described a mathematical model based on the discrete vortex method to predict the performance of a stationary and a rototary Savonius configuration. BACKGROUND RESEARCH Recently, some preliminary research works in the area of Savonius wind turbine has been carried out by Sharma (2001). The main objective was to study the performance of Savonius rotor in terms of its rotational speed. Straight blade turbine was tested for comparing its performance with Savonius rotor by keeping the swept area constant. In both the designs, the emphasis was mainly on reducing the cost and simplifying the design and installation, hence to make them affordable to common people in rural areas. Altogether, eight blades have been designed and tested. Experiments include testing of two-bladed Savonius rotor, three-bladed Savonius rotor and three-straight-bladed rotor. From the work of Sharma (2001), both straight and curved blades have been redesigned and tested (Grinspan et al., 2001). In the course of experimentation with these blades, an aerofoil shaped blade and a twisted blade have been designed, fabricated and tested in the same set-up. Thus, in the second phase of research, four different types of blades viz., curved, straight, aerofoil and twisted blades have been tested. Results obtained show that twisted blades at a certain setting angle yield higher rotor speed in comparison to other blade configurations. PRESENT OBJECTIVE In the present investigation, bamboo strips, a local raw material, have replaced the material for the twisted blades. The choice of the material was obviously crucial. Different criteria were considered for this choice: low price, ease of fabrication, low weight and good rigidity. 2

3 Rotational performance assessment of the three-bladed rotor with twisted bamboo blades has been carried out, and results have been compared and analyzed with those of earlier data of Grinspan et al with metal blades. The entire study was carried out at zero load condition and torque was not being measured. The main aim is to explore the feasibility of the twisted bladed rotor as compared to rotors with semicircular blades. BLADE FABRICATION Sheets made out of bamboo strips are used for the fabrication of blades. These sheets are cut to a dimension of 550 mm 350 mm, and all the edges have been stitched properly to ensure proper strength and durability. Both the surfaces of the blades are covered with paper sheets for a good surface finish. In order to ensure the required geometric profile, holes were drilled at the top corners of the blades, which were then pulled by a thin wire to make the required curvature. Similar technique has been used at two other locations along the blade height to maintain the desired twist angle. The pictorial view of the developed twisted bladed Savonius rotor is shown in Fig: 1. Three bladed rotor system is used in the present investigation. The blade shape is shown in Fig. 2 and its top and bottom perimeters are depicted in Fig: 3 Fig 1: Rotor with the twisted blade Fig 2: Profile of the tested blade. 3

4 mm Top Bot t om mm 58 Fig 3: Geometry of the top and bottom perimeters. 0 TEST SETUP The test set-up apparatus consists of the rotor assembly, bearing housing to hold the rotor and the support structure (Fig. 4). This set-up developed initially by Sharma (2001) and has been slightly improved later by Grinspan et al. (2001). The rotor assembly consists of a cylindrical solid mild steel shaft where blades are clamped by means of strips. The shaft consists of three equally spaced mild steel strips to hold the aluminum brackets. The brackets of different lengths are used to hold the twisted blades at different angular positions. A four bladed exhaust fan operating at 220 V and 50 Hz was used as an air source in the present study. The upstream and the downstream velocity profile of the jet was obtained by measuring the velocity of the jet at various points with the help of a pitot-probe. The upstream air velocity is varied by translating the fan axially. The rotational speed of the rotor was measured by a contact digital tachometer. Fig 4: Twisted bladed rotor in the set-up FLOW QUALITY Upstream velocity profile: The surface plot of the velocity profile taken at a distance of 420 mm upstream of the rotor axis (Z-direction) and at a particular setting angle of the blades, θ =18.4 is shown in Fig: 5. The X-axis in the graph represents the distance measured (from the center of the rotor axis) in horizontal direction, and is perpendicular to the rotor axis (Y-axis). The part of the X-axis in the positive side produces the negative torque (Fig. 6). The surface plot shows nearly a symmetric velocity profile on both sides of the rotor axis. Figure 7 shows the average 4

5 velocity (obtained from Simpson s rule) distribution along the X- axis only. The velocity is minimum at the center, and increases with an increase in distance from the rotor axis. Down-Stream Velocity Profile: Similarly, the downstream velocity profile (taken at a distance of 420 mm downstream of the rotor axis, i.e., in Z-direction) at θ =18.4 is shown in Fig: 8. It has been observed that the velocity is more in the positive wetted area side (negative side of X- axis). This is because of the fact that the rotor movement helps the airflow in the positive wetted area side, whereas the flow gets deflected away from its original direction in the negative wetted area side. Figure 9 depicts the average velocity distribution along X-axis only. The downstream velocity profile shows that velocity is more in the positive wetted area side (left of rotor axis). Fig: 5 Upstream air velocity profile at θ = 18.4 Fig: 6 Illustration of positive torque and negative torque 5

6 Fig: 7 Velocity distribution in two dimensions at θ =18.4 Fig: 8 Downstream air velocity profile at θ =18.5 Fig: 9 Velocity distribution of downstream air at θ =18.4 6

7 RESULTS AND DISCUSSION Figure 10 shows the variation of rotational speed (RPM) of the twisted bamboo bladed rotor at various angular settings. It has been seen that at higher setting angles, i.e., when θ = 21.5 and 25.6 the rotational speed (RPM) decreases. The reduction in RPM at higher value of θ can be explained by the less energy capture due to the reduction of both negative and positive wetted area. At setting angle, θ = 18.4 the twisted bamboo blade has shown superior performance as compared to all other setting angles. R.P.M Deg 18.4-Deg 21.5-Deg 25.6-Deg Velocity in m/s Fig: 10 Variation of RPM with velocity for bamboo bladed rotor. The variation of RPM with air velocity for the metal bladed rotor shows its optimum performance at θ = 18.4 (Fig. 11). A direct comparison has also been made for both the cases for optimum setting angle θ = 18.4 (Fig. 12). Similar trends have been observed over the tested range of air velocity with bamboo blades showing lower rotational speed as compared to metal blades. The performance of the rotor was studied by changing the blade surface finish (Table-1). Table-1 Type Blade Surface Characteristics Weight Case-A Covered with sheets on both sides gms Case-B Covered with sheet on concave sides gms Case-C Covered with sheet on convex sides gms Case-D Without covering gms The variation of rotational speed for each of the above cases at setting angle, θ = 18.4 is shown in Fig. 13. It has been observed that the blades with both the surfaces covered with paper sheets shows maximum RPM as compared to other three cases. It is to be noted that the results of bamboo blades with surfaces covered with paper sheets at all the tested setting angles have been presented in Fig

8 R.P.M Deg 18.4-Deg 21.5-Deg 25.6-Deg Velocity in m/s Fig. 11: Variation of RPM with velocity for metal bladed rotor R.P.M Bamboo blades Metalic blades Velocity in m/s Fig: 12 Comparison of RPM for the tested rotors θ = 18.4 R.P.M Velocity in m/s 1. Case-A 2. Case-B 3. Case-C 4. Case-D Fig: 13 Variation of RPM with velocity for different blade configurations at θ =

9 CONCLUSIONS In recent times, there has been an increasing effort to harness the electrical energy from the wind power. In most cases, principally because of their higher efficiency, two-or three-bladed fast running wind turbines are preferred. In general, Savonius rotors are considered as high productive and low technical wind machines and may be the reason why they are often used for water pumping, especially in poor countries and in isolated sites. The present paper deals with the conception of a small Savonius rotor (i.e., of low power) for local production of electricity. Our challenge is to design, develop and ultimately build a small prototype of Savonius rotor in order to produce autonomous electricity (for example to charge batteries). This type of wind machine has not been developed for small installations. Further, in order to increase the energy capture capability, ease the fabrication and reduce the cost of the machine, the conventional semicircular blades are given a twist, and the blades are fabricated out of bamboo mats. The performance of this bamboo bladed rotor has been compared to that of metal bladed rotor having similar shape and setting angle. The rotational speed is found to be marginally lower, however, this may be compensated against the advantages of easy fabrication and low cost. From the present investigation, it can be said that in the tested range of velocity, the available dynamic wind pressure was found to be utilized optimally at θ= The performance of the twisted blades can further be improved by giving a proper surface finish or by providing a smooth surface lamination. However, in order to get more accurate data and verify the design performances of the rotor system, the blades are to be tested in a wind tunnel. Currently, wind tunnel experiments are underway to optimize the twist of the blades on the basis of its performance parameters like torque, coefficient of performance, efficiency etc. REFERENCES Alexander, A. J, and Holownia, B. P., (1978), Wind Tunnel Test on a Savonius Rotor, Journal of Industrial Aerodynamics, Vol.3, pp Fernando, M. S. U. K., and Modi, V. J., (1989), A Numerical Analysis of the Unsteady Flow Past a Savonius Wind Turbine, Journal of Wind energy and Industrial Aerodynamics, Vol.32, pp Fujisawa, N., and Gotoh, F., (1992), Pressure measurement and flow visualization study of Savonius rotor Journal of Wind Energy and Industrial Aerodynamics, Vol.39, pp Fujisawa, N., and Gotoh, F., (1994), Experimental study on the Aerodynamic Performance of a Savonius Rotor, ASME Journal of Solar Energy Engineering, Vol.116, pp Grinspan, A.S., Suresh Kumar, P., Saha, U. K., Mahanta, P., Ratnarao, D.V., and Veda Bhanu, G., (2001), Design, Development and Testing of Savonius Wind Turbine Rotor with Twisted Blades, Proc. of 28 th Nat. Conference on Fluid Mechanics & Fluid Power, Chandigarh, December 13-15, pp Grinspan, A.S., Kumar, P.S., Saha, U.K., and Mahanta, P., (2003), Performance of Savonius Wind Turbine Rotor with Twisted Bamboo Blades, 19 th Canadian Congress of Applied Mechanics, CANCAM 2003, June 01-06, Calgary, Alberta, Canada. 9

10 Huda, M. D., Selim, M. A., Sadrul Islam, A. K. M., and Islam, M. Q., (1992), The Performance of an S-shaped Savonius Rotor with a Deflecting Plate, RERIC International Energy Journal, Vol. 14, pp Kumar, A., and Grover, S., (1993), Performance Characteristics of a Savonius Rotor for Wind Power Generation-A Case Study, Alternate Sources of Energy, Proceeding of Ninth National Convention of Mechanical Engineers, IIT Kanpur. Menet, J. L., (2004), A Double-step Savonius Rotor for Local Production of Electricity: A Design Study, Renewable Energy, Vol. 29, pp Modi, V. J., Roth, N.J., (1982), Prototype Design of a Wind Energy Operated Irrigation System, Proc. of 17 th Intersociety Energy Conversion Engineering Conference, pp Modi, V. J., and Fernando, M. S. U. K., (1989), On the Performance of the Savonius Wind Turbine, ASME Journal of Solar Engineering, Vol. 111, pp Mojola, O. O., (1985), On The Aerodynamics Design of The Savonius Windmill Rotor, Journal of Wind Energy and Industrial Aerodynamics, Vol.15, pp Ogawa, T., Yoshida, H., and Yokota, Y., (1989), Development of Rotational Speed Control Systems for a Savonius-Type Wind Turbine, ASME Journal of Fluids Engineering, Vol. 111, pp Reupke, P., and Probert, S. D., (1991), Slatted-blade Savonius wind-rotors, Applied Energy, Vol. 40, pp Sadrul Islam, A. K. M., Islam, M. Q., Mandal, A. C., Razzaque, M. M., (1993), Aerodynamic Characteristics of a Stationary Savonius Rotor, RERIC International Energy Journal, Vol.15, pp Saha, U.K., and Rajkumar, M.J., (2004), Performance Studies of Twisted Bladed Savonius Rotor, Proc. of 2 nd BSME-ASME International Conference on Thermal Engg., January 2 4, Dhaka, Bangladesh, pp Saylers, A. T., (1985), Blade Configuration Optimization and Performance Characteristics of a Simple Savonius Rotor, Proceeding of Institution of Mechanical Engineers, Vol. 199, pp Sharma, P. K., (2001), Vertical Axis Wind Turbine: Design, Fabrication and Experimental Study of Savonius Rotor and Straight Blade Wind Turbines, B. Tech. Project Report, Mechanical Engineering Department, IIT-Guwahati. Sheldahl, R.E., Blackwell, B. F., and Feltz, L. V., (1978), Wind Tunnel Performance Data for Two and Three Bucket Savonius Rotor, ASME Journal of Energy, Vol.2, pp Spera, D. A., (1994), Wind Turbine Technology, ASME Press. Ushiyama, I., Nagai, H., and Mino, M., (1982), The Optimum Design Configurations of Savonius Wind Turbines, Proceeding of 17 th Intersociety Energy Conversion Engineering Conference, pp Vishwakarma, R., (1999), Savonius Rotor Wind Turbine for Water Pumping-An Alternate Energy Source for Rural Sites, Journal of Institution of Engineers (India), Vol.79, pp