Journal of Applied Science and Engineering, Vol. 16, No. 2, pp. 159-164 (2013)
DOI: 10.6180/jase.2013.16.2.07
Experimental Investigation on Small Horizontal Axis Wind Turbine Rotor Using Winglets Saravanan P.1*, Parammasivam K. M.2 and Selvi Rajan S.3 1
Department of Aeronautical Engineering, Tagore Engineering College, Chennai 600048, India 2 Department of Aerospace Engg, MIT, Anna University, Chennai 600044, India 3 Wind Engineering Laboratory, CSIR-Structural Engg Research Centre, Chennai 600113, India
Abstract The present study explores the possibility of increasing the efficiency of the small horizontal axis wind turbine rotor by adding winglets at the tip of the blade. The effects of changing the winglet configuration with the blade on the power performance of small wind turbine rotor models were investigated experimentally. The blades with four different configurations of winglets are fabricated using Glass Fibre Reinforced Plastic materials and are used for the study. Experiments were conducted for all the rotor models with and without load conditions in the wind tunnel for various conditions. The power output is measured for the rotor models with load conditions. The maximum power coefficient obtained for an effective winglet configuration is about 0.43. It is observed that presence of winglet at the tip of the wind turbine blade will improve the power coefficient for low wind speed regions. It is recommended that the smaller curvature radius with sufficient winglet height added to the wind turbine rotor captures more wind energy in low wind speed region as against wind turbine rotors without winglets. Key Words: Wind Turbine Rotor, Winglets, Rotation Rates, Tip Speed Ratio, Power Coefficient
1. Introduction The force of the wind can be very strong, as can be seen after the passage of a cyclone and its damage over the structures. Historically, people have harnessed this wind force peacefully, it’s most important usage is probably the propulsion of ships using sails before the invention of the steam and internal combustion engine. Wind has also been used in windmills to grind grain or to pump water for irrigation [1]. Small capacity wind turbines need to be affordable, reliable and almost maintenance free for a common man to consider installing one. In a large capacity wind turbine, a generator will be used as a motor to start and accelerate the rotor when the *Corresponding author. E-mail:
[email protected]
wind is strong enough to begin producing power, while in the small capacity wind turbines; the torque produced by the wind acting on the blades of the fan is used to generate power. The small capacity wind turbines are generally located in populated areas, where the wind is weak, due to the presence of buildings and other adjacent obstructions. To yield a reasonable power output from these locations, a small wind turbine should be modified to improve its energy capture, particularly at low wind speeds [2]. This study aims to increase the utilization of small wind turbine rotor, which is most suitable for weak grid areas or remote areas where the electricity is not available and also available average wind speed is low. Normally, the wind power available in the wind is directly proportional to cubic value of wind speed
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(1) Using a wind turbine system, it is possible to extract 59.3% of the total energy in a stream of air according to Betz limit [3]. However, in reality it is possible to have only about 30% efficiency using wind turbine rotor system. As experimentally proved by van Bussel, the presence of winglet shifts the vorticity wake in the downstream direction, leading to power augmentation for horizontal axis wind turbine rotors. Further, the theory shows that significant enhancement in the power can be achieved for higher tip speed ratios [4]. To contradict this statement, Gaunaa & Johansen have summarized in their findings as: “the positive effect of winglets on power production is due to a reduction of tip losses, and is not connected with a downwind shift of wake vorticity [5]”. The objective of the work is to study the behavior of small wind turbine rotor with provision of winglets in terms of self starting of rotor models, rotor rotational rate effect and power coefficient of rotor models as compared to rotors without winglets using wind tunnel.
1.1 Winglets A winglet is a small attachment at the tip of the blade which has the same cross section as the blade. The purpose of attaching the winglet to wind turbine blades is to decrease the total drag from the blades and to increase the aerodynamic efficiency of the turbine. The design of winglet optimizes drag reduction, maximizes power production and minimizes thrust increase. In 1970s Richard Whitcomb first invented winglet concept for aircrafts to decrease drag and increase lift. The provision of winglet for aircrafts yielded 7% increase at cruise speeds [6]. Subsequent research conducted by Mr. Peter Masak of the Soaring Association of Canada has yielded substantial insight into the optimization of size of winglets for low-speed applications [7]. The parameters considered for the design of a winglet are its height, sweep angle, cant angle, curvature radius, toe angle and twist angle as shown in Figure 1 [8]. The winglet concept improves the performance of wind turbine system as predicted in Risø National Laboratory using computational fluid dynamics (CFD) software [9]. It is further reported that winglet height and curvature radius are dominant than the
other parameters. Hence, the present study focuses only on the effects due to variation of winglet height and curvature radius on the performance of a small capacity wind turbine rotor based on this aerodynamic point of view.
2. Experimental Programme 2.1 Wind Turbine Rotor Models Five rotor models of 340 mm diameter with 20 mm hub diameter and 140 mm as the length of the blade are fabricated using Glass Fibre Reinforced Plastic as shown in Figure 2. NACA 4412 profile is used from the blade root to tip with a pitch angle of 5°. The same profile is maintained for all winglet configurations. These winglets are bent towards the suction side with a cant angle of 75°. As mentioned only the effects due to variation in winglet height and curative radians on the performance of the wind turbine are studied. The dimensionless geometrical parameters of winglet configurations W1, W2, W3 and W4 used in this study are presented in Table 1. W0 denotes the case without winglet rotor.
Figure 1. Different parameter involved in design of a winglet.
Figure 2. Blades with different winglets.
Experimental Investigation on Small Horizontal Axis Wind Turbine Rotor Using Winglets
Table 1. The geometrical parameters of the winglets under study Rotor with winglet Winglet height Curvature radius configuration (% rotor radius) (% winglet height) W0 W1 W2 W3 W4
4% 4% 2% 2%
25% 12.5% 25% 12.5%
2.2 Test Setup The experiments were carried out in an open circuit low speed wind tunnel facility at Anna University Chennai. The size of the test section is 0.6 m ´ 0.6 m ´ 2 m. The axial propeller blade is driven by a motor with a speed control unit whose least count is 1 rpm. The velocity at the test section can be adjusted easily by changing the rpm of the axial propeller. The maximum velocity attached in the test section is 40 m/s at 1400 rpm. The turbine blades are attached to hub and this rotor-hub assembly is connected to the rotating horizontal shaft. The rotor shaft is mounted on two support arm using a low friction roller bearing to ensure free rotation at no load condition. The rotor shaft assembly is further mounted on a vertical tower of height of 300 mm and a digital laser tachometer of accuracy 0.02% is housed inside the tower to measure the rotational rates of the rotor. The mean velocity in the wind tunnel test section is measured using a Pitot static tube connected to a digital pressure meter of accuracy 0.2%. The tunnel is calibrated for different propeller rpm with corresponding test section velocities using the digital pressure meter prior to conducting the wind tunnel experiments. The blockage ratio of the experimental arrangement in wind tunnel is estimated to be about 4%. The experimental setup inside the wind tunnel is shown in Figure 3. The electric generator is coupled to the rotor shaft to measure the power output. The load test of all rotor models are carried out under the variable load ranging from 0 W to 4000 W and the corresponding voltage and current are measured using a voltmeter and an ammeter. The wind turbine rotor models are connected with electric load to produce electrical power. The schematic diagram representing the electric circuit is shown in Figure 4.
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2.3 Experimental Procedure The velocity at which each wind turbine rotor model starts rotating is recorded and the rotational rates for all the rotor models at various velocities under no load condition are also recorded. These values of initial starting velocity are shown in Table 1 and plotted in Figure 5. The propeller speed of the wind tunnel is varied from 150 to 500 rpm in steps of 50 rpm to alter the test section velocity in order to measure the rotational rates of the rotor models. The starting velocity at the test section for all the rotor models is presented in Table 2. Power output of the rotor models and their rotational rates are measured keeping for loading the same velocities under electric load conditions (Figure 4). The results of differential rates with load for various values are plotted in Figure 6.
3. Estimation of Power Coefficient The power output rotor models are measured for va-
Figure 3. Experimental setup.
Figure 4. Electric circuit for load test.
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Figure 5. Rotor models rotations without load.
Figure 6. Rotor model rotations with load.
Table 2. Starting velocity at the test section of wind tunnel for the selected winglet configurations Winglet configurations
W0
W1
W2
W3
W4
Initial starting velocity at test section of the wind tunnel (m/s)
4.7
3.7
2.8
4.11
3.7
rious velocities with load. The swept area of all the rotor models is taken as constant since the diameter of all rotor models are kept same. The rotor swept area is 0.091 m2 and this value is used for the estimation of available power in the wind using Eq. (4). Tip speed (Vc) of the rotor blade and tip speed ratio (l) are calculated using the standard equations as given below [10]
(5)
(2)
4. Results and Discussion
(3)
4.1 Self Starting of Rotor Models It is observed that the starting velocities are lower for all the rotor models with provision of winglets compared to rotor without winglet (W0). Among the rotor models with winglets, W1 and W2 configurations are having lower starting velocities compared to the other rotor models with W3 and W4. This is attributed due to the difference in winglet height and curvature radius. The winglet configuration W2 has the lowest starting velocity among all other configurations, since the initial torque needed is more for smaller winglet curvature radius. However it is clearly observed that the self starting velocity is observed
where, N = rotational rates of rotor model (rpm), d = rotor diameter (m), and Vi = initial velocity (m/s). The recorded voltages and currents are used for the computation of power output (P) of all wind turbine rotor models with a constant value of 0.82 as generator efficiency (hg). (4) where V is Voltage (V) and i the current (mA).
The power coefficient (Cp) is estimated from the available power from wind to the generated power from the rotor model and the same is plotted in Figure 7 for various tip speed ratio, l.
Experimental Investigation on Small Horizontal Axis Wind Turbine Rotor Using Winglets
Figure 7. Variation of power coefficient with tip speed ratios.
to be less when winglets are added to the small capacity horizontal axis wind turbine, which is one of the main requirements for low wind speed regimes. The least value of self starting velocity is observed for the winglet configuration of W2 due to its chosen winglet height and curvature radius further research is required to optimize the geometry of a winglet.
4.2 Rotor Rotational Rate Effect There are no significant changes in the measured rotational rates of rotor models for various velocities without load condition based on Figure 5. for all the winglet configurations, compared to rotor with no winglet. In the case of rotor models with load condition, the rotational rates are found to be increased for all the velocity ranges compared to the rotor model without winglet (W0) as can be seen from Figure 6. Among the chosen configurations of winglets, a W2 configuration is found to be more effective for all the range of velocities under rotor model rotations with load. 4.3 Power Coefficient of Rotor Models It is observed from Figure 7 that the values of power coefficient are high for low range of tip speed ratio for rotor models with winglet compared to without winglet. (W0) upto l = 3.0, beyond which the provision of winglet is found to be ineffective. Variation in power coefficient of Figure 7 with the tip speed ratio above 3 is very small and almost all configurations above the tip speed ratio of 3.47 give the same result. That is the reason why the tip speed ratio only covers up to 3.47 in Figure 7 and did not further extend to higher values. Comparing the power coefficient of all the winglet configurations, the
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W2 configuration is found to be more effective. The advantage of extracting power at low wind speed regime can be effectively made use by providing winglets to such small capacity wind turbine. Once the tip speed ratio of 3.05 (the corresponding test section speed is 10 m/s) is reached, the power extraction from the wind stabilizes and the maximum power coefficient is found to be about 0.427. It is here pointed out that beyond a velocity of 9.5 m/s in the test section, significant violent vibration is observed in the rotor models. At tip speed ratio of 1.91, rotor model with winglet configuration W2 produces about 12.8% increase in Cp compared to rotor model without winglet (W0). In low tip speed ratios W2 extracts more power output compared to other rotor models due to its selected geometrical configuration. A maximum power coefficient of 0.43 is reached at optimum tip speed ratio of 3.05 for all the winglet configurations. The winglet height increases the power output of the rotor model and vice versa as observed in the present study. The power coefficient is also improved by decreasing the curvature radius of the winglet. It is noted and recommended that the wind turbine rotors with winglets having smaller curvature radius and higher winglet height captures more wind energy in the low wind speed region as against plain wind turbine rotors without the provisions of winglets.
5. Conclusion The present study indicates a strong relation between winglet height, rotation rate, and curvature radius for the power extraction from wind to have more power co-efficient. The self starting wind speed requirement is less in the case of winglets added to the small horizontal axis wind turbine (W2), which is one of the main requirements for low wind speed regimes. The winglet added configurations have reduced the rotation rates significantly for all tested wind speeds under load conditions. As the winglet height increases the power output also increases and vice versa. The power coefficient is improved by decreasing the winglet curvature radius. It is recommended that the smaller curvature radius with sufficient winglet height added to the wind turbine rotor captures more wind energy in low wind speed region as against plain wind turbine rotors without the provision of winglets.
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6. Application This winglet configuration can be implemented in existing small wind turbine with slight modification in tip to enhance the power production upto about 2% to 6%. The inclusion of winglet in small wind turbine is economically feasible and is best suited where the available wind speed is less and in villages with dense population, which are difficult to reach. Also this type of small wind turbine may be installed in agricultural farms to meet the electricity requirements like preservation of agricultural products for drying the seeds and for charging the battery to improve the farmer’s economy in developing nations, etc.
References [1] Martin, O. L. Hansen., Aerodynamics of Wind Turbines, 2nd ed., Earthscan USA, p. 181 (2008). [2] Shane, M. and Jason G., “Wind Tunnel Analysis of a Counter-Rotating Wind Turbine,” Proc. of ASEE GSW Annual Conference 2009, Texas USA (2009). [3] Betz, A., Wind Energy and Their Utilization by Wind Mills, Vandenhoekk and Rupprecht, Goettingen, Germany (1926). [4] Van Bussel, A., “Momentum Theory for Winglets on Horizontal Axis Wind Turbine Rotors and Some Comparison with Experiments,” Proc. of 4 th IEA Symposium on Aerodynamics of Wind Turbines, Rome, Italy,
pp. 1-18 (1990). [5] Mac, G. and Jeppe, J., “Determination of the Maximum Aerodynamic Efficiency of Wind Turbine Rotors with Winglets,” J. Physics: Conference Series, Vol. 75, p. 12 (2007). doi: 10.1088/1742-6596/75/1/ 012006 [6] Whitcomb, R., A Design Approach and Selected Wind-Tunnel Results at High Subsonic Speeds for Wing-Tip Mounted Winglets, NASA TN D-8260, Washington USA (1976). [7] Peter, M., “Winglet Design for Sailplanes,” Free Flight, Vol. 2, No. 8, ISSN 0827-2557, Retrieved 2006-01-07, Apr/May (1992). Information on: http:// www.soaridaho.com/Schreder/Technical/Winglets/ Masak.htm. [8] Dreese, J., Aero Basics and Designfoil, User guide, European Institute of Education, Capitola, California USA (2000). [9] Jeppe, J. and Sørensen, N. N., Aerodynamic Investigation of Winglets on Wind Turbine Blades Using CFD, Roskilde, Denmark: Risø-R-1543 (EN), Risø National Laboratory, p. 1, Denmark (2006). [10] Vardar, A. and Eker, B., “Principle of Rotor Design for Horizontal Axis Wind Turbines,” Jl. of App. Sci., Vol. 6, No. 7, pp. 1527-1533 (2006). doi: 10.3923/jas. 2006.1527.1533
Manuscript Received: Jan. 10, 2012 Accepted: Sep. 10, 2012
