WIND-TUNNEL TESTS OF VERTICAL-AXIS WIND TURBINE BLADES. School of Engineering University of Saint Thomas 2115 Summit Ave Saint Paul, MN

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1 Proceedings of the ASME th International Conference on Energy Sustainability ES2011 August 7-10, 2011, Washington, DC, USA WIND-TUNNEL TESTS OF VERTICAL-AXIS WIND TURBINE BLADES ES B.D. Plourde a, J.P. Abraham a,1, G.S. Mowry a, W.J. Minkowycz b a School of Engineering University of Saint Thomas 2115 Summit Ave Saint Paul, MN b Department of Mechanical Engineering University of Illinois, Chicago Chicago, IL Corresponding Author: jpabraham@stthomas.edu ABSTRACT An ongoing research project is investigating the potential of locating vertical-axis wind turbines (WT) on remote, off-grid cellular communication towers. The goal of the WT is to provide local power generation to meet the electrical needs of the tower. While vertical-axis devices are less efficient than their more traditional horizontal-axis counterparts, they provide a number of practical advantages which make them a suitable choice for the present situation. First, the direction of their axis is aligned with the existing tower and its rotation does not interfere with the tower structure. Second, verticalaxis devices are much less susceptible to the direction of wind and they do not require control-systems to ensure they are oriented correctly. Third, vertical-axis turbines have very low start-up wind speeds so that they generate power over a wide range of speeds. Fourth, since vertical-axis turbines rotate at a slower speed compared with horizontal counterparts, they impart a lessened vibration load to the tower. These facts, collectively, make the vertical-axis turbine suitable for the proposed application. The design process involved a detailed initial design of the turbine blade using computational methods. Next, a trio of designs was evaluated experimentally in a large, low-speed wind tunnel. The wind tunnel is operated by the University of Minnesota s St. Anthony Falls Fluid Laboratory. The tunnel possesses two testing sections. The larger section was sufficient to test a full-size turbine blade. Accounting was taken of the blockage effect following the tests. The experiments were completed on (1) a solid-wing design (unvented), (2) a slotted-wing design (vented), and (3) a capped-and-slotted design (capped). Conditions spanned a wide range of wind speeds ( m/s). The turbines were connected to electronics which simulated a range of electrical loads. The tested range was selected to span the expected range of resistances which will be found in practice. It was discovered that over a range of these wind speeds and electrical resistances, slots located on the wings result in a slight improvement in power generation. On the other hand, the slotted-and-capped design provided very large increases in performance (approximately % compared with the unvented version). This large improvement has justified commercialization of the product for use in powering remote, off-grid cellular communication towers. KEYWORDS: Vertical-axis wind turbines, cellular communication towers, wind-power production 1 Copyright 2011 by ASME

2 INTRODUCTION The location of cellular communication towers in remote regions present a significant challenge to the tower operation because grid-connected electricity may be unavailable, or may be intermittent. Such a situation occurs both in the developed and developing worlds. There are slight differences in applications in these two sectors of the world. In the developed world, off-grid towers typically are rurally located and no grid-connected electricity is available. On the other hand, in the developing world, towers may be located in urban areas however the reliability of the grid electricity is low and frequent power outages occur. is difficult to attached to a vertical structure such as the communication towers of the present research project. The proposed VAWT is designed to avoid the abovereferenced shortcomings. It is to be positioned on existing communication towers so that tower construction costs would be avoided. The power generation goal for the devices is 1-1.5kW in windspeeds of m/s. An illustration of the wind turbine on a communication tower is shown in Figure 1. In either case, power must be provided intermittently or permanently by an external power source, typically diesel generation. Diesel power has a number of severe drawbacks. First, it is expensive with fuel costs alone amounting to thousands of dollars per year. Second, diesel fuel is extremely valuable in the developing world so that communications companies often require guards at the tower. Finally, diesel power leads to the emissions of airborne particulates and greenhouse gases (carbon dioxide) which are well known to cause climate change. These challenges have motivated the design, testing, and fabrication of a novel, vertical-axis wind turbine (VAWT) which is particularly suited for use on communication towers. The VAWT is rotated by drag forces and is often referred to as a Savonius turbine. The turbine is designed to provide local power to the tower and has the capability of storing power when the rate at which energy is generated exceeds the rate at which power is utilized by the tower. VAWT systems differ markedly from their more common horizontal-axis counterparts. Horizontal wind turbines are used to generate electricity either locally (small scale) or are used in large wind farms with multi-megawatt turbines. The mode of operation for horizontal-axis wind turbines (HAWT) differs from those considered here. First, HAWT systems allow rotation from lift forces generated at the wing-surface. Lift-forces allow the turbine to rotate at very high rates and generate a commensurate amount of electrical power. Because HAWTs utilize lift forces, they have a relatively high efficiency. This fact makes them the standard windturbine system for many applications. On the other hand, there are some key deficiencies with HAWT that prohibit their use in the present application. First, HAWTs are sensitive to the direction of wind flow. They must be actively controlled to face the wind. Second, HAWTs have a startup wind speed which is required to begin rotation. When wind speeds are below the start-up speed, HAWTs are unable to generte power. Third, HAWTs fast rotation can cause damaging vibrational loading to be imparted to the tower and this may cause mechanical failure. Finally, the HAWT rotor Figure 1: A VAWT positioned alongside an existing communication tower. The VAWT shown schematically in Figure 1 and dealt with in this study uses wind drag forces to cause the rotation (Savonius style turbine). The turbine positions a large frontal area into the air stream and the contour of the blade preferentially captures air on the retreating side of the blade. The unequal drag forces on the retreating and advancing blades gives rise to a torque and subsequent rotation. More information on the blade design will be given later in this report. In order to accommodate a wide range of powerconsumption requirements and wind speeds, the device was designed in a modular fashion. Each module consists of a pair of advancing and retreating blades separated by 180 degrees. Adjacent modules are offset by approximately 30 degrees so that the modules are staggered. Modules can be easily added or removed if more or less power is required by the communication 2 Copyright 2011 by ASME

3 tower. The photograph of an early version VAWT shown in Figure 2 clearly illustrates the modular fashion of the turbine system and the staggered arrangement of the adjacent modules. Each module is approximately 1.3 meters in height and 1.1 meters in diameter. The electrical and mechanical systems are designed to be able to support up to four modules. correction factors must be employed to deal with the blockage effect of turbines in the wind tunnel. More recent work has investigated multistage rotor systems and the use of one-way valves positioned in the turbine blades. These valves allow air to pass through the blade during its return stroke, which should provide an increase in generation performance. Other studies have investigated the effect of blade twist on the power generation capacity of turbines [11, 13-15]. The work presented here is an advancement in the present state-of-the-art. Our study involves the use of full-size turbine systems tested in a large wind tunnel. The testing included an electrical system which simulated the presence of a communication tower. In addition, our study tested three versions of the turbine. The first version is equivalent to the rotor shown in Figure 1 and is hereafter termed unvented. A second version (vented) was modified by incorporation of a slit-like vent in the rotor. The vent was tailored to promote air flow through the blade, particularly during the return stroke. The last version (capped) incorporated caps which were located at the top and bottom of the rotor blade to reduce air bypass and increase efficiency. Figure 2: A photograph showing a three-section turbine installed on a communication tower. The staggered arrangement aids in the start-up of the turbine in low wind speeds and avoids the potential of turbine stall. In an effort to create a viable communication tower power system, an efficient blade design is required. The design process involved a collaborative effort of experimentation and numerical simulation. The focus of the present work is on the experimental aspects of the process. The work presented here complements other research which has been carried out in the past. A number of optimization studies have been completed and [1-8] are representative of that body of work. Most of those studies deal with halfcylinder blades fabricated by cutting a cylindrical drum in halves. In some occasions, there were novel features such as deflection plates which were intended to increase powergeneration efficiency. A number of studies have used wind tunnel tests to experimentally determine the turbine efficiency [2, 9-12]. In some instances, the turbines were placed downstream of the wind tunnel exit. In other cases, the turbine was placed with an enclosed test section of the tunnel. In those cases, THE EXPERIMENTS Most commonly, Savonius turbines have been tested on small-scale replicates in wind tunnels. As mentioned earlier, the turbines are often positioned at the wind tunnel exit. Sometimes, the turbine is located within the tunnel itself but this practice requires the use of a blockage correction factor which is typically not known with great certainty. All experiments were carried out in a large-scale wind tunnel whose test section dimensions are 2.44 by 2.44 meters. This size was sufficient to test one of the rotor modules. An illustration of the wind tunnel facility is shown in Figure. 3. The figure shows a closed wind tunnel with two long test sections. The test sections differ in cross-sectional size. Test section 2 was used for the present experiments. A more detailed description of the tests is provided in [16]. 3 Copyright 2011 by ASME

4 Figure 5 has been prepared to provide information on the rotor dimensions. The image shows a cross-sectional view with relevant dimensional notation which is linked to the values set forth in Table 1. Figure 3: Schematic of the wind tunnel [16]. The wind tunnel is capable of providing wind speeds up to 19m/s; the tests which were completed ranged from 4.53 to 11.8 m/s. At each wind speed, the electrical resistance was varied from 19.4 to 78.7 ohms to replicate a wide range of potential communication tower electronics. All three windturbine models were tested over the wind-speed and electrical resistance ranges and the power produced by each variant was determined. Figure 5: Computer generated view of venting slot and blade dimensions. The image shown in Figure 5 illustrates that the vents are chamfered so that it is easier for wind pass in one direction compared to the other. The positioning of the vents was based on fluid dynamic simulations. These simulations determined how vents could be incorporated to reduce negative drag and thrust loading on the tower structure, without a reduction in power generation. Figure 4 shows a photograph of the capped rotor. It is seen that there are longitudinal vents aligned with the axis of rotation. Also, there are circular caps at the upper and lower edges of the rotor. Table 1: Relevant dimensions for the turbine blade L1 L2 L3 R θ 0.92 m (36 in.) 0.41 m (16 in.) 0.10m (4 in.) 0.20 m (8 in.) 120 degrees A list of the full suite of operating parameters is contained in Table 2. They show four wind speeds and four electrical resistance settings. Values of wind speeds listed in the table are uncorrected because they have not been augmented by the increase in wind which occurs because of the blockage. The effect of this blockage is that the actual wind speed must be increased by a ratio of the total wind tunnel cross section to the open (unblocked) area. That factor was calculated to be As a consequence, corrected wind speeds are 23% higher than the uncorrected values listed in Table 2. Table 2: List of operating parameters used in experiments Electrical Resistance Uncorrected Wind Speed (ohm) (m/s) Figure 4: Photograph of a capped and vented blade [16]. 4 Copyright 2011 by ASME

5 RESULTS AND DISCUSSION The first set of results to be presented is for the unvented case. Those results are provided in Figure 6. There, four individual curves are seen, each of which corresponds to a different electrical resistance. It can be seen that there is a strong, and expected, dependence of power generation with wind speed. It is also seen that there is only a moderate dependence of power generation on the electrical load. One common measure of performance is the powergeneration efficiency. That efficiency is defined as the ratio of the extracted electrical power compared to the total rate at which kinetic energy flows past the turbine. = (1) Efficiency values of 6-10 % were calculated for the unvented wind turbine. This range of efficiencies is significantly lower than other experimental investigations that often report efficiency values in the 10-15% range. There are some reasons for the low performance of the present unvented design. First, it does not have a cap on the top and bottom edges, a feature which most Savonius-style turbines possess, at least in some form. Second, our experiments included a proper account of the blockage effect and this correction is sometimes not made in other presentations of turbine efficiency. It should be noted that the efficiency values of this VAWT are much lower than that of a HAWT, which are often in the 20-40% range. The use of HAWT style turbines is not appropriate for the physical constraints that are present for this application. The results presented in Figure 6, and in the subsequent figures correspond to a single turbine module. These figures allow predictions of power generation which could be achieved if multiple modules were used by a simple multiplication. Figure 6: Power generation results for a single section of an unvented turbine [16]. Figure 7 shows results for the vented turbine. These values of power production are very close to those of the unvented case and suggest that vents alone are not capable of producing large improvements in performance. It also shows that when venting is used to reduce thrust loading on the tower, it can be successfully employed without a reduction in power production. For both Figures 6 and 7, it is seen that there is a mild dependence of performance on electrical load. This dependence suggests that some further improvements in performance can be achieved by optimizing the electrical system. The dependence of power generation on electrical load is not surprising because the electrical load dictates the rotational speed of the turbine and thereby affects the aerodynamic performance. Figure 7: Power generation results for a single section of a vented, uncapped turbine [16]. Last, results for the capped turbine are presented. These data, presented in Figure 8, display key differences compared to the results of the two uncapped varieties. First, the power levels are significantly higher than for the uncapped cases. In fact, the amount of power generated is % higher when caps are employed. A second feature is the very strong dependence of power on the electrical load. It can be see that as the load is increased, the amount of power generated also increased significantly. It is expected that even greater rates of power production can be achieved by optimizing the electrical load to improve performance. This final design achieved efficiencies in the range of 15-25%. The results of Figure 8 allow a rational design of turbine systems to meet a communication tower electrical requirements. For instance, for wind speeds of 12 m/s, a three-module turbine would be capable of providing nearly 900 Watts of power with an electrical load of Copyright 2011 by ASME

6 ohm. This power requirement is typical of modern communication towers. A second comparison between lab and field tests was achieved by evaluating the rotational rates of the turbine for the two testing scenarios. Those rotational rates were then compared and presented in graphical form in Figure 10. The same annotation scheme is employed in Figures 9 and 10 with SAFL referring to wind-tunnel results and Sebeka to in-field tests. It is seen that there is very good agreement between the results which adds further credibility to the independent tests. It is also seen that the rotational speed of the turbine varies nearly linearly with wind speed, at least for the unvented turbine design. Figure 8: Power generation results for a single section of a vented, uncapped turbine [16]. Results of In-Field Tests A second set of tests was completed on an in-field unit installed on a communication tower in Minnesota, USA. That test unit was attached to a 150 foot tower and was outfitted with instrumentation to read and record wind speed, turbine rpm, and power generation. The design was equivalent to the unvented variety previously disclosed. Data was recorded for approximately 8 months and a regression analysis was applied to the data to extract power generation information. Results from the in-field study are shown in Figure 9. Discrete data symbols represent the wind tunnel tests and are annotated as SAFL which designates the St. Anthony Falls Laboratory which houses the wind tunnel of Figure 3. The infield data is annotated as Sebeka, which is the town in which the in-field tests were carried out. The in-field tests had an electrical load of 24 ohm and a three-stage rotor. It was found that there was generally good agreement between the windtunnel tests and the in-field results with the wind-tunnel data slightly higher than those obtained in-field. Figure 10: A comparison of turbine rotational speeds for wind turbine (SAFL) and in-field (Sebeka) tests. CONCLUDING REMARKS The present investigation presents two sets of experimental data on the performance of Savonius style wind turbines with various added design features for improved aerodynamic performance. Those features were vents which were aligned with the axis of rotation and caps which were positioned at the top and bottom edges of the turbine. One set of tests were carried out in a large wind tunnel so that a full-scale turbine rotor could be evaluated, along with the accompanying electrical system. A second set of tests was completed on an in-field unit attached to an operating tower. The wind tunnel tests show that the employment of caps leads to a significant improvement in the performance of the turbine while the use of venting gave very minor changes to the performance. It was also found that the dependence of wind-turbine performance on the electrical system is very strong when caps are used. This dependence suggests that significant future improvements in performance are achievable by optimizing the electrical system. Figure 9: Comparison of wind tunnel (SAFL) and in-field power generation results. Next, a multi-month study was carried out on a turbine affixed to an operating communication tower. Those tests allowed independent verification of the wind tunnel tests. It was found that the power generated during the in-field 6 Copyright 2011 by ASME

7 tests was slightly less than the power generated during the wind tunnel experiments. It was also found that the rate of rotation observed in the in-field tests was nearly identical to that from the wind-tunnel experiments. The results presented here show that Savonius-style turbines are a viable means for providing electrical power to off-grid communication towers or towers in regions that have unreliable electrical connections. Acknowledgements The authors gratefully acknowledge West Central Telephone Association for providing access to cellular communication towers. References [1] Sayers, A.T., 1985, Blade Configuration Optimization and Performance Characteristics of a Simple Savonius Rotor, Proc. Instn. Mech. Engrs., Vol. 199, [2] Alexander, A.J. and B.P. Holownia, 1978, Wind Tunnels Tests on a Savonius Rotor, J. Industrial Aerodynamics, Vol. 3, [3] Ushiyama, I. and H. Nagai, 1988, Optimum Design Configurations and Performance of Savonius Rotors, Wind Engineering, Vol. 12, [4] Mowry, G.S., R.A. Erickson, and J.P. Abraham, 2009, Computational Model of a Novel, Two-Cup Horizontal Wind-Turbine System, Open Mechanical Engineering Journal, Vol. 3, [10] Fujisawa, N., and F. Gotoh, 1992, Pressure Measurements and Flow Visualization Study of a Savonius Rotor, J. Wind Engineering and Industrial Aerodynamics, Vol. 39, [11] Saha, U.K., S. Thotla, and D. Maity, 2008, Optimum design Configurations of Savonius Rotor Through Wind Tunnel Experiments, J. Wind Engineering and Industrial Aerodynamics, Vol. 96, [12] Grinspan, A., P. Mahanta, and U.K. Saha, Design, Development, and Calibration of a Low-Speed Wind Tunnel, 7 th International Symposium on Fluid Control, Measurement, and Visualization, FLUCOME 3. [13] Saha, U.K. and M.J. Rajkumar, 2004, Performance Studies of Twisted Bladed Savonius Rotor, 2 nd BSME- ASME International Conference on Thermal Engineering, Dhaka, India, January 2-4. [14] Saha, U.K. and M.J. Majkumar, 2006, On the Performance of Savonius Rotor with Twisted Blades, Renewable Energy, Vol. 31, [15] Rajkumar, M.J. and U.K. Saha, 2006, Valve-Aided Twisted Savonius Rotor, Wind Engineering, Vol. 30, [16] Plourde, B.D., J.P. Abraham, G.S. Mowry, and W.J. Minkowycz, An Experimental Investigation of a Large, Vertical-Axis Turbine: Effects of Venting and Capping, Wind Engineering (in press). [5] Abraham, J.P., Mowry, G.S., and Erickson, R.A., Design and Analysis of a Small-Scale Vertical-Axis Wind Turbine for Rooftop Power Generation, Climate Change Technology Conference, 2009, Hamilton, Ontario, May 12-15, [6] Abraham, J.P., G.S. Mowry, and R.A. Erickson, Design and Analysis of a Small-Scale Vertical-Axis Wind Turbine, Clean Technology Conference and Expo 2009, Houston, TX, May 3-7, [7] Modi, V.J., and M.S. Fernando, 1989, On the Performance of the Savonius Wind Turbine, J. Solar Engineering, 1989, 111, [8] Ogawa, T., H. Yoshida, and Y. Yokota, 1989, Development of Rotational Speed Control Systems for a Savonius-Type Wind Turbine, J. Fluids Engineering, Vol. 111, [9] Fujisawa, N., and F. Gotoh, 1994, Experimental Study on the Aerodynamics Performance of a Savonius Rotor, J. Solar Energy Engineering, Vol. 116, Copyright 2011 by ASME