A COMPUTATIONAL FRAMEWORK FOR THE DESIGN AND ANALYSIS OF SAVONIUS WIND TURBINE

Transcription

1 A COMPUTATIONAL FRAMEWORK FOR THE DESIGN AND ANALYSIS OF SAVONIUS WIND TURBINE A report submitted by Sailesh Kolachana (ED07B015) in partial fulfillment of the requirements for the award of the degree of Bachelor of Technology (Engineering Design) and Master of Technology (Engineering Design) Under the guidance of Dr. Palaniappan Ramu Department of Engineering Design Indian Institute of Technology Madras Chennai June 2012 i

2 ABSTRACT Vertical Axis Wind Turbines (VAWT) are ideal options for small scale generation of electricity from wind. Savonius, one of the widely developed and used VAWTs are suitable for rooftops and self sustaining light posts. Most research in designing a Savonius turbine is focused on the experimental side. Power calculated using brake dynamometers provide a reasonable guess of the performance, but it cannot capture all the aerodynamic characteristics on the blade. Also, results from an experimental study are limited to the design that is being tested. When a particular input is changed, the change in the output is not readily available unless sensitivity information is available. An attractive alternative is a computer model in the place of the experimental model. Computer models provide the user with the privilege to change inputs and see the outcome instantly as against fabricating a prototype as in experimental studies. However, computer models are only approximations of the experiments and are limited in modeling the actual physics. In the case of vertical axis wind turbines like the Savonius, one needs to model it in a transient multiple reference frame approach which requires the rotational velocity as an input. There are no explicit equations that relate the angular velocity of the turbine to the input wind velocity. In this work, we have obtained these relationships through the experimental results available in the literature. This work aims at developing a transient computational model following certain standards that are important to transient turbomachinery simulations, with inputs from industry experts. The performance metrics obtained from the computational model is compared to existing experimental results available and corroborated. The computational model that is developed allows for estimating performance metrics for a new design. This framework allows for propagating uncertainties in variables like wind speed and enable one for a robust or reliable design. i

3 THESIS CERTIFICATE This is to certify that the thesis titled A Computational Framework for the Design and Analysis of Savonius Wind Turbine, submitted by Sailesh Kolachana to the Indian Institute of Technology, Madras, for the award of the degree of Bachelor of Technology and Master of Technology in Engineering Design, is a bonafide record of the research work done by him under my supervision. The contents of this thesis, in full or in parts, have not been submitted to any other Institute or University for the award of any degree or diploma. Dr. Palaniappan Ramu Project Guide Department of Engineering Design IIT Madras, Chennai Place: Chennai Date: 22 nd June 2012 ii

4 ACKNOWLEDGEMENT The first person I should be thanking is Dr.Palaniappan Ramu for giving me this opportunity and for pushing me to do this kind of work. I would not have accomplished whatever I did without his guidance and motivation. His regular inputs helped me structure my project step by step. I would like to extend my gratitude to Dr.Saravana Kumar and Dr.Ramanathan for helping me with the workspace and facilities at the department.. I express my heartful thanks to Mr. Satya Narayana, an industry expert in CFD simulations with wind turbines, who guided me with difficulties through the simulations. I would also like to thank the authors mentioned in the reference page, who provided me with base data on which I could build my project. I owe my parents and family for their constant support and encouragement, without whom I would probably never have stepped into an institute like IIT Madras in the first place. Sailesh Kolachana June 2012 iii

5 TABLE OF CONTENTS ABSTRACT... i NOMENCLATURE... vii LIST OF FIGURES... vii LIST OF TABLES... ix CHAPTER 1: WIND ENERGY AND WIND TURBINES Wind Energy- Use and Advantages Energy in the Wind Wind Turbines Wind Turbine Classification Horizontal Axis Wind Turbines HAWT advantages HAWT disadvantages Vertical Axis Wind Turbines VAWT advantages VAWT disadvantages Types of Vertical Axis Wind Turbines Darrieus Savonius Thesis Outline...7 CHAPTER 2 : THE SAVONIUS VAWT Savonius- Advantages and Disadvantages Savonius- Applications Savonius- Aerodynamics Savonius Performance and Efficiency Motivation of the Project...12 iv

6 2.6 Project Objective...12 CHAPTER 3: LITERATURE SURVEY...13 CHAPTER 4: THE COMPUTATIONAL MODEL Introduction- CFD analysis Features of the CFD analysis CFD analysis Steps D- Geometry D Mesh Mesh Extrusion: Analysis type and timestep details: Boundary conditions: Solver conditions:...21 CHAPTER 5: RESULTS AND DISCUSSIONS Comparison- Aerodynamic vs Experimental Values Observation on Tip Speed Ratios Torque Characteristics Stress Characteristics Variation in Size Effect of diameter on the aerodynamic output at 10 m/s: Effect of height on the aerodynamic output at 10 m/s: Effect of aspect ratio on the aerodynamic output at 10 m/s: Variation in Overlap Ratio Variation in Blades...29 CHAPTER 6: SUMMARY...31 APPENDIX Variation in Sizes Experiments on Rotor v

7 7.1.2 Experiments on Rotor Experiments on Rotor Variation in Overlap Ratio Overlap Ratio 18 % Overlap Ratio 20 % Overlap Ratio 22 % Overlap Ratio 25 % Overlap Ratio 30 % Overlap Ratio 40 % Variation in the number of blades Rotor 1 with three blades at 6 m/s windspeed Rotor 1 with three blades at 7.2 m/s windspeed Rotor 1 with three blades at 8.3 m/s windspeed Rotor 1 with three blades at 9.4 m/s windspeed Rotor 1 with three blades at 10 m/s windspeed...45 REFERENCES...46 vi

8 NOMENCLATURE Symbol A D d h o v ω λ T P C d C p Re OR Explanation Projected Rotor Area Diameter of Rotor Diameter of blade Height of rotor Overlap Windspeed (m/s) Angular Velocity Density of air Tip speed ratio Torque of rotor Power Drag coefficient Power coefficient Reynolds number Overlap Ratio vii

9 LIST OF FIGURES Figure 1: Wind Turbine Classification... 3 Figure 2: Darrieus VAWT [11]... 6 Figure 3: The Savonius VAWT [12]... 8 Figure 4 : Top View of the Savonius [13] Figure 5 : Relative Performance of various VAWT's [1] Figure 6 : 2D geometry view in ICEM CFD Figure 7: 2D mesh view Figure 8: 2D extrusion view with boundaries Figure 9: Variation of power coefficient with velocity for a two bladed Savonius system Figure 10 : A typical graph of power coefficient versus tip speed ratio obtained by experiments [3] Figure 11 : A typical torque graph of a 2 blade Savonius Figure 12 : A graph showing sum of the torques of the blades Figure 13 :A pressure contour with the rotor horizontal axis at 0 degrees to the windspeed Figure 14: A pressure contour with the rotor horizontal axis at 30 degrees to the windspeed Figure 15: A pressure contour with the rotor horizontal axis at 60 degrees to the windspeed Figure 16 : A typical stress contour on the surface of the blades Figure 17 : Increase of the power coefficient with the diameter Figure 18 Variation of the power coefficient with the aspect ratio Figure 19 : Variation of power coefficient with overlap ratio Figure 20 : Comparison of the 2 blade and 3 blade performance of Rotor 1 with windspeeds Figure 21 : Comparison of the torque characteristics exhibited with an increase in blades Figure 22 : Torque of Rotor 1 at 6 m/s windspeed Figure 23 : Torque of Rotor 1 at 7.2 m/s windspeed Figure 24 : Torque of Rotor 1 at 8.3 m/s windspeed Figure 25 : Torque of Rotor 1 at 9.4 m/s windspeed Figure 26 : Torque of Rotor 1 at 10 m/s windspeed Figure 27 : Torque of Rotor 2 at 10 m/s windspeed Figure 28 : Torque of Rotor 3 at 4.82 m/s windspeed viii

10 Figure 29: Torque of Rotor 3 at 6.57 m/s windspeed Figure 30: Torque of Rotor 3 at 7.28 m/s windspeed Figure 31 : Torque of Rotor 3 at 7.77 m/s windspeed Figure 32: Torque of Rotor 3 at 8.85 m/s windspeed Figure 33 : Torque of Rotor 1 at 18 % overlap Figure 34 : Torque of Rotor 1 at 20 % overlap Figure 35 : Torque of Rotor 1 at 22 % overlap Figure 36 : Torque of Rotor 1 at 25 % overlap Figure 37 : Torque of Rotor 1 at 30 % overlap Figure 38 : Torque of Rotor 1 at 40 % overlap Figure 39 : Three bladed Rotor 1 at 6 m/s windspeed Figure 40 : Three bladed Rotor 1 at 7.2 m/s windspeed Figure 41 : Three bladed Rotor 1 at 8.3 m/s windspeed Figure 42 : Three bladed Rotor 1 at 9.4 m/s windspeed Figure 43 : Three bladed Rotor 1 at 10 m/s windspeed LIST OF TABLES Table 1 : Applications of the Savonius Wind Turbine... 9 Table 2: Description of the various sized rotors Table 3 : List of the experiments conducted over number of blades, overlap, wind velocities and sizes Table 4 : Aspect Ratio details of different rotors Table 5 : Torque comparison with two and three blades Table 6: Properties of Rotor 1 at 6 m/s Table 7 : Properties of Rotor 1 at 7.2 m/s Table 8: Properties of Rotor 1 at 8.3 m/s Table 9: Properties of Rotor 1 at 9.4 m/s Table 10: Properties of Rotor 1 at 10 m/s Table 11: Properties of Rotor 2 at 10 m/s ix

11 Table 12: Properties of Rotor 3 at 4.82 m/s Table 13 : Properties of Rotor 3 at 6.57 m/s Table 14 : Properties of Rotor 3 at 7.28 m/s Table 15 : Properties of Rotor 3 at 7.77 m/s Table 16 : Properties of Rotor 3 at 8.85 m/s Table 17 : Conditions of the overlap ratio test Table 18 : Properties for an overlap ratio of 18 % Table 19: Properties of an overlap ratio of 20 % Table 20 : Properties of an overlap ratio of 22 % Table 21 : Properties of an overlap ratio of 25 % Table 22 : Properties of an overlap ratio of 30 % Table 23 : Properties of an overlap ratio of 40 % Table 24 : Properties of three bladed Rotor 1 at 6 m/s Table 25 : Properties of three bladed Rotor 1 at 7.2 m/s Table 26: : Properties of three bladed Rotor 1 at 8.3 m/s Table 27 : Properties of three bladed Rotor 1 at 9.4 m/s Table 28 : Properties of three bladed Rotor 1 at 10 m/s x

12 CHAPTER 1: WIND ENERGY AND WIND TURBINES 1.1 Wind Energy- Use and Advantages Wind energy has been used since the earliest civilization to grind grain, pump water from deep wells and power sailboats. From as early as the thirteenth century, horizontal-axis windmills were an integral part of the rural economy and only fell into disuse with the advent of cheap fossil-fuelled engines. The 21 st century has seen a rise in the demand for renewable sources of energy owing to the shortage and cost of non-renewable resources like fossil fuels. The advantages of wind energy are as follows: The wind blows day and night, which allows windmills/wind turbines to produce electricity throughout the day. Energy output from a wind turbine will vary as the wind varies, although the most rapid variations will to some extent be compensated for by the inertia of the wind turbine rotor thus obtaining a continuous output in many cases. Wind energy is a domestic, renewable source of energy that generates no pollution and has little environmental impact. Up to 95 percent of land used for wind farms can also be used for other profitable activities including ranching, farming and forestry. The decreasing cost of wind power and the growing interest in renewable energy sources should ensure that wind power will become a viable energy source worldwide. 1.2 Energy in the Wind The energy that can be extracted from the wind is directly proportional to the cube of the wind speed. Thus, an understanding of the characteristics of the wind (velocity, direction, variation) is critical to all aspects of wind energy generation, from the identification of suitable sites to predictions of the economic viability of wind farm projects, to the design of wind turbines themselves, all is dependent on characteristics of wind. The most striking characteristic of the wind is its random nature. The wind is highly variable, both geographically and temporally. Moreover this variability exists over a very wide range of scales, both in space and time. This is important because extractable energy from wind varies with the cube of wind velocity. Generally more wind is witnessed on the tops of hills and mountains than in low level areas. Even more 1

13 locally, wind velocities are altered by obstacles such as trees or buildings. For any location there is variation of wind pattern, wind speed may vary from year to year, also wind distribution will change from decade to decade. These long-term variations are not well understood, and thus make it difficult to make predictions of the economic viability of wind-farm projects. Wind distribution is more predictable over shorter time spans like a year, but on shorter time frame like few days the wind energy is difficult to predict. On the whole, it is important that we understand, the wind carries some energy with it, out of which some part is harnessed by the wind turbine. The kinetic energy of air mass moving at a velocity can be expressed mathematically as is Eq 1. E = 1 2 mv2 (1) P = de dt (2) The rate of change of kinetic energy of the air flowing at a certain velocity v through a certain volume is the power possessed by the moving wind. This is the amount of wind energy passing through cross-section per unit time. At a certain given velocity v, P = 1 dm 2 dt v2 (3) We understand that the rate of change of mass of the air flowing can be determined in Eq 4 as dm dt = ρav (4) As we substitute Eq 4 in Eq 3, we get Power = 1 2 ρav3 (5) This is the theoretical energy possessed by the wind flowing at a certain velocity v. 1.3 Wind Turbines Wind Turbines are wind energy converters that harness the kinetic energy contained in air masses. Most modern wind energy converters are equipped with rotors to extract wind power, and consist of one or several rotor blades. The extracted wind power generates rotation and is thereby converted into mechanical power at the rotor shaft. Mechanical power is taken up at the 2

14 shaft in the form of a moment at a certain rotation and is transferred to a machine (such as a generator). Thus there are entire wind power stations that consist of several wind turbines each having a wind energy converter (rotor), a mechanical gear and a generator. Every wind turbine design works with a certain efficiency. As time evolved, wind turbines of various designs came into picture. The variation of the design is mostly inherent in the geometry of the blade of the rotor, the aerodynamic properties of which are crucial in determining the efficiency of the blade. A broad classification of wind turbines exists this way, which is discussed below in Figure 1. 1 blade Horizontal axis 2 blade 3blade (or more) Wind Turbines 2 blade Darrieus 3blade Vertical axis H Rotor Savonius 2 blade 3 blade Figure 1: Wind Turbine Classification 1.4 Wind Turbine Classification Wind Turbines have various classifications among them, the main classifications being horizontal and vertical axis wind turbines. 1.5 Horizontal Axis Wind Turbines Horizontal-axis wind turbines (HAWT) get their name from the fact that their axis of rotation is horizontal. They have the main rotor shaft and electrical generator at the top of a tower, and are pointed into the wind. The variability of wind distribution and speed brings up the requirement of 3

15 a gear system connected to the rotor and the generator. The gear system enables a constant speed of rotation to the generator thus enabling constant frequency generation. Turbine blades are made stiff in order to prevent the blades from being pushed into the tower by high winds. Downwind machines have also been built, as they no longer require a yaw mechanism to keep them facing the wind, and also because in high winds the blades can turn out of the wind thereby increasing drag and coming to a stop. Most of the HAWT's are upwind as downwind systems cause regular turbulence which may lead to fatigue HAWT advantages Higher efficiency, better performance Variable blade pitch, which gives the turbine blades the optimum angle of attack. As wind energy increases with height, the tall tower in the HAWT gives access to higher wind speed. More commercially viable HAWT disadvantages Due to inherent large structures, construction costs are very high and so are transportation costs. Wind turbine operation often leads to production of noise. In case of downwind HAWTs the regular turbulence produced leads to structural failure. HAWTs require an additional yaw control mechanism to turn the blades toward the wind. 1.6 Vertical Axis Wind Turbines Vertical-axis wind turbines (or VAWTs) have the main rotor shaft arranged vertically as the plane of rotation is vertical. Blades are also vertical in this arrangement. The biggest advantage of VAWTs is they don t require a yaw control mechanism to be pointed into the wind. Thus these are useful in sites where wind direction is random or there is presence of large obstacles like trees, houses etc. Also VAWTs don t require a tower structure and can be placed nearby a ground enabling access to electrical components. Some drawbacks are the low efficiency of wind production and the fact that large drag is created for rotating the blades in a vertical axis VAWT advantages A massive tower structure is not required, as VAWTs are mounted closer to the ground 4

16 They don t require yaw mechanisms. These are located closer to the ground and hence easier to maintain. These have lower startup speeds than their horizontal counterparts. These have a lower noise signature. Are more efficient in gusty winds already facing the gust. Can be installed in more locations - on roofs, along highways, in parking lots. Do not kill birds and wild - life - slow moving and highly visible. Can be significantly less expensive to build are inherently simpler VAWT disadvantages VAWTs have lower efficiency as compared to HAWT s Even though VAWTs are located closer to the ground, the equipment now resides at the bottom of the turbines structure thus making it inaccessible. Because of their low height they cannot capture the wind energy stored in higher altitudes. 1.7 Types of Vertical Axis Wind Turbines Darrieus Invented by F.M. Darrieus in the 1930s, Darrieus turbines shown in Figure 2, are lift-based turbines designed to function on the aerodynamic principle of airplanes.the rotor blades are designed as an airfoil in cross section, so the wind travels a longer distance on one side (convex) than the other side (concave). As a result, the wind speed is relatively higher on the convex side. If Bernoulli s equation is applied, it can be shown that the differential in wind speed over the airfoil creates a differential pressure, which is used to pull the rotor blade around as the wind passes through the turbine. The Darrieus VAWT is primarily a lift-based machine, which is a feature that makes it compete in performance with the conventional HAWTs. Figure 2shows a typical Darrieus wind turbine characterized by its C-shaped rotor. It is normally built with two or three rotor blades. It has a low starting torque, but high rotational speed, making it suitable for coupling with an electrical synchronous generator. For a given rotor size, weight and cost, its power output is higher than any drag-based VAWT.But the Darrieus VAWT suffers disadvantage by not self-starting. For this purpose, there exists one more design called the 5

17 Savonius, which has an easy starting capability. Experimental studies of Savonius Darrieus wind turbines have been conducted. The result of the combined designs shows an improvement in power generation efficiency. Figure 2: Darrieus VAWT [11] Savonius Savonius wind turbines are drag based VAWTs that operate on the theory and principle of a paddle propelling a boat through water. It was invented by a Finnish engineer, S.J. Savonius. If no slip exists between the paddle and water, the maximum speed attained will be the same as the tangential speed of the paddle. Similarly, in a drag based VAWT, the speed at the tip of the blade can seldom exceed the speed of the wind. In other words, the drag can also be described as the pressure force or the thrust on the blades created by the wind as it passes through it. Various types of drag based VAWTs have been developed in the past which use plates, cups, buckets, oil drums, etc. as the drag device. The Savonius rotor is an S - shaped cross section rotor which is predominantly drag based, but also uses a certain amount of aerodynamic lift. Drag based VAWTs have relatively higher starting torque and less rotational speed than their lift based counterparts. Furthermore, their power output to weight ratio is also less.because of the low speed, these are generally considered unsuitable for producing electricity, although it is possible by selecting proper gear trains. Drag based windmills are useful for other applications such as grinding grain, 6

18 pumping water and a small output of electricity. A major advantage of drag based VAWTs lies in their self starting capacity, unlike the Darrieus lift based vertical axis wind turbines. 1.8 Thesis Outline Chapter 2 helps us understand the Savonius vertical axis wind turbine, its aerodynamics properties and how its performance is measured. Chapter 3 explains the existing literature survey that was referred to. Chapter 4 describes the computational model, the boundary and solver conditions that was built using ANSYS modules ICEM CFD and CFX. Chapter 5 describes the set of computational experiments that was carried out spanning the geometrical parameters and the results of each. Chapter 6 analyses the data obtained from the experiments in Chapter 5, and deduces necessary inferences. Chapter 7 presents future scope of work and conclusion. 7

19 CHAPTER 2 : THE SAVONIUS VAWT Savonius turbines are one of the simplest turbines. Aerodynamically, they are drag-type devices, consisting of two or three scoops. Because of the curvature, the scoops experience less drag when moving against the wind than when moving with the wind. The differential drag causes the Savonius turbine to spin. Because they are drag-type devices, Savonius turbines extract much less of the wind's power than other similarly-sized lift-type turbines. Much of the swept area of a Savonius rotor is near the ground, making the overall energy extraction less effective due to lower wind speed at lower heights. Figure 3: The Savonius VAWT [12] 2.1 Savonius- Advantages and Disadvantages The Savonius turbine is of simple geometry and its construction is cheap. It starts rotating at lower wind speeds as compared to its counterpart wind turbines, having a high starting torque. It produces low noise and can make use of the wind flowing in any horizontal direction to its rotation. However, in spite of these advantages, this wind turbine faces one main disadvantage of having low efficiency. 8

20 2.2 Savonius- Applications Savonius turbines are used whenever cost or reliability is much more important than efficiency. Table 1 : Applications of the Savonius Wind Turbine Anemometers Deep Water Buoys Flettner Ventilators Most anemometers are Savonius turbines, because efficiency is completely irrelevant for that application. Much larger Savonius turbines have been used to generate electric power on deep-water buoys, which need small amounts of power and get very little maintenance The most ubiquitous application of the Savonius wind turbine is the Flettner Ventilator which is commonly seen on the roofs of vans and buses and is used as a cooling device. The ventilator was developed by the German aircraft engineer Anton Flettner in the 1920s. It uses the Savonius wind turbine to drive an extractor fan. The vents are still manufactured in the UK by Flettner Ventilator Limited. Other applications The upcoming needs of small scale electricity production are coming handy in urban sites. So we see their use in self sustaining street lights and rooftops 2.3 Savonius- Aerodynamics From an aerodynamic point of view, the different VAWT, have a number of aspects in common that distinguish them from the HAWT. Most of the rotation of the Savonius turbine is caused due to drag forces. The turbine consists of two blades (or more blades), the advancing blade and the retreating blade in the case of a two-bladed Savonius. The blades of a Savonius rotate on a 9

21 Figure 4 : Top View of the Savonius [13] horizontal plane whose axis is at right angle to the wind direction. The aerodynamic angle of attack of the blades varies constantly during the rotation. So during rotation, one blade moves on the downwind side and the other on the upwind side. The torque extraction from the blade that moves on the downwind side is less compared to the energy extracted by the upwind blades. This describes the characteristic of the torque. Hence, power generation is less in the downwind sector of rotation. The torque caused by drag forces is generally lesser than that produced by lift forces. In one revolution, a single rotor blade generates a mean positive torque but there are also short sections with negative torque. Various approaches, with a variety of weightings of the parameters involved have been published in the literature. Menet et al [1] specify values of to be the maximum value of power coefficient C p for the single stage Savonius type wind turbine. The following Figure 4 [13] shows us a characteristic of the flow and the basic terminology of the blades. The equations result in saying that the C p can be expressed as a function of the drag coefficients and the tip speed ratio. As the Savonius torque is generated by the difference of drag forces as in Eq 6 we can say, 10

22 F d = F dr F da (6) F d = 1 2 ρac dr Vsinθ ωr ρac da Vsinθ + ωr 2 (7) π T = F d Rdθ 0 C p = Tω/( 1 2 ρav3 ) (9) (8) C p = λ/3π[c dr 6λ 2 3λ π C da 6λ 2 + 3λ π ] (10) Figure 5 : Relative Performance of various VAWT's [1] Eq 10 explains that, for obtaining C p as a function as expressed in the above, a knowledge of the drag coefficient of each of the blades is necessary. However the drag coefficient (C d ) is not a value that we can define with respect to time. Hence this cannot be taken further unless the drag coefficients are known with respect to time. 2.4 Savonius Performance and Efficiency The performance of the Savonius wind turbine is determined by the coefficient of performance (C p ). It is theoretically defined as the ratio of the aerodynamic power generated by the wind turbine to the power possessed by the wind incoming on the the surface of the rotor. 11

23 However, Cp is indicated as a function of the tip speed ratio(λ), as the performance of wind turbines reaches an optimum for a certain tip speed ratio. λ = ωr/v (11) 2.5 Motivation of the Project Most research on the Savonius has been on the experimental side. The results from an experimental setup are limited to the design being tested. An attractive alternative is a computer model in the place of the experimental model. Computer models provide the user with the privilege to change inputs and see the outcome instantly. 2.6 Project Objective To develop a computational model that can capture the physics demonstrated by experimental Savonius models available in the literature. Identify design variables, make perturbations and observe changes. This thesis answers the following questions. Given a range of wind speeds, how do you decide upon your overall size? Does there exist an optimal overlap for the blades? Does performance increase when the number of blades are increased? What are the stresses exerted by the winds on the blade surfaces? 12

24 CHAPTER 3: LITERATURE SURVEY J L Menet[1] presented a study wherein he suggested the performance of one particular design of the Savonius rotor. It was a small Savonius rotor (i.e. of low power) for local production of electricity. They designed, developed and ultimately built a prototype of such a rotor, which was a complete electromechanical system. An optimised configuration was chosen for the geometry of the prototype. The building data were calculated on the basis of the nominal wind velocity V = 10 m/s. He indicates that studies done on the prototype have confirmed the high efficiency and the low technicality of the Savonius rotors for local production of electricity. It means that for urban usage, Savonius rotors are of low performance, but looking at it over a longer period of time, they make use of the lesser and multi directional wind speeds and proves more effective. They present a decision matrix for opting the material for the rotor, the choice of geometry, shaft and generator details etc. They consider nominal parameters and present a certain relationship of the power coefficient with angular velocity. This was interpolated to a relationship between wind velocity and angular velocity. However the interpolations are done neglecting the electrical and mechanical losses. UK Saha et al. [2] conducted wind tunnel tests on 14 prototypes of Savonius rotor models, all of the same overall dimension. The variation of the prototypes came in the number of blades, the number of stages and their respective combinations. Both semicircular and twisted blades have been used in either case. A family of rotor systems was manufactured with identical stage aspect ratio keeping identical the projected area of each rotor. The overlap ratio, being another important geometrical parameter has been suggested to be around 20% to 30% for the models. Experiments were carried out to optimize the different parameters like number of stages, number of blades (two and three) and geometry of the blade (semicircular and twisted). Further attempt was made to investigate the performance of two-stage rotor system by inserting valves on the concave side of blade. Velocities of wind speeds were considered between the range of 6 m/s and 11 m/s. Spanning 84 experiments on all these parameters, the 2 stage 2 blade Savonius model was considered to be showing the highest value of power coefficient. Among a comparison between the semicircular and the twisted ones, the twisted model was found to be more efficient than the semicircular one. 13

25 Sargolzaei et al. [9] simulated Savonius wind turbine using a concept called artificial neural networks (ANNs) to estimate power ratio and torque. They experimentally investigated seven prototype Savonius wind turbine and compared their overlap ratios with their predicted ANNs results. Their predicted results were in good agreement with their experimental results that existed in the literature. They found that the increase of wind speed causes torque increase.however; this necessarily need not increase the power coefficient. For all their models they found maximum torque was at 60⁰ and minimum torque was at 120⁰. Burcin Deda Altan et al. [8] introduced a new curtain arrangement to improve the performance of Savonius wind rotors. The curtain arrangement was placed in front of the rotor preventing the negative torque opposite the rotor rotation. The geometrical parameters of the curtain arrangement were optimized to generate an optimum performance. The rotor with different curtain arrangements was tested out of a wind tunnel, and its performance was compared with that of the conventional rotor. The maximum power coefficient of the Savonius wind rotor is increased to about 38.5% with the optimum curtain arrangement. The experimental results showed that the performance of Savonius wind rotors could be improved with a suitable curtain arrangement. Gupta et al. [10] compared a three-bucket Savonius wind turbine with a three-bucket Savonius Darrieus wind turbine. They found that the power coefficient of the combined turbine decreases as the overlap ratio increases. The maximum power coefficient of 51% was found where there was no overlap. They claimed that the combined rotor without overlap, which showed 51% efficiency, was the highest efficiency of a Savonius wind turbine at any overlap condition under these test conditions. Md. Imtiaj Hassan et al. [7] present a Computational Fluid Dynamics (CFD) analysis of a Twisted Savonius rotor. Simulations were performed in a CFD software Flow-3D, using Reynolds Averaged Navier-Stokes Equations (RANSE) solver with structured rectangular mesh. Expected performance of the twisted Savonius rotor has been determined, this includes starting 14

26 characteristics, static torque and rotational speed of the turbine. Simulation results show better performance of twisted Savonius rotor as compared to the other conventional Savonius rotors. M.A. Kamoji et al. [4], modified forms of the conventional Savonius rotors in an effort to improve the coefficient of power and to obtain uniform coefficient of static torque. To achieve these objectives, the rotors are being studied with and without central shaft between the end plates. Tests in a closed jet wind tunnel on modified form of the conventional Savonius rotor with the central shaft were to have a coefficient of power of They studied the effect of geometrical parameters on the performance of the rotors in terms of coefficient of static torque, coefficient of torque and coefficient of power. The parameters studied are overlap ratio, blade arc angle, aspect ratio and Reynolds number. 15

27 CHAPTER 4: THE COMPUTATIONAL MODEL 4.1 Introduction- CFD analysis As a consequence of the rapid evolution of computational power within the last two decades, the significance of numerical flow simulations in the design of hydraulic machinery has grown to a considerable extent. At present CFD simulations can often replace elaborate experiments due to the fact that even complex geometries and entire machines can be modeled. Separate numerical investigations of single turbo-machinery components are common practice, yet these simulations do not account for interactions between the components which have strong influence on the operational behavior of the entire machine. Due to these interactions e.g. between the stationary guide vanes and the rotating runner, the so called rotor-stator interactions the flow in turbo-machines is unsteady and highly turbulent. The investigation of these phenomena requires transient simulations and special numerical treatment with respect to the interface between the components. The approach used in an extruded 2D computational fluid dynamics analysis of the Savonius blades. The model used in analysis consists of a stream of air impinging on the blade system. 4.2 Features of the CFD analysis Multiple Frames of Reference: The CFD simulation requires the interfacing of a rotor domain and a stator domain to effectively determine performance, as the rotor is in a rotating frame and the incoming wind in a non rotating frame. Transience: This features a transient simulation of the rotation over every single degree of rotation. 4.3 CFD analysis Steps 2D geometry 2D mesh Mesh extrusion Boundary conditions Interfacing Solution monitoring and control 16

28 Post processing results D- Geometry The geometry is a combination of 3 domains- the blades, the inner fluid domain and the outer fluid domain. The thickness of the blades needs to be defined, which is 3mm in our case. The model was made with the dimensions as indicated by UK Saha [2] and ML Simmonds[3]. Figure 6 : 2D geometry view in ICEM CFD The inner fluid domain is shaped as a circle and the outer fluid domain is shaped as a rectangle. This is because the wind flow direction is rotary is the inner domain and linear of the outer domain. Inner domain constraints: The diameter of the inner fluid domain needs to be % greater than the diameter of the rotor. We have considered 10 %, which means the diameter of the inner fluid domain is 1.1 time that of the rotor. Outer domain constraints: The outer domain being rectangular must be 10D 5D (10D in the wind direction). This gives 5D for upstream and 5D for downstream. However, the size was 17

29 bought down to 6D 4D (6D in the wind direction, shown in Figure 6) as it exhibited the same result characteristics as to that of the 10D 5D dimension Figure 7: 2D mesh view D Mesh The mesh chosen for the 2D mesh geometry is a quad dominant geometry. The choice of a quad mesh was as a quad mesh gives better control and also will reduce numerical error so to say that the results obtained will mostly due to the flow behavior and not due to the numerical error as compared to the triangular mesh.(shown in Figure 7) 18

30 Mesh for Inner domain: The flow patterns inside the inner domain exhibit variations, due to which the mesh pattern needs to be uniform inside. The size of the mesh should necessarily be lesser than the thickness of the blade. We have considered the mesh element size to be one third the blade thickness. Mesh for Outer domain: The mesh size for the outer domain goes increasing in value as we move radially outwards. The outer fluid mesh must have a node-wise connection to the inner fluid mesh. Figure 8: 2D extrusion view with boundaries Mesh Extrusion: The simulation cannot just be carried out in a 2D fashion and the pre processing software CFX- Pre requires a certain thickness of the fluid domain to be present to calculate the torque over the blade surface. So the existing mesh is extruded by more than one layer(shown in Figure 8), specifying a certain height for each layer. We have considered 2 layers each of height h/2. 19

31 4.3.4 Analysis type and timestep details: The analysis is transient and it needs to be run in timesteps. So an ideal timestep in turbomachinery applications is 0.5 to 2 degrees especially when the rpm is greater than 100. The length of a timestep is inversely proportional to the rpm. Length of each timestep =Number of degrees moved per timestep/(6 rpm) (12) Total run time : 360 Number of degrees moved per timestep/(6 rpm) (13) Boundary conditions: The following are the boundary conditions that have been applied. The rotating and stationary domains are specified. Rotating Domain: The rotating domain has a certain angular velocity. The fluid is atmospheric air with atmospheric pressure. The domain requires initialization conditions to be preset (the wind velocity and the atmospheric pressure). The turbulence mode is set to the k-epsilon model. It is a two equation model. That means it includes two extra transport equations to represent the turbulent properties of the flow. This allows a two equation model to account for history effects like convection and diffusion of turbulent energy. Stationary Domain: The stationary domain has a free stream velocity. The domain is set to stationary. Adding to that, the atmospheric pressure conditions are applied and the initialization is done. Inlet, Outlet: Inlet and Outlet are default boundary conditions in ANSYS CFX. Inlet requires the speed of inlet air and the outlet requires the relative pressure, 0 Pa, at the initial conditions. Blade surfaces: The blade surfaces are enabled a wall condition. This condition enables the calculation of properties such as force, torque on the surface. Interfacing: Once the domains have been specified a default fluid-fluid interface is detected between the rotating and stationary domain. Here the transient rotor stator is selected. This model should be used any time it is important to account for transient interaction effects at a sliding (frame change) interface. It predicts the true transient interaction of the flow between a stator and rotor passage. It ultimately accounts for all interaction effects between components 20

32 that are in relative motion to each other. The interface position is updated each timestep, as the relative position of the grids on each side of the interface changes. The principle disadvantage of this method is that the computer resources required may be large, in terms of simulation time, disk space and quantitative post processing of the data. The resource requirement problem is exacerbated if unequal pitch between components occurs. In these situations, spatial periodicity cannot formally be used to limit the analysis to a single blade passage per component Solver conditions: Convergence loops: The solver calculates parameters the torque(required parameter) for every timestep. Each timestep is run for a certain number of loops for convergence. As per general guidelines, this is set from 7 to 10 for the simulations. Transient results (velocity contours, pressure contours) have to be defined with a certain frequency of timesteps. This helps record the contour details of pressure, velocities and forces. 21

33 CHAPTER 5: RESULTS AND DISCUSSIONS The power coefficient (C p ) is a crucial parameter that determines the performance of a Savonius. Following is the parameters that have an effect on the power coefficient (C p ) Number of blades Overlap Ratio Overall Size Wind Velocity To evaluate the effect that these parameters have, a set of experiments was conducted spanning all the parameters. The inputs of these experiments have been taken from the literature. Table 2: Description of the various sized rotors Reference Description Reference 1 ( dimensions and data adopted from UK Saha) Reference 2 ( dimensions and data adopted from JL Menet ) Reference 3( dimensions and data adopted from Simmonds) Rotor Dimension ( H D) 17 cm 19.8 cm cm 44.5 cm 177 cm 94.5 cm Table 3 : List of the experiments conducted over number of blades, overlap, wind velocities and sizes Rotor Number of Blades Range of wind velocities tested Number of Experiments Overlap Ratio 1 2-semicircular 6m/s 10 m/s semicircular 10 m/s semicircular 4.8 m/s m/s semicircular 6m/s-10 m/s semicircular 10 m/s For details in any particular experiment, refer Appendix. The results of experiments performed over variables size, overlap, and the number of blades are given section-wise. 22

34 Coefficient of Performance 5.1 Comparison- Aerodynamic vs Experimental Values Experiments by Simmonds et al.(rotor 3) Computational Result- Rotor 3 Experiments by Saha et al.(rotor 1) Computational Result- Rotor Velocity (m/s) Figure 9: Variation of power coefficient with velocity for a two bladed Savonius system From the results, we can say that the computational results have similar trend to the experimental results available in the literature. The values are off by a factor of 2 to Observation on Tip Speed Ratios The power coefficient is a function of the tip speed ratio given certain geometry of a Savonius rotor. For 95% of the experiments found in literature run over the rotor models over various sizes and overlaps, the tip speed ratio for almost all practical cases stands between So, for 95% of the experimental cases that have been run, 0.6 < λ < 0.9 (14) 0.6v R 0.9v < λ < R (15) 23

35 Figure 10 : A typical graph of power coefficient versus tip speed ratio obtained by experiments [3] As the transient CFD simulation requires the rotational speed of the rotor as an input, the above approximation could be a good measure to predict the range of the rotational speed given the geometry and the windspeeds as shown in Eq 14 and Eq Torque Characteristics We see a pattern convergence after crossing 100 degrees of rotation. The initialization of the Savonius has been done at 0 degrees. We see the first peak of the first blade at 210 degrees and the first peak of the second blade at 390 degrees of rotation. With a uniformity in cases, the Savonius rotor peaks the torque of its advancing blade at 30 degrees to the axis along which the wind speed is travelling. This pattern repeats itself over every 360 degrees of rotation. 24

36 Torque 4.00E-01 2 blade Savonius- Torque Characteristic 2.00E E E-01 Torque_Blade1 Torque_Blade2-4.00E-01 Degrees of Rotation Figure 11 : A typical torque graph of a 2 blade Savonius 2.00E E E E E+00 2 blade Savonius - Sum of Torque Characteristic Torque_Blade1 + Blade2 Figure 12 : A graph showing sum of the torques of the blades Figure 13 :A pressure contour with the rotor horizontal axis at 0 degrees to the windspeed 25

37 Figure 14: A pressure contour with the rotor horizontal axis at 30 degrees to the windspeed Figure 15: A pressure contour with the rotor horizontal axis at 60 degrees to the windspeed Here is a characteristic of the pressure contour at 30 degrees which shows a high pressure difference on the advancing blade, hence causing it to turn. The pressure contours around the blade show the characteristics wherein the advancing blade experiences positive torque due to a pressure difference and the retreating blade experiences resistance. The pressure patterns when the Savonius axis is at three different orientations are shown by Figures 13, 14 and15 26

38 5.4 Stress Characteristics Figure 16 : A typical stress contour on the surface of the blades The values are with respect to atmospheric pressure. The above case is for Rotor 3 at 8.85 m/s. The maximum increase pressure on the surface is around 100 Pa. 5.5 Variation in Size Three models of various geometries were taken under consideration. Table 4 : Aspect Ratio details of different rotors Rotor Diameter Height Aspect Ratio Velocity range of testing cm 17.3 cm m/s cm cm m/s cm cm m/s Rotor 3, unlike Rotor 1 has been showing a constant aerodynamic power coefficient with respect to the range of wind speeds between ( ) m/s. We extend this graph to 10 m/s and compare performance of the three rotors at 10 m/s. 27

39 Power Coefficient Power Coefficient Effect of diameter on the aerodynamic output at 10 m/s: Diameter (cm) Figure 17 : Increase of the power coefficient with the diameter Effect of height on the aerodynamic output at 10 m/s: The increase of height of the rotor does not affect the aerodynamic performance as observed in Eq 16 and Eq 17. Hence the value of C p remains unaltered. C p T ; C p 1/A (16) T h ; A h (17) Effect of aspect ratio on the aerodynamic output at 10 m/s: Aspect Ratio Figure 18 Variation of the power coefficient with the aspect ratio 5.6 Variation in Overlap Ratio A study of the performance has been observed at various overlap ratios has been observed from 10 to 40 % of overlap, and the performance has been recorded to find an optimum at 22 % 28

40 Power Coefficient overlap. The simulations have been carried out in certain steps and the results are shown in Figure Blade overlap charateristics Overlap Ratio (%) Figure 19 : Variation of power coefficient with overlap ratio 5.7 Variation in Blades Table 5 : Torque comparison with two and three blades 2 blade - torque 3 blade torque The torque values are largely positive We see an equally high contribution due to negative torque, which brings down the overall torque. The overall torque is that of the sum over 2 The overall torque is that of the sum over 3 blades blades. For the Rotor 1 model that was compared with respect to 2 blades and 3 blades (according to Figure 21), we find that the optimal performance for both cases is shown at around 8 m/s. But we cannot assure the similar optimal performance pattern as the wind speed increases, as the performance curve becomes flatter as the size increases. In spite of the 3 blade obtaining torque as a sum of that from three different blades, the power coefficient values of a 2 blade are greater than that of a 3 blade rotor. So we infer that owing to large negative torque values (Figure 22), the overall efficiency of a blade in a 3 blade Savonius is lesser than that of a 2 blade Savonius. Table 5 summarizes these differences. 29

41 Torque Torque Coefficient of Performance 2 blades versus 3 blades Velocity (m/s) Experiments by Saha et al.(2 blade) Computational Output (2 blade) Experiments by Saha et al.(3 blade) Computational Output (3 blade) Figure 20 : Comparison of the 2 blade and 3 blade performance of Rotor 1 with windspeeds 4.00E-01 Rotor 1-10 m/s 2.00E E E E-01 Degrees of Rotation Torque_Blade1 Torque_Blade2 Torque_Blade3 3.00E-01 Rotor 1-10 m/s 2.00E E E+00 Torque_Blade1 Torque_Blade2-1.00E-01 Degrees of Rotation Figure 21 : Comparison of the torque characteristics exhibited with an increase in blades 30