Modulation of Vertical Axis Wind Turbine Apurwa Gokhale 1, Nehali Gosavi 2, Gurpreet Chhabda 3, Vikrant Ghadge 4, Dr. A.P.Kulkarni 5 1,2,3,4 Vishwakarma Institute of Information Technology, Pune. 5 Professor, Vishwakarma Institute of Information Technology, Pune. Abstract Vertical axis wind turbines (VAWT) are capable of producing a lot of power, and offer many advantages. The mechanical power generation equipment can be located at ground level, which makes for easy maintenance. Also, VAWT are omni-directional, meaning they do not need to be pointed in the direction of the wind to produce power. In recent years more focus is put on the applications of wind turbines in the urban environment. The modern equivalent which is based on lift producing blades only exists for 30 years. In this period airfoils for this application have been developed, but still much work can be done in this field. Our main objective is to modify the design to make it self-starting and to bring it to production stage. The purpose of this project is easy installation in areas where electricity is not yet available. optimization of the methodology is achieved for the mass production of different parts of vertical axis wind turbine. This is achieved using numerous CATIA models analysed by ANSYS Workbench. Index Terms S Swept Area, m 2 ; R Radius of rotor m ; L Length of blade, m ; P w Power available from wind ; V o Velocity of the wind, m/s ; ρ Air density, kg/m 3 ; C p Power coefficient ; λ Tip speed ratio ; ω Angular speed, rad/s ; σ Solidity ; N No. of blades ; c Chord length I. INTRODUCTION Vertical axis wind turbines (VAWT) are advocated as being capable of catching the wind from all directions, and do not need yaw mechanisms and downwind coning. Their electrical generators can be positioned to the ground, and hence easily accessible. A disadvantage is that some designs are not selfstarting. There have been two distinct types of VAWT: The Darrieus and Savonius types. The Darrieus was researched and developed extensively by Sandia National Laboratories in the USA in the 1980 s. The concept of VAWT can have differently shaped blades. As the forces of the blades can be large, the ideal blade has a Troposkien (nearly parabolic) shape with which the centrifugal force is translated through the blade to the shaft. This type of blade is mainly used in large turbines and prevents the blade from failing because of too large rotational speeds. A large disadvantage is the decreasing radius near the top and the bottom of the turbine. These parts experience only low rotational speeds and therefore generate almost no power. Another concept is the H-Darrieus or Musgrove VAWT. The blades are straight and therefore the radius is equal over the total length of the blade, see figure 1.1(b). The power is now generated over the complete length of the blade. In contrast to the Troposkien shape blade extra strength is necessary to cope with the centrifugal forces. The blades can be rotated slightly to disperse the moment forces on the axis over a larger angle. The first prototypes of the H-Darrieus were developed in 1986. [2] The typical VAWT consists of the following parts: Supporting mast Central Shaft Supporting struts for the blades Blades Generator The blades of a VAWT have to develop lift and must have enough thickness to withstand the loads. To achieve this they have a certain shape, comparable to aircraft wings. This shape determines how the wind energy is conversed to forces on the blade. The goal of this study is to develop a new airfoil profile for an H-Darrieus vertical axis wind turbine. In most of the existing turbines of this type standard profiles like the NACA 0015 and NACA 0018 are used. These profiles were developed in the 1930 s by the NACA as standard profile series for turbulent flow. A. Overview II. LITERATURE REVIEW Horizontal axis wind turbines are typically more efficient at converting wind energy into electricity than vertical axis wind turbines. For this reason they have become dominant in the commercial utility-scale wind power market. However, small VAWT are more suited to urban areas as they have low noise level and because of the reduced risk associated with their slower rates of rotation. VAWT cost will come down appreciably once they are mass produced on production line scale. The economic development and viable use of HAWT would in the future be limited, partly due to high stress loads on the large blades. It is recognized that although less efficient, vertical axis wind turbines do not suffer so much from constantly varying gravitational loads. VAWT with rated power output of 10 MW could be
developed, with at least the same availability as a modern horizontal axis wind turbine, but at a lower cost per unit of rated power. B. How turbines work? The wind imposes two driving forces on the blades of a turbine; lift and drag. A force is produced when the wind on the leeward side of the airfoil must travel a greater distance than that on the windward side. The wind travelling on the windward side must travel at a greater speed than the wind travelling along the leeward side. This difference in velocity creates a pressure differential. On the leeward side, a lowpressure area is created, pulling the airfoil in that direction. This is known as the Bernoulli s Principle. Lift and drag are the components of this force vector perpendicular to and parallel to the apparent or relative wind, respectively. The swept area limits the volume of air passing by the turbine. The rotor converts the energy contained in the wind in rotational movement so as bigger the area, bigger power output in the same wind conditions. 2. Power and power coefficient The power available from wind for a vertical axis wind turbine can be found from the following formula: 3 P w = (½)* ρsv o... (2) Where V o is the velocity of the wind [m/s], ρ is the air density [kg/m3], the reference density used its standard sea level value (1.204 kg/m3 at 15ºC). The power the turbine takes from wind is calculated using the power coefficient: C p = (Captured mechanical power by blades / Available power in wind)... (3) C p value represents the part of the total available power that is actually taken from wind, which can be understood as its efficiency. For small VAWT, the value of maximum power coefficient has been found to be usually ranging between 0.15 and 0.22. This power coefficient only considers the mechanical energy converted directly from wind energy; it does not consider the mechanical-into-electrical energy conversion, which involves other parameters like the generator efficiency. Fig 1. Aerodynamic loads on VAWT Blade in terms of lift and drag [5] C. General parameters considered in design of VAWT [4] The wind turbine parameters considered in the design process are: Swept area Power and power coefficient Tip speed ratio Blade chord Solidity Initial angle of attack 1. Swept Area : The swept area is the section of air that encloses the turbine in its movement. The shape of the swept area depends on the rotor configuration. So the swept area of an HAWT is circular shaped while for a straight-bladed vertical axis wind turbine the swept area has a rectangular shape and is calculated: S=2RL... (1) Where S is the swept area [m2], R is the rotor radius [m], L is the blade length [m]. 3. Tip Speed Ratio (TSR) The power coefficient is strongly dependent on tip speed ratio, defined as the ratio between the tangential speed at blade tip and the actual wind speed. TSR = Tangential Speed at the blade tip / actual wind speed TSR = λ = Rω / V o... (4) where ω is the angular speed [rad/s], R the rotor radius [m], V o is the ambient wind speed [m/s]. Each rotor design has an optimal tip speed ratio at which the maximum power extraction is achieved. 4. Blade Chord The chord is the length between leading edge and trailing edge of the blade profile. The blade thickness and shape is determined by the airfoil used, in this case it will be a NACA airfoil, where the blade curvature and maximum thickness are defined as percentage of the chord. 5. Solidity The solidity σ is defined as the ratio between the total blade area and the projected turbine area. It is an important non
dimensional parameter which affects self-starting capabilities and for straight bladed VAWTs is calculated with, σ = (N*c)/R...(5) where N is the no. of blades, c is the blade chord (m), R is the radius of rotor (m). This formula is not applicable for HAWT as they have different shape of swept area. Solidity determines when the assumptions of the momentum models are applicable, and only when using high σ 0.4 a self starting turbine is achieved. 6. Initial Angle of Attack The initial angle of attack is the angle the blade has regarding its trajectory, considering negative the angle that locates the blade s leading edge inside the circumference described by the blade path. D. Comparison and effects of variations in performance characteristics There is decrease of aerodynamic performance due to the increment of rotor solidity. Maximum power coefficient of VAWT depends on both wind speed and rotor solidity. This is illustrated in fig below. Fig No. 2.5 : Evolution of rotor power coefficient as a function of the tip speed ratio [7] Following graph shows the effect of varying the chord length on the power captured from the wind. Each line represents a different chord length with 6 inch producing the highest power and 1 inch producing the least. Hence from the graph it is clear that, greater the chord length greater will be the power output for increasing wing speed. Hence average chord length of 200 mm is considered for further design. Fig 3. Power vs. Wind Speed for Various Cord Lengths (1 inch to 6 inch) [8] Fig 2. Maximum rotor power coefficient as a function of both rotor angular velocity and wind speed [6] Following graph shows relation between power coefficient and tip speed ratio for various diameters of rotor. From the graph shown below, it can also be observed that the position of maximum rotor efficiency at (λ~2.4) is roughly constant. Thus tip speed ratio of 2.4 is selected for calculation. A. Design Parameters III. DESIGN AND ANALYSIS Sr. Name of the Parameter Value No. part 1 - Wind speed (V o) 4.3 m/s 2 - Density of air (ρ) 1.204 kg/m 3 3 - Angular speed 100 rpm (N) 4 - Radius of rotor 1000 mm (R) 5 Blade (NACA0018) i) Length of blade (L) 2000 mm 200 mm
ii) Chord Length 7 Rod i) Outer Diameter ii) Wall Thickness iii) Length 8 Shaft i) Outer Diameter ii) Wall Thickness iii) Length 9 Disc i) Outer Diameter 10 Bolt M16 (Qty- 24) ii) Thickness i) Minor Diameter ii) Pitch 33.7 mm 4.05 mm 1130 mm 76.1 mm 3.65 mm 440 mm 180 mm 3 mm 13.546 mm 2 mm 11 Nut (Qty-24) Thickness 13 mm 12 Washer (Qty-24) Thickness 3 mm 13 Tower i) Height ii) Base Area 6000 mm 1500*1500 mm 14 - Swept Area 4*10^6 mm 2 15 - Tip Speed Ratio 2.4353 16 - Wind Power 191.4528 Watt 17 - Mechanical Power 18 - Solidity (for 6 blades) 113.453 Watt 0.6 Fig 5. Catia Model for centre shaft B. CATIA Model Fig 6. Catia Model for rods Various parts of vertical axis wind turbine were designed using CATIA. Above design parameters were considered for the same and standard sizes of pipe, nut, bolt, washer etc. were chosen according to standard catalogues and westerman table. CATIA models for some of the parts and their final assembly is shown below. Fig 7. Assembly of VAWT Fig 4. Catia Model for blades C. Material Selection TABLE.1 Sr. No. Part Name Material / Specification 1 Shaft Mild Steel 2 Rods Mild Steel 3 Bolt, Nut, Washer M16 Mild steel 4 Blades Fibre glass
D. Analysis of Components As the material used us mild steel, yield strength is taken as S yt = 250 N/ mm 2.. Factor of safety was considered to be 2 and design is modified considering manufacturing aspects. Analysis of various components was done using ansys workbench and design was finalised and checked for safety. Following figures show some of the data of analysis for various parts. Fig 11. Equivalent stress on Nut Equivalent stresses on each part are less than the permissible stresses. Hence the design is safe. E. Summary of the work The design of VAWT was finalised and tested on ANSYS. Factor of safety is 2. Various forces were applied on the parts and individual part is tested for safety. Materials were selected by referring previous research Fig 8. Equivalent stress on shaft papers. Market survey was done for the same. Further CFD analysis is to be done in order to confirm design under dynamic loads and then fabrication will be started. Using 3,4 and 6 blades on the same shaft, the various parameters will be studied. REFERENCES Fig 9.Equvalent stress on rod 1. The Design and Testing of Airfoils for Application in Small Vertical Axis Wind Turbines by M.C. Claessens, TUDelft 2. Vertical Axis Wind Turbines: History, Technology and Applications by Marco D Ambrosio, Marco Medaglia 3. Small-Scale Vertical +Axis Wind Turbine Design by Javier Castillo, Tampere University of Applied Sciences 4. Performance Prediction and Dynamic Model Analysis of Vertical Axis Wind Turbine Blades with Aerodynamically Varied Blade Pitch by Dhruv Rathi, North Carolina State University 5. Evaluation of the Effect of Rotor Solidity on the Performance of a H- Darrieus Turbine Adopting a Blade Element-Momentum Algorithm by G. Bedon, M. Raciti Castelli, E. Benini, World Academy of Science, Engineering and Technology International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:6, No:9, 2012 6. Effect Of Shaft Diameter On Darrieus Wind Turbine Performance, Strickland, J. H. (1975) The Darrieus Turbine: A Performance Prediction Model Using Multiple Streamtube, SAND75-0431. 7. Trade Study: The effect of Cord Length and Taper on Wind Turbine Blade Design John Larson Group C4: Turbinator Technologies AME 40463 Senior Design Fig 10.Equivalent stress on bolt