PERFORMANCE CHARACTERISTICS OF AN INDUSTRIAL CROSS FLOW WIND TURBINE

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 5, May 2017, pp , Article ID: IJMET_08_05_111 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed PERFORMANCE CHARACTERISTICS OF AN INDUSTRIAL CROSS FLOW WIND TURBINE Rama Prabha D School of Electrical Engineering, VIT University, Vellore, India Narendiranath Babu T School of Mechanical Engineering, VIT University, Vellore, India Raj Kumar E School of Mechanical Engineering, VIT University, Vellore, India ABSTRACT Today, only a small fraction of the world's electricity is generated by wind, however, demand for this renewable energy resource will continue to increase with the depletion of fossil fuels. Fast-response porous polymer paint was selected for the blade coating. This paint is a polymer/ceramic based PSP, which was designed specifically to improve the diffusivity of the binder material, thus allowing faster response times. Its kinetic characteristics were good and its response time can be as fast as 3.6 khz, which is sufficient to study the flow field on airfoils with oscillating free stream frequencies less than 20Hz.A wind turbine is a type of turbo machine that transfers fluid energy to mechanical energy through the use of blades and a shaft and converts that form of energy to electricity through the use of a generator. Depending on whether the flow is parallel to the axis of rotation (axial flow) or perpendicular (radial flow), determines the classification of the wind turbine. Each type of wind turbine has its strengths and weaknesses, but in the end, all wind turbines accomplish the same task. With mounting global pressure for sustainable energy technologies, there is a growing need to restructure traditional wind, solar and geothermal technologies to increase deployment over a greater range of conditions. Although efficient under open conditions, Traditional horizontal axis wind turbines (HAWTs) are not optimal for use in dense urban settings where wind speed and direction are more variable. Conversely, vertical axis wind turbines (VAWTs) can take advantage of incoming wind from any angle, and typically perform better at lower wind speeds associated with the topography of existing infrastructure. A new hybrid blade, blending Darrieus and Savonius qualities, has been investigated using numerical and computational fluid dynamics methods. There are other people that are connected to the grid but still have energy cuts. For these people, it seems to be cheaper to have a grid connection than to have an independent energy source, reasons for that are that in an isolated installation the energy has to be chemically stored in batteries, with limited lifetime that makes renewal costs comparable to those of a grid connection. Furthermore, there are places which will not have electricity access, these places editor@iaeme.com

2 Performance Characteristics of an Industrial Cross Flow Wind Turbine coincide with the poorest and isolated rural areas of developing countries, and these people cannot afford the cost of a wind turbine, nor small nor bigger. Maybe a free turbine design, that anyone with common workshop materials could build, would make a difference in the living conditions of this people. The purpose of this paper is to provide an alternative to these people. Finally, the structural behaviour of the turbine is analysed. Keywords: Cross flow, wind turbine, structural characteristics, turbine design, vertical axis. Cite this Article: Rama Prabha D, Narendiranath Babu T and Raj Kumar E. Performance Characteristics of an Industrial Cross Flow Wind Turbine. International Journal of Mechanical Engineering and Technology, 8(5), 2017, pp INTRODUCTION Throughout history, wind has often been harvested as a plentiful, inextinguishable energy source. It was first used to crush grains and pump water using windmills, which greatly decreased the manpower required for those tasks. At its peak, there were approximately 200,000 windmills throughout Europe [1-3]. However, the industrial revolution brought a new wave of engines powered by fossil fuels, which had a far greater energy density. The proliferation of gas and oil powered engines nearly wiped out the once-dominant windmills. However, recent concerns about fossil fuel depletion as well as greenhouse gas emissions have brought increasing interest in wind power. Modern wind turbines operate under the same principles as historic windmills. They convert the momentum of the wind into kinetic energy of rotating blades, which is then channelled into a rotating shaft. Within this general definition, there are two subgroups: horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWTs).Horizontal axis wind turbines are similar to windmills in that their axis of rotation is parallel to the incoming wind. This leads to a high efficiency, but the turbines are limited to utilizing wind that blows on that axis. Wind approaching from other directions cannot be converted into useful power, and can even cause structural damage to the turbine in extreme cases. Thus, HAWTs are suited to locations where wind blows from a near-constant direction, like the Great Plains of the United States. Conversely, vertical axis turbines rotate perpendicular to the wind. While their efficiencies are significantly lower than HAWTs, they can utilize wind from any direction. For this reason, they are best suited to locations where wind direction can vary significantly. Additionally, within VAWTs, there are two main categories: lift- or Darrieus-type turbines and drag- or Savonius-type turbines. The lift type uses principles of flight derived from airplanes, while the drag type uses scoops to catch the wind and transfer its energy to useful work. The Savonius-type VAWT was developed and patented in 1928 by Sigurd J. Savonius.It generally consists of two or more scoops that attach to a central, vertical mast (Savonius 1928). The scoops are curved in such a way that they generate substantially more drag when traveling with the wind than against the wind. This drag imbalance induces a nonzero net force on the scoop, or a net torque about the mast. Because the scoops are typically symmetrical, the net torques are additive, and they combine to force the assembly to rotate around the mast. This rotation is then used to drive a generator and create power. Because they take advantage of a drag imbalance, they are often simply referred to as drag-type turbines. A simple schematic of their operation can be found in Figure editor@iaeme.com

3 Rama Prabha D, Narendiranath Babu T and Raj Kumar E Figure 1 Schematic of the Savonius type turbine When the wind enters from the right, it flows relatively easily around the right scoops, but it is trapped by the left scoop. This drag slows the wind, and transfers some of its momentum on the scoop which creates torque. The advantage of this design is its simplicity. Savonius turbines are often in used in locations where routine maintenance is a challenge, and where reliability is paramount. Additionally, because they function via drag, they can operate at very low wind speeds. Because of this, they are occasionally used underwater, where low speed currents provide consistent power to offshore machinery and equipment. However, their main drawback is their extremely poor efficiency. While a modern HAWT can achieve efficiencies approaching 50%, a Savonius turbine will generally only reach 15%. That is, only 15% of the available power in the wind is converted into useful work. Because of this, Savonius turbines are rarely economical for large-scale power production. The Darrieus-type turbine was patented in 1926 by Georges Jean Marie Darrieus, French aeronautical engineer (Darrieus 1926). Instead of scoops that capture wind through drag, the Darrieus turbine uses airfoils that rotate and create tangential lift that propels the turbine around. The lift force is analogous to the lift force that an airfoil creates on an airplane [4-8]. For this reason, these turbines are often referred to as lift-type turbines. Figure 2 illustrates a simple schematic of a generic Darrieus-type turbine. Figure 2 Darrieus type turbine In this schematic, the motion of the wind through the turbine creates a varying angle of attack, and the lift force generated by the airfoil acts ahead of the turbine throughout most of the rotation. While the schematic above illustrates a two-blade turbine, Darrieus turbines are often found with three blades editor@iaeme.com

4 Performance Characteristics of an Industrial Cross Flow Wind Turbine 2. MATERIALS AND METHODS In a very practical type of application, self-cleaning coatings would be used on painted surfaces preventing the deposition of dirt or on optically transparent materials such as the surface of solar cells, etc. The method involves the in situ polymerization of common monomers in the presence of a porogenic solvent to afford surfaces with the desired combination of micro and nano scale roughness. The method is applicable to a variety of substrates and is not limited to small areas or flat surfaces. Therefore in this work, the following chemical composition was used to predict the performance characteristics. Porous poly(butyl methacrylate-co-ethylene dimethacrylate) via photoinitiation butyl methacrylate (24% wt.), ethylenedimethacrylate (16% wt.), 1-decanol (39% wt.), cyclohexanol(20% wt.) and 2,2-dimethoxy-2-phenylacetophenone (1% wt. with respect to monomers). 3. METHODOLOGY 3.1. VAWT Design Parameters The wind turbine parameters considered in the design process are: Swept area Power and power coefficient Tip speed ratio Blade chord Number of blades Solidity Initial angle of attack 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. For a straight bladed vertical axis wind turbine the swept area has a rectangular shape and is calculated using: where S is the swept area [m2], R is the rotor radius [m], and L is the blade length [m]. Power and Power Coefficient The power available from wind for a vertical axis wind turbine can be found from the following formula: where Vo is the velocity of the wind [m/s] and ρ is the air density [kg/m3], the reference density used its standard sea level value (1.225 kg/m^3 at 15ºC), for other values the source editor@iaeme.com

5 Rama Prabha D, Narendiranath Babu T and Raj Kumar E (Aerospaceweb.org, 2005) can be consulted. Note that available power is dependent on the cube of the airspeed. The power the turbine takes from wind is calculated using the power coefficient: Tip Speed Ratio 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. Where, ω is the angular speed [rad/s], R the rotor radius [m] and Vo the ambient wind speed [m/s]. Each rotor design has an optimal tip speed ratio at which the maximum power extraction is achieved. Solidity The solidity σ is defined as the ratio between the total blade area and the projected turbine area (Tullis, Fiedler, McLaren, &Ziada). It is an important nondimensional parameter which affects self-starting capabilities and for straight bladed VAWTs is calculated with (Claessens, 2006): Where N is the number of blades, c is the blade chord, L is the blade length and S is the swept area; it is considered that each blade sweeps the area twice. 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. Blade Performance Initial Estimations An initial estimation had to be made to provide guidance in the design process. In order to do that, the extracted data from similar market competitors (see Table 1) has been analyzed in order to find out reasonable values. With all this data, an estimation of the required dimensions as well as a rotational speed has been done, as it is not possible to approximate the average wind speed of the whole developing countries. This value has been estimated to be 6 m/s looking at the data provided. Also, it is interesting to have a low value of design wind speed as it will ensure more availability of electrical energy supply with averaged and frequent airspeeds. The turbines found on the market are rated by their maximum power output; this could give false expectative on the final user as the rated wind conditions can be higher than the average wind conditions editor@iaeme.com

6 Performance Characteristics of an Industrial Cross Flow Wind Turbine Table1 below shows what were the initial estimated performances of the wind turbine, the power coefficient and tip speed ratios. Table 1 Initial estimated performances of the wind turbine Aerodynamic Model In order to model the performance of a vertical-axis wind turbine there are four main approaches (Cooper, 2010): - Momentum models - Vortex models - Local circulation models - Viscous models Computational Fluid Dynamics (CFD) Each model has its advantages and drawbacks, the main advantage of momentum models is that their computer time needed is said to be much less than for any other approach (Spera, 2009). The model chosen for the aerodynamic analysis is the double-multiple stream tube with variable interference factor, often abbreviated DMSV, which is enclosed in the momentum models category and is based in the conservation of momentum principle, which can be derived from the Newton s second law of motion. It has been used successfully to predict overall torque and thrust loads on Darrieus rotors. 4. VALIDATION The Matlab Algorithm has been validated using the known results of one of the aerodynamic prediction models developed by I. Paraschivoiu. This model called CARDAAV considers the interference factor varying in function of the azimuth angle and also can consider secondary effects. It should be noted that the reference data was taken reading a plot because the numeric results were unavailable, furthermore the code version CARDAAV which has been used to compute the reference plot consider secondary effects like the rotating tower and the presence editor@iaeme.com

7 Rama Prabha D, Narendiranath Babu T and Raj Kumar E of struts, which makes a difference in maximum power coefficient of 6% (Paraschivoiu I., 2002, p. 185), the developed algorithm gives a difference in maximum Cp of 4.7%. A comparison between the qualitative accuracy of the algorithm compared with the reference turbine give us a good approximation of the turbine performance at TSRs lower than 3.5, which will be the range of our turbine. The reference turbine is found on Table 2 (Paraschivoiu, Trifu, &Saeed, 2009) and its parameters are: Table 2 Parameters of reference turbine Figure 3 Aerodynamic model Figure 3 shows aerodynamic model. The own version of the aerodynamic model takes into account a varying interference factor in function of the azimuth angle but doesn t consider the following effects that should improve the accuracy of the model: Presence of struts and mast. Vertical variation of the free stream velocity. Expansion of the stream tubes. Dynamic stall effects editor@iaeme.com

8 Performance Characteristics of an Industrial Cross Flow Wind Turbine Rotor Design Air foil Selection The airfoil has been selected considering the availability of airfoil data for angles of attack between -30 and 30º and the final thickness of the blade which is associated with its ability to withstand the loads. The selected air foil is the NACA0021, which aerodynamics characteristics were determined using an air foil property synthesizer code. This profile is one of the thickest available (21% of chord) and when compared with NACA0015 (thickness is 15% of chord) it can be seen that the self-starting behaviour is improved with thicker air foils. Figure 4 shows Performance comparison between two different airfoils Figure 4 Performance comparison between two different airfoils Starting Parameters The usual rated wind speed value the fixed parameters will be the design blade swept area. The design airspeed is kept 6 m/s or 9m/s or 12m/s, because the purpose of the application is to take profit of the average wind speed rather than higher speeds, which are not so frequent. Design Speed The target performance is 100 W at 6 m/s, which should correspond to the rated wind speed, and therefore the maximum rpm of the turbine should be the corresponding to the optimum tip speed ratio of the turbine. But, in order to take advantage also of higher wind speeds, the rated wind speed will be set to 9 m/s but maintaining the initial performance target at 6 m/s. The value ranges from 11.5 to 15 m/s, but a lower value may produce more energy overall because it will be more efficient at wind speeds between cut-in and rated. Figure 5 shows Cp depending on ambient airspeed (Vo) and Tip Speed Ratio (TSR) editor@iaeme.com

9 Rama Prabha D, Narendiranath Babu T and Raj Kumar E 3.00E E E E-01 Cp 1.00E-01 Vo = 12 m/s Vo = 9 m/s Vo = 6 m/s Vo = 3 m/s 5.00E E E TSR Figure 5 Cp depending on ambient airspeed (Vo) and Tip Speed Ratio (TSR) Torque [N m] Vo = =3m/s Vo = 6 m/s Vo = 9 m/s Vo = 12 m/s TSR Figure 6 Torque depending on ambient airspeed (Vo) and Tip Speed Ratio (TSR) The target performance is 100 W at 6 m/s, which should correspond to the rated wind speed, and therefore the maximum rpm of the turbine should be the corresponding to the optimum tip speed ratio of the turbine. But, in order to take advantage also of higher wind speeds, the rated wind speed will be set to 9 m/s but maintaining the initial performance target at 6 m/s. The usual rated wind speed value ranges from 11.5 to 15 m/s, but a lower value may produce more energy overall because it will be more efficient at wind speeds between cut-in and rated.figure 6 shows torque depending on ambient airspeed (Vo) and Tip Speed Ratio (TSR) Rotor Dimensions The blade length and rotor radius have a major contribution in the torque behaviour of the turbine as can be deduced from the torque equation. In general as bigger these parameters, editor@iaeme.com

10 Performance Characteristics of an Industrial Cross Flow Wind Turbine bigger the torque produced. The radius and blade variation analysis is done maintaining constant swept area. It can be seen in Figure below that an increase in rotor radius leads to greater maximum power coefficients, but they are achieved at greater tip speed ratios, so the little the radius, the less tip speed ratio is necessary to work at maximum power coefficient. The blade tip losses are not contemplated in the model, in fact, one of the initial assumptions of the model is that there is no interaction between stream tubes, so in a real model, an increase on blade length should make the blade more efficient. The optimization needs the input of a fourth factor, the maximum load due to both aerodynamic loads and rotational speed, as the radius will be the one determining them. Figure 7 shows power coefficients dependent on rotor radius (R) and tip speed ratio (TSR) maintaining constant Swept area. Figure 8 shows torque dependent on rotor radius (R) and tip speed ratio (TSR) maintaining constant Swept area 4.00E E-01 Cp 0.00E E E R = 0.5 R = 0.75 R =1.0 R = E-01 R = E E+00 TSR Figure 7 Power coefficients dependent on rotor radius (R) and tip speed ratio (TSR) maintaining constant Swept area 35 Torque [N m] R = 0.5 m R = 0.75 m R = 1 m R = 1.25 m R = TSR Figure 8 Torque dependent on rotor radius (R) and tip speed ratio (TSR) maintaining constant Swept area editor@iaeme.com

11 Rama Prabha D, Narendiranath Babu T and Raj Kumar E 5. NUMBER OF BLADES The other parameter affecting solidity is the number of blades affects solidity, keeping in mind the possible interference between blades and also a low solidity value, only three and four-bladed designs will be analyzed. Furthermore, the algorithm doesn t account for the wake turbulence that is created behind each blade and can seriously affect the adjacent blade s lift and drag forces so a big number of blades may give too optimistic results. Figure 9 shows Number of blades effect in average torque at several rotational speeds, the two models had blade chords of 0.28 m, NACA0021 airfoils and Vo is 9 m/s Average Torque [N m] N = 4 BLADES N = 3 BLADES Rotational speed [rpm] Figure 9 Number of blades effect in average torque at several rotational speeds, the two models had blade chords of 0.28 m, NACA0021 airfoils and Vo is 9 m/s. As seen from the graphs, a bigger number of blades lead to a bigger deceleration of the air, and this effect is greater than the increase in torque that can be produced having more blades. In this case, three blades are more efficient than four for the same rotational Speed; the final decision will be made considering the acceleration behaviour, which will provide a better view of the self-starting characteristics. The rotor torque curves at different rotational and wind speeds are presented. Figure 10 Average torque curves for several wind and rotational speeds Average Torque [N m] wind speed = 6 m/s wind speed = 9 m/s wind speed = 12 m/s Rotational speed [rpm] Figure 10 Average torque curves for several wind and rotational speeds editor@iaeme.com

12 Performance Characteristics of an Industrial Cross Flow Wind Turbine A mechanism to mechanically stop the wind turbine before the structural limit is surpassed has to be designed. In HAWT models, the over speed control is achieved turning away the frontal area of the turbine from the wind direction or changing the blade pitch (angle of attack) in order to stall the blades. These mechanisms may present spring-loaded devices, and although a fixed pitch was considered for the initial design, perhaps a furling mechanism of the rotor blades when certain centrifugal force is achieved can be considered in the design. Final Design and Operation With all the previous considerations, keeping in mind that a prototype must be built in order to validate the algorithm results and possibly make further refinements. The following are taken as the parameters for the model: Table 3 SYMBOL PARAMETER VALUE V rs Rated speed 9 m/s TSR Optimum tip speed ratio 2.86 Cp Maximum power coefficient R Rotor Radius 1.5 m C Blade chord 0.3 m L Blade length 3 N Number of blades 3 W Rotor angular speed 17 rad/s α o Initial angle of attack 0 0 OUTPUT POWER = kw OUTPUT TORQUE = N/m OUTPUT POWER COEFFICIENT = If overall efficiency is taken as 0.9 then also output power available is kw 6. CONCLUSIONS 1) Porous polymer coating is a suitable material for blade construction in small-scale wind turbines and the construction of an affordable wind turbine with common workshop tools can be contemplated. 2) A three-bladed design is more efficient than a four-bladed rotor; a low solidity wind turbine may present self-starting problems as rotor efficiency is poor at low tip speed ratios. 3) There is an optimum turbine rotational speed for each ambient wind speed at which the maximum efficiency is achieved. The energy production of a fixed pitch wind turbine can be improved adjusting the rated airspeed to the place of installation average wind conditions in order to reach its maximum efficiency. 5) Larger radius turbines are more efficient than small turbines at same rotational speed as the tangential airspeed increase leads to smaller angles of attack, bigger Reynolds numbers and thus bigger blade lift coefficients. 6) For 1kW rotor radius = 1.5m Blade length = 3m At rated wind speed of 9m/s and angular velocity =17 rad/s editor@iaeme.com

13 Rama Prabha D, Narendiranath Babu T and Raj Kumar E REFERENCES [1] Guerri, 0., A. Sakout, and K. Bouhadef, Simulations of the Fluid Flow around arotating Vertical Axis Wind Turbine. Wind Engineering, (3): p [2] Gupta, R. and A. Biswas, Computational fluid dynamics analysis of a twisted three-bladed H-Darrieus rotor. Journal of Renewable and Sustainable Energy, [3] Brahimi, M. T., Allet, A., & Paraschivoiu, I. (1995). Aerodynamic Analysis Models for Vertical-Axis Wind Turbines. International Journal of Rotating Machinery, Vol.2 No. 1 pp [4] Tong, W. (2010). Fundamentals of wind energy. In W. Tong, Wind power generation and wind turbine design (p. 112). WIT Press. [5] Abhilasha Rathod, Nalin Raut, Sai Patil, Kajol Kamble, Shailendra Shisode, Aerodynamic Analysis of Morphing Blade for Horizont al Axis Wind Turbine. International Journal of Mechanical Engineering and Technology, 8(1), 2017, pp [6] Wahl, M. (2007). Designing an H-rotor type Wind Turbine for Operation on Amundsen- Scott South Pole Station. Uppsala: Uppsala Universitet. [7] Wood, D. (2011). Small Wind Turbines Analysis, Design, and Application. Springer- Verlag. [8] Small-Scale Vertical Axis Wind Turbine Design by Javier Castillo [9] Rajasri Alloli, Pratibha Dharmavarapu and Chunchu Sravanthi, Design and Manufacture of Vetrical-Axis Wind Turbine Based on Magnetic Levitation. International Journal of Mechanical Engineering and Technology, 7(6), 2016, pp editor@iaeme.com