CHAPTER 2 REVIEW OF LITERATURE

Transcription

1 21 CHAPTER 2 REVIEW OF LITERATURE 2.0 INTRODUCTION The horizontal wind turbine converts maximum of 59.3% of available kinetic energy. In practice, the conversion efficiency may be in the range of only 30 to 35%. Hence, there is an excellent opportunity to increase the efficiency of wind turbine using a nozzle system. The nozzle increases the efficiency of wind turbine due to increased velocity of air. Both horizontal axis wind turbine and vertical axis wind turbines are developed with single nozzle system. The efficiency of the horizontal axis wind turbine can be enhanced further using multiple nozzle system. The literature is based on single nozzle wind turbines and symmetrical NACA (National Advisory Committee for Aeronautics) airfoils. Angle of attack is the direction in which air forces on to the rotor. The blades used in the investigation are symmetrical in shape and are flat. The survey is made for the best angle of attack when such blades are used. 2.1 LITERATURE SURVEY OF SINGLE NOZZLE WIND TURBINES Wind turbines with single nozzle systems are developed duct wind turbines, mantle wind turbines, diffuser augmented wind turbines, etc Duct Wind Turbines The concept of duct system is introduced to increase the efficiency of wind turbine using funnel or conduit around the rotor. Rise in air velocity at the inlet of turbine increases the efficiency of wind turbines, since power in the wind is directly proportional to the cube of air

2 22 velocity at the inlet of rotor. Betz 8 (1938) stated that the duct designs are not viable due to increased cost. Due to the high cost, fabrication of such ducts is not taken up till But the observation of Sanuki 9 (1950) and Iwasaki 10 (1953) produce a favorable outcome. First time, Lilley 11 et al. (1956) introduced the duct type wind turbine. The efficiency can be increased by at least 65% from the conventional wind turbine. Lilley found that the increase in axial velocity and reduction on the tip loss of shrouded turbine. After the investigations of Kogan 12 et al. (1962) the experimentation work is drastically reduced because of the oil crisis till The investigations of Fletcher 13 (1981) and Riegler 14 (1983) indicated the rise in efficiency because of duct design. The augmentation is approximately three times to conventional wind turbine. Wright 15 et al. (2004) stated in a research paper that energy input does not propose to capture, only the energy. Some amount of energy is wasted due to stall at the root of blades of rotor. Hua 16 et al. (2008) conducted an experiment on duct wind turbine using single convergent nozzle system. Grant 17 et al. (2002) conducted a survey on building integrated duct wind turbine module. The duct wind turbine is installed over a residential building. Lawn 18 (2003) optimized the power output of a duct type wind turbine. Varol et al. 19 (2001) proposed steering airfoil in front of wind turbine results in high energy production from the turbine. Hu 4 et al. ( 2006) contributed the effort in investigations of duct type wind turbines by simulations using CFD. Burton 20 et al. (2001) mentioned ducted type wind turbine in the wind energy hand book. Grass man 21 (2003) upgraded the wind turbine

3 23 efficiency using duct wind turbine. In all investigations, the performance of wind mill is increased almost by 50% to 60% from conventional wind turbine using optimal design duct. Of course, the cost of wind turbine increases due to additional fabrication. But, the design improves silent features like power factor, reducing capitalization, accessibility, etc. Weisbrich 22 (1977) suggested applicability and implacability of the economic theory of duct wind turbines to a variety of structures and environment Mantle Type Wind Turbines Frankovi 23 et al. (2001) conducted an experiment to increase the efficiency of wind turbine using mantle nozzle. A wind turbine is built in regulated mantle s nozzle. Frankovi stated that high velocity and high mass flow rate of air will boost the production of the turbine. The economic analysis of mantle wind turbine is also discussed by Frankovi for the island of Lastovo. The profit due to mantle wind turbine is equal to five times to the conventional wind turbine. The installation cost of the turbine is 8000 pounds to pounds for one kilowatt power generation. When the wind is blowing briskly in the range of 8.5 m/s, such turbine produces 1.5 kilowatts of electrical power. In the investigation, Frankovi states that mantle wind turbine produces 1.3 GWh more than conventional wind turbine per hour. Molly 24 (1978), Jhonson 25 (1985) and Taylor 26 (1983) contributed efforts in the design of propeller type wind turbines using mantle nozzle. They concluded that mantle nozzle increases axial velocity of air and thus high mass flow.

4 VAWT using Convergent Nozzle Although vertical axis wind turbines have been neglected till now, they can prove to be highly promising in the case of the new rotor design. In the last two decades, attention has been focused on improving the efficiency of vertical axis wind turbines. According to Bergey 27 (1979), Griffiths 28 (1977) and Paor 3 (1982), the maximum conversion efficiency of the horizontal axis wind turbine is 16/27. Air is treated stationery, non-viscous and incompressible in the analysis of Betz limit. Paul 29 (1983) of Van Nostrand Reinhold Company stated that the vertical axis turbine will have efficiency almost identical to the horizontal axis wind turbine. Shikha 30 et al. (2003) reported a new vertical axis wind rotor for low wind speed areas with a convergent nozzle for the amplification of wind speed. Such novel rotors can be built in small units instead of large central power plants suitable for different load requirements. The importance of the work is especially due to the current trend of high power generation. Another success has been made to analyze the optimal nozzle dimensions for a better performance of the system. Shikha et al. 31 (2005) presented the idea of convergent nozzle with different shape. The article of Shikha brings a comprehensive theoretical and empirical study of air concentrating nozzle in the area of wind energy. In the analysis, five different nozzle models are used. In stepwise, the length and exit area of the nozzle is changed to determine its effect on the increase in the wind speed. The variations of outlet speed by varying inlet and outlet diameters of nozzle, varying

5 25 inlet velocities of air at the inlet and different distances from wind tunnel are discussed by Shikha. Touryan 32 et al. (1987), Macpherson 33 et al. (1972) and Newman 34 (1983) conducted few investigations on the vertical axis wind turbine to enhance the power coefficient using a nozzle system. In investigations, the efficiency of the turbine is increased approximately to 33%. But, according to Modi 35 et al. (1984) the power coefficient of a fairly streamlined vertical axis wind turbine is only 22%. Sharpe 36 (1984) developed a theoretical model using the computer programme for aerodynamic analysis of various stream tube for a vertical axis wind turbine. Sabzevan 37 (1977) proposed the idea of placing a flat plate to collect a large amount of air on to the drum of VAWT. The power coefficient is thus, increased by 15%. The rise in power coefficient can be compared with the simulated version of Obeidat 38 (1987). According to Obeidat, the rise in power coefficient of VAWT is 17%. Opawa 39 (1984) applied the discrete-vortex method for the analysis of flow separation for the generation of vortices around the rotor of VAWT. Fujisawa 40 et al. (1987) compared the flow velocities, pressures and vortices with flow visualization technique for the rotor of identical dimensions Swift Wind Turbines Wind turbines typically turn from tall towers on hills and planes at high elevations. In the recent days, some companies manufactured smaller turbines for domestic spots like homes and garages. The rooftop turbines generate electricity for 1/3rd of demand of a residence. Under the guidance of Anderson, Renewable Devices

6 26 Company produced a novel design of Swift Wind Turbine. The noise produced by conventional wind turbine is 35 decibels. The sound is diffused by a ring mounted round blades. The air is pushed towards the diffuser ring and then dispersed. Such swift turbine supplies energy suitable to produce 1.5 kw of electric power with the inlet air velocity in the range of 8.33 m/s. It is enough, for instance, to glow fifteen 100W bulbs. The output of turbine hugely increases with the rise in air velocity at the inlet of the wind turbine. Over a year, the energy production in windy locations should be roughly 2000 kilowatt hours, for a residence which requires kwh in a year. At that instance, energy costs would be reduced about 18 percent Diffuser Augmented Wind Turbines According to Van Holten 41 (1981) and Dick 42 (1986), augmentation means increasing the mass flow of air and mixing of turbulence with external flow. A conference was held in the year 1979 in USA based on DAWT. Due to diffuser assembly the power output can be augmented, but the cost of shrouded type turbine increases due to the complexity in design. According to Kogan 43, 44 et al. (1962 &1963), power output is a function of the rotor disc loading, diffuser inlet and exit pressure. In their analysis, the diffuser was characterized by the exit to the entry area ratio of 3.5. Moeller 45 et al. (2008) of Clarkson University also proposed diffuser augmented wind turbine (DAWT). The investigation concludes that the power output of DAWT is 4.25 times more than the power produced by conventional wind turbine. High output from the turbine is expected because of placing a diffuser at

7 27 the outlet of the turbine to control the flow rate of air producing at sub-atmospheric pressure. Moeller analyzed Clarkson s wind tamer, also. Thus, the power coefficient of wind mill can be increased to 0.5 from 0.39 when compared to conventional wind turbine. The lowstatic pressure induces greater mass flow of air through the turbine in contrast to a conventional turbine design of the same diameter. The analysis is based on two diffuser design concepts. One is directed toward the unconventional, extremely small and cost effective configurations. This approach is based on the active external wind, to prevent separations of the diffuser internal boundary layer. Another concept used high lift airfoil contours for the diffuser wall structure. Test results show that the power produced by DAWT is almost two times the power produced by conventional wind mill. Moeller concluded that the DAWT configuration is found to be cheaper than conventional wind mill for rotor diameters between 50 m and 20 m. The optimum design of diffuser is incorporated in the analysis. Gilbert 46 et al. (1978) contributed efforts in connection with the company vortex energy, New Zealand. The experimental results are compared with the CFD analysis of Flay 47 et al. (1999) According to Flay the maximum power coefficient is 1.0. But according to Vortec Energy Company the maximum power coefficient is significantly less than Betz limit. Philips 7 (1999) submitted a thesis to University of Auckland based on the concept of diffuser Augmented wind Turbines. Oman 48 et al. (1978) studied the fluid flow through diffuser augmented wind turbine. Pressure and velocity variation through the

8 28 cross section of DAWT is discussed in the article. Lift and drag on the blade of diffuser augmented wind turbine is studied by Phillips 7 et al. (2008). Igra 49 (1984) submitted a research paper on shrouded wind turbines in the Energy Conference EWEC 84, held in Hamburg. Igra compared the conventional wind mill and shrouded wind turbine. Hansen 50 et al. (2000) studied the efficiency of a horizontal axis wind turbine by using the diffuser around the rotor using CFD. According to Hansen the power coefficient can exceed Betz limit, through mass augmentation. Wele 51 et al. (2008) conducted experiments on duct type water and wind turbines. The augmentation of power is discussed in their research. Van Bussel 52 (2007) studied the effect of torque in the rotor of a wind turbine using a diffuser round the rotor. Abe 53 et al. proposed the concept of flanged diffuser geometry in various articles. Metins 54 (2006) published a thesis on nozzle concentration effects for buildings with diffuser augmented wind turbines. The research of Bussel 55 concludes that low back-pressure and high diffuser exit area are highly beneficial. In the investigation, the optimal pressure drop is 8/9 of local dynamic pressure and is equal to the pressure drop of bare wind turbines without mass flow augmentation. According to Gerard, the augmentation factor 3.0 cannot be achieved. Vries 56 (1979) developed a theoretical model using diffuser augmented wind turbine. The negative back-pressure obtained in the earlier experiments is used in the analysis. According to study, the power factor 2.0 can be achievable.

9 Vortex Augmented Wind Turbines Kinetic energy concentrator in the form of congestion produces a similar sense of diffuser. Thus, the air with high vortex occurs across the rotor drum. The congestion or obstruction can also be in the form of space between buildings in an urban environment. Rutherford Appleton laboratory, U.K (2007) installed a wind turbine of 2 m rotor diameter between kidney shaped tall buildings. Sforza 57 (1977) used delta concentrator to signify the vortex augmentation theory. Loth 58 (1977) stated that the upstream of the vortex core results collapsing pressure field, reversal of flow and, therefore, no augmentation. Olivieri 59 (1996) obtained experimental and numerical predictions of power coefficient as 0.9 and 1.5, where the Betz limit is only Hence, from the above two authors, it is debatable whether the power coefficient is increased or decreased due to vortex augmentation theory Wind Turbine with Tip Vanes Another method to increase the power coefficient of the horizontal axis wind turbine is by incorporating small auxiliary wings on blade tips. Such design increases the amount of lift force. Vanholten 60 (1976) investigated using tip vanes of length almost equal to 1 to 2% of structural material. Vanholten s 42 experimental studies conclude that tip vortices almost become zero at high tip speed ratios. This resembles ejector type augmentation. The power coefficient can be so, increased to 1.5. Bussel 61 (1977) conducted another inspection using tip vanes to increase the power coefficient of wind turbine to 2.0. In both the above cases, the power coefficient exceeds Betz limit.

10 Convergent Nozzle to Solar Chimney The article of Grant 6 et al. (2002) presented the design of a solar stack. Such designs are suited in rural areas of developing countries. The model involves heating of air using solar energy and the chimney action to keep the hot air moving up through chimney stack. Thus, the velocity of hot air can be increased suitable to drive a turbine. The kinetic energy of the hot air is then converted in to electricity by a wind turbine. Particularly solar energy utilization in now a days becomes a challenge in developing countries. The design of a small solar chimney, which employs the convergent nozzle to increase the air to a suitable speed, is aimed at addressing these problems. During winter, the stack requires kwh/day/ m 2 of collector area, having collector slope of 30 o. For a solar chimney, 36 meters high and 4 meters diameter, the air velocity of 3.53 m/s, the maximum theoretical power output is found to be watts. Extra development of convergent nozzle increases the speed of air to 15 m/s and thus, power to watts. Turbine blade calculations are based on a design of a NACA blade rotor with a diameter of 0.97 m. The cost of electricity becomes approximately equal to Rs 7.80 per kwh generation. Such technology is experimented for the situation of Botswana. The technology is economically viable when compared to current technologies of power generation. The generation cost per kwh reduces for large capacity plants.

11 FLAT NACA AIRFOILS AND LIFT COEFFICIENT Experiments are conducted to increase the power coefficient of wind turbine at different angles of attack using NACA blades. NACA has developed airfoils under 4 digit series and 5 digit series. For 4 digit series, the camber as a percentage of the chord length is represented by the first digit. The second digit represents the maximum camber from the end of leading edge of airfoil in ten s of percentage of the chord length. The last two digits represent maximum thickness of airfoil as a percentage of the chord length. For example, NACA 2414 will have maximum camber of 2%. It is occurred at 40% of chord length from leading edge. The maximum thickness of an airfoil is equal to 14% of chord length. NACA 0009 is symmetrical airfoil. 00 indicates no camber and 09 indicate maximum thickness of 9% of chord length. For 5 digit series, the first digit multiplied by the factor 0.15 gives the lift coefficient. The second and third digit when divided by 2, gives the maximum camber from leading edge as a percentage of chord length. Fourth and fifth digits represent the maximum thickness as a percentage of chord length. For example, NACA will have the maximum thickness of 25% of chord length. Maximum camber is located at 10% of chord length. The lift coefficient for such airfoil becomes 0.3. High lift coefficient indicates the maximum power produced by the turbine. The selection of an airfoil is beneficial to run the wind turbine satisfactorily. Abott 62 (1958) analyzed the properties of airfoil using

12 32 theory of airfoil section. According to Grifith 28 (1977), the output power of the turbine depends on lift/drag ratio. But, according to Hassanien 63 et al. (2000) the selection of airfoil should be so that location along the blade ensures highest contribution to the overall performance. Sepera 64 (1935) studied the Wilson s methods of blade design. The main objective of the study is to improve the power coefficient of wind turbine using appropriate blade design at rated wind speed. But, due to time variation characteristics of wind speed, the blades not achieved maximum annual energy production. Design results are not so accurate to obtain optimum chord length and twist distributions. Due to variations in results, the blade design poses problems. Maalawi 65 et al. (2001) studied the theoretical optimum distribution of the inflow angle using trigonometric function methods which are based on ideal condition of Glauert 66 (1935). The study has developed iterative solutions for relationship of angle of attack. Nagai 67 et al. (2009) developed a prototype wind turbine of capacity 3 KW having blade diameter of 4 meters. Such systems are developed in the regions where typhoons occur. In such locations wind turbines are installed with variable pitch control systems. No, mechanical breaking system is provided. The turbine is controlled by adjusting the pitch angle and maximum electric load. The turbine is investigated using variable pitch angle and regulation of field current. According to the simulation results the wind turbine shows power coefficient of with the average wind speed of 7.3 m/s.

13 33 Liu 68 et al. (2007) presented the optimization model of the horizontal axis wind turbine with an intention to deliver maximum annual energy. Extended Generic Algorithm (EGA) is used in the optimum design. The simulations results can be used in installing the wind turbine in the ranges of 1.3 MW capacities. Habali 69 (1995) developed the design procedure for airfoil sections of small capacity wind turbines. Five meters length of blades, of two different airfoils mixed at the outer span of the span will be sufficient for improving aerodynamic characteristics. Lanzafame 70 et al. (2007) developed a mathematical model based on blade element moment theory. The simulations results are then, compared with experimental results. The comparison is made under on-design and off-design conditions. Many researchers contributed work insight into rotor design. Particularly the attention is focused on lift and drag coefficients. The study of Snel 71,72 (1988, 2003) gives over view of different methods, to evaluate the aerodynamic performance of wind turbine. Tangler 73 (2000) and Fuglsang 74 (2002) presented the overview of rotor design investigations. Tangler of National Renewable Energy Laboratory also stated the guidelines for performance characteristics of airfoil based on blade element momentum theory. The test results of unsteady aerodynamic analysis are used to establish steady state data. According to Tangler, the flat or symmetrical blades of aero turbine produce high lift coefficient at 45 degrees of attack.

14 34 The rise in productivity of wind turbine based on blade design is also studied by Johansen and Madse 75 (2005). From the analysis of Johansen, it is found that new root designs of turbine blade do not increase the power coefficient significantly. However, the effect of various parameters in raising the efficiency of wind turbine like tip speed ratio, Reynolds number and blade twist are simulated. Another wind turbine of capacity 5 kw is manufactured by Silkroad International Company considering the lift coefficient as 1.1. The lift and drag coefficients are chosen from Abbott 62. The Reynolds number is assumed as 3 x 10e6. During study, NACA four digit series and five digit series airfoils are used. The blade is divided into ten elements. Later, the efficiency of wind turbine is estimated considering every element using the computer program. The analysis proved that stresses are high at the root of the blade. Rajkumar 76 et al. (2010) conducted an experiment using NACA 4420 airfoil from 0 o to 12 o of attack using CFD. Kumar concluded that 5 0 of attack resulted high lift/drag ratio. Colman 77 et al. (2008) studied the aerodynamic performance of HQ 17 airfoil with and without Gurney flap. In the analysis, they have used two different turbulent flows of intensity 1.8% and 3.5 % of the same wind speed. The Reynolds number during the study is 3x10 5 for two different regimes. The results reveal that the lift coefficient is increased as gurney flap increases and thus the power coefficient of wind turbine. The results were contrasted with the results of Bechert 78 et al. (2000) for the same HQ 17 airfoil. Ahmadi 79 et al. (2009) investigated the simulation study

15 35 of NACA 0012 airfoil which is oscillating in different amplitudes. Simulation is carried out using ANSYS software. During the study, the Reynolds number and angle of attack range from 9.71e5 to 22.65e5 and to respectively. The forward wind speed is treated as an independent parameter. Ahmadi concluded that Shear stress Transport model (SST model) yields better results with least error. Also, it is found that loss in lift occurred due to dynamic stall phenomenon. Maldonado 80 et al. (2009) increased the efficiency of wind turbine using synthetic jet actuator. They passed air from wind tunnel to turbine using jet actuator. Thus, they reduced the loss of energy in wind. Also, the flow separation is reduced. The method of Particle Image Velocimetry is used to determine the flow field of wind. Maldonado studied the variations in moments, forces and vibrations. It is concluded that synthetic jet actuator governs the above variations. Sheldahl 81 et al. (1981) of Sandia National Laboratories investigated the performance of NACA 0009 and NACA 0012 airfoils for different angles of attack. Angle of attack is varied from 0 0 to They concluded that 45 0 of attack yields maximum lift coefficient. Whereas, NACA 0012H and NACA 0009H airfoils work better at 10 to 12 degrees of attack. It is slightly more than at 45 degrees of attack. Design of NACA 0009H and NACA 0012H indicate that no change in the ratio of thickness to chord length. But, retaining 10 to 12 degrees of attack for a longer time is extremely difficult. Since, at this situation the direction of wind is exactly perpendicular to the rotor. But, when the angle of attack is 30 to 55 degrees the performance of the turbine

16 36 is stable. In the investigation, though 12 degrees of attack produce more lift coefficient, but it is not preferred. At the state of optimum lift coefficient, the power coefficient of wind turbine also becomes high. 2.3 USE OF CFD IN THE WORK Computational fluid dynamics is a software tool used to simulate flow phenomenon of gases and liquids. Modeling software GAMBIT and Analysis software Fluent are used in the study. It is useful in simulating fluid flow for the determination of heat and mass transfer coefficients, moving bodies, multiphase physics, chemical reactions, fluid structure interactions and acoustics through computer modeling. A vertical prototype of any system can be built using the simulated results of CFD software. The CFD software provides the images and information, which predict the performance of that design. In the present thesis, CFD is used for the validation test of air velocity from nozzle system. Variations of other fluid properties such as pressure, intensity of turbulence etc, are also studied using CFD along the length of multiple nozzle system. 2.4 Thematic Representation of Multiple Nozzle System The multiple nozzle system used in the analysis is represented in Figure 2.1. The creation of multiple nozzle system was awarded design patents. Multiple nozzle system is assembled to wind tunnel instead of wind turbine in the experiment. Thus, extra load stresses on the rotor can be minimized. The effect of conversion of heat energy into kinetic energy is also studied as on objective. The multiple nozzle system, if it

17 37 is used, may convert the heat energy of wind into kinetic energy to some extent. Experimentally, increase in velocity due to multiple nozzle system is compared using computational fluid dynamics. In the present study, heat absorbed by air is also estimated. Figure 2.1 Thematic Representation of Multiple Nozzle System Literature is collected for single nozzle wind turbines. Yet, multiple nozzle system is not used in the field of wind energy. In the next chapter, the test set up including the manufacture of multiple nozzle system and method of conducting research is furnished.