Alternate Energy Engineering - Turbine Blades - Research Paper Example

Alternate Energy Engineering: Specific Topic: Turbine Blades Alternate Energy Engineering: Specific Topic: Turbine Blades Introduction Wind turbines have been in use for many years. Wind turbines were used earlier in the 14th, 15th, 16th, 17 and 18th centaury to grind grains and for pumping water form wells. Turbine blades were made from wood and cloth to extract as much wind energy as the blade can. Previously the designs of the wind turbine blades are not very impressive to attain maximum amount of energy from wind. As the technology progresses, the designs of the wind turbine blades also changed. In the mid 19th centaury, wind turbines with multiple blades made from steel were found to be very useful at far regions. These wind turbines are also known as American Farm windmills. These turbines were used to pump water from ground. Later, the similar model was used to generate electrical energy on small scale. Theses wind turbines had turbine blades wider from outer edges and narrower from the inner edges. The blade edges nearer from the rotor were narrower as compared to that of the edges at the other side. Blades were made at an angle to capture maximum amount of aerodynamic energy. These types of wind turbines are still made as DIY projects. Wind generators are now used to generate electrical energy. Wind generators use wind turbine to attained mechanical energy from winds and then transform the mechanical energy into electrical energy with the help of alternator and gear assembly. The main design aspects of ancient and modern wind turbines are the same but modern wind turbines are far more efficient as compared to that of the ancient turbines. On the other hand, technology also focused on the economic efficiency of the wind turbines. Modern wind turbines are capable of generation more than 5 MW of power. Wind turbine blades are carefully manufactured to collect as much wind energy as possible. On the other hand, advanced electronics are focused to maximize the energy efficiency by collecting the wind data over time and implement the data after the installation of the wind turbines (Burton, 114). The air resistance angles of blades are adjusted with respect the speed of wind in order to maintain a constant rotational motion this type of motion is often referred as pitch of the wind turbine or pitch of the blades. Moreover, the modern wind turbine generators are capable of changing the direction of the turbine as the direction of wind changes. This type of motion is often referred as Yaw. Either PLCs or computers or both are there to control all the parameters of a win turbine generator to maximize the output of the wind generator. Vertical Axis Wind Turbine (VAWT) is under research phase nowadays that are designed to maximize the energy efficiency. The research models of VAWT proved to be more efficient as compared to that of the Horizontal Axis Wind Turbine models. However, commercial VAWTs are not marketed yet nor any big name in wind turbine manufacturers proposed any project regarding VAWT. However, some small companies are making VAWTs on small scale and marketing it for individual use. Both the types of wind turbines have different orientation of blades. For three blade design of either VAWT or HAWT, there is a 120o angular difference between the blades. For two blade design, the angular difference between the blades is 180o and for four blade design, the angular difference between the blades is 90o (Burton, 118). Fig 1: Wind Turbine Orientation (Burton, 118). Importance of turbine blades Turbine blades are the most important part of the wind turbine. Conventionally turbine blades are manufactured with tensile metals which are then reinforced with ceramics, other metals, fiber glass or then some materials to enhance the durability of the blades and reduce the metallic characteristics of fatigue, rupture and corrosion (Quarton, 6). Turbine blades collect the kinetic energy of wind and transform it into mechanical energy by reacting against the kinetic energy of the wind. The energy transferred from the wind is directly proportional to the speed of wind. On the other hand, the reacting surface area of the wind turbine blades also matters a lot. On the other hand, it is important to design the turbine blades in the manner that they produce no hurdle in transferring energy from the wind to the rotor. A wind turbine may have multiple turbine blades but wind turbine should have at least two turbine blades that should be properly balanced with respect to each other and with respect to the rotor of the wind turbine. Any imperfection in the wind turbine blades results in effective power loss and rupturing with continuous use. Wind turbines that are commercially utilized for generating power often have three blades (Burton, 119). Theoretical efficiency of wind turbines blades The theoretical efficiency of the wind turbines can be mathematically expressed as: Here, P is the power that can be collected from turbine, A is the Swept area, V is the velocity of wind, and is the air density. The swept area can be mathematically expressed as: Here r is the swept radius or the length of one turbine blade from center of the rotor to the tip of the blade. In this manner, we can deduct that larger the length of blades, the more power can be attained from the wind turbine. In this way, length of blades is directly proportional to the power output of the wind turbine. However, it is better to carefully deduce the other factors like effective length of the turbine tower and power to weight ratio of the blades before expanding the lengths of the blades (Yurdusev, Ata, Cetin, 2153). Practical loses in efficiency The evaluation is for the 100% attained power. However, practically, the output power of a conventional wind turbine remains lesser than 59.3% as the maximum value of power coefficient (Cp) has a maximum value 0.593. Following are the blade losses that further reduce the efficiency of the turbine. Lower lift to drag ratios Higher turbulence of blades Defects in blade shape like unsmooth surface, etc HWAT Blades Design parameters Tip speed ratios Tip speed is the speed of the rotor blade. Tip speed velocity is the “relationship between the rotor blade velocity and relative wind velocity”. Mathematically, the relation can be expressed as: Where, is the tip speed ratio, is the rotational speed in radian per second, r is the length of the blades from center of the rotor to the tip of the blade, and is the wind speed. Other aspects that may affect the tip speed are torque, stress and losses related to aerodynamics. The higher the tip speed, the higher efficiency of wind turbine can be attained. Reducing the chord width enhances the tip speed. On the other hand reducing the chord width also reduces the blade’s width. Wind turbine blades with narrow profiles are manufactured to enhance the efficiency by enhancing the tip speed of the turbine. On the other hand, narrowing the turbine blades also reduces the cost and material usage in the wind turbine blades. However, enhancing the tip speed of the turbine blades increases the aerodynamic force on blades and centrifugal force on blades. Most commercial wind turbines have the tip speed ratio within the range of 6 to 9 for three blade turbines (Yurdusev, Ata, Cetin, 2154). Blade Plan shape and Quantity According to the Betz approximation: Here, r is the length of the blade in meters, n is the quantity of blades; CL is the lift coefficient λ is the tip speed ratio of the turbine, Vr is the resultant air velocity of the region, U is the wind speed, Uwd is the designed wind speed and Copt is the Optimum cord length (Yurdusev, Ata, Cetin, 2155). Fig 2: Three Blade Regions (Yurdusev, Ata, Cetin, 2155) Betz optimization suggests that as the tip speed ratio increases the chord length decreases. On the other hand, as the tip speed ratio decreases, the solidity ratio of blades increases. Here, ratio of solidity is the ratio of area of blade to the swept rotor area. It is better to limit the ratio of solidity as it helps in reducing the production costs and material cost but reducing the width of the blades. It is also a better option to enhance the efficiency of the turbine blades. The image below depicts the relationship between the tip speed ratio, chord lengths and number of blades (Yurdusev, Ata, Cetin, 2156). Fig 3: Optimal Blade Plan (Yurdusev, Ata, Cetin, 2156) While designing the blades for the optimum efficiency it is better to consider the number of blades negligible and then compute the blade’s chord length. Aerodynamics of the blades Aerodynamic performance of the blades is the basic characteristic of the blade that affects the efficiency of the wind turbine. While considering the aerodynamic properties, two basic aerodynamic properties are lift and drag. Lift is the force that is attained when wind strikes the surface of the blade. Lift is the force that is responsible for the rotation in the rotor as lift transfers the energy from wind to the rotor. While, drag is the opposing force that acts against the lift or drag is the force that tries to stop the energy transfer from blades to the rotor. Both the forces act in opposite direction and try to cancel each other. In order to attain the maximum energy efficiency from the turbine blades, the lift to drag ratio should be significantly large. In general, the lift to drag ratio can be simplified as: The aerodynamic performance is affected as the aerofoil thickness varies. Previously aerofoil thickness conditions were selected with respect to the models available for the aviation industry and the similar Reynolds number as that used for the aviation. However, modern technology suggests that aerofoil thickness patterns should have to be considered according to the available conditions. On the other hand, aerofoil thickness patterns are carefully selected with respect to the specific mechanical load patterns (Kishinami, 2099). Table 1: Airfoils required for blade section (Timmer, van Rooij, 488) In order to make the blades structurally fit, thick aerofoil sections must be used at the root sections of the blades. Thick aerofoil sections are generally considered to have lower lift to drag ratio. Therefore, thick aerofoil sections are carefully selected and designed to attain maximum efficient and higher lift to drag ratio. Specifically designed airfoils like “Deft University”, “LS”, “SERI-NREL and FFA” and “RISO” are designed to enhance energy efficiency of the turbine blades. These airfoils are specifically designed to suit the wind turbine blades (Timmer, van Rooij, 488). It is improper to have the aerofoil with similar thickness all over the turbine blade, as it reduces the efficiency of the turbine blades. Varying the aerofoil thickness at various sections enhances energy efficiency (Duquette and Visser, 432). Angle of twist Angle of twist is usable angle that is the function of angle of attack. When the stream of wind strikes the surface of the blade, the angle of attack of the wind strikes the blades. At certain angle of twist of blade, the response of the blade with respect to the angle of attack in the perspective of angle of twist is different. The angle of twist is dependent on two basic factors: Tip speed ratio Desired aerofoil angle of attack In general, the sections nearer to the hub are made at angle to maximize power to maximize “high ratio of wind speed to radial velocity of blade”. On the other hand, the tip of the blade is made to react normally with the wind (Timmer, van Rooij, 488). Wind turbines have a mechanism to maintain a constant rpm of the rotor. As the velocity of wind changes the wind turbine changes the angle of twist by rotating the blades vertically increase or decrease the reactivity of the turbine blades with respect the velocity of the wind. This strategy prevents the turbine from damages and maintains the constant rpm of the turbine. Previously gear assembly was utilized to change the gear ration with respect to the velocity of wind (Griffiths, 326). Blade material Conventionally steel was considered the bets material to manufacture wind turbine blades due to its extreme rigidity. However, as the technology progresses, material like glass fibers and carbon fibers and their composites have presented better performance as compared to that of the conventional materials. Carbon fiber composites are considered to be best for the wind turbine blade manufacturing due to its better properties (Kong, Bang, Sugiyama, 2010). Carbon fiber materials have better: Load bearing capacity Energy transfer to weight ratio Corrosion proof and rust proof properties Better shape maintaining properties Less influence with thermal expansion A commercial 2.0 MW wind turbine blades have the following configuration: Table 2: Configuration of Commercial 2.0 MW Wind Turbine Blades (Barlas and Lackner) Fiber glass materials are also utilized to further strengthen the blade structure. Aluminum and fiber glass composites were also researched at initial stages and found to have better properties a compared to the conventional materials. The composites are found to have better structural properties as compared to that of the aluminum. Aviation industry utilizes the aluminum and fiber glass composite materials to develop airplanes (Barlas and Lackner). Blade manufacturer World’s famous wind turbine blade manufactures are: Siemens AG (Erlangen, Germany) is one of the biggest manufacturers of the wind turbine blades. The company is focused to develop the largest ever wind turbine blades for its largest ever wind turbine. The wind turbine blades will have the length of about 75 meters or 246 feet. The turbine blades will have no joints or no week points. In this manner, the blades will have 20% lesser in weight as compared to of he propose weight. LM Wind Power (Kolding, Denmark) is one of the biggest manufacturers of wind turbine blades. LM wind power supplies wind turbine blades to the other turbine manufacturers. LM wind power is developing the wind turbine blades which have significantly low noise as compared to that of the previous model and a significantly higher efficiency as compared to that of the previous models. LM wind power is also researching for high efficiency coating for the wind turbine blades that not only enhance the performance of the wind turbine blades but also enhance the life expectancy of the wind turbine blades. Molded Fiber Glass Cos. (MFG, Ashtabula, Ohio) 3M’s Renewable Energy Div. (St. Paul, Minn.), researched and manufactured a unique coating for the wind turbine blades that protect the blades from harsh impacts of weather. In particular, the conditions of surface roughing, scaling and chemical reactivity is limited with the use of special wind turbine blade coating acrylic chemical epoxy based emulsion. On the other hand, the quick drying agent in the coating makes it possible to coal several layers where desired in order to change the aerodynamic behaviors of the blade. Vestas Wind Systems A/S (Aarhus, Denmark) has its own name for its extensive research on wind turbine blades and performance of various models of wind turbine blades. The researches projects, the company are handling are suitable to manufacture the wind turbines which have low noise but high efficiency and low cost. The company promises to develop a glass fiber wind turbine blades which will have low radar signal disturbance ratio. The company developed a special type of “ink” that is sandwiched between the fiber glass layers. Such type of wind turbine blades would produce low sensitive signals when a radar signal strikes the wind turbine blades. GE Wind Energy (GE’s R&D center, Munich, Germany) /TPI Composites, is one of the largest organizations that are working in the energy and power generation sector for many years. GE is operational in many countries and TPI Composites provides rotor blades for all these locations. The renewable energy sector of the GE is working to promote renewable power generation methods. GE is determined to develop sustainable solutions for energy sector. GE is making wind turbine blades for GE R&D center is working to develop the wind turbine blades with low sensitivity for radar systems. TPI Composites is developing the wind turbine blades, which have significantly higher efficiency but radar have low sensitivity for the blades for GE. GE is using several layers of materials between the fiber glass reinforced wind turbines blades that have the capability of absorbing the radar radiations instead of reflecting them back to the station. In this manner, the turbine blades are unable to be sensed by the radar detection system. In 2012, GE sold 1000 1.6 MW wind turbines and thus TPI Composites manufactured 3000 wind turbine blades in 2012. Blade Loads Aerodynamic load Aerodynamic load is due to the drag of the blade; while the like also produce aero dynamic load. The aerodynamic load is directly proportional to the speed of wind, velocity of blade, angle of attack, angle of twist and yaw. Aerodynamic loads can be dangerous in cases like rapid change in speed of wind, rapid change in direction of wind, improper and unsynchronized angle of twist of the blades. As the wind turbine attains a constant rotational speed, the aerodynamic loads of the wind turbine blades reduce; while at initially, the aerodynamic loads are way too much. In some of the wind turbine, a head start is given to the turbine with the help of a motor that reduces the aerodynamic loads on the turbine blades (Kishinami, 2100). Gravitational load and Centrifugal load Gravitational force always tries to oppose the motion of the turbine blades by consistently neglecting the upward motion of the blades. On the other hand, centrifugal force always tries to act in outward direction. Since there is not force that is acting inside direction. Bother the forces are directly proportional to the mass of the wind turbine blades. The heavier the blades are, the larger will the gravitational force and centrifugal force. In the similar manner, radial velocity also impacts on increasing the centrifugal and gravitational forces. Gyroscopic load Gyroscopic loads are due to flap wise bending, edge wise bending and fatigue loads. These types of loads not only influence the symmetry of the wind turbine blades but also reduce the efficiency of the turbine and reduce the overall performance of the wind turbine. Conclusion The performance of wind turbine blade depends on various factors like angle of twist, thickness to chord ratio, structural load bearing requirement, geometrical compatibility, maximum lift to insensitive to leading edge roughness, design lift close to maximum lift off-design, maximum CL and post stall behavior and low aerofoil noise. The change in factor makes brings alteration in the other factors too. Modern wind turbine blades are well researched and an efficiency of more than fifty percent can be attained. Aerodynamic load, gravitational load and centrifugal load and gyroscopic load affect the performance of wind turbine blade. However, carefully designed wind turbine blades may have least values of loads. Works Cited Burton, T. Wind Energy Handbook; John Wiley & Sons Ltd.: Chichester, UK, 2011. Barlas, T.; Lackner, M. The Application of Smart Structures for Large Wind Turbine Rotor Blades. In Proceedings of the Iea Topical Expert Meeting; Delft University of Technology: Delft, The Netherlands, 2006. Duquette, M.M.; Visser, K.D. Numerical implications of solidity and blade number on rotor performance of horizontal-axis wind turbines. J. Sol. Energy Eng.-Trans. ASME 2003, 125, 425–432. Griffiths, R.T. The effect of aerofoil characteristics on windmill performance. Aeronaut. J. 1977, 81, 322–326. Kong, C.; Bang, J.; Sugiyama, Y. Structural investigation of composite wind turbine blade considering various load cases and fatigue life. Energy 2005, 30, 2101–2114. Kishinami, K. Theoretical and experimental study on the aerodynamic characteristics of a horizontal axis wind turbine. Energy 2005, 30, 2089–2100. Quarton, D.C. The Evolution of Wind Turbine Design Analysis—A Twenty Year Progress Review; Garrad Hassan and Partners Ltd.: Bristol, UK, 1998; pp. 5–24 Timmer, W.A.; van Rooij, R.P.J.O.M. Summary of the Delft University wind turbine dedicated airfoils. J. Sol. Energy Eng. Trans. ASME 2003, 125, 488–496. Yurdusev, M.A.; Ata, R.; Cetin, N.S. Assessment of optimum tip speed ratio in wind turbines using artificial neural networks. Energy 2006, 31, 2153–2161. Read More