Product Design of Wind Turbine Blade - Essay Example

Product Design and Development of a 10kW Horizontal Axis Wind Turbine Blade January 2009 Introduction Wind power generation techniques are being promoted all over the world as a beneficial alternative among other renewable sources (Li, Chen 71). A wind turbine is defined as a rotating machine that is used to generate power by means of converting kinetic energy to mechanical energy. Kinetic energy is formed when the blades of the turbine captures wind, making the blades rotate in its axis. This motion of the blades creates mechanical energy that is used to generate electricity (Mahoney 1). Wind turbines are very useful in power generation especially in places where wind velocity ranges from intermediate to high level. According to studies, wind contributes only less than 1% of the world's energy needs (Grose 1). Even if the percentage of contribution is very low, the use of wind turbines as means to generate electricity grow continuously. This is because people nowadays discover the good benefits of using wind turbines in terms of environmental factors. However, some disadvantages still prevent other people from using wind turbines and few of these reasons are related to high cost of manufacture and noise created by these machines (Mahoney 2). Ongoing improvements on the blade's design and material are continuously done to further develop the machine in terms of its aerodynamic properties and performance. 2.0. Preliminaries to Product Design 2.1. Definition of Wind Turbine A wind turbine is a machine that can generate electricity through the use of wind power. A wind turbine has large rotating blades that are capable of capturing wind and creating kinetic energy. This energy is converted to mechanical energy which is used to generate electricity for various purposes (Mahoney 1). Shown below are the parts of a typical wind turbine: Among all the parts of a wind turbine, the most important part is the rotor blade. If the blade is poorly designed, the performance of the wind turbine will not be efficient (NRC 12). 2.2. Types of Modern Wind Turbine 2.2.1. Horizontal Axis Wind Turbine The horizontal axis wind turbine (HAWT) is also known as the classical or most commonly used wind turbine in the world. The axis of rotation of these turbines is horizontal to the ground and parallel to the wind stream (Mathew 16). Shown is a diagram showing a sample of a horizontal axis wind turbine. The advantages of using HAWT are its structural stability and changeable blade pitch that allows greater control of the turbine and aids the blades in catching the maximum amount of wind. Versatility is also seen as one of its advantage in terms of its ability to be feathered in case of storms (Mahoney 1). 2.2.2. Vertical Axis Wind Turbine Vertical axis wind turbine (VAWT) is different from HAWT because it is vertical to the ground and almost perpendicular to the wind direction. The VAWT can receive wind from any direction and so, complicated devices are avoided. The advantage of using VAWT is that they are not needed to be built very high. Maintenance is also easier since the VAWT is located nearer to the ground. The design is also not complicated and blades can easily be seen by birds (Mathew 19). On the other hand, VAWTs are less efficient than HAWTs since they can only produce energy that is 50% of what HAWTs can produce. Another downfall of VAWTs is that it can only rotate faster in higher elevations and with high wind velocity. Lastly, the turbine must be dismantled first in order to change or repair some parts when necessary (Mathew 19). Shown in diagram 2 is a sample of a vertical axis wind turbine. 2.3. Types of Horizontal Axis Wind Turbine Blades Wind turbine can be classified as single bladed, double bladed, and three bladed. The cheapest among the four classifications is the single bladed wind turbine since it only consumes small amount of material and labor. On the other hand this design is not ideal since balance is also an important factor in wind turbine construction and single bladed wind turbines does not have anything to counter the weight of the blade. Furthermore, it has higher rotational speed and creates noise and visual intrusion problems (Mathew 17). Double bladed is better than single bladed in term of balance but also has some drawbacks. Double bladed wind turbine requires more complex design with a hinge rotor that can act as shock absorber. Also, like the single bladed, it also has high rotational speed and noise level. Nowadays, most of the wind turbines are three bladed due to the fact that it exhibits better balance and has lower rotational speed and noise level than the previous two (Mathew 17). 2.4. Upwind and Downwind Wind for Horizontal Axis Turbines Horizontal axis wind turbines have machines that can be classified as upwind or downwind. Upwind machines have the rotor facing the wind directly. As the wind stream passes the rotors first, they do not have the problem of tower shadow (Mathew 18). By far the vast majority of wind turbines have this design. The basic drawback of upwind designs is that the rotor needs to be made rather inflexible, and placed at some distance from the tower. In addition an upwind machine needs a yaw mechanism to keep the rotor facing the wind (Quaschning 203). Downwind machines have the rotor placed on the lee side of the tower. This is better than upwind machines since they can be built without a yaw mechanism, if the rotor and nacelle have a suitable design that makes the nacelle follow the wind passively (Mathew 18). Another advantage is that the blade may be made more flexible and it may be built somewhat lighter than an upwind machine. However, the fluctuation in the wind power due to the rotor passing through the wind shade of the tower may give more fatigue loads on the turbine than with an upwind design (Quaschning 203). 2.4. Material for Wind Turbine Blade and its Properties 2.4.1. Glass Fibers Glass Fibers are the cheapest and most widely used reinforced fibers. Glass fibers are produced by drawing molten glass through small opening in a platinum die. The most common type of glass fiber that is used in wind turbine blades is the E-glass fiber. Another type of glass fiber that is used in matrix composites is the S type which has a 40% higher strength and stiffness but is more expensive than E-glass fiber. (Kalpakjian 245). Shown in Table 1 is a summary of the mechanical, physical, thermal, optical and acoustical properties of various types of glass fibers When glass fibers are used for polymer products, the resulting composite material is called Glass Reinforced Plastic or GRP. GRP is most known as "Fiber glass" and is used for many purposes especially for manufacturing wind turbine blades (NRC 44). 2.4.1.1. Properties of Glass Reinforced Plastic The use of fiberglass in wind turbine blade manufacturing is preferred because fiberglass exhibits superior properties like resistance to heat, corrosion and chemical resistance, durability and outstanding electrical properties. Moreover, fiberglass is inorganic, inexpensive and economical making it more compatible for competitive blade manufacturing (Barker 449). 2.4.2. Carbon Fibers Many aerospace applications choose carbon fiber due to its superb properties. Despite its expensive, carbon fibers have higher specific strength and fatigue resistance than E-glass fibers. (Committee 35) It is also five times stronger than grade 1020 steel but still five times lighter. It is also has superior fatigue properties and its heat resistance makes it preferable to be used in manufacturing wind turbine blades for high temperature areas Unlike glass fibers, carbon fibers provide higher specific modulus and specific strength (NRC 35), but it is brittle and it is considered as electrical conductor and corrosion may occur when it is in contact with metals. 2.4.2.1. Properties of Carbon Fiber Composite Carbon Fiber Composite is formed from the mixture of different fillers, one of which is carbon fiber. The matrix for the composite is usually a polymer, a carbon, a ceramic or combination of different materials. The property of carbon fiber composite depends on the orientation of matrix. For unidirectional composite, the longitudinal tensile strength is independent of the fiber matrix bonding but the transverse tensile strength and flexural strength increase with increasing fiber-matrix bonding. In the case of carbon composite that consists of ductile matrix such as metals and polymers, a crack that is initiating in the brittle fiber tends to be blunted when it reaches the ductile matrix even if the bonding is strong (Chung 81). At 300K, the thermal conductivity of carbon fiber composite is about four times greater than fiberglass (Chung 110). Shown in Table 2 is the comparison of mechanical properties and density of different fiber composites specifically carbon and glass fibers. Based on the table, the tensile modulus and compression of carbon is greater than that of glass. 2.5. Principles of Wind Turbine Blade Design 2.5.1. Theoretical Power of the Wind that Goes to the Rotor As stated, wind turbine generates power from the conversion of kinetic energy to another form of energy. The kinetic energy that comes from the speed of the wind creates power and this power is the maximum power that is usable and convertible (Vardar and Eker 1531). 2.5.2. Wind Turbine Blade's Tip Speed Ratio Tip-speed ratio is the ratio of the speed of the rotating blade tip to the speed of the free stream wind. The design tip speed ratio is said to be closely dependent to the number of blades. For a three bladed motor, the maximum tip speed ratio is reached between 7 and 8. For a two bladed rotor, the value is 10 while it is 15 in single bladed rotor (Quaschning 200). Based on studies, there is an optimum angle of attack which creates the highest lift to drag ratio. This angle of attack is dependent on wind speed; there is an optimum tip-speed ratio (Vardar and Eker 1531). 2.5.3. Wind Turbine Blade Power Coefficient The entire computed theoretical power of the wind cannot be converted into practical power. It is because during the process, some power losses occur. Power coefficient is just the ratio of the theoretical wind power to axle power and is dependent on some factors like the profile of the rotor blades, blade arrangement and materials used (Mathew 14). The table below shows these principles and respected formulas (Vardar and Eker 1531). 2.6. Manufacturing Processes for Wind Turbine Blades 2.6.1. Resin Transfer Molding Resin Transfer Molding (RTM) is done by transferring resin into a dry preformed of aligned fibers that are constrained by matched inner and outer surface tools. This transfer is done by means of vacuum and pressure (Commission). Shown in Diagram 7 on the next page is a typical resin transfer molding in which several layers of dry continuous fibers are placed in the bottom half of a two-part mold manually using hand lay up or automatically using laying machine. The mold is then closed and catalyzed resin is injected slowly via a centrally located sprue. Injection pressure usually ranges from 70 - 700 kPa. Careful injection is done so that any sudden movement of the fibers inside the mold is avoided (Kalpakjan 553). The mold is held under vacuum so that air can be removed and resin can spread freely inside. As an alternative, the edges of the mold can be vented to allow air to escape while the resin is being injected. When excess resin begins to flow from the vet areas of the mold, the resin flow is stopped and the molded part begins to cure. Curing time takes several minutes to several hours depending on the size of the part being molded. When the part is already cured, it is taken out from the mold and another process begins again (Mitchell 799). 2.6.2. Pultrusion Pultrusion is a process in which a continuous dry preformed pulled through a matched die while resin is injected under high pressure. The preform is first heated for cure and then cooled in the die to a temperature where it possesses sufficient strength to support the pulling clamp pressure (NRC 86). Shown in Diagram 8 is an illustration of a typical Pultrusion process. The most common material used for Pultrusion is polyester with glass reinforcements (Kalpakjan 555). 2.6.3. Fiber Placement Fiber Placement that is used for manufacturing wind turbine blades is similar to filament winding in which a continuous tape of resin-impregnated fibers is wrapped over a mandrel to form the part. Layers and layers of resin-impregnated fibers are added to achieve the desired thickness (lee, lee and lee 319). Shown in Diagram 9 on the next page is an illustration of filament winding. The process of fiber placement offers all the design freedom of manual lay up with a uniform quality of machine fabrication (NRC 83). Parts with strength at several directions and with high pressure ratings can also be made through this process. Efficient material usage also makes this process preferable for blade manufacture (lee, lee, and lee 321). 3.0. Concept Selection and Specifications of the Wind Turbine Blade 3.1. Dimensions of the 10kW Horizontal Axis Wind Turbine Blade Computation was done with the aid of a computing device available known as "Blade Calculator 2006" from Warlock Engineering. Since average speed varies in different locations, the average wind speed that was considered was that of London. Airfoil for a NACA 2412 was also used. Aside from the fact it is most widely used in wind turbine blade designs, NACA 2412 airfoil has a maximum camber of 2% located 40% (0.4 chords) from the leading edge with a maximum thickness of 12% of the chord; excellent for turbine blade designs (Palmer 281). Values used (in SI units) are shown below in Diagram 10 on the next page. For TSR value, the Tip Speed Ratio Guide included in the software was followed. Based on the software guide, since a three bladed design was chosen, tip speed ratio equal to 7 and blade efficiency equal to 0.4 were used. NACA airfoil of CI equal to 0.85 and equal to 6 was set by the software. During the computation, the measurement of blade radius was altered in order to meet the requirement of 10kW amount of power generated. Using the raw data (shown in blue boxes) and the software; generator power, generator torque and generator speed were calculated (shown in lower red box). Based from the calculated values, for a three bladed 10 kW horizontal axis wind turbine blade with TSR = 7, blade efficiency = 0.4 and running at wind velocity = 5.53m/s, the required radius should be 8.873 meters. Generator speed is 4.41 rad/s and generator torque is 2267.57 Nm as calculated by the software. Significant values for the dimension of the blade were also computed. These are the blade chord and blade angle for various distances along the blade's radius. These values are shown in Table 4 along with Diagram 11 for better visualisation. 4.2. Design and Analysis of the 10 kW Horizontal Axis Wind Turbine Blade The AutoCAD for the design of one blade is shown below. After the establishing the design for the wind turbine blade, a CFD software was then use to analyse the performance of the blade. Graphs and analysis are shown below. Again, since the design was patterned to NACA 2412, the CFD analysis was also a bit similar to it. Shown in Diagram 13.a the blade's airfoil profile of the. The upper edge is shown as well as the lower edge of the blade. CFD Analysis: Based from the values used in the computation of the blade design, the value of CI used is 0.85 while the angle of attack is 6 degrees. Under these conditions, the following are the results of the simulation done. As shown on Diagram 13.b (below), the pressure distribution on the lower part of the blade is greater than the amount of the incoming flow stream. This pushes the airfoil upward normal to the incoming flow stream. On the other hand, based on Diagram 13.c on the next page, the upper surface on the airfoil experiences higher velocity (as shown by Mach numbers) compared to the lower surface of the blade. Diagram 14 shows the graph of pressure distribution on the top and bottom of the blade. 3.3. Selection of Appropriate Material In terms of materials, a glass fiber reinforced composite (Fiberglass) was selected as the material to be used in manufacturing the blade 3.4. Selection of Appropriate Manufacturing Process Fiber Placement or specifically, filament winding would be recommendable because of its efficient material usage but for competitive blade manufacturing, resin transfer molding is more suggested because of its manufacturing cost and efficiency. 4.0. Resin Transfer Molding for the Wind Turbine Blade Manufacture 1. Pattern Making - a pattern is an object which is exactly the shape of the blade. It is used to make moulds for the blades. Patterns can be made of wood or metal. For fiberglass, it is suggested to use mould made of wood or aluminum. 2. Fabric Stacking - When the mould is ready, fabrics are laid up as a dry stack of materials. These fabrics are sometimes pre-pressed to the mould shape, and held together by a binder. 3. Resin Injection - A second mould tool is then fastened over the first. Resin is injected carefully into the cavity using a computer generated tube. The amount of resin is controlled in order to maintain the uniformity of resin on the material inside the mould. Sometimes, vacuum is used to assist resin in being drawn into the fabrics. 4. Curing - The resin inlets are closed for many hours to allow the laminate to cure. Both injection and cure can take place at either ambient or elevated temperature. 5. Trimming - As soon as the resin cured, the component is taken out from the mould and trimming takes place. Excess parts are trimmed and surface finish is done. References: Books: Barker, Roger and Gerard Coletta. Performance of Protective Clothing: A Symposium Sponsored by ASTM Committee F-23 on Protective Clothing. Raleigh, NC. ASTM International, 1986. Chung, Deborah. Carbon Fiber Composites. Butterworth-Heinemann, 1994. Kalpakjian, Serope. Manufacturing Engineering and Technology (3rd Edition). Addison-Wesley Publishing Company, 1995. Lee, Stuart and Y. Ed. Lee. Handbook of Composite Reinforcements. Wiley_Default, 1992. Mathew, Sathyajith. Wind Energy: Fundamentals, Resource Analysis and Economics. Birkhauser,2006. Mitchell, Brian. An Introduction to Materials Engineering and Science for Chemical and Materials Engineers: For Chemical and Materials Engineers. Wiley-IEEE, 2004 National Research Council (NRC). Assessment of Research Needs for Wind Turbine Rotor Materials Technology. National Academies Press, 1991. Palmer, Grant. Physics for Game Programmers. Apress, 2005. Quaschning, Volker. Understanding Renewable Energy Systems. Earthscan, 2005 Articles: Grose, Thomas. "Blowing in the Wind". 1 Nov. 2006. Mahoney, John. "Aleternative Source of Energy: What is the Background" Environmental Issues Community. 8 Nov. 2008. National Renewable Energy Laboratoty. "Airfoil Behavior".Energy in the Wind. 14 May 2005. Read More