ENGINEERING
Abstract
Wind turbines have become a practical and efficient fossil fuels replacement for the generation of electrical energy. Characteristics optimized include weight, ease of manufacture and structural integrity. Ease of manufacturing has a direct relationship to costs. Lighter materials for blade construction are necessary to decrease blade weight and lengthen blades. Carbon fibers or composites of glass and carbon fibers are lighter than glass fibers. Hypothesis: controlling the speed of the infusion will allow the capacity to manage the speed of the infusion process and in that way control of the final composite material. Results showed that 68.5 meters has needs to be designed with a blade weight of over 12,000 kg and that the longest blade at 68 meters needs to be designed with a blade weight of over 15,000 kg. Therefore the hypothesis was incorrect. Prepreg is a better choice for integrating the fibers into the resin.
(wind turbines, blade length, blade wide, infusion, prepeg)
Figure 3 Blade configurations and blade tip to blade root relationship to the blade 8
Figure 4 Length of wind turbine blade v. weigh for glass and carbon fiber structures 9
Figure 5 Blade length (m) versus Blade weight (kg) using epoxy prepreg 12
Figure 6 Blade length (m) versus Blade weight (kg) using PE (polyethylene) epoxy 12
1.0 WIND TURBINE BLADE CONSTRUCTION DESIGN
1.1 Introduction
The first wind turbines designed in the early 1980s were much smaller than the giant wind turbines that can built today; the largest machines using rotors for rotating history are wind turbines located in the seas and oceans. (Jamieson 2011: 75) The demand for wind turbines has become greater as fossil fuel based energy is prices rise. Another fact causing wind power to grow in popularity is the push for sustainability so that the problem of global warming will not be negatively impacted. All the locations are unique. The locations may be good for potential wind turbine fields. For some locations only one or more turbines are needed. A rule of thumb is that the fewer turbines to reach energy output needs, the more efficient and cost effective the project. For example, a large field of turbines with small blades would provide less efficient and cost effective energy to a smaller field of turbines with large blades. The length of the blades on the turbines has a direct relationship with the ability for increased power generation. That is because the surface area of the blade is directly linked to the amount of wind power in the path of the blade.
1.2 Manufactured Wind Turbine Blades
A number of manufacturing companies offer a large variety of wind turbines. A comparison of the blade lengths per model shows that the energy capacity and the area swept by the blades change with a change in the blade length. GE 1.5s has a 1.5 MW capacity, and 3,904 m2 area swept by the blades when the blade length is 32.25 meters. (AWEO) The GE 1.5sle has a 38.5 m blade length but the energy capacity is the same, 1.5 MW and the area of blades sweep is larger, 4,657 m2. AWEO) Another variable that is critical is the height of the turbine. In fact, designing a wind turbine is a complex project that requires knowing the rated wind speeds at the location and structural measures such as the rotor diameter, and angle of rotor to hub among many other variables that are necessary to calculate.
1.3 Effect of resins on Blade Length to Blade Weight
A basic design challenge is to find the best ratio of blade length to total blade size. The problem is complex because the characteristics that need to be optimized include weight, ease of manufacture and structural integrity. A fourth factor related to ease of manufacture is cost. Lighter materials for blade construction are necessary to decrease blade weight. Even the resins used to coat the blades for longevity must be evaluated, so one layer of resin must work as well as three or four resins from the 1980s and 1990s. (See fig. 1) The structure of the blade plus the coatings is pictured in layers from the shell made of prepeg infusion to the surface finishing with colored epoxy gel coat. From the middle to the left the image shows the spar, made of a glass/carbon composite (which is at the internal surface, bottom and top of the shell) and the shear web that is perpendicular to the internal surface and offers strength to the hollow blade. Modern blades are hollow and made with newly invented materials so that the weight of the blade will be as small as possible while still meeting design specifications.
Figure 1 Design for large-scale blade (Stackpole, 2014: para. 7)
1.4 Optimization of Blade Design
Research on optimizing the size of blades with respect to making the longest blade for energy generation is done at many laboratories like those at academic institutions, NASA and the Sandia National Labs. Computer simulations are used to enhance blades and evaluate the best size and length for optimum energy generation. The structural dynamics are tested with a computer model before prototypes are built. (See fig. 2) The computer generated image figure 2shows a turbine blade that is strengthened with two parallel
Figure 2 Structural design for stronger and larger blade (Stackpole, 2014: para. 2)
‘beams’ of different height. On the other hand, figure 3 is a black and white drawing, but at the bottom left it shows how the classic I-beam has been adapted for use in the shear web configuration. The shear web is the perpendicular piece in the hollow blade while at each end the spar caps stabilize the web. The shear web cuts down on weight compared to the box beam (at the top of the image in figure 3). The perpendicular components in the box beam are almost equal in size but designing with the shear web component offers more adaptability to meet the shear force requirements of the blade. That is because the using the ‘I-beam’ style of strengthening the designer can place more than one shear web where most needed. Manufacturing processes become more sophisticated active-flow aerodynamic controls are developed and embedded into a blades designs. Sensor devices have been added strategically to wind generation turbines to send information so the blade will adapt to optimum wind direction and strength.
Figure 3 Blade configurations and blade tip to blade root relationship to the blade
(Guillermni, 2011: para. 5 & 6)
1.5 Materials: Glass Fibers, Carbon Fibers, Composites
The “self-load weights” are the design drivers when the types of materials and the size of the blade are the determining factors (Jamieson, 2011, p. 84). But, the length of the blade can be determined for design purposes by “scaling with geometric similarity largely works from a stress point of a view” (Jamieson, 2011, p. 84). Figure 4 shows the difference between glass and carbon fiber used to build a blade impacts the weight and so the length of the blade is impacted. The use of carbon fiber instead of glass fiber allows the blade to be longer and the blade will weigh less than if glass fibers were used. Composites of glass and carbon fibers can be adapted to meet special requirements. The length of the blade determines the type of material chosen as wells a the manufacturing
Figure 4 Length of wind turbine blade v. weigh for glass and carbon fiber structures
(Brondsted and Nijssen 2013)
process. (WE, page 4-1) Older technologies showed that the weight of a blade increased to about the power of 2.5 as the blade becomes longer. (WE, page 4-1) This is a large rate of increase that quickly causes problems with load weights on the bearings, the blade structure, and in the towers. (WE, page 4-1) That is the reason alternatives to glass fibers were needed. Carbon fibers are stiffer and lighter than glass, so they have become a useful alternative. Carbon is approximately three times stiffer than glass, and the density is less. The density for glass is 2.4kg/m3 whereas the density for carbon is 1.8 kg/m3. (WE, page 4-1) The combination of the difference in stiffness and density result in carbon fibers having a specific stiffness of about four times higher than glass. (WE, page 4-1) Therefore, the use of carbon fiber is an advantage that has made it possible to build the long lighter weight blades on the huge turbines that are built today. On the hand, a disadvantage of carbon fibers is the cost; the cost ranges from 7 to 8 times higher than glass. (WE, page 4-1) The design and manufacturing processes necessary using carbon fibers are more difficult because of the amount of precision required. That is why for every advantage the trade off s (disadvantages) must be carefully balanced in order to reach the energy requirements expected of the wind turbine.
1.5.1 Composites and Reliability
One more caution is that the smaller the blades the reliability of composite materials is higher. The design driver for a small blade of about 2- to 35 meters is dependent on the stiffness properties of the material used. The longer and larder blades are “much more sensitive to fatigue loads which are much more dependent on the resin in the composite". (WE, page 4-2) But the sensitivity to fatigue loads can be decreased by choosing the correct resin in the composite. The speed of the infusion process is approximately equal to the (Density times the Gradient Pressure) divided by the viscosity of the resin system. In other words, the speed of infusion can be controlled. Infusion is the process when the resin is ‘sucked’ into the reinforcing (glass or carbon) fibers and the fabrics (web) in a vacuum.
1.6 Hypothesis
A higher concentration of carbon fibers than glass fibers will allow a longer blade to be designed while at the same time controlling the weight of the blade. Additionally, controlling the speed of the infusion will allow more the designer the capacity to manage the speed of the infusion process so the Infusion allows more control over the properties of the final composite material over using the prepreg process. Prepeg is when the resin is pre-impregnated with fibers in order to add structural strength reinforcement to an item. In prepreg, temperature is the main property a designer can control when producing an epoxy. The ability to change the speed of infusion offers a designer to make a design of higher precision, because the manufacturing process choice, infusion, offers more precision.
1.7 Results
Figures 5 and 6 compare the difference between the epoxy prepeg process and the infusion process in reference to blade weight and blade length. Figure 5 shows that the longest blade at 68.5 meters has needs to be designed with a blade weight of over 12,000 kg. Figure 6 shows that the longest blade at 68 meters needs to be designed with a blade weight of over 15,000 kg. Therefore the epoxy prepreg is a better choice. The assumptions about the precise nature of using the speed of infusion do not always hold.
Figure 5 Blade length (m) versus Blade weight (kg) using epoxy prepreg (Data WE Handbook)
Figure 6 Blade length (m) versus Blade weight (kg) using PE (polyethylene) epoxy (data from WE handbook
1.8 Conclusion
The epoxy prepreg is a better choice for integrate the fibers into the resin, than the infusion process. Although further research is needed because, possibly, using carbon reinforced fibers without making a composite could solve the weight problem and then the infusion method would be suitable.
1.9 References
AWEO web page [online] http://www.aweo.org/windmodels.html [24 March 2014]
Brondsted, P. and Nijssen, R.P.L. (2013) Advances in Wind Turbine Blade Design and Materials. Philadelphia, PA: Woodhead Publishing
Guillermin, O. (2011, Jan. 24) “Developing Composite Wind Blades That Will Stand the Test of Time” [online] Design News, accessed from http://www.designnews.com/document.asp?doc_id=230009 [24 March 2014] [2- Mar. 2014)
Jamieson, P. (2011) Innovation in Wind Turbine Design. West Sunset, UK: John Wiley & Sons, Ltd.
Stackpole, B. (2011, Jan. 24) “Sandia Sizes up Wind Turbine Blades Design” [online] Design News accessed from http://www.designnews.com/document.asp?doc_id=230008 [24 March 2014]
Wind Turbine Blade, WE Handbook (2014) Chapter 4 Blade Manufacturing Processes [online] Gurit www.guit.com/files/documents/4_blade_structure.pdf [24 March 2014]
