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Design and Fabrication of a Vertical Axis Wind Turbine University of Notre Dame Department of Aerospace and Mechanical Engineering Mark Paluta, Dan Reitz, Ryan Snelling, Joe Gadient Advisors: Richard Strebinger, Dr. Mihir Sen May 10, 2013 1 Background The commercial wind turbine business mostly targets large scale projects, producing power to feed into the grid, but limiting the availability of personal turbines for homes or businesses. This is particularly problematic in developing countries, where electricity can be the difference between life and death and there is no grid to draw from. The goal of this project is to design and fabricate an inexpensive wind turbine for small scale use, built out of parts that can be easilyfound throughout the world. The design process will be outlined to allow for easy replication. Planning The first step was to simply brainstorm various ideas for the turbine design and specifications. Individual research was conducted in order to fully understand the problem before trying to tackle it. As we began to to pinpoint the various users and uses of the turbine, several design requirements were defined: 1. The turbine is to be inexpensive in terms of both components and manufacturing. 2. The design is to use components that, in the event of failure, can be easily replaced with everyday objects. 3. The customer will be able to easily find parts, as opposed to ordering specialized parts. 4. The turbine is to be lightweight and modular to allow for relocation. These requirements focus on userfriendliness, since this is a main concern with this project. From these conclusions, we began looking into manufactured models as well as individually made “doityourself” (DIY) designs. We took the pieces from each of these that best matched our needs and constraints to craft a preliminary design. After studying various designs, we determined that the wind turbine would either have to be driven by lift or drag, a Darrieus or Savonius wind turbine, 2 respectively. From there we spoke with several experts in the field of turbine design and performance and with their guidance, and the aforementioned considerations, we chose a Savonius vertical axis wind turbine (VAWT). Darrieus turbines employ airfoils to generate lift and spin, whereas dragdriven designs, such as the Savonius, use the geometry of the blades to create a drag differential between both sides of the shaft. Since an airfoil has a very precise shape, it is not something that can be easily manufactured in a rural area or replaced with miscellaneous parts. Because of this, liftdriven turbine designs were determined to be inappropriate for our goals. A goal output was not specified for the turbine as we visualize the design being used in many different applications, from powering lights to pumping water. Our decisions after this point were largely based on low cost, easy fabrication, and availability of parts, while still maintaining a decent efficiency and power output. Final Design With a Savonius turbine in mind, we began making initial sketches. The design called for an Sshaped formed by two curved blades mounted between two discs with a shaft running through the middle of the assembly. Two of these assemblies were to be mounted on top of each other with the blades on the top tier offset by 90 degrees from those on the bottom. This helps to ensure that regardless of initial orientation, the turbine will have at least one tier of blades that can catch the wind allowing it to start autonomously. Figure 1 below shows the tier concept with the dividing discs but without the blades installed. The Sshape of the blades is also illustrated from the vertical perspective: 3 Figure 1: SShaped Blades and Tier Layout Concept See Appendix B for CAD drawings of the parts. Many ideas were discussed for the material of the Sshaped blades. A barrel or trash can cut in half initially seemed to be the best option because of its simplicity. Another idea proposed was using sturdy “ribs” with a material draped over them. This soon became the popular idea for two reasons. The first is that a cloth or tarplike material would be a bit lighter than a solid plastic or metal trash can. The second reason is that the rib design would provide extra design flexibility. With a rigid trash can, there is a prescribed shape and lower flexibility in size. With ribs, spacing can be changed depending on the desired height of the blades. With the turbine type and tier design decided upon, the next area of design was the transmission system. Transmission from the shaft of the turbine to the generator was initially designed to use many parts from a bike transmission. The gears on a bike would be mounted on the shaft of the turbine and 4 power would be transmitted using the bike chain. This would allow for a mechanical advantage between the turbine shaft and generator. A second idea was to use a friction band along the outside of a bike wheel to connect the generator and turbine. This idea was pursued because it allowed for a larger mechanical advantage than the gear system since the bicycle wheel has a large diameter. Bicycle parts were the focus of the design because of their availability worldwide. This greatly aids in keeping costs down and replacing any broken parts. The bearings in the bicycles were suitable for our purposes while keeping costs down. The third and final component is the frame, a housing to hold the blade assemblies and the transmission system. Thin frames were considered in order to limit the amount of wind blocked and allow maximum force from the wind. Three or four legged designs were considered to allow for structural stability while still using few materials. For the bottom of the legs, two types of systems were examined. The first was a foot, either a plus sign or horizontal pole for stability. The second is a stake into the ground at the bottom of each leg. The foot is advantageous for flat ground or if the ground is too hard to stake into. The stake is advantageous if the ground is soft or sloped, where feet would not lie flat. The final design drew from all of the ideas for each of the three components addressed above. For each of the three components described above, we will describe the assembly, justify decisions made, and list materials used with their associated costs. Since the sections were modular and easily disassembled, modifications could be made to any section with little effect on the others. 5 1. Blades Although the turbine was designed with a set of blades for each of the two tiers, for simplicity and low cost, a single Sshape was used in the prototype in only a single tier. It was created using ¾” PVC pipe, 13” diameter buckets, chicken wire, and duct tape. The 3/4" PVC pipe was cut to the sizes specified by Figure 5 in appendix B and glued together using PVC cement. The buckets were cut into 2” half cylinder ribs and attached to the PVC frame. The rib structure was then covered in one layer of chicken wire to add stability while adding very little weight. Then, several 3/4"#8 sheet metal screws and washers were used to connect the chicken wire to the ribs. A detailed depiction if these connections can be found in Figure 13 in appendix C. To connect the blades to the axle of the bike rims, a flange nut was epoxied into a PVC bushing which was glued into the blade assembly. The chicken wire skeleton was then covered in duct tape. Duct tape was determined to be preferable to a tarp of cloth because it is cheap, easy to replace in the case of a tear, and lighter than any tarps that were considered. Also, because it covers the turbine strip by strip as opposed to a blanket effect,it can mold more closely to the desired shape. The buckets for ribs were chosen because they are inexpensive, sturdy, and readily available almost everywhere. Because the chicken wire may not be as easy to find everywhere, it became a useful but optional component. The blades would lose some rigidity without it, but would still retain its shape reasonably well. Because efficiency is not a primary concern of the design, this slight reduction in sturdiness would be acceptable. PVC was chosen as the frame material for its light weight and sturdiness. It is also easy to cut into pieces of any length. Were PVC not available, the bicycle frames or wood could be cut into strips and built into a frame for the blades. The total blade assembly cost under $50, so this component works effectively toward the goal 6 of an inexpensive turbine. Table 1 shows a list of parts that were used in construction of the blade assembly. Table 1: Blade Materials, Cost, and Quantity (One Tier) Material Quantity Cost Total Cost of Parts ¾” PVC Pipe 15 ⅙ ft. $0.23 / ft. $3.49 ¾” PVC 90° Elbow 8 $0.47 $3.76 ¾” PVC Tee 4 $0.47 $1.88 ¾” PVC Cross 2 $2.37 $4.74 ¾” x ½” PVC Bushing 4 $0.66 $2.64 8 oz. PVC Primer and Solvent Cement 1 $6.97 $6.97 Duct Tape 2 Rolls $2.89 $5.78 5Gallon Bucket 2 $2.60 $5.20 2 ft. x 5 ft. x ¼ in. Chicken Wire 1 $10.38 $10.38 #8 x ¾” Sheet Metal Screw 18 $4.41 / (100Pieces) $0.80 #8 x ¾” Flat Washer 18 $4.24 / (100Pieces) $0.77 ⅜ 16 Flange Nut 2 (Found on Bike) (Found on Bike) Total Cost: $46.41 2. Transmission An additional assembly was created to transmit the torque generated by the wind. A third wheel was connected to the axle of the lower stationary rim in a similar manner to the blade assembly. 7 A nut was glued to a PVC assembly that was bolted to the inside of the spinning wheel. A band, wrapped around the rim, transmitted the torque to a nylon pulley which drove the generator shaft. The generator was held vertically by an Lbracket. The materials used in the transmission system can be found in Table 2. Table 2: Transmission Materials, Cost and Quantity Material Quantity Cost Total Cost of Part ¾” PVC Pipe 2 ⅓ ft. $0.23 / ft. $0.57 ¾” PVC 90° Elbow 2 $0.47 $0.94 ¾” PVC Cross 1 $2.37 $2.37 ¾” x ½” PVC Bushing 2 $0.66 $1.32 ⅜ 16 Flange Nut 2 (Found on Bike) (Found on Bike) Bicycle Tire 1 (Found on Bike) (Found on Bike) Round Belt 7 ft $2.64 $18.84 Belt Connector 1 $5.55 / (20Pieces) $0.28 3” Pulley 1 $15.59 $15.59 Generator 1 N/A N/A Total Cost $39.91 3. Frame To create a frame, three legs were made to attach to the outside of the wheels attached to the blade assembly. The 1" PVC pipe was cut to the lengths specified in Figure 9 in appendix B.Unlike the blade assembly, the frame is not glued together with PVC cement. This allows for easy disassembly as well as ease of access to parts that need to be replaced. As can be seen in Figure 10 of appendix B, 8 the blade assembly was mounted between two bicycle tire rims as previously described. The threaded axles of the wheels attached to the blades to spin together, while the actual wheels were held stationary by the frame. The stationary wheels were then attached to the PVC frame. Three ½” holes were drilled evenly around the two bike frames. Similar holes were drilled into the PVC end caps so that the caps would connect to the bike frame using a ⅜ inch hex bolt, threaded through a washer, the PVC cap, and the tire, all held in place by two hex nuts. This connection can be seen in more detail in the Figure 2, while the photograph can be seen in Figure 19 of appendix C. Figure 2: Detail of Rib & Frame Connection Table 3 provides a summary of the parts used in construction of the frame. 9 Table 3: Frame Materials, Cost, and Quantity Material Quantity Cost Total Cost of Part 1” PVC Pipe 13 3/16 ft. $0.90 / ft. $11.87 1” PVC 90° Elbow 3 $0.57 $1.71 1” PVC Tee 3 $0.86 $2.58 1” PVC Cap 6 $0.66 $3.96 ⅜ x 1 ½” Hex Bolt 6 $0.25 $1.50 ⅜ 24 Hex Nut 12 $0.12 $1.44 ⅜” Flat Washer 6 $0.14 $0.84 Bicycle Frames 3 (Found on Bike) (Found on Bike) Total Cost: $23.90 Performance Without the generation system attached, the frame spun well in moderate wind conditions on a four story rooftop. With low wind at ground level, the blades would occasionally spin, but had trouble starting. From a video taken without the generator attached, the average rotational speed of the blades was estimated. This was converted to about 200400 RPM in the generator, using the mechanical advantage, had the generator been attached. This rotational speed would have produced between 12.5 and 25 Watts according to the manufacturer’s chart, shown in Appendix A. Once the generator system was attached via wrapping the belt around the wheel, the friction to turn the generator became too much for the system to start. With a push, the blades would turn roughly half a rotation, but the system would not spin freely. This made it impossible to get any power output from the turbine. 10 A second major problem was that the epoxy used to attached the pvc components of the main shaft to the axles on the wheels failed when subjected to large forces. Figure 20 in Appendix C shows one of the central shaft to wheel axle adaptors that failed. The nut on the wheel axle was supposed to remain glued inside the adaptor, but as can be seen in the figure, the epoxy ripped off and fell and the nut came loose. It supported the weight of the structure well and could withstand the force of the rotation, but if a person stopped it suddenly with their hand while it was spinning or gave it a very strong spin on their own, the glue would weaken a bit, causing some wobbling in the shaft and inefficiency. Eventually, the top connection wore down to the point of failure, causing the turbine to only be supported by the bottom bearing. While the turbine was still capable of spinning, it is important that this problem of weak epoxy be addressed for future turbines. Future Improvements Several improvements to this turbine design need to be made to make it a viable power generation device. First is the issue of the weak joints from inadequate epoxy. One possible solution might be to use set screws to hold the hex nuts to the PVC instead of the glue. This should support a higher load and avoid failure even due to human forces. Another idea was to use a press fit between the shaft of the tire rims and a connector on the blade assembly. While this may require special equipment, it does eliminate the need to purchase set screws or epoxy. The second and more important problem was how to make the turbine spin with the added resistance of the generation system. A couple ideas were thought of. The first was to reduce the mechanical advantage by using a smaller object to drive the belt than a whole bicycle wheel. This would 11 greatly reduce the torque required in the blades to spin the generator shaft. The bicycle wheel on the bottom of the shaft could be replaced with a similar common round object such as the unused bottom of the bucket, which is closer to the size of the pulley on the generator. A second possiblity was to make the turbine shaft connect directly to the generator shaft. A PVC pipe connector could act as an adapter for the two different threads by holding two nuts, with one nut connected to the wheel axle, and the other connected to the generator shaft. This would be a similar connection to how the scoop currently connects to the wheel axles. Another problem that needs to be addressed is the amount of torque generated by the blades themselves. One way to do this could be to use a wider windcatching area to generatemore force. This would require a largerdiameter bucket or similar object. Another way to achieve more torque would be to offset the blades horizontally from the shaft. In this case, they would produce the same force due to pressure, but it would be applied at a larger radius, creating more torque around the axis. Further improvement could also be made in the frame strength to reduce wobbling. Instead of PVC, iron could be used. Iron rods with threaded ends were easy to find at local stores and were not too expensive. While iron and PVC may not always be readily available for makeshift repairs, things like welded bike frame parts could make for a good substitute without adding cost, assuming bicycles are already obtained. 12 The weights of the different components were recorded and are shown in the following table: Table 4. Weights of Turbine Components Component Weight (kg) Blades 3.6 Frame 6.9 Transmission 2.3 Total 12.8 13 Acknowledgements We sought the help of experts in various areas. Below is a list of people whom we contacted: ● Dr. Thomas Corke Professor of Wind Turbine Performance, Control, and Design course ● Justin Kurtich Manages the wind turbine at the Notre Dame power plant ● Dr. Anthony Serianni Led the team that designed the wind turbine at the power plant ● Michael Schafer Electrical engineering professor ● Officer Foust NDPS officer who provided free bicycles These people were an invaluable resource toward helping us shape our ideas. Their guidance helped us make many decisions along the way and we thank them for giving us their time. 14 Appendix A: Generator power output Specs Figure 3: Generator Specifications 15 Appendix B: Technical Drawings Figure 4: Drawing of one constructed Tier 16 Figure 5: Assembly Drawing of One Tier 17 Figure 6: Single Blade Tier Bill of Materials and Exploded View 18 Figure 7: Transmission Adaptor Assembly Drawing 19 Figure 8: Constructed Frame Drawing 20 Figure 9: Frame Leg Assembly Drawing 21 Figure 10: Full Turbine Assembly Drawing 22 Appendix C: Photographs of Constructed Turbine Figure 11: Fully Constructed Turbine Without Generator Figure 12: Detail of Swoop Rib Configuration 23 Figure 13: Detail of Rib and Chicken Wire Set Screw Figure 14: Swoop to Wheel Connection Detail 24 Figure 15: Center Shaft Rib Connection Detail Figure 16: Belt Drive Adaptor 25 Figure 17: Belt on Belt Drive Figure 18: Single Frame Leg 26 Figure 19: Frame Leg to Wheel Connection Figure 20: Failed Epoxy in Central Shaft Connector 27
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