Design Course Windmill Project
As I stated in the summary, this windmill project was for my first design course, sophomore year at Iowa State University (ISU). A single project is worked on for the entire semester. The purpose of this course was to teach engineering knowledge, more than to deliver a working solution. However, the intention was to create real steps toward to a functional solution. Students are guided through each step in the standard design process for a product. From brainstorming and research, to solution evaluation, design steps and a fabrication plan, then fabrication, testing and a business plan. Everything needed to be documented and explained, as it would in reality. At the end of every semester, the Mechanical Engineering department at ISU hosts a Design Expo for the student projects in design courses (sophomore and senior). Participating in this expo is a requirement for the design course but its mostly about students showcasing their projects. Anyone can attend, students, faculty and outside visitors. If a project was very successful, it had the possibility of being picked up by Engineers Without Borders or another non-profit organization.
The sophomore design course is geared toward creating a product to solve an issue faced by people living in under developed countries. Some of the suggested subject areas were: water retrieval or irrigation, electricity and light, and food processing or cooking. Students were also provided with examples of past-student projects and projects from engineering non-profits. The critical design requirements were: low cost, strong usability, and ease of repair. Students chose their own project. Once it was approved, the design steps outlined above were followed while providing regular updates to the professor. Every group was expected to create a prototype of their project for testing. The key word is prototype. Depending on the size and scale of the entire project, it would not be feasible to build a full design within the time and budget limit. The University provided $100 for each project to fabricate the prototype. All items purchased had to be justified and relevant to the project and prototype. Students were provided the tools and a lab to fabricate their prototype on their own; plus storage in between class periods. Tests would then need to run on the prototype to compare the design against theoretical requirements. Certain testing tools were also provided to the students.
The Requirements
My group decided to build a windmill water pump to replace buckets or other human powered pump systems for drawing water from a well. Since no-one in my group knew about building windmills or pumps we performed extensive research on both. After researching ideas, we decided we should use a mechanical pump to avoid possible issues with electricity. An electric pump would need considerable power and if the
pump failed it would be unlikely the user could repair it. We based the blade design on examples of existing mechanical windmill pump systems (Aermotor). The blades on those were basic, flat blades angled into the wind. The pumps were considerably complicated. Our group decided to use the blade design and create a different, simple pump. The water would likely be drawn from a considerable depth (30 feet or more), we needed a pump design that could draw in and lift water to the surface. The simplest type of pump that could perform that was a two-stroke pump (at right). We decided we wanted to pump 20 gallons per hour at a low wind speed, about 10 MPH. Those requirements were chosen based on data from existing pumps systems, an estimated water usage for a person and the average wind speed in locations like Africa. Africa was chosen over other under developed locations because water is scarce and wind speeds were within our requirements.
The Design
Even though we had considerable information, we needed to create a low cost, durable solution. We were not able to find any specific design information for existing systems. But, working with pictures and a general idea, I created a model in Solidworks (at left). This model needed to be refined but it became the basis for our final design. Even without having an idea for our pump, it was going to be powered by the windmill. A two-stroke pump uses a piston that requires an up and down movement. The easiest method to achieve this with a windmill is to create an offset in the blade-hub axel, making it a crank shaft (left, picture 2). When the wind pushes the blades, the crankshaft will turn, moving the piston on the pump. Determining the shape of the blades required additional research. Our group discovered the coverage area and the angle of attack for the blades was very important. The coverage area refers to the circle of area created by the blades as they rotate. Using more of the coverage area (more blades) will increase the energy drawn from an air flow. Our requirement for wind speed was fairly low, so we decided using the maximum amount of the coverage area would be necessary. This left the shape of the
blades to be designed. The product would need to have minimum cost, be easy to build and repair; therefore using flat wooden blades would be the best option. A single 8-foot by 4-foot sheet of plywood could be used to create all the blades, which would be about $25. Metal and PVC pipe were considered but even for a small design the cost would be significantly higher. The angle of attack was up for debate. There is a balance between the amount of blade that faces the wind and the amount of rotation force gained. If the blades have no angle (perpendicular or parallel to the wind) its unlikely the windmill would function. The group researched equations for wind force and used knowledge of basic force analysis to attempt to find the best angle of attack. Our calculations struggled since airflow analysis is typically complicated. The numbers were showing the design would not work. But we knew real products existed that accomplished what we desired. Using that knowledge, we decided to wait until we could build a model to determine the best design. Luckily, the pump was easier to design. Through additional research into
two-stroke pumps we found a simple design we could emulate. It is similar to the pump in typical upright soap or lotion dispensers. The nozzle is pushed downward, the liquid comes out. As the nozzle rises (spring powered), more liquid is drawn into the tube. This design uses a piston and check valve. Our pump is similar but different. The length of pipe required meant a large volume of water would be held. Priming a system from ground level would be near impossible. The system would need to be perfectly sealed and use huge vacuum pressure. So we flipped the design upside down and added a second check valve. The piston and both check valves would be at the water source, the bottom of a well. The piston also acts as the delivery tube; the shaft is a pipe instead of a solid rod. The first check valve is in front of the piston, where water is first drawn in (right, picture 2). The second, is part of the piston shaft (right, picture 3). As the piston moves upward, the lower check valve opens, water is pulled in. When the piston stops, the weight of the water closes the valve immediately. The piston moves down, the upper valve opens and water moves up the pipe. At the bottom of the stroke, the upper valve closes. Our check valve would use a loose ball over an opening smaller than the diameter of the ball (right, picture 4). The water flow lifts the ball and the ball drops when the flow stops (the ball is trapped from moving far upward by a small rod across the pipe [right, picture 4]). A seal for the piston would be made of a flexible ring connected to the bottom of the piston shaft. The length of the stroke and size of the pipe determines the flow rate.
test platform and a single set of blades would be created. The platform would hold the axel and hub (left, picture 1). The hub and blades would have pre-drilled holes for bolts, making it possible to attach and remove the blades quickly and easily. The shape of the blade would be an elongated trapezoid (smaller width at the axel, wider at the end). In order to fit many blades on a central hub, the end connected to the hub must be much smaller than the rest of the blade. We chose to test three things: attack angle, blade length and blade count (coverage area). Each group would have several iterations. The three angles were: 15, 30 and 45 degrees. We wanted a range over the possibly effective angles. Less than 15 the projected area is nearly flat plane, greater than 45 its nearly a line and 30 is in the middle. The three lengths were: 8, 12, 15 and 17.5 inches. These were chosen based on the test platform. The maximum clearance was 18 inches and we needed a range. The three blades counts were: 4, 8 and 12. Our design held 12 blades, using 4 and 8 made an even spread between 0 and 12. Every test would involve measuring the rotational force on the axle and wind speed. The rotational force was measured by using a force gauge connected to a string, which wrapped around the axle (left, picture 2). The force measurement recorded was the holding force. The term holding force is to mean the maximum rotational force a constant wind flow can exert where the blades no longer rotate. I realized this would be equal to the maximum force produced under a constant load. The way the force gauge was used, the load was not constant. It
Fabrication and Testing
When considering fabrication options, the group realized building a full sized windmill or a functioning model would not be productive for determining the effectiveness of the design. We decided to create a full sized section of the pump (above, picture 1) and a scaled version of the blades (below, picture 1). Earlier I mentioned our group would use testing to decide blade orientation. Since we had limited material, a single
would go from zero to the maximum. This allowed the blades to reach a high speed before the gauge began pulling. If this force were to be measured, it would be significant (caused by the weight and angular momentum). Wind speed was measured with an anemometer so a range of wind speeds could be used (above, picture 3). I had just completed my course in statistics. I knew having several, and a range of values would produce more accurate results. First, each angle would be tested: using the full set of blades at the greatest blade length. The most effective angle would be used in other tests. The different angles were achieved by using various wood wedges placed between the blade and hub, where both are bolted in place (above, picture 4). Then the blade count was tested with the greatest blade length and the most effective angle (above, picture 5,6). Using the most effective blade count and angle, the blades were cut the blades shorter and the test repeated (above, picture 7). Unsurprisingly, we found that an increased blade count and longer blades created the most force. The data for the angle provided interesting results. After graphing the force data, we saw the 30-degree angle was providing the greatest force. I found this suspicious because I thought 45 degrees would be the best. The conditions for that test were not ideal, the wind was not consistent, there were constant gusts. However, being under a time constraint, 30 degrees was used as the most effective
angle. The other tests went quickly and there was extra time. I decided to retest the blade angles. More favorable wind conditions were found and the data showed 45 degrees was the better angle. Now moving on to the pump. Previous calculations had been done for the pump to determine the pipe size and stroke length. The design was to use a 4-inch stroke. Explaining the pump assembly with words is a bit complicated, please view the pictures to the right. There is one difference from the fabricated design and the model. The material for the piston seal was planned to be rubber. In practice, the seal we could obtain did not function well. However, in anticipation of this issue had ordered another material in advance. Through our research, we discovered designs using leather washers for the seal. We had to build a custom leather punch to make the seals from a leather sheet. The tests needed were: piston pull force and flow rate. We had calculated the weight of the water plus the pipe but we needed the force of friction on the piston. This force would be measured by assembling the pump, wetting the parts (to simulate actual conditions) and pulling the piston with a force gauge. The friction was calculated to be 3.4 lbs. The flow rate we required was 20 gallons per hour. For our test we measured gallons per minute (GPM). One group member moved the piston up
and down at the specified stroke length for one minute at one stroke per second, from a bucket with a known amount of water. After the test, the water was measured again, the difference would be the flow rate. Using the 4-inck stroke the flow rate was 1.34 GPM. This test is considered accurate. The rotational speed (RPM) for the windmill was measured for a given wind speed. Converting RPM to rotations per second, the strokes per second could be determined (one rotation=1 stroke). However, after calculating the lift force needed for the full design it was determined the windmill could not function with a 4-inch stroke. We then tested a 2-inch stroke, the data showed this would be possible. With the same conditions, the smaller stroke still produced 1 GPM.
Conclusions
Considering the length of this page, I will keep the explanations short. Over all the windmill design was a success. Had we been another semester to test and design, we could have created a much better product. If you visit the Dropbox page linked at the top, in the folder 'Documents', there are model drawings, detailed test data, the testing summary, the business plan and the full report on the project.