Christopher Brinkerhoff and Professor Julie Vanderhoff, Mechanical Engineering
Introduction
Currently, there is much interest involving technologies dealing with clean and renewable energy sources. The current goal in the United States is to produce 20% of its electrical needs by wind power by the year 2030. Wind energy is seen as a viable option for further research because of its low carbon footprint and virtually limitless supply of wind, but in order for it to be economically viable, there are certain aspects of wind technology that need improvement.
Researchers have claimed that “[t]he blades of a wind turbine rotor are generally regarded as the most critical component of the wind turbine system”. It is the blades of a wind turbine that interact with the environment during the production of energy. Due to the turbulent nature of wind, the forces imposed on the turbine blade can be significant and damage the turbine. These forces not only act on the blades, but can also be transferred to the gearbox causing considerable wear on the gears and significantly shorten the operating life of the turbine. The purpose of my ORCA project is to come up with a way to measure the deflection of a small wind turbine blade (approx. 4 feet long) to better understand how the aerodynamic forces affect the wind turbine.
Theory and Procedure
In order to proceed, I used a Whisper 200 wind turbine (approx. 9 feet in diameter). I determined the most practical way of measuring the deflection of a wind turbine blade would be to utilize a strain gauge and incorporate it into a Wheatstone bridge. A Wheatstone bridge consists of four resisters in a specific arrangement. When all of the resistors are equal to each other, the voltage output is zero. The strain gauge will act as a type of variable resistor. As the blade moves, it will pull on the strain gauge causing the resistance of the strain gauge to change. By replacing one of the resistors with a strain gauge, it is possible to correlate a relationship between the voltage output and the deflection of the blade.
The first step was to attach the strain gauges to the blades. To try and minimize impact of the strain gauges on the air flow across the blade, I glued them onto the back side of the blade. One difficulty arose when attempting to connect the strain gauges to the circuit. The blades would be in motion and unless something was done, the wire would entangle and could cause damage to the strain gauges. The solution for this was the use of a slip ring, which allows part of an electrical circuit to rotate and the other part to stay stationary without tangling the wires.
This caused another problem because I needed to come up with a way to mount the slip ring to the wind turbine. I built an adapter for the slip ring which consisted of two disks, a cylindrical dowel, and a number of nuts and bolts. The slip ring was placed around the cylindrical dowel, which was bolted to the disks, with one on each side. Holes were drilled in the disks so they could be bolted to the turbine and the blades to the adapter. Once the adapter was on the wind turbine and the electrical circuit was wired together, it was time to program the computer to collect the data by using LabVIEW. This program measures the strain from the strain gauge and writes the data to Excel for further analysis.
Results and Discussion
The wind turbine was placed behind a wind tunnel to produce a wind of approximately 15 mph and the results are shown below. As can be seen from Figures 1a-d, there is a lot of variability in the measured strain. This is reasonable because the air is turbulent and the blade could be modeled as a spring, which would oscillate under these conditions. It is interesting to note that both the intensity and variability of the strain increases as you move closer to the center of the turbine. This is due to the blades being bolted down near the center of the wind turbine. This data confirms known patterns with wind turbines. The method used is a valid way to measure strain.
Future Work
I will share my work with the Wind Energy Club and use this work as a foundation for future research and experimentation. Possible alterations would be to run the experiment at different temperatures and conditions (such as wet to simulate rain or ice). It will also be a basis to encourage others within the university to get involved with wind energy.
References
- United States. Dept. of Energy. Office of Energy Efficiency and Renewable Energy., & National Renewable Energy Laboratory (U.S.). (2008). 20% wind energy by 2030 increasing wind energy’s contribution to U.S. electricity supply.
- Kong, C., Bang, J., & Sugiyama, Y. (2005). Structural investigation of composite wind turbine blade considering various load cases and fatigue life. Energy, 30(11-12), 2101-2114.
- Schreck, S. J., & Robinson, M. C. (2007). Horizontal axis wind turbine blade aerodynamics in experiments and modeling. IEEE Transactions on Energy Conversion, 22(1), 61-70.
- Special thanks to Dr. Julie Vanderhoff, Kevin Cole, and Bryce McEwen for their help in this project.