Aerodynamic Testing of Micro-scale Wind Turbines

Open Access
Author:
Lihn, Christine Marie
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Richard Randolph Auhl, Thesis Advisor
Keywords:
  • wind turbine
  • aerodynamics
  • blockage
  • wind tunnel
  • field test
  • Sunforce
  • All-Power
Abstract:
In efforts to develop a platform and methodology for testing wind turbines at Penn State, wind tunnel and field experiments were conducted with micro-scale turbines. Testing was aimed to expand on the results collected in previous work by Kylie Flickinger [1]. This research focused on gathering a broader spectrum of data using a 3-phase electrical generator with a simplified control arrangement. The factory installed, internal speed controller was removed from an All-Power brand 400 watt turbine. This allowed for non-adaptive control of the AC generator in the form of a user applied electronic load from an adjustable resistor bank across the DC output of an external rectifier. Bench-top dynamometer testing was conducted to collect performance data and calculate the generator's efficiency. In this round of experimentation, 12 different load resistor settings were used. The 0.333 ohm loading case had consistently high efficiencies over the full range of generator speeds tested, but at higher rotational velocities, the 1.0 and 1.5 ohm loads were the most efficient, at 83\% and 82\%, respectively. Wind tunnel testing of the generator was then conducted in the Penn State Multi-Purpose Low-Speed Wind Tunnel at the Academic Projects Building. The test section was not large enough to accommodate the fully assembled turbine, so it was placed in the second diffuser. Three different blade shapes were tested in the tunnel, in both a two and three blade configuration. The load resistances tested were simplified to eight different settings due to safety concerns of testing under 0.333 ohms and in efforts to reduce the duration of the wind tunnel tests. The blade designs included the All-Power and Sunforce blades, as well as a newly fabricated set. The new blade design, referred to as the Micro Whisper, was a scale model of the Whisper 500, which is a research turbine located at the Penn State Center for Sustainability. This facility was developed and maintained by Brian Wallace, whose Masters and Ph.D research has focused on this system [2]. The Micro Whisper blades were manufactured for this project in an attempt to explore the effects of scaling on performance. The model blades were scaled from the original 15 foot diameter down to the size of the Sunforce and All-Power blades, which are 3.83 feet in diameter. Unfortunately the Whisper blades did not hold their proper shape during wind tunnel testing, so comparison to the full scale turbine was impractical. The blades were not designed for the scale they were operating at and as a result, they also remained in a stalled condition for the majority of test cases, until they twisted during testing. The data for electrical power output collected in the wind tunnel was corrected for losses due to the generator efficiency, as calculated during bench-top dynamometer testing of the three phase generator. Corrections to the wind speed at the location of the turbine were made using a method developed to more accurately calculate the effects of blockage based on turbine performance. This method focused on adjusting the wind speed for the amount of power that was extracted from the tunnel, allowing for a correction that would adjust appropriately to any turbine. The corrected wind tunnel results show that increasing the loading resistance resulted in reduced blade power. This also caused the turbine to reach greater angular velocities at lower wind speeds. At a 3.0 ohm load, the maximum mechanical blade power reached was 232.6 watts, at a wind speed of 33.1 ft/s and turbine angular speed of 2024 RPM. The greatest mechanical power was 444.6 watts, achieved under a 0.8 ohm load on the generator, at 38.1 ft/s and 1822 RPM. A new mobile field test facility was developed for micro-scale wind turbines. Tests were conducted using the Sunforce two and three blade configurations under 0.8, 1.5, and 3.0 ohm loads on the generator. This data was corrected for generator efficiency losses and then compared to the wind tunnel corrected results to determine the accuracy of the new blockage calculation method. The field data appeared to be comparable to the corrected wind tunnel results, but when the coefficient of power was plotted as a function of the tip speed ratio, it became apparent that the processed data exhibits significant problems. Several data points from the wind tunnel produce power coefficients that exceeded the Betz limit for an optimum rotor, of 0.59. The highest value recorded was 0.78 for the All-Power three blade configuration at a corrected wind speed of 7.7 ft/s. The solution to this problem was not discovered, but will be explored in future work.