Title: Using rotor or tip speed in the acoustical analysis of small wind turbines Authors
Submitted to: American Society of Mechanical Engineers
Publication Type: Proceedings
Publication Acceptance Date: December 29, 2007
Publication Date: January 7, 2008
Citation: Vick, B.D., Clark, R.N. 2008. Using rotor or tip speed in the acoustical analysis of small wind turbines. In: Proceedings of 46th AIAA Aerospace Sciences Meeting and Exhibit, January 7-10, 2008, Reno, Nevada. AIAA 2008-1334. 2008 CD-ROM. Interpretive Summary: Wind energy has been the fastest growing source of energy in the world for the past 6 years, but most of this growth has been in the installation of large megawatt size wind turbines with blade rotor diameters the length of a football field (~ 300 ft). Sales of small wind turbines (less than 100 kW) have been at a much slower pace, and one of the obstacles to the increased sale of these wind turbines is their reputation for producing high levels of noise. For small wind turbines most of the noise usually originates from the fast moving blades, so noise data were recorded for different blades that were tested on 1 and 10 kW wind turbines (10 and 23 ft rotor diameters respectively) that were used for remote water pumping systems (e.g. no utility electricity available). All of the testing was accomplished at the USDA-ARS Conservation and Production Research Laboratory at Bushland, Texas (located 10 miles east of Amarillo in the Texas Panhandle). Three different blades were tested on the 1 kW wind turbine. When the noise level was plotted versus wind speed, it was difficult to determine if the noise level was reduced due to a large amount of scatter in the data. However, when the noise level was plotted versus wind turbine rpm, the amount of data scatter was decreased significantly and it became obvious that each successive blade design had a significantly lower sound level. In addition, when the blades began fluttering, the rpm could be determined for each blade design. The decrease in data scatter by binning the data with blade rpm instead of wind speed is logical since most of the noise comes from the blades. For the larger 10 kW wind turbine two different blades were tested. For both sets of blades the noise level was seen to increase significantly when the electrical load was disconnected by the controller (wind turbine disconnected from pump when the power generated by wind turbine exceeded 10 kW). The newer blades tested on the 10 kW wind turbine were shown to produce a lower sound level than the original blades when the pump was disconnected from the wind turbine although the sound level was about the same for both sets of blades when the wind turbine was connected to the pump. When the noise level was compared between the two different wind turbines, the 1 kW wind turbine blades were quieter than the 10 kW wind turbine blades as long as flutter was prevented on the 1 kW wind turbine with proper electrical loading via the controller. Prior to this work, the noise level was always plotted versus wind speed, but now engineers at wind turbine companies and research laboratories may begin plotting the noise level versus rotor rpm or blade tip speed which will result in a better understanding of the wind turbine noise and the design of blades with lower noise levels which should lead to more wind turbines being installed near populated areas.
Technical Abstract: Acoustical noise data have been collected and analyzed on small wind turbines used for water pumping at the USDA-ARS Conservation and Production Research Laboratory (CPRL) near Bushland, Texas. This acoustical analysis differed from previous research in that the data were analyzed with rotor or tip speed being the independent variable in addition to analyzing the data with wind speed as the independent variable. Acoustical noise generation was analyzed for two different wind turbines which were tested with different blades. The averaging period for acoustical noise data was one second instead of one minute (smallest time increment recommended in IEC wind turbine noise standard) since the sound pressure level of small stand-alone wind turbines can vary significantly over just a few seconds. Disconnecting the wind turbine from the water pump motor by the pump controller was shown to significantly increase the noise of the wind turbine.