Title: Effect of blade flutter and electrical loading on small wind turbine noise Authors
|Broneske, Sylvia -|
Submitted to: Renewable Energy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 20, 2012
Publication Date: February 1, 2013
Citation: Vick, B.D., Broneske, S. 2013. Effect of blade flutter and electrical loading on small wind turbine noise. Renewable Energy. 50:1044-1052. Interpretive Summary: Wind energy was the fastest growing energy source in the United States in 2010. This is primarily due to wind energy's cost effectiveness compared to other energy alternatives. Wind turbines do not require water to produce electricity, and they do not produce any emissions or waste when they generate electricity. The energy alternatives that wind energy is replacing are natural gas, coal, and nuclear and these fuels require large amounts of water for electricity generation and do generate emissions and waste. Wind turbines can however produce a noise level high enough that some people find them objectionable. The noise level can be reduced with changes in blade design and controller electrical loading. The noise level was measured on two wind turbines: one rated at 1 kW and the other rated at 10 kW. Both of these wind turbines were tested with three different blade designs. When the noise data were analyzed with wind speed conventionally, it was difficult to determine which blade designs had a lower noise level. When the blades were analyzed with blade angular velocity (e.g. rotor speed in rpm), the newer blade designs were shown to produce less noise. In addition, analyzing the sound level with rotor speed helped determine whether the wind turbine blades were fluttering, and at what rpm the flutter would begin to occur. Fluttering for wind turbine blades is usually caused by a rapid oscillation of the blades normal to the direction of rotation. In this study, fluttering was shown to increase the sound level 15 to 25 dB for a 1 kW wind turbine, and 5 dB for a 10 kW wind turbine. The first two wind turbine blade designs tested on the 1 kW wind turbine were shown to flutter whether the wind turbine was electrically loaded or unloaded. The third and final blade design tested on the 1 kW wind turbine did not flutter as long as the wind turbine was electrically loaded. For the 10 kW wind turbine, the original blades would flutter if the wind turbine was disconnected from an electrical load. Two other newer blade designs were tested on this 10 kW wind turbine, and neither showed signs of fluttering whether or not there was an electrical load. Both these newer blade designs when analyzed with rotor speed were shown to produce approximately the same noise level. One of these newer blade designs was installed on a 10 kW wind turbine connected to the utility grid, and the other blade design was installed on an off-grid irrigation pump. For wind speeds less than 20 mph, the noise level of the grid-tie system was shown to be 1 to 5 dB less than that of the off-grid system due to a difference in off-grid and on-grid electrical loading. Reducing the noise for the grid-tie systems is especially important since the wind turbine owner is more likely to have nearby neighbors. Reduced wind turbine noise should result in an increase in small wind turbine sales. More wind turbine sales will improve the environment because increased wind turbine electrical generation will result in a reduction in the amount of fossil fuel burned in steam power plants. More small wind turbine sales will also increase the number of American jobs since the major small wind turbine manufacturers in the world are based in the United States.
Technical Abstract: The effect of blade flutter and electrical loading on the noise level of two different size wind turbines was investigated at the Conservation and Production Research Laboratory (CPRL) near Bushland, TX. Noise and performance data were collected on two blade designs tested on a wind turbine rated at 1 kW, and there also was a third blade tested on an updated version of the same wind turbine. The 1 kW wind turbines were used for off-grid water pumping. On a 10 kW wind turbine, noise and performance data were collected on two blade designs (BW03bl1 and SH3055bl3) which were used for off-grid water pumping. Data were also collected on a third blade design (SH3055bl4) with an updated version of the 10 kW wind turbine, and this wind turbine was connected to the utility grid via an inverter. For the 1 kW wind turbine, binning the sound pressure level (SPL) data with rotor speed, instead of wind speed, was necessary to determine if the blade design affected the likelihood of blade flutter. For the 1 kW wind turbine with an electrical load, blade flutter occurred at a higher rpm for the shorter blade, and no flutter occurred when the shorter blade was stiffened by adding more fiber. However, if there was no electrical load, the shorter stiffer blade design could flutter, and the sound power level (SWL) could increase above 110 dB. For the 10 kW wind turbine, the original blades (BW03bl1) fluttered when the wind turbine was offline (e.g. no electrical load), but two later blade designs (SH3055bl3 and SH3055bl4) did not flutter up to the highest wind speed that data were collected (16 m/s). A procedure was developed to calculate SWL with rotor speed as the independent variable. This procedure helped determine which of the three blade designs tested on the 10 kW wind turbine had the lowest noise level even though the tower heights and electrical loadings varied. For the 10 kW wind turbine, the SWL of the SH3055bl3 blade design (tested on an off-grid system) was approximately the same as that of the SH3055bl4 blade design (tested on the grid-tied system) for the same rotor speed. For wind speeds below 10 m/s, the SWL of the on-grid 10 kW wind turbine was lower than the off-grid 10 kW wind turbine because the electrical loading was greater. However, for wind speeds above 10 m/s, the SWL of the on-grid 10 kW wind turbine was higher due to the electrical loading being less than that of the off-grid 10 kW wind turbine (as long as the off-grid 10 kW wind turbine was connected to an electrical load).