Arxiv – Order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays by John O. Dabiri of the California Institute of Technology
Modern wind farms require significant land resources to separate each wind turbine from the adjacent turbine wakes. These aerodynamic constraints limit the amount of power that can be extracted from a given wind farm footprint. We conducted full-scale field tests of vertical-axis wind turbines in counter-rotating configurations under natural wind conditions. Whereas wind farms consisting of propeller-style turbines produce 2 to 3 watts of power per square meter of land area, these field tests indicate that power densities approaching 100 Watts per square meter can be achieved by arranging vertical-axis wind turbines in layouts that enable them to extract energy from adjacent wakes. In addition, we calculated that the global wind resource available to 10-meter tall turbines based on the present approach is approximately 225 trillion watts (TW), which significantly exceeds the global wind resource available to 80-m tall, propeller-style wind turbines, approximately 75 TW. This improvement is due to the closer spacing that can be achieved between the smaller, vertical-axis wind turbines. The results suggest an alternative approach to wind farming, in which many, smaller vertical-axis wind turbines are implemented instead of fewer, large propeller-style turbines.
We conducted a series of field tests on six 10-m tall x 1.2-m diameter VAWTs on a tract of flat, vacant land in the Antelope Valley of Los Angeles County, California from June to September 2010. The turbines were a commercially available model with 4.1-m span airfoil blades and a 1200-Watt generator connected to the base of the turbine shaft. Three of the turbines rotated around their central shaft in a clockwise direction (e.g. from a top view) in winds above 3.8 m s-1; the other three rotated in a counter-clockwise direction when the wind speed exceeded the same threshold.
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