BioDreamMachine Exploring Nature's Solutions

A team at Caltech led by John O. Dabiri (Professor of Aeronautics and Bioengineering) has been testing vertical axis wind turbines in configurations where the positioning of the turbines enhances performance. Individually, these turbines are less efficient that a horizontal axis turbines. Pairs of counter-rotating turbines arranged in an array inspired by how schooling fish align themselves minimize energy consumption are expected to deliver power densities of 20 to 30 watts per square meter.

Existing wind farms are limited in the amount of energy that they can extract - turbulence caused by the propellers of one turbine affect the performance of surrounding turbines, requiring large spaces between turbines.  As turbines become larger and taller to improve efficiency and tap the increase in wind energy with distance from the ground, spacing needs to be increased as well.  Even though horizontal axis wind turbines are approaching the theoretical maximum efficiency of 59%, a typical wind farm is only able to extract about 2 to 3 watts of power per square meter.  Each turbine needs to be spaced apart such that it effectively is isolated from other turbines. 

Field tests using turbines 10 meters tall and 1.2 meters in diameter have been carried out.  In spite of relatively low mean wind speeds (5.7  meters/second), power densities of 6 to 30 watts per square meter were observed.  Power loss from the front to the back of a five-row array was only about 5%.  Additional improvements can be obtained by increasing the height of the turbines which does not require an increase in spacing.  In contrast, increasing the blade length of horizontal axis turbines requires a similar increase in spacing. 

The increasing height and footprint of horizontal axis turbines are forcing wind farms into remote or offshore areas.  This design of vertical axis wind turbines may be able to effectively and efficiently generate power closer to urban centers.

References: 

• John O. Dabiri, Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays, Journal of Renewable and Sustainable Energy 3, 043104 (2011)
• Robert W. Whittlesey, Sebastian Liska and John O. Dabiri, Fish schooling as a basis for vertical axis wind turbine farm design, Bioinspiration & Biomimetics, 5 (2010) 035005