Enhanced energy capture by a vertical axis wind turbine during gusty winds in an urban/suburban environment

Abstract

Wind resource maps of annual-averaged wind speeds, typically used in the siting of wind turbines and wind farms, can grossly underpredict the actual wind energy potential in areas with highly transient or gusty winds. The goal of the present study is to quantify the additional energy capable of being harvested by a small vertical axis wind turbine using control strategies that exploit the excess energy in wind gusts. A transient-response numerical model is utilized with input from actual wind data acquired by cup-vane and sonic anemometers located in both an urban and suburban area. The total energy captured by the turbine during the year 2013, along with its overall efficiency, was determined for two different control strategies: constant rotational speed (ω) controller and ideal tip speed ratio (TSR) controller. For the case of the constant ω controller, the turbine achieves maximum efficiency when it is operated at an optimal overspeed setting, ωopt. Results indicate that ωopt can be estimated from the turbulence intensity of the wind. For the case of the ideal TSR controller, turbine efficiency was observed a plateau to a maximum value when the nondimensional turbine response parameter ζ dropped below a critical value ζc. This is as expected since turbines with fast response times (ζ < ζc) are capable of instantaneously tracking fluctuations in the wind and thus effectively capturing the high energy content contained within each wind gust. The value of ζc exhibits a dependence on the characteristic gust time scale. Over the course of the year, the turbine in conjunction with the constant ω controller, operating at ωopt, was capable of harvesting six times the energy of a naive controller operating a fixed ω based on the annual-averaged mean wind speed. The same turbine operating in conjunction with an ideal TSR controller, however, was successful in harvesting nearly thirteen times the energy of that with the naive controller. This may have significant implications on the viability of small turbines in relatively gusty urban/suburban areas, even when the annual-averaged wind speed is below a threshold typically considered practical for harvesting wind energy.