Effects on the Performance of Vertical Axis Wind Turbines with Unsteady Wind Inflow: A Numerical Study

Abstract

Numerical simulations of a micro scale vertical axis wind turbine (VAWT) rotating at constant speed were performed with steady and unsteady wind conditions. The aim of the study was to determine both the performance of a VAWT in unsteady wind conditions and also to determine the flow physics that causes the performance. Frequencies used for the unsteady incoming wind ranged from high values equal to the VAWT rotational frequency to low rates of up to an order of magnitude slower. Mean and standard deviation of wind velocity data from a local wind station were used as mean and amplitude of the variation in wind speed. In general the results show a decrease in performance versus steady wind conditions. The worst cases show a 75% drop in power coefficient (CP) for wind fluctuation rates equal to a quarter of and an order lower to the VAWT rotational frequency. Increasing wind velocities induced earlier stalling and loss of lift on VAWT blades while decreasing wind velocities reduced the apparent angle of attack and subsequently the lift generated by the blades. A hysteresis in the CP was observed when the VAWT was in the low tip speed ratio (λ) region while negative performance dominated the high λ region. For the conditions tested here, any time variation in the wind tends to decrease the energy yield of a VAWT.