An experimental investigation was carried out on a wind tunnel scale vertical axis wind turbine with unsteady wind conditions. The wind speed at which testing was conducted was 7m/s (giving a Reynolds number of around 50,000) with both 7% and 12% fluctuations in wind velocity at a frequency of 0.5Hz. Rotational speed fluctuations in the VAWT were induced by the unsteady wind and these were used to derive instantaneous turbine rotor power. The results show the unsteady power coefficient (CP) fluctuates following the changes in wind speed. The time average of the unsteady CP with a 7% fluctuation in wind velocity was very close to that with steady wind conditions while 12% fluctuations in wind speed resulted in a drop in the mean CP, meaning unsteady winds of such amplitudes are detrimental to the energy yields from these wind turbines. At mean rotational speeds corresponding to tip speed ratios (λ) beyond peak CP, no significant hysteresis was observed for both 7% and 12% fluctuations. However, substantial hysteresis is seen for conditions where mean λ is below peak CP.
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