The wind shear exponent in a Mediterranean island climate

Abstract

The difficulties in estimating the long term mean wind speed and subsequently wind turbine energy output derive from the fact that more often than not, available data is taken at a level other than machine hub height. The 1/7th power law has been recognised as a handy tool to carry out vertical wind speed extrapolation to the desired hub height. It is also understood that using an exponent of 1/7th could lead to underestimation of the actual long-term mean wind speed aloft. This paper strives to evaluate the power law with respect to wind data taken on a 25 m mast on the central Mediterranean island of Malta. Whilst deriving a site-specific factor affiliated to a typical terrain type, it also strives to determine characteristic variations of the power law exponent over appropriate sampling intervals.