Determination of the power law exponent for southern highlands of Tanzania

Abstract

The 1/7th power law is among the methods that have been used to extrapolate wind speed to the hub heights of wind turbines from the measuring levels. However, it tends to underestimate the actual long-term average wind speeds. In this paper, the power law exponent for Makambako, a site located in the southern highland zone of Tanzania, was established using wind speeds measured at heights Z1=2 m and Z2=7 m. The average power law exponent obtained at the test site was 0.47, a value that is substantially higher than the 1/7th power law exponent. It is low during the dry season and high during the rainy season. Values of the exponent higher than the overall mean were observed during the night while lower values were detected during the daytime. The overall mean value of the exponent determined from this trial site can be used to project wind speed to desired heights in areas with similar topographical features especially those in the southern highlands of Tanzania, in which the experimental site is situated.

Tanzania Journal of Science Vol. 32 (1) 2006: pp. 103-108