DETERMINATION OF THE POWER LAW EXPONENT AT MAKAMBAKO-IRINGA, TANZANIA
HH Mwanyika and RM Kainkwa
Department of Physics, Faculty of Science,
University of Dar Es Salaam,
P.O.BOX 35063, Dar es Salaam, Tanzania.
ABSTRACT
The knowledge of wind characteristics can be used to identify suitable locations for generating electricity from this energy source for the rural population in Tanzania. These characteristics include variation of wind speed and direction with height, wind regime, turbulence, and wind gusts. Ideally the wind speed and direction should be measured at hub heights (20 m above the ground level) of most Electric Wind Energy Conversion Systems (EWECS). However, this is not achieved in practice because of high cost associated with measuring wind speed and direction at such heights. Lack of data at these heights is a reason methods have been developed and used to extrapolate wind speed to these heights from lower levels. The 1/7th power law is among the methods that have been used to make such extrapolations. However it tends to underestimate the actual long-term average wind speeds. In general, the power law exponent varies in space and time depending on relief features, stability of the atmosphere and roughness length of a place. In this paper, two approaches were used to establish the power law exponents at Makambako using wind speed data recorded at two different heights (Z1=2 m and Z2=7 m above the ground level). In one method we used only Z1 and Z2 together with the roughness length Z0=0.25 m for Makambako. In the other, we used a combination of Z1 and Z2 and the speeds measured at corresponding heights. The exponent obtained for the two methods were not significantly different since they varied by about 0.47 %. The power law exponents obtained can now be used to extrapolate wind speeds at lower levels to hub heights.