P.V.C. Luhanga, P. Monowe, N. Nijegorodov, A.R. Akande, and J.G. King (Botswana)
Solar radiation components in Botswana
During the last 17 years main solar radiation parameters have been measured at the University of Botswana main campus, Gaborone. It was observed that instantaneous direct-normal radiation at Solar-Noon can be very high, in fact it can exceed 1000 W/m2; and that half an hour before sunset, it can be greater than 500 W/m2. At sunset (or sunrise) moment it can be up to 100 W/m2. The direct normal solar radiation can exceed 45 MJ/m2per day, whereas mean-daily-global radiation varies from over 30 MJ/m2 in December to over 15 MJ/m2 in June. These high values of daily direct-normal and global radiation are attributed to low humidity and low turbidity. Depending on the meteorological conditions, the hourly and daily direct and global radiation for clear days can vary within 20-25%, depending on the mean meteorological parameters used in the simulation. It was observed that the uv-component has over the years increased by 4-5%. It was noted that the filter used in the instrumentation for uv-radiation measurement degrades with time, necessitating frequent calibration. There were cases of anomalous phenomena observed too: wherein direct normal radiation vastly increases (and diffuse radiation decreases), especially soon after Solar-Noon. When the humidity is low and visibility high, the pyranometer recordings of hourly global radiation can be less than the expected (Ibncosθx+ Id) values. This discrepancy, which may be common for semi-desert areas, with low humidity and turbidity, is discussed. The results obtained are important for practical purposes and efficient utilisation of solar energy.
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