REVIEW OF 17 YEARS OF CONSTANT MONITORING OF MAIN SOLAR RADIATION COMPONENTS AT UB CAMPUS (BOTSWANA): PECULIARITIES AND DISCREPANCIES

摘要

During the last 17 years main solar radiation parametersrnhave been measured at the University of Botswana mainrncampus, Gaborone. It was observed that instantaneousrndirect-normal radiation at Solar-Noon can be very high, inrnfact it can exceed 1000 W/m~2; and that half an hour beforernsunset, it can be greater than 500 W/m~2. At sunset (orrnsunrise) moment it can be up to 100 W/m~2. The directnormalrnsolar radiation can exceed 45 MJ/m~2 per day,rnwhereas mean-daily-global radiation varies from over 30rnMJ/m~2 in December to over 15 MJ/m~2 in June. These highrnvalues of daily direct-normal and global radiation arernattributed to low humidity and low turbidity. Dependingrnon the meteorological conditions, the hourly and dailyrndirect and global radiation for clear days can vary withinrn20-25%, depending on the mean meteorologicalrnparameters used in the simulation. It was observed thatrnthe uv-component has over the years increased by 4-5%.rnIt was noted that the filter used in the instrumentation forrnuv-radiation measurement degrades with time,rnnecessitating frequent calibration. There were cases ofrnanomalous phenomena observed too: wherein directrnnormal radiation vastly increases (and diffuse radiationrndecreases), especially soon after Solar-Noon. When thernhumidity is low and visibility high, the pyranometerrnrecordings of hourly global radiation can be less than thernexpected (I_(bn) cosθ_z + I_d ) values. This discrepancy,rnwhich may be common for semi-desert areas, with lowrnhumidity and turbidity, is discussed. The results obtainedrnare important for practical purposes and efficientrnutilisation of solar energy.