Estimating Global Solar Radiation in Bale Robe Town Using Angstrom-Prescott and Hargreaves-Samani Models

Abstract

The demand for energy, consumption of forest for firewood and charcoal, and the pollution rate of atmosphere are increasing in alarming rate as worldwide particularly in developing countries like Ethiopia. Following hydro energy the forest is the main source of energy for the communities of Robe town. The town is growing rapidly and facing high shortage of energy source from household to organization level. Therefore, for many reasons like solar energy which is free of environmental pollution, renewable and abundantly accessible has to be considered and studied for the development of project on energy harvesting. The first objective of this study was to determine seasonal value of global solar radiation GSR by using Angstrom-Prescott AP and Hargreaves-Samani HS models in Bale Robe town. The second one is to estimate variation between the two models for this town. The data were obtained from Robe meteorological station which was measured over a period of the year 2010 to 2014. The measured data of the daily sunshine duration and daily maximum and minimum temperature were used to estimate seasonal mean values of GSR and their percentage difference between Angstrom-Prescott AP and Hargreaves-Samani HS models in Bale Robe town and analyzed using linear regression. The findings of the study in general, revealed that, GSR in different seasons summer, autumn, winter and spring season were 18.80 MJm^-2day^-1, 22.76 MJm^-2day^-1, 26.64 MJm^-2day^-1 and 23.94 MJm^-2day^-1 respectively using Angstrom-Prescott model. Using Hargreaves-Samani model the value of GSR found to be in summer, autumn, winter and spring seasons were 20.60 MJm^-2day^-1, 24.42 MJm^-2day^-1, 28.60 MJm^-1day^-1 and 26.12 MJm^-2day^-1 respectively. The results of the study showed that, there were peak values of GSR estimated in winter and spring and low value of global solar radiation has been observed in summer and autumn in both models. The percentage difference between the two models showed that AP and HS models were favorable models that predict seasonal value of GSR in Bale Robe town in the absence of instrumental installations which are important to measure GSR directly. Based on the finding, peak value of GSR obtained in a seasons of winter and spring with AP and HS models. Hence, both models used successfully to estimate seasonal value of GSR with relative accuracy.

Keywords: GSR in Robe, sunshine duration, air temperature, AP and HS model