# Solar Potential in Saudi Arabia for Southward-Inclined Flat-Plate Surfaces
## 4 Conclusions and Discussion  
The present study investigated the solar availability across Saudi Arabia on flat-plate solar panels with south orientation. The main objective was to find the optimum tilt angles of solar panels that produce maximum annual energy under all-sky conditions. This was achieved by calculating the annual energy sum on flat-plate surfaces with tilt angles in the range 5\({}^{\circ}\)-55\({}^{\circ}\) towards south with an increment of 5\({}^{\circ}\) at 82 sites across Saudi Arabia; the solar availability on a horizontal plane was also included. The calculations of the energy received on the tilted surfaces were repeated for both a ground albedo equal to 0.2 (reference value) and a near-real ground albedo. The first outcome of the work was that the optimum tilt angles over Saudi Arabia are 20\({}^{\circ}\), 25\({}^{\circ}\), and 30\({}^{\circ}\) towards south. The second finding of the study was that these 3 optimum tilt angles group the 82 sites into 3 (solar energy) zones (SEZ), i.e., SEZ-A with 20\({}^{\circ}\) S, SEZ-B with 25\({}^{\circ}\) S, and SEZ-C with 30\({}^{\circ}\) S. The third conclusion was related with the variation of the annual maximum solar energy in each SEZ, i.e., 2192-2595 kWhm\({}^{-2}\)year\({}^{-1}\) (SEZ-A), 1612-2977 kWhm\({}^{-2}\)year\({}^{-1}\) (SEZ-B), 1702-2587 kWhm\({}^{-2}\)year\({}^{-1}\) (SEZ-C), and 1612-2977 kWhm\({}^{-2}\)year\({}^{-1}\) (all SEZs). Beside the annual energy sums, monthly solar energy ones averaged over all locations belonging to the same SEZ as well as to all SEZs were estimated under all-sky conditions. Regression equations were provided as best-fit curves to the monthly mean energy sums that estimate the solar energy potential per SEZ (and all SEZs) with great accuracy (R\({}^{2}\)\(\geq\) 0.90). These expressions may prove very useful to architects, civil engineers, solar energy engineers, and solar energy-systems investors in order to assess the solar energy availability in Saudi Arabia throughout the year. Seasonal solar energy sums were also calculated. They were averaged over all sites in the same SEZ as well over all sites (all SEZs) as in the case of the annual and monthly sums, under all-sky conditions. For every case, regression curves that best fit the mean values were estimated with greatest accuracy (R\({}^{2}\) = 1). Maximum sums were found in the summer (730 kWm\({}^{-2}\)), and minimum ones in the winter (450 kWm\({}^{-2}\)), as expected. Figure 12: Variation of the correction factor, CF, as function of the tilt angle, \(\beta\), of an inclined surface towards south (S) in (**a**) SEZ-A, (**b**) SEZ-B, (**c**) SEZ-C, and (**d**) all SEZs, averaged over the period 2005–2016. The black solid lines are the mean CF values, while the red and blue ones correspond to the mean + 1\(\sigma\), and mean \(-\) 1\(\sigma\), respectively. The green dotted lines are the best fits to the mean curves and are hardly seen as they coincide with the mean curves. A correction factor, CF, was defined as the ratio of the maximum solar energy sum derived from calculations with near-real ground albedos, \(\rho_{\rm g}\), to that derived with the reference value, \(\rho_{0}\). A graph of CF as function of the tilt angle (in the range \(15^{\circ}\)-\(55^{\circ}\)) showed an exponential growth for sites having ratios \(\rho_{\rm g}/\rho_{0}>1\), or an exponential decay in the cases of \(\rho_{\rm g}/\rho_{0}<1\). Such curves are assumed to be universal (nomograms), but this remains to be proved for other locations in the world with different climate and terrain characteristics. Graph of CF as function of \(\rho_{\rm g}/\rho_{0}\) was prepared for different values of the tilt angle (in the range \(15^{\circ}\)-\(55^{\circ}\)). Best-fit lines to the data points were estimated and found to be linear of decreasing slope with decreasing tilt angle. These curves are also assumed to be nomograms, but this also has to be proved for other locations in the world with different climate and terrain characteristics.