Calculation of urban solar radiation based on simulation and fitting of ASHRAE model constants

Karachi is one of the largest metropolitan cities in the world. It is located along the coast of the Arabian Sea. This article investigates the variability of monthly solar radiation in Karachi and Badin (nearby coastal regions) using the ASHRAE model. Clear sky ASHRAE model contains constants, A (Apparent solar radiation constant), B (atmospheric coefficient), and C (diffused sky factor) which are the central three constant values used to find solar radiation on any surface, either inclined or horizontal. We have used two alternate methods of finding these constants by a 9th-degree polynomial technique and simulation approach. The values of ASHRAE constants are determined by both ways and are compared with actual values, resulting in a good agreement. Furthermore, the direct, normal, and global radiations were calculated for two cities of Sindh, namely, Karachi and Badin, which lie in the Coastal region of Pakistan. Subsequently, the results were compared with those calculated by the ASHRAE model; a good agreement was also found between them. Multiple statistical analyses such as Root Mean Square Error (RMSE), Mean Bias Error (MBE), Chi-square and t–statistics, and coefficient of correlation were used to validate the results. RMSE results show the uncertainty of less than 2%; Chi-square values validate our results, t-statistic values ± 0.06, show that mean values of our products and calculation by the ASHRAE model are same.