Determining the solar reflectance coefficients of terrain from satellite images and creating a tilted radiation model based on a digital elevation model

In many solar radiation models, a combination of physical measurements and mathematical models is used to achieve results close to actual values. However, since these calculations are typically based on a horizontal surface, terrain irregularities and shading effects are often overlooked. Additionally, total solar radiation can only be calculated considering three dimensional (3D) terrain model and albedo effect since albedo effect is ignored in classical solar radiation calculation formulas. In two-dimensional (2D) evaluations, total radiation is generally defined as the sum of direct and stray radiation. Moreover, reflected radiation appears when the studies are conducted in three dimensional (3D) model. To compute reflected radiation, the albedo value of the corresponding pixel and slope must be taken into account. This study aims to improve global solar radiation calculations—one of the most critical indicators in site selection—by utilizing a 3D Digital Surface Model (DSM) which can instead of traditional formulas that operate on a horizontal plane. It is desired to obtain real radiation and sunshine duration thanks to this approach. Additionally, the study seeks to analyze the impact of reflected radiation on total radiation by incorporating albedo values into the global solar radiation computation. As a case study, a 1.600 km² test area located south of Karaman was selected due to its high solar energy potential, varied topography, and diverse land surface types with different albedo values. Using the Google Earth Engine (GEE) platform, albedo values were calculated for summer and winter seasons based on atmospherically corrected SENTINEL-2A satellite imagery with a spatial resolution of up to 10 meters. The results showed that the albedo value for snow-covered surfaces in winter was 0,86, while for light-colored buildings, it was 0,36 in summer and 0,28 in winter. These findings, which closely align with reference values, it is examined that the albedo value for the surfaces may change due to angle of incidence in summer and winter by obtaining closely aligned results to the reference values. The slopes of pixels were determined using 5-meter resolution digital surface models to compute reflected radiation after calculating pixel-based albedo values for both seasons. 3D solar radiation maps were generated using the ArcGIS Area Solar Radiation Module to calculate shading effects in required total radiation while reflected radiation is calculated. As a result, it was determined that reflected radiation can contribute to an increase of approximately 5% to 7% in total radiation on especially steep sloped terrain.