*2.1. The r.sun Solar Radiation Model*

The r.sun model implemented in the open GRASS GIS environment is one of the commonly used GIS-based solar radiation models [18]. The r.sun model is based on a comprehensive methodology for spatially and temporally distributed solar radiation and irradiance calculations developed by Hofierka and Šúri [8]. The model can calculate direct (beam), diffuse, and reflected solar radiation components of the total solar radiation for a specific location on land surface, given day, latitude, and atmospheric conditions. The module is designed to meet the needs of users in different scientific fields, such as environmental sciences, photovoltaics, agriculture, or forestry. Its applications range from local to regional scales. Another typical feature of this module is that it considers the shadow effect of local topography, which can be switched on and off according to the type and need of a given calculation. Solar irradiation maps are calculated by integrating the corresponding irradiances in selected time steps between the sunrise and sunset times for a given day.

The r.sun module works in two modes. In the first mode, it calculates the angle of incidence of solar radiation (expressed in degrees) and the solar irradiance values (W/m2) for the set local time. In the second mode, the daily solar irradiation amounts are calculated for the set day. By scripting, these two modes can be used separately or in combination to provide estimates for any desired time interval [27].

Together with the r.sun model, the PVGIS online tool was developed to assess the photovoltaic potential of chosen locations within the regions of Europe and Africa [28]. The r.sun model was also used in the assessments of photovoltaic potential in urban areas [2]. The most commonly investigated surfaces include roofs of buildings. Thus, we can determine, for example, the photovoltaic potential for installing PV systems on these rooftops [2].

The basic input parameters of the r.sun model include a DSM and raster maps of slope steepness, aspect, and land surface albedo, as well as Linke's turbidity coefficient. Another input parameter is the specific hour of the day and day of the year for which calculations of global solar radiation, or its individual components, will be performed.

The advantages of this model according to Šúri et al. [27] are as follows:


The growing trend of solar applications in urban areas requires the use of the most efficient models that can evaluate the solar potential of each surface. Therefore, it is important to evaluate the complex morphology of urban areas using models that exploit 3D environments. While the r.sun model can be applied to 2D surfaces in the form of rooftops, the v.sun model has been developed to detect the solar potential of vertical surfaces and facades [5].
