# Modeling Photovoltaic Potential for Bus Shelters on a City-Scale: A Case Study in Lisbon
## 2 Materials and Methods
### Bus Shelter Photovoltaic Potential
#### 2.2.1 Assessing the City's Solar Potential  
The availability of solar radiation is affected by the time of the day and season, the atmospheric conditions, the latitude and the local topography. Several solar spatial databases are available for Europe, including Meteorom ([[https://meteororm.com/](https://meteororm.com/)]([https://meteororm.com/](https://meteororm.com/))), S@tel-Light ([[http://www.satel-light.com/](http://www.satel-light.com/)]([http://www.satel-light.com/](http://www.satel-light.com/))), NASA POWER ([[https://power.larc.nasa.gov/](https://power.larc.nasa.gov/)]([https://power.larc.nasa.gov/](https://power.larc.nasa.gov/))), SOLEMI ([[https://www.dlr.de/](https://www.dlr.de/)]([https://www.dlr.de/](https://www.dlr.de/))) and PVGIS ([[https://re.jrc.ec.europa.eu/](https://re.jrc.ec.europa.eu/)]([https://re.jrc.ec.europa.eu/](https://re.jrc.ec.europa.eu/))). The data provided are intended for continental/national analysis. However, for regional and local scale analysis, while the periodicity of the measurement (i.e., time resolution) provides good accuracy (ranging from 30 min to hourly, daily and monthly averages), the spatial detail does not. For large spatial and temporal scales, computing radiation and accounting for obstructions to sunlight can only be performed using computational modeling of the physical context [18]. In these situations, solar availability is usually modeled in a Geographic Information System (GIS) environment [19], generally through a Digital Surface Model (DSM) derived from LiDAR data. The level of detail is essential in the urban analysis, since the incident solar radiation varies along the surface (1) due to the presence of buildings with distinct morphologies, (2) on an hourly basis and (3) on a seasonal basis due to sun-path movements. For a list of existing solar mapping tools, see [18]. In a state-of-the-art review performed by [18], 12 numerical models out of 21 were identified as suitable for urban spaces. From this set, ArcGIS Solar Analyst [20] and GRASS GIS r.sun [21] have been widely applied.  
Figure 2: Datasets used for studying bus shelters’ photovoltaic (PV) potential: ortho-photo (**a**), bus stops and bus shelters (**b**) and Digital Surface Model (**c**).  
In this work, the solar radiation is modeled for the whole city surface, using ArcGIS' Solar Analyst (Esri), following the work of [10,22]. This model requires as input the city's DSM and local climatological parameters such as transmissivity and diffuse radiation fractions, compiled by [23,24].  
The Area Solar Radiation Tool--available in Solar Analyst--calculates, for each location (pixel) on the surface, the direct radiation and the global radiation (direct and diffuse portions). In the present work, the worst solar scenario was investigated by selecting the 21 st of December, the winter solstice. Then, by taking into account the site's latitude and elevation, the local aspect, the shifts in the Sun's angle and the shadows cast by surrounding features (like buildings or trees), the model produces a map of the solar irradiation (direct + diffuse fractions) at each location, for the shortest day of the year.  
Although the project's focus is the public space, in this initial task, all elements above ground must be considered, mainly buildings, since they affect the access to sunlight in the surrounding areas, a key factor when sizing PV systems.