# Modeling Photovoltaic Potential for Bus Shelters on a City-Scale: A Case Study in Lisbon
## 3 Results and Discussion  
### The City's Solar Bus Shelters  
To obtain the location of the bus shelters eligible for PV installation, the shapefile with the bus stops available in the city had to be analyzed and refined. Of the 1880 available locations, 1558 were identified as having a shelter. Of these, 369 were eliminated due to the presence of trees compromising the available radiation (Figure 4). The remaining 1189 point locations were subjected to visual analysis using the ortho-photo and the Google Street View tool. In this analysis, all identifiable shelters were digitalized, and those that were under structures like balconies and bridges were identified. Furthermore, the shelters that were under trees, and that were not classified as so, were also signalized (29). From this analysis, one concludes that the initial tree classification had an overall accuracy of 93%. From this cleaning and refining stage, the output was a polygon file with the 1132 bus shelters available in Lisbon. Such bus shelters have the geographical characteristics that make them eligible for PV installation. The next step was to evaluate the solar resources available at each shelter. Based on the city's DSM, a solar map was derived for the 21 st of December (the northern hemisphere's shortest day) from 8 a.m. to 5 p.m. The parameters for determining solar insolation in the Solar Analyst tool were sky size resolution of 200 cells and viewshed calculated using 360 azimuth directions directions. The atmospheric parameters of direct radiation transmissivity and diffuse radiation fraction were the same as those determined for Lisbon by [23; 24]. The solar map quantifies the direct and global radiation in each location of the surface (kWh/m\({}^{2}\)/day) (Figure 5). The calculation time was 705 h using a standard desktop PC. The solar resource of each bus shelter was determined by using the Direct Irradiance Fraction (DIF). To characterize the shadowing effect, the local Direct Radiation value was compared with the maximum Direct Radiation value found at the 1132 bus shelters (1.198 kWh/m\({}^{2}\)/day), i.e., the value of a pixel that did not suffer any shading. The central point of each shelter was used as a proxy for the shelters' area potential. Other metrics were tested, such as the maximum radiation in the shelter's area, the mean or the mode values. However, problems arose due to the misalignment between the raster file with the radiation information and the polygons obtained from the visual identification of shelters over the ortho-photo (Figure 6). This is a common problem when detailed vector and VHR raster datasets are compared (e.g., [27]). The high insolation bus shelters in Lisbon were selected based on a threshold of DIF \(>\) 70% (Figure 7). The results showed that 612 shelters out of 1132 (54%) received the required energy, making them suitable for PV panel installation (Figure 8). These are the city's solar bus shelters. For the selected high insolation shelters, the PV potential was assessed by considering the bus shelter area, the Global Radiation that reaches each shelter, and a system energy conversion efficiency of 10%. The PV module efficiency, battery charger efficiency and battery charging efficiency are  
Figure 4: Tree map derived from the object-oriented classification of the ortho-photo for the whole city (**a**) and a neighborhood close-up (**b**). dependent on the manufacturers, system configuration and also operating conditions. The values provided are conservative, at the lower end of the spectrum: the PV system power conversion efficiency was 13% [28], and the battery charger system efficiency was 80% [29]. The chosen values indicate the expected minimum energy that could be generated and stored. These are easily altered for specific system configurations. The results indicated that all the shelters produced more than 0.7 kWh/day but fewer (63) produced 1.8 kWh/day or more. In this set, eight shelters presented very high PV potential (>100 kWh/day) due to their large dimensions (e.g., interface stations).