## 3 Results
### Environmental Impacts of Transportation  
In analyzing the environmental impacts of transportation, we offered two steps of analysis. First (Figure 2a), we compared the global warming potential (GWP or kg CO2-eq or carbon footprint) environmental impacts of the most common method of transportation \"truck\" with other ways of transportation. Second (Figure 2b), we compared the total environmental impacts of four types of transportation methods.  
Figure 2a shows the GWP impacts of transportation of different PV technologies for 1000 km. As shown, GWP impacts of CIGS modules are always higher than c-Si and CdTe PV modules due to the higher weight of the CIGS modules. Furthermore, using a diesel-fueled truck has the highest GWP impacts among all transportation methods considered here. On the other hand, GWP impacts of shipping have the lowest GWP impacts and are about 15 times less compared to transportation by the diesel-fueled trucks. These results indicate that overseas processing could be an environmentally friendly approach in the recycling of PV waste if we prioritize the GWP impacts from transportation, mainly if the PV plants are located close to or in coastal regions and truck transport to a US port can be minimized.  
Figure 2b gives a more complete picture of all the environmental impacts related to the transportation of PV waste. For this analysis, we extracted the data for truck, train, and ship transportation from the EcolInvent database Then, we normalized the environmental impacts of  
Figure 2: Environmental impacts of transportation of PV module (**a**) global warming potential (GWP) emissions of transportation (**b**) total normalized environmental impacts of PV transportation. Note that the calculations for transportation vehicles were performed assuming the average weight of PV module as 20 kg.  
different transportation options (train with electricity and diesel, and ship) using the diesel-fueled truck as a reference point. To use the impacts of diesel-fueled truck as a reference point for the normalization, we divided each EPA-TRACI impact category for the environmental impacts of all means of transportation options to the those of diesel-fueled trucks assuming each environmental indicator category has equal impact. Thus, we calculated the total normalized environmental impacts of diesel fuel truck's as 10 since there are 10 impact categories in EPA TRACI. The results are somewhat similar to what we observed in the comparison provided in Figure 2a. The total normalized environmental impacts of the diesel-fueled trucks are drastically (65%-95%) higher compared to the other transportation options. The higher environmental impacts associated with trucks are mostly due to the diesel production (-40%) and construction of roads (-20%). While the total environmental impacts of trains are more evenly affected by the processes related to train transportation. For example, the impacts from diesel consumption, construction of railway tracks and production of wagons contribute 33%, 28%, and 25% of the total environmental impacts of trains fueled by diesel, respectively. On the other hand, the total impacts of electric trains are mostly due to construction of railway tracks (29%), production of wagons, and consumption of high voltage electricity (17%). Most of the environmental impacts of the ship transportation are associated with the emissions due to port activities (-45%) and consumption of heavy fuel oils (-35%).  
The difference between the total environmental impacts correlates with the amount of fuel consumed in each transportation alternative. For example, the trucks consume 37 g of fuel while the trains consume 2.7 kg of fuel during the transportation of each metric ton per km. Similarly, diesel train consumes 17 g of fuel of diesel fuel, while electric train consumes 0.7 g of fuel and 0.31 MJ of electricity per metric ton per km.