## 4 Conclusions  
This study presents financial and environmental analyses for transportation-related impacts of silicon-based, CdTe and CIGS solar photovoltaic modules at their EoL phase. The results from this analysis demonstrate that c-Si modules are the most economical PV technology (USD 2.3 per m2 PV module (or 0.87 e/W) for transporting in the US for 1000 km. Additionally, it was found that there is not much difference (only 12%) in the cost of transportation for carrying Si-based, CdTe, or CIGS PV modules for either 1000 km or 2000 km using trucks. The similarities are related to the relationship between distance and unit cost of transportation in the US, which is non-proportional (higher distances result in lower unit costs of transportation). An important finding from the assessment conducted is that, considering the assumptions made, the CO2-eq emissions associated with transporting PV modules are lower for c-Si and CdTe than for CIGS because of the higher weight of the CIGS modules. We also found that the transportation associated CO2-eq emissions contribute 47%, 28%, and 40% of overall EoL impacts of c-Si, CdTe, and CIGS PV wastes, respectively. Besides, it was found that, comparing the different types of fuel used for the transportation and considering all the impacts analyzed, using ships (consumes heavy fuel oil) is the most environmentally friendly alternative (considering acidification, ecotoxicity, eutrophication, global warming, particulate matter, human toxicity, and ozone depletion potential) among the alternatives of diesel-fueled trucks and diesel or electricity fueled trains. Finally, a hot spot analysis on the entire life cycle CO2-eq emissions of different PV technologies showed that the EoL phase-related emissions are more significant for thin-film PV modules compared to crystalline silicon PV technologies, so more environmentally friendly material recovery methods should be developed.  
Conceptualization, I.C., M.L. and R.C.; methodology, I.C.; software, I.C.; validation, I.C. and R.C.; formal analysis, I.C.; investigation, I.C. and M.L.; data curation, I.C. and A.F.; writing--original draft preparation, I.C., M.L. and A.F.; writing--review and editing, I.C., M.L., A.F. and R.C.; project administration, I.C. and R.C.; funding acquisition, R.C. All authors have read and agreed to the published version of the manuscript.  
This research was partly funded by the Australian Renewable Energy agency, grant number SRI-001 and the SAIF and URSCA grants provided by Office of Research and Sponsored Programs at the University of Wisconsin-Platteville.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{2}{c}{**GWP Impacts of 1 m\({}^{2}\) PV Module (kg CO2-eq)**} \\ \cline{2-4}  & **c-Si** & **CdTe** & **CIGS** \\ \hline Cradle-to-Gate [50; 51] & 130.2 & 56.3 & 51.4 \\ EoL & 7.79 & 8.51 & 13.85 \\ Total & 137.99 & 64.81 & 65.25 \\ \hline \hline \end{tabular}
\end{table}
Table 5: The comparison of associated CO2-eq emissions of entire life cycle of different PV technologies. The contribution of transportation to the total GWP impacts are found to be \(\sim\)3%, 4%, and 9% for c-Si, CdTe, and CIGS PVs, respectively.