The cost–emission trade-off in the optimization of a mixed-fleet public bus system

This paper presents a multi-objective optimization framework that helps bus operators in planning the composition of a decarbonized mixed-fleet bus system. The framework combines environmental aspects, namely greenhouse gas emissions and cumulative energy demand, with economic aspects represented by total life cycle costs. The underlying life cycle assessment accounts for efficiency gains and changes in the electricity mix over time (2030 - 2050). The optimization framework determines the optimal fleet composition considering seven different technology options and provides detailed insights into the incremental trade-offs between environmental impact and costs. As application of our general model, we analyze the bus system of the city of Graz, Austria (pop. 300,000). Compared to emission-minimal solutions, we identified fleet compositions that offer comparable levels of emission reduction at cost savings of 25 million euros. Depending on the assumed electricity mix, these fleet compositions consist of battery-electric and fuel cell buses, or synthetic fuel buses.