Mixed-fleet operation of battery electric bus and hydrogen bus: Considering limited depot size with flexible refueling processes

Electrified transit is crucial for promoting zero-emission transportation in metropolitan cities. However, this initiative faces considerable challenges in Hong Kong due to limited space available for parking and refueling bus fleets. Despite the inexpensive operational costs of battery electric buses (BEBs), plenty of space is required to park and refuel BEBs. Hydrogen buses (HBs) can partially address the space issue with a longer driving range and shorter refueling time compared to the BEBs, whereas more operational costs must be spent. Therefore, a mixed fleet with BEB of HB tends to be a more cost-effective solution. In consideration of the distinctive characteristics of BEBs and HBs, this study develops an integrated vehicle scheduling and refueling of mixed fleets with multiple depots (IVSR-MFMD) restricted by the limited depot size (i.e., space provided for bus parking and bus refueling), where flexible refueling processes, allowing for uncertain refueling start times and refueling amounts, are endogenously incorporated into the operational cycle. The speed-up techniques are further used to reduce problem dimensions and reformulate the model. A two-route-two-depot example and a real-world case in Hong Kong are used to verify the model. The results demonstrate that adopting HB can efficiently reduce depot size and achieve lower operational costs when routes consume massive energy. Additionally, it is found that flexible refueling processes lead to lower operational costs, and the customized speed-up techniques can significantly improve computational efficiency.