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or refuelling. As such this allows for much greater route flexibility. This appears to be eas- ily achieved for FC buses, however for overnight e-buses this is not always the case and will again be dependent on the size of the battery. Refuelling time: Opportunity e-buses require frequent recharging throughout the entire route. Although each recharges for the opportunity e-bus only takes up to 15 minutes, it is still considered as a drawback due to the requirement for regular recharging. Overnight e-buses require a longer recharging time (average >4 hours) after each operation due to the increased battery capacity. The recharging time is heavily dependent on the charging power. Trolley e-buses are charged through overhead wires so that they require no refuelling time. FC buses are refuelled with gaseous hydrogen, which can be completed quickly (<10 minutes) [91]. Infrastructure: Opportunity e-buses and trolley buses require corresponding infrastructure along the route and each end of the routes. Therefore, opportunity e-buses and trolley buses require a comprehensive infrastructure network. Overnight e-buses and FC buses both require infrastructure to recharge/refuel at the end of daily operation. This can, however, be central- ised at the service depot and hence does not need to be as comprehensive. It appears, however, that the current recharging times for overnight e-buses presents a problem since it is likely that a significant number of recharging points and a massive recharging power would be needed to recharge the batteries of a large fleet in time for the next day’s service. This could potentially be an issue for the electrical grid infrastructure if the number of buses grows significantly, while this would not be a problem for FC buses because of their short refuelling time. Fuel availability: All three battery electric bus technologies use electricity to recharge their batteries. This electricity could be central managed and distributed locally through the local electricity grids; however, widespread electric bus deployment could significantly stress this infrastructure. FC buses will likely require the development of a comprehensive distribu- tion network for hydrogen, although on-site hydrogen production has been demonstrated. Additionally, hydrogen fuel storage would also create additional cost. Clean source: Real zero emissions bus technology needs to be clean throughout the manu- facturing process, fuel production and bus operation. Currently, battery electric and FC bus Zero emission option Opportunity E-bus Overnight E-bus Trolley E-bus Fuel cell bus Daily range 4 3 1 2 Route flexibility 3 1 4 1 Refuelling time 2 3 Not available 1 Infrastructure 3 2 4 1 Fuel availability 1 1 1 4 Clean source 1 1 1 4 Cost 3 1 2 4 Notes: 1, best; 4, worst. Table 4. High level comparison of operational performance of zero emission bus concepts. Development of Bus Drive Technology towards Zero Emissions: A Review http://dx.doi.org/10.5772/68139 51