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the availability upgrade program carried out in 2014. The availability is expected to improve to over 85% by the end of the CHIC project as operators gain more operational and problem- solving experience. Apart from the technical and economic improvements, the London trail buses have also proven that the technology became more viable because of the full working schedule, direct diesel replacement, centralised infrastructure and high public acceptance [86]. The trial test of FC-powered buses projects has provided promising performance as a long-term solution to zero emission transportation. 5. Comparison study This part aims at to provide a comparison of the current state of low emission and zero emis- sion bus systems. Diesel hybrid buses have been developed and deployed as a means of achieving emissions reductions, where a number of advantages in terms of efficiency, emis- sions and fuel consumption can be seen over diesel buses. There are, however, a number of problems associated with their widespread deployment. The first of these is the cost and is due to the additional components necessary for the electrical system. Second, the inclusion of the electrical system necessitates a significantly more complicated configuration [19]. Third, although diesel hybrid buses can offer significant improvements in terms of CO2 and NOx emissions, the primary energy source is still the ICE. As such, they fail to address the underly- ing source of emissions and are therefore fundamentally limited in the improvements that can be achieved. As such, they can only really be considered as a transitional technology to reduce emissions but are not a viable option for meeting zero emissions targets. In order to meet the requirements for zero emissions buses, which is the ultimate objective for a clean transporta- tion network, technologies such as electric and FC buses have been developed as a long term solution for city bus transportation needs. Therefore, this section will mainly compare the battery electric bus (opportunity, overnight and trolley) and FC bus technologies as the two most promising zero emission solutions in terms of the operational requirements and is sum- marised in Table 4. The rankings are based on the authors’ opinions with reasoning given in the paragraphs below. Range: Opportunity e-buses have a smaller energy storage that requires frequent recharging, which equates to poor performance in terms of daily range. Overnight e-buses utilise a much larger battery, which increases the range with reported values of over 300 km per charge. Trolley e-buses are continuously powered with electricity by overhead lines along the route which effectively gives unlimited range. FC buses use hydrogen cylinders as the fuel tanks, which allow the range to be greatly extended (up to 450 km) for as much as hydrogen fuel cylinder weight and size allows [91]. Route flexibility: Opportunity and trolley e-buses require recharging infrastructures along the route which greatly limits their route flexibility. This is somewhat dependant on the size of the on-board battery and will likely be more acute for trolley e-buses. The overnight e-buses and FC buses are expected to be able to operate for an entire day’s service without recharging Hybrid Electric Vehicles50