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technologies can achieve zero operating emission but the lifetime emissions are much harder to quantify. It is hard to forecast how the emissions from new technology manufacturing will change, but the fuel production method can be roughly estimated. In the UK, the GHG emis- sions for electrical energy were 0.44932 kgCO2/kWh in 2014 [92]. This is likely to change as the UK’s energy mix changes, where in 2015, 24.6% of electricity was generated from renewable energy sources [93]. Similarly, for FC buses, the source of hydrogen is critical in determin- ing the overall emissions. Currently, about 96% of hydrogen is derived from fossil fuels [94] which results in 13.7 kgCO2/kgH2 [95]. Despite this, investigations into the use of renewable energy for hydrogen production through the process of electrolysis have been carried out offering potential for a low carbon source of hydrogen. Currently, electricity for battery elec- tric buses is a cleaner fuel than hydrogen for FC buses. Cost: Both electric and FC buses have higher capital costs than a conventional diesel bus; how- ever, FC buses are currently far more expensive than electric buses. The capital cost of electric buses is somewhat dependant on the type of operation expected, where overnight buses will have higher costs than opportunity and trolley buses due to the increased battery capacity. This does, however, need to be weighed up against the cost of infrastructure, where opportu- nity and trolley buses require a comprehensive and expensive charging network. Overnight electric and FC buses on the other hand can make use of a centralised recharging/refuelling infrastructure. Throughout this chapter, the main technologies being implemented to meet the low emissions requirements have been presented. The most promising for these in terms of zero emissions are electric and FC buses; however, it is clear that there are still significant barriers to their widespread implementation. Following on from the challenges identified in the comparison section a number of challenges for future developments have been identified. For electric buses, it is clear that further improvements to battery technology are required in terms of their energy densities and lifetime as well as the development of an effective charg- ing infrastructure. The challenges are somewhat dependant on whether the bus is intended to use the overnight or opportunity charging schemes. For overnight charging, the charging infrastructure can be centralised; however, this necessitates very large power requirements for the charging infrastructure, additionally the range of the buses needs to be addressed through battery developments. The opportunity charging schemes a comprehensive and dis- tributed charging network. In most cases, this requires the development of high efficiency and power wireless charging technologies. The future development of FC buses requires development in a broader range of areas. This includes further work on individual components such as the FC stack and hydrogen storage. The FC stack is still the most expensive component of the FC bus. The further development of the control strategies for hybridised buses held significant promise in reducing the size of the required FC stack and improving the fuel economy. Hydrogen storage is a key area for future research for bus applications, where technologies such as solid state storage offer potential to improve the storage density of hydrogen. For widespread implementation, the development of the hydrogen infrastructure is vital. This includes the production of hydrogen, particularly from clean sources, the distribution of hydrogen or on-site production and purification. Hybrid Electric Vehicles52