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Environmental concerns and limited fossil fuels reserves have fostered an increased interest in alternative propulsion systems. In this scenario, electric traction, with its inherent zero local emissions, high efficiency and improved operational performance (acceleration and hill climbing potential), emerges as a desired option for public transport systems. Transit buses, the prevailing transport system in cities, and, hence, strong contributors to traffic environmental impact on urban areas, can reduce considerably their environment burden with the use of electric traction. This means less local pollutants, specially particulate matter - PM and nitrogen oxides - NOx, currently the “Achilles heel” of diesel engines, as well as CO2 greenhouse emissions - GHG.

The increasing demand for urban mobility, combined with the constriction of investment capacity of transit authorities and private companies make bus based systems a great option for public transport systems, since they allow the provision of high quality services at a fraction of the costs of rail based systems. In this scenario, Bus Transit System - BTS and Bus Rapid Transit - BRT allow the implementation of transport networks at considerably lower costs than their rail system counterparts. This is specially true to developing-nation cities, that have infrastructure costs as a pre-eminent decision-making factor in technology selection. From an environmental perspective, traction technology and fuel option are decisive to define systems' performance.

Intensive use of fossil fuels in densely populated areas has caused adverse environmental effects in cities all over the world. This has fostered the evaluation of alternative technologies for transit applications, like hydrogen fuel cells - electrochemical energy conversion devices that operate with zero emission, quieter and with higher efficiencies than internal combustion engines, specially at part load regimes. Transit bus market is particularly well suited to technology innovations because they are i) centrally fueled and maintained, ii) professionally operated on fixed routes and schedules, iii) tolerate weight and volume requirement of new technologies and, finally, whenever necessary, iv) can be subsidized by government. In this scenario, considerable research, development and testing effort has been dedicated to hydrogen fuel cell bus technology, with the engagement of governments and transit authorities, bus industry and operators.

The growing concentration of population in world metropolis caused by increasing urbanization rates has pushed the demand for high capacity and efficient public transport systems. At the same time, environmental concerns have led to increasingly stricter emission standards. In this context, transit authorities have become strongly focused on making their bus fleets more efficient and cleaner, by incorporating new alternative fuels and clean propulsion technologies. This has led to increased interest in electric driven technologies, with their intrinsic efficient, quiet and environment friendly features. Trolleybuses, a well proven mature electric technology already adopted in some cities, although efficient and clean, are burdened by high infrastructure costs and operational inflexibility.