Page - 41 - in Hybrid Electric Vehicles

and hence require relatively regular replacement [34]. In addition to a battery pack, some buses utilise supercapacitors in conjunction with a battery as supercapacitors are much more effective in shielding batteries from high current load and thus increase battery life [35]; however, their low energy density means they are unsuitable to be used as the primary energy source, as shown in Figure 1. They do, however, have several key advantages over existing battery technologies, such as very high power densities and discharge rates as well as very long cycle life [34]. There is no simple answer to which battery technology is best, as it will depend on the application. Mahmoud et al. [36] carried out a detailed comparison study of different electric powertrains and concluded that a single technological choice would not satisfy the varied operational demands of transit services because electric buses are highly sensitive to the energy profile and operational demands. Electric buses are zero emission at the point of use and therefore offer great emission savings particularly in terms of local air pollution when compared to ICE or hybrid buses, as well as very high efficiency. However, there are a number of barriers to widespread deployment, the main ones are recharging time, vehicle range, infrastructure and cost [34]. Battery electric buses normally operate in one of two different forms: opportunity and over- night [32]. Opportunity e-buses have a smaller energy storage capacity that offers limited range but can be charged much quicker (5–10 minutes); while overnight e-buses have a much larger energy storage but at the cost of longer charging time (2–4 hour) [36]. These repre- sent two different approaches for electric buses in the urban environment. The opportunity approach aims to minimise the weight of the battery pack by utilising frequent and fast recharging at points along the bus route, such as bus stops or the end of route [32]. This holds the promise of high efficiency and lower projected bus costs but requires a comprehensive recharging network [37]. Route flexibility of the bus is, however, limited, as it is required to follow the assigned bus route to recharge the battery. The overnight method utilises a large energy storage system to extend the range so that the bus can drive the entire route/day without recharging [37]. This holds the promise of greater route flexibility and convenience as well as utilising a centralised recharging infrastructure, but suffers from passenger loss due to increased battery weight as well as battery lifetime issues [38] and battery cost [34]. An alternative approach is offered by the Trolleybus, which has a small battery but receives power from overhead cables along the assigned route. This overcomes problems associated with range and recharging times but is very limited in terms of route flexibility. Figure 6. Battery electric drive bus basic configuration. Development of Bus Drive Technology towards Zero Emissions: A Review http://dx.doi.org/10.5772/68139 41