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utilise regenerative braking [74]. An additional benefit of such an approach is that the size of the expensive fuel cell stack can be reduced [75]. The energy storage implemented is usu- ally either electrochemical battery technology such as Li-ion or NiCd batteries or electrostatic supercapacitors (sometimes referred to as ultracapacitors). The choice between these depends on the particular design and requirements of the system, with batteries offering reasonable power and energy densities although they have a relatively short cycle life and superca- pacitors offering poor energy densities but excellent power densities, as shown in Figure 1. Additionally, supercapacitors have very long lifetimes of up to 40 years [31]. In a series configuration, there are three main modes of operation that can be utilised to pro- vide for the buses power demands, as shown in Figure 10. Although these are the main modes of operation, the way these modes are utilised will depend on the control strategy imple- mented [76]. • Mode 1: The SC discharges to supplement the FC to provide for high transient power de- mands. This type of operation is expected to occur under heavy loads such as during ac- celeration or going uphill. • Mode 2: The FC will both power the load and use excess power to charge the SC. This is expected to occur under low loads, when the FC power output is higher than the required load. • Mode 3: The power from the FC and generated power from regenerative brake will both be used to charge the SC. This is only expected to occur when the drive motor is operating as a generator in the regenerative brake mode. There have been a number of projects aimed at utilising FC technology for bus propulsion applications. Table 3 lists many of the projects currently in operation along with the FC size and energy storage used. The projects are split into two main categories depending on the relative size of the FC and energy storage systems. The majority of the current projects are FC Figure 10. Modes of operation for a series hybrid FC drive train [77]. Hybrid Electric Vehicles46