Page - 45 - in Hybrid Electric Vehicles

4. Hydrogen fuel cell hybrid bus 4.1. Basic theory Hydrogen fuel cells (FCs) are considered a clean energy source with the main benefits over ICEs of zero harmful emissions during operation and high efficiency [56]. Although many types of FCs exist, this paper will only consider the application of FCs in transportation, considering the operating temperature, start-up time and technology maturity, Proton Exchange Membrane Fuel Cell (PEMFC) offer most promising solution [57]. Significant research into solid oxide fuel cells (SOFCs) in transportation has been carried out [58–60], although these have yet to been applied in real world bus applications. A PEM FC uses hydrogen as the fuel, which, through an electrochemical reaction with oxygen (usually from air) generates electricity with water as the only by-product from the chemical process [61]. By replacing the internal combustion engine in conventional buses, FCs can be used as the primary energy source to power a bus with electri- cal energy, therefore, achieving zero operating emissions. An additional advantage over ICE’s comes from the higher efficiencies exhibited by FCs [62, 63]. However, there are a number of barriers that need to be overcome before widespread deployment can be achieved. These are primarily cost and infrastructure [64, 65]. FC powered buses cost approximately five times more than a conventional diesel bus with the similar power output [66], where they typically cost in excess of £1,000,000 [67], due primarily to the expensive FC stack and the small scale of produc- tion [68]. In addition, the widespread deployment of FC buses would require a significant invest- ment in hydrogen refuelling infrastructure [64]. The implementation of FC buses has shown that the technology is a promising solution for zero emissions buses if these barriers can be overcome. Figure 9 shows the configuration usually used in FC vehicles. The basic drive train utilises a FC to power the propulsion motor; however, FCs are not well suited to providing for the transient power demands associated with city driving buses [69–73]. As such, most FC buses utilise a form of energy storage in a series configuration to both address this and also to Figure 9. Simplified architectures of FC drivetrain. Development of Bus Drive Technology towards Zero Emissions: A Review http://dx.doi.org/10.5772/68139 45