Hydrogen Fuel Cell Transit Bus Technology into a Technical-Economical Perspective 2013-36-0270
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. Early non hybrid fuel cell transit bus designs were focused in reliability rather than fuel economy, with efficiency penalties and reduced fuel cell lifetime, due to the impossibility of regeneration of kinetic energy and dynamic requirement on the power plant. Moreover, this design required larger fuel cells to supply all the power requirements of the vehicles, with direct effects into life cycle costs. In the light of these problems, fuel cell bus designs have evolved to hybridized concept, with fuel cell operating associated with energy storage devices/buffers (batteries, supercapacitors or both), with the possibility of recovery of part of kinetic energy and, hence, improvement of efficiency and fuel cell lifetime. Some state of art hybrid fuel cell drivetrains use a low power fuel cell working in steady state condition, recharging traction batteries, also recharged by the grid, in the so called plug-in configuration. This enables smaller fuel cells, working in a controlled regime, which means lower capital expenditures, higher efficiency and, hence, lower lifetime costs. However, when compared to non hybridized architecture, hybrid systems still present low availability, related to problems in power electronics and energy storage systems, as opposed to the fuel cell itself. Fortunately, ongoing industry effort to optimization of hybrid diesel electric concept, which uses the same electric hybrid architecture, will help to address this hurdle. To reach commercialization of Fuel Cell Transit Buses, few barriers need to be surpassed, with the optimization of i) fuel cell durability, ii) initial purchase cost, and iii) hydrogen production and delivery technology. On a Total Cost of Ownership - TCO perspective, current fuel cell buses are three to four times more costly than modern diesel buses. This additional cost is not acceptable to bus operators, unless with heavy public subsidies. This means that much has to be done in terms of technological improvement and increased volume production scale, to achieve, in the medium term, a reasonable TCO that would make fuel cell buses competitive with well established technologies on environmentally sensitive areas. This work is supposed to present an analysis of technical and economical features and performance of fuel cell bus technology, with a survey of the main barriers that need to be surpassed, and an overview of some ongoing fuel cell bus projects.