On direct hydrogen fuel cell vehicles : modelling and demonstration
Abstract: In this thesis, direct hydrogen Proton Exchange Membrane (PEM) fuel cell systems in vehicles are investigated through modelling, field tests and public acceptance surveys. A computer model of a 50 kW PEM fuel cell system was developed. The fuel cell system efficiency is approximately 50% between 10 and 45% of the rated power. The fuel cell auxiliary system, e.g. compressor and pumps, was shown to clearly affect the overall fuel cell system electrical efficiency. Two hydrogen on-board storage options, compressed and cryogenic hydrogen, were modelled for the above-mentioned system. Results show that the release of compressed gaseous hydrogen needs approximately 1 kW of heat, which can be managed internally with heat from the fuel cell stack. In the case of cryogenic hydrogen, the estimated heat demand of 13 kW requires an extra heat source. A phase change based (PCM) thermal management solution to keep a 50 kW PEM fuel cell stack warm during dormancy in a cold climate (-20 °C) was investigated through simulation and experiments. It was shown that a combination of PCM (salt hydrate or paraffin wax) and vacuum insulation materials was able to keep a fuel cell stack from freezing for about three days. This is a simple and potentially inexpensive solution, although development on issues such as weight, volume and encapsulation materials is needed Two different vehicle platforms, fuel cell vehicles and fuel cell hybrid vehicles, were used to study the fuel consumption and the air, water and heat management of the fuel cell system under varying operating conditions, e.g. duty cycles and ambient conditions. For a compact vehicle, with a 50 kW fuel cell system, the fuel consumption was significantly reduced, ~ 50 %, compared to a gasoline-fuelled vehicle of similar size. A bus with 200 kW fuel cell system was studied and compared to a diesel bus of comparable size. The fuel consumption of the fuel cell bus displayed a reduction of 33-37 %. The performance of a fuel cell hybrid vehicle, i.e. a 50 kW fuel cell system and a 12 Ah power-assist battery pack in series configuration, was studied. The simulation results show that the vehicle fuel consumption increases with 10-19 % when the altitude increases from 0 to 3000 m. As expected, the air compressor with its load-following strategy was found to be the main parasitic power (~ 40 % of the fuel cell system net power output at the altitude of 3000 m). Ambient air temperature and relative humidity affect mostly the fuel cell system heat management but also its water balance. In designing the system, factors such as control strategy, duty cycles and ambient conditions need to taken into account. An evaluation of the performance and maintenance of three fuel cell buses in operation in Stockholm in the demonstration project Clean Urban Transport for Europe (CUTE) was performed. The availability of the buses was high, over 85 % during the summer months and even higher availability during the fall of 2004. Cold climate-caused failures, totalling 9 % of all fuel cell propulsion system failures, did not involve the fuel cell stacks but the auxiliary system. The fuel consumption was however rather high at 7.5 L diesel equivalents/10km (per July 2004). This is thought to be, to some extent, due to the robust but not energy-optimized powertrain of the buses. Hybridization in future design may have beneficial effects on the fuel consumption. Surveys towards hydrogen and fuel cell technology of more than 500 fuel cell bus passengers on route 66 and 23 fuel cell bus drivers in Stockholm were performed. The passengers were in general positive towards fuel cell buses and felt safe with the technology. Newspapers and bus stops were the main sources of information on the fuel cell bus project, but more information was wanted. Safety, punctuality and frequency were rated as the most important factors in the choice of public transportation means. The environment was also rated as an important factor. More than half of the bus passengers were nevertheless unwilling to pay a higher fee for introducing more fuel cell buses in Stockholm’s public transportation. The drivers were positive to the fuel cell bus project, stating that the fuel cell buses were better than diesel buses with respect to pollutant emissions from the exhausts, smell and general passenger comfort. Also, driving experience, acceleration and general comfort for the driver were reported to be better than or similar to those of a conventional bus.
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