• Energy Research

In this paper, the humidification issues of proton exchange membrane (PEM) fuel cells were experimentally analyzed using three fuel cell systems (FCSs) based on stacks of different sizes (2.4, 6.2 and 14 kW). Both internal and external humidification strategies were considered. External humidification was performed on the air stream using the following techniques: air saturation at different temperatures (bubbler), water injection into the cathode manifold and heat and mass exchange by selective polymeric membranes. The internal humidification analysis focused on the self-humidification approach. The effect of humidification strategies on membrane hydration was evaluated by analyzing the stack performance and its power loss rate. The external humidification strategy was effective at most operative conditions, but it exhibited limitations at typical conditions which favored membrane dry-out (i.e., low load and high stack temperature). At a high load and temperature, the external humidification was effective when the saturation temperature of the inlet air stream was maintained at values close to the stack temperature (temperature difference < 5 K). The selfhumidification technique was shown to be the most practical choice for application in hybrid fuel cell vehicles, though it requires accurate control of the stack temperature profile in the range of 303e328 K for normalized powers (P/Pmax) between 20 and 90%.

Three developmental catalytic converters, provided by different companies, were tested at the exhaust of a SI (spark ignition) NG (natural gas) engine for bus application. The catalysts were all based on noble metals: Pt (platinum), Pd (palladium), Rh (rhodium) and differed in size, metal loading and active phase composition. Emission evaluation was performed according to the European ECE-R49 procedure (13 mode cycle), in stoichiometric and lean-burn conditions. In addition to regulated emission measurement, speciation of NMHC (non-methane hydrocarbons) and carbonyl compounds was performed. The results showed that all the catalyst compositions considered allowed the European emission limits to be complied when the engine operated in stoichiometric conditions, while the overall best performance in the lean region was obtained on the catalysts with noble metal composition Pd:Rh=21:1. For NMHC and carbonyl compounds, not limited by regulation, conversion efficiency results always close to 100%. About NOx (nitrogen oxides) emissions in lean operation, the effect of the different noble metals on the catalyst conversion efficiency was investigated and compared with results obtained on cobalt-based de-NOx catalysts.

The dynamic performance of a laboratory fuel cell system based on a 20 kW H2/air proton exchange membrane (PEM) stack was investigated on test cycles compatible with automotive applications, with particular reference to the effect of different air management strategies on cell voltage uniformity and fuel cell system efficiency. The air management strategies were varied by imposing different stoichiometric ratio values as function of stack current, and were studied on two test cycles characterized by current variation rates ranging from 2 to 50 A/s, with maximum stack current of 240 A. Stack temperature and reactant pressure during the tests were maintained below 330 K and 150 kPa, respectively. The best compromise between fuel cell system efficiency and dynamic response in terms of cell voltage regularity was obtained with an air management strategy characterized by stoichiometric ratio values slightly superior to those optimized for steady state conditions. This management strategy determined an efficiency decrease in steady state conditions of maximum 3% for the sub-system stack + compressor in the range 0–200 A. The individual cell voltage uniformity was continuously monitored by a statistical indicator (coefficient variation Cv), which was always lower than 3% also at 50 A/s, indicating a satisfactory dynamic stack operation.

A laboratory fuel cell system based on a 20 kW H2/air proton exchange membrane stack was designed, realized and characterized with the aim to elucidate specific concerns to be considered for both hydrogen stationary power systems and automotive applications. The overall system characterization permitted the effect of the main operative variables (temperature, pressure and stoichiometric ratio) on stack power and efficiency to be evaluated. Reactant feeding, humidification and cooling problems are discussed evidencing in particular the role of air compressor, fuel purge, stack temperature and humidification strategy in the system management. The characterization results are analysed in terms of H2 consumption and available power evidencing the energy losses of the individual fuel cell system components. In particular the data obtained on key components (stack, reactants, heat and water management devices) are used for a critical discussion about their specifications and operation characteristics as demanded by both stationary and mobile applications.

This work aimed to examine the possibility of creating natural gas urban bus fleets by applying lean burn technology. Different engine configurations were tested, keeping in consideration the necessity to ensure suitable performance, and to meet the European regulations. With the target torque and power, the severe European limits were not met only for methane emissions. In addition, the GWI (global warming impact) values were also computed and compared with proposed limits expressly conceived for natural gas engines. The results showed that the NG lean burn engine at the present state of development does not appear able to meet the future requirements of both low NOx emissions and GWI, because of the difficulty of resolving the trade-off between NOx and HC.

This paper deals with the application of lithium ion polymer batteries as electric energy storage systems for hydrogen fuel cell power trains. The experimental study was firstly effected in steady state conditions, to evidence the basic features of these systems in view of their application in the automotive field, in particular charge–discharge experiments were carried at different rates (varying the current between 8 and 100 A). A comparison with conventional lead acid batteries evidenced the superior features of lithium systems in terms of both higher discharge rate capability and minor resistance in charge mode. Dynamic experiments were carried out on the overall power train equipped with PEM fuel cell stack (2kW) and lithium batteries (47.5 V, 40 Ah) on the European R47 driving cycle. The usage of lithium ion polymer batteries permitted to follow the high dynamic requirement of this cycle in hard hybrid configuration, with a hydrogen consumption reduction of about 6% with respect to the same power train equipped with lead acid batteries.

An experimental study was carried out on a fuel cell propulsion system for minibus application with the aim to investigate the main issues of energy management within the system in dynamic conditions. The fuel cell system (FCS), based on a 20 kW PEM stack, was integrated into the power train comprising DC–DC converter, Pb batteries as energy storage systems and asynchronous electric drive of 30kW. As reference vehicle a minibus for public transportation in historical centres was adopted. A preliminary experimental analysis was conducted on the FCS connected to a resistive load through a DC–DC converter, in order to verify the stack dynamic performance varying its power acceleration from 0.5 kW/s to about 4 kW/s. The experiments on the power train were conducted on a test bench able to simulate the vehicle parameters and road characteristics on specific driving cycles, in particular the European R40 cycle was adopted as reference. The “soft hybrid” configuration, which permitted the utilization of a minimum size energy storage system and implied the use of FCS mainly in dynamic operation, was compared with the “hard hybrid” solution, characterized by FCS operation at limited power in stationary conditions. Different control strategies of power flows between fuel cells, electric energy storage system and electric drive were adopted in order to verify the two above hybrid approaches during the vehicle mission, in terms of efficiencies of individual components and of the overall power train. The FCS was able to support the dynamic requirements typical of R40 cycle, but an increase of air flow rate during the fastest acceleration phases was necessary, with only a slight reduction of FCS efficiency. The FCS efficiency resulted comprised between 45 and 48%, while the overall power train efficiency reached 30% in conditions of constant stack power during the driving cycle.

In this paper the performance of two polymeric electrolyte fuel cell systems (FCS) for hybrid power trains are presented and discussed. In particular, an experimental analysis was effected on 2.4 and 20 kW stacks with the aim to investigate the energy management issues of the two FCSs for utilization as power sources in electric power trains for scooter and minibus, respectively. The stack characterizations permitted the effect of the main operative variables (temperature, pressure and stoichiometric ratio) on mean power density of cells to be evaluated. The FCS efficiency was evaluated and compared for the two traction systems, individuating the optimal operative conditions for automotive application and specifying the energy losses of the auxiliary components. The efficiency of both fuel cell systems resulted higher than 40% in a wide range of loads (100–600 mA/cm2), with maximum values close to 50%. The experimental characterization of the two power trains was carried out on dynamic test benches, able to simulate the behaviour of the two vehicles on the European R40 driving cycle. The characterization of the two propulsion systems on R40 driving cycle evidenced that the overall efficiency was not affected significantly by the hybrid configuration adopted, as the efficiency values ranged from 27 to 29% in the different procedures analyzed.