• Energy Research

In this paper some preliminary experimental results on a Zebra battery based propulsion system for urban bus applications are presented. The tests were carried out using a laboratory 1:1 scale test bench, composed by a 65 kW electric drive, specifically designed for urban bus applications, supplied by two 20 kWh Zebra batteries connected in parallel. The electric power train was tested on a laboratory bench, connected through a fixed ratio gear box to a 100 kW regenerative electric brake provided with speed and torque controls, in order to evaluate the propulsion system performance in steady state operative conditions. The obtained preliminary experimental results were utilized to implement a Matlab-Simulink model of urban bus, which might be powered by the same electric propulsion system studied. Thanks to this model it was possible to evaluate the dynamic behavior of the urban bus, working on standard driving cycles, taking into account the resistant forces represented by proper vehicle/road/aerodynamic parameters. An evaluation of the expected real vehicle driving range was also estimated in different road conditions.

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%.

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 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.

In this paper experimental results obtained on a small size fuel cell power train (1.8 kW) based on a 500 W Proton Exchange Membrane (PEM) stack are reported and discussed, with specific regard to energy management issues to be faced for the attainment of the maximum propulsion system efficiency. The fuel cell system (FCS) was realized and characterized investigating the effect of main operative variables on efficiency. This resulted higher than 30% in a wide power range with a maximum of 38% at medium load. The efficiency of the overall fuel cell power train measured during both steady state and dynamic conditions (European R40 driving cycle) resulted about 30%. A discussion about the control strategy to direct the power flows is reported with reference to two different test procedures used in dynamic experiments, i.e. load levelled and load following.