Fuel stack characteristics

While the reformer and burner can be considered as Gibbs reactors (delivering thermodynamic equilibrium values), the flow sheet simulation of the overall process requires the implementation of a confirmed stack characteristic. Key figures for the stack are power output, fuel utilization and electrochemical efficiency at the desired operation point. Thus, a Staxera Mk200/ESC4 stack was evaluated in a stack-test-bench with different fuel gas compositions and throughputs. Figure 4 shows the measured U/I-curves, Table 1 summarizes the stack performance data for the different operation points. 

DaimlerChrysler developed a methanol processor for the NeCar 3 experimental vehicle. This was demonstrated in September 1997 as the world s first methanol-fuelled fuel cell car. It was used in conjunction with a Ballard 50-kW fuel cell stack. Characteristics of... 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

In this paper we report the effect of varying loads on a small size DMFC stack (10 cells with 9 cm active-area each). The transient responses of the stack voltage have been investigated upon variable current load conditions to obtain the information on the dynamic characteristics of the stack. Also, the transient responses of the stack current upon changing fuel flow rates have been monitored to obtain the optimal operating conditions for the staek. 

Figure 20. Performance characteristics of the International Fuel Cells 10-cell DMFC stack. (After Ref. 112 reproduced by permission of The Electrochemical Society, Inc.)...

PEFC) stacks, components and entire systems, in off-grid, and grid-connected configurations, with a capacity of up to 100 kW electrical power output. The facility consists of an automated and computerised fuel cell test station, gas analysers, a multi-axial vibration system which is housed in a walk-in environmental chamber (for controlling temperatures, humidity, shocks and vibrations) and ancillary equipment. The data obtained are complementary to and validate fuel cell simulations and models with reference to operation modes, components and system characteristics 1 ... 

Pei et al.12 studied the hydrogen pressure drop characteristics in a PEM fuel cell in order to use the pressure drop as a diagnostic tool for prediction of liquid water flooding in fuel cell stacks before flow channels have been blocked. 

This section examines some experimental evidence for the transient behavior of individual cells evaluated on single-cell test stands and stacks contained within full systems. Results from models developed to help understand some of the detailed physical effects that influence cell performance are also examined. The goal is to introduce, albeit in brief, some of the principle dynamic characteristics of single cell and full stack performances. Single cell studies are important since they isolate the cell in a well understood and controlled environment thereby removing the effects of other external processes (e g., reforming) which may have their own transient behavior that affects cell operation (e g., controlling cell input fuel composition). Such... 

The major market for the product fuel oil for the demonstration plant and near-term future commercial plants is expected to be existing power plants in the coastal metropolitan areas, where the physical and environmental costs of conversion to coal make such a conversion impractical. A significant characteristic of the SRC-II fuel oil for this application is its low sulfur content and thus the capability to meet stringent emission limits in urban areas. Coal-derived residual fuels will, in general, not meet these requirements without stack gas cleanup. 

Subsequent deployment of the new catalyst in the cathode layer of small-area MEAs first, then large-area MEAs, and finally fuel cell stacks represents the typical series of performance tests to check the practical viability of novel ORR electrocatalyst materials. Figure 3.3.15A shows the experimental cell voltage current density characteristics (compare to Figure 3.3.7) of three dealloyed Pt-M (M = Cu, Co, Ni) nanoparticle ORR cathode electrocatalysts compared to a state-of-the-art pure-Pt catalyst. At current densities above 0.25 A/cm2, the Co- and Ni-containing cathode catalysts perform comparably to the pure-Pt standard catalyst, even though the amount of noble metal inside the catalysts is lower than that of the pure-Pt catalyst by a factor of two to three. The dealloyed Pt-Cu catalyst is even superior to Pt at reduced metal loading. 

Ballard assembles and tests stacks, to confirm their characteristics and optimise such water management problems as electrolyte humidity control and cathode water removal, via wettable materials in the porous electrode, to the external air flow. Potential difficulties, exclusive to the water-producing PEFC, can be seen with 100% humidity in the tropics, and with freezing conditions in cool climates. High altitudes can be difficult for all fuel cell types, via low oxygen density. 

The combustion characteristics of the bio-fuel oil from BTG has been assessed in a 300 kW(th) fiunace at Rostock University. This combustion facility is depicted in Figure 8 and consists of a burner head, an bum out chamber immediately under the burner. Connected to the flame chamber is a horizontal heat exchanger segment and finally the exhaust flue gases are diluted with air to be released by the stack. 

The MCFC is a promising power generating source because of its unique characteristics such as high fuel efficiency and ability to use various carbonaceous fuels. Although Ni-10wt% Cr is used in the state-of-the-art MCFC as anode, it needs to be improved in terms of better creep and sintering resistance. In spite of the development in the alternate cathode material research, lithiated nickel oxide has been the choice of cathode material in the kilowatt-level MCFC stacks developed by many companies. Continuous research in the development of stable electrolyte retention matrix, identification of suitable molten carbonate electrolyte composition, and additives to the electrolyte will be a significant milestone. Also, research in the area of current collector/bipolar plate to overcome... 

Other configurations of fuel cell vehicles can be realized combining the advantages of different types of storage systems. As an example, the Fig. 5.24 shows the combination of rechargeable batteries with a super capacitor system. In this case, a three-way converter is required to connect the two storage systems with the fuel cell stack and interface the different voltage versus current characteristics of the devices interconnected [46]. 

For the fuel cell the situation is quite different. One of the characteristics of the fuel cell is its modular construction. This opens wide opportunities. For a certain type of fuel cell, 0.8 V for example, the voltage is a given parameter. The current then depends on the area of the cell. When stacking a number of these cells by connecting them in series, the voltage depends on the number of interconnected cells. In other words, by serial connection of a number of cells, the voltages is multiplied while, by enlarging the area, the current rises. 

Nevertheless, classical heterogeneous catalysts like particulate noble metals may be immobilized on the nanotube surface as well. Nanoparticles of platinum or rhodium, for instance, can be deposited on cup-stacked carbon nanotubes by reductive precipitation (Figure 3.114b). The catalysts obtained this way suit an application in fuel cells run on methanol. Electrodes made from the nanotube material exhibit twice the efficiency as compared to the classical material XC-72-carbon. The particles of noble metal on the nanotube surface catalyze the direct conversion of methanol into CO2 (MeOH -1- H2O CO2 -1- 6 H -1- 6e ). A material to be employed in such fuel cells has to meet some essential requirements, including a maximal specific surface, a defined porosity and a high degree of crystalhnity. Carbon nanotubes are endowed with exactly these characteristics, which is why they are the most suitable material for electrodes. Their high price, however, is still prohibitive to an industrial scale application. 

F. Barbir, M. Fuchs, A. Husar and J.Neutzler, Design and Operational Characteristics of Automotive PEM Fuel Cell Stacks , Fuel Cell Power for Transportation, SAE SP-1505, Proc. SAE International Congress Exhibition, Detroit, March 2000, pp. 63-69, SAE, Warrendale, PA, 2000... 

The combustion roar associated with flares typically peaks at a frequency of approximately 63 Hz while combustion roar associated with burners can vary in the 200-500 Hz range. Burner noise can have a spectrum shape and amplitude that can vary with many factors. Several of these factors include the internal shape of the furnace, the design of the burner muffler, plenum and tile, the acoustic properties of the furnace lining, the transmission of the noise into the fuel supply piping, and the transmissive and reflective characteristics of the furnace walls and stack. 

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