Utility systems fuel density

A prototype system-integrated microfiuidic fuel cell stack based on the air-breathing direct methanol laminar flow fuel cell technology [42] has been reported by INI Power Systems. A combination of planar and vertical stacking methods was employed to scale the system and increase its power output. With respect to fuel utilization, a fuel and electrolyte separation and recirculation system was proposed at the cost of added complexity and reduced energy density of the complete fuel cell system. 

The performance of fuel cells is affected by operating variables (e.g., temperature, pressure, gas composition, reactant utilizations, current density) and other factors (impurities, cell life) that influence the ideal cell potential and the magnitude of the voltage losses described above. Any number of operating points can be selected for application of a fuel cell in a practical system, as illustrated by Figure 2-4. 

Since each system achieves the same total fuel utilization (90%) across the same total area, each stack has the same average current density. Irreversible voltage loss is mainly a function of current density and stack temperature. Since these parameters are equivalent in each stack, it is assumed that the Nemst potential of each stack would be reduced by the same amount. 

Another study (32) maximized the efficiency of conventional and series-connected fuel cell systems by optimizing cell voltage and current density. The study found that the optimum fuel utilization in the series-connected system was higher than that in the conventional system. Most importantly, the higher fuel utilization and lower current density of the series-connected system combined to give more efficient performance than the conventional system. 

A fuel cell utilizing UDMH as fuel and N02 as oxidant is reported. Operated intermittently over a 3-month period with degradation, it consistently produced a power density of 40mw/ cm2 (40w/ft2). The cell consists of a sandwich of Zr acid phosphate in a polyvinylidene fluoride (PVF) binder and diffuse-catalyst layers of Pt black, Zr acid phosphate and PVF. Pt screens are used as current collectors] 3) G.R. Eske-lund et al, Chemical-Mechanical Mine , PATR 3724 (1968) [A mine feasibility study is reported in which the hypergolic system UDMH—... 

As for the power density of SOFC systems, a comparison between the different kinds of cells has been made (Vora, 2006). The measured specific power (at 1000 K, fuel utilization ratio of 80% and 0.65 V) of a tubular SOFC bundle (24 cells) is around 0.13 in W/cm3, whereas that of an HPD5 bundle (six cells) is of 0.17 in W/cm3. The Delta9 configuration, with bundles of nine cells, reaches over 0.4 W/cm3. A comparison is shown in Figure 7.9. 

In volume limited applications, high density propellant combinations are favored and some appropriate trade-off between performance and density is established. In a truly volume limited system as shown in section IV. A. 1., the appropriate performance parameter is the product of the specific impulse and the propellant bulk density, a quantity usually labeled the density impulse. Conceivably, mixture ratio may be determined by yet other vehicle system considerations. If a new propellant combination is to be utilized in an existing vehicle, the optimum mixture ratio may be influenced by such considerations as existing pump flow rate capacities, tank volumes, and structure load carrying capacities. Even other system considerations, such as the desirability of operating at equal fuel and oxidizer volume flow rates to allow interchange of fuel and oxidizer flow hardware, may determine the propellant mixture ratio. 

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