Fuel cell sizing

Electroless deposition of the catalytic Pt or Pt-Ru layer was proposed for the preparation of electrodes in microdirect methanol fuel cells.53 A porous silicone substrate is prepared by the anodic etching in HF-ethanol-water (1 1 1) solution. After the etching, at the surface of porous silicon substrate, a thin film of titanium is sputtered and then a film of Pt or Pt-Ru alloy with thickness of about 150-200 nm was electroless deposited. The electrodes prepared in this way helped in minimization of the fuel cell size and increased the reactive area of the catalyst over the silicon electrode surface. 

Water height experiments were performed with fuel elements (638 gm U-235 fuel cell size 3 X 3 X 36 in. metal-to-water ratio > 0.298) arranged in various slab cohfigura-tions, either rod-free or with a single, fully inserted, B" -SS, box-shaped control rod. Reactivity control was by water height only and measurements included those of critical hei t, he, and Incremental water height worth. Experimental results are given in the table, where lattices are described by the array of circles for elements and crosses for poison rods. 

Fuel Cell Size (in.) Thermal UUlizaHon UneasureiQ (theory -Sd ... 

Major issues that influence the development of a fuel processor are 1) choice of commercially available fuels suitable for specific applications 2) fuel flexibility 3) catalyst tolerance 4) fuel cell size, and 5) vaporization of heavy hydrocarbons. Heavy hydrocarbons, such as diesel, require vaporization temperatures much in excess of 350 to 400 °C, at which temperature some of the heavier fuels pyrolyze. 

Use of Polarization Curve for Fuel Cell Sizing Example... 

Because of the low operating temperature and ease of fabrication for low power units, PFFCs are the most likely fuel cell to be introduced in portable power packs. PFFCs in sizes of 300—500 W are being considered as a power source, eg, 4-h duration, 300 W, 1.2 kW, for the modem soldier operating in the enclosed environment of a self-contained protective suit, which has faciUties for air conditioning, radio communication, etc. Analytic Power Corp. (Boston) is assessing the use of PFFCs for this appHcation. 

Not all of the gas is wasted. About 300 MW of electricity is generated from landfills. A variety of electric generation systems have been employed by a small number of developers. Most projects use simple technology and are small (2—10 MW). However, an EPRI study has estimated that landfill gas resources in the United States could support 6,000 MW of generation if utilized in 2-MW-sized carbonate fuel cells. Constmction on the world s first utihty-scale direct carbonate fuel cell demonstration was begun in California. If successful, EPRI estimates that precommercial 3-MW plants based on this design could become available by the end of this decade at an installed cost of 17,000/kW. 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

Example. The Pechini method for fuel cell electrode preparation. La, Ba, Mn niU ates - - CgHgO — citrate complex - - C2FI6O2 — gel. Metal nitrates are complexed with citric acid, and then heated with ethylene glycol to form a transparent gel. This is then heated to 600 K to decompose the organic content and then to temperatures between 1000 and 1300K to produce tire oxide powder. The oxide materials prepared from the liquid metal-organic procedures usually have a more uniform particle size, and under the best circumstances, this can be less than one micron. Hence these particles are much more easily sintered at lower temperatures than for the powders produced by tire other methods. 

Fuel cells, which rely on electrochemical generation of electric power, could be used for nonpolluting sources of power for motor vehicles. Since fuel cells are not heat engines, they offer the potential for extremely low emissions with a higher thermal effidency than internal combustion engines. Their lack of adoption by mobile systems has been due to their cost, large size, weight, lack of operational flexibility, and poor transient response. It has been stated that these problems could keep fuel cells from the mass-produced automobile market until after the year 2010 (5). 

Cell size depends strongly on the fuel and mixture composition more reactive mixtures result in smaller cell sizes. Table 3.2 shows that a stoichiometric mixture of methane and air has an exceptionally low susceptibility to detonation compared to other hydrocarbon-air mixtures. 

A deflagration-detonation transition was first observed in 1985 in a large-scale experiment with an acetylene-air mixture (Moen et al. 1985). More recent investigations (McKay et al. 1988 and Moen et al. 1989) showing that initiation of detonation in a fuel-air mixture by a burning, turbulent, gas jet is possible, provided the jet is large enough. Early indications are that the diameter of the jet must exceed five times the critical tube diameter, that is approximately 65 times the cell size. 

As with a high explosive, a fuel-air mixture requires a minimum charge thickness to be able to sustain a detonation wave. Hence, a fully unconfined fuel-air charge should be at least 10 to 13 characteristic-cell sizes thick in order to be detonable. If the charge is bounded by a rigid plane (e.g., the earth s surface) the minimum charge thickness is equal to 5 to 6.5 characteristic-cell sizes (Lee 1983). 

The characteristic magnitudes of detonation cells for various fuel-air mixtures (Table 3.2) show that these restrictive boundary conditions for detonation play only a minor role in full-scale vapor cloud explosion incidents. Only pure methane-air may be an exception in this regard, because its characteristic cell size is so large (approximately 0.3 m) that the restrictive conditions, summarized above, may become significant. In practice, however, methane is often mixed with higher hydrocarbons which substantially augment the reactivity of the mixture and reduce its characteristic-cell size. 

The PAFC is, however, suitable for stationary power generation, but faces several direct fuel cell competitors. One is the molten carbonate fuel cell (MCFC), which operates at "650°C and uses an electrolyte made from molten potassium and lithium carbonate salts. Fligh-teinperature operation is ideal for stationary applications because the waste heat can enable co-generation it also allows fossil fuels to be reformed directly within the cells, and this reduces system size and complexity. Systems providing up to 2 MW have been demonstrated. 

System integration involves numerous miscellaneous development activities, such as control software to address system start-up, shut-down and transient operation, and thermal sub-systems to accomplish heat recovei y, heat rejection and water recoveiy within the constraints of weight, size, capital and operating costs, reliability, and so on. Depending on the application, there will be additional key issues automotive applications, for example, demand robustness to vibrations, impact, and cold temperatures, since if the water freezes it will halt fuel cell operation. 

Many of the world s major automakers, prompted by both this consumer demand and progress in reducing the inherent cost and size of fuel cells, are now committed to developing and commercializing fuel cell vehicles. 

It is expected that the fuel cell should be able to compete with an IC engine in terms of size and weight. As an added advantage, many fuel cell components can be configured into a relatively wide array of shapes to take advantage of space onboard the vehicle. 

The methane conversion and hydrogen yield were investigated as a function of with respect to methane flow rate and both of the two were very high more than 90%. Particle size and sinface area of synthesized carbon were strongly dependent on methane flow rate. Hydrogen produced finm thermal plasma can be applied to fuel cell due to its high purity and carbon black can be applied for the synthesis of rubber industry. 

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