Fuel-cell systems

Besides chemical catalytic reduction of carbon dioxide with hydrogen, which is already possible in the laboratory, we are exploring a new approach to recycling carbon dioxide into methyl alcohol or related oxygenates via aqueous eleetrocatalytic reduction using what can be called a regenerative fuel cell system. The direct methanol fuel cell... 

A regenerative fuel cell system can also be a single electrochemical cell in which both the oxidation of fuels (i.e., production of electric power) and reduction of CO2 (to obtain fuels) can be carried out by simply reversing the mode of operation. 

Fuel cell is an ambiguous term because, although the conversion occurs inside a fuel cell, these cells need to be stacked together, in a fuel cell stack, to produce useful output. In addition, various ancillai y devices are required to operate the stack properly, and these components make up the rest of the fuel cell system. In this article, fuel cell will be taken to mean fuel cell system (i.e., a complete standalone device that generates net power). 

A complete fuel cell system, even when operating on pure hydrogen, is quite complex because, like most engines, a fuel cell stack cannot produce power without functioning air, fuel, thermal, and electrical systems. Figure 3 illustrates the major elements of a complete system. It is important to understand that the sub-systems are not only critical from an operational standpoint, but also have a major effect on system economics since they account for the majority of the fuel cell system cost. 

Heat rejection is only one aspect of thermal management. Thermal integration is vital for optimizing fuel cell system efficiency, cost, volume and weight. Other critical tasks, depending on the fuel cell, are water recovery (from fuel cell stack to fuel processor) and freeze-thaw management. 

Even in a simple hydrogen fuel cell system, capital cost reduction requires improvements in many diverse areas, such as catalyst loadings, air pressuriza-... 

A fuel cell system also needs ancillaries to support the stack, just as an IC engine has many of the same type of ancillary subsystems. Major subsystems are needed for providing adequate humidification and cooling, and for supplying fuel and oxidant (air) with the correct purity and appropriate c uantity. 

These subsystems profoundly affect the fuel cell system performance. As an example, the inherently slow air (oxygen) electrode reaction must be acceler-... 

Ford Motor Company. (1997). Direct Ilydrogcn-Fuclcd Proton Exchange Membrane Fuel Cell System for Transportation Applications Hydrogen Vehicle... 

Ultradeep desulfurization of fuel oils is used for producing not only clean fuels but also sulfur-free hydrogen used in fuel-cell systems, in which the hydrogen can be produced potentially through the reforming of fuel oils. Fuel-cell systems must be run with little-to-no sulfur content, because sulfur can irreversibly poison the precious metal catalysts and electrodes used [12]. 

Binary liquid metal systems were used in liquid-metal magnetohydrodynamic generators and liquid-metal fuel cell systems for which boiling heat transfer characteristics were required. Mori et al. (1970) studied a binary liquid metal of mercury and the eutectic alloy of bismuth and lead flowing through a vertical, alloy steel tube of 2.54-cm (1-in) O.D., which was heated by radiation in an electric furnace. In their experiments, both axial and radial temperature distributions were measured, and the liquid temperature continued to increase when boiling occurred. A radial temperature gradient also existed even away from the thin layer next to the... 

A fuel cell system for automobile application is shown in Figure 1.5 [41]. At the rated power, the PEMFC stack operates at 2.5 atm. and 80°C to yield an overall system efficiency of 50% (based on lower heating value of hydrogen). Compressed hydrogen and air are humidified to 90% relative humidity at the stack temperature using process water and heat from the stack coolant. A lower system pressure is at part load and is determined by the operating map of the compressor-expander module. Process water is recovered from spent air in an inertial separator just downstream of the stack in a condenser and a demister at the turbine exhaust. 

Wee, J.H., Applications of proton exchange membrane fuel cell systems. Renewable Sustainable Energy Rev., 11,1720-1738,2007. 

Ahluwalia, R.K. and X. Wang, Direct hydrogen fuel cell systems for hybrid vehicles. /. Power Sources, 139(1-2), 152-164,2005. 

Lehrhofer, J. et al., Integrated analysis of the "sponge iron reactor" and fuel cell system, Proc. 1996 Fuel Cell Seminar, Orlando, FL, 710,1996. 

Cavallaro, S. Freni, S., Syngas and electricity production by an integrated autothermal reforming molten carbonate fuel cell system. Journal of Power Sources 1998, 76,190-196. 

The NFPA Research Foundation, in a collaborative project with the DOE and the telecommunications industry, completed a draft report on diffusion of hydrogen leaks from cabinet-enclosed hydrogen storage tanks. The purpose of this research is to establish a better scientific foundation for setback requirements for hydrogen fuel cell systems used in telecommunication applications. 

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