Fuel cell flow-through

The coolant of fuel cells flows through a fuel cell stack and there is a danger of a short circuit of cells or a leakage of current if electrical conductivity of the coolant is high. Thus, ion exchanged water with an electrical conductivity of 1 mS or less, or a low-conductive antifreeze, is used as coolant. 

A rather dangerous situation arises when individual fuel cells of a multicell fuel-cell stack deviate. Such nonuniformity is most often because of problems in reactant supply. Two systems of gas supply exist parallel and series. In parallel supply, the gas reaches each fuel cell through a narrow channel coming from a common manifold. The pressure in these channels is the same for all elements where they leave the manifold, but on account of differences in gas flow resistance, the amounts of gas (or the pressure) reaching each fuel cell may differ. In series supply, gas is fed to a first individual cell, flows through it, and continues to the next cell, and so on. In each fuel cell in series the amount of gas needed for the... 

We discussed fuel cells in Section 16.1. Electric vehicles powered by fuel cells may one day replace infernal combustion vehicles. Fuel cells are like batteries, but the reactants are constantly replenished. Normal batteries lose their voltage with use because the reactants are depleted as electrical current is drawn from the battery. In a fuel cell, the reactants—the fuel—consfantly flow through the battery, generating electrical current as they undergo a redox reaction. 

As a PEM fuel cell is a complicated system, the effect of RH is also related to other operating conditions, such as temperature, pressure, flow field design. This chapter will address the effect of RH on fuel cell performance through theoretical analysis and typical experimental examples. 

In most cases the flow through the fuel cell flow field is laminar, which means that the pressure drop is linearly proportional to velocity, that is, to flow rate. However, in a fuel cell channel there are some deviations from the uniform pipe flow ... 

This proton exchange membrane is used in both hydrogen and methanol fuel cells, in which a catalyst at the anode produces hydrogen from the methanol. Because the membrane allows the protons, but not the electrons, to travel through it, the protons flow through the porous membrane to the cathode, where they combine with oxygen to form water, while the electrons flow through an external circuit. 

The stability of ceramic materials at high temperatures has made them useful as furnace liners and has led to interest in ceramic automobile engines, which could endure overheating. Currently, a typical automobile contains about 35 kg of ceramic materials such as spark plugs, pressure and vibration sensors, brake linings, catalytic converters, and thermal and electrical insulation. Some fuel cells make use of a porous solid electrolyte such as zirconia, Zr02, that contains a small amount of calcium oxide. It is an electronic insulator, and so electrons do not flow through it, but oxide ions do. 

Figure 8.31. Principle of a Polymer Electrolyte Membrane (PEM) fuel cell. A Nation membrane sandwiched between electrodes separates hydrogen and oxygen. Hydrogen is oxidized into protons and electrons at the anode on the left. Electrons flow through the outer circuit, while protons diffuse through the...

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... 

Initially, both electrodes are at equilibrium. Since the anode has accumulated electrons and the cathode has depleted electrons, electrons begin to flow from electrode from the anode to the cathode. The thermodynamic driving force for the electron flow is the electrode potential difference, which for the fuel cell reaction is 1.23 Y at standard conditions. In addition to electron flow, H + ions produced at the anode diffuse through the bulk solution and react at the cathode. The reaction is able to continue as long as H2 is fed at the anode and 02 at the cathode. Hence, the cell is not at equilibrium. The shift in electrode potential from equilibrium is called the overpotential (>/). 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

The flow-through cathode is the result of a tailored-to-the-process evolution of the GDE structure, which is available also in two additional configurations double-sided (originally developed for fuel cell servicing) and single-sided (see Fig. 9.7). The double-sided type is particularly suited for the electrochemical process where the product should not be released on to the back surface of the cathode, as in the case of oxygen-depolarised chlor-alkali electrolysis, discussed in Section 9.3. 

The oxygen from the dual gas regulator module flows directly through two ports into a closed-end manifold in the fuel cell stack to provide optimum oxygen distribution in the cells. All the oxygen that flows into the stack is consumed, except during purge operations. 

In a proton exchange membrane (PEM) fuel cell, protons travel through a film 18 microns thick which is the proton exchange membrane. Electrons are blocked by the film and take another path which provides the electric current flow. Over time and usage tiny holes can form on the film which reduces fuel cell performance. If the film is made thicker and stronger, then performance suffers. 

Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. The basic physical structure or building block of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in Figure 1-1. 

Other important parts of the cell are 1) the structure for distributing the reactant gases across the electrode surface and which serves as mechanical support, shown as ribs in Figure 1-4, 2) electrolyte reservoirs for liquid electrolyte cells to replenish electrolyte lost over life, and 3) current collectors (not shown) that provide a path for the current between the electrodes and the separator of flat plate cells. Other arrangements of gas flow and current flow are used in fuel cell stack designs, and are mentioned in Sections 3 through 8 for the various type cells. 

Ohmic Polarization Ohmic losses occur because of resistance to the flow of ions in the electrolyte and resistance to flow of electrons through the electrode materials. The dominant ohmic losses, through the electrolyte, are reduced by decreasing the electrode separation and enhancing the ionic conductivity of the electrolyte. Because both the electrolyte and fuel cell electrodes obey Ohm s law, the ohmic losses can be expressed by the equation... 

Combined Brayton-Rankine Cycle The combined Brayton-Rankine cycle. Figure 9-14, again shows the gas turbine compressor for the air flow to the cell. This flow passes through a heat exchanger in direct contact with the cell it removes the heat produced in cell operation and maintains cell operation at constant temperature. The air and fuel streams then pass into the cathode and anode compartments of the fuel cell. The separate streams leaving the cell enter the combustor and then the gas turbine. The turbine exhaust flows to the heat recovery steam generator and then to the stack. The steam produced drives the steam turbine. It is then condensed and pumped back to the steam generator. 

The effectiveness of the regenerative Brayton cycle performance will depend on the efficiency of the fuel cell, compressor, and turbine units the pressure loss of gases flowing through the system the approach temperatures reached in the recuperative exchanger and, most importantly, the cost of the overall system. 

NETL looked at improving upon conventional MCFC system designs, in which multiple stacks are typically arranged in parallel with regard to the flow of reactant streams. As illustrated in Figure 9-19a, the initial oxidant and fuel feeds are divided into equal streams which flow in parallel through the fuel cell stacks. 

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