Fuel cell critical parameters

A control subsystem is needed not only to control the fuel cell operating parameters (flow rates, temperature, humidity, etc.) but also to communicate with the load and other electrical components of the system. This is particularly critical for applications where a fuel cell operates integrated within an electrical grid. Tbe functioning of the electric and control subsystems depends on application [e.g., stand-alone, grid integrated, combined with another power source, backup power). 

All the examples discussed in the following subsections are meant to be taken simply as an indication of how important it is to determine the correct hydro-phobic content for a specific DL. It is also critical to take into account all the possible operating conditions in which the fuel cell will function because these parameters will have a direct impact on the overall performance of the cell. 

The following subsection will briefly discuss the main methods used to measure in-plane and through-plane electrical conductivity for diffusion layer materials. This parameter is critical for optimal fuel cell performance. 

In PEM fuel cells, uniformity of the current density across the entire active area is critical for optimizing the fuel cell performance. A non-uniform current density in the fuel cell can drastically affect different parameters of the fuel cell, such as reduced reactant and catalyst utilization along the active area, decrease in total efficiency and lifetime, and durability failure modes. Thus, determination of the current density distribution information is vital for designing PEM fuel cells that achieve higher performance and longer life.130 A number of methods for measuring current distribution in PEM fuel cells have been demonstrated the following sections discuss some of these methods in further detail. 

Current density distribution along the active area in a fuel cell is a very critical parameter that needs to be understood completely in order to improve the design of each component of a cell. So far, one limitation prevailing in much of the published work regarding segmented cells is the difficulty in producing a similar level of current... 

Commercial alkaline electrolysis occurs at temperatures up to 150 °C and pressures to 30 bar,96 and super critical electrolysis to 350 °C and 250 bar.102 Although less developed than their fuel cell counterparts which have 100 kW systems in operation and developed from the same oxides,103 zirconia and related solid oxide based electrolytes for high temperature steam electrolysis can operate efficiently at 1000 °C,104,105 and approach the operational parameters necessary for efficient solar... 

Perform a sensitivity analysis on supply and pricing to critical parameters in the model related to fuel cell markets and technology advances... 

A further factor has to be considered, namely, the purity of the hydrogen. As a fuel for internal combustion engines, purity is not a prime consideration and hydrogen from almost any source will be suitable, provided that sulfur is removed. With low-temperature types of fuel cell, however, purity is a critical parameter since the electrocatalysts are subject to poisoning by many contaminants, several of which are found in fossil fuels see Section 6.3, Chapter 6. In this regard, hydrogen produced by the electrolysis of water is much purer and may prove to be the preferred source for this application, despite its higher cost. 

Critical heat flux (CHF), also known in the literature as burnout point, is generally related to a drastic decrease in the heat transfer coefficient and is observed not only under pool boiling but also under convective boiling conditions. The CHF condition is observed when the liquid supply to the heated surface is blocked and the surface is covered by a layer of vapor, such that the heat is transferred from the surface to the liquid by conduction and convection through a vapor layer. When heat is dissipated from a device which the imposed parameter is the heat flux, viz. microprocessors, fuel cells, spacecraft payloads and fuel elements in nuclear reactors, exceeding the CHF may result in an irreversible damage of the thermally controlled device. 

One phase cif. this research invoived critical experirhents with low-enriched uranium fuel rods with slabs of uranium or lead on two sides immersed in water. Rather interesting results were noted in this research. Bierman observed that a lead or a depleted uranium wall (0.2 wt%< U) backed by water was a better reflector than water alone. Moreover, for the case of uranium shielding walls there was an optimum water gap spacing between fuel and wall that resulted in a more efficient reflector combination than if a d< y fitting uranium wall were used. The lead wall did not show this effect but was most reactive when positioned at the fuel cell boundary. As a result of these findings, a calculational study was made to examine the effects oh criticality of changing various parameters. These induded distance fiom wall to fuel, lattice pitch, wall tiiickness, and fuel compoa-tion. The results of this study substantiate the experimental results found by Bierman, and further indicate tiiat the effect of the metal reflector on criticality varies with all of the above-mentioned parameters. 

Table 23.1 lists the typical parameters and critical current densities for the three electrodes. As can be seen, for the polymer electrolyte membrane fuel cell (PEMFC) cathode, fait is large and under a typical current density of about 1 A cm the cathode works in the Tafel regime. However, if the CL proton conductivity were to decrease by a factor of 3, jcnt would be three times lower and the CL would enter the transition region, in which the polarization voltage increases. 

Table 23.1 Typical Parameters and Critical Current Densities for Electrodes in Various Fuel Cells.

In the previous section, the underlying principles of the behavior of the electrolyte were laid out. The main intention of this section is to discuss the consequences following from these considerations on the parameters of fuel-cell models and mediate a critical discussion. For this purpose, the widely used standard approach is applied, where the polarization curve of a HT-PEFC is described by the following... 

The electrodes in the fuel cell should provide solid-liquid-gas three-phase boundary to reduce overpotential. Porous type electrodes are designed and the carbonate electrolytes are dispersed in the electrode by capillary forces. The anode has a higher contact angle and lower wetting with carbonates than the cathode, which allows a smaller pore size at the anode. As mentioned above, the anode has a very high H2 oxidation rate. So, the active surface area and electrolyte filling in the anode are not critical parameters for its performance. Therefore the anode behaves as an electrolyte reservoir in the MCFC. 

The terms E, and Xj are termed the critical parameters, and the Kjj are termed the critical specifications that are required to meet the functional requirements of the particular component under consideration. The term critical is of course an indication that the specification in question is one of the significant few specifications that will be tracked all the way from the voice of the customer down to the factory floor. In a complete PEM fuel cell system, there can be hundreds of specifications that will need to be tracked during the development of the system. Of these, perhaps a few dozen will remain critical even after the product has launched and is in production. 

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