The PEFC

6 Bar 600 °C MCFC with Air Cross Flow and Fuel Upflow 

Compressed Fuel and Steam Inlet to Anode Reform and Oxidation 

1] The temperature entropy chart is tiiat of an existing 20 MW gas turbine. 

2] An ideal machine would have a conpressor outlet temperature equal to die MCFC, namely 600 C. 

3] The mismatch could be overcome by burning a little fuel at the combustion chamber inlet, and supplying die fuel cell widi slightly depleted air, at 600 C. Better to select, for simplicity, a matched gas turbinel. 

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Polymer electrolyte fuel cells can be obtained from several developers. These fuel cells deliver about 5 kW of power and measure 30 by 30 by 70 cm (12 X 12 X 28 in.). For the large produc tion volume anticipated if the automotive industry were to adopt the PEFC, a system cost of less than 100/kW may be reached eventually. 

Can an ORR mechanism at Pt metal in an acid electrolyte with the Reaction (1.2) as the first and rate-limiting step be defended in light of the recently reported apparent Tafel slope and reaction order for ORR in the PEFC cathode ... 

PEFC The PEFC, like the SOFC, has a solid electrolyte. As a result, this cell exhibits excellent resistance to gas crossover. In contrast to the SOFC, the cell operates at a low 80°C. This results in a capability to bring the cell to its operating temperature quickly, but the rejected heat cannot be used for cogeneration or additional power. Test results have shown that the cell can operate at very high current densities compared to the other cells. However, heat and water management issues may limit the operating power density of a practical system. The PEFC tolerance for CO is in the low ppm level. 

The balance of plant contains all the direct stack support systems, reformer, compressors, pumps, and the recuperating heat exchangers. Its cost is low by comparison to the PEFC because of the simplicity of the reformer. However, the cost of the recuperating heat exchangers partially offsets that. 

Polymer electrolyte fuel cells (PEFC) deliver high power density, which offers low weight, cost, and volume. The immobilized electrolyte membrane simplifies sealing in the production process, reduces corrosion, and provides for longer cell and stack life. PEFCs operate at low temperature, allowing for faster startups and immediate response to changes in the demand for power. The PEFC system is seen as the system of choice for vehicular power applications, but is also being developed for smaller scale stationary power. For more detailed technical information, there are excellent overviews of the PEFC (1,2). 

The electrochemical reactions of the PEFC are similar to those of the PAFC hydrogen at the anode provides a proton, freeing an electron in the process that must pass through an external circuit to reach the cathode. The proton, which remains solvated with a certain number of water molecules, diffuses through the membrane to the cathode to react with oxygen and the returning electron (5). Water is subsequently produced at the cathode. 

When used in a PEFC system, the reformate must pass through a preferential CO catalytic oxidizer, even after being shifted in a shift reactor. Typically, the PEFC can tolerate a CO level of only 50 ppm. Work is being performed to increase the CO tolerance level in PEFC. At least two competing reactions can occur in the preferential catalytic oxidizer ... 

Ambient air is compressed in a turbocharger, powered by the expansion of the hot pressurized exhaust gases. Following this first compression stage, the air is intercooled by a fm fan air cooler and fed into a second turbocharger. The high-pressure air is fed directly to the PEFC... 

The fuel cell itself liberates heat that can be utilized for space heating or hot water. The reference article did not list any operating conditions of the fuel cell or of the cycle. The PEFC is assumed to operate at roughly 80°C. Another recent article (49) published by Ballard shows numerous test results that were performed at 3 to 4 atmospheres where fuel utilizations of 75 to 85% have been achieved. Performance levels for an air fed PEFC are now in the range of 180 to 250 mW/cm. Ballard Power Systems has performed field trials of 250 kW systems with select utility partners. Commercial production of stationary power systems is anticipated for the year 2002. Similarly sized transportation cycles also are anticipated for commercial production in the same year. 

J. M. Song, H. Uchida, and M. Watanabe. Effect of wet-proofing treatment of carbon backing layer in gas diffusion electrodes on the PEFC performance. Electrochemistry 73 (2005) 189-193. 

S. Ge and C. Y. Wang. Liquid water formation and transport in the PEFC anode. Journal of the Electrochemical Society 154 (2007) B998-B1005. 

Polymer Electrolyte Fuel Cell The PEFC, also known as the... 

As mentioned, the primary motivation for the PEFC development was the anticipated applicability in transportation. However, the economics of stationary use are more forgiving, and commercialization of the technology will likely begin as grid-independent power supphes. Figure 24-51 shows a 5-kW PEFC system operating on natural gas. 

A particular version of the PEFC is the direct methanol fuel cell (DMFC). As the name implies, an aqueous solution of methanol is used as fuel instead of the hydrogen-rich gas, eliminating the need for reformers and shift reactors. The major challenge for the DMFC is the crossover of methanol from the anode compartment into the cathode compartment through the membrane that poisons the electrodes by CO. Consequently, the cell potentials and hence the system efficiencies are still low. Nevertheless, the DMFC offers the prospect of replacing batteries in consumer electronics and has attracted the interest of this industry. 

The most important electrolyte property is ionic conductivity. For the PEFC system, water and proton transport in the polymer electrolyte occurs concurrently. Springer et al. correlated the proton conductivity (in S/cm) in the polymer membrane with its water content as follows... 

Two other important electrolyte properties for the PEFC system are the water diffusion coefficient and electro-osmotic drag coefficient. These two param-... 

Diffusion medium properties for the PEFC system were most recently reviewed by Mathias et al. The primary purpose of a diffusion medium or gas diffusion layer (GDL) is to provide lateral current collection from the catalyst layer to the current collecting lands as well as uniform gas distribution to the catalyst layer through diffusion. It must also facilitate the transport of water out of the catalyst layer. The latter function is usually fulfilled by adding a coating of hydrophobic polymer such as poly(tet-rafluoroethylene) (PTFE) to the GDL. The hydrophobic polymer allows the excess water in the cathode catalyst layer to be expelled from the cell by gas flow in the channels, thereby alleviating flooding. It is known that the electric conductivity of GDL is... 

In the PEFC system, the mean pore radii of catalyst layers are of the order of 0.1 pm. The Knudsen diffusion coefficients at 80 °C for O2 and H2O through the catalyst layer are thus estimated to be 0.32 and 0.43 cm /s, respectively. These values are comparable to the respective ordinary diffusion coefficients, indicating that Knudsen diffusion further restricts the rates of oxygen and water transport through the cathode catalyst layer in PEFCs and should be taken into account. 

In this chapter, the development of a mesoscopic modeling formalism is presented in order to gain fundamental insight into the structure-wettability influence on the underlying liquid water transport and interfacial dynamics in the PEFC CL and GDL. 

Carbon-fiber based porous materials, namely non-woven carbon paper and woven carbon cloth, shown in Fig. 5, have received wide acceptance as materials of choice for the PEFC GDL owing to high porosity ( 70% or higher) and good electrical/thermal conductivity. Mathias et al.32 provided a comprehensive overview of the GDL structure and functions. In this work, the reconstruction of non-woven carbon paper GDL is presented. 

Before discussing the details of the numerical experiments performed in this study, the primary mechanisms governing the two-phase transport in the PEFC catalyst layer and gas diffusion layer are discussed, which essentially build the foundations behind the specific assumptions and justifications pertaining to the subsequent two-phase simulations. 

The aforementioned numerical experiments, namely quasi-static drainage and steady-state flow simulations, are specifically designed to study the influence of microstructure and wetting characteristics on the underlying two-phase behavior and flooding dynamics in the PEFC CL and GDL. 

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