PAFC system development

PAFC systems achieve about 37 to 42% electrical efficiency (based on the LHV of natural gas). This is at the low end of the efficiency goal for fuel cell power plants. PAFCs use high cost precious metal catalysts such as platinum. The fuel has to be reformed external to the cell, and CO has to be shifted by a water gas reaction to below 3 to 5 vol% at the inlet to the fuel cell anode or it will affect the catalyst. These limitations have prompted development of the alternate, higher temperature cells, MCFC, and SOFC. 

What are the lessons learned from the failure of PAFC to become a commercial success and how do these lessons apply to other stationary fuel cell systems in development and demonstration Was the cause of failure only the high cost relative to the other DG systems The PAFC systems appeared to perform well. The federal government had spent more than 411 million on PAFC. Should it have continued... 

The section on intermediate temperature fuel cells has just one entry on each fuel cell type. With decreasing operation temperature, the Molten Carbonate Fuel Cell technology is critically discussed (Molten Carbonate Fuel Cells) before two related systems relying on the unique protrui conducting properties of phosphoric acid are described. While the well-established phosphoric acid fuel cell (PAFC) is developed for stationary applications (Phosphoric Acid Fuel Cells for Stationary Applications), polybenzimidazole (used as a matrix for phosphoric acid) fuel cells even have some potential for mobile and small applications (Polybenzimidazole Fuel Cell Technology). 

PAFCs are mostly developed for stationary applications. Both in the U.S. and Japan, hundreds of PAFC systems were produced, sold, and used in field tests and demonstrations. It is still one of the few fuel cell systems that are available for purchase. Development of PAFC had slowed... 

Sugano et al. [46] reported the analysis of the dynamic behavior of a PAFC stack cooling systems. Miki and Shimizu [47] reported the results of the dynamic characteristics of a fuzzy control based stack cooling system. An analytical, exergetic, and thermoeconomic analysis of a 200 kWel PAFC power plant was presented by Kwak et al. [48]. In [49], a novel optimization tool was developed that realistically described and optimized the performance of a PAFC system. Zhang et al. [50] presented an analytical model to optimize several parameters using a thermodynamic-electrochemical analysis. In [51], a dynamic model was developed to simulate a PAFC system and associated components. 

Other recent significant developments in PAFC technology are improvements in gas diffusion electrode construction and tests on materials that offer better carbon corrosion protection. Of course, many improvements can be made in the system design, with better BOP components such as the reformer, shift reactors, heat exchangers, and burners. Much of this is covered in the chapters that follow. For example. Figure 8.4 shows a schematic arrangement of the essential components in a PAFC system. The actual fuel cell stack is a small part of the total system. 

Fuel Cells (UTC Fuel Cells). Worldwide, Fuji Electric Company and Mitsubishi Electric Company in Japan developed PAFC systems for residential and stationary power applications. The PAFC demonstration units have been developed for a wide variety of backup power and even transportation applications. In the 1990s Georgetown University helped operate a PAFC bus fueled by reformed methanol. The original stack was produced with a Fuji Electric fuel cell stack, and a second system was installed with an IFC 100-kWe PAFC stack, shown in Figure 7.15. This bus was operated successfully for a number of years and then sent to the University of Califomia-Davis. However, large relative system size and rapid development of the PEFC have since limited development of the PAFC to stationary power applications [37]. 

Both systems have an acid-based electtolyte (PEFC is sulfuric acid based), although the PAFC is a liquid electrolyte solution system and the PEFC electrolyte exists as a partially bound solution in a solid polymer matrix. Between the PEFC and PAFC, the anode HOR and cathode ORR are the same. A schematic of the materials and electrochemical reactions in the PAFC system is shown in Figure 7.20. Both systems use a noble metal catalyst or alloy with noble metals on the electrodes, and both suffer from poor ORR kinetics relative to alkaline-based systems. Ironically, since operation of the PEFC at 80°C results in catalyst poisoning from CO as well as water management issues that the PAFC avoids, developers seek higher temperature PEFC membranes that can operate at 120-200°C like the PAFC but maintain the high power density advantage of the PEFC. 

Demonstrated High Reliability and Developed System The PAFC system is the first fuel cell to reach the consumer production stage and has millions of operation hours accumulated with hundreds of 200-kWe units. This system has demonstrated high service reliability of over 95% as well as combined thermal efficiency of over 80% in the cogeneration mode. 

The PAFC was the next system to be seriously developed following the AFC and is the first mass-produced fuel cell system to reach the consumer market in premium power applications. Through several generations of design, stationary 200-kWe PAFC systems showed reliable (>95%) service and high combined efficiency (>80%). Ultimately, the total system cost has limited development of this technology. 

Because of this extreme sensitivity, attention shifted to an acidic system, the phosphoric acid fuel cell (PAFC), for other applications. Although it is tolerant to CO, the need for liquid water to be present to facilitate proton migration adds complexity to the system. It is now a relatively mature technology, having been developed extensively for stationary power usage, and 200 kW units (designed for co-generation) are currently for sale and have demonstrated 40,000 hours of operation. An 11 MW model has also been tested. 

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

A typical system that is commercially available in the United States is the 200 kilowatt (kW) PAFC unit produced by UTC Fuel Cells. This is the type of unit used to provide electricity and heat to the U.S. Postal Service s Anchorage Mail Handling Facility. In 2000, the Chugach Electric Association installed a 1 Megawatt (MW) fuel cell system at the U.S. Postal Service s Anchorage Mail Handling Facility. The system consists of five natural gas powered 200-kW PC25 fuel cells developed by UTC Fuel Cells. 

Several types of fuel cell are currently under development, using different electrolyte systems phosphoric acid (PAFC), alkaline, molten carbonate (MCFC), regenerative, zinc-air, protonic ceramic, (PCFC), proton exchange membrane (PEM), direct methanol (DMFC), and solid oxide (SOFC). The last four contain solid electrolytes. 

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