PAFC Fuel Cell

Source U.S. Department of Energy, Energy Efficiency and Renewable Energy 

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Product FT Gasoline and Diesel Methanol Hydrogen Boiler Turbine PAFC Fuel Cell MCFC SOFC PEFC... 

Figure 1-6 Relative Emissions of PAFC Fuel Cell Power Plants Compared to Stringent Los Angeles Basin Requirements...

The current technological status of small (0.5-5 kWe) PEM, SOFC, AFC and PAFC fuel cell stationary units, was recently reviewed by Staffed et al. (2007) (Table 3.12). The system efficiency refers to electrical losses of the fuel-cell system (fans, pumps, control) and the current conditioning unit (transformer, inverter), while total efficiency refers to both electrical power and heat cogeneration. Cost estimations refer to mass-produced fuel-cell systems according to today s state-of-the-art manufacture technologies and materials, while current retail prices of demonstration fuel-cell systems are in the range of 10,000-100,000 / kWe (Staffed et al., 2007). 

Various combinations of applications, including different choices of photovoltaic panels and electrolysers were tested. The cumulative operating times logged for the various plant subsystems differed considerably according to the test programs run, ranging from 6000 h for the alkaline low-pressure electrolyser, to 2000 h for the membrane electrolyser, 5200 h for the catalytic heater, 3900 h for the PAFC fuel cell plant and 900 h for the LH2 filling station. 

Figure 9. In situ Pt L3 XANES data taken in a PAFC and PEMFC. The PEMFC (grey) are taken at 0.66 V (dotted) and 0.72 V (solid) vs. RHE. The PAFC fuel cell data (black) were taken at 0.6 (dotted), 0.75 (long dotted) and 0.85 V (dot-dashed) vs. RHE. Electrochemical cell data at 0.74 V vs RHE in H2SO4 is also shown (thin, continuous line). The regions for OH, OOH and anion adsorption are indicated.

PAFC fuel cells are a namral At for combined heat and power applications. The use of CHP fuel cell systems in commercial buildings such as supermarkets, office towers, schools, data centers, industrial buildings, etc., improves overall efficiency... 

Participant PAFC Fuel cell technology PEMFC MCFC SOFC... 

Polymer Electrolyte Fuel Cell. The electrolyte in a PEFC is an ion-exchange (qv) membrane, a fluorinated sulfonic acid polymer, which is a proton conductor (see Membrane technology). The only Hquid present in this fuel cell is the product water thus corrosion problems are minimal. Water management in the membrane is critical for efficient performance. The fuel cell must operate under conditions where the by-product water does not evaporate faster than it is produced because the membrane must be hydrated to maintain acceptable proton conductivity. Because of the limitation on the operating temperature, usually less than 120°C, H2-rich gas having Htde or no (

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

In a typical PAFC system, methane passes through a reformer with steam from the coolant loop of the water-cooled fuel cell. Heat for the reforming reaction is generated by combusting the depleted fuel. The reformed natural gas contains typically 60 percent H9, 20 percent CO, and 20 percent H9O. Because the platinum catalyst in the PAFC can tolerate only about 0.5 percent CO, this fuel mixture is passed through a water gas shift reactor before being fed to the fuel cell. 

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. 

The PAFC is, however, suitable for stationary power generation, but faces several direct fuel cell competitors. One is the molten carbonate fuel cell (MCFC), which operates at "650°C and uses an electrolyte made from molten potassium and lithium carbonate salts. Fligh-teinperature operation is ideal for stationary applications because the waste heat can enable co-generation it also allows fossil fuels to be reformed directly within the cells, and this reduces system size and complexity. Systems providing up to 2 MW have been demonstrated. 

The most promising fuel cell for transportation purposes was initially developed in the 1960s and is called the proton-exchange membrane fuel cell (PEMFC). Compared with the PAFC, it has much greater power density state-of-the-art PEMFC stacks can produce in excess of 1 kWA. It is also potentially less expensive and, because it uses a thin solid polymer electrolyte sheet, it has relatively few sealing and corrosion issues and no problems associated tvith electrolyte dilution by the product water. 

An attempt has been made in Table 1 to present the status of fuel cell technologies. For terrestrial applications, the PAFC power plant is the most advanced, and a 200-kW system manufactured by ONSI, a division of United Technologies, Inc., has reached commercialization. Its main applications are focused on on-site integrated energy systems that could... 

PAFC, phosphoric acid fuei ceii MCFC, moiten carbonate fuei ceii SOFC, soiid oxide fuei ceii PEMFC, proton exchange membrane fuei ceii DMFC, direct methanoi fuei ceii AFC, alkaiine fuel cell. 

Medium-temperature phosphoric acid fuel cells (PAFCs). The electrolyte is 85 to 95% phosphoric acid the working temperatures are 180 to 200°C. Such systems were used to build numerous autonomous power plants with an output of up to about 250 kW, and even some with an output of up to 4 MW, in the United States, Japan, and other countries. 

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