PAFC stack

Westinghouse Electric Corp. initiated a program to develop air-cooled PAFC stacks, containing cooling plates at six-ceU intervals. Full size 100-kW stacks (468 cells, 0.12-m electrode area) were built, and a module containing four of these stacks was tested. An air-cooled stack operated at 0.480 MPa yielded a cell voltage of 0.7 V at 267 m A /cm (187 mW/cm ). Demonstration of this technology is plarmed for a site in Norway. 

From the standpoint of commercialization of fuel ceU technologies, there are two challenges initial cost and reHable life. The initial selling price of the 200-kW PAFC power plant from IFC was about 3500/kW. A competitive price is projected to be about 1500/kW orless for the utiHty and commercial on-site markets. For transportation appHcations, cost is also a critical issue. The fuel ceU must compete with conventional mass-produced propulsion systems. Furthermore, it is not clear if the manufacturing cost per kilowatt of small fuel ceU systems can be lower than the cost of much larger units. The life of a fuel ceU stack must be five years minimum for utiHty appHcations, and reHable, maintenance-free operation must be achieved over this time period. The projection for the PAFC stack is a five year life, but reHable operation has yet to be demonstrated for this period. 

Several designs for the bipolar plate and ancillary stack components are used by fuel cell developers, and these are described in detail (9, 10, 11, and 12). A typical PAFC stack contains... 

Ghouse M, Abaoud H, Al-Boeiz A, (2000). Operational experience of a 1 kW PAFC stack. Applied Energy 65 303-314... 

The operating temperature of the PAFC stack has gradually evolved from 165 °C to 180 °C and then to the present 190 °C. Several factors favor operating the stack at higher temperatures ... 

The ability to operate the PAFC stack at 190 °C has in large part enabled a reduction in Pt loading to 0.25 mg Pt/cm2 electrode area. In addition, the efficacy for increasing the temperature removes the need for producing more advanced alloy electrocatalysts to overcome the carbon monoxide poisoning problem. That is not the case for PEMFCs operating at lower temperatures below 100 °C. 

Figure 6.6 Model of a water-cooled PAFC stack. (Courtesy of The Royal Society).

Breault RD (2003) PAFC stack materials and stack design. In Vielstich W, Lamm A, Gasteiger HA (eds) Handbook of fuel cells fundamentals, technology, and applications,... 

When natural gas fuels are used in a PAFC or a PEFC, the reformate must be water-gas shifted because of the high CO levels in the reformate gas. A PAFC stack can tolerate about 1 percent CO in the cell before having an adverse effect on cell performance due to catalyst poisoning. 

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. 

The PAFC stack consists of a repeating arrangement of a ribbed bipolar plate, the anode, electrolyte matrix, and cathode. In a similar manner to that described for the PEM cell, the ribbed bipolar plate serves to separate the individual cells and electrically connect them in series, whilst providing the gas supply to the anode and the cathode, respectively, as shown in Figures 1.9 and 1.10. Several designs for the bipolar plate and ancillary stack components are being used by fuel ceU developers, and these aspects are described in detail elsewhere (Appleby and Foulkes, 1993). A typical PAFC stack may contain 50 or more cells connected in series to obtain the practical voltage level required. 

All PAFC stacks are fitted with manifolds that are nsnally attached to the ontside of the stacks external manifolds) We shall see later that an alternative internal manifold arrangement is preferred by some MCFC developers. Inlet and outlet manifolds simply enable fnel gas and oxidant to be fed to each cell of a particnlar stack. Carefnl design of inlet fnel manifold enables the fuel gas to be supplied uniformly to each cell. This is beneficial in minimising temperature variations within the stack thereby ensuring long lifetimes. Often a stack is made of several sub-stacks arranged with the plates horizontal. 

Sulphur in the fuel stream, usually present as H2S, will similarly poison the anode of a PAFC. State-of-the-art PAFC stacks are able to tolerate around 50 ppm of sulphur in the fuel. Sulphur poisoning does not affect the cathode, and poisoned anodes can be reactivated by increasing the temperature or by polarisation at high potentials (i.e. operating cathode potentials). 

Ghouse, M., H. Abaoud, A. Al-Boeiz and S. Al-Zaharani. Fabrication and characterization of the graphite bi-polar plates used in a 0.25 kW PAFC stack. International Journal of Hydrogen Energy 23(8) 721-730, 1998. 

PAFC stack has coolant channels to remove heat generated during cell operation, and these channels are located about every fifth cell. Heat is removed by either liquid (two-phase water or a dielectric fluid) or gas (air) coolants that are routed. Liquid cooling requires complex manifolds and coimections. 

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]. 

There have been many internal configurations of the PAFC stack plates, the most recent involving the use of carbon paper substrate as an electrolyte reservoir and a flow distributor, as discussed. The flow field design in a PAFC is similar to a PEFC, but there is no special provision for flooding, since this is not an issue with the medium-temperature PAFC. Typically, the flow fields are aligned in plane and perpendicular to one another (cross-flow), and external manifolding is used, as shown in Figures 7.21 and 7.22. By... 

Figure 7.21 Illustration of PAFC stack flow fleld assembly with stored electrolyte. (Reproduced with permission from Ref. [26].)...

PAFC stack external manifolding concept. (Reproduced with permission from Ref. 

A basic PAFC stack configuration is given in Fig. 13. The anode-acid holder matrix-cathode sandwich is held between two bipolar plates and this array is repeated to the end. At each end there is an one-side grooved plate—one with anode groove and the other end with cathode groove. Each of these plates have through holes, when assembled act as reactant gas flow headers (inlet and outlet). The inlet header of one gas has an opening in one side of each bipolar plate, and similarly the outlet header of the same gas collects excess unreacted reactants/products from the same side of the bipolar plates. The inlet and outlet headers of the other gas similarly feed and collect gas from the other side of the bipolar plates. 

With all these efforts, the recent PAFC stacks perform anywhere between 200 and 300 mA/cm current density at a cell potential of 500 to 700 mV during the initial phase of its life, at atmospheric pressure. Though pressurized cells are reported and can perform at a higher current density, they pose other problems (Bloomfield and Cohen, 2000). For example, the bulky components and added compressor power may overshadow the higher power density of the stack. The system that uses pressure canopy faces additional gas leakage problems and safety related issues. Hence, with present technology, atmospheric PAFCs are more realistic than their pressurized counterparts. 

The waste heat from the PAFC stack can be utilized for other purposes. Miki and Shimizu, 1998, have reported a dynamic simulation study of water/steam based 50 kW PAFC cooling. The study uses lumped parameter models and suggested fuzzy logic type controls for better performance. The study considers an upstream reformer and the steam generated from the fuel cell waste heat is used for feeding the shift reactor. 

The waste heat management of PAFC is greatly enhanced if there is a methanol reformer based hydrogen provider in the upstream. The vaporization heat load of the methanol and the reforming process steam demand can be taken care of by the fuel cell waste heat as the temperature of the PAFC stack is sufficiently high to allow such waste heat recovery. 

PAFC needs to be maintained at temperatures above 60°C to prevent acid freezing leading to frozen acid crystals from getting inside the hydrophobic pores. Under operation, these crystals melt and remain inside the hydrophobic pore. Such blockages, can offer very high gas diffusion resistance. Major damage occurs to the PAFC stacks during the shutdown and startup only, and hence it should happen only a minimum number of times. 

NMRL has mastered the technology of PAFC stacks, ranging from 100 W to 10 kW. These compact systems have all necessary subsystems, namely, built-in acid management systems, catalytic pre-heaters with controlled gas diversion, sandwich type humidifiers (for certain applications) based on waste heat and tail gas catalytic burners. 

Fig. 15 The 1 kW PAFC stack (Courtesy Naval Materials Research Laboratory, India).

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