Small fuel cells system requirements

A number of approaches can be used to purify reformate fuel (including pressure swing adsorption, membrane separation, methanation, and selective oxidation). Selective or preferential oxidation (PROX) is usually the preferred method for CO removal in the relatively small fuel cell systems because of the parasitic system loads and energy required by other methods. In selective oxidation, the reformed fuel is mixed with air or oxygen either before the... 

The complexity of the overall fuel processing system for carbonaceous fuels indicates the challenge associated with adapting conventional fuels for use in small fuel cell systems. Considerable effort is required in designing and optimizing the system to achieve the requisite miniaturization and low cost in these applications. 

The requirements of the small fuel cell system translate into a fuel cell stack that is compact, externally air-cooled and utilizes unconditioned ambient air as a reactant. For hydrogen-fueled systems (which are the majority to date) the anode compartments of the stack are virtually dead-ended. The need for compactness calls for internal reactant manifolding and bolting as well as thin end-plates. A representative stack for a small fuel cell system is shown in Fig. 43.3. The cell components, of course, should also be as thin as possible. 

However, most fuel cell systems can tolerate methane concentrations up to at least 1% in the reformate, no special purification reactions are required. In contrast, hence, removing small residual amounts of carbon monoxide from pre-purifled reformate applying the methanation reaction may be considered as an alternative to the preferential oxidation of carbon monoxide, provided that the CO concentration is low enough to have no significant impact on the hydrogen yield. However, no applications of methanation for CO clean-up in micro structured devices appear to have been reported, hence the issue is not discussed in depth. Finally, during hydrocarbon reforming all hydrocarbon species (saturated and unsaturated) smaller than the feed molecule may be formed. 

Nowadays, the most common small-scale application of hydrogen is the use in residential or mobile fuel cell systems. Special requirements of this application are compact design, integrated CO-removal, high energetic efficiency, quick start-up and fast transient behavior. The proposed solutions comprise unit-operation-based concepts as well as multifunctional, micro-structured reactors. 

Small systems of a few kWe are not likely to operate under high pressure. Currently available high-temperature fuel-cell systems reach electrical capacities of around 250 kWe (for these systems, the integration of a gas turbine can raise electric efficiency up to 60%). Table 3.8 indicates the current investment costs for stationary high-temperature fuel cells (IEA, 2005 Blesl et al., 2004 Alanne el al., 2006). Today, the manufacturing cost of PEM fuel cells is reported to vary depending on scale, power electronics requirements, and reformer requirements, with retail prices varying between 3000/kW and 6000/kW (Cotrell et al., 2003 Fuel Cells, 2000). 

The parasitic load required to run the auxiliary BoP components rednces the overall efficiency of the system. This is clearly evident when the power reqnired to run auxiliary components such as air compressors, coolant pumps, hydrogen circulation pumps, etc., is included in the efficiency calculations. Additionally, the weight and size of fuel cell systems will need to be reduced in order for fnel cells to become compatible with onboard transportation applications and small-scale portable applications. 

The core of a fuel cell power system is the electrodes, the electrolyte, and the bipolar plate that we have already considered. However, other parts frequently make up a large proportion of the engineering of the fuel cell system. These extras are sometimes called the balance of plant (BOP). In the higher-temperature fuel cells used in CHP systems, the fuel cell stack often appears to be quite a small and insignificant part of the whole system, as is shown in Figure 1.19. The extra components required depend greatly on the... 

Small fuel cells that provide power at ratings below 1000 watts, are being introduced into and groomed for application areas such as portable power sources, mobile power sources, remote or unattended power sources, and propulsion power for small, off-the-road vehicles. These existing and candidate applications may include power systems that are fuel-cell-only, fuel-cell/battery hybrids, and fuel-cell/solar/battery hybrids, depending on the nature of the system s requirements. Representative examples of such applications are illustrated in Table 43.1. 

The application of small fuel cells could clearly be greatly enhanced if compact systems using conventional fuels are implemented. In most cases, this requires a fuel processor to convert the fuel into a hydrogen-rich gas that would be delivered to the fuel cell. Much of the challenge in such an approach relates to attaining a sufficiently compact and low-cost fuel processor. In the case of methanol in particular, there is also the potential for systems that use direct fuel feed to the fuel cell anodes. 

The selection of a preferred fuel processor type depends heavily on the application requirements. For example, a conventional stationary fuel cell system operating continuously on natural gas might be best suited to the SR processor to minimize fuel cost, while a small, mobile system requiring rapid start-up might be best served via a POX, or ATR, system. 

---