Stack volume

Velocity and Volumetric Flow Rate The U.S. EPA has published Method 2 as a reference method for determining stack-gas velocity and volumetric flow rate. At several designated sampling points, which represent equal portions of the stack volume (areas in the stack), the velocity and temperature are measured with instrumentation shown in Fig. 25-27. 

As will be reported, as result of continuous efforts to optimize cell components and cell structure in developing DMFCs for portable power applications, we were able to demonstrate 30-cell DMFC stacks operated at 60 °C and fed with ambient air at 2-3 times faradaic stoichiometry, generating a power density of 320 W (active stack volume) during at least the initial week of testing in our laboratory. 

Fig. 56 System volume for a 1-W DMFC power source versus number of watt-hours stored (= hours of use time at 1-W to the load per single fuel cartridge) compared to the volume of a prismatic Li-ion battery of the same number of watt-hours. The three cases shown are for three different volumes of the nonfuel-containing components of the DMFC system, that is, stack + auxiliaries. Assumptions for the DMFC system are stack power density of 50-100 W L-1 (0.05-0.1 W cm-3), auxiliaries volume = stack volume, and system conversion efficiency = 30%.

Resistant to corrosion in an environment of acidic electrolyte, hydrogen, oxygen, heat, and humidity Reasonably high stiffness, E> 25 MPa As little cost as possible Thin dimensions for maximum stack volume Light weight for minimum stack mass Effective production process with a short cycle time... 

Specific details of the design are listed in Table 4. The most electrically efficient (highest cell potential) will be that of the highest cell area (lowest current density). For a 200 V stack voltage, a current of 500 A is needed to produce 100 kWe. At the maximum cell area of 900 cm, the current density is 0.556 A/cm, which then locates point A on the polarization curve of Fig. 3. The potential at this point is 0.75 V, and therefore 267 cells are required to achieve 200 V. With an assumed cell thickness of 0.3 cm, the stack volume is 72.03 1, thus giving a power density of 1.39 kWe/1 and a fuel cell plant cost of 7,203 according to Eq. 23 and TCI of 28,813 by Eqs. 22 and 24. 

Basically, there are three approaches to stack modelling. The first is fully 3D modelling, taking into account the hydrodynamics of flows in channels, heat transport and potential distribution over the stack volume. This approach leads to extremely time-consuming CFD codes to our knowledge there is only one model of that type (Liu et al., 2006). 

Below we will show that fully 3D Laplace equations for voltage and temperature distribution over the stack volume can be split into a number... 

The efficiency and lifetime of the DMFC stack depend on a large number of design and operational parameters stack temperature T is among the most important. The rates of kinetic and transport processes in DMFC rise exponentially with T. Improper thermal management increases current nonuniformity over the stack volume, which may dramatically lower stack performance and lifetime. 

Figure 5.22 shows the local current density and the temperature disturbance induced by the spot AT = Tgpot — Tbase over the stack volume Tspot is the temperature obtained with the spot). The physical resistive spot in cell 8 is clearly seen on the map of local current in the spot region, j drops to zero (Figure 5.22, left column). 

This propagation is further facilitated by thermal insulation of the stack side surface and by high thermal conductivity of MEA and BP. The adiabatic side surface of the stack reduces the effect of cold spot smoothing by the conductance mechanism of heat transport. Literally speaking, the adiabatic side surface traps the cold spot in a stack volume. At the same time, the large thermal conductivity of the stack components along z facilitates spot penetration. 

A combination of synergistic improvements in the catalyst, support, gas diffusion layers, membrane, and essentially the entire porous electrode structure in conjunction with bipolar plates/flow fields is expected to improve the mass-transport of reactant gases, protons, and water management. Thus, an increase in the peak current density (A/cm ) and peak power density (W/cm ) will result this in turn will lower the stack volume, the amount of catalyst, and membrane material used and raise the kW/L, kW/kg, and lower the /kW stack metrics. It should be noted that the rated or peak power for automotive stacks is based in part on maintaining an electrical efficiency of >50% this dictates that the cell voltage has to be maintained above 0.60 V. At this time, volumetric power densities of practical stacks in fuel ceU vehicles have been reported to be as high as 2 kW/L... 

High and efficient insulation that increases the stack volume and weight... 

The real catalyst is a porous substance when it is heaped up in a vessel (reactor). Its volume includes solid skeleton volume (Vsk), inner pore volume (Vpo) and the interstitial volume (V p) between particles. Therefore, the total stacking volume of catalyst (Uc) is... 

The voidage of the catalyst bed is closely related to pressure drop, which is an important engineering parameter. Voidage of the catalyst bed is the ratio of the void volume (Kp) between particles to total stacking volume (14) of per unit mass catalyst, represented by (e)... 

The bipolar plates used to realize the second principle, which contain flow fields for reactant delivery, are very expensive, often not sufficiently corrosion stable, and lead to a considerable increase in the stack volume and weight. 

In spite of the progress, the ORR in the cathode still incurs about 40% of all irreversible energy losses in the cell, as well as a proportional fraction of voltage losses. Moreover, at the current mass loadings required for high cell performance, Pt is responsible for 30-70% of the total cost of a fuel cell stack, although it only amounts to about 0.1 % of the stack volume. The foremost challenge in PEFC research remains to maximize performance with a minimal amount of Pt. 

The bipolar plate must be slim, for minimum stack volume. 

Figure 2 PCF measured from the maximally air stacked volume (MIC) in supine position in a neuromuscular patient. Abbreviations. PCF, peak cough flow MIC, maximum insufflation capacity.

For a fuel ceU stack, mass- and volume-specific power densities are more useful parameters. The mass power density is the ratio of power to stack weight, measured in kilowatts per kilogram. Clearly, light-weight stack materials are required to increase the mass power density of a stack. Volume power density is the ratio of power to stack volume, measured in kilowatts per liter or kilowatts per cubic meter. Thus, for increasing the volume power density, one requires to reduce the stack size and simplify the stack system. 

Figure 634 Typical PEFC channel/land design features. The draft angle can be used to tailor the channel cross-sectional area without affecting the depth (stack volume) or land contact area.

The manifold must be compact to minimize stack volume yet have minimal pressure... 

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