Stacking in Series

If all individual cells are identical, the stack voltage can be expressed as  

The overall charge Qs(stadc) density accumulated over the stack can be calculated as  

Equations (2.86) and (2.87) indicate that the energy and power densities of a stack containing n identical single cells are the sums of all individual cells energy and power densities. 

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Each cell generates a maximum potential of just over one volt. The cells are stacked in series to achieve higher voltages as required. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

The process flow stream through a commercial demineralizer, incorporating two stacks in series demineralized water, is shown in Figure 3. Several of the refinements required for continuous-flow operational systems are shown on this diagram, representing a two-stage demineralizer. 

Stack columns are a modular process system that allows several column units to be stacked in series. This column system is well suited for packing soft compressible gels in short segments that can then be hooked in series. The... 

Reverse osmosis/electrodeionization (RO/EDI) plants are available in modular form to suit any desired input-output water quality and flow rate. A RO/EDI system should be capable of producing high-purity water of perhaps 5 to 20 xS/cm conductivity (0.2-0.05 MO/cm resistance). By providing a second EDI stack in series, it is possible to achieve even higher quality of up to 0.055 xS/cm conductivity (18.2 Mfl/cm resistance). 

Hydrogen production from wind energy has not been implemented in large-scale WFs yet. The main reason for this, apart from the high cost, is that the present commercially available electrolyzers are designed to operate at lower capacities. An increase in the size of an electrolyzer is achieved by connecting electrolysis stacks in series. 

In an improved design, called an MCFC network, reactant streams are ducted such that they are fed and recycled among multiple MCFC stacks in series. Figure 9-19b illustrates how the reactant streams in a fuel cell network flow in series from stack to stack. By networking fuel cell stacks, increased efficiency, improved thermal balance, and higher total reactant utilizations can be achieved. Networking also allows reactant streams to be conditioned at different stages of utilization. Between stacks, heat can be removed, streams can be mixed, and additional streams can be injected. 

Stacks in series approach reversibility. MCFC stack networks produce more power than conventional configurations because they more closely approximate a reversible process. To illustrate this fact, consider Figure 9-20, which compares the maximum power that could be generated by three different MCFC systems having identical feed stream compositions. ... 

In system C, many stacks are connected in series. Very small currents are generated at still higher voltages. As the number of stacks in series is increased, the maximum achievable power quickly approaches the power which a reversible system would generate, i.e. complete conversion of the available free energy. (A reversible system is reversible at every point in each stack, not just at the stack outlets.) The shaded area in the graph nearly fills the entire area under the curve - the reversible power. 

Figure 9-21 shows an MCFC network. The arrangement of stacks in series, as well as a unique recycle scheme, allows an MCFC network to meet all the requirements of an MCFC power system, while achieving high efficiency. 

Another potential disadvantage of an MCFC network is the interdependence of the stacks in series. A problem with one stack could alter the performance of succeeding stacks. 

Key to the concept of networking is the arrangement of multiple fuel cell stacks relative to the flow of reactant streams. Conventional fuel cells systems have been designed such that reactant streams flow in parallel through fuel cell stacks. In a fuel cell network, however, reactant streams are ducted such that they are fed and recycled through stacks in series. 

Standard cell sizes are up to 30 by 45 in. In an individual stack the compartments are in parallel, but several stacks in series are employed to achieve a high degree of ion exchange. The ion exchange membrane is not depleted and does not need regeneration. The mechanism is that an entering cation under the influence of an emf replaces an H+ ion from the resin and H+ from solution on the opposite face of membrane replaces the migrating cation. 

Single stack, MK II, four stages Single stack, MK III, three stages Single stack, MK III, one stage Three stacks in series, MK III ... 

The IP-SOFC bundle is then the basic unit of the IP-SOFC stack (Figure 6.2), where several bundles are coupled to a reformer in which the fresh fuel (CH4) reacts with a part of the water-rich SOFC anode off-gas, in order to produce an H2 and CO-rich gas mixture. The fuel produced by the reformer is fed in parallel to the various electrochemical bundles. The oxidant stream crosses all the bundles of the stack in series. The hot oxidant stream exiting the last electrochemical bundle flows through the reformer as well, in order to provide the heat necessary for the endothermic reforming reaction. 

The second typical technology applied for d. of water is -> electrodialysis. After appropriate pretreatment (as above), the feed solution is pumped through the unit of one or more stacks in series or parallel. The concentrated and depleted process streams leaving the last stack are recycled, or finally collected in storage tanks. The plants operate unidirectionally, as explained, or in reverse polarity mode, i.e., the current polarity is changed at specific time intervals (minutes to hours), and the hydraulic flow streams are reversed simultaneously, thus preventing the precipitation in the brine cells. 

The use of two stacks in series without interstage pumping has two important advantages ... 

However, when two subcells are stacked in series, the photocurrent is determined by the cell generating the smaller current. Therefore a current balance (or resistance match) is required between the front and back cell. Another approach is the so-called "stacking solar cell" in which two separate solar cells with transparent electrodes are fabricated and stacked together, with the front and back cell connected in parallel to give a larger short-circuit current. In this case, open-circuit voltage balance, which now becomes the key feature, can be fairly easily achieved. This will significantly increase the number of material systems applicable for efficient utilization of the solar spectrum. 

In order to achieve water splitting, some PEC devices require a potential bias. In some standalone configurations, such as the tandem hybrid device, such a bias can be provided internally by a photovoltaic junction stacked in series with the PEC junction. The determination of the saturated photocurrent density achievable by a PEC material, as well as the bias required to achieve this value, is crucial in identifying possible integration schemes for the material of interest. 

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