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Section 6.5 Evaluation of all twenty-four cell designs The relation, presented in Eq. (6-7) is illustrated in Figure 6-8 for the stoichiometric flow rate calculated from a tank SoC of 80 % and a current density of 100 mAcm-2, multiplied by different flow factors. For a given flow factor, the fluid velocity increases with the electrode area. If the electrode area is increased by a factor of four, the fluid velocity doubles for a given flow factor, as shown in Figure 6-8 a). If we want to obtain a certain fluid velocity, we have to apply a larger flow factor for a smaller electrode area, as shown in Figure 6-8 b). Figure 6-8: Impact of electrode area on fluid velocity for a given flow factor and vice versa The fluid velocity directly affects the mass transfer coefficient, as shown in Eq. (2-70) on page 44. For a given flow factor, a larger electrode yields a higher fluid velocity. Hence, the mass transfer coefficient is increased, the concentration overpotential is decreased and the voltage efficiency rises. Or to rephrase: If a particular mass transfer coefficient should be reached to limit the concentration overpotential, a larger electrode requires a lower flow factor. The dependence of the fluid velocity on the electrode area explains the increasing voltage efficiency of a larger electrode for a given flow factor, as shown in Figure 6-7. Note, that these considerations only apply, if we compare electrodes with different areas, but identical aspect ratios. However, the flow factor affects the voltage efficiency significantly stronger than the electrode area. Hence, we can compensate the negative effect of a smaller electrode area on the voltage efficiency by applying a larger flow factor. Naturally, this comes at the costs of an increased pump power demand. The holistic considerations in this design study reveal that the smallest electrode area can be operated as efficiently as the largest electrode area, if all loss mechanisms are taken into account, as shown in Table 6-6 on page 103. Nevertheless, the afore- mentioned phenomenon is present in many facets in this work. Amongst others, it also 97 Fluid velocity in cms-1 Flow factorPDF Image | Model-based Design Vanadium Redox Flow Batteries
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