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Section 6.6 Conclusion Table 6-6: Current-weighted average RTSE values for all twenty-four designs Channel variation Area Electrode 123456 short short short long long long wide medium narrow wide medium narrow 73.6% 74.2% 74.7% 74.5% 74.9% 75.0% 74.2% 75.0% 75.1% 74.8% 75.2% 75.1% 74.8% 74.9% 75.0% 75.1% 75.1% 75.0% 74.9% 75.0% 74.9% 75.0% 75.0% 74.8% variation area 1 1000cm2 2 2000cm2 3 3000cm2 4 4000cm2 6.6 Conclusion The single-stack design study reveals three important correlations: 1. Enlarging the electrode and increasing the channel geometry factor using a long and narrow channel is equally effective to limit the impact of shunt currents. 2. For a constant aspect ratio, a larger electrode requires a lower flow rate to obtain the same fluid velocity and thus the same mass transfer coefficient as a smaller electrode. Hence, referred to the electrode area, the optimal flow rate for a smaller electrode is larger than for a larger electrode. 3. The large electrolyte demand of a large electrode slightly lowers the auxiliary efficiency. This means that the relative pump power demand tends to increase. However, if the presented design methodology is obeyed, electrode areas between 1,000 and 4,000 cm2 can be used to construct almost equally efficient VRFBs. The key is to equip the electrode with a channel design adapted to its area and to identify optimal flow factors. In this case, the different loss mechanisms, namely shunt currents, concentration overpotential and pump power demand can be balanced in a way that all electrode areas obtain comparable efficiencies. This fact promotes an excellent scalability of the redox flow technology. The presented approach includes a simple flow rate optimization by identifying the optimal flow factors for each design and each current density. It is inevitable to include a flow rate optimization into the design study. If all designs are operated with the same flow factor, smaller electrode areas would suffer from the aforementioned phenomenon of lower fluid velocities. Hence, in this study, larger optimal flow factors are identified and deployed for these designs. The additional pump power demand of the larger flow factors is tolerable, due to the lower pressure drop across the channels of the cells with a smaller electrode. Consequently, the smaller electrode areas are surprisingly competitive, compared to the larger electrodes, given their advantages in terms of both, shunt currents and concentration overpotential. For a single-stack system, which uses all reasonable power levels equally frequent, a long but medium wide channel in combination with an electrode area of 2000 cm2 is the best choice. However, as the total difference between all studied designs is rather small, additional parameters in terms of costs and manufacturing can be considered to identify 103PDF Image | Model-based Design Vanadium Redox Flow Batteries
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