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Section 8.5 Variable flow rate – Conventional approach With a ØRTSE of 76.5 %, a flow factor of 1.5 yields the highest current-weighted average RTSE. However, this flow factor yields a nominal discharge capacity of only 11.8 WhL-1. The nominal capacity increases with the flow factor, as long as the pump capacity is not maxed out, as shown in Figure 8-3 b). Hence, the maximum reasonable flow factor of 4.35 is deployed to maximize the nominal discharge capacity. This flow factor yields a nominal discharge capacity of 15.4 WhL-1, while yielding a current-weighted average RTSE of 70.2 %. Hence, the additional nominal discharge capacity of 3.6 WhL-1 (+31 %) reduces the current-weighted average RTSE by 6.3 %-points. 8.5 Variable flow rate – Conventional approach 8.5.1 Methodology The principal advantage of a variable flow rate is its ability to consider the specific requirements of different operational conditions of the battery, e.g., different SoCs and currents. The conventional variable FRCS constantly computes the stoichiometrically required flow rate for the present combination of tank SoC and current, as shown in Eq. (8-6) [94]. Due to Coulomb losses and for optimization purposes, this value is scaled with a flow factor larger than one. In this work, the flow factors for the charging and discharging process are always identical. The conventional variable FRCS is comprehensively presented in [17, 30]. Q C (8-6) FF⋅IC , for charging F(1 SoCT)cV FF⋅IC , for discharging FSoCT cV Similar to the constant FRCS, we can either optimize the flow factor to maximize system efficiency or discharge capacity. In this work, flow factors between 1.5 and 8 are studied in steps of 0.25 for the conventional variable FRCS. Here, a flow factor below one is not reasonable, while the flow factor is basically not limited upwards. This is because certain operation regions, such as the charging of the battery from a low SoC, require very small stoichiometric flow rates. Hence, it is possible to scale them with a very large flow factor without reaching the limiting pump capacity. For every studied current density, the flow factor which yields the highest efficiency and the flow factor which yields the highest discharge capacity is identified using a parameter sweep. The optimization result is a look-up table consisting of tank SoC, charging/discharging current and associated flow factor, either to operate with highest efficiency or maximum discharge capacity. In practice, the BMS would select the appropriate flow factor from this look-up table. 125PDF Image | Model-based Design Vanadium Redox Flow Batteries
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