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Section 8.4 Constant flow rate process and a lower tank SoC at the end of the discharging process, increase the discharge capacity. Hence, the higher the flow rate, the larger the useable SoC range. However, in practice, this correlation is hindered by two reasons. First, the pump capacity is limited. Secondly, the pump power demand increases quickly with an increasing flow rate, as illustrated in Figure 2-23 on page 56. Hence, system efficiency is negatively affected. For very large flow rates, it is most likely that the additional pump energy demand completely consumes the gained additional discharge capacity. In this work, the maximization of system efficiency and discharge capacity is considered as two separate optimization objectives. It is of course possible to combine both objectives. However, without knowing the intended battery use-case, it is not possible to decide whether a certain loss in system efficiency is economically compensated by the additionally provided discharge capacity or vice versa. Hence, the two extreme optimization objectives are studied to give the boundaries for any combination of the two. Flow rate optimization is carried out for cell design 2.5, which yields the highest average RTSE of all studied designs in a single-stack system. Hence, electrode area is 2000 cm2, channel length is 1,129 mm and channel width is 10 mm. A tank volume of 1,000 L per half-side is deployed. Again, the method of deriving the RTSE by means of simulating a successive charging and discharging process is applied. A pre-discharging process is carried out before the actual cycle. 8.4 Constant flow rate 8.4.1 Methodology In terms of system complexity and control, a constant flow rate is the simplest approach. One advantage of this method is that we can design the hydraulic circuit for one specific flow rate. We can select the pumps to have their best efficiency point at this flow rate and no inverter is required to run the pump motor. Furthermore, we do not require flow rate or pressure sensors, which lowers the system costs. While determining the constant flow rate, Faraday’s law has to be obeyed as well. The constant flow rate has to be large enough to enable both the charging and discharging operation towards the highest and lowest SoC, respectively, to be reached with the nominal current. If the SoC limits are non-symmetrical to a SoC of 50 %, we have to select the larger of the two flow rates. A VRFB using cell design 2.5 has a nominal current of 200 A. It shall be possible to obtain a tank SoC of 80 % during a charging process with this current. According to Eq. (8-3), the minimally required flow rate for the deployed electrolyte with a total vanadium concentration of 1.6 molL-1 for reaching a SoC of 80 % with a charging current of 200 A is 0.389 Lmin-1. This flow rate is denoted as the basic flow rate. Nevertheless, it is possible to deviate from this flow rate in order to increase system efficiency or discharge capacity. In this work, the deviating flow rates are referred to the 122PDF Image | Model-based Design Vanadium Redox Flow Batteries
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