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Section 8.1 Current state of science Chapter 8 Flow rate optimization Flow rate optimization 8.1 Current state of science The simplest flow rate control strategy (FRCS) of a VRFB is deploying a constant flow rate for all SoCs and currents. Another simple approach is to adapt the flow rate according to the instantaneously derived stoichiometric requirements of the electro- chemical reactions. These requirements are given by Faraday’s first law of electrolysis. To compensate for imperfections and losses, the actual flow rate has to be larger than the stoichiometric one. Hence, the instantaneous stoichiometric flow rate is multiplied by a constant factor, commonly referred to as the ‘flow factor’ [30]. The two approaches mentioned above have already been studied by NASA for the iron- chromium flow battery in 1982 [94]. More than 30 years later, a similar comparison is presented for the VRFB [30]. In terms of discharge capacity and system efficiency, a variable flow rate using the scaled instantaneous stoichiometric flow rate is found to be advisable for the VRFB. One drawback of the presented study is the over-estimation of the pump efficiency. In [30], a constant pump efficiency of 80 % is assumed. This is more than twice as high as the best efficiency point of real pumps, as shown in Section 2.12.6 on page 54. In [17, 95], a third FRCS is presented, representing a simple and straightforward optimization approach. If we charge the battery with a constant power, we have to adapt the flow rate depending on the SoC with the goal of maximizing the applied charging current. This is reasonable because the charging progress depends on the applied current, not on the applied power. Hence, the claim for a maximum current at a given power implies a low charging voltage, which benefits the efficiency. If we discharge the battery with a constant power, we adapt the flow rate depending on the SoC to minimize the absolute value of the discharging current. If we charge the battery with a constant current, the goal is to minimize the input power. If we discharge the battery with a constant current, the goal is to maximize the absolute value of the output power. This methodology is also used in [96]. Herein, the impact of temperature and pipe diameter on the efficiency is studied in addition. However, due to additionally considered loss mechanisms, namely shunt currents and vanadium crossover, the previously presented approach of minimizing and maximizing current and power, respectively, is no longer adequate. In addition to the presented model-based studies, some experimental studies have been published as well. By increasing the flow rate at pre-defined cell or stack voltage levels, a good compromise between system efficiency and discharge capacity can be yielded [97]. Herein, an experimental study is carried out with a kW-class VRFB. However, the values of the pre-set voltage limits and the values of the applied low and high flow rates are not varied. Hence, it can be assumed that this method represents a 119PDF Image | Model-based Design Vanadium Redox Flow Batteries
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