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Section 3.4 Efficiency definitions not store in the reservoir in the associated charging process, is taken out of the tank. In this case, the Coulomb efficiency might exceed 100 %. To sum up, if we intend to precisely state on the round-trip efficiency, the initial tank SoC must be equal to the value at the end of the discharging process. In the case of cycles which are limited by SoC limits, this is fulfilled per definition. However, in this case, the SoC of the stack(s) at the beginning and the end of the cycle also affects the RTSE, but to a lower extent [9]. If the cell voltage limits the cycles, it is proposed to conduct the presented pre-discharging process before the actual cycle starts. Note that this process does not relate to the conditioning of the electrolyte. The pre-discharging process starts from a tank SoC of 50 %. It is carried out with the same discharging current or discharging power, as the relevant discharging process and with the same flow rate control strategy. When the lower SoC or voltage limit is reached during the pre- discharging process, the charging process and thus the actual cycle starts, as shown in Figure 3-1. The lower SoC or voltage limit can be chosen to match the purpose of the experiment. 3.4.3 Operation point efficiency Although we can easily determine the round-trip efficiency in simulated and real systems, it faces some drawbacks. If the efficiency at lower currents and power is of interest, round-trips are very time-consuming. Furthermore, the influence of design or operational parameters, such as the flow rate, is only evaluated in an integral manner over the SoC range used during the cycle. We cannot evaluate the individual contribution of an individual SoC value to the overall round-trip efficiency. Hence, parameters cannot be adapted to specific requirements of an individual SoC value, e.g., in the middle or at the end of the charging and discharging process. Consequently, a concept of operation point efficiency is introduced in [16], which determines the system efficiency in any operation point defined by tank SoC and charging or discharging current. To determine the operation point efficiency, tank SoC is kept constant. This corresponds to assuming an infinite large tank volume of the positive and negative electrolyte tanks. In general, the instantaneous flow battery efficiency is defined as shown in Eq. (3-5). Herein, PT is the electrochemical tank power and PSys is the system power. Both quantities are introduced in the next sections. In the following, the equations are provided for a single-stack system. (3-5) Electrochemical tank power PT In order to calculate the electrochemical tank power, first, the equivalent electric tank current is computed. By multiplying the ionic flux, J, of V2+ and VO2+ ions with η PSys PT , for charging Sys PSys , for discharging PT 61PDF Image | Model-based Design Vanadium Redox Flow Batteries
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