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Section 2.9 Overpotential due to deviations between tank OCV and cell EMF 4. Finally and most importantly, for state-of-the-art flow battery stacks, the activation overpotential can obviously be neglected, because the model without the activation overpotential shows a good agreement with the experimental data as shown in Section 4.3.3 on page 69. The large surface area and the possible thermal or plasma activation of the graphite felt as well as additional improvements as described in [47, 60] effectively minimize the activation overpotential. 2.9 Overpotential due to deviations between tank OCV and cell EMF Because of the charging and discharging process, the electrolyte in the cell naturally has a different SoC than the electrolyte in the reservoir. This we can only prevent, if we replace the charged or discharged electrolyte with electrolyte from the tank with an infinite velocity. During the charging process, the average cell SoC is higher than the tank SoC. During the discharging process, the average SoC of the cell is lower than the tank SoC. The SoC variations reflect in corresponding variations of the voltage, calculated with the Nernst equation. Normally, this voltage is called open circuit voltage (OCV). However, as the cell does not operate under open circuit conditions, technically, this term is not applicable here. Hence, it is denoted as electromotive force (EMF). Figure 2-14: Difference between cell and tank OCV for different flow rates for a cycle between tank SoC 20 % and 80 % with 200A We have to consider the difference between cell EMF and tank OCV as an additional overpotential. This overpotential mainly depends on the ratio between the applied current and the applied flow rate, as shown in Figure 2-14. For a high flow rate of 1.5 Lmin-1, the voltage difference between cell and tank OCV is significantly smaller. Voltage in V Voltage in V Voltage in mV Voltage in mV 41PDF Image | Model-based Design Vanadium Redox Flow Batteries
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