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Section 2.5 Shunt currents Therein, the simulation results of a charging/discharging cycle between a tank SoC of 20 % and 80 % are shown. For the cycle simulation, a tank volume of 10 L per half- side, a total vanadium concentration of 1.6 molL-1, a constant flow rate of 1.5 Lmin-1 and a charging/discharging current of ±200 A is used. Additional parameters which remain unchanged throughout this work, including the diffusion coefficients of the simulated Nafion 115 membrane, are given in the Appendix B.2 on page 152. In Figure 2-4, the ionic fluxes due to the vanadium crossover are interpreted as equivalent electric currents. The magnitude of the currents strongly depends on the SoC. A higher SoC increases the crossover losses. This is reasonable, as a higher SoC corresponds to a higher concentration of V2+ and VO2+ ions. Thus, more V2+ and VO2+ ions are going to cross the membrane. In the half-cell from which they originate, this is equivalent to a discharging process. Also, they trigger self-discharge reactions in the half-cell in which they arrive, which corresponds to a discharging process there as well. From the current magnitude, we see that the self-discharge processes result in a higher equivalent discharging current than the diffusion of the ions itself. The crossover of V2+ and VO2+ ions is nearly zero when the SoC is low. However, at this point, the crossover of V3+ and VO2+ ions is dominant, because of their higher concentrations. In the half-cell in which they arrive, these ions also trigger self-discharge reactions. As shown in Figure 2-4 c), the parasitic currents are different for the negative and the positive half-side. Hence, in long-term operation, the SoCs of the two half-sides might deviate from each other. 2.5 Shunt currents 2.5.1 Shunt current phenomenon In order to obtain a reasonable battery voltage, we have to connect several cells electrically in series. Regarding the hydraulic interconnection, both series and parallel connections are possible in principle. However, in a hydraulic series connection, the total flow rate, required for the operation of all cells, has to pass every single cell. This leads to a very high pressure drop and thus to a very high pump power demand. Therefore, in general, the parallel connection is preferred [17]. The disadvantage of combining an electric series connection and a hydraulic parallel connection is the occurrence of shunt currents. Within a stack, mutual manifolds usually supply the cells with electrolyte. Hence, the electrolyte path connects cells with different electric potentials to each other. As the electrolyte is a relatively good ionic conductor, shunt currents evolve. Using the electrolyte supply path, hydrogen protons are now able to skip one or several cells. Thus, they do not partake in the desired electrochemical reaction in the skipped cells. The stored energy, namely the product of the electric charge of the hydrogen proton and the sum of the voltages of the skipped cells, is converted into heat and thus lost for the battery operation. Therefore, shunt currents lower the battery efficiency. 26PDF Image | Model-based Design Vanadium Redox Flow Batteries
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