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Section 2.5 Shunt currents The shunt current phenomenon was first observed in electrochemical processes such as chloralkaline electrolysis [34]. For this and comparable processes from the field of electro synthesis, a series arrangement of several cells is common as well [35]. Beside energy loss, shunt currents have also be found to cause additional negative effects, such as material corrosion [36, 37]. 2.5.2 Shunt current modeling Within the stack, four manifolds, two per half-side, transport the electrolyte to the cells and collect it when it leaves the cells, as shown in Figure 2-5. The so-called channels connect each cell to its inlet and outlet manifolds. The channels are usually carried out in the shape of a meander. This intentionally extends the electrolyte path to increase the shunt current resistance. The disadvantage of the channels is an increased pressure drop within the cell and hence an increased pump power demand [38]. + + Stack Half cell 2 Half cell 1 - Negative tank Cell 2 EC2' RCh RC IC2 Membrane RCh Cell 1 RM RCh RC IC1 RM EC1' RCh Channel Manifold RM R R Figure 2-6: Equivalent electric circuit of the negative half-side in a two-cell stack E nal RM E x xt te e r r n Figure 2-5: Schematic of the negative electrolyte supply of a two-cell stack To compute the shunt currents, the equivalent electric circuit of the stack with its electric and hydraulic components is set up, as shown in Figure 2-6 [9, 31, 39, 40]. It contains the actual cell resistance (RC), as well as the effective resistances of the manifolds (RM), the channels (RCh) and the external hydraulic circuitry (tanks, pipes, etc. – RExternal). While the cell resistance is assumed to be constant, the resistances of the manifolds and the channels vary with the SoC of the negative and the positive electrolyte. 2.5.3 Conductivity of negative and positive electrolyte The conductivities of negative and positive electrolyte, σ− and σ+, differ strongly from each other. Further, they are temperature- and SoC-dependent [40, 41]. The conductivity 27 - + - + -PDF Image | Model-based Design Vanadium Redox Flow Batteries
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