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Section 2.5 Shunt currents 2.5.8 Multi-stack systems 14 12 10 8 6 4 2 0 Figure 2-11: Equivalent shunt current over the number of stacks in series connection using cell design 4.6 For manufacturing reasons and the fact that the electrolyte supply via the common manifold is going to evolve inhomogeneously if too many cells are supplied at once, the number of cells per stack is limited. Hence, if a higher voltage than achievable with a single stack is required, we have to connect several stacks electrically in series. Unfortunately, this amplifies the occurrence of shunt currents, as shown in Figure 2-11. Although cell design 4.6 with the largest geometry factor of all studied designs is deployed, for a series connection of five stacks, shunt currents rise to 5.91 A. This corresponds to a loss in Coulomb efficiency of 2.0 %-points for an applied current of 300 A, one way. Again, a curve fitting is deployed to estimate the shunt current losses of the stack series arrangement from the shunt currents of the single stack, as shown in Eq. (2-40). If eight stacks are connected in series, an equivalent shunt current of 12.5 A has to be expected. IShunt, Design 4.6NS 0.479NS1.57A (2-40) However, this time, we cannot apply the curve fitting to a general case. This is because shunt currents in multi-stack arrangements strongly depend on the length and the diameters of the external electrolyte piping [9]. For the simulations corresponding to Figure 2-11, the pipe length between the stacks is 1 m. The stacks are connected to the pipe by a tube with a length of 1.5 m. Both tube and pipe have a diameter of 60 mm. If for example the tube diameter is reduced to 30 mm, the equivalent shunt current of five serially connected stacks decreases from 5.91 A to 3.95 A for the considered operation. However, it has to be evaluated if the additional pressure drop due to the decreased tube diameter is tolerable regarding safety and efficiency. Cell design 4.6 Fitting 34 12345678 Number of stacks in series Equivalent shunt current in APDF Image | Model-based Design Vanadium Redox Flow Batteries
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