Model-based Design Vanadium Redox Flow Batteries

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Model-based Design Vanadium Redox Flow Batteries ( model-based-design-vanadium-redox-flow-batteries )

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Section 2.10  Concentration overpotential To transform experimental results, obtained with other redox systems, we can use the dimensionless Sherwood number, which relates to the mass transfer coefficient as shown in Eq. (2-65) [62]. Sh 􏰵 k dF MT Deff Wherein: dF Fiber diameter of the graphite felt Deff Effective diffusion coefficient in the electrolyte (2-65) We can calculate the effective diffusion coefficient from the measured diffusion coefficient using the Bruggeman correction, as shown in Eq. (2-66) [18, 64]. Deff 􏰵 DElε3􏱮2 DEl Measured diffusion coefficient in the electrolyte (2-66) (m2s-1) (-) Wherein: ε Porosity of the graphite felt We can experimentally relate the Sherwood number to the Reynolds number, Re, as shown in the Eqs. (2-67) and (2-68) [61, 62]. In these two references, the mass transfer coefficient towards carbon fiber electrodes is studied using the reduction of [Fe(CN)6]3- to Fe(CN)6]4-. It is widely assumed that the mass transfer coefficient which is obtained using that reaction is also applicable to the particular vanadium reactions [8]. Sh 􏰵 7.00Re0.40 (2-67) Sh 􏰵 6.13Re0.36 (2-68) We can express the Reynolds number in quantities, which are commonly used to describe a VRFB, as shown in Eq. (2-69). Re 􏰵 ρElvEldF (2-69) (m) (m2s-1) Wherein ρEl Electrolyte density μEl Electrolyte dynamic viscosity vEl Electrolyte velocity μEl (kgm-3) (Pas) (ms-1) If we combine the Eqs. (2-65), (2-67) and (2-69), we derive an actual relation for the mass transfer coefficient that only contains known quantities. sectional area. μEl Deff ρ dF 0.40 Q 0.40 44 Deff dF ρ dF 0.40 7.00􏱲 El 􏱵 v0.40 (2-70) kMT 􏰵 We can further replace the fluid velocity by the flow rate over the electrode cross kMT􏰵 7.00􏱲El􏱵􏱲C􏱵 dF μEl CSAE (2-71)

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