A Multiscale Flow Battery Modeling Approach Using Mass Transfer Coefficients

Abstract

Driven by the transformation of the energy system and the need to store fluctuating power generation from renewable energy sources, stationary storage systems become more and more important. [1] Flow batteries are promising candidates for balancing the demand and supply in the electrical grid and thus providing continuous and reliable power supply. The unique concept of flow batteries is based on the spatial separation of electrolyte and electrode which results in the independent scalability of power and capacity. [2,3] This makes them applicable in a wide energy range of the grid infrastructure. Moreover, long operating life cycles, environmental friendliness, and nonflammability are further advantages toward predominant storage systems. [4,5] However, high capital costs and low energy density predominantly have been preventing a deep market penetration up to the present. [6] A typical flow battery consists of two independent reservoirs holding separated electrolyte solutions and two porous electrodes separated by an ion transport membrane. During operation, the electrolytes are pumped through the electrochemical cell, where the redox reaction takes place at the surface of the porous electrodes. Subsequently, the charged or discharged electrolyte flows back into the reservoir. [7,8] The all-vanadium system is so far the most studied and developed flow battery system, which convinces by the absence of material degradation and capacity fade. [9,10] However, it suffers from the comparatively high prices and enormous price fluctuations regarding the electrolyte, [11] and from the fact that vanadium is a critical raw material. [12] Recently, organic redox-active materials have emerged as a promising alternative to metal-based systems due to their low cost, structural designability, and the independence on metal mining. [13,14] For this reason, this contribution focuses on an electrolyte for aqueous organic redox flow batteries, namely 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4-OH-TEMPO). [11,15] Simulation methods are powerful tools for gaining insight into the multiphysical processes inside the battery and for predicting its performance for various operating conditions. They provide a substantial alternative to experimental investigations as these can be very challenging and costly in terms of raw materials, physical resources, and time. Modeling approaches for flow battery components and performance apply to a wide range of length scales. This contribution focuses on a detailed microscale model (MSM) and an upscale connection to a homogenized cell-scale model (HCSM).Three-dimensional MSMs resolve the actual electrode geometry to gain insights of the micro-structural processes within the battery. The structure of the electrode is obtained by image processing and reconstructing, mostly using X-ray-computed