## 1 Introduction
At the cells, the electrochemical conversion takes place and then catholyte and anyolite are collected back in the tanks. Figure 1: SRFB with a n n-type photoelectrode integrated directly with RFB; the redox couples of RFB are shared with the PVC. R and O are the reduced and oxidized form of the redox coupes of catholyte (positive electrolyte I) and anolyte (negative electrolyte II), respectively. X is the exchanged across the membrane ion. The current collectors are put together with electrodes, making flow-through or flow-by configurations of cells, while the flow-by configuration is considered to be the best option [14]. The flow-by electrode can be realized in different ways by an interdigitated pattern of channels prepared in a porous electrode material or in the current collector. [PERSON] and coworkers [16] have analyzed the influence of electrolyte flow hydrodynamics on VRFB performance. The two patterns of channels (Figure 3) were prepared in a graphite current collector, and the VRFB's performance was examined experimentally and also in CFD simulations. They developed a full 3D model in ANSYS Fluent using several custom user-defined functions (UDFs) for the implementation of the electrochemistry, along with the standard fluid dynamics module. Earlier, several successful applications of the finite volume method in the VRFB performance simulations have been reported in the literature [17; 18; 19]. Recently, [PERSON] and coworkers [20; 21] used STAR-CCM+, a finite volume method package, to investigate the influence of the different flow patterns of electrolytes on the industrial scale VRFB cell performance and the total costs of the device. All of the research results presented in the above-mentioned works lead to the conclusion that channels pattern and electrolyte flow hydrodynamics have a great influence on VRFB performance. As a final result of construction optimization, the total costs of device production were reduced by half while maintaining the assumed efficiency of the device [21]. Figure 3: Flow channel designs of VRFB cell: (**a**) Serpentine flow configuration; (**b**) Interdigitated flow field configuration. Figure 2: SRFB with stacked together n-type PVC an RFB; each module operates autonomously. R and O are reduced and oxidized form of the redox coupes of catholyte (positive electrolyte I), anolyte (negative electrolyte II), and PVC electrolyte III. X is the exchanged across the membrane ion. Another configuration of RFB and fuel cells, which perform to an even larger degree, depends on electrolyte flow hydrodynamics are membrane-less cells, which were originally developed for microfluidic fuel cell systems. The working principles of membrane-less cell designs rely on (a) single or multiphase phase co-laminar flows (Figure 4a); (b) co-laminar flows of electrolytes separated by a flowing stream of a supporting electrolyte (FSE) (Figure 4b); (c) half-cell reaction involving the deposition of solid active species on an electrode surface; and (d) the application of ionically inert solid separators [22]. The (a) and (b) system configurations are highly dependent on microflows hydrodynamic and the correct selection of the hydrodynamic conditions and channel geometry, which allows for the limitation of the unfavorable phenomenon of electrolyte mixing. It is worth emphasizing here that the advances in the theory of the fluid flow and cell design of microfluidic fuel cells (MFC), rather than in electrolyte and electrode kinetics, have been responsible for the continuous performance improvement (from power densities of \(<\)1 to \(>\)700 mW/cm\({}^{2}\)) in the period of 2004-2016 [22]. Such impressive growth of MFC performance was possible to be achieved, among other means, through the use of numerical methods in the analysis of microflows and parameters influencing them [23]. The first integrated with the RFB type of PVC was dye-sensitized solar cell (DSSC) [24; 25; 26].