## 3 Results and Discussion

Table 6: Change in cell voltage and pressure drop across a 200 mm long unit-width cell, relative to conventional uniform reference geometry, when charged at 160 mA cm\({}^{-2}\) from 90% SOC. Figure 5: Concentration (mol m\({}^{-3}\)) of V\({}^{4+}\) (left) and V\({}^{3+}\) (right) on the unit-width cell domain boundary showing the mixing action in the positive and negative half-cells respectively over a truncated section. impact on cell efficiency was seen at the highest current density. Cell resistance testing confirmed that the improvement seen below was not due to cell construction with lower contact resistance. With regards to the pressure drop, the results of this study are consistent with the finding in the previous mixer study [26]--the mechanical benefit, or penalty, of the mixers depends on the electrolyte flowrate. While the stoichiometric flowrate used here was higher in relative terms, the absolute velocity is lower in this case as the cells here have a smaller active area (400 cm\({}^{2}\) vs. 900 cm\({}^{2}\)). Further optimization of felt parameters and mixer geometries could preserve this advantage in larger cells, some of which exceed 2.5 m\({}^{2}\), which are being developed to deliver economies of scale [33]. Simulations of one geometry variation, a helical design, recently showed improved mixing with reduced pressure drop [32]. Possible concerns for the application of mixers in flow cells include the manufacturability of the geometries, and potential damage to thin structurally weak membranes. There are commercially available mixers for pipe applications on this scale, and Figure 7 shows a grey mixer prototype 3D printed in Accura(r) Xtremet(tm) using stereolithography (SLA), sitting atop a black cube printed using fused filament fabrication (FFF) for contrast. This material is rigid, but we have previously demonstrated the ability to print at the same scale with flexible Visijet(r) CE-NT using material jetting (MJP), a soft material with a Shore Hardness of 27-33A [34]. The alternative helical mixer geometry fabricated using MJP [35] is also shown below. A toroidal vanadium redox flow battery (VRB-T) concept was first introduced at the International Flow Battery Forum [18] based on the simulation and experimental testing of wedge-shaped cells. Figure 8 shows development on this concept with internally contained pumps, providing a visualization of how wedge-shaped cells with mixers could be applied in an assembly of multiple 400 cm\({}^{2}\) cell stacks. This novel RFB system is only possible with wedge-shaped cells, and would realize the technical advantages promised by the simulation results presented here. Figure 6: Charge–discharge curves demonstrating extended energy capacity in experimental studies over 10 cycles at 120 mA cm-2 in 25 cm\({}^{2}\) cells using conventional uniform and wedge flow-frames [18; 32]. Public acceptance of new energy generation and storage technologies is crucial to their long-term uptake and viability within social settings, especially if they are to be seamlessly integrated into highly visible urban environments [36]. Lessons learnt from the early installations of wind turbines are instructive in this regard [37]. Without adequate engagement with the communities that the technologies were located in, significant public push back occurred. Social surveys were used to identify and resolve some of these concerns for existing and future installations. For urban environments, where deployment of these new energy storage technologies is anticipated, public acceptance will be even more important. For this reason, it is instructive to use simulations and scenario planning to ensure that adequate groundwork is carried out to facilitate the seamless integration of these new energy storage systems into existing urban landscapes and the built environment.