*2.4. Salient Features of the Redox Flow Battery*

An in-house vanadium redox flow battery stack has been used as the battery in the present study. The stack is rated at 1 kW power and has 8 cells of 900 cm2 active area in each cell and features a specially designed flip-flop serpentine flow field [37] on the graphite plates. Nafion 117 membrane is used between the porous electrodes (SGL GFD4.6 4.6 mm thickness) with 35% compression as separator between anode and cathode. Full details of the stack construction can be found in [38–40]. Furthermore, 1.6 M vanadium electrolyte, procured from Oxkem, UK in an oxidation state of 3.5 is used as the electrical energy storage medium. In all the experiments, a constant flow rate of electrolyte, which corresponds to a stoichiometric factor of 9 at a current density of 60 mA/cm2, was maintained on each side [22,39]. Comprehensive testing over a range of flow conditions and state of charge (SoC) shows that, over a wide range of SoC, the stack can deliver 1200 W in charging and 750 W in discharging when operated at a current density of 100 mA/cm2. The maximum amount of energy stored in the battery depends on the electrolyte volume. Experiments have been conducted with electrolyte volumes of 30, 35 and 40 liters on each side. At a rated discharge energy of 25 Wh/litre, these electrolyte volumes amount to energy rating of 750 to 1000 Wh and about 50% more in charging.

A couple of runs have also been done with a commercially available lead-acid battery having a rating of 150 Ah, 12 V with a recommended C-rating of 10. Two of these batteries were connected in series thus giving a nominal power and energy rating of 360 W and 3.6 kWh, respectively for the lead-acid battery system. Compared to the VRFB, the leadacid battery has one-third power and about four times energy storage capacity. Thus, the P/E ratio of the two storage systems varies by more than an order of magnitude.
