*3.5. Flow Batteries*

Utility-scale energy storage has some promise thanks to flow batteries. There are many different compositions, but they all have energy-producing cells with electrode material stored remotely, making it possible for very large storage batteries to be made [38,39]. Vanadium redox batteries (VRB) are made up of cells with carbon composite electrodes submerged in a fluid containing aqueous acid and vanadium sulfate, with different valence states separated by an ion-selective membrane. At the positive electrode during discharge, V5+ is converted to V4+, while V2+ is converted to V3+ at the negative electrode. The volume of the vanadium sulfate solution, and hence the battery's capacity, is potentially limitless because it is kept in a storage tank. Recharging causes reverse reactions, which replenish the materials. The batteries are complicated to use and made of heavy materials, but their expected lifespan is very lengthy. Only a few prototype systems have been implemented so far, and given the size of the battery, VRB batteries are only practical for utility energy storage. Figure 10 depicts the chemistry and main parts of a vanadium redox flow battery.

Another kind of flow battery is the zinc–bromine (Zn-Br2) battery, as shown in Figure 11. Zinc bromide is synthesized when Zn reacts with Br2 within the cell. Br2 is injected into cells with carbon electrodes and a microporous plastic separator in an aqueous solution as an organic complexing agent [40,41].

**Figure 11.** Chemistry and principal components of a zinc–bromine battery.

Metallic Zn is formed on charging, and while Br2 is housed in tanks, the Zn electrode enforces a limit on the capacity for any specific design. The price is cheaper than VRB batteries, but the average lifetime is less. The discharge of bromine is a perceived threat that must be avoided. Zn-Br2 batteries, like other flow batteries, have only been employed in moderate numbers for utility usage. There are a few similar types of flow batteries, such as iron–chromium batteries; however, they are not broadly utilized.

The technical comparison of the aforementioned battery technologies has been tabulated as demonstrated in Table 2.


**Table 2.** Technical comparison of battery technology in South Africa [23–37].
