2.2.2. Flow Batteries

Carbon-free hydrogen gas production is extremely flexible; however, inherent energy losses mean it is not always the most efficient means of providing secure and reliable power from renewable energy technologies [32]. Batteries have a significant role to play, although there are serious sustainability concerns for the widespread adoption of lithium batteries [33,34]. Large-scale deployment of this incumbent technology will face battery materials constraints in a global-scale energy transition [35]. There are other battery technologies suitable for grid-scale energy storage, such as the vanadium redox flow battery (VRB/VRFB).

VRBs are a hybrid between fuel cell and conventional battery technology. Energy is stored in liquid electrolyte tanks for power conversion in cell stacks that operate in a similar manner to a fuel cell. The power is stored (or recovered) from the change in state of vanadium ions in an aqueous sulfuric acid solution which changes color based on its state of charge. Figure 4 provides a simplified visual explanation of this process. By separating power (kW/MW) from energy storage (kWh/MWh), a VRB system is highly scalable and can readily be configured to suit the needs of the application. sulfuric acid solution which changes color based on its state of charge. Figure 4 provides a simplified visual explanation of this process. By separating power (kW/MW) from energy storage (kWh/MWh), a VRB system is highly scalable and can readily be configured to suit the needs of the application.

*Sustainability* **2020**, *12*, x FOR PEER REVIEW 6 of 17

**Figure 4.** Simplified schematics of a vanadium redox flow cell during charge and discharge showing electrolyte color change with hydrogen and electron flows. **Figure 4.** Simplified schematics of a vanadium redox flow cell during charge and discharge showing electrolyte color change with hydrogen and electron flows.

The aqueous electrolyte is non-flammable by nature, eliminating the potential fire safety hazard of lithium batteries. The reduced fire risk means these systems are more suitable for enclosed spaces, such as underground car parks. Electrolyte contamination is eliminated because the electrolyte is the same on both sides of the system, giving VRBs a long lifetime (>25 years) with low capacity fade. Allvanadium batteries' tolerance for practically unlimited charge–discharge cycles over their lifetime makes the technology ideal for high-use applications, such as supporting renewables and electric vehicles. The valuable vanadium in the electrolyte can be easily recovered for use in a new battery system or other applications. The balance of material is predominately carbon, metals and polymers that can also be recovered and recycled at end of life [36]. The aqueous electrolyte is non-flammable by nature, eliminating the potential fire safety hazard of lithium batteries. The reduced fire risk means these systems are more suitable for enclosed spaces, such as underground car parks. Electrolyte contamination is eliminated because the electrolyte is the same on both sides of the system, giving VRBs a long lifetime (>25 years) with low capacity fade. All-vanadium batteries' tolerance for practically unlimited charge–discharge cycles over their lifetime makes the technology ideal for high-use applications, such as supporting renewables and electric vehicles. The valuable vanadium in the electrolyte can be easily recovered for use in a new battery system or other applications. The balance of material is predominately carbon, metals and polymers that can also be recovered and recycled at end of life [36].

### 2.2.3. Hybrid Batteries 2.2.3. Hybrid Batteries

summarized in Table 2 below.

The ubiquitous lead-acid battery (LAB) has no moving parts and is the standard for uninterruptible power supplies (UPS). The conventional format of this technology, however, is not well suited to new power demands. Hybrid battery installations are being used to take advantage of the strengths of different technologies [37]. Pairing battery components with integrated supercapacitors creates new opportunities for mature LAB technology by improving its peak power The ubiquitous lead-acid battery (LAB) has no moving parts and is the standard for uninterruptible power supplies (UPS). The conventional format of this technology, however, is not well suited to new power demands. Hybrid battery installations are being used to take advantage of the strengths of different technologies [37]. Pairing battery components with integrated supercapacitors creates new opportunities for mature LAB technology by improving its peak power capacity.

capacity. This hybrid technology was invented by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) and commercially acquired in 2010. It has since been deployed for a range of kW- and MW-scale storage applications. Similar to vanadium-sulfuric acid electrolyte flow batteries, hybrid lead-sulfuric acid batteries are almost completely recyclable. The Environmental Protection Agency in the United States found that lead-acid batteries are consistently one of the most recycled products [38]. This and the other energy storage technologies discussed above are This hybrid technology was invented by the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) and commercially acquired in 2010. It has since been deployed for a range of kW- and MW-scale storage applications. Similar to vanadium-sulfuric acid electrolyte flow batteries, hybrid lead-sulfuric acid batteries are almost completely recyclable. The Environmental Protection Agency in the United States found that lead-acid batteries are consistently one of the most recycled products [38]. This and the other energy storage technologies discussed above are summarized in Table 2 below.


**Table 2.** Storage technologies for healthcare and their key strengths, weaknesses, and opportunities.
