Go with the flow18 Mar 2019 23:13
https://www.windpowerengineering.com/business-news-projects/how-three-battery-types-work-in-grid-scale-energy-storage-systems/
Redox-flow batteries
NASA studied the use of redox-flow batteries (RFB) for the space program during the 1970s, and the concept of using chemical reduction and oxidation reactions for energy storage dates back even further.
In RFBs, two chemical components are dissolved in liquids within the system, and are separated by a membrane. The membrane facilitates the ion exchange and the electric current flows, while the liquids are kept separate in anolyte and catholyte tanks. The chemical reduction and oxidation reactions that take place in these tanks store the generated energy in a liquid electrolyte solution and are what the “redox” (reduction, oxidation) name refers to.
Similar to lithium batteries, there are multiple types of flow batteries with a variety of chemistries. Most commercial efforts for grid-scale solutions are using some form of vanadium, iron, bromine, or sodium solution.
RFBs are unique compared to traditional batteries because the power (kW) rating of the system is based on the power stack size selected, and the energy (kWh) capacity is independently selected based on the storage tank size and volume of electrolytes in the tanks. In principle, this means that any combination of energy and power can be configured. In practice, however, the infrastructure required for pumping and managing the tanks becomes economically viable for systems with four hours or more of energy, when compared to the rated power of the power stacks.
Many flow batteries are characterized by extremely long cycle life—tens of thousands of cycles, or (theoretically) unlimited cycle life. For example, there is no inherent degradation of the vanadium solutions as ions are exchanged between the tanks, and there is an assumed residual value of the vanadium solution at the end of life of the system. Although they typically have a higher initial cost relative to other batteries, the lifetime costs may be lower for flow batteries, especially in high-cycle applications.
Flow batteries have a lower energy density than a comparable lithium solution and are typically characterized by a larger footprint, or the area of land required for a particular storage device or system. However, for many sites, the footprint is not a drawback or determining factor of the viability of a project. For example, utility-owned property can usually accommodate stacked tanks (15 to 20 feet high).
In these cases, four-hour flow battery systems can have a smaller footprint than a comparable lithium-based storage system. The weight of lithium battery containers typically makes stacking impractical.
There are some misconceptions that since flow batteries are sized for four hours or more of discharge at rated power, they cannot perform frequency regulation or other short duration tasks. In fact, flow batteries are ideally suited for long-durati
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