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Advanced Energy Storage
ZincFlow Technology

Zinc-Flow® Technology

Regenerative Fuel CellsThe zinc bromide flowing electrolyte energy storage system stores and releases electricity though a reversible electrochemical reaction. Flowing electrolyte systems are typically made up from three sub-systems (cell stacks, electrolyte tank system, and an electronics and control system.) The Zinc-Flow® technology is a flowing electrolyte energy storage system based on zinc bromide chemistry. Zinc-Flow systems derive their energy and power from the reaction between two aqueous solutions containing zinc and bromide ions that differ only in their concentration. Zinc-Flow technology is totally reversible with no degradation of the electrolyte or other components from the reaction; this strength provides these products with an unrivaled operational lifespan of 30 years, even under harsh conditions.

In the Premium Power zinc bromide energy storage system, electrolyte is pumped from two electrolyte reservoirs through the cell stack in two circuits, one for anode half-cells and the other for cathode half-cells. This is shown schematically below.

The electrolyte in the anode loop is commonly called anolyte; the electrolyte in the cathode loop is called the catholyte. Anolyte and catholyte are in contact through microporous cell separators. Although ionic components in the electrolyte can readily pass through the cell separator, bulk mixing of anolyte and catholyte is prevented.

Initially the electrolyte is a homogeneous aqueous solution of zinc bromide, zinc chloride, potassium chloride and quaternary organic bromide salts. As the zinc bromide energy storage system is charged, zinc ion is reduced to metal on the anodes, and bromide ion is oxidized to molecular bromine on the cathodes. The anolyte and catholyte gradually develop different compositions. Elemental bromine produced in the cathode half-cells forms a polybromide complex with quaternary salts in the catholyte. The polybromide complex separates from the catholyte aqueous phase as a high density oily liquid phase. This is collected in the bottom of the catholyte reservoir. The charging process stores chemical energy in separate locations, inside the cell stack as zinc metal and outside the cell stack in the catholyte reservoir as polybromide complex. During discharge these processes are reversed. Zinc metal oxidizes reforming the zinc ion and bromine is reduced to bromide ion. Bromine available in the catholyte for reduction to bromide ion will be consumed in a short period of time unless polybromide complex from the bottom of the catholyte reservoir is fed back into the circulated catholyte. If the complex is not pumped back into the cathode half-cells, the stack discharge voltage will quickly drop to a value too low to provide useful DC power.

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