Distribution of electrolytes in a flow battery

1. A method of determining a distribution of electrolytes in a flow battery, the method comprising:

(a) providing a flow battery with a fixed amount of fluid electrolyte having a common electrochemically active specie, the common electrochemically active specie having multiple reversible oxidation states, a portion of the fluid electrolyte serving as an anolyte and a remainder of the fluid electrolyte serving as a catholyte;

(b) determining an instant average oxidation state of the common electrochemically active specie in the anolyte and the catholyte; and

(c) responsive to the determined instant average oxidation state, adjusting an instant molar ratio of the common electrochemically active specie between the anolyte and the catholyte to increase a relative energy discharge capacity of the flow battery for the determined instant average oxidation state, the relative energy discharge capacity being an instant energy discharge capacity based on the instant molar ratio relative to a maximum capacity based on a fully balanced molar ratio between the anolyte and the catholyte, and wherein the adjusting includes partially mixing the anolyte and the catholyte together into non-equal volumes of the anolyte and the catholyte in the flow batter.

2. The method as recited in claim 1 , wherein the anolyte and the catholyte define a fully balanced average oxidation state with respect to equal molar ratios of the anolyte and the catholyte, and the determined instant average oxidation state is different than the fully balanced average oxidation state.

3. The method as recited in claim 2 , wherein the instant molar ratio is represented by N − /(N − +N + ), where N − is moles of the common electrochemically active specie in the anolyte and N + is moles of the common electrochemically active specie in the catholyte, and the adjusted molar ratio is less than or greater than 0.5.

4. The method as recited in claim 1 , wherein said step (b) is conducted at a 100% state of charge of the anolyte and the catholyte.

5. The method as recited in claim 1 , wherein said step (c) includes adjusting the instant molar ratio such that the increased relative energy discharge capacity of the flow battery is less than a full energy discharge capacity of the flow battery at the fully balanced average oxidation state with respect to equal molar ratio of the anolyte and the catholyte.

6. The method as recited in claim 1 , including mixing less than 50 vol % of the anolyte with the catholyte and then mixing less than 50 vol % of the catholyte with the anolyte.

7. The method as recited in claim 1 , wherein the common electrochemically active elemental specie is selected from the group consisting of vanadium, iron, and chromium.

8. The method as recited in claim 1 , wherein said step (b) includes directly determining the instant average oxidation state from concentrations of different oxidation states of the common electrochemically active elemental specie in the anolyte and the catholyte.

9. The method as recited in claim 1 , wherein said step (b) includes determining the instant average oxidation state as a function of molar concentrations of different oxidation states of the common electrochemically active elemental specie in the anolyte and the catholyte divided by a total molar amount of the common electrochemically active elemental specie.

10. The method as recited in claim 1 , wherein said step (b) includes collecting measurements representing volumes of the anolyte and the catholyte and concentrations of different oxidation states of the common electrochemically active specie in the anolyte and the catholyte.

11. The method as recited in claim 10 , wherein the measurements are selected from the group consisting of optical measurements, conductivity measurements, density measurements, viscosity measurements, and combinations thereof.

12. The method as recited in claim 1 , further including adjusting the instant molar ratio in response to a concentration of the common electrochemically active specie in one of the anolyte and the catholyte exceeding a defined threshold.

13. The method as recited in claim 1 , further including adjusting the instant molar ratio in response to an external environmental air temperature of the flow battery being below a defined threshold.

14. The method as recited in claim 1 , wherein said step (c) includes partially mixing the anolyte and the catholyte together as a function of an external environmental air temperature of the flow battery such that volumes that are mixed are inversely related to the external environmental air temperature.

15. The method as recited in claim 1 , further including adjusting the volume ratio of the anolyte according to volume ration bands on a chart of anolyte concentration versus average oxidation state, wherein the volume ratio is adjusted to the volume ratio band that contains a plot point of an instant anolyte concentration and instant average oxidation average oxidation state.

16. The method as recited in claim 1 , wherein the flow battery includes a supply/storage system external of the at least one electrochemical cell, the supply/storage system including first and second vessels fluidly connected with the at least one electrochemical cell, the first and second vessel having respective volumetric sizes, wherein at least one of the volumetric sizes is selected, at least in part, according to a state of charge range of the flow battery.

17. The method as recited in claim 16 , wherein the state of charge range of the flow battery is from a first value that is less than 100% charge to a second value that is greater than 0% discharge.

18. The method as recited in claim 16 , wherein the state of charge range of the flow battery is 80% charge to 20% discharge.

19. The method as recited in claim 1 , including mixing less than 50 vol % of the anolyte with the catholyte and then mixing less than 50 vol % of the catholyte with the anolyte, and the common electrochemically active elemental specie is vanadium.

20. The method as recited in claim 19 , further including adjusting the instant molar ratio in response to a concentration of the common electrochemically active specie in one of the anolyte and the catholyte exceeding a defined threshold.

21. The method as recited in claim 20 , wherein the instant molar ratio is represented by N − /(N − +N + ), where N − is moles of the common electrochemically active specie in the anolyte and N + is moles of the common electrochemically active specie in the catholyte, and the adjusted molar ratio is less than or greater than 0.5.

22. The method as recited in claim 20 , wherein said step (c) includes adjusting the instant molar ratio such that the increased relative energy discharge capacity of the flow battery is less than a full energy discharge capacity of the flow battery at the fully balanced average oxidation state with respect to equal molar ratio of the anolyte and the catholyte.

23. A method of determining a distribution of electrolytes in a flow battery, the method comprising:

(a) providing a flow battery with a fixed amount of fluid electrolyte having electrochemically active species, the electrochemically active species having multiple reversible oxidation states, a portion of the fluid electrolyte serving as an anolyte and a remainder of the fluid electrolyte serving as a catholyte;

(b) determining an instant average oxidation state of the electrochemically active species in the anolyte and the catholyte; and

(c) responsive to the determined instant average oxidation state, adjusting an instant molar ratio of the electrochemically active species between the anolyte and the catholyte to increase a relative energy discharge capacity of the flow battery for the determined instant average oxidation state, the relative energy discharge capacity being an instant energy discharge capacity based on the instant molar ratio relative to a maximum capacity based on a fully balanced molar ratio between the anolyte and the catholyte, and wherein the adjusting includes partially mixing the anolyte and the catholyte together into non-equal volumes of the anolyte and the catholyte in the flow battery.

24. A method of determining a distribution of electrolytes in a flow battery, the method comprising:

(a) providing a flow battery with a fixed amount of fluid electrolyte having electrochemically active vanadium species, the electrochemically active vanadium species having multiple reversible oxidation states, a portion of the fluid electrolyte serving as an anolyte and a remainder of the fluid electrolyte serving as a catholyte;

(b) determining an instant average oxidation state of the electrochemically active vanadium species in the anolyte and the catholyte; and

(c) responsive to the determined instant average oxidation state, adjusting an instant molar ratio of the electrochemically active vanadium species between the anolyte and the catholyte to increase a relative energy discharge capacity of the flow battery for the determined instant average oxidation state, the relative energy discharge capacity being an instant energy discharge capacity based on the instant molar ratio relative to a maximum capacity based on a fully balanced molar ratio between the anolyte and the catholyte, and wherein the adjusting includes partially mixing the anolyte and the catholyte together into non-equal volumes of the anolyte and the catholyte in the flow battery.