Redox flow battery system and cell stack

Details Redox flow battery system and cell stack Redox flow battery system and cell stack

2000-04-24

2002-11-05

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Fluid active material or two-fluid electrolyte combination...

Active material in solution

C429S072000

Reexamination Certificate

active

06475661

ABSTRACT:

This invention relates in general to renewable electrochemical energy storage by redox flow battery systems and more in particular to vanadium redox secondary batteries.
Electrochemical systems because of their theoretically high efficiency have long been looked at as ideal energy conversion systems. In particular secondary batteries are by definition extremely interesting candidates for energy storage systems. Load levelling and peak-shaving in electric power generation, distribution and use are all areas where secondary batteries may offers very efficient solutions.
Among secondary batteries, the so-called redox flow battery or more briefly redox (cells) batteries employ solutions for storing the energy; the cell hardware simply providing an appropriate support for the parallel reduction and oxidation (redox) half-cell reactions, during both modes of operation, that is during the charging and the discharging processes.
The use of redox couples of the same (multivalent) element, that is for the a negative electrode redox couple as well as for the positive electrode redox couple, offers a great simplification in the handling and storage of the dissolved species.
The vanadium redox flow battery also referred to as the all-vanadium redox cell or simply the vanadium redox cell or battery, employs V(II)/(III) and V(IV)/(V) as the two redox couples, in the negative (sometime referred to as the anolyte) and positive (sometime referred to as the catholyte) half-cell electrolyte solutions, respectively.
Numerous publications on the all-vanadium redox cell have recently been published. Among these, the following provide an update overview of the secondary battery field, also including comparative cost analysis with alternative renewable energy storage systems, as well as among the most promising redox flow batteries that are being developed.
GB-A-2,030,349-A discloses a process and an accumulator for storing and releasing electrical energy based on a solid polymer electrolyte flow redox battery; Chromium-chromium redox couples and Vanadium-vanadium redox couples being indicated as viable choices.
U.S. Pat. No. 4,786,567, EP-A-0,517,217-A1, U.S. Pat. Nos. 5,250,158, 5,318,865, as well as the following articles:
“Improved PV System Performance Using Vanadium Batteries” by Robert L. Largent, Maria Skyllas-Kazacos and John Chieng, Proceedings IEEE, 23rd Photovoltaic Specialists Conference, Louisville, Ky., May 1993;
“Electrochemical Energy Storage and Vanadium Redox Battery” by Maria Skyllas-Kazacos, unpublished article freely distributed for general information purposes;
“The Vanadium Redox Battery for Efficient Energy Storage” by Maria Skyllas-Kazacos, unpublished article freely distributed for general information purposes; and
“Status of the Vanadium Redox Battery Development Program” by C. Menictas, D. R. Hong, Z. H. Yan, J. Wilson, M. Kazacos and M. Skyllas-Kazacos, Proceedings Electrical Engineering Congress, Sydney, November 1994;
are all pertinent to the so called “Vanadium Redox System”.
The publication WO 95/12219 describes methods for preparing stabilized solutions of vanadium and related redox systems.
EP-A-0,566,019-A1 describes a method for producing vanadium electrolytic solutions.
WO 95/17773 describes a combined system for producing electric energy in a biofuel cell, based on a vanadium redox flow system.
Typically and in general a redox flow battery systems includes two separate tanks, namely a catholyte tank and an anolyte tank and a plurality of cell stacks or batteries.
The capacity of the two tanks must be sufficient to provide for the required renewable energy storage capacity.
The overall cell area and the number of cells must be such as to satisfy the peak current and the “nominal” DC voltage requisites, respectively, thus dictating the electrical configuration (series and/or parallel) of the plurality of stacks or batteries.
The two hydraulic circuits of the catholyte and of the anolyte, respectively, is must be substantially separated from one another, each having its own circulation pump or pumps.
In a system employing single catholyte and anolyte tanks, that is functioning in a recirculation mode, the catholyte and the anolyte flow through the respective compartments of the unit cells of each stack or battery. Depending on whether the secondary battery is being discharged by flowing a current in an external electrical circuit that includes an electrical load, or being charged by forcing a current through the battery, both the catholyte and the anolyte are respectively discharged or charged.
Conventionally, a positive half-cell electrolyte solution (catholyte) is said to be charging when the redox couple therein is being oxidized more and more to the higher of the two valence states and to be discharging when the redox couple therein is being reduced more and more to the lower of the two valence states. Conversely, a negative half-cell electrolyte solution (anolyte) is said to be charging when the redox couple therein is being reduced more and more to the lower of the two valence states and to be discharging when its redox couple is being oxidized more and more to the higher of the two valence states.
As an alternative, instead of been operated in a recirculation mode, a redox flow system may be operated in a “batch mode”.
According to this alternative mode of operation, both the negative half-cell electrolyte circuit and the positive half-cell electrolyte circuit include two tanks, respectively for the relatively spent or discharged solution and for the relatively charged solution. Pumps will be commanded to pump the positive half-cell electrolyte and the negative half-cell electrolyte from their respective spent electrolyte tanks to their respective charged electrolyte tanks during a charging phase of the battery and viceversa, when the battery is operated as an electrical energy source, to invert the direction of flow of the negative half-cell electrolyte and of the positive half-cell electrolyte streams so that the solutions be flown from the respective charged solution tanks to the respective spent solution tanks.
The batch mode of operation provide for a “volumetric” indication of the state of charge or of discharge of the system.
The stacks or batteries of individual cells comprise a plurality of cells in electrical series defined by a stacked repetitive arrangement of a conductive intercell separator having a generally bipolar function, a positive electrode, an ion exchange membrane, a negative electrode and another conductive intercell separator.
Each electrode is confined in a flow compartment, usually having an inlet manifolding space and an outlet manifolding space.
The actual voltage of each unitary redox flow cell during discharge when an electrical load is connected as well as the voltage that is needed to force a current through the cell during a charging phase, depends on the specific half-cell reactions (basically on the redox couple been used), however such a standard cell potential will be diminished during discharge and increased during charge by the energy losses associated with the internal resistance (R) of the cell, the overvoltage losses due to the finite kinetic of the half-cell reactions (activation overvoltage: &eegr;
a
)) and the mass transport limitations (concentration overvoltage: &eegr;
c
).
In practice, the actual voltage needed to charge the battery and the voltage delivered by the battery during discharge (charge), will be given in first approximation by the following equations:
E
o
cell
=E
o
cathode
−E
o
anode
−iR−n
a
−n
c
E
o
cell
=E
o
cathode
−E
o
anode
+iR+n
a
+n
c
While the terms E
o
cathode
and E
o
anode
representing the standard half-cell potentials will depend on the state of charge of the positive half-cell electrolyte and of the negative half-cell electrolyte besides temperature, the other terms reflect the kinetic limitations of the electrochemical reactions and the ohmic losses through the cell.
Redox flow batteries are customarily realized in the for

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