Page 61 - Energize March 2022

TECHNICAL

The flow battery comprises two elements: where the reactant is plated out as a metal on the anode during
• The cell, consisting of anode and cathode electrodes immersed charge, and the capacity is limited by the size of the anode.
in the active fluids, and the ion or proton exchange membrane While the emphasis of the industry has been toward grid
which allows passage of ions between the two fluids. The size of scale, these batteries are also well suited to many smaller energy
the electrodes determines the power rating of the cell. applications. The capacity requirements for off-grid applications in
• Active material storage. There are two storage areas, associated remote villages, farms and islands ideally suits flow batteries.
with the anode and cathode sections of the battery. The size of
the storage (volume) will determine the capacity of the battery. Characteristics of redox flow batteries
By convention, the fluid containing the active material is referred Before looking at different technologies, let us consider the
to as the electrolyte, or positive and negative electrolytes characteristics of flow batteries.
(anolytes and catholytes). Cells are generally stacked together as
shown in Figure 2 to form a battery. Commercial batteries range Cycle count
in size from 10 kW/40 kWh self-contained modular units to large This is the number of charge/discharge cycles that a battery can
400 MW/800 MWh multi-unit installations. endure before performance deteriorates. With conventional
batteries, the depth of discharge affects the cycle count negatively.
This does not apply to redox flow batteries, where the anode and
cathode material is unaffected by charge/discharge cycles. Claims of
full discharge cycle lifetimes of 20 000 or more are common.

Storage duration and capacity
In full flow, redox flow batteries’ storage duration is limited only
by the storage capacity, which may be limited by space in modular
transportable systems. Typical duration for modular units typically
ranges from 6 to 12 hours, although claims of 16 h have been made. 5
Hybrid redox flow battery duration is limited by the anode size.
Figure 2: Cells stacked together (CIT) Claims of 15 times maximum power capacity have been made for
these (15 MWh for 1 MW power capacity). The full state-of-charge
Electrolyte capacity is claimed to be available for discharge.
Active materials of redox flow batteries are salts of metals that
exhibit different oxidation states, such as iron which forms ferric Energy vs. power flexibility
(Fe X 2) and ferrous ( FeX 3) salts and vanadium which exhibits four Perhaps the most important characteristic is that energy capacity can
oxidation states (+2 to +5) .The salts are dissolved in acids to be scaled independently of the power, and cell characteristics such as
provide ionic conductivity and enable electrochemical reactions. electrode area do not need to be changed to modify capacity. Since
During the discharge mechanism in redox flow batteries, an the electrolyte is stored separately from the cell, energy capacity is
electron is released through an oxidation reaction on the anodic side disconnected from power output, and the redox flow battery can be
of the cell. The electron then passes through an external circuit and dimensioned to give any required combination of power and storage
is accepted at the cathode side of battery via a reduction reaction. capacity. Most modular units have a limit on storage capacity.
The discharge process involves the move from a higher chemical
potential state to a lower potential state. The charging mechanism Geographical flexibility
reverses both the direction of the current and chemical reactions. The redox flow battery is not limited by geographic features, other
than perhaps access to site, and can be transported in modular form
The general half-reaction mechanism for a redox flow cell is as and assembled on site.
follows:
(C=Catholyte, A=Anolyte): Modularity
These batteries are available as standard sized modules, including
Discharge: electrolyte storage, which can be combined to provide the required
C + e → C (Reduction) capacity. Manufacturers focus on standard sized cells which could be
2+
3+
–
A → A + e (Oxidation) combined with custom sized storage to provide very large systems.
–
2+
3+
Charge: Modules can be combined in any required combination to give the
C → C + e (Oxidation) size combined. Multiple cells can combined with common storage
3+
–
2+
A + e → A (Reduction) tanks to simplify design.
3+
2+
–
There are two types of redox flow batteries, one where the reactants Cost
remain in solution during charge and discharge cycles and the The lifetime cost of large systems is claimed to be competitive with,
capacity is limited only by the storage volume, and hybrid redox or lower than, other chemical technologies.
energize | March 2022 | 59