IEEE Power & Energy Magazine - Grid Edge 2023 - 58

As shown in Figure 4, an RFB is a type of energy storage
device consisting of separate power and energy modules. The
power module is the stack, which provides energy conversion
between chemical and electrical energy. The stack typically
comprises multiple cells to meet power demand, each with
positive and negative half-cells separated by an ion-exchange
membrane. The electrolyte tanks constitute the energy module,
where chemical energy is stored in the liquid electrolyte.
Energy conversion occurs when liquid electrolytes are pumped
from the tanks to cells, where the electrochemical reaction
happens in the electrodes. The ion-exchange membrane prevents
the electrolytes from mixing and transports charged ions
to form an inner pathway between the positive and negative
half-cells. Taking VRB as an example, two redox couples are
formed by four oxidation states of vanadium ions, with V4+/V5+
in the positive electrolyte and V3+/V2+ in the negative electrolyte.
During charging, V4+ is converted to V5+ in the positive
half-cell, and electrons are introduced, converting V3+ to V2+ in
Positive
Half-Cell
Ion-Exchange
Membrane
Negative
Half-Cell
the negative half-cell. During discharge, the process is reversed.
Figure 5 illustrates a typical charge-discharge curve.
Compared with other rechargeable batteries (e.g., lead-acid
and Li-ion), commercial RFBs currently have lower energy
density. However, they show several distinct advantages.
✔ RFBs' power and energy capacity are completely independent
of each other. The power is determined by the
stack, while the energy capacity is determined by the
electrolytes. An RFB is thus easy to scale up to multimegawatts
and megawatt hours by modular design.
Furthermore, the requirements for RFB management,
especially the equalization circuitry/algorithm against
cell variability in state of charge (SOC) and/or temperature,
are substantially lower than those of other batteries.
✔ Because the electrodes of an RFB provide only an
electrochemically active surface for redox reaction
and only liquid-phase reaction during charge/discharge,
RFB chemistry has an exceptionally long calendar/cycle
lifespan. For example, a VRB has been
verified to withstand more than 200,000 cycles and
over ten years in demonstration projects in Japan and
Europe, respectively.
Tank
+
Positive
Electrolyte
Tank
-
Negative
Electrolyte
Tank
✔ There is no metallic dendrite formed on either electrode
during charge/discharge, averting internal shortcircuit
risks. Moreover, for most types of RFB, such as
VRB, the electrolyte is aqueous and nonflammable.
Therefore, the RFB intrinsically has excellent safety,
reliability, and large-scale applicability.
Pump
Pump
figure 4. A schematic representation of an RFB.
✔ RFBs exhibit good transient behavior, with very fast
response speed. The switching time between charge
and discharge is often less than 0.02 s. It is, therefore,
well-suited for balancing highly variable renewables.
Numerous demonstrations of RFB energy storage have
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
Charge Stage
Discharge Stage
been showcased around the world, predominantly for VRB.
In Japan in 2005, a 4-MW/6-MWh VRB system was installed
by Sumitomo Electric Industries at the Subaru WindFarm for
wind energy storage and power stabilization. In the United
States, a 1-MW/8-MWh VRB system for load-leveling application
was funded by the U.S. Department of Energy at the
Painesville, Ohio, municipal power station in 2010. In China,
several VRB energy storage stations at the mega-watt scale
have been established in the past ten years, as VRB was chosen
to be the extension energy storage technology by the government
of the People's Republic of China. Table 4 lists some
representative installations of VRB in recent years.
Voltage
05 10 15 20
Time (m)
25 30 35
figure 5. The voltage response of an all-VRB under charge
and discharge operations.
58
ieee power & energy magazine
Summary
Overall, NaS, Li-ion, and flow batteries are currently the major
battery technologies used in utility-scale energy storage.
To expedite large-scale deployment of EVs, Li-ion batteries
are gaining momentum in overcoming technological and cost
bottlenecks. Chemistry, materials science, manufacturing
skills, system integration techniques, and applications of
Li-ion batteries are being developed at a much quicker pace
than other batteries. Li-ion batteries are most widely used in
september/october 2017
Cell Voltage (V)
IEEE Power & Energy Magazine - Grid Edge 2023

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