IEEE Power & Energy Magazine - September/October 2017 - 26

1.4

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.
✔✔ 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.
✔✔ 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
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.

1.3

Summary

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

Tank
-

+

Negative
Electrolyte
Tank

Positive
Electrolyte
Tank

Pump

Pump

figure 4. A schematic representation of an RFB.

1.8

Charge Stage

Discharge Stage

Cell Voltage (V)

1.7
1.6
1.5

1.2
1.1

Voltage
0

5

10

15

20

25

30

35

Time (m)

figure 5. The voltage response of an all-VRB under charge
and discharge operations.
26	

ieee power & energy magazine	

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



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