Page 62 - Energize March 2022

TECHNICAL

Longevity and maintenance electrolyte storage capacity. The vanadium redox battery (VRB) is the
Lifetimes in excess of 25 years are claimed for commercial products main type in this category.
and very little maintenance is required. The electrolyte can be easily The first commercial flow battery, the vanadium redox battery,
replaced and regenerated. appeared on the market in the 1980s and there are numerous
models available on the market. The all-vanadium redox battery is
Self-discharge illustrated in Figure 4.
Self-discharge is low because the electrolyte is stored separately
from the cell.

Environmental risk
Environmental risk is low because the materials used are non-toxic.
All components can be easily recycled, and the materials used do not
pose a fire hazard.

Maturity
The vanadium redox battery is a mature product, with numerous
installations. Other technologies on the market are at the initial com-
mercialisation stage, and developing products are at laboratory stage.

Construction time
No onsite construction other than assembly and electrolyte filling is
required. Figure 4: Vanadium/Vanadium RFB 2

Round trip efficiency The cathode electrolyte is an oxide of vanadium, and the anode
Power is consumed in the pumping operation, which could account electrolyte a vanadium salt. The charge-discharge process of
for the round trip efficiency of 70 to 75%. vanadium redox flow batteries (VRFBs) is commonly represented
by a combination of the following half-cell reactions:
Energy density
2+
+
−
+
Energy density is lower than other chemical batteries, usually in the (cathode) VO + e + 2H ↔ VO 2 + 2H 2O
3
range of 30 to 50 Wh/l compared to 150 to 200 Wh/l for Li-ion batteries. (anode) V − e ↔ V
−
2+
Figure 3 shows a comparison between flow batteries, PWS and
Li-ion batteries when used for bulk long duration storage. The VRFB is a mature product, with a number of manufacturers
world-wide. In South Africa there is one manufacturer of VRFBs.
4
The first installation is a 1 MW/4 MWh unit. It is due to be installed
as part of a hybrid solar/ESS system at the Vametco alloys mine later
this year. The same company is establishing a 200 MWh per annum
vanadium electrolyte production facility at Coega in the Eastern
Cape which is due to come on line this year. 4
Internationally a 200 MW/800 MWh VRFB is planned for
Dalian City in China, while a 100 MW/500 MWh VRFB plant is
under construction. Several other plants of similar capacity have
been planned. Development in marketing includes the rental of
5
the electrolyte, which comprises 35% of the cost of the battery, to
customers.

Figure 3: Comparison between technologies 2 Hybrid flow batteries
In these batteries the active material is plated onto the anode as a
Technologies metal during the charge cycle, and this limits the capacity. Capacity
Numerous versions of the redox flow battery have been investigated, is still higher than conventional chemical batteries.
but the most common technologies in current use are vanadium,
zinc bromine and iron. Zinc-bromine battery (ZB)
The zinc-bromine battery is a hybrid redox flow battery.
Full flow redox flow batteries Electrolytes are made to flow past carbon composite electrodes
In these batteries the active material remains in solution during in two adjacent compartments which are separated by a fine
both charge and discharge cycles, and capacity is limited only by the polyolefin membrane. The anode side electrolyte is water-based.

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