Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 679

4.5.1 VR fl ow battery
A VR battery is made up of a cell stack, electrolyte tank system, control system
and a PCS (see Fig. 10). These batteries store energy by interconnecting two forms
of vanadium ions in a sulphuric acid electrolyte at each electrode; with V 2+ / V 3+ in
the negative electrode, and V 4+ / V 5+ in the positive electrode. The size of the cell
stack determines the power capacity (kW) whereas the volume of electrolyte (size
of tanks) indicates the energy capacity (kWh) of the battery.
As the battery discharges, the two electrolytes fl ow from their separate tanks to
the cell stack where H + ions are passed between the two electrolytes through the
permeable membrane. This process induces self-separation within the solution
thus changing the ionic form of the vanadium as the potential energy is converted
to electrical energy. During recharge this process is reversed. VR batteries operate
at normal temperature with an effi ciency as high as 85% [2, 3]. As the same chem-
ical reaction occurs for charging and discharging, the charge/discharge ratio is 1:1.
The VR battery has a fast response, from charge to discharge in 0.001 s and also a
high overload capacity with some claiming it can reach twice its rated capacity for
several minutes [2]. VR batteries can operate for 10,000 cycles giving them an
estimated life of 7-15 years depending on the application. Unlike conventional
batteries they can be fully discharged without any decline in performance [21]. At
the end of its life (10,000 cycles), only the cell stack needs to be replaced as the
electrolyte has an indefi nite life and thus can be reused. VR batteries have been
designed as modules so they can be constructed on-site.
4.5.1.1 Applications of VR fl ow battery
As the power and energy capacities are decoupled, the VR fl ow battery is a very
versatile device in terms of energy storage. It can be used for every energy storage
requirement including UPS, load levelling, peak shaving, telecommunications,
electric utilities and integrating renewable resources. Although the versatility of
fl ow batteries makes it extremely useful for a lot of applications, there are a number
of competing devices within each area that perform better for their specifi c appli-
cation. Consequently, although capable of performing for numerous applications,
Figure 10: Vanadium redox fl ow battery [ 20 ].