Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 682

Figure 12: Zinc-bromine battery [ 22 ].
are separated by a microporous membrane. Unlike VR and PSB fl ow batteries, the
electrodes in a ZnBr fl ow battery act as substrates to the reaction. As the reaction
occurs, zinc is electroplated on the negative electrode and bromine is evolved at
the positive electrode, which is somewhat similar to conventional battery opera-
tion. An agent is added to the electrolyte to reduce the reactivity of the elemental
bromine. This reduces the self-discharge of the bromine and improves the safety
of the entire system [22]. During discharge the reaction is reversed; zinc dissolves
from the negative electrode and bromide is formed at the positive electrode. ZnBr
batteries can operate in a temperature range of 20-50°C. Heat must be removed by
a small chiller if necessary. No electrolyte is discharged from the facility during
operation and hence the electrolyte has an indefi nite life. The membrane however,
suffers from slight degradation during the operation, giving the system a cycle life
of approximately 2000 cycles. The ZnBr battery can be 100% discharged without
any detrimental consequences and suffers from no memory effect. The effi ciency of
the system is about 75% [2] or 80% [3]. Once again, as the same reaction occurs
during charging and discharging, the charge/discharge ratio is 1:1, although a slower
rate is often used to increase effi ciency [3]. Finally, the ZnBr fl ow battery has the
highest energy density of all the fl ow batteries, with a cell voltage of 1.8 V.
4.5.3.1 Applications of ZnBr fl ow battery
The building block for ZnBr fl ow batteries is a 25 kW, 50 kWh module con-
structed from three 60-cell battery stacks in parallel, each with an active cell area
of 2500 cm 2 [22]. ZnBr batteries also have a high energy density of 75-85 Wh/kg.
As a result, the ZnBr batteries are relatively small and light in comparison to other
conventional and fl ow batteries such as LA, VR and PSB. Consequently, ZnBr is
currently aiming at the renewable energy backup market. It is capable of smooth-
ing the output fl uctuations from a wind farm [2], or a solar panel [22], as well as