Environmental Engineering Reference
In-Depth Information
During discharge, the two electrolytes fl ow from their tanks to the cell where
the reaction takes place at a polymer membrane that allows sodium ions to pass
through. Like VR batteries, self-separation occurs during the discharge process
and as before, to recharge the battery this process is simply reversed. The voltage
across each cell is approximately 1.5 V.
PSB batteries operate between 20 and 40°C, but a wider range can be used if a plate
cooler is used in the system. The effi ciency of PSB fl ow batteries approaches 75%
according to [2, 3]. As with VR batteries, the discharge ratio is 1:1, since the same
chemical reaction is taking place during charging and discharging. The life expectancy
is estimated at 2000 cycles but once again, this is very dependent on the application.
As with VR batteries the power and energy capacities are decoupled in PSB batteries.
4.5.2.1 Applications of PSB fl ow battery
PSB fl ow batteries can be used for all energy storage requirements including load
levelling, peak shaving, and integration of renewable resources. However, PSB
batteries have a very fast response time; it can react within 20 ms if electrolyte is
retained charged in the stacks (of cells). Under normal conditions, PSB batteries can
charge or discharge power within 0.1 s [2]. Therefore, PSB batteries are particularly
useful for frequency response and voltage control.
4.5.2.2 Cost PSB fl ow battery
The power capacity cost of PSB batteries is $1094/kW and the energy capacity
cost is $185/kWh [ 3 ].
4.5.2.3 Disadvantages PSB fl ow battery
During the chemical reaction small quantities of bromine, hydrogen, and sodium
sulphate crystals are produced. Consequently, biweekly maintenance is required to
remove the NaS by-products. Also, two companies designed and planned to build
PSB fl ow batteries. Innogy's Little Barford Power Station in the UK wanted to use
a 24,000 cell 15 MW 120 MWh PSB battery, to support a 680 MW combined-
cycle gas turbine plant. Tennessee Valley Authority (TVA) in Columbus wanted a
12 MW, 120 MWh to avoid upgrading the network. However, both facilities have
been cancelled with no known explanation.
4.5.2.4 Future PSB fl ow battery
Like the VR battery, PSB batteries can scale into the MW region and therefore must
have a future within energy storage. However, until a commercial demonstration
succeeds, the future of PSB batteries will remain doubtful.
4.5.3 Zinc-bromine (ZnBr) fl ow battery
These fl ow batteries are slightly different to VR and PSB fl ow batteries. Although
they contain the same components: a cell stack, electrolyte tank system, control
system, and a PCS (see Fig. 12) they do not operate in the same way.
During charging the electrolytes of zinc and bromine ions (that only differ in
their concentration of elemental bromine) fl ow to the cell stack. The electrolytes
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