Handbook of Energy Storage for Transmission or ... - W2agz.com

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Figure 4
Typical flow configuration (Courtesy Telepower Australia)
It is likely that future VRB systems will be manufactured in several standard AC
configurations to eliminate project-specific engineering costs. Today’s systems,
however, include custom-specified PCSs and project specific DC designs.
As the battery is charged and discharged, the DC bus varies in voltage. The open circuit
voltage varies with the battery state-of-charge, and charging or discharging produces a
corresponding increase or decrease in bus voltage. The PCS must be designed to handle
the full voltage “window”. At the Stellenbosch demonstration, for example, the DC bus
voltage ranged from 650 to 850 VDC (1.08 to 1.42 volts per cell). Since the battery is
occasionally fully discharged to 0 VDC (for maintenance and transport), a mechanism
such as switched DC resistive loads must be provided to accommodate voltages below
the operating range of the PCS.
As charged electrolyte is stored in separate anolyte and catholyte tanks, no self-discharge
occurs during extended periods of downtime. This would be advantageous in
applications such as spinning reserve that require availability of stored energy, but do not
require instantaneous power on demand. Under these conditions, the pumps would be
powered down, causing the stacks to drain back into the tanks, and the battery would
retain its full charge without incurring ongoing parasitic pump losses. It could be
restored to full power in a matter of minutes by restarting the pumps and flooding the
stacks.
While it would be possible to design the hydraulic system to retain active electrolyte in
the stacks when the pumps were off, the battery would self-discharge over a period of
hours, depending upon the stack (and associated manifold) volume, the number of cells
(stack voltage), and the concentration of electrolyte. Furthermore, the energy storage
capacity would be negligible.
Vanadium Redox Battery 13