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

EPRI Proprietary Licensed Material
plumbing, structural elements, power electronics, and controls would have longer useful
lifetimes. It is possible to replace only the stacks, and keep the remainder of the system
in place.
Efficiency
Several losses must be accounted for in characterizing the VRB performance:
• Transformer losses. Most utility scale and industrial PCSs are designed with
outputs around 480 VAC. To connect to utility distribution voltages, a
transformer must be installed resulting in losses of a few percent. Even for nonutility
systems, isolation transformers are installed to prevent DC injection into
the AC grid.
• PCS losses. Whether charging or discharging, power flow through the PCS is
subject to losses related to voltage drops across the switching devices. PCS
throughput efficiency depends somewhat on load and PCS design, but is typically
in the 92-96% range.
• Battery DC losses. The energy to charge the battery is typically 20% greater
than the energy delivered during discharge. Internal battery losses include voltaic
losses such as ionic flow resistance and coulombic losses such as cell-to-cell shunt
currents (stray ionic flow through the stack manifold). Actual DC losses depend
on rate of charge and discharge (the system is slightly more efficient at lower
rates).
• Pumping losses. Pumping power is a relatively constant auxiliary load that is
drawn whenever electrolyte must be supplied to the stacks, i.e., during charge and
discharge. In some applications such as backup power, it is possible to charge the
battery, then turn the pumps off for long periods of time. The actual efficiency
penalty for pumping depends upon the operation of the pumping, the frequency of
cycling, and the pump design. At the 250 kW Stellenbosch demonstration in
South Africa (see below), four pumps each drew 2.2 kW (8.8 kW total).
The “round trip” (“turnaround”) efficiency – including transformer losses during charge,
PCS losses during charge, battery DC losses, PCS losses during discharge, transformer
losses during discharge, and pumping losses – is on the order of 70%.
Response Time
The battery can is capable of transitioning from zero output to full output in
microseconds – virtually instantaneously – provided the stacks are primed with reactants.
However, the power electronics respond within milliseconds, and the response time of the
controls and communications (sensing the load requirements and signaling the PCS to
take action) can be even longer.
Where response time is important, the control system must be programmed to keep the
pumps on and electrolyte flowing through the stacks. This requirement imposes a small
performance penalty due to the constant auxiliary losses of the pumps. If response time
is not critical, such as in peak shaving applications, then the stacks can be drained and the
Vanadium Redox Battery 11