more on an in place Vanadium redox flow storage system. (Couldn't make the link work.)
ELECTRIC OASIS from - IEE Power Engineer February/March 2005
www.iee.org/power
It’s been called the ‘Duracell in the Desert’, but a pilot
energy storage system in south-eastern Utah, US, is
anything but your average battery. Currently being
put through its paces by ScottishPower subsidiary
PacifiCorp, the so-called Castle Valley VRB
(vanadium redox battery) project is the first of its
kind in the US and has been designed to enhance voltage
control, power quality and local electrical service in the
region.
For more than 50 years, power has been delivered to
south-eastern Utah via a 209 mile, 25kV distribution line
known as ‘Rattlesnake’. The feeder is the longest
distribution circuit in PacifiCorp’s service area, situated
between Arches National Park, Manti-La Sal National
Forest and along the Colorado River scenic area. However,
as electrical demand on the circuit has increased, the utility
has been forced to find a way to increase load, and improve
reliability and power quality.
During its investigations, the company found that
increasing loads along the middle and end of the long line
only caused voltage regulation problems, so other options,
such as upgrading the line and building a new substation,
were considered. Unfortunately the traditional
alternatives to add capacity and improve service were
found to be costly and time consuming, requiring up to
five years just to obtain permits. What’s more, the use of
traditional distributed generation (DG), such as
microturbines or reciprocating engine generators, wasn’t
feasible due to additional permitting issues and a lack of
remote fuel sources.
So what was the answer? PacifiCorp finally decided that
an energy storage technology would make the most
attractive solution. A review of storage technologies –
including lead-acid, nickel-cadmium and sodium-sulphur
modular batteries – revealed these options could not provide
the necessary storage duration; eight hours of peak output.
As an alternative, the utility turned to flow batteries,
which proved more suitable. Zinc bromine and sodium
polysulphide-sodium bromide technologies were
considered, but the business finally selected vanadium
reduction-oxidation (redox) batteries, based on their cost,
scalability and environmentally friendly nature.
With the right technology chosen, the next step was to
find a suitable product. PacifiCorp selected a system
manufactured by Canada-based electrochemistry energy
storage business, VRB Power, known as the VRB Energy
Storage System. The utility’s decision was based on the life
span of the system as well as its low operating temperature,
speed of response to waveform deviations and ability to
compensate for flicker-producing loads. The system can also
easily be relocated and has a modular design,which means
components can be added or removed depending on
capacity and energy needs. Remote control and monitoring
is also possible.
~~
~~
DESERT REGENERATION
So what exactly is the VRB energy storage system? It is an
electrical energy storage system containing a vanadiumbased,
redox, regenerative fuel cell, with a proton exchange
membrane (PEM) (fig 2).
The system consists of two electrolyte storage tanks,
each containing active vanadium-sulphuric acid solutions,
with the vanadium in different oxidation states. These
energy-bearing liquids are circulated through the cell stack
by pumps (fig 3).
The stack consists of many cells, each of which is
separated by a membrane. This separation creates two halfcells.
In the half-cells, the electrochemical reactions take
place on inert carbon-felt polymer electrodes from which
current may be used to charge or discharge the battery.
The system employs vanadium ions in both half-cell
electrolytes. As a result, cross-contamination of ions
through the membrane separator has no detrimental
influence on the battery capacity, as is the case in flow
batteries employing different metals between electrolyte
types.
The vanadium half-cell solutions can also be re-mixed,
bringing the system back to its original state. Indeed,
mixing the electrolytes does not destroy their usefulness
and has no harmful effects. Electrolyte service life is
indefinite, making the vanadium system particularly
suitable for remote locations with extreme temperatures.
At a concentration of two moles per litre for each
species, the open circuit cell voltage is 1.6V when fully
charged. The relatively fast kinetics of the vanadium redox
reaction allows high Coulombic and voltage efficiencies to
be achieved without costly catalysts. Other system
advantages include high efficiency, long cycle life and ease
of scalability.
A key advantage is that the vanadium redox battery has
the lowest ecological impact of all energy storage
technologies and doesn’t rely on toxic substances such as
lead, zinc or cadmium. The storage system at Castle Valley
is rated at ± 250kW and ± 250kVAR with eight hours of
storage at peak loads. Electrolyte storage capacity is
140,000 litres and the system footprint is 200m2. Voltage
response is less than 5ms and the operating temperature
ranges from 5°C to 40°C.
The charge/recharge cycle – which can be adjusted
remotely or on-site – relies on a time-of-day, local time
algorithm, with control override based on battery state-ofcharge.
The system operates with defined charge and
discharge times each day, during the summer and winter
months. The scheduled kW output changes have a defined
ramp and fall rate to give the step-type voltage regulators
sufficient time to track any system voltage changes.
~~
The system’s actual response and state-of-charge
curves have correlated well with predictions. The
current system is capable of delivering 350kVA and has
a capacity of 15,500Ah or 2.3MWh, which exceeds initial
specifications. Further storage capacity increases may
be obtained by increasing the molarity of the existing
electrolyte.
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