The vanadium redox flow battery
By Dr. Adam H. Whitehead
Small power plants
It is obvious that the vanadium redox battery particularly well suited
for stationary applications, which provides an operating time of
several years have and require frequent charging / discharging cycles.
The former, large-vanadium redox batteries were used as load balancing
in the network or as a large uninterruptible power supplies (UPS) for
entire factories - eg a battery pack with 3 MW and 1.5 MWh in Sanyo's
LCD plant in Tottori, Japan. The new generation of smaller batteries
is better suited for small island power plants - "remote area power
supplies" (RAPS) - together with renewable energy sources or diesel
generators. This RAPS provide enough power available for homes,
businesses and telecom equipment.
When considering a RAPS system with a diesel generator, a typical
total run time of the diesel generator set with approximately 18 000
hours - assuming regular maintenance. The diesel generator is very
efficient when it is operated close to maximum capacity, but with
greatly reduced efficiency at lower loads. By adding a vanadium redox
battery, the diesel generator can be operated in two points: with
maximum efficiency and off. Energy that is not used by the load is
stored in the battery.
When the battery is charged to a predetermined level, the diesel
generator is switched off and the load supplied from the battery.
Although for a diesel generator with a slightly higher power is
needed, as in the operation without battery, but this investment pays
for itself very quickly, because the fuel consumption goes down much
lower. With the lower fuel consumption also results in lower CO2
emissions. The on / off operation extends the lifetime of the diesel
generator and the service intervals, since the daily operation of the
diesel engine is reduced. This is specifically designed for unmanned,
remote applications important to represent where an exchange of parts
or fuel supplement difficult and expensive processes.
Figure 3 A small island power plant with a 5-kW redox battery (20
kWh), combined with a 6-kW diesel generator: The diesel generator is
only 40% of the time for supplying the load and charge the battery
(positive battery power). This operation works only with an energy
storage device that is suitable for frequent deep discharge cycles.
Figure 3 shows the charge state of a vanadium redox battery (5 kW, 20
kW) with a 6-kW diesel generator in a simulation with a non-constant
load of about 40 kWh / day and 3.5 kW peak power. The battery runs
through periodic deep discharge cycles - approximately two full cycles
/ day. The diesel generator is running in the optimal operating point
with maximum efficiency, but only 40% of the time. 60% of the time it
is turned off. This is a typical mode of operation, wherein the
vanadium redox battery last longer than a lead acid battery. A larger
amount of electrolyte - more energy - would reduce the frequency of
the diesel starts even further, but would have no effect on the
In areas with suitable climatic conditions, photovoltaic modules
represent a useful addition to a remote small power plant (RAPS).
Complementary services of the solar energy saves diesel fuel and leads
to even more durability and longer service intervals. The correct
dimensioning of the system of energy storage, solar cells and diesel
generator is crucial for an economic operation. Fortunately for some
software packages are available - eg HOMER  and RETScreen .
Also, wind turbines can be included as a current generator with. A
correct simulation is more difficult here but, since local influences
- buildings, trees, etc. - can have a significant impact on earnings.