Saturday, December 3, 2011

The vanadium redox flow battery

Each charge / discharge cycle batteries usual stressed out because of
the charge transfer changes the volume of the electrodes. They are
usually not suitable for operation with many cycles and deep
discharge. Redox batteries, however, work with two liquid electrolyte
and can withstand changes in volume without stress. They are suitable
for many cycles, rapid and deep discharge cycles consequences.
By Dr. Adam H. Whitehead
A vanadium redox flow battery had been with the appearance of
conventional batteries and accumulators little in common. Rather, it
resembles a small chemical plant. Here you can find tanks that are
filled with green liquid blue - the color changes during charging -,
pumps, valves, sensors and reactors. And the battery is large: the
most commercial systems in the power range of 0.1 - 4 MW have a mass
of some 100 t. Recently, however, a new generation of redox batteries
in the range of 1 to 10 kW, small enough to be found in standard ISO
containers place.
What is now a vanadium redox flow battery? Like all rechargeable
batteries converts it to load the electricity into chemical energy and
makes electricity by reversing the chemical reaction during discharge
ready. The main difference from conventional batteries is that here
the chemical reactions take place only in solutions. In the vanadium
redox battery are two solutions, called electrolytes, are used: one
for the positive and negative reactions for. Chemically speaking, both
solutions are very similar: vanadium salts that are dissolved in
sulfuric acid - the same acid, which is also used in conventional lead
acid batteries.
Cell Power
Figure 1 Core of a redox battery is the reactor. In it, the charge
carrier exchange between the two liquid electrolytes, which are pumped
through the reactor.
The bulk of the electrolyte is stored in external tanks. During the
charging and discharging process, these pumped through the reactors
and then flow back into the same tank. In the reactors (Fig. 1) run
the critical chemical reactions take place for the loading and
unloading. Each reactor consists of a plastic block that contains many
individual cells, which are electrically connected in series. The
cells in turn have two chambers, filled with porous graphite
electrodes, but by a very thin, about 100 microns strong membrane are
separated. This membrane prevents contact and thus the internal short
circuit of the graphite electrodes. Among themselves, the cells are
connected with conductive bipolar plates, end plates which make the
connection to the external electrical connections.
While the positive and negative electrolyte flow through the reactor
internal distribution channels, the distribution of the river to take
on the individual cells. Within the cells, they flow through the
graphite electrodes, but remain separated by the membrane. Such a
mixing of the electrolyte. But this electric current can flow, the
selected ion membrane enables the transition from one electrolyte to
This exchange of electrons between the electrodes and electrolytes,
the salts of vanadium are chemically reduced or oxidized - hence the
name "Redox". The degree of oxidation of vanadium in the solution is
indicated numerically. For example, the metallic vanadium oxidation
state 0 The vanadium in the electrolyte, the positive oxidation states
4-5, while the negative in two stages - there are third This results
in each cell to increase the potential difference to about 1.4 V. This
potential difference changes continuously from 1.25 V to 1.55 V while
the cell is loaded. This effect is exploited for the precise
measurement of the charge state with the help of a simple cell.
The size of the electrodes and the membrane determines the performance
of the battery. By increasing the number of cells or the cells may
increase the overall performance will be raised. In contrast, the
energy content is determined by the amount of dissolved vanadium. Will
be increased by the addition of liquid electrolyte in the tanks, the
amount of energy. Power and energy of the battery can be sized
independently of each other and optimized for each application.

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