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Comparison with
Other Vanadium Batteries
Bypass Currents
Electrical bypass currents can flow in the channels distributing the
electrolytes to the cells. In a conventional battery with electrolytes
fed to the cells in parallel these currents span not only individual
cells but also the whole stack. Because the channels must be wide to
feed the electrolytes uniformly to many cells at once the bypass
resistances are low. Moreover, the bypass potentials range from the
voltage of a single cell to the voltage of the whole stack.
Consequently, relatively large currents can occur. These currents
reduce the electrical storage efficiency of the battery.
A second undesirable effect of the bypass currents in batteries with
parallel electrolyte flow is corrosion of the electrodes, which are
normally made of carbon. This occurs when there is a high current
density in the region of an electrode near a channel carrying a bypass
current at a sufficiently high voltage.
In the Squirrel architecture with electrolytes flowing in series
through the cells the effects of bypass currents are negligible. The
voltage across the channels is never more than that of one cell and
wide channels are not needed so they have high electrical resistances.
There are no bypass currents between any non-adjacent cells in the
stack and the conditions for corrosion of the electrodes do not occur.
Variations in Operating Conditions between Cells
In parallel feed designs it is difficult (or impossible) to ensure that
all the cells receive the electrolytes at the same flow rate. Since all
the cells carry the same electrical current, the electrolyte in a cell
with a lower flow rate than average will be raised to a higher state of
charge than the electrolyte in the other cells. This electrolyte is
then partly discharged by mixing with the electrolyte from other cells
and there is a loss of efficiency.
In the most extreme case an undetected blockage in one cell during
charging will cause the evolution of hydrogen and oxygen gases and a
large pressure difference across the membrane. There will be an
explosion if the membrane breaks and the gases are ignited electrically.
Another cause of lost efficiency related to variations in the flow is
variation in the degree of polarization due to the formation of
concentration gradients within cells.
In the Squirrel series feed design all cells receive the electrolytes
at the same flow rate. Losses due to the mixing of electrolytes at
different states of charge cannot occur. If there is a blockage during
charging the whole flow is reduced or stopped and the charging current
must be cut for safety. This can be done with a single pressure switch
- a much simpler operation than monitoring many cells at once in a
parallel feed system. If it happens that hydrogen and oxygen gases are
produced by accident, then the Squirrel vertical stacking arrangement
allows the gases to flow naturally upwards (in the same direction as
the pumping of the electrolyte) where they can be released through
one-way valves.
Number of Cells
per Stack
In conventional vanadium battery designs the problems described above
associated with parallel feed limit the size of the stack to not more
than 20-30 cells. In the Squirrel series feed design, on the other
hand, more than 100 cells can be stacked together without ill effects
to give simple and compact high-voltage units.
Electrolyte
Flow Rate and Pumping Energy
In conventional parallel feed designs the rate of flow of the
electrolytes is typically 20 times the rate actually required for
charging and discharging. The purpose of this is (a) to minimize
variations in the flow rate between cells and guard against individual
cells becoming blocked, and (b) to maintain sufficient mass transfer at
the electrodes and reduce polarization losses. The large flow rate
demands for pumping at least 10% of the total power. Furthermore, since
in a single pass through the stack each portion of the electrolyte is
charged or discharged in only one cell, the electrolytes must be
recirculated through the stack many times for complete charging or
discharging.
In the Squirrel series feed design all the cells in the stack have the
same electrolyte flow rate. There is no need to pump the electrolytes
faster than the rate required for charging and discharging. The cells
have a low flow resistance, and the total power needed for pumping at
the optimum rate is only 1% of the total power. Volumetric pumps are
used to keep the flow rate steady. However, although the total flow
rate is low, the flow through individual cells is high because the
whole of the electrolyte flows through each cell. Consequently, mass
transfer at the electrodes is good and polarization effects are small.
Total Stack
Voltage
In parallel feed designs the total voltage across the stack depends on
the state of charge of the electrolytes. For example, in a stack of 20
cells the total voltage is 22V in the fully discharged state and 32V in
the fully charged state. This large variation in the voltage creates
serious difficulties in many applications.
In the Squirrel series feed design the problem does not occur. If fully
discharged electrolytes are fed into the bottom of a stack, the
operating voltage of the first cell is 1.1V. The flow rate and charging
current can be adjusted so that the electrolyte leaves the top of the
stack fully charged and the operating voltage of the last cell is 1.6V.
The total voltage is the average voltage per cell times the number of
cells. In a stack of 100 cells the total voltage will be constant at
135V throughout the whole process. To prevent mixing of the charged and
discharged electrolytes each electrolyte tank can be divided into two
separate tanks or compartments (not shown in Fig. 3), one for the
charged electrolyte and the other for the discharged electrolyte. |
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