|
|
 |
 |
|
|
|
|
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. |
|
 |
|
