(218d) Energy Efficiency of Battery Systems with Forced Convection Between Electrodes

The energy
efficiency of an alkaline battery system with forced convective flow between
the electrodes has been investigated. Rechargeable alkaline batteries, which depend
on the reaction between zinc and manganese dioxide (Zn/MnO₂), have
been widely used because of their excellent properties such as inexpensiveness,
long shelf-life, high-temperature performance, and environmental friendliness comparing
to the Leclanche cell or zinc
chloride type batteries.¹ A full recharge of commercial alkaline AA
or AAA type batteries takes, however, more than 10 hours; but, the charge/discharge
time is difficult to be reduced because the limiting current i_L (the maximum
charge-transfer rate) is fixed in a battery system.

In 2010
and 2011, Suppes introduced an interesting new battery configuration, which can
in principle reduce charge/discharge times by using coaxial liquid flow to
control the limiting current, i_L.^2,3
The Suppes system uses a mechanical pump to drive fluid between porous
electrodes as the battery operates, so as to manipulate the concentration
distributions of ions in the separator region. We applied the Nernst-Planck
dilute-solution theory^4-7 to model this battery configuration. The
model allows the limiting currents, i_L,
to be calculated in terms of a Peclet number, Pe_forced, which
quantifies the dimensionless rate of forced convection. In this presentation, we
will show quantitatively how a pump can be used to adjust the maximum charge/discharge
rate of the Suppes flow battery.

Even
though forced convection can in principle reduce the charge/discharge time as much
as needed, the rate increase is necessarily accompanied by a power loss due to
concentration overpotential, as shown in Fig. 1. The power loss is increased because
the larger Pe_forced leads to a non-uniform concentration
distribution in the battery, inducing a diffusion potential ΔΦ_diffusion. In addition, an analysis with the Ergun equation⁸
shows that significant power may be lost to fluid friction in the porous
electrodes and separator. The fluid friction causes a significant pressure-head
loss that increases with the forced-convection rate parameter Pe_forced;
therefore, the use of forced flow to enhance rates may be more effective for
increasing battery charge rates than for increasing rates of discharge.

Once the
energy losses are calculated, the task of evaluating the energy and power efficiency
of the battery presents itself. We investigate the optimal fluid velocity needed
to get the highest efficiency from the Suppes flow battery. Also, the comparison
of this battery efficiency with the Carnot efficiency will be discussed.

References

1. K. Kordesh and M. Weissenbacher, J. Power Sources 51(1-2)
(1994) 61-78

2. G. J. Suppes, B. D. Sawyer, M. J. Gordon, AIChE J. (2010) (in press).

3. B. D. Sawyer, G. J. Suppes, M. J. Gordon, M. G.
Heidlage, J. Appl. Electrochem.  (2011) (online).

4. W. Nernst, Z. Physik. Chem. 2 (1888), 613-637.

5. M. Planck, Ann. Physik, 275(2) (1890) 161-186; 276(8)
(1890) 561-576.

6. J. S. Newman
and K. E. Thomas-Alyea, 3^rd ed. Wiley-IEEE, (2004) p. 271.

7. A. J. Bard and
L. R. Faulkner, 2^nd ed. Wiley, (2000) p. 138.

8. S. Ergun,
Chem. Eng. Prog. 48 (1952) 89.