Environmental Engineering Reference
In-Depth Information
The overall discharge reaction is
PbO
2
+ Pb + 2H
2
SO
4
→ 2PbSO
4
+ 2H
2
O
As shown, the basic electrode processes in the positive and negative elec-
trodes involve a dissolution-precipitation mechanism and not a solid-state
ion transport or film formation mechanism. As the sulfuric acid in the elec-
trolyte is consumed during discharge and produces water, the electrolyte is
an “active” material and in certain battery designs can serve as the capacity-
limiting material. As the cell approaches full charge and most of the PbSO
4
has been converted to Pb or PbO
2
, the cell voltage on charge becomes greater
than the gassing voltage (about 2.39 V per cell) and the overcharge reactions
begin, resulting in the production of hydrogen and oxygen (gassing) and the
resultant loss of water. In sealed lead-acid cells, this reaction is controlled to
minimize hydrogen evolution and the loss of water by recombination of the
evolved oxygen with the negative plate.
Stationary batteries are generally used to provide direct current (dc) power
for controls and switching operations, as well as standby emergency power,
in utility substations, power generation plants, and telecommunications sys-
tems. For the most part, these batteries operate under what is known as float
charge. A charger keeps them at the full charge voltage with a small charg-
ing current, so that they are ready for use when needed. A battery experi-
ences occasional discharges when a relay, breaker, or motor is energized and
during outages. In this application, energy and power density are of second-
ary importance to long life and low maintenance. Partly for these reasons,
stationary batteries have seen comparatively little development since their
introduction in the early twentieth-century. The construction of these batter-
ies tends to be very conservative. The care used in construction is reflected
in their extremely long service lives, often extending to 30 to 40 years. The
water lost to electrolysis during long periods of float charge must be replaced
regularly. The batteries contain large electrolyte reservoirs with a quantity
of excess electrolyte to extend the intervals between such maintenance
operations.
Sodium-Sulfur (NaS) Batteries
Ford Motor Company is credited with the first research into the potential of
the sodium-sulfur battery based on a β-alumina (β-Al
2
O
3
) solid electrolyte
in the 1960s.
5
The basic cell structure and associated electrochemistry of
the NaS cell are depicted in Figure 6.1. Liquid sodium is the active material
in the negative electrode and the ceramic β-Al
2
O
3
functions as the electro-
lyte. The cell is made in a tall cylindrical configuration, enclosed entirely
by an inert metal container, and sealed at the top with an air-tight alumina
lid. Such cells become more economical with increasing size. In commercial
applications, the cells are arranged in blocks for better conservation of heat.
