WIND ENERGY SYSTEMS - Cd3wd

Chapter 7—Asynchronous Loads 7–26
sulfate, which deposits as a white substance on the metallic lead. The reaction for this is
described in chemical terms by
Pb 2+ +SO 2−
4 −→ PbSO 4 (18)
The reaction at the lead dioxide electrode can be considered to proceed in two stages.
First, the lead dioxide combines with hydrogen ions from the sulfuric acid and electrons from
the external circuit to form lead ions and water, according to the equation
PbO 2 +2e − +4H + −→ Pb 2+ +2H 2 O (19)
Removing electrons from this electrode gives it a positive charge.
In the second part of the reaction, the lead ions just formed combine with sulfate ions
from the sulfuric acid to form lead sulfate.
Pb 2+ +SO 2−
4 −→ PbSO 4 (20)
The overall reaction of the cell during discharge can be written as
Pb + PbO 2 +2H 2 SO 4 −→ 2PbSO 4 +2H 2 O (21)
We see that both the lead and lead dioxide electrodes become covered with lead sulfate during
discharge. We also see that the concentration of sulfuric acid becomes lower during discharge,
since the chemical reaction uses up the sulfuric acid and produces water. The reaction will
slow down and eventually stop as the plates become covered with lead sulfate and as the
sulfuric acid is depleted.
The reverse process occurs when an external source of electricity is connected to the
terminals so that current flow is reversed. The lead sulfate is converted to lead and lead
dioxide on the appropriate electrodes and the concentration of sulfuric acid is increased. In
practice, the process is not completely reversible since some lead sulfate tends to flake off the
electrodes and sink to the bottom of the cell where it can not participate in future cycles.
Several hundred cycles are possible, however, in a properly built cell that is never allowed to
be fully discharged.
The density of sulfuric acid is higher than the density of water, so hydrometer (density)
measurements are commonly made to determine the state of charge of a cell. The quantity
actually used is the specific gravity, which is the ratio of the density of the electrolyte to the
density of water at 4 o C. The specific gravity of pure sulfuric acid is about 1.8, but this is
substantially higher than what is actually needed in a cell. The proper specific gravity of a
cell is a matter of engineering design. There must be enough sulfuric acid to meet the chemical
requirements of cell operation and not so much that the acid would destroy the cell materials.
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001