Environmental Engineering Reference
In-Depth Information
The storage of magnetic energy is complicated by two effects. Maintaining the coil current
I
requires the expenditure of electric power to overcome its resistive losses, unless a superconducting
coil is used. There is a practical upper limit to the magnetic induction
B
related to the fact that
the coil windings are subject to an outward force per unit length proportional to
IB
that must
be supported by a containing structure having a volume comparable to
AL
. This limits magnetic
induction to values less than about 6 weber/m
2
and corresponding energy densities of 1.43E(7) J/m
3
and 1.8E(3) J/kg.
Superconducting magnetic energy storage systems have been proposed for use with elec-
tric utility systems. Designs project energy storage densities of 1 MJ/m
3
, prospective costs of
$180/kWh
=
$50/MJ, and an energy efficiency of 95%.
4.4.3
Electrochemical Energy Storage
We are familiar with the employment of batteries to supply electric power for a myriad of uses:
flashlights, portable radios, watches, hearing aids, heart pacemakers, toys, and so on. Batteries that
convert the energy of reactant chemicals to electrical work and are then discarded when discharged
are called
primary batteries
. In contrast,
secondary batteries,
whose chemical constituents can
be reconstituted by recharging with electrical power, are commonly termed storage batteries. The
most common storage battery is that used in the automobile, primarily for supplying engine starting
power.
A storage battery consists of two electrodes, a positive one and a negative one, each of different
chemical composition, immersed in an electrolyte. The electrodes provide the store of chemical
energy that is converted to electrical work as the battery is discharged. The electrolyte provides
an internal electric current of negatively charged anions and/or positively charged cations closing
the external electric circuit that consumes or provides electric work as the battery is discharged or
charged. In contrast with the fuel cell, which also converts chemical energy to electrical work but
which requires an external source of fuel and oxidant flow, the storage battery has only a limited
amount of chemical energy stored within it that, once consumed, brings to an end its supply of
electrical work. In this manner it resembles the other energy storage systems that we have been
discussing in this chapter.
To illustrate the principle of operation of a storage battery, we will consider the example of the
lead-acid storage battery. In its completely charged configuration, it consists of a positive electrode
composed of lead dioxide (PbO
2
), a negative one of pure lead (Pb), and an electrolyte that is
a concentrated solution of sulfuric acid (H
2
SO
4
) in water. The sulfuric acid is dissociated into
hydrogen cations (H
+
) and hydrosulfate anions (HSO
4
). The conversion of chemical energy to
electrical work occurs partially at each electrode. At the negative electrode, a hydrosulfate ion at
the electrolyte potential
el
reacts with the lead to form lead sulfate (PbSO
4
), a hydrogen ion (H
+
)
at the electrolyte potential
el
, and two conducting electrons in the negative electrode at its electric
potential
n
:
HSO
4
{
el
}→
H
+
{
el
}+
2
e
−
{
n
}
Pb
+
PbSO
4
+
(4.15)
At the positive electrode, two electrons at its potential
p
and a lead dioxide molecule combine
with three hydrogen ions and a hydrosulfate ion at the electrolyte potential
el
to form a lead sulfate
