Environmental Engineering Reference
In-Depth Information
TABLE 4.1
Properties of Battery Systems
Energy/Mass
Peak Power/Mass
Cost
Efficiency
Battery Type
(Wh/kg)
(W/kg)
($/kWh)
(%)
Lead acid
40
250
130
80
Nickel/cadmium
50
110
300
75
Nickel/metal hydride
80
250
260
70
Sodium/sulfur
190
230
330
85
Lithium-ion
100
250
200
95
The lead-acid storage battery is a well-developed technology. There are about 200 million
lead-acid batteries installed in U.S. road vehicles, each storing about 1 kWh of energy, for a total of
about 7E(4) GJ of electric energy stored. This is about 1% of the daily electric energy produced in
the United States. If storage batteries were used to level the daily electric utility supply, a very large
increase in battery production, above that required for the automotive market, would be needed to
satisfy this requirement.
The energy efficiency of lead-acid batteries is about 75% at low (multi-hour) charge and
discharge rates. Manufacturing cost is about $50/kWh
$14/GJ. Battery life is usually about 1000
full charge-discharge cycles, which makes storage batteries more expensive than pumped storage
for electric utility storage systems.
If lead-acid batteries are overcharged, they emit hydrogen gas, which can be an explosion
hazard.
Other well-developed storage batteries employ an alkaline electrolyte, a metal oxide positive,
and a metal negative electrode. The nickel-cadmium battery, commonly used in portable electronic
equipment, consists of a NiOOH positive and Cd negative electrode, with a KOH electrolyte. Such
batteries can provide more energy storage per unit weight than lead-acid batteries, but at greater
economic cost. Table 4.1 lists several electric storage battery systems and their properties.
=
4.4.4
Mechanical Energy Storage
The common form of storing energy for use in electric utility systems is that of pumped hydropower.
It consists of a normal hydroelectric plant that is supplied with water impounded behind a dam at
high elevation and discharging to a body of water at a lower elevation through a turbine that drives
an electric generator. But unlike the usual hydropower plant, the water flow may be reversed and
pumped from the lower to the higher reservoir using electric power available from the utility system
during times of low demand. Operating on a diurnal cycle, the pumped storage system undergoes
no net flow of water but does not deliver as much electrical energy as it uses during the pumping
part of the cycle because its components (turbogenerator and pump-motor) are less than 100%
efficient.
Energy in a pumped hydropower system is stored by lifting a mass of water through a vertical
distance
h
in the earth's gravitational field. The gravitational energy stored in the reservoir water,
per unit mass of water, is
gh
, while the energy per unit volume is
gh
/ρ
, where
g
is the gravitational
8E(2) J/m
3
and
gh
acceleration. For
h
98 J/kg. These are extremely low
energy storage densities, requiring very large volumes of water to store desirable amounts of energy.
=
100 m,
gh
=
9
.
/ρ
=
0
.
