Environmental Engineering Reference
In-Depth Information
TABLE 4.2
Properties of Energy Storage Systems
Energy/Volume
Energy/Mass
Cost
Efficiency
(J/m
3
)
Type
(J/kg)
($/MJ)
(%)
Capacitor
4E(7)
4E(4)
95
Inductor
1E(7)
2E(3)
50
95
Pumped hydro
1E(3)
1
25
70
Lead-acid storage battery
3E(7)
2E(5)
15
75
Flywheel
2E(8)
2E(5)
80
fuel, based upon its heating value, of 5E(7) J/kg. It is this great energy density of fossil fuel that
makes highway vehicles, ships, and aircraft such productive transportation devices. On the other
hand, the energy needs of a portable digital computer can be satisfied easily by high-grade storage
batteries having a mass that is small compared to the computer mass.
The capital cost of energy storage is not greatly different among the systems listed in Table 4.2.
If we value electric energy at three cents per kilowatt hour (
$/MJ), then the capital
cost of a typical storage system is about 3000 times the value of the stored energy. If the stored
energy is discharged each day, as it would be in a supply leveling system for an electric utility,
the capital charge for storage would about equal the cost of electric energy, thus doubling the cost
of the stored energy in this instance. The economic cost of storing energy is an important factor
limiting its use in electric utility systems.
The efficiency of energy storage systems—the ratio of the output energy to the input energy—is
exhibited in the last column of Table 4.2. These are relatively high values, reflecting the best that
can be done with optimum energy management.
=
2.8E
(
−
2
)
4.5
CONCLUSION
The generation and transmission of electric power is a necessary component of the energy supply of
modern nations. Most such power is generated in thermal plants burning fossil or nuclear fuel, or in
hydropower plants. In either case, steam, gas, or hydro turbines supply mechanical power to electric
generators that feed electric power via transmission and distribution lines to the end consumer. The
electricity generation and distribution process is very efficient, with overall percentage losses being
in the single-digit numbers.
There is very little electric energy stored in the generation and transmission system, so electric
power must be generated at the same rate as it is consumed if line voltages and frequencies are to
be maintained. The diurnal pattern of electricity demand requires that the electric power network
be capable of supplying the peak demand, which may be 25% or more above the average demand.
Pumped hydroelectric plants are used to store energy that may be used to supply daily peak electric
power demand.
Electric energy may be stored for various purposes in storage batteries, capacitors, and induc-
tors, but the only significant amount of energy storage of these types is that of lead-acid batteries
in motor vehicles. Cost and weight of storage batteries has limited their use for primary power in
vehicles.
