Environmental Engineering Reference

In-Depth Information

Because water is essentially a free commodity, the cost of storage is related to the civil engineering

costs of the reservoir and power house.

This system is by far the most commonly used energy storage in electric power utility systems.

The largest pumped storage system in the United States is the Luddington, Michigan plant that stores

15 GWh (

5.4E(4) GJ) of energy, providing 2000 MW of electrical power under an hydraulic

head of 85 m. But the total U.S. pumped storage power is about 2% of the total U.S. electric power,

implying an energy storage of 170 GWh (

=

6E(5) GJ). Because normal hydropower provides

about 11% of U.S. electrical power, pumped storage hydropower is not a negligible application of

hydropower machinery.

The overall energy efficiency of hydropower storage is about 70%; that is, the energy delivered

to the electric power system is 70% of that withdrawn during the storage process. This creditably

high value stems from the fixed speed and hydraulic head of the turbomachinery used to fill and

withdraw water stored in the reservoir.

Hydroelectric machinery is safe, reliable, and cheap to operate. The capital cost of pumped

storage installations is significantly related to the capital cost of the civil works (dam, reservoir,

etc.). If the capital cost is $500/kW of power, then the capital cost of pumped storage energy is

$23/GJ.

Flywheel energy storage systems are being developed for use in road vehicles and for emer-

gency electric power supplies. In this device, an axially symmetric solid material is rotated about

its axis of symmetry at a high angular speed

=

. The material at the outer edge of the flywheel rim,

2

whose radius is
R
, moves at the speed

R
and has the kinetic energy per unit mass of

(

R

)

/

2

2

and the kinetic energy per unit volume of

is the flywheel mass density. If the

flywheel rim has a radial thickness small compared with
R
, then it would experience a tangential

stress

ρ(

R

)

/

2, where

ρ

2
, which is twice the kinetic energy per unit volume. In other words, the max-

imum kinetic energy per unit volume equals

σ
=
ρ(

R

)

σ/

2 and the kinetic energy per unit mass is

σ/

2

ρ

,

where

σ

is the maximum allowable tangential stress in the flywheel. For high-strength steel, where

8E(3) kg/m
3
, the kinetic energy per unit volume is 4.5E(8) J/m
3

σ
=

9E(8) Pa and

ρ
=

and the

kinetic energy per unit mass is 5.63E(4) J/kg.

The energy input and output are usually in the form of electric power. These systems have very

high rotational speeds that decline as energy is withdrawn from them. Their overall energy efficiency

is comparable to other forms of storage. Energy storage densities are about 50 Wh/kg

0.18 MJ/kg.

To reach these energy storage densities, high-strength-to-weight materials, such as carbon fiber,

are used. In the event of a stress failure, the flywheel components become dangerous projectiles

moving at the flywheel peripheral speed, so safety can be a problem.

=

4.4.5

Properties of Energy Storage Systems

For some energy storage systems the storage capacity per unit volume or mass (J/m
3
, J/kg) are

important characteristics. For example, in electric-drive highway vehicles the mass or volume of a

battery or flywheel system needed to supply enough traction energy for a desirable trip length may

be too great for a practical design. Also, the capital cost of the energy storage system is relatable

to its mass and affects the dollar cost per unit of stored energy ($/MJ). Values of these properties

for the energy storage systems considered above are listed in Table 4.2.

Among the systems listed in Table 4.2, there is a large range in stored energy per unit volume and

per unit mass. Yet even the best of them does not approach the high energy density of a hydrocarbon