Civil Engineering Reference
In-Depth Information
If P and V represent the air pressure and volume, respectively, and if the
air compression from pressure P
1
to P
2
follows the gas law PV
n
= constant,
then the work required during this compression is the energy stored in the
compression air. It is given by the following:
(
)
nPV
−
PV
22
11
Energy stored
=
(10-8)
n
−
1
And the temperature at the end of the compression is given by the following:
n
−
1
=
T
T
P
P
n
2
2
(10-9)
1
1
When the elevated temperature air at the end of the constant volume
compression cools down, a part of the pressure is lost with the corresponding
decrease in the stored energy.
The smaller the value of n, the smaller the energy stored. The isentropic
value of n for air is 1.4. Under normal working conditions, n is about 1.3.
The electrical power is generated by venting the compressed air through
an expansion turbine which drives the generator. The compressed air system
may work under constant-volume or constant-pressure.
In the constant-volume compression, the compressed air is stored in pres-
sure tanks, mine caverns, depleted oil or gas fields, or abandoned mines.
One million cubic feet of air storage at 600 psi provide an energy storage
capacity enough to supply about million kWh
e
. This system, however, has
a disadvantage. The air pressure reduces as the compressed air is depleted
from the storage, and the electrical power output decreases with the decreas-
ing pressure.
In the constant-pressure compression, the air storage may be in an above
ground variable-volume tank or an underground aquifer. One million cubic
feet of air storage at 600 psi provide an energy storage capacity enough to
supply about 0.07 million kWh
e
. A variable-volume tank maintains a con-
stant pressure by the weight on the tank cover. If aquifer is used, the pressure
remains approximately constant while the storage volume increases because
of water displacement in the surrounding rock formation. During electric
generation, the water displacement of the compressed air causes a decrease
of only a few percentages in the storage pressure, keeping the electrical
generation rate essentially constant.
The energy storage efficiency of the compressed air-storage system is a
function of a series of component efficiencies, such as the compressor efficiency,
the motor-generator efficiency, heat losses, and the compressed air leakage.
The overall round trip efficiency of about 50 percent has been estimated.
1
4
⁄
