Environmental Engineering Reference
In-Depth Information
CAES system
Compressor train
Expander/generator train
Air
Exhaust
P
c
P
G
Intercoolers
Heat recuperator
P
C
= Compressor
power in
Fuel (e.g. natural gas, distillate)
P
G
= Generator
power out
Aquifer,
salt cavern,
or hard rock
h
s
= Hours of storage
(at P
G
)
Air
Storage
FIGURE 5.2
CAES system configuration.
approximately two thirds of the output power from the expansion stage to
run compressors.
During the compression (storage) mode, electricity is used to run a chain of
compressors that inject air into an uninsulated storage reservoir, thus storing
the air under high pressure and at the temperature of the surrounding for-
mation. The compression chain makes use of intercoolers and an aftercooler
to reduce the temperature of the injected air, thereby enhancing compression
efficiency, reducing the storage volume requirement, and minimizing ther-
mal stress on the storage volume walls.
Despite the loss of heat via compression chain intercoolers, the theo-
retical efficiency for storage at formation temperatures in a system with a
large number of compressor stages and intercooling can approach that for
a system with adiabatic compression and air storage in an insulated cavern.
Furthermore, despite the higher input energy required per unit mass due to
cooling needs, overall fuel consumption is still dramatically lower since the
net output of CAES is three times that of a conventional turbine [32].
During expansion (generation) operation, air is withdrawn from storage
and fuel (typically natural gas) is combusted in the pressurized air. The
combustion products are then expanded (typically in two stages), thus re-
generating electricity. Fuel is combusted during generation for capacity, effi-
ciency, and operational considerations. Expanding air at the wall temperature
of the reservoir would necessitate much higher air flow to achieve the same
turbine output, thus increasing the compressor energy input requirements
to the extent that the charging energy ratio would be reduced by roughly a
factor of four [33]. Furthermore, in the absence of fuel combustion, the low
