Environmental Engineering Reference
In-Depth Information
electrical work [73]. For this measure of the thermal efficiency, the round-
trip efficiency is 81.7%.
An alternative formulation η
RT,2
of an electrical round-trip storage effi-
ciency introduces an output correction term E
F
* η
NG
. Instead of express-
ing the fuel input as an effective electrical input, the electrical output is
adjusted by subtracting the assumed contribution to the output attributable
to the fuel. Correspondingly the output attributable to the electrical input
is E
T
- E
F
* η
NG
[74].
E
−
E
E
η
η
,2
=
T
F NG
M
(5.5)
RT
However, this interpretation is a bit different since the term in the numerator
attempts to estimate the residual output of the plant not attributed to natural
gas output. Therefore, the conversion efficiency factor serves to separate this
component of the output that would otherwise be generated in a conven-
tional turbine from the overall output of the CAES unit. The efficiency of a
stand-alone natural gas fired combustion turbine (38%) is a more appropriate
benchmark than an exergy-based measure of the CAES system fuel conver-
sion. Using the same assumptions as for η
RT,1
and a CT conversion efficiency,
the round-trip efficiency is 82.8% which is consistent with the measure
derived above. Thus, depending on the index chosen for its measure, the
round-trip efficiency for CAES is ~82%.—in the same range as the round-trip
efficiencies cited for other bulk energy storage technologies such as pumped
hydroelectric storage (74%) and vanadium flow batteries (75%) [72].
AdditionalApproaches
Still another measure of the efficiency of CAES proposed by Schainker et al.
may be useful for an economic evaluation of CAES in load leveling or arbi-
trage applications. This approach is similar to η
RT,1
in that it adjusts the fuel
input by a correction factor:
E
E CR E
(5.6)
η
AD
=
T
/
+
F
M
In this case, however, the fuel input is converted to equivalent electricity
not by using the primary energy conversion efficiency for natural gas, but
