Environmental Engineering Reference
In-Depth Information
rather by using the cost ratio CR ≡ (off-peak electricity price)/(fuel price) [75].
Although this index may be helpful for deciding how to operate a given
CAES unit over time, it varies significantly over both time and geographical
region and so is not a useful general plant characterization. A final descrip-
tion of CAES efficiency compares CAES output to the output of a thermody-
namically ideal CAES plant operating between ambient temperature T
o
and
T
max
[67]:
E
η
II
=
T
(5.7)
E
T REV
,
E
=
E
+
E
−
T
⋅
∆
S E
=
+
E
−
T E T
⋅
(5.8)
T REV
,
M
F
o
M
F
o
F MAX
Analysis of a conventional CAES system yields a second law efficiency of
η
II
= 68% with a recuperator and 59 to 61% without one.*
Ultimately, the choice of efficiency measure remains an open question
because thermal energy and electrical energy quantities cannot be com-
bined by algebraic manipulation. The formulations provided in this section
provide only a basis for comparison with other storage technologies. As
indicated above, the relevant expression is determined in large part by each
specific application.
AdvancedTechnologyOptions
Although commercial CAES plants have been operating for several decades,
the technology is still in an early stage of development. This is reflected in
the fact that the two existing plants are based largely on conventional gas
turbine and steam turbine technologies. Consequently, various technologi-
cal improvements may eventually enhance performance and reduce costs
over relatively few product cycles.
One option that has attracted interest is to reduce (and perhaps elimi-
nate) the CAES fuel requirements and associated GHG emissions by recov-
ery and storage of the high-quality heat of compression in thermal energy
storage (TES) systems. Heat recovery may be implemented at some or all
compression stages, thus allowing stored heat to be used in place of fuel to
reheat air withdrawn from the CAES cavern, thereby partially or completely
*
The range of efficiencies for a system without a recuperator reflects changes in system perfor-
mance due to varying storage pressures (p
S
= 20 to 70 bar). The change in efficiency was <1%
for a system with a recuperator.
