Environmental Engineering Reference
In-Depth Information
important in the lp expansion stage, in which most of the enthalpy drop
occurs and where approximately three quarters of the power is generated [68].
Increased turbine inlet temperatures (e.g., by using expander blade cooling
technologies) would enhance the turbine and CAES electrical efficiencies as
well [69].
TowardaSingleCAESPerformanceIndex
Several single-parameter performance indices have been proposed for CAES
(see Table 5.2). The simplest is an efficiency η index defined as the ratio of
energy generated by the turbine (ET)
T
) to the sum of electrical energy delivered
to the compressor motor (E
M
) and the thermal energy in the fuel (E
F
):
E
η =
T
(5.2)
E
E
+
M
F
Typical HR and CER values of, respectively, 4220 kJ/kWh and 1.5 imply
η = 54%. However, because of the substantial difference between the energy
qualities of the thermal energy in the fuel and the electrical energy supplied
to the compressor, their sum is not a meaningful number. To estimate the
total energy input to CAES, it is necessary to express both the fuel and com-
pressor electricity on an equivalent energy basis. One approach is to express
the electrical input as an equivalent quantity of thermal energy.
Primary Energy Efficiency
When CAES is used to convert baseload thermal power into peaking power
(in place of gas turbines or other peaking units) one can introduce a primary
energy efficiency η
PE
defined in terms of the thermal efficiency of the base-
load plant (η
T
). Compressor motor energy input E
M
is replaced by an expres-
sion for the effective thermal energy input required to produce E
M
. Thus,
the overall efficiency value reflects the system (grid + CAES) efficiency of
converting primary (thermal) energy into electrical energy:
E
(5.3)
η
=
T
PE
E
E
/
η
+
M
T
F
This methodology has been applied to CAES units charged by nuclear and
fossil fuel plants [33], CHP plants [65], and grid-averaged baseload power
