Environmental Engineering Reference
In-Depth Information
This term increases with p
S2
/p
S1
as shown in Figure 5.13. Thus, selecting for-
mations that can accommodate large pressure swings and high maximum
reservoir pressures will reduce land area requirements for CAES through
increased storage energy density.
Typical numbers for E
GEN
/V
S
are 2 to 4 kWh/m
3
for lower pressure ratios
such as those at Huntorf (p
S2
/p
S1
= 1.38; p
S2
= 66 bar; E
GEN
/V
S
= 3.74) and 6 to
9 kWh/m
3
for the newer designs such one proposed by Alstom that features
higher operating pressures and larger pressure ratios (p
S2
/p
S1
= 2.0; p
S2
= 110
bar; E
GEN
/V
S
= 8.44) [1,66].
Performance Indices for CAES Systems
The energy performance of a conventional fossil fuel power plant is easily
described by a single efficiency: the ratio of electrical energy generated to
thermal energy in the fuel. The situation is more complicated for CAES due
to the presence of two very different energy inputs. On the one hand, elec-
tricity is used to drive the compressors; natural gas or oil is burned to heat
the air prior to expansion. This situation makes it difficult to describe CAES
performance via a single index in a way that is universally useful—the most
helpful single index depends on the specific application for CAES. Before
turning to a discussion of alternative options for a single CAES performance
index, we should consider two performance indices that apply to each energy
input separately: the heat rate and the charging electricity ratio.
Heat Rate
The heat rate (HR) or fuel consumed per kilowatt hour of output for a CAES
is a function of many system design parameters, but the design choice that
most critically affects heat rate is the inclusion of a heat recovery system.
The addition of a heat recuperator allows the system to capture the exhaust
heat from the lp turbine to preheat the air withdrawn from the storage res-
ervoir. Heat rates for CAES operations without heat recovery systems are
typically 5500 to 6000 kJ/kWh LHV (e.g., 5870 kJ/kWh LHV for Huntorf).
See Table 5.2. Heat rates with a recuperator are typically 4200 to 4500 kJ/
kWh LHV (e.g., 4330 kJ/kWh for McIntosh). By comparison, a conventional
gas turbine consumes at least twice this level of fuel (~9500 kJ/kWh LHV)
because two thirds of the electrical output is used to run the compressor.
Because the CAES compression energy is supplied separately, the system can
achieve a much lower heat rate [1,56].
The addition of a heat recuperator reduced fuel consumption at McIntosh by
22% relative to operation without the component [58], but a high pressure com-
bustor was still required. Newer CAES designs feature higher inlet tempera-
tures at the lp turbine. The added heat generated at this stage facilitates the
removal of the hp combustor from the design. In addition to further reducing
