Environmental Engineering Reference
In-Depth Information
eliminating the need for natural gas [67]. To be economically feasible, fuel
cost reductions must offset the additional capital cost associated with a TES
system. Early studies found that very high fuel prices would be required
to justify such systems, making adiabatic CAES too costly for commercial
use [76-80].
More recent studies, however, suggest that new TES technologies along
with improvements in compressor and turbine systems may make the so-
called advanced adiabatic CAES (AA-CAES) technology economically
viable (see Table  5.3) [17,81]. One such AA-CAES concept with a high effi-
ciency turbine and a high capacity TES, achieves a round-trip efficiency of
approximately 70% with no fuel consumption (see Figure  5.14) [39]. But it
should be noted that the efficiency gain of adiabatic systems over multistage
compression with intercooling is small [82], and both the fuel use and GHG
emissions for wind/CAES systems are already very modest [10].
Another proposal is to use biomass-derived fuels to reheat the air with-
drawn from storage. This could reduce GHG emissions and decouple plant
economics from fuel price fluctuations [83]. It may also allow CAES to be
run on fuel produced locally, thereby facilitating the use of energy crops in
remote, wind-rich areas and eliminating the need for natural gas supplies.
However, as in the adiabatic case, the emissions benefit would be small
because the emissions level of wind/CAES is already quite low (~2/3 the
rate for a coal IGCC plant with CCS) [10]. Moreover, a biofuel plant dedi-
cated to a wind/CAES system would require fuel storage because biofuels
must be produced in large-scale plants that run flat-out in order to be cost
effective, while CAES expander capacity factors for backing wind are typi-
cally modest [10].
A CAES variant proposed for wind applications is to replace the electrical
generator in a wind turbine nacelle with a compact compressor. This would
enable the wind turbine to generate compressed air directly, thereby elimi-
nating two energy conversion processes. However, the reduced losses and
potential drop in turbine capital cost would have to offset the added capital
cost of the compact compressors and the considerable cost of the high pres-
sure piping network needed to transport the compressed air from each tur-
bine to the storage reservoir.
In contrast to the option of coupling intermittent wind to CAES to obtain
baseload electricity, CAES might also be coupled to baseload power systems
to facilitate the use of such systems to provide load-following and/or peak-
ing power—the function originally envisioned for CAES—e.g., by coupling
CAES to a coal IGCC plant [84,85].
Improving CAES turbo machinery is a promising area for innovation.
CAES turbine operating temperatures might be increased, thereby increas-
ing their efficiency by introducing turbine blade cooling technologies rou-
tinely deployed in conventional gas turbines but not in commercial CAES
units. Other advanced CAES concepts include various humidification and
steam injection schemes that may be used to boost the power output of a
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