Compressed Air Energy Storage - Large Energy Storage Systems - page 136

[70]. Assuming η T = 40% (as might characterize a modern supercritical steam
electric plant) and the other parameters considered in the earlier calculation
of η implies η PE = 35%.
In principle, this formulation of system efficiency can be applied to a wind/
CAES by using the atmospheric efficiency of the wind turbines η WT in place of
the thermal plant efficiency η T . This formulation proposed by Arsie et al. yields
a system efficiency of 39% [71]. However, the use of atmospheric efficiency in
this case does not serve the same function as using thermal efficiency. In the
case of fossil fuel or nuclear power as the source of compressor energy, thermal
efficiency provides a measure of the amount of primary fuel needed to deliver
a quantity of electrical energy E M . In contrast, the extraction of “fuel” in the
case of wind energy does not affect the environment or overall cost of the plant.
Consequently, this measure of the amount of atmospheric kinetic energy cap-
tured in providing E M is not very helpful and in the case of CAES supporting
renewables, is not the optimal formulation for CAES efficiency.

Round-Trip Efficiency

A CAES unit powered by wind energy may be compared to other electri-
cal storage options that might be considered for wind back-up, for example,
electrochemical or pumped hydroelectric storage. Such alternative storage
systems are typically characterized by a round-trip electrical storage effi-
ciency (η RT ) calculated as η RT = (electricity output)/(electricity input). To facili-
tate comparisons of CAES to other electrical storage devices, a round-trip
efficiency that employs an “effective” electricity input ≡ E M + η NG * E F may be
introduced. The second term is the amount of electricity that could be have
been made from the natural gas input E F , had that fuel been used to make
electricity in a stand-alone power plant at efficiency η NG instead of firing a
CAES unit. The round-trip efficiency η RT,1 so defined is

E

η

,1 =

T

(5.4) RT

E + η E

M NG F

This methodology has the advantage of providing an electricity-for-
electricity round-trip storage efficiency that isolates the energy losses in the
conversion of electricity to compressed air and back to electricity. Several
values for η NG have been proposed including the hypothetic Carnot cycle
efficiency [67] and the efficiencies of commercial simple cycle and com-
bined cycle power plants [10,72]. For typical natural gas power systems,
(heat rates in the range 6700 to 9400 kJ/kWh), CAES round-trip efficiencies
are in the range of 77 to 89%, assuming a 1.5 ratio of output to input electric-
ity and a heat rate of 4220 kJ LHV/kWh. An exergy analysis of conventional
CAES systems indicates that 47.6% of the fuel energy input is converted into