Environmental Engineering Reference
In-Depth Information
Table 4.1 An example of output of the thermodynamic database MALT. (http://www.kagaku.
com/malt/)
T
Δ r
Δ r
Δ r
K
J/K·mol
kJ/mol
kJ/mol
a) H 2 and 1/2O 2 to H 2 O as vapour
298.15
− 44.366
− 241.826
− 228.600
373.15
− 46.585
− 242.567
− 225.186
473.15
− 48.869
− 243.529
− 220.408
573.15
− 50.652
− 244.458
− 215.428
673.15
− 52.068
− 245.338
− 210.290
773.15
− 53.205
− 246.158
− 205.024
873.15
− 54.126
− 246.914
− 199.656
973.15
− 54.872
− 247.601
− 194.205
1073.15
− 55.479
− 248.222
− 188.686
1173.15
− 55.973
− 248.776
− 183.113
1273.15
− 56.374
− 249.267
− 177.495
b) H 2 and 1/2O 2 to H 2 O as liquid
298.15
− 163.179
− 285.830
− 237.178
373.00
− 156.047
− 283.439
− 225.240
373.00
(H2O:bp)
originating water molecules (H 2 O). At ultra-high temperatures, where the value of
Δ r G becomes positive, H 2 O will spontaneously decompose to 1/2O 2 and H 2 .
Definition of Efficiency The efficiency η of the fuel cell or thermal power system is
defined as the ratio of the extracted electric power (work = W ) to the heat available
from the combustion of the fuel (Δ r H ) at the standard temperature; η = W r H .
Here, care should be taken in the choice of the reference value Δ r H . In this case,
the value at the temperature of 298.15 K in Table 4.1a is a virtual value because
at 1 atm, H 2 O vapor becomes liquid water at room temperature; the latent heat of
condensation is thus included in Δ r H (Table 4.1b ) which thus has a higher value than
the figure for the vapour phase in Table 4.1a .
The nominal efficiency of the system depends on which reference value is used.
To avoid confusion, the value of Δ r H which is free of the heat of condensation is re-
ferred to as the Lower Heating Value (LHV); and the Higher Heating Value (HHV)
includes the heat of condensation. The efficiency which defines the basis on which
it was calculated then becomes η LHV or η HHV . When not distinguished, efficiency
values are in many cases calculated using the LHV to give the appearance of higher
efficiency so it is necessary to be careful when comparing different systems.
Efficiency of a Heat Engine Heat engines produce mechanical work by repeatedly
absorbing high-temperature heat obtained from the combustion of the fuel, dis-
charging a part of the heat at low temperatures, and returning to the original state.
Considering the balance of heat supply and use, the work ( W ) obtained from the
 
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