Therefore, the irreversibility of the cycle isThe total exergy destroyed during the cycle could also be determined from
Eq. 10–21. Notice that the largest exergy destruction in the cycle occurs
during the heat-addition process. Therefore, any attempt to reduce the
exergy destruction should start with this process. Raising the turbine inlet
temperature of the steam, for example, would reduce the temperature differ-
ence and thus the exergy destruction.
The exergy (work potential) of the steam leaving the turbine is determined
from Eq. 10–22. Disregarding the kinetic and potential energies, it reduces towhereThus,Discussion Note that 449 kJ/kg of work could be obtained from the steam
leaving the turbine if it is brought to the state of the surroundings in a
reversible manner.10–8 ■ COGENERATION
In all the cycles discussed so far, the sole purpose was to convert a portion
of the heat transferred to the working fluid to work, which is the most valu-
able form of energy. The remaining portion of the heat is rejected to rivers,
lakes, oceans, or the atmosphere as waste heat, because its quality (or grade)
is too low to be of any practical use. Wasting a large amount of heat is a
price we have to pay to produce work, because electrical or mechanical
work is the only form of energy on which many engineering devices (such
as a fan) can operate.
Many systems or devices, however, require energy input in the form of
heat, called process heat.Some industries that rely heavily on process heat
are chemical, pulp and paper, oil production and refining, steel making,449 kJ/kgc 4 1 2403.071.355 2 kJ>kg 1 290 K 231 6.74500.2533 2 kJ>kg#K 4
s 0 s@ (^) 290 K,100 kPasf (^) @ (^) 290 K0.2533 kJ>kg#K
h 0 h@ (^) 290 K,100 kPahf (^) @ (^) 290 K71.355 kJ>kg
1 h 4 h 02 T 01 s 4 s 02
c 4 1 h 4 h 02 T 01 s 4 s 02
V^24
2
gz 4
1524 kJ/kg
0 1110 kJ>kg 0 414 kJ>kg
xdest,cyclexdest,12xdest,23xdest,34xdest,41
414 kJ/kg
1 290 K2c11.21326.7450 2 kJ>kg#K
2018.6 kJ>kg
290 K
d
xdest,41T 0 as 1 s 4
qout,41
Tsink
b
578 | Thermodynamics
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