Exergy Transfer and Balance. Exergy transfer by heat transfer, Q. The Carnot efficiency η c = 1-T 0 /T is the fraction of Q from T that can be converted to work, so…. Heat transfer through a finite temperature difference. Exergy transfer by work, W.
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The Carnot efficiency ηc = 1-T0/T is the fraction of Q from T that
can be converted to work, so…
Xwork = W – Wsurr for boundary work where Wsurr = P0(V2 – V1)
Xwork = W for other types of work
Work done by or against the
atmosphere is not available for
any useful purpose, and thus
should be excluded from
(or increase in entropy principle)
ΔEsystem = E2 – E1 = 0
ΔSsystem = S2 – S1 = Sgen
ΔXsystem = X2 – X1 ≤ 0
Or, the exergy of an isolated
system during a process always
decreases or in a reversible
process, remains constant.
Or exergy is always destroyed
in a real process.
Any system and its surroundings
constitutes an isolated system
Xdestroyed = T0Sgen≥ 0
Xdest is also called lost
work or irreversibility.
So, exergy of the universe
is always decreasing.
The general exergy balance is Xin – Xout – Xdestroyed = ΔXsystem
For a closed system, reduces to Xheat – Xwork – Xdest = ΔXsystem
Or: Σ(1-T0/Tk)Qk – [W – P0(V2 – V1)] – T0Sgen = X2 – X1
Can be used to find Wrev by setting Xdest = T0Sgen = 0
destroyed only in the system. If Xdest = 0 then the system is
Exergy destroyed outside
system boundaries can be
accounted for by writing an
exergy balance on the
extended system that
includes the system and its
You only need to multiply
Sgen by T0 to get
exergy of an incompressible substance.