
where dQ ¼ heat transferred to system and dQ
0
¼ heat rejected from system, as in an engine (E
R
)
undergoing an infinitesimal Carnot cycle.
In reality, dQ
0
is the minimum amount of heat that can be rejected because processes 1 to 2 and 3 to
4 are both isentropic, i.e. adiabatic and reversible.
Hence the amount of energy that must be rejected is
E
unav
¼
Z
dQ
0
¼ T
0
Z
dQ
T
R
¼ T
0
DS (4.14)
Note that the quantity of energy, dQ, can be written as a definite integral becau se the process is an
isentropic (reversible) one. Then E
unav
is the energy that is unavailable and is given by cdefc. The
available energy on this diagram is given by abcda and is given by
E
av
¼ Q