Page 85 - Entrophy Analysis in Thermal Engineering Systems

Endoreversible heat engines 77

Novikov’s engines. For the Novikov’s engine, the thermal efficiency mono-
tonically increases, whereas the entropy production consistently decreases
with T EH /T L . The power output of the engine peaks at T EH /T L ¼2.1.
For the modified Novikov’s engine, with an increase in T EL /T L , the thermal
efficiency decreases whereas the entropy production increases. The power
output attains its maximum at T EL /T L ¼2.1. From this discussion, we arrive
at the conclusion that for the engine models of Novikov and modified
Novikov, the minimization of the entropy production is equivalent to
the maximization of the thermal efficiency, a similar conclusion that we
reached in Section 6.2 for the Curzon-Ahlborn engine.

6.5 Carnot vapor cycle
The operation of the Carnot vapor cycle is shown on a T-s diagram in
Fig. 6.7. Unlike the heat engine models examined in previous sections,
which interact with fixed temperature reservoirs, the model of Fig. 6.7
exchanges heat with a hot fluid and a cold fluid whose specific heat is den-
oted by c p,h and c p,l , respectively. The cycle undergoes an isothermal evap-
oration process at temperature T EH , and an isothermal condensation process

Fig. 6.7 A schematic Carnot vapor heat engine on a T-s diagram.