Thermodynamics
SECOND LAW OF THERMODYNAMICS
Causes of Inefficiency
In the preceeding sections, cycle and component efficiencies have been discussed, but the actual
causes or reasons for the inefficiencies have not been explained. In this section we will compare
some of the types and causes for the inefficiencies of real components and cycles to that of their
"ideal" counterparts.
Components
In real systems, a percentage of the overall cycle inefficiency is due to the losses by the
individual components. Turbines, pumps, and compressors all behave non-ideally due to
heat losses, friction and windage losses. All of these losses contribute to the non-
isentropic behavior of real equipment. As explained previously (Figures 24, 25) these
losses can be seen as an increase in the system’s entropy or amount of energy that is
unavailable for use by the cycle.
Cycles
In real systems, a second source of inefficiencies is from the compromises made due to
cost and other factors in the design and operation of the cycle. Examples of these types
of losses are: In a large power generating station the condensers are designed to subcool
the liquid by 8-10°F. This subcooling allows the condensate pumps to pump the water
forward without cavitation. But, each degree of subcooling is energy that must be put
back by reheating the water, and this heat (energy) does no useful work and therefore
increases the inefficiency of the cycle. Another example of a loss due to a system’s
design is heat loss to the environment, i.e. thin or poor insulation. Again this is energy
lost to the system and therefore unavailable to do work. Friction is another real world
loss, both resistance to fluid flow and mechanical friction in machines. All of these
contribute to the system’s inefficiency.
Rev. 0
Page 95
HT-01
