From second law of thermodynamics
we found that complete conversion of heat into work is not possible in a
continuous process. Also it has been proved that the most efficient cycle to
produce work is a reversible power cycle (Carnot cycle). Even in carnot cycle,
the efficiency of conversion can never be unity and hence to establish a
comparison of the work-energy conversion in actual processes, the maximum
theoretical work obtainable with respect to some datum must be determined.
Available and Unavailable Energy
The energy content of a system can be divided into two parts
· Available energy, which
under ideal conditions may be completely converted into work
· Unavailable energy which is usually rejected as waste.
Consider Q units of heat
energy available at a temperature T. Available part of energy can be obtained
by assuming that the heat is supplied to a Carnot engine. Work obtained from
the carnot engine is the available part. The quantity is the unavailable part. In a T-S diagram these quantities
can be represented as shown in the fig 6.1. The term T0 is the ambient temperature.
Hence it can be concluded that the available and unavailable part of energy
content of a system depends on the ambient conditions also.
Availability
The maximum useful work that can be obtained from the system such
that the system comes to a dead state, while exchanging heat only with the
surroundings, is known as availability of the system. Here the term dead state
means a state where the system is in thermal and mechanical equilibrium with
the surroundings.
Availability Change Involving Heat Exchange with Reservoirs
Consider a system undergoing a change of state while interacting
with a reservoir kept at TR and atmosphere at pressure po and temperature To. Net heat transfer to the system.
Irreversibility
Work obtained in an irreversible process will always be less than
that of a reversible process. This difference is termed as irreversibility
(i.e) the difference between the reversible work and the actual work for a
given change of state of a system is called irreversibility.
I=Wrev- Wact
Let a stationary closed
system receiving Q kJ of heat is giving
out Wact kJ of
work. From first law of thermodynamics.
