20-10-2012, 05:31 PM
Advanced Gas Turbine Cycles
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Ideal (Carnot) power plant performance
The second law of thermodynamics may be used to show that a cyclic heat power plant
(or cyclic heat engine) achieves maximum efficiency by operating on a reversible cycle
called the Carnot cycle for a given (maximum) temperature of supply (T-) and given
(minimum) temperature of heat rejection (Tmin)S. uch a Carnot power plant receives all its
heat (QB) at the maximum temperature @.e. TB = Tmm) and rejects all its heat (QA) at the
minimum temperature (i.e. TA = Tmin)t;h e other processes are reversible and adiabatic
and therefore isentropic (see the temperature-entropy diagram of Fig. 1.8).
Two objectives are immediately clear. If the top temperature can be raised and the
bottom temperature lowered, then the ratio T = (Tmin/Tmmis) decreased and, as with a
Carnot cycle, thermal efficiency will be increased (for given p). The limit on top
temperature is likely to be metallurgical while that on the bottom temperature is of the
surrounding atmosphere.
A third objective is similarly obvious. If compression and expansion processes can
attain more isentropic conditions, then the cycle ‘widening’ due to irreversibility is
decreased, cr moves nearer to unity and the thermal efficiency increases (for a given 7).
Cycle modifications or innovations are mainly aimed at increasing 6 (by increasing & or
decreasing lA
REWERSIBILITY AND AVAILABILITY
Introduction
In Chapter 1, the gas turbine plant was considered briefly in relation to an ideal
plant based on the Carnot cycle. From the simple analysis in Section 1.4, it was explained
that the closed cycle gas turbine failed to match the Carnot plant in thermal efficiency
because of
(a) the ‘6 effect’ (that heat is not supplied at the maximum temperature and heat is not
rejected at the minimum temperature) and
(b) the ‘u effect’ (related to any entropy increases within the plant, and the consequent
‘widening’ of the cycle on the T, s diagram).
Since these were preliminary conclusions, further explanations of these disadvantages
are given using the second law of thermodynamics in this chapter. The ideas of
reversibility, irreversibility, and the thermodynamic properties ‘steady-flow availability’
and ‘exergy’ are also developed.
Reversibility, availability and exergy
The concepts of reversibility and irreversibility are important in the analysis of gas
turbine plants. A survey of important points and concepts is given below, but the reader is
referred to standard texts [ 1-31 for detailed presentations.
A closed system moving slowly through a series of stable states is said to undergo a
reversible process if that process can be completely reversed in all thermodynamic
respects, i.e. if the original state of the system itself can be recovered (internal
reversibility) and its surroundings can be restored (external irreversibility). An irreversible
process is one that cannot be reversed in this way.
The objective of the gas turbine designer is to make all the processes in the plant as near
to reversible as possible, i.e. to reduce the irreversibilities, both internal and external, and
hence to obtain higher thermal efficiency (in a closed cycle gas turbine plant) or higher
overall efficiency (in an open gas turbine plant). The concepts of availability and exergy
may be used to determine the location and magnitudes of the irreversibilities.