Power Plant Engineering

9.4.1 REGENERATION
In regeneration, the heat energy from the exhaust gases is transferred to the compressed air
before it enters the combustion chamber. Therefore, by this process there will be a saving in fuel used in
the combustion chamber if the same final temperature of the combustion gases is to be attained and also
there will be a reduction of waste heat. Fig. 9.10. shows a regenerative cycle.

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GAS TURBINE POWER PLANT
277
A.C.
Exhaust
Fuel
Alternator
S.M.
Gas
Turbine
Air in
1
2
3
6
5
4
Starting motor
Regenerator
2a
2
3
6
5
5a
4
Temp. Reduced
due to Transfer
of Heat in H.E.
H
Regeneration cycle
Fig. 9.10
Fig. 9.11
For regeneration to take place T
5
should be greater than T
2
.
In the heat exchanger, the temperature of air is increased from T
2
to T
3
, and the temperature of
the exhaust gases is reduced from T
5
to T
6
. If the regeneration is perfect, the air would be heated to the
temperature of the exhaust gases entering the H.E. the effectiveness of the regeneration is defined as:
ε
= effectiveness
= 
Rise in air temperature
Max. possible rise
= 
3
2
5
2
T
T
T
T
−
−
For ideal regeneration,
T
3
= T
5
and T
6
= T
2
The common values of effectiveness would be from
70 to 85%. The heating surface of the generator, as well
as the dimensions and price of the gas turbine increases
with the regeneration fraction. But to justify the regen-
eration economically, the effectiveness should atleast be
50%. The regenerative cycle has higher efficiency than
the simple cycle only at low-pressure ratios. If the pres-
sure ratio is raised above a certain limit, then the regen-
erator will cool the compressed air entering the combus-
tion chamber instead of heating it and the efficiency of
the regenerative cycle drops. This is clear from Fig. 9.12.
It is clear from Fig. 9.11, that the compressor turbine works are not affected by regeneration.
However, the heat to be supplied in the combustion chamber is reduced and also it is added at higher
temperature as compared to the cycle without regeneration. Thus, the thermal efficiency of the cycle
increases. It will be equal to,
η
t
= 
4
5
2
1
4
3
C (T
T )
C (T
T )
C (T
T )
p
p
p
−
−
−
−
For ideal regeneration, T
3
= T
5
η
t
= 1 – 
2
1
4
5
(T
T )
(T
T )


−


−


0
0
10
20
30
40
5
10
15
20
Pressure Ratio
T
her
m
al
E
ff
ic
ienc
y
%
Regenerative
Simple