Exergy analysis of gas turbine trigeneration system for combined production of power heat and refrigeration Abdul Khaliq* Department of Mechanical Engineering, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi 110025, India article info Article history: Received 31 August 2007 Received in revised form 30 May 2008 Accepted 21 June 2008 Published online 28 June 2008 Keywords: Trigeneration Gas turbine Absorption system Heat recovery Calculation Thermodynamic cycle Exergy abstract A conceptual trigeneration system is proposed based on the conventional gas turbine cycle for the high temperature heat addition while adopting the heat recovery steam generator for process heat and vapor absorption refrigeration for the cold production. Combined first and second law approach is applied and computational analysis is performed to investigate the effects of overall pressure ratio, turbine inlet temperature, pressure drop in combustor and heat recovery steam generator, and evaporator temperature on the exergy destruction in each component, first law efficiency, electrical to thermal energy ratio, and second law efficiency of the system. Thermodynamic analysis indicates that exergy destruction in combustion chamber and HRSG is significantly affected by the pressure ratio and turbine inlet temperature, and not at all affected by pressure drop and evaporator temperature. The process heat pressure and evaporator temperature causes significant exergy destruc- tion in various components of vapor absorption refrigeration cycle and HRSG. It also indi- cates that maximum exergy is destroyed during the combustion and steam generation process; which represents over 80% of the total exergy destruction in the overall system. The first law efficiency, electrical to thermal energy ratio and second law efficiency of the trigeneration, cogeneration, and gas turbine cycle significantly varies with the change in overall pressure ratio and turbine inlet temperature, but the change in pressure drop, process heat pressure, and evaporator temperature shows small variations in these param- eters. Decision makers should find the methodology contained in this paper useful in the comparison and selection of advanced heat recovery systems. ª 2008 Elsevier Ltd and IIR. All rights reserved. Syste ` me de trige ´ne ´ ration a ` turbine a ` gaz utilise ´ pour produire de I’ e ´nergie, du chauffage et du froid : analyse de I’ exergie Mots cle ´s : Trige ´ne ´ ration ; Turbine a ` gaz ; Syste `me a ` absorption ; Re ´ cupe ´ ration de chaleur ; Calcul ; Cycle thermodynamique ; Exergie * Tel./fax: þ91 11 26328717. E-mail address: [email protected]www.iifiir.org available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ijrefrig 0140-7007/$ – see front matter ª 2008 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2008.06.007 international journal of refrigeration 32 (2009) 534–545
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i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 2 ( 2 0 0 9 ) 5 3 4 – 5 4 5
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Exergy analysis of gas turbine trigeneration system forcombined production of power heat and refrigeration
Abdul Khaliq*
Department of Mechanical Engineering, Faculty of Engineering and Technology, Jamia Millia Islamia, New Delhi 110025, India
second law efficiency and electrical to thermal energy ratio.
Fig. 3 – Effect of variation of Turbine inlet temperature on first law efficiency, second law efficiency and electrical to thermal
energy ratio.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 2 ( 2 0 0 9 ) 5 3 4 – 5 4 5540
_ED;TV ¼ _mrðf � 1Þðj14 � j15Þ (46)
The terms used in Eqs. (1)–(46) have been defined in
nomenclature.
Fig. 4 – Effect of variation of % pressure drop on first law efficie
ratio.
5. Results and discussion
The effects of pressure ratio across the compressor (pC), tur-
bine inlet temperature (TIT), percentage pressure drop
ncy, second law efficiency and electrical to thermal energy
Fig. 5 – Effect of variation of process heat pressure on first law efficiency, second law efficiency and electrical to thermal
energy ratio.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 2 ( 2 0 0 9 ) 5 3 4 – 5 4 5 541
(Dp/p), process heat pressure ( pp), and evaporator tempera-
ture (TE) on the first law efficiency and electrical to thermal en-
ergy ratio (RET) is obtained by energy balance approach or the
first law analysis of the cycle. However, the exergy destruction
Fig. 6 – Effect of variation of evaporator temperature on first law
energy ratio.
or thermodynamic losses in each component, and the second
law efficiency of the trigeneration cycle have also been inves-
tigated under the exergy-balance approach or the second law
analysis of the cycle. To examine the effect of these operating
efficiency, second law efficiency and electrical to thermal
Table 1 – Effect of variation of pressure ratio on exergy destruction in different components of the cycle for TIT [ 1500 K,Dp/p [ 4%, pP [ 5 bar, TE [ 5 8C, patm [ 1 bar, Tatm [ 298 K
Table 3 – Effect of variation of percentage pressure drop in combustion chamber on exergy destruction in differentcomponents of the cycle for pC [ 16, TIT [ 1500 K, pP [ 5 bar, TE [ 5 8C, patm [ 1 bar, Tatm [ 298 K
Table 5 – Effect of variation of evaporator temperature on exergy destruction in different components of the cycle forpC [ 16, TIT [ 1500 K, Dp/p [ 4%, pP [ 5 bar, patm [ 1 bar, Tatm [ 298 K
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 2 ( 2 0 0 9 ) 5 3 4 – 5 4 5544
of exergy between the combustion products and compressed
air but its difference with exergy carried by fuel drops. It is
further shown that as the pressure ratio increases the exergy
destruction in HRSG decreases. This is because the higher
pressure ratio results in higher exergy of combustion products
and lower turbine exhaust exergy which leads to the higher
turbine output.
Table 2 shows the variation of magnitude of exergy de-
struction in each component of the system with the change
in turbine inlet temperature (TIT) for fixed values of pC¼ 16,
Dp/p¼ 4%, pP¼ 5 bar, TE¼ 5 �C. As the TIT increases the exergy
destruction in combustion chamber increases because the
mean temperature of heat addition increases. The exergy
destruction in HRSG increases because the temperature
difference between the two heat exchanging fluids (flue gas
and water/steam) increases, and for the given pressure ratio
of the cycle, more process heat is produced due to more steam
generated by HRSG at higher TIT. The exergy destruction in
compressor and turbine is constant. The exergy destruction
in generator, condenser, throttling valve, evaporator,
absorber, and heat exchanger is also constant.
Table 3 shows the variation of magnitude of exergy de-
struction in each component of the system, with respect to
pressure drop in combustion chamber and HRSG [pc¼ 16,
TIT¼ 1500 K, pP¼ 5 bar, TE¼ 5 �C]. It is shown that the exergy
destruction in all components of the system are more or less
independent of pressure losses in combustion chamber. It is
further shown that increase in pressure drop in combustion
chamber causes little increase in exergy destruction in
combustion chamber and HRSG. But the exergy destruction
in turbine decreases insignificantly with the increase in
pressure drop in combustion chamber.
The effect of process heat pressure on exergy destruction
in each component of the system is shown in Table 4. It has
been observed that the exergy destruction in all components
of gas turbine and cogeneration cycle is more or less indepen-
dent of the process heat pressure but increase in process heat
pressure causes significant decrease in the exergy destruction
in HRSG. This is consistent with the fact that larger process
heat pressure will lead to more entropy generation in HRSG.
The exergy of the flue gas coming to the refrigeration cycle in-
creases due to increase in process heat pressure. So, more
exergy is added to the refrigeration cycle. As a result, more
refrigerant will evaporate from the generator. So, exergy de-
struction in each component of refrigeration cycle increases.
It is further observed that the exergy destruction in each com-
ponent of vapor absorption refrigeration increases with
increase in process heat pressure. This is expected because
higher pressure for process heat results in higher flue gas
temperature. Consequently more heat will be added to the
generator and hence mass flow rate of refrigerant increase,
and that would result in higher exergy destruction in each
component of vapor absorption system.
Table 5 shows the variation of magnitude of exergy
destruction in each component of the system with respect to
evaporator temperature [pC¼ 16, TIT¼ 1500 K, pP¼ 5 bar,
Dp/p¼ 4%]. It has been observed that the exergy destruction
in the components of gas turbine cycle and cogeneration cycle
is constant with the increase in evaporator temperature but
there is slight variation of exergy destruction in the compo-
nents of refrigeration cycle.
6. Conclusion
The exergy-balance equation, which is applicable to any ther-
mal system has been applied to the trigeneration cycle for
combined production of power, heat and refrigeration. From
thermodynamic point of view, the combination of gas turbine
with absorption chilling machine in these trigeneration sys-
tems proves to be highly efficient, because the flue gas from
heat recovery steam generator is used as a heat source for
vapor absorption refrigeration as described in this study.
Combined first and second law analysis of the given system
leads to the following conclusions:
1. Maximum exergy is destroyed during the combustion and
steam generation process; it represents over 80% of the to-
tal exergy destruction in the overall system.
2. The exergy destruction in combustion chamber and heat
recovery steam generator decreases significantly with
the increase in pressure ratio but increases significantly
with the increase in turbine inlet temperature.
3. At a given TIT, pressure drop, process heat pressure, and
evaporator temperature, the exergy destruction in com-
pressor and turbine increases with the increase in
pressure ratio.
4. The exergy destruction decreases in HRSG and increases
significantly in the vapor absorption refrigeration compo-
nents with the increase in process heat pressure. The
exergy destruction seems constant in the compressor,
the combustion chamber, and the turbine.
5. The exergy destruction in the generator, the absorber, and
the condenser increases slightly with the evaporator tem-
perature, while it decreases in the throttling valve, the
evaporator and the heat exchanger solution.
6. The first law efficiency of cogeneration and trigeneration
decreases with the increase in pressure ratio but the
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 2 ( 2 0 0 9 ) 5 3 4 – 5 4 5 545
second law efficiency and electrical to thermal energy ra-
tio for these systems increases with the same.
7. The first law efficiency, electrical to thermal energy ratio,
and second law efficiency of cogeneration and trigenera-
tion increases with the increase in turbine inlet
temperature.
8. The first law efficiency, electrical to thermal energy ratio,
and second law efficiency of gas turbine, cogeneration and
trigeneration cycles are not at all affected with the pres-
sure drop in combustion chamber and HRSG.
9. The first law efficiency of cogeneration and trigeneration
decreases slightly with the increase in process heat pres-
sure but the second law efficiency and electrical to
thermal energy ratio increases with the same.
10. The first law efficiency and second law efficiency for co-
generation and trigeneration is found to be almost con-
stant with the variation of evaporator temperature but
the electrical to thermal energy ratio for trigeneration
decreases slightly with the increase in TE.
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