Exergy Based Analysis of an Open Cycle Gas Turbine Power Plant

Canadian Journal of Basic and Applied Sciences

©PEARL publication, 2015

CJBAS Vol. 03(10), 273-282, October 2015

ISSN 2292-3381

Exergy Based Analysis of an Open Cycle Gas Turbine Power Plant

Mukesh Gupta , Raj Kumar

Department of Mechanical Engineering, YMCA University of Science & Technology Faridabad, Haryana, India-121006

Keywords: Abstract

Exergetic analysis,

Open cycle gas turbine,

Exergy destruction

Open cycle gas turbine power plants play a significant role in the power generation

industry. In India alone, 10.5% of the total thermal power capacity is generated by gas

turbine power plants. Hence, analysis of these power plants is of significant interest

from thermodynamic point of view. Over the years, many researchers have analysed

the performance of open cycle gas turbine power plants using the approach based on

first law of thermodynamics which uses energy as the criterion for defining the

performance of the power plants. However use of this approach has its limitations as it

is unable to take into account the irreversibilities which are inherent part of the system.

To take into account these irreversibilities, a new approach was developed based on the

second law of thermodynamics. In this approach, exergy is the criterion for defining the

performance of a thermal system. This approach allows us to take into consideration

the irreversibilities associated with the various components of the system. In the current

study, an exergy based approach has been illustrated for an open cycle gas turbine

power plant. For formulation, a 25 MW open cycle gas turbine power plant has been

considered as an example. Detailed exergy analysis has been done for the variou s plant

components. Exergy destruction has been calculated for various components and the

effect of thermodynamic variables on the exergy destruction in various components has

been analyzed. Finally, equations have been developed which provide a correlatio n

between the exergy destruction in different components as a function of the

thermodynamic variables under consideration. The current study provides a robust

method which can be used to analyze open cycle gas turbines of different capacities.

1. Introduction

Open cycle gas turbine power plants play a significant role in the power generation industry. In

India alone, 10.5% of the total thermal power capacity is generated by gas turbine power plants.

Corresponding Author :

E-mail, [email protected] – Tel, (+91) 9999766752

Mukesh Gupta et al. - Can. J. Basic Appl. Sci. Vol. 03(10), 273-282, October 2015

274

Many researchers have put forward different approaches to analyze the performance of gas turbine

power plants. Most of these approaches are based on the first law of thermodynamics. However,

there is a major drawback with these approaches. In the first law of thermodynamics, energy is the

criterion based on which the performance of a thermal system is defined. However, energy based

approach fails to consider the effects of irreversibilities which are inherent with any thermal system.

Hence, for better understanding of the performance of a thermal system, a new approach based on

the second law of thermodynamics was proposed. In this approach, exergy is the criterion for

analyzing the performance of a thermal system.

Exergy analysis and its applications in calculating entropy generation has been described [1].

Exergy based methods have been used for optimization of a single and double effect vapor

absorption refrigeration system [2, 3, 4]. An elaborate method to analyze the operation of a plant

using various exergetic variables such as exergetic efficiency, the rates of exergy destruction,

exergy destruction ratio has been provided [5]. Different researchers have used thermodynamic

relations between the energy and exergy losses to analyze the performance of a modern coal fired

electrical generation station [6, 7, 8, 9, 10]. Various efficiencies of fossil-fuel power plants have

been studied in detail using the exergy concepts [11]. Comparison between conventional and

fluidized bed power plant have been made and improving techniques have also been given using

exergy based methods for the conventional plants [12] Graphical exergy analysis has been used to

locate inefficient segments in the combined cycle plant [13]. Exergy based analysis has been used

for performance analysis of different processes such as production of hydrogen and hydrogen-

derived fuels, electrical and thermal power generation, thermal energy storage [14, 15, 16]. To

evaluate the exergy losses in the individual components of a cogeneration system, exergy analysis

for each component in the subsystems has been done [17]. Estimation of avoidable and unavoidable

exergy destruction and investment costs associated with different thermal components has been

done [18]. The primary way of keeping the exergy destruction, in a combustion process, within a

reasonable limit is to reduce the irreversibility in heat conduction [19].

2. Methodology

A typical open cycle gas turbine power plant is shown in Figure 1.


275

Figure 1. Schematic layout of an open cycle gas turbine plant

Steady flow conditions can be closely approximated by devices that are considered for

continuous operation such as compressor, combustor and gas turbine of the power plant. The

conservation of mass principle for a general steady flow system with multiple inlets and outlets is

given in Equation 1.

. .

i em m (1)

Where m denotes the mass flow rate and subscripts i and e stand for inlet and exit respectively.

A general exergy balance equation, applicable to any kth component of a thermal system, has

been formulated [1] and is given by Equation 2.

, , ,e k k q k i k

e i

E W E E (2)

The thermo-mechanical exergy of any stream may be decomposed into its thermal and

mechanical components and is represented by Equation 3 and Equation 4.

. .

0 0

0

[ (ln )]T

p

TE mc T T T

T (3)

Where

.TE is the thermal exergy of a component.

. .

0

0

lnM

pE m RT

p (4)

Where . M

E is the mechanical exergy of a component.

The exergy destruction for the kth component is calculated from the exergy balance as given in

Equation 5.

. . .

, , ,D k i k e k

i e

E E E (5)


276

2.1. Illustrative Example

For analysis purpose, an open cycle gas turbine power plant of 25 MW capacity has been

considered. The system comprises of an air compressor, a combustion chamber and a gas turbine.

The mass flow rate of air is 212.95 kg/ s and air enters the compressor at a temperature of 200 C

and a pressure of 0.981 bars. The pressure increases to 4.81 bars through the compressor whose

isentropic efficiency has been taken as 80%. The inlet temperature to the gas turbine is 11230C and

a pressure of 1.01325 bars. The isentropic efficiency of the turbine has been taken as 80%. The

exhaust gases from the turbine are at 8170C and 1.10 bars. The fuel (natural gas) is injected at 200C

and 22 bars.

3. Results and Discussion

3.1. Exergy calculations for plant

The net flow rates for different streams entering and leaving the system are shown in Table 1.

Positive values indicate the exergy flow rates of the products and the negative values represent the

exergy flow rates of resources or fuel for a particular component.

Table 1. Property values and thermal, mechanical, chemical and net exergy flow rates at various state points in the

gas turbine power plant

State .

m (kg/s) P (bar) T (K)

.TE (MW)

. M

E (MW)

.CE (MW)

.

E (MW)

1 212.95 0.981 293.00 0.00 0.00 0.00 0.00

2 212.95 4.2 481.60 10.52 26.146 0.00 36.916

3 216.66 1.01325 1123.00 108.768 23.89 0.7488 133.406

4 216.66 1.1 817.60 55.80 1.5 0.7488 58.04

5 3.71 22 293.00 0.00 0.8240 190.53 191.39

Where .CE represents the chemical exergy [1] and

.

E represents the net exergy flow for a

stream.

Exergy balance values for each component are given in Table 2.

Table 2. Exergy balance for each component in the gas turbine power plant

Component .WE (MW)

.CE (MW)

.TE (MW)

. M

E (MW)

.DE

Compressor -40.363 0.00 10.52 26.146 3.697

Combustion chamber 0.00 -189.78 98.248 -3.083 94.615

Gas Turbine 66.498 0.00 -52.698 -22.39 8.59

Overall Plant 26.135 -189.78 56.07 0.673 106.902


277

The values of exergy destruction calculated in Table 2 are plotted in Fig. 2.

Figure 2. Exergy Destruction in various components and plant

From Figure 2, it can be seen that maximum exergy destruction takes place in the combustion

chamber followed by the gas turbine and the air compressor. Hence the combustion chamber is least

efficient from the exergetic view point.

3.2 Effect of thermodynamic variables on plant performance

The next step in the analysis is to study the effect of thermodynamic variables on the

performance of the gas turbine power plant. For this the following two thermodynamic variables

have been considered:

1. Compressor pressure ratio

2. Air inlet temperature

The effects of these two thermodynamic variables have been analyzed with respect to the

exergy destruction values for various components.

3.2.1. Effect of compressor pressure ratio (rp)

The effect of variation of the compressor pressure ratio on exergy destruction in compressor,

combustion chamber and gas turbine are shown in Fig. 3, Fig. 4 and Fig. 5 respectively.


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Figure 3. Exergy destruction in compressor Vs. Compressor pressure ratio

Figure 4. Exergy destruction in combustion chamber Vs. Compressor pressure ratio

Figure 5. Exergy destruction in Gas turbine Vs. Compressor pressure ratio

From Fig. 3, 4 and 5 it is clear that the exergy destruction increases for all the three components

of the plant with increase in compressor pressure ratio. The effect of increase in compressor

pressure ratio is felt most in the gas turbine. For the given range of compressor pressure ratios, the

increase in exergy destruction ratio is terms of percentage are given in Table 3.

Table 3. Maximum exergy destruction variation in various components


279

Component Maximum exergy destruction variation (% )

Compressor 70.002

Combustion chamber 90.03

Gas turbine 230.39

Based on the analysis done, following equations have been developed which express the exergy

destruction in compressor, combustion chamber and gas turbine as a function of compressor

pressure ratio. These equations have been checked for different ranges of compressor pressure ratio

and the results have been fairly satisfactory. These are represented as Equations (6), (7) and (8).

(6) .

= 15.5271 + 49.8326ln(r )D

CC pE (7)

.

= -22.1874 + 88.9799ln(r ) D

GT pE (8)

Where. D

CompE , . D

CCE ,. D

GTE represent the exergy destruction in compressor, combustion chamber and

gas turbine.

3.2.2. Effect of air inlet temperature

The effects of variation in inlet air temperature on the exergy destruction in compressor,

combustion chamber and gas turbine are shown in Fig. 6, Fig. 7 and Fig. 8.

Figure 6. Exergy destruction in compressor Vs. inlet air temperature

.

= 1.9308 + 1.9505ln(r ) D

Comp pE


280

Figure 7. Exergy destruction in combustion chamber Vs. inlet air temperature

Figure 8. Exergy destruction in gas turbine Vs. inlet air temperature

From Fig. 6, 7 and 8 it is clear that the exergy destruction decreases for all the three

components of the plant with increase in inlet air temperature. The effect of increase in inlet air

temperature is felt most in the combustion chamber. For the given range of compressor pressure

ratios, the increase in exergy destruction ratio is terms of percentage are given in Table 4.

Table 4. Maximum exergy destruction variation in various components

Component Maximum exergy destruction variation (%)

Compressor 82.9

Combustion chamber 86.07

Gas turbine 79.27

Based on the analysis done, following equations have been developed which express the exergy

destruction in compressor, combustion chamber and gas turbine as a function of inlet air

temperature. These equations have been checked for different ranges of inlet air temperatures and

the results have been fairly satisfactory. These are represented as Equations (9), (10) and (11).

.

= 5.4154 - 0.1596ln(T ) D

Comp aE (9)

.

=110.1955 - 2.5805ln(T ) D

CC aE (10)

.

= 117.5092 - 3.9577ln(T ) D

GT aE (11)


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4. Conclusions

This study makes use of this concept of exergy to analyze the open cycle gas turbine power

plant. It shows in detail the performance of various components of the open cycle gas turbine power

plant. Exergy destruction has been taken as an important parameter to understand the performance

of different components of the power plant. Combustion chamber has the maximum exergy

destruction followed by the gas turbine and compressor. It means that the combustion chamber is

the least efficient among the three components from exergetic view point.

Further the effect of two thermodynamic variables; (1) Compressor pressure ratio and (2) Inlet

air temperature, on the exergy destruction in different components have been studied in detail.

Maximum effect of variation in compressor pressure ratio is felt in the gas turbine and is felt least in

the compressor. Maximum effect of inlet air temperature is felt in the combustion chamber and least

in the gas turbine.

Finally, equations have been developed to correlate the exergy destruction in different

components as a function of the two thermodynamic variables. These equations provide a robust

mathematical model which is valid for different ranges of the thermodynamic variables under

consideration.

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Documents

Exergy Based Analysis of an Open Cycle Gas Turbine Power Plant