14

CHAPTER 2

LITERATURE REVIEWThe importance of energy and exergy analysis of different supercritical

cycles is presented in Chapter 1. In view of this, literature on

supercritical cycle/ultra supercritical cycle/advanced ultra supercritical

cycles, and their improvements have been reviewed thoroughly and

presented in this chapter.

2.1 ENERGY SCENARIO

Lodhi[18] revealed that fossil fuels account for about 80%, renewable

energy resources contribute 14% and nuclear 6% of world annual energy

use. These numbers will soon change as the world’s population grows,

energy demand rises, inexpensive oil and gas deplete, global warming

effects continue to rise and urban pollution worsens the living

conditions. The development of alternative energy sources and devices

will emerge more rapidly to address the world’s energy and

environmental situation. Thus, the establishment of a sustainable energy

is one of the most pressing tasks of humanity. With the exhaustion of

fossil resources, the energy economy will change to a chemical and an

electrical base. Mahendra Lalwani and Mool Singh[119] revealed that,

India consumes 7% of coal of the world where as China, U.S, Japan and

rest of the world consumes 43%, 9%, 4% and 20%, respectively. 68% of

world’s consumption of coal for the generation of electricity. Coal-fired

15

generation increases by an annual average of 2.3 percent, making coal

the second fastest-growing source for electricity generation. World net

electricity generation upto year 2007 and projected generation upto year

2035 by different fuels is shown in the Fig. 2.1.

Figure 2.1. World net electricity generation by fuel (trillionkilowatthours)

Source : International Energy Outlook 2010

Wikipedia [101] reveals that, the energy policy of India is largely

defined by the country's burgeoning energy deficit and increased focus

on developing alternative sources of energy, particularly nuclear, solar

and wind energy. About 70% of India's energy generation capacity is from

fossil fuels, with coal accounting for 40% of India's total energy

consumption followed by crude oil and natural gas at 24% and 6%

respectively. India is largely dependent on fossil fuel imports to meet its

energy demands; by 2030 India's dependence on energy imports is

16

expected to exceed 53% of the country's total energy consumption. In

2009-10, the country imported 159.26 million tonnes of crude oil which

amount to 80% of its domestic crude oil consumption where as 31% of

the country's total imports are due to oil.

Maximum efficiency of the power cycle together with a minimum

investment costs and highest reliability are the overall design targets of

power plant. According to International Energy Agency [135] the

worldwide demand for power will increase significantly over the next

decades, and the current power plant capacity will double by the year

2030. To save primary energy resources i.e. to reduce fuel consumption,

and to reduce emissions, maximum power plant efficiency is a crucial

parameter. Therefore, steam parameters will have to be maximized to an

economically reasonable extent, so that supercritical, ultra supercritical

and advanced ultra supercritical Rankine cycles are essential to improve

the efficiency.

2.2 RANKINE CYCLE: ENERGY AND EXERGY ANALYSIS

The Rankine power cycle which converts the thermal energy into

mechanical energy, does not differ between critical, sub-critical,

supercritical, ultra supercritical and advanced ultra supercritical cycles.

Energy can neither be created nor destroyed. It just changes forms

such as potential, chemical, electrical energy, heat and work. Energy

analysis based on the first law of thermodynamics embodies the principle

17

of conservation of energy and is the traditional method used to assess

the performance and efficiency of the energy systems and processes.

The word ‘Exergy’ was derived from Greek words ex (meaning out)

and ergon (meaning work). Exergy is the useful work potential of the

energy. Exergy is not conserved. Once the exergy is wasted, it can never

be recovered. When we use energy we are not destroying any energy; we

are merely converting it to a less useful form, a form of less exergy. The

useful work potential of a system is the amount of energy we extract as

useful work. The useful work potential of a system at the specified state

is called exergy (also called availability or essergy). Exergy is a property

and is associated with the state of the system and the environment.

Exergy losses are additive (i.e. the total exergy loss for the plant is the

sum of all the component losses), enabling attribution of the losses to

plant components. Exergy is always destroyed when a process involves a

temperature change. This destruction is proportional to the entropy

increase of the system together with its surroundings. Second law

analysis is about understanding irreversibility in systems. It focuses on

changes in the quality of energy. The quality of energy is measured by

exergy. As energy is used in a process it loses quality and its exergy

decreases. There cannot be an "energy crisis" as energy is always

conserved. A system that is in equilibrium with its surroundings has zero

exergy and is said to be at the dead state. At the dead state, a system is

at the temperature and pressure of its environment and it has no kinetic

18

or potential energy relative to the environment. In fact Gouy and Stodola

[52] independently showed that the absolute value of this loss of exergy

is equal to the entropy production multiplied with the temperature of the

surroundings. The exergy analysis is a tool to identify losses and

destructions so that appropriate measures can be implemented to reduce

the losses and destructions. An exergy analysis is a very powerful way of

optimizing complex thermodynamic systems.

Exergy analysis helps in improving plant efficiency by determining

the origin of exergy losses, and hence providing a clearer picture. Exergy

helps in identifying components where high inefficiencies occur, and

where improvements are merited. The thermodynamic cycle can often be

optimized by minimizing the irreversibilities.

The ability to perform useful work in a natural environment has

been suggested and investigated as a measure of energy quality by

Gibbs, A. Stodola, G. Gouy, J.H. Keenan, F. Bosnjakovic and many other

researchers [52]. The term exergy was suggested by Zoran Rant in 1956

to denote ‘technical working capacity’ but the concept was developed by

J. Willard Gibbs in 1873. A complete definition was given by H.D.Baehr

in 1965; exergy is that part of energy that is convertible into all other

forms of energy. Exergy is a measurement of how far a certain system

deviates from a state of equilibrium with its environment (Wall, 1977).

Szargut et al. [32] stated that, "exergy is the amount of work obtainable

when some matter is brought to a state of thermodynamic equilibrium

19

with the common components of the natural surroundings’’. The

fundamentals of the exergy method were laid down by Carnot in 1824

and Clausius in 1865 [52].

Kapooria et al.,[19] have carried out thermodynamic analysis of

Rankine cycle to enhance the efficiency and reliability of steam power

plant. Further, they identified factors such as reheating and regeneration

affecting efficiency of the Rankine cycle and analysed for improved

working of the thermal power plants in subcritical range. The

thermodynamic deviations resulting in non-ideal or irreversible

functioning of various steam power plant components have been

identified by Kapooria et.al. Turbine, boiler and a pump are the

components of a steam thermal power plant.

Srinivas et.al [20,21] have carried out thermodynamic analysis of

Rankine cycle with generalization of feed water heaters in subcritical

range. They studied the effect of number of feed water heaters and bled

temperature ratio on overall performance of the Rankine cycle in

subcritical range. They have developed computer code for the evaluation

of first law efficiency, irreversibilities and second law efficiency of

Rankine cycle with different number of feed water heaters. They

concluded that, greatest increment in efficiency is brought by the first

heater; the increments for each additional heater thereafter successively

diminish. An increase in feed water temperature reduces the heat

20

absorption from the outgoing flue gases in the economiser and may

cause a reduction in boiler efficiency.

Habib et al. [3] have discussed first and second-law procedure for

the optimization of the reheat pressure level in reheat regeneration

thermal-power plants in subcritical range. The procedure is general in

form and is applied for a thermal-power plant having two reheat pressure

levels (low and high pressure levels) and two open-type feedwater

heaters. The second-law efficiency of the steam generator, turbine cycle

and plant were evaluated and optimized the reheat pressure ratio in both

the pressure levels. The irreversibilities in the different components of

the steam generator and turbine cycle sections were evaluated and

discussed.

Ibrahim Acar [2] has used the second law analysis of the reheat-

regenerative Rankine cycle in the subcritical range. He performed the

energy and exergy analysis for each component in the system at

operating parameters of (i) turbine inlet pressure of 150 bar, (ii) turbine

inlet temperature of 6000C and (iii) condenser pressure of 0.1 bar. He

inferred that, exergy analysis is better in comparison with energy

analysis, as it gives a clear understanding of real losses in the system.

A.Srivastava [25] has discussed the exergy analysis of various

types of coal from major mines of the world. He concluded that the first

law analysis gives only the quantity of energy, while the second law

defines the quality of energy. The projected increase in coal utilization in

21

power plants makes it desirable to evaluate the energy content of coal

both quantitatively and qualitatively.

Energy analysis is traditionally used in industries to carry out

performance comparisons and optimizations. The conventional methods

of energy analysis are based on the first law of thermodynamics, which is

concerned with the conservation of energy [51]. Conservation of energy

indicates that energy flow into and out of components should be equal.

The most effective way to meet the energy demand is to use energy more

efficiently. Using energy methods to evaluate efficiency has been

recognized for years, but the exergy method examines efficiency change

in a more practical way.

Nag and Gupta [1] have carried out exergy analysis of Kalina cycle

both in first law efficiency and second law efficiency. The method of

finding exergy efficiency through irreversibilities of all the components in

the cycle was adopted. They have done exergy analysis to reduce thermal

irreversibilities of the thermodynamic cycle was conceptualized for the

bottoming part of a combined cycle by Kalina, which used NH3-H2O

mixture as the working substance.

Dincer et al. [4] have analyzed the Rankine cycle single reheat in

sub-critical ranges in terms of the energy and exergy analysis. The

system parameters of boiler temperature and pressure values were

selected in the range between 4000C and 5900C, and 100 bar and 150

22

bar respectively. The possibilities to further improve the plant efficiency

and hence reduce the inefficiencies were identified and exploited.

Kotas [5] discussed exergy method as a relatively new analysis

technique in which the basis of evaluation of thermodynamic losses

follows from the second law rather than the first law of thermodynamics.

Mean molar isobaric heat capacity for evaluating enthalpy changes and

mean molar isobaric exergy capacity for evaluating changes in the

physical exergy values have been presented by him. He made description

of the exergy as well as enthalpy of the flue gas inlet and outlet of the

boiler. The exergy analysis [5] method is a useful tool for promoting the

goal of more efficient energy-resource use, as it enables the locations,

types and true magnitudes of wastes and losses.

Horlock et al. [6] described the exergy analysis of modern power

plants. The definition of open cycle rational efficiency is unequivocally

based on the ratio of the actual shaft work output from a power plant to

the maximum work that could be obtained in a reversible process

between prescribed inlet and outlet states. However, different constraints

may be applied to such an ideal reversible process, and the maximum

work obtainable will then vary, as well as the value of the rational

efficiency.

Kotas et al. [7] gave the nomenclature for exergy analysis. These

authors concluded that the concept of the exergy is dependent on that of

the environment. For calculating loss of exergy or process irreversibility

23

or exergy balance, the Gouy Stodala relation can be used and also offers

a glossary of terms in exergy and in exergy analysis and shows the

symbols in the context of expressions in which they might be used. The

proposed nomenclature for exergy analysis is in its present form, the

results of successive developments which were shaped by contributions

and exchanges of gas between the working party and practitioners of

technique of thermodynamic analysis. Several mathematical models and

studies on thermodynamics of regeneration Rankine cycle power plants

are presented by optimum reheat pressures in thermal-power plants in

below supercritical range [8,9].

Khan[11] described the second-law assessment of regenerative-

reheat coal-fired electricity generation plant in terms of irreversibility

analysis. He reported reduction in irreversible losses with the addition of

backward, cascade type feedwater heater. He concluded that,

incorporating reheating in a regenerative steam power cycle in subcritical

range can further improve its efficiency and the total irreversible losses

in the plant. These improvements become slower as the number of feed

water heaters increase. The reduction in the total irreversible rate due to

backward cascade feedwater heating is nearly 18%, which correspond to

a 12% improvement in thermal efficiency. These estimates were

increased to 24% and 14% respectively, with incorporation of reheat in

addition to feedwater heating. The second-law indicates that maximum

24

exergy is destroyed in the boiler and these thermodynamic losses are

significantly reduced by the incorporation of feedwater heating.

The thermodynamic deviations resulting in non-ideal or irreversible

functioning of various steam power plant components have been

identified by Hermann [22]. He concluded that known exergy reservoirs

and flows within our sphere of influence are more than enough to provide

energy services for the increasing population and activity of humankind.

Siva Reddy V et al. [26] have reviewed on energy analysis and exergy

analysis of thermal power plants. They reviewed a thermodynamic

analysis of a coal based thermal power plant and gas based cogeneration

power plant in terms of energy and exergy analysis for the different

components of the power plants in subcritical range. They concluded

that, the major energy loss was found to occur in condenser. The exergy

analysis showed that combustion chamber in both steam and gas

turbine thermal power plants is main source of Irreversibility. The

Irreversibility in condenser is insignificant as the low quality energy is

lost in the condenser. An Exergy method of optimization gives logical

solution improving the power production opportunities in thermal power

plants.

Rosen [27] has analysed the energy analysis and exergy analysis

based comparison of Coal-fired and Nuclear Steam Power Plants in

subcritical range. Sciubba et al. [28] have studied the parametric effect of

second law analysis of thermal power plants in sub critical range. Stecco

25

et al. [29] have developed a computer program for exergy loss in steam

turbine power plants.

Bejan [37] gave outlines of the fundamentals of the methods of

exergy analysis and entropy generation minimization. He explained the

concept of irreversibility, entropy generation, or exergy destruction in the

subcritical range. With examples he has illustrated the accounting for

exergy flows and accumulation in closed systems, open systems, heat

transfer processes, and power and refrigeration plants below

supercritical range.

Marc A. Rosen and Cornelia Aida Bulucea [64] have analysed the

benefits of using exergy to understand the efficiencies of electrical

technologies systems and to guide improvement efforts. They concluded

that the concepts of exergy have a significant role to play in evaluating

and increasing the efficiencies.

The exergy analysis and software development have been carried

out for the thermal power plants by H Kwak et al. [30]. However, the

computer program was developed by the authors was not made public

but simply inferred that, the exergy and the thermo-economic analysis

carried out systematically.

Moran, M.J. and Sciubba, E [46] were explained the basic

principles of exergy and practice. Regulagaddaa, P and Dincer, I [47]

were explained in the exergy analysis of a thermal power plant with

measured boiler and turbine losses. A second law analysis of a thermal

26

power plant has been done in this paper, along with a parametric study

that considers the effects of various parameters like operating pressure

and pressure on the system performance in subcritical range. The power

plant's energy efficiency was is determined to be 30.12% for the gross

generator output. The plant exergy efficiency for the system was 25.38%

for the gross generator output. The maximum exergy destruction was

found to occur in the boiler. As a result the authors inferred that, efforts

at improving the performance of the power plant should be directed at

improving the boiler performance, since this will lead to the largest

improvement to the plant's efficiency.

Sergio Espatolero et al. [55] have analysed the reduced flue gas exit

temperature of boiler between 800C -900C. They concluded that around

1.11% more efficiency was improved due to the reduction of the boiler

flue gas exit temperature. M.K. Gupta and S.C. Kaushikb [56] have

carried out exergy analysis for various bleed pressure and mass fractions

of bleed steam of proposed conceptual direct steam generator solar

thermal power plant having one feed water heater (FWH). The

investigations for bleed pressure and mass fraction of bleed steam were

also carried out by incorporating two and three FWHs. They concluded

that, significant improvement in efficiency by using three FWHs and

further gain in efficiency.

Ibrahim Dincer and Yunus A. Cengel [63] have discussed the

concepts of energy, entropy and exergy concepts and their roles. The first

27

law of thermodynamics refers to the energy analysis which only identifies

losses of work and potential improvements or the effective use of

resources, e.g., in an adiabatic throttling process. However, the second

law of thermodynamics, i.e., exergy analysis takes the entropy portion

into consideration by including irreversibilities. During the past decade

exergy related studies have received considerable attention from various

disciplines ranging from chemical engineering to mechanical engineering,

from environmental engineering to ecology and so on. As a consequence

of this, recently, international exergy community has expanded greatly.

Exergy analysis is based on the second law of thermodynamics, and

generally allows process inefficiencies to be better pinpointed. Exergy is

often treated as a measure of economic value. Some researchers have

portrayed costs of energy conversion devices as functions of their exergy

efficiencies [77].

Sengupta et al. [121] have carried out exergy analysis of a coal-

based 210MW thermal power plant in subcritical range. They have split

entire plant cycle three zones for the sake of their analysis such as (i)

only the turbo-generator with its inlets and outlets, (ii) turbo-generator,

condenser, feed pumps and the regenerative heaters, (iii) the entire cycle

with boiler, turbo-generator, condenser, feed pumps, regenerative

heaters and the plant auxiliaries. The exergy efficiency is calculated

using the operating data from the plant at different conditions, viz. at

different loads, different condenser pressures, with and without

28

regenerative heaters and with different settings of the turbine governing.

The load variation is studied with the data at 100, 75, 60 and 40% of full

load. They concluded that, the boiler causes the maximum destruction of

exergy amounting to almost 60% at all loads. The contribution of the

turbine including its control valves comes next, while the contribution of

the regenerative feed cycle with all the feed water heaters and pumps is

the least in the above order. But when the boiler is included the exergy

efficiency decreases on the heater withdrawal. Comparison of the exergy

efficiencies shows that less throttling at the control valves with sliding

pressure mode of operation helps to reduce exergy destruction in the

plant.

Hasan Huseyin Erdem et al. [129] have analyzed comparatively the

performance of nine thermal power plants in Turkey, from energetic and

exergetic viewpoint. The power plants considered are mostly conventional

reheat steam power plant fed by low quality coal. Thermodynamic models

of the plants were developed, energetic simulation results of the

developed models were compared with the design values of the power

plants and design point performance analyses based on energetic and

exergetic performance criteria such as thermal efficiency, exergy

efficiency, exergy loss, exergetic performance coefficient.

P. E. Liley [16] has proposed an alternative analysis of Rankine

cycles in subcritical range based on the concept of mean temperature of

heat input in contrast to the varying temperatures which actually

29

occurred. Rankine cycle with a condenser pressure of 0.04 bar and a

boiler pressures of 20, 60, 80 and 100 bar and turbine inlet temperature

of 500°C has been considered in this analysis. He inferred that, this

concept of analysis is very simple in comparison with other types of

analysis. Further he stated this proposed analysis does not hold good in

case of Rankine cycle with reheating.

Therefore, exergy analysis is as important as energy analysis for

design, operation and maintenance of different equipment and systems of

a supercritical steam power plant. It is important that the performance

monitoring of an operative power station includes exergy analysis besides

the conventional energy analysis.

2.3 MOTIVATION AND OBJECTIVES

A thorough literature survey has revealed that a substantial amount

of work has been carried out on thermodynamic analysis of Rankine

cycle in subcritical ranges of temperatures and pressures and

established that the energy and exergy efficiency of the cycle can be

enhanced by operating the Rankine cycle at higher temperatures and

pressures. Further, reheating the steam in between the stages of the

turbine also enhances the performance of the Rankine cycle.

But so far, not much of focus has been made on energy and exergy

analysis of supercritical, ultra supercritical and advanced supercritical

cycles. The important hurdle in implementing the supercritical Rankine

cycle for the generation of power in steam based power plants is in

30

developing the material that can sustain very high pressures and

temperatures involved in supercritical/ultra supercritical/advanced ultra

supercritical cycles. However, the hurdle in carrying out the analysis on

supercritical Rankine cycle is in obtaining the values of steam properties

at these operating conditions of high pressures and temperatures.

Though the thermodynamic relations exist for the calculation of steam

properties, they are very complicated and require several iterations. Even

a small gain in efficiency of the power plant will lead to substantial

savings in the expenditure. At this stage, it is pertinent to recall that the

efficiency of the power plant can be enhanced by operating it under

supercritical conditions. This magnitude of saving that can be derived by

operating the power plant with increased efficiency demands thorough

concentrated efforts from research community in this area. As the

metallurgical scientists are significantly progressing in the development

of newer material that can withstand higher temperatures and pressures,

the present investigation got motivated to carryout of thermodynamic

analysis of Rankine cycle under supercritical/ultra

supercritical/advanced ultra supercritical conditions.

Literature also revealed that the reheating of the steam in between the

stages of the turbine also enhances the efficiency of the cycle under

subcritical conditions. In view of this, the present investigation has set

an objective of analyzing the Rankine cycle with single and multiple

reheats also.

31

Further, it is proposed to study and analyze the effect of operating

parameters, mentioned in the next page, on the performance of

supercritical, ultra supercritical and advanced ultra supercritical power

cycle without reheat, with single reheat and with double reheat.

i. Steam turbine inlet temperature

ii. Steam turbine inlet pressure

iii. Condenser pressure

iv. Reheat pressure ratio

v. Temperature of flue gas at the entry of boiler

vi. Temperature of flue gas at the exit of boiler.

It is further proposed to optimize the reheat pressure ratio for single and

multiple reheats.

In order to achieve the above objectives it is proposed to develop a

computer program for predicting thermodynamic properties of steam

such as mean molar isobaric heat capacity, mean molar isobaric exergy

capacity, enthalpy, entropy, specific volume, saturation temperature and

saturation pressure etc., at given values of turbine inlet pressure,

turbine inlet temperature, condenser pressure, boiler flue gas inlet

temperature and boiler flue gas exit temperature.