Thermodynamic

Characteristics of Power Plant

Doojeong Lee

Korea Atomic Energy Research Institute (djlee1@kaeri.re.kr

Contents

• Glossary of Terms

– Thermodynamic Properties

– Energy, Work and Heat

• Thermodynamic Systems and Processes

• Change of Phase

• Property Diagrams and Steam Tables

• Laws of Thermodynamics

• Steam Power cycles

Nuclear Power Plant

• Nuclear power plant produces electricity from heat by nuclear

reactions through energy conversion process

Nuclear

Energy

Thermal

Energy

Mechanical

Energy

Electrical

Energy Steam

Reactor Turbine S/G Primary System Generator

Reactor

Steam

Generator

Turbine

Coolant Pump

Condenser

Q=3,000 MW

Electricity

1,000 MW

2,000 MW Where?

Heat Tax

• The “Heat Tax”

– Conversion of

energy between

forms is inefficient

• Usually, some energy is

lost as heat

• The fewer energy

conversion, the better

Thermodynamics

• Thermodynamics

– the study of the effects of work, heat, and energy on a system

• Important Discoveries in Thermodynamics

– Work could be converted into an equivalent amount of heat and heat

could be converted into work

Classical Thermodynamics

only concerned with macroscopic (large-scale) changes and

observations

Glossary of Terms

Temperature

• A measure of the molecular activity of a substance

– The greater the movement of molecules, the higher the temperature

– It is a relative measure of how "hot" or "cold" a substance is

– It can be used to predict the direction of heat transfer

• Measured in Fahrenheit, Celsius, and Kelvin

Cold Hot

Temperature Scales

932 273

5

o o o oF C K C

Pressure

• Pressure is a measure of the force exerted per unit area on the

boundaries of a substance (or system)

– It is caused by the collisions of the molecules of the substance with the

boundaries of the system

• Units : N/m2

– Pascal, atm, bar, psi, mmHg

A

FP

F

A

Impact

Weight

abs atm gaugeP P P

Mass and Weight

• Mass (m)

– The measure of the amount of material present in that body

• Weight (wt)

– The force exerted by that body when its mass is accelerated in a

gravitational field

• Density ()

– Mass per unit volume

– units : g/cm3

V

M

High density Low density

wt mg

Volume and Specific Volume

• Volume (V)

– Volume is the quantity of three-dimensional space enclosed by some

closed boundary

– The volume of a container is generally understood to be the capacity of

the container

– Unit : m3 (cubic meter)

• Specific Volume ()

– Volume per unit Mass

– Unit : m3/kg

x

y

z

V xyz

1v

Energy

Energy (Joule)

– the capacity of a system to perform work or produce heat

• Kinetic energy : the energy of motion

• Potential energy : stored energy

– Rubber bands, Springs, Bows, Batteries, Gravitational Potential

2mv2

1KE

PE mgh

Internal Energy

• Macroscopic Forms of Energy

– Potential energy and kinetic energy

– can be visualized in terms of the position

and the velocity of objects

• Microscopic Forms of Energy

– Energy due to the rotation, vibration, translation, and interactions among

the molecules of a substance

– None of these forms of energy can be measured or evaluated directly,

but techniques have been developed to evaluate the change in the total

sum of all these microscopic forms of energy

• Internal Energy (U)

– These microscopic forms of energy are collectively called internal

energy

E U KE PE

P-V Energy, Enthalpy

• P-V Energy (Pressure times Volume)

– Because energy is defined as the capacity of a system to perform work,

a system where pressure and volume are permitted to expand performs

work on its surroundings

– Therefore, a fluid under pressure has the capacity to perform work

– P-V Energy also called flow energy

• Enthalpy (H)

x

P

P A

V

P = const

E KE PE U PV

KE PE H

H U PV

Enthalpy Internal Energy PV Energy

Work

• Work (W : Joule)

– Work is a form of energy, but it is energy in transit

– Work = Force Distance

– Work is not a property of a system

– Work is a process done by or on a system, but a system contains no

work

dFW cos21

Heat

• Heat (Q : Joule)

– Heat, like work, is energy in transit

– The transfer of energy as heat, however, occurs at the molecular level

as a result of a temperature difference

Heat Vibrating copper atom

Copper rod

T = 100oC

T = 0oC

Temperature

Profile in Rod

Heat vs. Internal Energy

• Heat

– The energy that is transferred from one body or location due to a

difference in temperature

– Heat is internal energy when it is transferred between bodies

• A Hot Potato

– does not possess heat

– rather it possesses a good deal of internal energy on account of the

motion of its molecules

Hot Potato Hot potato dropped in

a bowl of cold water

Heat Capacity

• Heat Capacity (C)

– The ratio of the heat (Q) added to or removed from a substance to the

change in temperature (T) produced

– Different mediums require different amounts of energy to produce a

given temperature change

Q - heat in Joules or calories

m - mass in kilograms

DT - change the temperature in Kelvin

C has units of J/kgK or kcal/kgK

1 calorie = 4.184 Joules

Tm

QC

Heat Capacity (Cv)

• for Constant Volume Processes (Cv)

• Heat is added to a substance of mass m in a fixed volume enclosure,

which causes a change in internal energy, U. Thus,

Heat, Q

added m m

T Insulation

2 1 vQ U U U mC T

Heat Capacity (Cp)

• for Constant Pressure Processes (Cp)

• Heat is added to a substance of mass m held at a fixed pressure,

which causes a change in internal energy, U, AND some PV work

m Heat, Q, added

T

m

x

pQ U P V H mC T

Entropy

• Entropy (S)

– A property of a substance like P, T, V, E

– can be defined as S in the following relationships

– Entropy quantifies the energy of a substance that is no longer available

to perform useful work

– Entropy is sometimes referred to as a measure of the inability to do

work for a given heat transferred

abs

QS

T

Classification of

Thermodynamic Properties

• Intensive Property

– Property which is independent of the amount of mass

– Temperature, Pressure, Density, Specific Volume

• extensive property

– Property which varies directly with the mass

– Mass, Total Volume

• It is customary to define some intensive properties associated with

extensive properties

– Specific Volume, Specific Internal Energy, Specific Enthalpy

– Intensive properties are useful because they can be

tabulated or graphed without reference to the amount of

material under study

Thermodynamic Systems and Processes

Thermodynamic System and

Surroundings

• Thermodynamic System

– A system in thermodynamics is nothing more than the

collection of matter that is being studied

• Thermodynamic Surroundings

– Everything external to the system

• System Boundaries

– The system is separated from the surroundings by the system

boundaries

– These boundaries may either be fixed or movable

– A control surface across that mass, momentum, heat and work flow

System

Surrounding

System Boundary

Types of

Thermodynamic Systems

• Isolated system

– System that is not influenced in any way by the surroundings

• No energy in the form of heat or work may cross the boundary of the system

• In addition, no mass may cross the boundary of the system

• Closed system

– System that has no transfer of mass with its surroundings,

– but may have a transfer of energy with its surroundings

• Open system

– System that may have a transfer of both mass and energy with its

surroundings

Control Volume and Control Surface

• Thermodynamic Equilibrium

– When a system is in equilibrium with regard to all possible changes in

state, the system is in thermodynamic equilibrium

• Control Volume

– A fixed region in space chosen for the thermodynamic study of mass

and energy balances for flowing systems

– The boundary of the control volume may be a real or imaginary

envelope

• Control surface : The boundary of the control volume

• Steady state

– that circumstance in which there is no accumulation of mass or energy

within the control volume, and the properties at any point within the

system are independent of time

Process and Cycle

• Thermodynamic Process

– The path of the succession of states through which the system passes is

called the thermodynamic process

• When the system changes its properties (T, P, V, etc.) from one value to

another as a consequence of work or heat or internal energy exchange, then it

is said that the fluid has gone through a "process.“

– The most common processes are those in which the temperature,

pressure, or volume is held constant during the process

• These would be classified as isothermal, isobaric, or isovolumetric processes,

respectively. Iso means "constant or one.“

• Cyclic Process or Cycle

– When a system in a given initial state goes through a number of different

changes in state (going through various processes) and finally returns to

its initial values, the system has undergone a cyclic process or cycle

Reversible Process

• Reversible Process

– A process that, once having taken place, can be reversed, and in so

doing leaves no change in either the system or surroundings

– In reality, there are no truly reversible processes. One way to make real

processes approximate reversible process is to carry out the process in

a series of small or infinitesimal steps

• Irreversible Process

– Friction, Unrestrained expansion of a fluid, Heat transfer through a finite

temperature difference, Mixing of two different substances

Not reversible unless

energy is expended

Dropping A Ball of Clay

Thermodynamic Processes

• Adiabatic process

– A process of no heat transfer into or out of the system

– When a system expands adiabatically, W is positive

– When a system compresses adiabatically, W is negative

• Isothermal process : constant temperature process

– Any heat flow into or out of the system must be slow enough to maintain

thermal equilibrium

• Isobaric process : constant pressure process

– Water boiling in a saucepan is an example of an isobar

process

• Isentropic process : constant entropy process

– This will be true if the process the system goes through is reversible and

adiabatic

Thermal Equilibrium

Two bodies are said to be at thermal equilibrium if they are at the same

temperature. → This means there is no net exchange of thermal energy

between the two bodies.

The two purple objects are at the same temp and, therefore are in

thermal equilibrium. There is no net flow of heat energy here.

hot cold heat

26 °C 26 °C

No net heat flow

Change of Phases

Phase Diagram (P-T Diagram)

Plasma Gas

Vapor

Liquid

Solid

Ttriple Tcritical

Ptriple

Pcritical

Pressure

Temperature

Critical

Point

Triple

Point

Boiling

Condensation

Sublimation

Melting

Freezing A point at which the density

of the liquid and vapor

phases are the same

A point at which all three phases

exist (solid, liquid and gas)

T-V Diagram

T

V

ABCD : Constant Pressure Line

(P=14.7 psia = 1 atm)

• Line AB : Sub-cooled Liquid

• Point B : Saturated Liquid

• Line BC : Saturation Line

• Point C : Saturated Vapor

• Line CD : Superheated Vapor

Saturation

• Saturation

– A condition in which a mixture of vapor and liquid can exist together at a

given temperature and pressure

– Saturation Temperature or Boiling Point

• The temperature at which boiling starts to occur for a given pressure

– Saturation Pressure

• The pressure at which boiling starts to

occur for a given temperature

• Relationship between saturation pressure and

saturation temperature

– The higher the pressure, the higher the

saturation temperature

Quality and Moisture of the Mixture

• Quality (x)

– The ratio of the mass of the vapor to the total mass of both vapor and

liquid

– Quality is an intensive property

• Moisture of The Mixture (M)

– The ratio of the mass of the liquid to the total mass of both liquid and

vapor

– The moisture content of a substance is the opposite of its quality

Property Diagrams and Steam Tables

Property Diagrams

• Property Diagram

– The phases of a substance and the relationships between its properties

are most commonly shown on property diagrams

• Basic Properties

– Pressure(P), Temperature(T), Specific Enthalpy(h), Specific Entropy(s),

Specific Volume(), Quality(x) for two phase

• Basic Property Diagrams

– P-T Diagram

– P- Diagram

– P-h Diagram

– h-T Diagram

– T-s Diagram

– h-s Diagram or Mollier Diagram

Pressure Temperature (P-T) Diagram

• The most common way to show the phases of a substance

P-T Curve for Pure Water

Pressure Specific Volume (P- )

Diagram

• Difference between P-T Diagram and P- Diagram

– There are regions on a P- diagram in which two phases exist together

P- Diagram for Water

Pressure Enthalpy (P-h) Diagram

• A P-h diagram exhibits the same features as a P-v diagram

P-h Diagram for Water

Enthalpy Temperature (h-T) Diagram

h-T Diagram for Water

Temperature Entropy (T-s) Diagram

• Diagram most frequently

used to analyze energy

transfer system cycles

– This is because the work done

by or on the system and the

heat added to or removed from

the system can be visualized on

the T-s diagram.

• By the definition of entropy,

the heat transferred to or

from a system equals the

area under the T-s curve of

the process

• T-s diagram exhibits the

same features as P-v

diagrams Entropy (s)

abs

QS

T

Tem

pera

ture

(T

)

Enthalpy Entropy (h-s) Diagram or

Mollier Diagram

• The Mollier diagram

– A chart on which enthalpy (h) versus

entropy (s) is plotted.

– It is sometimes known as the h-s diagram

– an entirely different shape from the T-s

diagrams.

• The chart contains

– a series of constant temperature lines

– a series of constant pressure lines

– a series of constant moisture or quality

lines

– a series of constant superheat lines

• The Mollier diagram is used only when

quality is greater than 50% and for

superheated steam

Entropy

Entropy

En

tha

lpy

Steam Tables

• Steam Tables

– Steam tables consist of two sets of tables of the energy transfer

properties of water and steam

• saturated steam tables

• superheated steam tables

– Tabulations of pressure (P), temperature (T), specific volume (ν),

specific enthalpy (h), and specific entropy (s)

• Saturated Steam Tables

– Divided into two parts

• temperature tables and pressure tables

– The values of enthalpy and entropy given in these tables are

measured relative to the properties of saturated liquid at 0oC

Saturated Steam Table

Superheated Steam Table

Laws of Termodynamics

Laws of Thermodynamics

• Zero Law of Thermodynamics

– If: Objects A and B are the same temperature and Objects B and

C are the same temperature

– Then: Objects A and C are the same temperature

• First Law of Thermodynamics

– Energy can neither be created nor destroyed

• Second Law of Thermodynamics

– Naturally occurring processes are directional

– These processes are naturally irreversible

• Third Law of Thermodynamics

– a temperature of absolute zero is not possible

Physics

• Physics (mechanics, thermodynamics, etc.) is our model of how the

universe operates

• It is not obvious that the universe should obey the “laws” of

physics

• The laws represent the accumulated observations on the universe.

Since all observations so far indicate that physics applies, we expect

that physics to apply in all cases.

Observation

(Experiment)

General Laws

(Theory)

Analytical Solution

Numerical Solution

The Zeroth Law of Thermodynamics

When two objects are separately in thermodynamic equilibrium with a

third object, they are in equilibrium with each other

Objects in thermodynamic equilibrium have the same temperature

Thermodynamic Equilibrium

The First Law of Thermodynamics

• The First Law of Thermodynamics

– referred to as the Conservation of Energy principle

– meaning that energy can neither be created nor destroyed, but rather

transformed into various forms as the fluid within the control volume is

being studied

• Energy Balance

in outE E Q W

W Q

System Boundary

inE out

E

E Q W

Internal Energy

Experiment (Joule)

• Cyclic Process of A Control Mass

• Observation

Q W Gas Gas

WQ

Internal Energy

P

V

1

2 A

B

C

2

1

2

1

)()( CA WQWQ

1

2

2

1

1

2

2

1

BABA WWQQ

A cycle of Path A and B

1

2

2

1

1

2

2

1

BCBC WWQQ

2 2 2 2

1 1 1 1

A C A CQ Q W W

A cycle of Path C and B

dE Q W

The First Law of Thermodynamics

• Any thermodynamic system in an equilibrium state possesses a

state variable called the internal energy (E)

• Between two equilibrium states, the change in internal energy is

equal to the difference of the heat transfer into the system and work

done by the system

The Second Law of Thermodynamics

• The Second Law of Thermodynamics : by R. Clausius

– It is impossible to construct a device that operates in a cycle and

produces no effect other than the removal of heat from a body at

one temperature and the absorption of an equal quantity of heat by

a body at a higher temperature

• The Second Law of Thermodynamics : by Max Planck

– It is impossible to construct an engine that will work in a complete

cycle and produce no other effect except the raising of a weight and

the cooling of a heat reservoir

Observation

• Work into Heat

– work may be converted to heat completely

• Heat into Work

– Heat cannot be converted to work completely

• The first law of thermodynamics can not provide any information

about the direction of the process

Q

W Gas

Gas

Direction of Spontaneous Process

Final State

Sucrose dissolves

(heat given off to surroundings)

Initial State Process

Heat flows

between blocks

Ice melts

(heat flows from water)

Cards shuffled

Partition removed

Water evaporates

(heat flows from dish)

Second Law of Thermodynamics

• There exists a useful thermodynamic variable called entropy (S). A

natural process that starts in one equilibrium state and ends in another

will go in the direction that causes the entropy of the system plus the

environment to increase

Sf = Si (reversible) Sf > Si (irreversible)

Entropy

• The second law of thermodynamics introduces the notion of entropy

(S), a measure of system disorder (messiness)

• Internal Energy (U) is the quantity of a system’s energy, Entropy (S)

is the quality of a system’s energy

• C.P. Snow :

– not knowing the 2nd law of thermodynamics is the cultural

equivalent to never having read Shakespeare

Heat Engine

• The Second Law of Thermodynamics : by Max Planck

– It is impossible to construct an engine that will work in a

complete cycle and produce no other effect except the raising of

a weight and the cooling of a heat reservoir

• Heat Engine

– A system that performs the

conversion of heat or thermal

energy to mechanical work

Heat

Engine

High Temperature reservoir

Low Temperature reservoir

1H C C

hot H H

Q Q QWEfficiency

Q Q Q

Carnot’s Principle

• Question

– What’s the maximum efficiency such a heat engine can be?

• Carnot’s Principle

– No engine can be more efficient than a reversible engine

operating between the same high temperature and low

temperature reservoirs

– The efficiencies of all reversible engines operating between the same

constant temperature reservoirs are the same

– The efficiency of a reversible engine depends only upon the

temperatures of the heat source and heat receiver

Carnot's Cycle Efficiency 1 1C C

H H

Q T

Q T

Carnot Cycle

• Process 1-2

– Adiabatic compression from TC to TH due to

work performed on fluid

• Process 2-3

– Isothermal expansion as fluid expands

when heat is added to the fluid at

temperature TH

• Process 3-4

– Adiabatic expansion as the fluid performs work

during the expansion process and temperature

drops from TH to TC

• Process 4-1

– Isothermal compression as the fluid contracts

when heat is removed from the fluid at

temperature TC

The Third Law of Thermodynamics

• No system can reach absolute zero

• This is one reason we use the Kelvin temperature

scale. Not only is the internal energy proportional to

temperature, but you never have to worry about

dividing by zero in an equation!

• There is no formula associated with the 3rd law of thermodynamics

Carnot's Cycle Efficiency 1 1C C

H H

Q T

Q T

Steam Power Cycle

The CARNOT Vapor Cycle

• Process 1-2

– isentropic compression in a

pump

• Process 2-3

– isothermal heat addition in a

boiler

• Process 3-4

– isentropic expansion in a

turbine

• Process 4-1

– isothermal heat rejection in a

condenser

• Which processes here would cause problems?

Problems of the Carnot Vapor Cycle

• The Carnot cycle is the most efficient cycle operating between two

specified temperature limits but it is not a suitable model for power

cycles. Because:

• Process 1-2

– It is not practical to design a compressor that handles two

phases

• Process 2-3

– Limiting the heat transfer processes to two-phase systems

severely limits the maximum temperature that can be used in the

cycle (374°C for water)

• Process 3-4

– The turbine cannot handle steam with a high moisture content

because of the impingement of liquid droplets on the turbine

blades causing erosion and wear

Ideal Rankine Cycle

• This cycle follows the idea of the Carnot cycle but can be practically

implemented

• Process 1-2

– isentropic pump

• Process 2-3

– constant pressure heat

addition

• Process 3-4

– isentropic turbine

• Process 4-1

– Constant pressure heat

rejection

Energy Analysis of

the Ideal Rankine Cycle

Steady-flow energy equation

The thermal efficiency can be interpreted as

the ratio of the area enclosed by the cycle on a

T-s diagram to the area under the heat-addition

process.

( ) ( ) (kJ/kg)in out in out e iq q w w h h

pump in 2 1 2 1

1 1

( 0) : ( )

@ , @f f

Pump q w h h v P P

h h P v v v P

3 2

3 4

4 1

( 0) :

( 0) :

( 0) :

in

turbine

out

Boiler w q h h

Turbine q w h h

Condenser w q h h

1

net in out turbine pump in

net outth

in in

w q q w w

w q

q q

Deviations from Ideal in Real Cycles

• The actual vapor power cycle differs from the ideal Rankine cycle as

a result of irreversibilities in various components.

• Fluid friction and heat loss to the surroundings are the two common

sources of irreversibilities

2 1

2 1

s sp

a a

w h h

w h h

3 4

3 4

a at

s s

w h h

w h h

Thermal Efficiency of Power Cycle

• The basic idea behind all the modifications to increase the thermal

efficiency of a power cycle

– Increase the average temperature at which heat is transferred to the

working fluid in the boiler, or

– Decrease the average temperature at which heat is rejected from the

working fluid in the condenser

• Way of increasing thermal efficiency

– Lower condenser pressure

– Superheat the steam more

– Increase boiler pressure (with same Tmax)

– Reheat Cycle

– Regeneration Cycle

Lowering the Condenser Pressure

• To take advantage of the

increased efficiencies at low

pressures, the condensers of

steam power plants usually

operate well below the

atmospheric pressure

• There is a lower limit to this

pressure depending on the

temperature of the cooling

medium

• Side effect: Lowering the condenser pressure increases the

moisture content of the steam at the final stages of the turbine

Superheating the Steam

to High Temperature

• Both the net work and heat input

increase as a result of superheating

the steam to a higher temperature

• The overall effect is an increase in

thermal efficiency since the average

temperature at which heat is added

increases

• Both the net work and heat input increase as a result of Superheating to higher temperatures decreases the moisture content of

the steam at the turbine exit, which is desirable

• The temperature is limited by metallurgical considerations.

Presently the highest steam temperature allowed at the turbine

inlet is about 620°C.

Increasing the Boiler Pressure

• The net work increases as a

result of increasing the boiler

pressure

• The overall effect is an increase

in thermal efficiency since the

average temperature at which

heat is added increases

• For a fixed turbine inlet temperature, the cycle shifts to the left and

the moisture content of steam at the turbine exit increases. This side

effect can be corrected by reheating the steam.

Supercritical Power Plant

• Today many modern steam

power plants operate at

supercritical pressures (P >

22.06 MPa) and have thermal

efficiencies of about 40% for

fossil-fuel plants

Reheat Rankine Cycle

• How can we take advantage of the increased efficiencies at higher

boiler pressures without facing the problem of excessive moisture at

the final stages of the turbine?

– Superheat the steam to very high temperatures

– Expand the steam in the turbine in two stages, and reheat it in between

Evaluation of Reheat Rankine Cycle

• The single reheat in a modern power plant improves the cycle

efficiency by 4 to 5% by increasing the average temperature at

which heat is transferred to the steam

– The use of more than two reheat stages is not practical

– The theoretical improvement in efficiency from the second reheat is

about half of that which results from a single reheat

• Reheat Cycle Design

– The reheat temperatures are very close or equal to the turbine inlet

temperature

– The optimum reheat pressure is about one-fourth of the maximum cycle

pressure

The Ideal Regenerative Rankine Cycle

• Heat is transferred to the working

fluid during process 2-2’ at a

relatively low temperature

• This lowers the average heat-

addition temperature and thus the

cycle efficiency

• In steam power plants, steam is extracted from the turbine at

various points. This steam, which could have produced more work

by expanding further in the turbine, is used to heat the feedwater

instead.

• The device where the feedwater is heated by regeneration is

called a regenerator, or a feedwater heater (FWH)

Regenerative Rankine cycle with

an open feedwater heater

• An open (or direct-contact) feedwater heater is basically a mixing

chamber, where the steam extracted from the turbine mixes with the

feedwater exiting the pump

• Ideally, the mixture leaves the heater as a saturated liquid at the

heater pressure

Regenerative Rankine cycle with

a closed feedwater heater

• The closed feedwater heater is basically a heat exchanger in which

heat is transferred from the extracted steam to the feedwater without

any mixing taking place.

• The two streams now can be at different pressures, since they do

not mix.

Open and Closed Feedwater Heaters

• The closed feedwater heaters

– more complex because of the internal tubing network, and thus they are

more expensive.

– Heat transfer in closed feedwater heaters is less effective since the two

streams are not allowed to be in direct contact.

– However, closed feedwater heaters do not require a separate pump for

each heater since the extracted steam and the feedwater can be at

different pressures.

• Open feedwater heaters

– simple and inexpensive and have good heat transfer characteristics

– For each heater, however, a pump is required to handle the feedwater

• Most steam power plants use a combination of open and closed

feedwater heaters

steam

Feed water

steam generator

HP turbine LP turbine G

moisture separator

reheater

steam

LP extracted steam

Deaerater

steam

condensate pump

condenser

LP extracted steam

feedwater pump HP extracted steam

A Typical PWR Steam Power Cycle

• Reheat Cycle – 1 reheater (moisture separator)

• Regeneration Cycle

– 1 open feedwater heater (Deaerater)

– 6 closed feedwater heater

Thank you