Energy Conversion Technologies(Part 3)

Source: General Electric

Brunel University 2013, Dr. Z. Dehouche1

1. Conversion of thermal to mechanical energy2. Conversion of thermal and chemical toelectricity3. Nuclear energy and its operation4. Environmental impact of power plant operation

CONTENTS Part III

2

4. Environmental impact of power plant operation

Reading List:1. Stephen R. Turns, Thermodynamics - Concepts And Applications, Cambridge University Press, 20062. A.W. Culp, Principles of Energy Conversion, McGraw-Hill Series in Mechanical Engineering, 2nd Edition, 19913. T. D. Eastop and A. McConkey, Applied Thermodynamics for Engineering Technologists - Prentice Hall, fifth

edition (1996)4. M.J. Moran and H.N. Shapiro, Fundamentals of Engineering Thermodynamics, John Wiley & Sons, 4th

edition (2000)5. R. OHayre, W. Colella, SukWon Cha and F.B. Prinz, Fuel Cell Fundamentals, John Wiley & Sons; 2nd edition

(2009)

Learning Objectives Understand the principles of different thermodynamic powercycles Be able to calculate the thermal properties at state pointsunder some conditions Be able to assess the overall performance of power cycles Be aware of the principle of the direct conversion of thermal

3

Be aware of the principle of the direct conversion of thermalto electricity Know the basic principle of fuel cells; their types andapplications Be aware of the basic concepts of nuclear reactors and theiroperations Be aware of the pollutions caused by the energy conversionsystems

The first law of thermodynamics defines therelationship between

the various forms of energy present in a system(kinetic and potential):

IntroductionBasic Concepts and Definitions

the work which the system performs and thetransfer of heat

The 1st Law of Thermodynamics is a statement ofthe Conservation of Energy Principle:

in terms of heat, work, and internal energy

4

Energy is neither created nor destroyed during aprocess; it can only change forms Energy of the universe (system + surroundings) isconstant

The heat Q transferred to the control volume isequal to the shaft work W and the change in the

Introduction

5

equal to the shaft work W and the change in theinternal energy U

Universe=System+ Surroundings

Second Law - Heat Engines The second law of thermodynamics asserts that energy hasquality as well as quantity, and

that processes occur in the direction of decreasing quality ofenergy

It is impossible to extract an amount of heat QH from a hotreservoir and use it all to do work W Some amount of heat QC must be exhausted to a cold reservoirThis precludes a perfect heat engine

6

Chapter 1 - Conversion of Thermal toMechanical Energy The devices or systems used to produce a net poweroutput are often called engines, and

the thermodynamic cycles they operate on arecalled power cycles

Thermodynamic cycles often use quasistatic (ideal)Thermodynamic cycles often use quasistatic (ideal)processes to model the workings of actual devices A thermodynamic cycle is a series of thermodynamicprocesses transferring heat and work, while

varying pressure, temperature, andother state variables, eventually returning asystem to its initial state

7

Thermodynamic cycles are categorized as gas cycles andvapour cycles, depending on the phase of the workingfluid:

In gas cycles, the working fluid remains in thegaseous phase throughout the entire cycle, whereas In vapour cycles the working fluid exists in thevapour phase during one part of the cycle andin the liquid phase during another part

8

in the liquid phase during another part Thermodynamic cycles are also categorized as closedand open cycles:

In closed cycles, the working fluid is returned to theinitial state at the end of the cycle and is recirculated In open cycles, the working fluid is renewed at theend of each cycle instead of being recirculated

Heat engines are categorized as internal and externalcombustion engines,

depending on how the heat is supplied to the working fluid In external combustion engines (such as steam powerplants), heat is supplied to the working fluid from an externalsource:

furnace, geothermal well, nuclear reactor, or solarradiation

9Geothermal power plant

In internal combustionengines (such asautomobile engines), thisis done by burning the

10

is done by burning thefuel within the systemboundaries

Heat engines are designedfor the purpose ofconverting thermal energyto work:The thermal efficiencyth is the ratio of the net

11

th is the ratio of the network produced by theengine to the total heatinput:

( )( ) a

netth Q

WkJheatadded

kJoutputworknet==

The working fluid (such as an ideal gas or water)moves through many thermodynamic states in anever-ending cyclic processThe detailed structure of the heat engine containsthe following four steps:

1. Isothermal absorption of heat from the high-temperature reservoirtemperature reservoir2. Adiabatic (isentropic) production of work3. Isothermal rejection of heat to the low-temperature reservoir4. Adiabatic work done on the fluid to return itto the state at the start of step 1

12

The mathematical equation for an ideal fluid undergoing areversible (i.e., no entropy generation) adiabatic process:

where P is pressure, V is volume, and k the specific heat ratiodefined as:

CPV k =

pck =

CP being the specific heat for constant pressure and CV beingthe specific heat for constant volume

For a monatomic ideal gas, k = 1.66, and for a diatomic gas(such as nitrogen and oxygen, the main components of air)k =1.4

13

v

p

c

ck =

http://buphy.bu.edu/~duffy/semester1/c27_process_adiabatic_sim.html

For reversible adiabatic processes (no heattransferred), it is also true that

where T is an absolute temperature

CTVCTP kkk == 11 ;

Using the ideal gas law, this can also be written as:

kkk

k

VV

PP

TT

VV

TT

PP

=

=

=

2

1

1

21

1

2

1

1

21

2

1

2

1 ;;

14http://www.youtube.com/watch?v=t6cGw1scLvc

The Ideal Power Cycle Carnot Heat Engine Cycle Two reversible isothermal processes alternated with tworeversible adiabatic (isentropic) processesThermal efficiency:

H

Lth T

T=1max,

P-v and T-s diagrams of a Carnot cycle15

EXAMPLE 1The turbine in a power station extracts kinetic energy fromsteam at a temperature of 800 K. The steam emerging fromthe turbine has a temperature of 370 K. What is the Carnotmaximum efficiency of the turbine?The Carnot efficiency of the heat engine is given by:

%75.535375.037011 orTL ===

16

This will be the maximum efficiency of the power station as awhole

%75.535375.080037011max, orT

T

H

Lth ===

1.2 Internal combustion engine cyclesPrinciple of the reciprocating internal combustionengine The reciprocatingengine (basically apistoncylinder device)is the powerhouse ofis the powerhouse ofthe vast majority of:

automobiles,trucks, light aircraft,ships, and electricpower generators

17Reciprocating engine

The piston reciprocates in the cylinderbetween two fixed positions called:

The top dead center (TDC): the positionof the piston when it forms the smallestvolume in the cylinder (vmin), andThe bottom dead center (BDC): theposition of the piston when it forms thelargest volume (vmax) in the cylinder

The distance between the TDC and the BDC The distance between the TDC and the BDCis the largest distance that the piston cantravel in one direction, and it is called thestroke of the engine The diameter of the piston is called the bore

Nomenclature forreciprocating engines

18

The airfuel mixture is drawn into the cylinder through the intakevalve, and the combustion products are expelled from the cylinder through theexhaust valve

The minimum volume formedin the cylinder when the pistonis at TDC is called theclearance volume The volume displaced by thepiston as it moves betweenTDC and BDC is called thedisplacement volume The ratio of the maximum The ratio of the maximumvolume formed in the cylinderto the minimum (clearance)volume is called thecompression ratio rv of theengine:

min

max

v

vrv =

Displacement and clearance volumes ofa reciprocating engine

19https://www.youtube.com/watch?v=Nplacsumrfw

Another term frequently used inconjunction with reciprocatingengines is the mean effectivepressure (MEP), given by:

( )kPavv

WMEP netminmax

=

The net work output of a cycle isequivalent to the product of themean effective pressure and thedisplacement volume:

MEP is the theoretical pressureexerted on the top of the pistonduring the power stroke

20

The engine with a larger value of MEP delivers morenet work per cycle (the power output of the engine)

Reciprocating engines are classified as:spark-ignition (SI) engines or compression-ignition (CI) engines, depending on how the combustionprocess in the cylinder is initiated:

In SI engines, the combustion of the airfuel

21

In SI engines, the combustion of the airfuelmixture is initiated by a spark plugIn CI engines, the airfuel mixture is self-ignited

as a result of compressing the mixtureabove its self-ignition temperature

Otto cycle: The ideal cycle for spark-ignition engines The Otto cycle is the ideal cycle for spark-ignition reciprocating engines In most spark-ignition engines, the piston executes four completestrokes within the cylinder for each thermodynamic cycle

Actual and ideal cycles in spark-ignition engines and their P-v diagrams 22

Initially, both the intake and the exhaust valves are closed, and thepiston is at its lowest position (BDC): During the compression stroke, the piston moves upward, compressingthe airfuel mixture:

Shortly before the piston reaches its highest position (TDC), thespark plug fires and the mixture ignites, increasing the pressure andtemperature of the system

The high-pressure gases force the piston down,

which in turn forces the crank-shaft to rotate, producing a usefulwork output during the expansion or power stroke

At the end of this stroke, the piston is at its lowest position (BDC), andthe cylinder is filled with combustion products

Now the piston moves upward one more time, purging the exhaustgases through the exhaust valve (the exhaust stroke),and down a second time, drawing in fresh airfuel mixture through theintake valve (the intake stroke)

23

P-v and T-s diagrams of the ideal Otto cycle

The ideal Otto cycle consists of four internally reversibleprocesses:1-2 Isentropic compression2-3 Constant-volume heat addition3-4 Isentropic expansion4-1 Constant-volume heat rejection

24

The Otto cycle is executed in a closed system, the overallenergy balance for any of the processes is expressed as:

No work is involved during the two heat transfer processes(2-3 and 4-1) since both take place at constant volume Therefore, heat transfer to and from the working fluid can be

( ) ( ) ( )kJUWWQQ rara =+

Therefore, heat transfer to and from the working fluid can beexpressed as

( )( )1414

2323

TTmcUUQTTmcUUQ

vr

va

==

==

Where m is the mass of the working fluid 25

The thermal efficiency of the ideal Otto cycle

=

=

==

1

111

2

32

1

41

23

14

TTT

TTT

TTTT

QQQ

QW

a

ra

a

netth

Processes 1-2 and 3-4 are isentropic, and v2=v3 and v4=v1 .Thus,

3

41

4

31

1

2

2

1

TT

VV

VV

TT

kk

=

=

=

Substituting these equations into the thermal efficiency relation give

26

( )kvth r

=11

Where rv is the compression ratio :

and k is the specific heat ratio

3

4

2

1

min

max

v

v

v

v

v

vrv ===

and k is the specific heat ratio

27

v

p

c

ck =

Diesel cycle: The ideal cycle for compression-ignition engines The Diesel cycle is the idealcycle for CI reciprocatingengines In spark-ignition engines, theairfuel mixture is compressedto a temperature that is belowto a temperature that is belowthe auto-ignition temperatureof the fuel, and the combustion processis initiated by firing a sparkplug

In diesel engines, the spark plug isreplaced by a fuel injector, and only airis compressed during the compressionprocess

28

In CI engines, the air is compressed to a temperature that is above theauto-ignition temperature of the fuel, and combustion starts on contact as the fuel is injected into this hot air

T-s and P-v diagrams for the ideal Diesel cycle

The ideal Diesel cycle processes are: 1-2 isentropic compression, 2-3constant-pressure heat-addition, 3-4 isentropic expansion, and 4-1constant-volume heat rejection The thermal efficiency of the ideal Diesel cycle

( )

=

=

===

1

11)(1)(11

2

32

1

41

23

14

23

14

TTkT

TTT

TTkTT

TTmcTTmc

QQ

QW

p

v

a

r

a

netth

29

We now define a new quantity, the cut-off ratio rcf, as theratio of the cylinder volumes after and before the combustionprocess:

Utilizing rcf and the isentropic ideal-gas relations forprocesses 1-2 and 3-4, the thermal efficiency relation reducesto

2

3

v

vrcf =

to

Where rv is the compression ratio : 2

1

min

max

v

v

v

vrv ==

30

( )( )11

1 1

=

cfk

v

kcf

thrkr

r

Take the cv=0.7176 kJ/(kg K) and the specific heat ratio, k=1.4 for the compressionand the expansion processesSolution: The Otto cycle and given data are shown infollowing p-v and T-s plots,

31

following p-v and T-s plots,

The temperature and pressure at state 1 are given:T1=27C=300 K, p1=100 kPa The process from state 1 to state 2 is isentropic, sothe reversible adiabatic relationships for an idealgas can be used, as

rV

andCTV k 811 ===

32

KT

VV

TT

rVV

andCTV

k

v

k

2.689x8300

8

0.42

1

2

1

1

2

2

11

==

=

===

kPapVV

pp

k

9.1837x8100 1.422

1

1

2

==

=

The process from state 2 to state 3 is constant volume, sothe heat transfer is equal to the change of internal energyaccording to the first law,

CPV k =

33

( ) ( )( )

kPaTTpp

KT

TKkg

kJkgkJ

TTcuuq va

1.86762.6895.32539.1837

5.3253

2.689.

7176.01840

2

323

3

3

2323

=

=

=

=

=

==

CTP

=

The process from state 3 to state 4 is isentropic, so thefollowing relationships hold

VV

TT

rVV

andCPV

kv

k 1

0.4

1

4

3

3

4

4

3

=

==

34

KT 2.1416815.3253

0.4

4 =

=

kPap

VV

pp

k

1.472811.8676

4.1

4

4

3

3

4

=

=

=

Thermal efficiency

( ) %5.56565.081111 4.01 or

rk

v

th ===

35

I.2 - An ideal Otto cycle has a compression ratio of 8(see Fig. I.2). At the beginning of the compressionprocess, air is at 100 kPa and 17C, and 800 kJ/kg ofheat is transferred to air during the constant-volumeheat-addition process. Take the cv=0.72 kJ/(kg K)and the specific heat ratio, k=1.4 for thecompression and the expansion processes.Determine:

36

Determine:1)The temperature and pressure at the end of eachprocess of the cycle.2)The heat rejected qout, in kJ/kg3)The net work output, in kJ/kg4)The thermal efficiency

37

I.3 - An engine operates on the air-standard diesel cycle(seeFig. next slide). The conditions at the start of compressionare 27C and 100 kPa. The heat supplied is 1840 kJ/kg, andcompression ratio is 16. Take the cp=1.0047 kJ/(kg K),cv=0.7176 kJ/(kg K) and the specific heat ratio, k=1.4 for thecompression and the expansion processes. Determine:a)The maximum temperature and pressure of the diesel cycleb)The thermal efficiency

38

c)The mean effective pressure (MEP) in (kPa)d) The power output at 2000 rpm for an engine displacementof 5000 cm3

Assumptions1.The air in the piston-cylinder is a closed system2.The air in the system is an ideal gas with constant specific heats3.Kinetic and potential energy changes are negligible

Solution: The ideal Diesel cycle and given data are shown infollowing p-v and T-s plots,

39

a)The temperature and pressure values at the end of eachprocess can be determined by utilizing the ideal-gasisentropic relations for processes 1-2 and 3-4But first we determine the volume at state 1 for ideal-gas,where, T1=300K, p1=100 kPa, and

kgmp

RTv /861.0

100300x287.0 3

1

11 ===

Process 1-2 (isentropic compression of an ideal gas,constant specific heats), hence:

kgmvvrvandCPv k /0538.0861.0;16 311 ======

40

KT

v

v

TT

CTv

kgmvvrv

vandCPv

k

k

v

k

4.909x16300

/0538.016861.0

16;16

0.42

1

2

1

1

2

1

312

2

1

==

=

=

======

kPapv

v

pp

k

3.4850x16100 1.422

1

1

2

==

=

Process 2-3 (constant-pressure heat addition to an ideal gas):( ) ( )TTchhq pa 2323 ==

41

( ) ( )( )

kgmp

RTv

kPappp

KTT

Tpa

/1622.03.4850

8.2740x287.03.4850

8.27404.9090047.11840

3

max

max3

2max3

max3

3

2323

===

===

==

=

Process 3-4 (isentropic expansion of an ideal gas, constantspecific heats), and v4=v1 , hence:

v

v

TT

v

vandCTvCPv

k

kk 188.0861.0

1622.0;;

1

4

3

3

4

4

31

=

====

42

KT 7.1405x0.1888.2740 0.4443

==

kPapv

v

pp

k

6.468x0.1883.4850 1.444

3

3

4

==

=

Process 4-1 is a constant-volume heat-rejection process (itinvolves no work interactions), and the amount of heatrejected is

( ) ( )( )

kgkJqq

TTcuuq

r

r

vr

/4.7937.14053007176.0

4141

=

=

==

43

Thus, the net work output of a cycle is

b) Then, the thermal efficiency of this Diesel cycle isdetermined from

( ) ( ) kgkJqqw ranet /6.10464.7931840 ==+=

%9.56569.01840

6.1046or

qw

a

netth ===

c) The mean effective pressure is determined from itsdefinition

d) The power output is calculated as

kPavv

w

vv

wMEP netnet 6.12960538.0861.06.1046

21minmax

=

=

=

=

( ) ( ) ( ) s2000 1

44

( )( ) ( ) ( ) kWsmkParpmntdisplacemepistonMEPPower 05.10860x2

2000x105000x6.1296

60x2

136-

=

=

=