Encyclopedia of Energy Engineering and Technology, Volumes 1, 2 and 3

Pub Physics of Energy Phot Milivoje M. Kostic Department of Mechanical Engineering, Northern Illinois University,DeKalb, Illinois, U.S.A.

Abstract The concept and definition of energy are elaborated, as well as different forms and classification of energy are presented. Energy is afundamental concept indivisible from matter and space, and energy exchanges or transfers are associated with all processes (or changes), thus indivisible from time. Actually, energy is “the building block” and fundamental property of matter and space, thus fundamental property of existence. Any and every material system in nature possesses energy. The structure of any matter and field is “energetic,” meaning active, i.e., photon waves are traveling in space, electrons are orbiting around an atom nucleus or flowing through aconductor, atoms and molecules are in constant rotation, vibration or random thermal motion, etc. When energy is exchanged or transferred from one system to another it is manifested as work or heat. In addition, the First Law of energy conservation and the Second Law of energy degradation and entropy generation are presented along with relevant concluding remarks. In summary, energy is providing existence, and if exchanged, it has ability to perform change.

INTRODUCTION:FROMWORK TO HEAT TO “energetic,”meaning active,i.e., photonwaves are GENERAL ENERGY CONCEPT traveling in space,electrons are orbiting aroundanatom nucleusorflowingthroughaconductor,atoms and Energy is afundamental concept indivisible from matter molecules are in constant rotation, vibration or random andspace, andenergyexchangesortransfers are thermal motion, etc., see Table 1and Fig. 1. Thus energy is associated with all processes(or changes), thus indivisible aproperty of amaterial system (further on simplyreferred from time. Actually, energy is “the building block” and to as system), and togetherwith other properties defines the fundamental propertyofmatterand space, thusa system equilibrium state or existence in space and time. fundamental property of existence. Energy transfer is Energy in transfer ( E transfer)ismanifested as work ( W ) neededtoproduceaprocess to change otherproperties. or heat ( Q )when it is exchanged or transferred from one Also, among all properties, energyisthe only onewhich is system to another, as explained next (see Fig. 2). directly related to mass and vice versa: E Z mc2 (known in some literature as mass energy,the c is the speed of light in Work avacuum),thus the mass and energy are interrelated.[1,2] Energy moves cars and trains, and boats and planes. It Work is amode of energy transfer from one acting body (or bakes foods and keeps them frozen for storage. Energy system) to another resistingbody (or system), with an lights our homes and plays our music. Energy makes our acting force (or its component) in the direction of motion, bodies alive and grow,and allows our minds to think. along apath or displacement. Abody that is acting Through centuries, people have learned how to use energy (forcing) in motion-direction in time,isdoing work on in different forms in order to do work more easily and live another body which is resistingthe motion (displacement more comfortably. No wonder that energy is often defined rate) by an equal resistanceforce, including inertial force, as ability to perform work,i.e., as apotential for energy in opposite direction of action. The acting body (energy transfer in aspecific direction (displacement in force source) is imparting (transferring away) its energytothe direction)thus achieving apurposeful process, as opposed resistingbody(energy sink), and the amount of energy to dissipative (less-purposeful) energy transfer in form of transfer is the workdone by the acting onto the resisting heat. The above definition couldbegeneralizedas: energy body, equal to the productofthe forcecomponent in the is providing existence, and if exchanged, it has the ability motion directionmultiplied with thecorresponding to perform change. [3] displacement, or vice versa (force multipliedbydisplace- Any and every materialsystem in nature possesses ment component in the forcedirection), see Fig. 3. If the energy.The structureofany matter andfield is force ð F ð Þ and displacement vectors ð d sð Z d rð Þ are not constant,then integration of differential work transfers frominitial (1)tofinalstate (2), defined by the Keywords:Energy; Entropy; Heat; Heat engine; Power; Thermal energy; corresponding position vectors rð ,will be necessary, see Total internal energy; Work. Fig. 4.

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Table 1 Material system structure and related forces and energies Pub Particles Forces Energies

Atom nucleus Strong and weak Nuclear Phot Electron shell, or electron ﬂow Electromagnetic Electrical, magnetic, electromagnetic Atoms/molecules Inter-atomic/molecular Chemical Molecules Inertial due to random collision Sensible thermal and, potential inter-molecular Molecules Potential inter-molecular Latent thermal Molecules Potential inter-molecular Mechanical elastic System mass Inertial and gravitational Mechanical kinetic and gravitational potential

The work is adirectional energy transfer. However, it is rubbing hands together or from mechanical energy during ascalar quantity like energy, and is distinctive from another friction,like mixingorsimilar. Finally,careful experi- energy transfer in form of heat,which is due to random ments by James P. Joulepublishedin1843,quantified motion(chaotic or randominall directions) and collisions correlation betweenmechanical workand heat, and thus of system molecules and their structural components. put the caloric theory to rest by convincing the skeptics Work transfer cannot occur without existence of the that heat was not the caloric substance after all. Although resisting body or system,nor without finite displacement the caloric theory was totally abandoned in the middle of in the forcedirection. This may not always be obvious. For thenineteenthcentury,itcontributed greatly to the example, if we are holding aheavy weight or pushing hard development of thermodynamics and heat transfer. against astationarywall, there will be no work done Heat maybetransferredbythree distinctive againstthe weight or wall (neglectingtheir small mechanisms: conduction, convection, and thermal radi- deformations).However, we will be doing work internally ation, see Fig. 5. Heat conduction is the transfer of thermal due to contraction and expansion of our muscles (thus energy duetointeraction betweenthe more energetic force with displacement),and that wayconverting particles of asubstance, likeatoms and molecules (thus at (spending) alot of chemical energy via muscle work, highertemperature), to the adjacent less energetic ones then dissipating it into thermal energy and heat transfer (thus at lower temperature).Heat convection is the transfer (sweating and getting tired). of thermal energy betweenasolid surface and the adjacent moving fluid, and it involves the combined effects of Heat conduction and fluid motion. Thermal radiation is the transfer of thermalenergydue to theemissionof Heat is another mode of energy transfer from one system electromagnetic waves (or photons) which are products at highertemperature to another at lower temperature due of random interactions between energetic particles of a to their temperature difference. Fire was civilization’s first substance(thus related to the temperature). During those greatinvention, long before peoplecould read and write, interactions the electron energy levelischanged,thus and wood was the main heat source for cooking and causing emission of photons, i.e., electromagnetic thermal heating for ages. However, truephysical understandingof radiation. the nature of heat was discovered rather recently, in the middleofthe nineteenth century, thanks to the develop- The Joule’s experiments of establishing equivalency mentofthe kinetic theory of gasses. Thermal energy and betweenworkand heat paved the way of establishing the heat are defined as the energy associated with the random concept of internal thermal energy, to generalize the motionofatoms and molecules.The prior concept of heat concept of energy, and to formulate the general law of was based on the caloric theory proposed by the French energy conservation. The total internal energy includesall chemist Antoine Lavoisierin1789. The caloric theory otherpossiblebut mechanical energy types or forms, defines heat as amassless, colorless, odorless, and including chemical and nuclear energy. This allowed tasteless, fluid-like substance called the caloric that can extensionofthe well-established lawofmechanical be transferred or “poured” from one body into another. energyconservation to thegeneral lawofenergy When caloricwas added to abody, its temperature conservation, known as the First Law of Thermodynamics, increased and vice versa. The caloric theory came under which includesall possibleenergy forms that asystem attacksoon after its introduction. It maintained that heat is couldpossess, and heat and all types of work as all asubstance that couldnot be created or destroyed. Yet it possibleenergy-transfers betweenthe systems. The Law was knownthat heat can be generated indefinitelyby of Energy Conservation will be elaborated later.

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Fig. 2 Energy as material system property and energy transfer from one system to another.

elastic, gravitational, and sound);where, for example, electro-mechanical energy may be kinetic or potential, while thermal energy represents overall potential and chaotic motionenergy of molecules and/or related micro structure. [3,4]

Energy, Work, and Heat Units

Energy,Work, and Heat Units.Energy is manifested via work and heat transfer, with corresponding Force!

Length dimension for work (N m, kgf m, and lbf ft, in SI, metric and English system of units, respectively); and the caloric units, in kilocalorie (kcal) or British-thermal-unit (Btu), the last two defined as heat needed to increasea unit massofwater (at specified pressure and temperature) for one degreeoftemperature in their respective units. Therefore, the water specific heat is 1kcal/(kg 8 C)Z 1Btu/(lb 8 F) by definition, in metric and English system of units, respectively. It was demonstrated by Joule that 4187 Nmof work, when dissipated in heat, is equivalent to 1kcal. In his honor, 1Nmofwork is named after him as 1Joule, or 1J,the SI energy unit, also equal to electrical work of 1Ws Z 1VAs.The SI unit for power, or work rate, is watt,i.e., 1J/sZ 1W,and also corresponding units in othersystem of units, like Btu/h, etc. The Horse Powerisdefined as 1HP Z

550 lbf ft/sZ 745.7 W. Other common units for energy, work and heat are given in Table 2. Fig. 1 Different types of energy: (A) Potential gravitational and electromagnetic radiation; (B) Organized energy as work transfer; (C) Disorganized thermal energy as heat transfer.

Energy

Energy is fundamental propertyofaphysical system and refers to its potential to maintain asystem identity or structure and to inﬂuence changeswith othersystems (via forced interaction)byimpartingwork(forced directional displacement)orheat (forced chaotic displacement/motion of asystemmolecularorrelated structures). Energy existsinmanyforms:electromagnetic (including light), electrical, magnetic, nuclear, chemical, thermal, and mechanical (including kinetic, Fig. 3 Work, force, and displacement.

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internal within asystem microstructure)and/or macro-

r 2 Pub scopic (or externalasrelated to the system massasawhole W 12 = F·ds = F·dr ∫ ∫ with reference to other systems). Furthermore, energy may S 12 r 1 Phot 2 be “quasi-potential” (associated with asystem equilibrium state andstructure, i.e., systemproperty)or“quasi- S r 2 kinetic”(energy in-transfer from onesystem or one F ds = dr structure to another, in form of work or heat). Every material system state is an equilibrium potential r + dr “held” by forces (space force-fields), i.e., the forces r “define”the potential and state—action and reaction; r 1 otherwise asystem will undergo dynamic change (in 1 time), or quasi-kinetic energy exchange towards another 0 stable equilibrium. Atoms(mass) are “held” by atomicand electromagnetic forces in small scale and by gravityin Fig. 4 Work along an arbitrary path. large scale, see Fig. 1A, otherwise mass would disintegrate ENERGY FORMS AND CLASSIFICATIONS: (“evaporate” or radiate into energy) like partly in nuclear ENERGY-TRANSFERVSENERGY-PROPERTY reactions—nuclear energy or electromagnetic radiation. Molecules are “held”byelectro-chemical bonding (valence) forces (chemical reactions—chemical energy). Any and all changesorprocesses (happeninginspace and Liquids are “held” by latent intermolecular forces (gas time)are caused by energy exchanges or transfers from condensation, whenkinetic energy is reduced by cool- one substance(system or subsystem)toanother, see Fig. 2. ing—latent thermal energy). Solids are “held” by “firm” Apart of asystem may be considered as asubsystemif intermolecular forces (freezing/solidificationwhenenergy energy transfer within asystem is taking place, and is further reduced by cooling—latent thermal energy inversely, agroup of interactingsystems maybe again). Sensible thermal energy represents intermolecular considered as alarger isolated system,ifthey do not potential energy and energy of random molecular motion, interact with the rest of the surroundings. Energy transfer and is related to temperature of asystem. “Holding” may be in organized form (different typesofwork transfer potential forces may be “broken” by energy transfer (e.g., due to different force actions)orinchaotic disorganized radiation, heating,high-energy particlesinteractions, etc.). form (heat transfer duetotemperature difference). Energy States and potentials are often “hooked” (i.e., stable)and transferintoasystem builds up energy–potential or thus need to be “unhooked” (or to “be broken”) to generalized-force (called simply potential for short, like overcome existing “threshold” or equilibrium, like in pressure, temperature, voltage,etc.) over energy—dis- igniting combustion, starting nuclear reaction, etc. placement (like volume, entropy, charge, etc.). Conver- As stated above, energy transfer can be directional sely,ifenergy is transferred from asystem, its energy (purposeful or organized) and chaotic (dissipative or potential is decreased.Thatiswhy net-energyis disorganized). Forexample, mass-in-motion, mechanical transferred from higher to lower energy potential only, kinetic energy, and electricity-in-motion, electrical kinetic due to virtually infinite probability of equi-partition of energy, are organized kinetic energies (Fig. 1B), while energyoversystem micro-structure,causing system thermal energy is disorganized chaotic energy of motion equilibrium,otherwisevirtually impossible singularity of of molecules and atoms (Fig. 1C). System energy may be energy potential at infinite magnitude would result.[4] defined with reference to position in avector-force field, There are many forms and classifications of energy, see like elastic potential (stress)energy, gravitational Table 3, all of which could be classified as microscopic (or potential energy, or electromagnetic field energy. There aremanydifferentenergyformsandtypes(seeTable3). We are usually not interested in (absolute) energy level, but in the change of energy (during aprocess) from an System 4 4 Q rad = A es ( T b − T surr ) initial state ( i )toafinal state ( f ), and thus zero reference E Sys values for different energy forms are irrelevant, and often taken arbitrarily for convenience. The followings are basic ∂ T correlations for energy changesofseveral typical energy Q cond =−Ak ∂ n Q conv = Ah ( T b − T fluid) forms, oftenencountered in practice:motion kinetic energy ( E K Z KE)asafunction of system velocity ( V );

Interface boundary potential elastic-deformational, e.g., pressure elastic or

spring elastic energy ( E Pdeff Z E PpZ E Ps)asafunction of Fig. 5 Heat as energy-transfer by conduction, convection, and spring deformationdisplacement ( x ); gravitational radiation is due to adifference in temperature. potential energy ( E PgZ PEg )asafunction of gravitational

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Pub Table 2 Typical energy units with conversion factors

Energy units JkWh Btu Phot 1Joule (J) 12.78! 10K 7 9.49! 10K 4 1Kilowatt-hour (kWh) 3.6! 106 13.412! 103 1Kilocalorie (kcalZ CalZ 1000 cal) 4187 1.19! 10K 3 3.968 1British thermal unit (Btu) 1055 2.93! 10K 4 1 K 7 K 3 1Pound-force foot (lbf $ ft) 1.36 3.78! 10 1.29! 10 1Electron volt (eV) 1.6! 10K 19 4.45! 10K 26 1.52! 10K 22 1Horse Power! second (HP sec) 745.7 2.071! 10K 4 0.707

[5] elevation ( z ); and sensiblethermal energy ( E U Z U )asa corresponding generalized-displacement : function of system temperature ( T ): hiX d E Transfer Z d Q netIN K d W netOUT 1 1 2 2 2 2 hiX D E k Z mVf K V i ; D E Ps Z kxf K x i 2 2 Z d Q netIN C d W netIN 2 D E Z mgð z K z Þ ; D E Z mc ð T K T Þ Pg f i U v f i 6 ð 1 Þ Z T d S CK4 P d V C s d A |fflfflffl{zfflfflffl} |{z} If the reference energy values are takentobezero when COMPR: STRECHING C t d ð AsÞ C V d q C E ð d ð V P ð Þ the above initial ( i )variables are zero, then the above |fflfflffl{zfflfflffl} |{z} |fflfflfflffl{zfflfflfflffl} equations will represent the energy values for the final SHEARING CHARGING # POLARIZATION values ( f )ofthe corresponding variables. If the C m H ð d ð V M ð Þ C corresponding parameters, spring constant k ,gravity g , |fflfflo fflfflfflfflffl{zfflfflfflfflfflfflffl} |{z} ETC or constant-volume specificheat c v ,are not constant,then MAGNETIZATION integration of differential energy changesfrom initial to ð 4 Þ final state will be necessary. Wherethe quantities after the last equalsignare: Energy transfer via work W (net-out), and heat transfer temperature and entropy; pressure and volume; surface Q (net-in), may be expressed for reversible processes as tension and area; tangential-stress and area with tangen- productofrelated energy–potentials (pressure P ,or tial-displacement, voltage and electrical charge; electric temperature T )and corresponding energy–displacements field strengthand volume-electric dipole moment per unit (change of volume V and entropy S ,respectively), i.e.: volume product; and permeability of freespace, magnetic field strengthand volume-magnetic dipolemoment per W Z F ð $ d ð 12 unit volume product; respectively.

Vð 2 The total system energy stored within the system, as Z ð P $ A n ð Þ $ d ð Z P $ D V j Z P d V ð 2 Þ energy property, is: |fflffl{zfflffl} 12 P s const D V V E Z E C E C E 1 Sys |fflfflfflfflfflfflfflK fflfflfflfflfflfflfflfflfflPgffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflPdeffffl}:

E Mechanical Sð 2 C E C E C E C E C E C . |{z}Uth Ch Nucl El Magn |{z} Q Z T D S j Z T d S ð 3 Þ Thermal 12 12 T s const |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflEtcfflffl: ffl} S 1 Internal ð totalÞ ð 5 Þ Note, in Eq. 2, that force cannot act at apoint but is distributed as pressure ( P )oversomearea A (with Where the quantities after the equal sign are: kinetic, orthogonalunit vector n ð ), which when displaced will potential-gravitational, potential-elastic-deformational, cause the volume change D V .Also note that it is custom in thermal, chemical,nuclear, electric, magnetic energies, etc. some references to denoteheat transfer “in” and work transfer “out” as positive (as they appearinaheat engine). In general, “in” (means “net-in”) is negative“out”(means THE FIRST LAW OF ENERGY CONSERVATION: “net-out”) and vice versa. WORK–HEAT–ENERGY PRINCIPLE In general,energy transfer betweensystems is taking place at the system boundaryinterface and is equal to the Newton formulated the general theory of motion of productofenergy–potential or generalized-force and the objects due to applied forces (1687).This provided for

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Table 3 Energy forms and classiﬁcations Pub ENERGY Phot Transfer Type (release)

Kinetic Kinetic Scale (motion) (motion) a a b Energy form (Energy storage) Energy process Macro/external (mass-based) Micro/internal (structure-based) Potential (state or field) Directional Chaotic dissipative Work directional Heat dissipative Mechanical XKinetic mV2 /2 Acceleration XX XGravitationalc mgz Elevation XX XElastic kx2 /2 or PVZ mP/r Deformation XX

Thermal U th

XSensible U thZ mcavgT Heating XX

XLatent U thZ H latent Melting, Evaporation XX

XChemical U ch Chemical Reaction XX

XNuclear U nucl Nuclear Reaction XX

Electrical E el XElectro-kinetic V(It) or LI2 /2 Electro-current ﬂow XX XElectrostatic (It)2 /(2C) Electro-charging XX

Magnetic E magn Magnetization XX d d X Electromagnetic E el_mag Radiation X X a Electro-mechanical kinetic energy type (directional/organized, the highest energy quality) is preferable since it may be converted to any other energy form/ type with high efﬁciency. b All processes (involving energy transfer) are to some degree irreversible (i.e., dissipative or chaotic/disorganized). c Due to mass position in agravitational ﬁeld. d Electromagnetic form of energy is the smallest known scale of energy.

concepts of mechanical work, kineticand potential The above correlation is knownasthe work–energy energies, and development of solid-body mechanics. principle.The work–energy principle couldbeeasily Furthermore, in absence of non-mechanical energy expended to include work of gravityforce and interactions, excluding friction and other dissipation gravitational potential energy as well as elastic spring effects, it is straightforwardtoderive (and thus prove) force and potential elastic spring energy.[3] energy conservation, i.e.: During afree gravityfall (or free bounce)without air friction,for example, thepotential energy is being ! ! ðs 2 ðs 2 ðs 2 d V d V d s converted to kineticenergy of the falling body (or vice F d s Z m s d s Z m s d s versafor free bounce), and at any time the total mechanical s d t d s d t s s |{z} s |fflffl{zfflffl} energy (sum of kinetic and potential mechanical energies) |fflfflfflffl1 ffl{zfflfflfflfflffl} 1 a 1 s V s isconserved,i.e.,staysthesame,seeFig.6.The W Fs mechanical energy is alsoconserved if amass freely ð 6 Þ Vð 2 1 vibrates on an ideally elastic spring, or if apendulum 2 2 Z mVs d V s Z mV2 K V 1 oscillates aroundits pivot, both in absence of dissipative |fflfflfflfflfflfflfflfflffl2 ffl{zfflfflfflfflfflfflfflfflfflffl} V effects, like friction or non-elastic deformation. In general, 1 KE K KE 2 1 for work of conservative forces only, the mechanical E Z W Z KE K KE Z E K E Transfer Fs 2 1 2 1 energy, E mech ,for N isolated systems, is conserved since

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At z = z , v = 0 z 0 , v 0 =0, P o Pub 0 z 0 → E Pg g

Phot During free fall E K g z , v =0 and free bounce E 1 1 Pg E mass m Mech is weightless ll fa E K + Pg FLUID Free bounce v > 0 v Free z 2 , v 2_IN =0 m E Pp v ( z ) =± 2 g ( z 0 − z ) z z 2 , v 2_OUT

v < 0 P = P E = E + E + E 2_OUT o v max_bounce Mech K Pg Pp z E Pp, E Pg,

E Mech0, = E Mech1, = E Mech2_IN, = E Mech2_OUT, v z = 0 = v max= ± 2 gz0 v , E K , E Pg, E K + Pg 2 P 2 P 2 P n ++gz n ++gz n ++gz r = r = r { 2 PE = 2 { 2 { Fig. 6 Energy and work due to conservative gravity force. g 00 { =0 =0 =E 0 =0 =E 2_IN KE=E 2_OUT Pg Pg k =0

P 2_IN = r g ( z o − z 2 )and n 2_OUT = 2 g ( z o − z 2 )

there is no dissipative conversion into thermal energy and Fig. 7 Conservation of ﬂuid mechanical energy: Bernoulli thus no heat transfer, i.e.: equation, hydrostatic equation, and Torricelli’s oriﬁce velocity.

E mech Z E k C E Pg C E Ps mechanicalenergyisconserved. However, work of N X 1 1 non-conservative,dissipative forcesisprocesspath- Z mV2 C mgz C kx2 i Z 1 2 2 i dependent and part of the mechanical energy is converted Z const ð 7 Þ (dissipated) to thermal energy, see Fig. 8A.

The mechanical work–energy concept couldalsobe expended to fluid motion by inclusion of elastic-pressure Q force and potential elastic-pressure energy (referred to in IN W OUT some references as flowwork; however,note that elastic- pressure energy is asystem property while the flowwork is Material SYSTEM Q related energy transfer), see the Bernoulliequation below. OUT For flowing or stationary fluid without frictional effects, E themechanicalenergyisconserved,including fluid Sys elastic—pressureenergy, PV Z mP= r (where V is BOUNDARY volume, whereas v is used for velocity here), as expressed Interface of System by the Bernoulli or hydrostatic equations below, see also W Fig. 7.[3] IN E 1 mech Z ð E C E C E Þ m m K Ps Pg netIN

2 W

v P Sys Sys Sys netIN E E E Z C C gz netIN ∆ ∆ ∆ W 2 r Q |fflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflffl} v Z 0 Bernoulli equation netIN Q P Process Dependant Z C gz Z const: ð 8 Þ r (A) Both Q&W (B) W only (No Q ) (C) Q only (No W ) |fflfflfflfflffl{zfflfflfflfflffl } Work-Heat interaction Work interaction Heat interaction Hydrostatic equation ð v Z 0 Þ (process dependant) (adiabatic process) (caloric process)

Work against inertial and/or conservative forces (also Fig. 8 System energyand energy boundary interactions knownasinternal, or volumetric, or space potential ﬁeld), (transfers) for (A) arbitrary, (B) adiabatic, and (C) caloric is path-independentand duringsuchaprocessthe processes.

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When work of non-conservativeforces W nc ,is The boundary energy transfers are process (or process Pub exchanged between N isolated systems, from an initial path)dependent for the same D E Sys change,except for ( i )tofinalstate ( f ), then the total mechanical energy of all special cases for adiabatic processes with work systems is reduced by that work amount, i.e.: interactiononly(no heat transfer), or forcaloric Phot !! processes with heat interaction only (no work transfer), X N X N see Fig. 8A to C. For the latter caloric processes W Z E K E ð 9 Þ nc; i / f mech; j mech; j without workinteractions (no volumetric expansion or j Z 1 j Z 1 i f othermechanical energy changes), the internal thermal Regardless of the traveled path (or displacement), the energy is conserved by beingtransferred from one workagainst conservativeforces (like gravity or elastic system to another via heat transfer only, known as spring in above cases) in absence of any dissipative forces, caloric.This demonstrates the value of the caloric will depend on the final and initial position (or state) only. theory of heat that was established by Lavoisier and However, the work of non-conservative,dissipative forces Laplace (1789),the great minds of the 18th century. ( W nc)dependsonthe traveled path sincethe energy is Ironically, the caloric theory was creatively used by dissipated during the forcedisplacement, and mechanical Sadi Carnot to develop the concept of reversible cycles energy will not be conserved,but in part converted (via for conversion of caloricheat to mechanical work as it dissipation and heat transfer) into thermal energy, see “flows” from high to low temperature reservoirs (1824) Eq. 9. This shouldnot be misunderstood with the total that later helped in dismantling the caloric theory. The energy conservation, which is always the case, and it must calorictheory was discredited by establishing the heat include both work and heat transfer, see below. equivalent of work,e.g., mechanical equivalent of heat As already stated, there are manydifferent typesof by Mayer(1842) and experimentally confirmed by worktransfer into (or out of) asystem which will change Joule(1843),which paved the way for establishing the the corresponding energy-form stored in (or released, First Law of Energy Conservation and new science of discharged out of) the system. In addition to work, energy Thermodynamics (Clausius, Rankine, and Kelvin, 1850 maybetransferredasheat caused by temperature and later). Prejudging the caloric theory now as a difference and associated with change of the thermal “failure” is unfair and unjustified since it made great energy of asystem. Furthermore, different forms of stored contributions in calorimetry and heat transfer, and it is energy are often coupled so that one type of energy valid for caloric processes(without work interactions). transfer may change more than one form of stored energy, The coupling of work–heatinteractions and conversion particularly duetoinevitable dissipative conversion of betweenthermal and mechanical energy are outside of worktoheat, and in turn to internal thermal energy. the calorictheory domain and are further developed Conversely,heat and thermal energy may be converted within theFirst and theSecond Laws of into otherenergy forms. In the absence of nuclear reaction Thermodynamics. (no conversion of mass into energy, E Z mc2 ), mass and The First Law of energy conservation for the control- energy are conserved separately for an isolated system, volume (CV, with boundary surface BS) flow process, see agroup of isolated systems, or for the Universe. Since the Fig. 9, is: material system structure is of particulate form, then systems’ interactions (collisions at different scale-sizes) d E will exchange energy during the forced displacement— CV |fflffld t ffl{zfflfflffl} and similarly to the mechanical energy conservation—the RATE OF ENERGY CHANGE IN CV totality of all forms of energy will be conserved,[8] see X Z Q _ Fig. 8, which couldbeexpressed as: netIN; i BS X |fflfflfflfflfflfflffl{zfflfflfflfflfflfflffl} BS TRANSFERRATE OF HEAT E i ; Trans: Z D E Change or X All i’s |fflfflfflfflffl{zfflfflfflfflffl } K W _ |fflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} SYSTEM netOUT; i BOUNDARY |fflfflBSfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflffl} X X BS TRANSFERRATE OF WORK W C Q Z D E Z D E X j ; netIN k ; netIN |fflfflfflfflfflfflnetIncreaseffl{zfflfflfflfflfflfflffl} Sys C m _ ð e C PvÞ All j’s All k’s j j |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl} SYSTEM |fflfflfflfflfflfflfflfflfflfflfflfflIN ffl{zfflfflfflfflfflfflfflfflfflfflfflfflffl} BOUNDARY ENERGY TRANSPORT RATE WITH MASS IN ð 10Þ X K m _ k ð e C PvÞ k ð 11Þ Energy interactions or transfers across asystem P OUT|fflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflffl} Pboundary, in theP form of work,P W netIN Z W INK ENERGY TRANSPORT RATE WITH MASS OUT W OUT Z K W P OUT K P W IN Z K W netOUT, and heat, Q netIN Z Q IN K Q OUT,will change The FirstLaw of energy conservation equation for a thesystemtotal energy, D E Sys Z E Sys; 2 K E Sys; 1 . differential volume per unit of volume around apoint

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volume V is: Pub ð _ W F Z F d V ð 15Þ Phot V NOTE : distinguish volume V ; form velocity V ð

THE SECOND LAW OF ENERGY Fig. 9 Control-volume (CV) energy and entropy, and energy DEGRADATION: ENTROPY ANDEXERGY and entropy flows through the boundary interface of the control volume. Every organized kinetic energy will, in part or in whole (and ultimately in whole),disorganize/dissipate within the ( x,y,z)inaflowing fluid is[6]: microstructure of asystem (in time over its mass and De space) into disorganized thermal energy. Entropy, as an r energy–displacement system property, representsthe Dt |ffl{zffl} measureofenergy disorganization, or random energy energy change in time redistribution to smaller-scale structure and space,per Z K V ð $ ðÞV P |fflfflfflfflfflffl{zfflfflfflfflfflffl} absolutetemperature level. Contrary to energyand mass, work rate of normal pressure stresses which are conserved in the universe, entropy is continu- ously generated (increased)due to continuous redistribu- ð C V $ V $ t ij C V $ ðÞk V T ð 12Þ tion and disorganization of energy in transfer and thus |fflfflfflfflfflfflffl{zfflfflfflfflfflfflffl} |fflfflfflfflffl{zfflfflfflfflffl} heat rate via thermal conduction work rate of shearing stresses degradation of the quality of energy (“spreading” of energy towards and over lower potentials in time until Where e Z u ^ C V ð 2 2 C gz NOTE: distinguishspecific equi-partitioned over system structure and space). Often, thermal energy u ^ ,from velocity component u below. we want to extract energy from onesystem in order to purposefully changeanother system,thus to transfer Eq. 12 after substitution, V $ V ð $ t Z V ð $ V $ t C ij ij energy in organized form (as work,thus the ultimate F ,and using the momentumequation, reduces to: energy quality). No wonder that energy is defined as D u ^ ability to perform work ,and aspecial quantity exergy is ð r Z K p V $ V C F k C F C V $ ðÞk V T ð 13Þ defined as the maximum possible work that may be Dt obtainedfrom asystem by bringingittothe equilibrium in ð 2 aprocess with reference surroundings (called dead state). Where, F k Z k V $ V is the bulk-viscosity dissipation, The maximum possiblework will be obtained if we and F is the shear-viscosity dissipation function, which is prevent energy disorganization, thus with limiting rever- the rateofmechanical work conversion to internal thermal sible processeswhere the existing non-equilibrium is energy for adifferential volume per unit of volume, with conserved within interacting systems. Since the energy is [ W/m3 ]unit, is given for Newtonian fluid as: conserved during any process, only in ideal reversible "#processesentropy (measure of energy disorganization or _ 2 2 2 d W F v u v v v w degradation) and exergy (maximum possible work with F Z Z C C d V v x v y v z regard to the reference surroundings) will be conserved, while in real irreversibleprocesses, the entropy will be " v v v u 2 v w v v 2 generated andexergy will be partly (or even fully) C m C C C destroyed. Therefore, heat transfer and thermal energy v x v y v y v z are universal manifestations of all natural and artificial # (man-made) processes, where all organized potential and/ v u v w 2 C C or quasi-kinetic energies are disorganizedordissipatedin v z v x the form of thermal energy, in irreversible and spontaneous processes. 2 K m V $ V ð 2 3 Reversibility and Irreversibility: EnergyTransfer ð 14Þ and Disorganization, and EntropyGeneration

The power or work rate of viscous dissipation (irreversible Energy transfer (when energy movesfrom one system or conversion of mechanical to thermal energy) in acontrol subsystemtoanother) through asystem boundary and in

10.1081/E-EEE-120042342—21/6/2007—18:56—VELU—221753—XML Taylor &Francis Encyclopedias Physics of Energy 1169 time, is of kinetic nature, and may be directionally expansion work) to the same pressure and volume, as organized as work or directionally chaotic and disorga- illustrated in Fig. 10Aand B, respectively. Pub nizedasheat. However, the net-energy transfer is in one If heat or work at higherpotential (temperature or direction only, from higher to lower energy–potential, and pressure) than necessary, is transferred to asystem,the Phot the process cannot be reversed. Thus all processes are energy at excess potential will dissipate spontaneously to irreversible in the direction of decreasing energy–potential alower potential (if left alone) before anew equilibrium (like pressure and temperature)and increasing energy– state is reached, with entropy generation, i.e., increaseof displacement (like volume and entropy) as aconsequence entropy (energy degradation per absolutetemperature of energy and mass conservationinthe universe. This level).Asystem will “accept” energy if it is transferred implies that the universe (as isolated and unrestricted at minimum necessary(infinitesimally higher) or higher system) is expending in space with entropy generation (or potential with regard to the system energy–potential. increase) as ameasureofcontinuous energydegradation, Furthermore, the higherpotential energywill i.e.,energyredistributionand disorganization. It is dissipate and entropy increasewill be the same as with possible in the limittohave an energytransfer process minimum necessary potential,like in reversibleheating with infinitesimal potential difference (still from higherto process, i.e.: infinitesimally lower potential, P ). Then, if infinitesimal ð change of potentialdifferencedirectionisreversed d Q d Q ( P C d P / P K d P ,with infinitesimally small exter- d S Z or S Z C S ref ð 16Þ nal energy, sincedP / 0), the process will be reversed too, T T which is characterizedwith infinitesimal entropy generation, thus in the limit, without energydegradation (no further energy disorganization) and no entropy generation—thus achieving alimiting, ideal reversible process. Such processesatinfinitesimalpotential differences, calledquasiequilibrium processes, maintainthe system equilibrium at any instant butwith incremental changesin time.Only quasiequilibrium processes are reversibleand vice versa.Ineffect, thequasiequilibriumreversible processes are infinitely slow processesatinfinitesimally small potential differences, but they couldbereversedto any previous state, and forwarded to any future equili- briumstate, without any “permanent” change to the rest of thesurroundings. Therefore,the changesare “fully” reversible, and alongwith their rate of change and time, completely irrelevant, as if nothing is effectively chan- ging—the time is irrelevantasifitdoes not exist since it couldbereversed (no permanent change and relativityof time). Since the real time cannot be reversed, it is a measure of permanent changes, i.e., irreversibility, which is in turn measured by entropy generation. In thisregard the time and irreversible entropy generation are related.[2] Entropy is also asystem property, which together with energy defines its equilibrium state, and actually represents the system energy–displacement or randomenergy disorganization (dissipation)per absolute temperature levelover its mass and space it occupies. Therefore, the entropy as property of asystem,for agiven system state, is the same regardless whether it is reached by reversible heat transfer, Eq.16, or irreversibleheat or irreversible worktransfer (adiabatic or caloric processes on Fig. 8B and C). For example, an ideal gas system entropy increase will be the sameduring areversible isothermal heat Fig. 10 (A) Isothermal reversible heat transfer and restricted transfer and reversible expansion to alower pressure (heat- reversible expansion; (B) Adiabatic unrestricted irreversible in equal to expansion work-out), as duringanirreversible expansion of the same initial system to the same final state (Thus adiabatic unrestricted expansion (no heat transfer and no the same system entropy change).

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However, the sourceentropy will decrease to asmaller generation ( S G Z D S S O 0). Similarly, during reversible Pub extent over higherpotential,thus resulting in overall adiabatic expansion, the system internal thermal energy entropygeneration forthe two(or all) interacting will be reduced and transferred in organized expansion Phot systems, which maybeconsidered as acombined work with no change of system entropy (isentropic isolated system (no energy exchange with the rest of process),since the reduction of disorganized internal the surroundings). Thesame is true for energy exchange energy andpotential reductionofentropy will be betweendifferent system parts (could be considered as compensated with equal increaseofdisorder and entropy subsystems) at different energy potentials(non uniform, in expanded volume. The process couldbereversed back- not at equilibrium at agiventime). Energy at higher and-forth (like elastic compression–expansion oscillations potential (say close to boundary within asystem) will of asystem) without energy degradation and entropy dissipate (“mix”) to parts at lower energy potential with change,thusanisentropicprocesses.Inreversible larger entropy increasethan decrease at higherpotential, processesenergy is exchanged at minimum-needed, not resulting in internal irreversibility and entropy gener- higher than neededpotential, and isolated systems do not ation, i.e., energy “expansion” over moremass and/or undergo any energy–potential related degradation/disor- space with lower potential. Entropy is not displacement ganization,and with total conservationofentropy. The for heat only as often stated,but also displacement for total non-equilibrium is conserved by reversible energy any energy dissipation (energy disorganization) and the transfer within interacting systems, i.e., during reversible measureofirreversibility. Examples are unrestricted or processes. throttling expansion with no heat exchange but entropy There are two classical statements of the Second Law generation. Therefore, entropy generation is fundamental (both negative, about impossibility), see Fig. 11. [7,8] One is measureofirreversibility or “permanent change.” the Kelvin–Plank statement which expressesthe never Even thoughdirectionally organizedenergy transfer as violated fact that it is impossible to obtain work in a work, does not possess or generateany entropy (no energy continuous cyclic process ( perpetual mobile )from asingle disorganization, Fig. 1B), it is possibletoobtain work thermal reservoir (100% efﬁciency impossible, since it is from the equal amount of disorganized thermal energyor notpossibletospontaneously create non-equilibrium heat if such process is reversible. There are two typical reversible processeswhere disorganized heat or thermal energy could be entirely transferred into organized work, and vice versa. Namely, they are[2]:

T 1 , (>T2 ) T 1 , (>T2 )

1. reversibleexpansion at constant thermal energy, Q Q e.g., isothermal ideal-gas expansion ( d W Z d Q ), 1,IN 1,IN Fig. 10A, and W OUT 2. reversibleadiabatic expansion ( d W Z K d U ). HEAT HEAT W OUT = Q − Q ENGINE ENGINE 1,IN 2,OUT = Q 1,IN Duringareversibleisothermalheattransferand POSSIBLE NOT expansionofanideal-gassystem(S),for example, POSSIBLE Fig. 10A, the heat transferred from athermal reservoir Q 2,OUT=0 Q 2,OUT ( R )will reduceits entropyfor ( D S magnitude), wile ideal R T T gas expansion in space (largervolume and lower pressure) 2 2 will further disorganize its internal thermal energy and (A) Kelvin-Plank statement of the Second Law

increasethe gas entropy for ( D S S ), while in the process an T , (>T ) T , (>T ) organized expansionwork, equivalenttothe heat 1 2 1 2 Q Q = transferred, will be obtained(W 12Z Q 12). The process 1,OUT 1,OUT = Q W + Q could be reversed, and thus it is reversible process with 2,IN IN 2,IN

zero total entropy generation ( S G Z D S S K D S R Z 0). On the REVERSED REVERSED other hand, if the sameinitial system (ideal gas) is W HEAT W =0 HEAT IN ENGINE IN ENGINE expanded without any restriction(Fig. 10B, thus zero NOT expansion work) to the same ﬁnal state, but without heat POSSIBLE POSSIBLE transfer, the system internal energy will remainthe same Q 2,IN Q but more disorganized over the larger volume, resulting in 2,IN thesameentropy increase as duringthe reversible T 2 T 2 isothermal heating and restricted expansion.However, (B) Clausius statement of the Second Law this process can notbereversed, since no work was obtainedtocompressback the system,and indeed the Fig. 11 The Second Law: (A) Kelvin–Plank, and (B) Clausius system entropy increaserepresents thetotal entropy statements.

10.1081/E-EEE-120042342—21/6/2007—18:56—VELU—221753—XML Taylor &Francis Encyclopedias Physics of Energy 1171 within the single reservoir equilibrium), and the second T h Clausius statement refers to the direction of heat transfer, T Pub expressing the never violated fact that it is impossible for Q h Phot heat transfer to take placebyitself (without any work W W input) from alower to highertemperature thermal C T reservoir(it is impossibletospontaneously create Q c non-equilibrium).Actuallythe twostatements imply T each otherand thus are the same, as well as they imply c that all reversiblecycles betweenthe two temperature W W T − W C h th, C = = reservoirs (or all reversibleprocesses between the two Q h Q h Q − Q T states)are the most and equally efficient with regards to = h c = 1– c T extracting the maximum work out of asystem,orrequiring Q h h theminimumpossibleworkintothe system (thus S conserving the existing non-equilibrium). Aheat-engine cycle, energy conversion efficiency is defined as: Fig. 12 Heat engine ideal Carnot cycle.

W Q K Q h Z cycle; netOUT Z IN OUT cycle Q Q IN IN X _ X Q d Q i OUT S Z C m _ s Z 1 K ð 17Þ CV j j Q d t T i IN rev |fflfflffl{zfflfflffl} |fflfflBSffl{zfflfflffl} |fflfflfflINffl{zfflfflfflffl} RATE OF ENTROPY T OUT CHANGE IN CV BS TRANSFER TRANSPORT RATE Z 1 K Z h rev Z h max X RATE WITH Q WITH MASS IN ð 21Þ T IN _ K m _ k s k C S gen OUT |{z} The reversiblecycle efficiency betweenthe twothermal |fflfflfflfflffl{zfflfflfflfflffl} IRREVERSIBLE TRANSPORT RATE GENERATION RATE reservoirs does not depend on the cycling system, but only WITH MASS OUT on the ratio of the absolutetemperatures of the two reservoirs ( T Z T IN and T refZ T OUT), knownas Carnot cycleefficiency (last part in the Eq.17, see also below). The latter defines the thermodynamic temperature with the Heat Engines following correlation: HeatEngines are devices undergoing thermo-mechan- T Q ical cycles(transformingthermal into mechanical Z ð 18Þ energy), similar to one on Fig. 12,with mechanical T ref Q ref rev cycle expansionand compressionnet-work(W Z Q h K Q c ), obtained as the difference betweenthe heat transferred The Second Law efficiency is defined by comparing the to the engine from high temperature heat reservoir (at real irreversible processes or cycles with the correspond- T h )and rejected to alow (cold) temperature heat ing ideal reversible processesorcycles: reservoir(at T c ), thus converting part of the thermal energy into mechanical work. In aclose-system cycle W W thenet-work-outisdue to net-workofthermal- h Z OUT or h Z IN; rev II; OUT W II; IN W expansionand thermal-compression.Therefore, heat |fflfflfflfflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflfflfflfflOUT; revffl} |fflfflfflfflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflfflfflfflINffl} engine cyclecannotbeaccomplishedwithouttwo Energy Production Process Energy Consumption Process thermal reservoirs at different temperatures, one at ð 19Þ highertemperature to accomplish thermal expansion and work out, and another at lower temperature to The irreversibility ( I )isdue to the entropy generation accomplish thermal compressiontoinitial volume and

( S gen)and represents the lost workpotential or lost exergy thus complete the cycle. The combustion process itself ( E X,loss)with regard to reference system (surroundings at is an irreversible one, where chemical energy (electro- T o absolutetemperature), as expressed by the following chemical energy binding atoms in reactants’ molecules) correlation: is chaotically released during combustion, i.e., converted into randomthermal energyofproducts’ I Z E Z T S ð 20Þ X ; loss o gen molecules, and cannot be fully converted into direc- The entropy balance equation for the control-volume tionalworkenergy. The Second LawofThermo- ﬂow process, complementing the related energy balance dynamics limitsthe maximum amountofwork that Eq. 11, see Fig. 9, is[5–7]: couldbeobtained from thermal energy betweenany

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two thermal reservoirs at different temperatures, hot T h , combustion enginesundergothe Otto cyclewith Pub and cold T c ,byusing the ideal, reversible Carnot cycle, gasolinefuel and the Diesel cyclewith diesel fuel, see Fig. 12,with thermal efficiency given by Eq. 22. As while the steam and gas turbine power plants undergo Phot an example, consider T c Z 293 Kand T h Z 2273 K: the Rankine and Brayton cycles, respectively. However, with improvements in material properties,effective W Q K Q componentcooling,and combining gasand steam h Z Z h c turbine systems, the efficiencies above 50% are being th; C_ad Q Q h h achieved,which is asubstantial improvementoverthe usual 30%–35% efficiency. The idealCarnot cycle is an T c Z 1 K important reference to guide researchers and engineers T h T h Z T adZ 2273 K ; T c Z 293 K to better understand limitsand possibilities for new concepts and performanceimprovements of real heat 293 Z 1 K Z 87: 1 % ð 22Þ engines. 2273

where, W Z W K W ,isthe net-work of expansion, T C CONCLUDING REMARKS: ENERGY PROVIDES usuallyturbine ( W T ), andcompression ( W C ). The maximumefficiency is achieved if heat is supplied at EXISTENCE AND IS CAUSE FOR CHANGE the highest possible temperature T ,and released at the h Energy is fundamental property of aphysical system and lowest possible temperature T c .However, both temperatures are limited by the fact that fuel combustion is refers to its potential to maintain asystem identity or performed using oxygen with ambient air, resulting in structure and to influence changeswithothersystems maximumsocalled adiabatic,stoichiometric combus- (via forcedinteraction)byimpartingwork(forced tion temperature T ,which is for mostfuels about directional displacement) or heat (forced chaotic dis- ad placement/motion of asystem molecularorrelated 20008 C, or T adZ 2273 K. Apart of the heat supplied at hot temperature T ,mustbereleased to the surround- structures).Energyexistsinmanyforms:electro- h magnetic (including light), electrical, magnetic, nuclear, ings at cold temperature about T c Z 208 C Z 293K,which results in aCarnot efficiency of 87.1%, see Eq. 22 and chemical,thermal, and mechanical (including kinetic, Fig. 12.However, the fuel heating valueenergy Q Z elastic, gravitational, and sound);where, for example, HV electro-mechanical energy may be kinetic or potential, Q ad_var ,isnot all available at the adiabatic temperature of the products,but is distributed over their variable while thermal energy represents overall potential and temperature range from initial surrounding temperature chaotic motionenergy of molecules and/or related micro structure. before combustion T c ,tofinal adiabatic temperature T ad. For such avariable heat reservoir, alarge number The philosophicaland practical aspects of energy and (infinite in the limit)ofidealCarnot engines operating entropy, including reversibility and irreversibility couldbe at differenttemperatures(with d W Z d Q ), must be summarized as follows: employedtoachieve areversible cycle, resulting in the variable-temperature Carnot cyclewith the maxi- † Energy is afundamental concept indivisible from mum possible combustion-products-to-work conversion matter and space,and energy exchanges or transfers are efficiency[1]: associated with all processes (or changes), thus indivisible from time. † Energyis“thebuildingblock”and fundamental h th; Cvar max property of matter and space,and thus afundamental property of existence. For agiven matter (system lnð T ad= T c Þ Z 1 K structure)and space(volume)energy defines the ð T = T Þ K 1 ad c T adZ 2273 K ; T c Z 293 K system equilibrium state, and vice versa. † Foragivensystem state (structureand phase)addition Z 69: 7 % ð 23Þ of energywillspontaneously tend to randomly redistribute (disorganize, degrade) overthe The above Eq. 23 is valid if the cyclic medium has system smaller microstructure and space it occupies, constant specific heat, otherwise integration will be called thermal energy, equalizingthe thermal required. Due to engine materialproperty limitations energy–potential (temperature)and increasing the and other unavoidable irreversibilities, it is impossible energy–displacement (entropy), and vice versa. to reachthe ideal Carnot efficiency. Different actual † Entropy may be transferred from system to system by heat enginesundergosimilar butdifferent cycles, reversible heat transfer and also generated due to depending on the system design. For example, internal irreversibility of heat and work transfer.

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† Energy and mass are conserved within interacting Exergy: It is the maximum system work potential if it is systems(all of which may be considered as acombined reversibly brought to theequilibrium with reference Pub isolated system not interacting with its surrounding surroundings, i.e., exergy is ameasure of asystem non- systems), and energy transfer (in time)isirreversible equilibrium with regard to the reference system. Phot (inone direction) from highertolower energy– Heat: It is inevitable (spontaneous) energy transfer due to temperature differences (from higher to lower level), to a potentialonly, which then results in continuous larger or smaller degree without control (dissipative) via generation (increase) of energy–displacement, called chaotic (in all directions, non purposeful) displacement/ entropy generation, which is fundamental measureof motion of system molecules and related microstructure, irreversibility, or permanent changes. as opposed to controlled (purposeful and directional) energy † Reversible energy transfer is only possible as alimiting transfer referred to as work (see below). case of irreversible energy transfer at infinitesimally Heat Engine: It is adevice undergoing thermo-mechanical cycle small energy–potential differences, thus in quasiequili- that partially converts thermal energy into mechanical work brium processes, with conservationofentropy. Since and is limited by the Carnot cycle efficiency. The cycle such changes are reversible, they are not permanent mechanical expansion and compression net-work is obtained and, along with time,irrelevant. due to difference between heat transferred to the engine from ahigh temperature heat reservoir and rejected to alow temperature reservoir, thus converting part of thermal energy In summary, energy is providing existence, and if into mechanical work. exchanged, it has ability to perform change. Mechanical Energy: It is defined as the energy associated with ordered motion of moving objects at large scale (kinetic) and Glossary ordered elastic potential energy within the material structure (potential elastic), as well as potential energy in gravitational Energy: It is fundamental property of aphysical system and refers field (potential gravitational). to its potential to maintain asystem identity or structure and to Power: It is the energy rate per unit of time and is related to work influence changes with other systems (via forced-displace- or heat transfer processes (different work power or heating ment interactions) by imparting work (forced directional power). displacement) or heat (forced chaotic displacement/motion of System: (also Particle or Body or Object)refers to any, arbitrary asystem molecular or related structures). Energy exists in chosen but fixed physical or material system in space (from a many forms: electromagnetic (including light), electrical, single particle to system of particles), which is subject to magnetic,nuclear, chemical,thermal,and mechanical observation and analysis. System occupies so called system (including kinetic, elastic, gravitational, and sound). volume within itsown enclosureinterface or system Energy Conservation: It may refer to the fundamental law of boundary, and thus separates itself from its surroundings, nature that energy and mass are conserved, i.e., cannot be i.e., other surrounding systems. created or destroyed but only transferred from one form or one Temperature: It is ameasure of the average quasi-translational system to another. Another meaning of energy conservation is kinetic energy associated with the disordered microscopic improvement of efficiency of energy processes so that they motion of atoms and molecules relevant for inter-particulate could be accomplished with minimal use of energy sources collision and heat transfer, thus temperature is related to and minimal impact on the environment. relevant particle kinetic energy and not to the particle density in Energy Conversion: Aprocess of transformation of one form space. of energy to another, like conversion of chemical to thermal Thermal Energy: It is defined as the energy associated with the energy during combustion of fuels, or thermal to mechanical random, disordered motion of molecules and potential energy energy using the heat engines, etc. due to intermolecular forces (also associated with phase Energy Efficiency: Ratio between useful (or minimally necess- change), as opposed to the macroscopic ordered energy ary) energy to complete aprocess and actual energy used to associated with moving objects at large scale. accomplish that process. Efficiency may also be defined as the Total Internal Energy: It is defined as the energy associated ratio between energy used in an ideal energy-consuming with the random, disordered motion of molecules and process vs energy used in the corresponding real process, or intermolecularpotential energy (thermal), “binding” vice versa for an energy-producing process. Energy, as per the potential energy associated with chemical molecular conservation law, cannot be lost (destroyed), but part of structure (chemical) and atomic nuclear structure (nuclear), energy input which is not converted into useful energy is as well as with other structural potentials in force fields customarily referred to as energy loss. (electrical, magnetic, etc.). It referstothe “invisible” Entropy: It is an integral measure of thermal energy redistribu- microscopic energy on the subatomic, atomic and molecular tion (due to heat transfer or irreversible heat generation) scale. within asystem mass and/or space (during system expansion), Work: It is atype of controlled energy transfer when one system per absolute temperature level. Entropy is increasing from is exerting force in aspecific direction and thus making a orderly crystalline structure at zero absolute temperature (zero purposeful change (forced displacement) of the other systems. reference) during reversible heating and entropy generation It is inevitably (spontaneously) accompanied, to alarger or duringirreversibleenergyconversion, i.e.,energy smallerdegree(negligible in idealprocesses), with degradation or random equi-partition within system material dissipative (without control) energy transfer referred to as structure and space. heat (see above).

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REFERENCES 4. Kostic, M. Treatise with Reasoning Proof of the Second Law Pub of Energy Degradation,Copyrighted Manuscript; Northern 1. Kostic, M. Work, power, and energy, In Encyclopedia of Illinois University: DeKalb, IL, 2006.

Phot Energy,Cleveland, C.J., Ed.; Elsevier Inc.: Oxford, U.K., 5. Moran, A.J.; Shapiro, H.N. Fundamentals of Engineering 2004; Vol. 6, 527–538. Thermodynamics,4th Ed. Wiley: New York, 2000. 2. Kostic, M. Irreversibility and reversible heat transfer: the 6. Bejan, A. Advanced Engineering Thermodynamics;Wiley: quest and nature of energy and entropy, IMECE2004. In New York, 1988. ASME Proceedings,ASME, New York, 2004. 7. Cengel, Y.A.; Boles, M.A. Thermodynamics, An Engineering 3. Kostic, M. Treatise with Reasoning Proof of the First Law of Approach,5th Ed.; WCB McGraw-Hill: Boston, MA, 2006. Energy Conservation,Copyrighted Manuscript; Northern 8. Zemansky, M.W.; Dittman, R.H. Heat and Thermodynamics; Illinois University: DeKalb, IL, 2006. McGraw-Hill: New York, 1982.

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