大连理工大学：《工程热力学》课程教学资源（PPT课件）5 火用分析基础 Exergy

5Exergy5.1 IntroductionQuantityEvaluationof heatQualityExergy and AnergyFull convertible energy:mechanicalPartial convertible energy:heatUnconvertible energy: environmental

5 Exergy Exergy and Anergy Evaluation of heat Quantity Quality 5.1 Introduction Full convertible energy: mechanical Partial convertible energy: heat Unconvertible energy: environmental

Exergy: useful work potential; available energy;The maximum useful work a system can deliveredfrom a specified state to the state of its environmentin theoryAnergy: unavailable energy, unconvertible energyConditionsfordefinitionofexergywork = f(initial state, path, final state)(1)basedonenvironment;exergyofenvironmentalenergyiszero(2)reversibleprocess(3) thereis nootherheatresourceintheprocess.e=e.+aE= E, +A

（1）based on environment; exergy of environmental energy is zero （2）reversible process （3）there is no other heat resource in the process. Exergy: useful work potential; available energy; The maximum useful work a system can delivered from a specified state to the state of its environment in theory. Anergy: unavailable energy, unconvertible energy E = Ex + An x an e = e + Conditions for definition of exergy work (initial state, path, final state) = f

5.2 Calculation of exergyTypeunbalanceChemicalChemical potentialPhysicalTemperature and pressureKineticVelocityPositionPositionConcentrationDistribution

5.2 Calculation of exergy Kinetic Distribution Chemical potential Temperature and pressure Velocity Position Concentration unbalance Position Physical Chemical Type

UnbalanceSourceElectricityvoltageWaterlevelHydraulicWindPneumaticpressureDifference betweenWaveinterior and surface

Source Electricity Hydraulic Pneumatic Wave voltage Water level Wind pressure Difference between interior and surface Unbalance

5.2.1WorkresourceElectricity, mechanical energy, pneumatic energy,hydraulic energy, can be converted to workentirely.Exergy of work resource = its total energy12E-mcxk2E, = mg△Zxp

5.2.1 Work resource Electricity, mechanical energy, pneumatic energy, hydraulic energy, can be converted to work entirely. Exergy of work resource = its total energy 1 2 2 xk xp E mc E mg Z = =  • • •

5.2.2 Heat exergyPotential work of heatT=(1-)8QSEroM=Q-T·△S=Q-TJExoWoAno =Q- Exo =T·△S中To

T T0 0 (1 ) x Q T E Q T   = − xQ 0 0 Q E Q T Q T S T  = − = −   5.2.2 Heat exergy Potential work of heat A Q E T S nQ xQ = − =  0 W0

TErQTo3......AnQ65sFor constant temperature heat resourceExo = Q(1 - 0)=Q-TAS=0=T.AS

0 0 (1 ) xQ T E Q Q T S T = − = −  0 nQ 0 T A Q T S T = =  For constant temperature heat resource T0

TI园ErQT.3AnQ56For finite heat resourceEx0o=J(1-)Q=J(1-)mcpT=Z-TT= mcp[(T, -T)- TIn0=Q(1-1

For finite heat resource 2 1 0 0 2 1 2 2 1 0 2 1 1 0 (1 ) (1 ) [( ) ln ] (1 ) T XQ P T P T T E Q mc dT T T T T T mc T T T T T T T Q T = − = −  − = − − − = −   1 2 2 1 ln T T T T T − = T0 T T0

Influential factors:Heat quantityHeat resource temperatureT个Ex ↑, △S ↓二T， Ex, △S个Environmental temperature T。T=T.Ex = 0If the system absorbs heat, it absorbs exergy;If the system discharges heat, it discharges exergy;

Influential factors: ⚫Heat quantity ⚫Heat resource temperature , , , xQ xQ T E S T E S           ⚫Environmental temperature T0 0 0 T T E = =  xQ If the system absorbs heat, it absorbs exergy; If the system discharges heat, it discharges exergy;

Example1kg air withtemperature of200Cwascooled to 4oC.Please calculate the heatTheisofairspecificheatexergy.theenvironmentalCp=1.004kJ/(kg.K)temperatureisT.=25C

Example 1kg air with temperature of 200℃ was cooled to 40℃. Please calculate the heat exergy. The specific heat of air is cp =1.004kJ/(kg.K). the environmental temperature is T0=25℃