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Thermodynamics and Efficiency An

Toolbox 6Sustainable Energy Energy chains and overall versus individual efficienc Playing by the rules

- First Law energy conservation

- Second Law - entropy generation- irreversibility

- Availability and exergy concepts max/min workPower generation via heat to work cycles

Rankine ( steam and other prime movers)BraytonCombined cycles

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Energy chains and efficienc

A linked or connected set of energy efficiencies from extraction to use:

n

Overall efficiency = = overall ii=1

overall = gas extractiongas proces singgas transmissionpower plantelectricity transm

for example for batteries: battery = rev,max rx voltagelosses

rev,max = Grx / Hfuel = nF / Hfuelo

G = nF RT

ln

(a )i

= n F rx e i species i e

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Energy Conservation and the F

of Thermodynamics

System and surroundings Heat and work interactions path dependent effec Mass flow effects First Law -- conservation of energy

E = Q + W +Hin min HoutmoutordE = Q + W + Hin min Houtmout

where E = total energy of the systemQ = net heat effect at system boundary W = net work effect at system boundary

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Figure removed for copyright reasons.

Source: Figure 4.6 in Tester, J. W., and M. Modell. Thermodynamics and its Applications. 3rd ed. Englewood

Cliffs, NJ: Prentice Hall, 1996.

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Energy and Enthalpy EnergyE contains the internal energy Usystem as well as other contributions eg. Kinertial velocity effects, PE due to body force

as gravity or electrostatic

For simple systems, that is those without ibody force effects

E = U

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Entropy and the Second La

Provides directionality for natural processe heat flows from a hot to a cold body rivers flow down hill

Describes in mathematical terms the maxamount of heat that can be converted into wo

Introduces the concept of entropy and dethe ratio of a reversible heat interaction to its

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Entropy and the Second La

Describes the maximum efficiency of a reversible Caengine in terms of heat source and heat sink temperat

Carnot = thermal = Max work produced / heat supplie

c = (T(hot) T(cold)) / T(hot)

For all reversible processes the total entropy is consFor all real processes the total entropy increases anassociated with increased levels of molecular disorder

mixture of two components versus two pure componenversus a liquid or solid phase

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Consider a fully reversible

process with no dissipative

effects that is all work is

transferred without loss and

all heat is transferred using an

ideal Carnot process to

generate additional work, The

resulting maximum work isSecondary system

given by Small Carnot engine

Ideal maximum work availa

Heat reservoir

Primary

system

QR

Qs

nout

B Hout Hin T (S Sin ) = HT o out oClearly, the availability B is a state function in the strictes

th ti l th i ( i i ) k

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Availability or Exergy Yields the maximum work producing potentialor the minimum work requirement of a processAllows evaluation and quantitative comparison ofsustainability context

B = change in availability or exergy= maximum work output orminimum work inp

,Tin PinB [ H T S]o ,Tout Poutnormally T ,P = ambient or dead state conditiout out

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Playing by the rules The 1st and 2nd Laws of thermodynam

relevant1st Law energy is conserved

2nd Law all real processes are irr

Heat and electric power are not the sa Conversion efficiency does not have adefinition

All parts of the system must work fu

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Consider three cases

Define efficiency asO output/input = (energy utilized) / (energy c

used)

+ geothermal heat pump

Case 3 DER CHP microturbine

Case 1 Central station generator

Case 2 DER fuel cell system

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Case 1 Central station generato

State of the art vs system average performan

Powerplant

T&D system Elecloa

100fuel

58 522932

electricityelectricity

O 52/100 52% t t f th t t

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64 waste heat Fuel

Converter Fuel Cell Electloa

100fuel

3660

hydrogen electricity

O = 36/100 or 36%

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100



65 heat

35 140heatfuel electricity

Micro

Turbine

generator

Geothermalheat pump

COP = 4

HVAloa

O = 185

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WithO (energy used) / (energy co

fuel)O= 52 to 29 %

O= 36 %


Case 1 Central station generator



Sustainable EnergySustainable EnergyT lb l t #6T lb l t #6

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Toolbox lecture #6Toolbox lecture#6

Thermodynamics and Efficiency Analysis MethodsThermodynamics and Efficiency AnalysisMethodsSupplementary notes to lecture materials and Chapter 3Supplementarynotes to lecturematerials and Chapter3

11.. FuFunnddamamenenttaall prpriinncciipplleess-- eennerergygy ccoonnsseerrvvaattiionon aanndd tthhee 11stst LaLaww ofof tthheerrmmoodydynnaammiiccss-- enenttrrooppyy pprroodduuccttiion anon and td thhe 2e 2ndnd LaLaww ofof tthheerrmmoodydynnaammiiccss-- rreevveerrssiibbllee CCaarrnnotot hheateat enenggiinneess

- maximum work / availability / exergy concepts -- B = H- To S- maximum work / availability / exergy concepts -- B = H- To S

2.2. EEffffiicciienencicieess-- mmeecchhaanniiccalal ddeevviiccee efefffiicciienencycy ffoorr ttuurrbbiinneses anand pud pummppss-- hheeaatt exexchchanange ege effffiicciienencycy

-- CCaarrnnoott eeffffiicciieennccyy-- ccyyccllee efefffiicciienencycy-- ffuueell eeffifficciieennccyy-- uuttiilliizzatatiioonn efefffiicciienenccyy

3.3. IIddealeal ccyycclleses-- CCaarrnnotot wwiitthh ffiixxed Ted THH anand Td TCC- Carnot with variable TH and fixed TC- Carnot with variable TH and fixed TC-- IIddealeal BBrrayayttoonn wwiitthh vvaarriiaabbllee TT

HH

anandd TTCC

44.. PPrraaccttiicacall ppoowweerr cycyclcleess- an approach to Carnotizing cycles- an approach to Carnotizing cycles-- RRaannkkiinne ce cyyclcleess wwiitthh ccoonndendenssiinngg sstteeaamm oror oorrganganiicc wwoorrkkiinng fg flluiuiddss

-- ssuub anb andd ssuuppeerrccrriittiiccaall ooppereratatiioonn-- ffeeed wed waatteerr hheateatiinngg-- wwiithth rerehheeaatt

-- BBrraayyttoonn nonon-n-ccoondndeennssiingng ggaass ttuurrbbiinnee ccyycclleess

-- CCoommbbiinneded gagass ttuurrbbiinnee anandd sstteeaamm RRaannkkiinne ce cyycclleses-- TTooppppiinngg anand bd boottttoommiinng ang andd dduualal cycycclleess

OOttttoo anandd ddiieesselel cycycclleess ffoorr iinntteerrnnalal ccoommbbuussttiionon eennggiinneess

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Lets look a little deeper iheat to work cycle analys

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Images removed for copyright reasons.

Source: Figure 14.7 in Tester, J. W., and M. Modell. Thermodynamics and its Applications. 3rd ed. Englewood Cliffs,

NJ: Prentice Hall, 1996.

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Images removed for copyright reasons.

Source: Tester, J. W., and M. Modell. Thermodynamics and its Applications. 3rd ed. Englewood Cliffs,

NJ: Prentice Hall, 1996. Figures 14.2-14.12, 14.16.

0303 s 05 therm effi

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