of 24
8/14/2019 0303 s 05 Therm Effi
1/24
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
8/14/2019 0303 s 05 Therm Effi
2/24
8/14/2019 0303 s 05 Therm Effi
3/24
8/14/2019 0303 s 05 Therm Effi
4/24
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
8/14/2019 0303 s 05 Therm Effi
5/24
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
8/14/2019 0303 s 05 Therm Effi
6/24
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.
8/14/2019 0303 s 05 Therm Effi
7/24
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
8/14/2019 0303 s 05 Therm Effi
8/24
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
8/14/2019 0303 s 05 Therm Effi
9/24
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
8/14/2019 0303 s 05 Therm Effi
10/24
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
8/14/2019 0303 s 05 Therm Effi
11/24
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
8/14/2019 0303 s 05 Therm Effi
12/24
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
8/14/2019 0303 s 05 Therm Effi
13/24
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
8/14/2019 0303 s 05 Therm Effi
14/24
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
8/14/2019 0303 s 05 Therm Effi
15/24
Case 2 DER fuel cell system
64 waste heat Fuel
Converter Fuel Cell Electloa
100fuel
3660
hydrogen electricity
O = 36/100 or 36%
8/14/2019 0303 s 05 Therm Effi
16/24
100
+ geothermal heat pump
Case 3 DER CHP microturbine
65 heat
35 140heatfuel electricity
Micro
Turbine
generator
Geothermalheat pump
COP = 4
HVAloa
O = 185
8/14/2019 0303 s 05 Therm Effi
17/24
WithO (energy used) / (energy co
fuel)O= 52 to 29 %
O= 36 %
+ geothermal heat pump
Case 1 Central station generator
Case 2 DER fuel cell system
Case 3 DER CHP microturbine
Sustainable EnergySustainable EnergyT lb l t #6T lb l t #6
8/14/2019 0303 s 05 Therm Effi
18/24
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
8/14/2019 0303 s 05 Therm Effi
19/24
Lets look a little deeper iheat to work cycle analys
8/14/2019 0303 s 05 Therm Effi
20/24
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.
8/14/2019 0303 s 05 Therm Effi
21/24
8/14/2019 0303 s 05 Therm Effi
22/24
8/14/2019 0303 s 05 Therm Effi
23/24
8/14/2019 0303 s 05 Therm Effi
24/24
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.