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EnergyTransferandConversionMethods
MIT10.391J/22.811J/ESD.166J/11.371J/1.818J/3.564J/2.65
9/16/2010
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MissionofthisSession
IntroducetheimportanceandchallengEnergyConversion
Diffuseener sources
Thermodynamiclimits
Rateprocesses
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EnergyConversion
EnergyConversionistheprocessofchangingenergyfromoneformtoano
Energy
Source
Use
Ene
Energy
Conversion
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HistoricEnergyConversionSeque
Biomassheat(esp.cooking) Solar heat,dryclothes,dryfood
Solarisstillmainlightsource,noneedforconversion
Solarissourceofbiomass,wind,hydro,etc.
Biomassfarmanimalshorsepower,foLater, people also did these conversions:
Coalheat Hydromillingflour,runningmachinery
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o mec ca e ec r c y
ModernEnergyConversionSequeHeatingofBuildings:
Gas,oil,biomassheat
SolarheatElectricityGeneration:
Coal,gas,nuclearheatmechanicalelectric
Windmechanicalelectricity
SolarElectricity
Transportation:
Oil
gasoline,diesel,jetfuel
heat
mechanic Biomassethanolheatmechanical
F l ll G h d l i i
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EnergySources
Type of Energy Examples
PotentialEnergy Hydro
KineticEnergy Wind,Tidal
ThermalEnergy Geothermal,OceanTh
RadiantEnergy Solar
ChemicalEnergy Oil,Coal,Gas,Biomas
Nuclear Energy Uranium Thorium
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Solar Photovoltaics
Wind, hydro,waves tidal
Oceanthermal
Biomass
fuels
Chemical
Nuclear
Heat
Mechanical
work ElectricitElectricit
Geothermal
Fission &
fusion To end uses:residential, industrial,
transportationo
urces
Energy
Forms
So
urces
Energy Sources and Conversion Processes
Photosynthesis
Direct
thermal
Climate
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hair dryer 1 500 W
Scalesofenergyflows
cellphone 2W
laptopcomputer 10W humanbody(2000Caloriediet) 100W
1horsepower 750W
a r ryer , automobile 130,000W
1windturbine 2,000,000W(
757jetplane 5,000,000W(
Largepowerplant 1,000,000,000W(
Global energy use 15 000 000 000 000 W (
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Energy versus Power
EnergyEnergy EE ( in BTU, joules(J) or cal)( in BTU, joules(J) or cal)
PowerPowerPP== dEdE//dtdt
( BTU/hr, Watts(W))( BTU/hr, Watts(W))1 Watt = 1 Joule/Second1 Watt = 1 Joule/Second
Heat Flows versus Work
Energy per time can be used to describe heatEnergy per time can be used to describe heat
flow and work but to distinguish between theseflow and work but to distinguish between theseenergy flows we use notation:energy flows we use notation:
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OrderofMagnitudeofEnergyResour
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EnergySupplyUSASource
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ImportantMetrics
Energy Sources Conversion Meth SpecificEnergy(MJ/kg) ConversionEffic
EnergyDensity(MJ/L) Formofenergyp
Phase CO2generation
Impurities Waterusage
Cost Landusage
Cost
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.
TypicalSpecificEnergyValueFuel Higher Heating Value (M
Hydrogen 141.8
Methane 55.5Gasoline 47.3
Diesel 44.8
.
Lignite 25.1
DouglasFirWood 20.4
CornStover 17.8
Bagasse 17.3WheatStraw 17.0
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Energy Content of Fuels
Energy content of fuel is characterized by the heat produced by
Dry Fuel
Air
CompleteCombustion
Vapor: CO2,N2, SO2Cool25C
Liquid: H2O
Higher Heating Value (HHV) or Gross Calor
Dry Fuel CompleteCombustion
Vapor: H2
O,CO2, N2, SO2Cool25C
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KeyMetric:ConversionEfficien
ConversionProcess
Useful Energy O
Energy Loss
Energy Input
Whenproducingwork(mechanicalorelectr
=WorkOutput/EnergyInputWh d i i (di l h d
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Solar Photovoltaics
Wind, hydro,
waves tidal
Ocean
thermal
Biomass
fuels
Chemical
Nuclear
HeatMechanical
workElectricitElectricit
GeothermalFission &
fusion
Fossil fuels: Fuel cells
To end uses:
residential, industrial,transportation
Sourc
es
EnergyForms
Sources
Energy Sources and Conversion Processes
Photosynthesis
Direct
thermal
Climate
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Conversion Efficiencies
Conversion Type Ef
Natural Gas Furnace Chemical
Heat 90Internal combustion engine Chemical Mechanical 15
Power Plant Boilers Chemical Heat 90
Electricity Generator Mechanical Electricity 98
Gas Turbines Chemical Mechanical 35
Hydro Grav. Potential Mechanical 60
Geothermal ThermalMechElectricity 6-Wind Kinetic MechElectricity 30
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Overall Efficiency includes Steps Upstr
& Downstream of the Energy Conversion A linked or connected set of energy efficiencies from extraction to use:
n
Overall efficiency= overall = ii=1
= overall gasextraction gasprocessin g gastransmission powerplant electricitytransmissionKey Efficiencies include:
Fuel production Fuel Transport Transmission Energy Storage
for example compressed air energy storage (CAES):
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Energy Conversion
Laws of Thermodynamics provide limits
Heat and work are not the same
They are both energy, but..cannot convert all heat to work
Each conversion step reduces efficiency
reversible processes All real processes are irreversible
Losses always occur to degrade the
efficiency of energy conversion and reduce
work/power producing potential
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em ca ec ons
RateProcessesinEnergyConvers
HeatTransfer MassTransfer
Ch i l tiem ca Reac ons
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Fluid mechanics
Fluxesofheat,material,electronsmustbe
bygradientsinfreeenergydT
Fourierslawofheatconduction q= kdx
Fickslawofdiffusion j= DdC
dx
Fluidmechanics Flowinpipe=
2
fRe
I dV=
Ohmslawofcurrentflow A dx
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Heat Transfer
For heat to be transferred at anappreciable rate, a temperaturedifference ( T) is required.
Q = U A T
The non-zero T guaranteesirreversibility
As T does to zero, area and costgoes to infinity
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Heatexchangers
Manyvarietiesofheat
exchangersusedinenergyconversion Heatrecoverysteamgenerators
(HRSG)
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Shell-and-tubeexchangers Plate-finexchangers
CoolingTower
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HumanitysMainEnergySourChemicalreactions
Virtuallyallfossilfuelsandbiofuelsareconvertedusefulenergyviachemicalreactionsatarateof~1
Energyreleasedbyconversionreactionscanbecon
Somereactionsareusedtoconvertaprimaryenerg
sourcestomoreusefulformsofchemicallystorede
SolidfossilfuelsLiquidfuels NaturalGas
Hydrogen BiomassLiquidfuels
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ChemicalReactions
Chemical reactions either require or release heat.
CH4 + 2 O2CO2 + 2 H2O Hrxn = -890 kJ/mol
Exothermic reaction: one that gives off energy. Hrxn < 0.Endothermic reaction: one that requires energy. Hrxn > 0.
Except in unusual situations (e.g. fuel cells, chemiluminescenceessentially all of the Hrxn is released or supplied as heat.
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Examples of Energy Conversion Reac
Fuel combustion
CH4 + 2 O2 = CO2 + 2 H2O natural gas C8H12 + 11 O2 = 8 CO2 + 6 H2O gasoline C6H12O6 + 6 O2 = 6 CO2 + 6 H2O cellulosic bi
Hydrogenproduction CH4 + H2O = CO + 3H2 steam reforming of m CO + H2O = CO2 + H2 water gas shift reaction
Hydrogenfuelcell H2 + O2 = H2O + electricity + heat
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CH O 0 33 H O CO 1 08 H
GasificationtoSyngasFossil fuelor biomass
Steam or oxygen
Gasifier800C
Electricity,CH4, H2,Gasoline/d
methanol
Syngas:CO
H2
CH O + 0.33 H OCO + 1.08 H . .wood steam syngas
Hr = +101 kJ/mol700-900C, 1 atm
Heat required to drive thisendothermic reactionusually provided by partial
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Water-gas-shift and methanation reacCOH2 WGS
reactor
H2CO2
CO + H OCO + HHr = -41 k400-500C
Water gas shift
H2O
COH2
Methanation
reactor
CO + 1 08 H 0 52 CH + 0 48 CO + 0 04 H OHr = -12400C 10
Methanation
CH4CO2H2O
Iron oxide c
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Fischer-Tropsch Reaction:Syngas to Liquid Fuels
Ideal FT reaction:
(2n+1) H2 + n COCnH2n+2 + n H2O
COH
2
F-T
Many simultaneous reactions alcohols, alkenes, etc.
Applications: Coal-to-liquids
reac or
Alkanes (gasoline, d
Alcohols
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RatesofchemicalreactionA + B C + D
Chemicalreactionrates
arefunctionsofthe Forward rateconcentrationof rf =kf[A]nA[B]nreactingspecies.
Backward rate
rb =kb[C]nC[D]n ForwardandBackwardOverall rate
Reactionsrunning
simultaneously:needa r=k [A]nA[B]nB k [f bfree-energydifferencetodriveinone Rate definition
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Reactionratesarestrongfunctionsoftemperature
Chemicalreactions rk=Aexp
ERT
generallyaccelerate
dramaticallywith ln ktemperature
TypicalvaluesofEA are
~200kJ/mol.
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catalysts can accelerate
Catalysts
Catalystsacceleratechemicalreactions.
Inmixtureswithmanyreactionspossible,
catalystscanacceleratedesiredreactionstoincreaseselectivity.
Catalystsdontchangethe
equilibrium. Catalystsdontchange
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Po
gen
Hy
rec
Transpo
fue
Wax
hydrocracking
Product
recoveryFT
process
WGS
Gas
treatment
Synthesis gas
production
Air sep.
plant
Prep
Coal
Air
Liquid
fuels
Mid-distillate
Liquid
fuels
Wax
Tail
gas
Ele
H2
N2O2
H2
CO2 H2S
CO
Coal-to-Liquids Conversion
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Po
gen
Hy
rec
Transpo
fue
Wax
hydrocracking
Product
recoveryFT
process
WGS
Gas
treatment
Synthesis gas
production
Air sep.plant
Prep
Coal
Air
Liquid
fuels
Liquidfuels
Wax
Tail
gas
Ele
H2
N2O2
H2
CO2 H2S
CO
Coal-to-Liquids Conversion
Reaction Kinetics - Key to Performance
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Po
gen
Hy
rec
Transpo
fue
Wax
hydrocracking
Product
recoveryFT
process
WGS
Gas
treatment
Synthesis gas
production
Air sep.
plant
Prep
Coal
Air
Liquid
fuels
Liquid
fuels
Wax
Tail
gas
Ele
H2
N2O2
H2
CO2 H2S
CO
Coal-to-Liquids Conversion
Mass Transfer: Key to Reactions & Separations
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Po
gen
Hy
rec
Transpo
fue
Wax
hydrocracking
Product
recoveryFT
process
WGS
Gas
treatment
Synthesis gas
production
Air sep.
plant
Prep
Coal
Air
Liquid
fuels
Liquid
fuels
Wax
Tail
gas
Ele
H2
N2O2
H2
CO2 H2S
CO
Coal-to-Liquids Conversion
Heat Transfer Size & Cost
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Pogene
Hy
rec
Transpofue
Wax
hydrocracking
Productrecovery
FTprocess
WGS
Gas
treatment
Synthesis gasproduction
Air sep.
plant
Prep
Coal
Air
Liquid
fuels
Mid-distillate
Liquid
fuels
Wax
Tail
gas
Ele
H2
N2O2
H2
CO2 H2S
CO
Coal-to-Liquids Conversion
Thermodynamics set the limits
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Cooli tower rate of eva ati LHV limit
CommonConversionEfficienChallenges,Part1
ThermoLimitonConversionofheattowor Work
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CommonConversionEfficienChallenges,Part2
EnergyResourcesFarFromUsers
Realsecurity,globaleconomyissues Takesenergyandmoneytomovetheresourceorelectricitytotheusers
Convenience,Reliability,EmissionsMatter SolidFuelsDifficulttohandle(sowedontusecoalforshipsanymore
EnergyDensityandSpecificEnergyMatters Lotsoflandneededtocollectdiffuseresourceslikesolar,wind,biomas
Transportcostsandtransportenergysignificantforlow-energy-densitynaturalgas,hydrogen)
PowerDensityMatters Energyconversionequipmentisexpensive,wanttodoalotofconversi
smallequipment:LargeFluxesrequired,soLargeFreeEnergyGradien Fortransportation,needtocarrytheenergyconversionequipmentwith
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22.081J / 2.650J / 10.291J / 1.818J / 2.65J / 10.391J / 11.371J / 22.811J / ESD.166JIntroduction to Sustainable EnergyFall 2010
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