Energy Transfer and Conversion Methods

8/13/2019 Energy Transfer and Conversion Methods

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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)

-

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


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


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


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


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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MIT OpenCourseWarehttp://ocw.mit.edu

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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Documents

Energy Transfer and Conversion Methods