alt 410 lecture 2 solar radiation 2010

8/4/2019 ALT 410 Lecture 2 Solar Radiation 2010

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LakshmiMunukutla 1ALT410

Solarradia7on

Lecture-2


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Outline

Radia&onproper&esSpectralirradiance

Blackbodyradia&onAnatomyofSunEffectofsunonEarthQuan&fyingsolarpowerSolarradia&onTerrestrialradia&onSunsenergypathtoEarthAtmosphericeffectsonsunsradia&onSolarradia&onspectrumAirMassconceptSolarradia&on

-Angulardependence-Earthrota&on

-Sunspath

-Azimuthangleandeleva&on

-Poweron&ltedsurface


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Radia7onProper7es

Theconceptsofwaveandenergyandtheircorrespondingequa&ons:

Energyunits:electron-volt(eV)

Powerdensitycanbecalculatedintermsofphotonflux.

Powerdensity:H(W/m2)

Photonflux:(photons/sec.m2)

Theproductofphotonflux&mesphotonenergygivesthe

powerdensity,H.


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Thespectralirradianceasa

func&onofphotonwavelength(orenergy),

denotedbyF,isthemost

commonwayof

characterisingalightsource.

Itgivesthepowerdensityat

apar&cularwavelength.The

unitsofspectralirradiance

areinWm-2m-1.TheWm-2

termisthepowerdensityat

thewavelength(m).

Therefore,them2refersto

thesurfaceareaofthelight

emiYerandthem-1

referstothewavelengthof

interest.

SpectralIrradiance

Thespectralirradianceofxenon(green),halogen(blue)andmercury(red)

lightbulbs(leZaxis)arecomparedtothespectralirradiancefromthesun

(purple,whichcorrespondstotherightaxis).


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BlackBodyRadia7on

Plancksblackbodyradia&oncurvesforincreasing

temperatures.Plancksworkonderivingthisequa&on

ledhimtoabreakthroughinunderstandingthe

quantumnatureofmaYer.Thesecurvesalsoshowthe

trendofshiZingpeakwavelengthsforincreasing

temperature,aspredictedbyWien.

Theblackbodyradia&onprovideswithasetof

preciseworkingequa&onsthatrelatethetemperatureofanobjecttothelightitemits.

Planckslawpredictstheenergydistribu&onacross

thespectrumforagiventemperature.

Stefan-Boltzmannlawcanbeusedtocalculatethetotal

emiYedpower.

Thewavelengthofthepeakemissiondominatedby

agivencolordominatedatthistemperatureisobtained

usingWiensdisplacementlaw.

Forincreasingtemperatures,thesequenceof

radiatedcolorsis:black,red,orange,yellow-

white,bluish-white.

Blackbodypowerdensityisgivenby

Wavelengthcorrespondingtopeakspectral

irradianceisgivenby

p (m) =Spectralintensity

Temperature


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Theore&calblackbodycurvefor5000K

BlackBodyRadia7on

Ablackbodyisatheore&calobjectthatabsorbs100%oftheradia&onthathitsit.

Therefore,itreflectsnoradia&onandappearsperfectlyblack.

Sofarnomaterialcameclosetobeconsideredasatrueblackbodyexceptcarbon

initsgraphiteformthatabsorbsapproximately97%oftheincidentradia&on.


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LakshmiMunukutla ALT410

Source:WWW.Space.com

AnatomyofSun

" Theprimarysourceofenergyfromthesuncomesfromthermonuclearreac&ons

(conver&nghydrogentohelium)inthecore.

" Typicalsunscoretemperaturerangeisfrom15to25milliondegrees.

" Theenergyfromthecoreradiatesthroughsunsmiddlelayer,thenbubblesandboils

tothesurfacethroughconvec&onprocess.

" Thechargedpar&cles,calledthesolarwindstreamoutatamillionmilesperhour.


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AnatomyofSun


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hYp://space.com/media/h_in_sun_energy_03.gif

hYp://space.com/media/h_in_sun_magnetosphere_03b.gif

TheEarth

SunsEffectonEarth

Sunasanenergysource


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SunSpots,SolarFlaresandEffectsonEarth

" SunSpots:Sunspotsarecoolerareaswhichappearasdarkpatcheswithinthesun.Sunspotsarecausedbyslowdownradia&onincertainareasdueto

magne&cfieldswithinthesun.

" SolarFlares:TheSunfrequentlyspewsplumesofenergy,essen&allyburstsofsolarwind.ThesesolarflarescontainGammaraysandX-rays,plusenergizedpar&cles

(protonsandelectrons).EnergyisequaltoabillionmegatonsofTNT

(Trinitrotoluene),isreleasedinamaYerofminutes.Flareac&vitypicksupassunspotsincrease." EffectonEarth:TheSun'scharged,high-speedpar&clespushandshapeEarth'smagne&cfieldintoateardropshape.Themagne&cfieldprotectsEarthfrommost

oftheharmfulsolarradia&on,butextremeflarescandisablesatellites

anddisruptcommunica&onsignals.Thechargedpar&clesalsoexcite

oxygenandnitrogenintheatmospheretocreatetheauroraborealis,

ornorthernlights.

Note:AgramofTNTbydefini&onforarmscontrolpurposesis1000

thermochemicalcalories,whichequals4.184kilojoules(KJ).

AmegatonofTNTis4.1841015joules=4.184petajoules(PJ).


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Quan7fyingSolarPower

hYp://ocw.mit.edu/termsSource:


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Quan7fyingSolarPower



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SolarRadia7onWhatissolarradia&on?

*Electromagne&cradia&onemiYedfromsun,whichpassesthroughthe

atmosphereandisreflectedinvaryingdegreesbyEarth'ssurfaceand

atmosphere.

*detectableonlyduringdaylight.

Sunisveryclosetobeconsideredasablackbodyatapproximately6000Kandwith

apowerdensitycloseto73MW/m2.

Usingtheequa&on

ThetotalpoweremiYedbythesuncan

beobtained.

Wherethesunsradiusis6.96x105km.Thepowerdensitycanbecalculatedatanygiven

distance,Dusing

Hsun=


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WhatisTerrestrialradia&on?

*EnergyemiYedfromtheEarthandatmosphere

*detectablebothdayandnight

Theimpactofearthsatmosphereonsolarradia&on

includes:

ScaYeringisoneoftheradia&onimpactmechanism.-DuetoscaYeringthesunlightreachestheearthssurface

atdifferentanglesfromthesky.

ThelightthatreachestheearthdirectlyfromsunwithoutscaYeringscaYeringiscalleddirectlight.

WhenscaYeringispresentonlydiffusedlightreachestheearth.Absorp&onissecondimpactmechanismthataltersthepowerdensityandterrestrialsolarradia&onspectraldistribu&on.

Athighphotonenergiesozoneabsorp&onoccurs.Ininfra-redregionabsorp&onofwatervaporandCO2takesplace.

Cloudsandotheratmosphericvaria&onsmayinducetemporaryvaria&onsinterrestrialsolarradia&on.

TerrestrialRadia7on


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SunsEnergyPathtoEarth


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AtmosphericEffectsonSunsRadia7on

Atmosphericeffectsincludeare:scaYering,absorp&onandreflec&on.


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Asthesolarradia&onpassestheatmosphere,apartoftheincidentenergyisremoved

byabsorp&onorscaYeringbymolecules,cloudsandaerosols.Wavelengthslessthan

300nmisfilteredoutbymoleculeslikeozone,nitrogenandoxygen.Dipsinthe

infraredareaarecausedbywaterandCO2. Source:WWW.Google.com/Images


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AirMass

" The Air Mass is the path length which light takes through the atmospherenormalized to the shortest possible path length (that is, when the sun is directly

overhead). The Air Mass quantifies the reduction in the power of light as it passesthrough the atmosphere and is absorbed by air and dust.

Theairmassrepresentsthepropor.onof

atmospherethatthelightmustpassthrough

beforestrikingtheEarthrela.vetoits

overheadpathlength,andisequaltoY/X

Entertheanglefromver&cal,=30

Airmass,AM=1.1547units

The Air Mass is defined as:

hYp://pvcdrom.pveduca&on.org/Source:

Duetovaria&onsinterrestrialsolarradia&on

astandardterrestrialsolarspectrumisdefined,

calledAM1.5G.(Gstandsforglobal,or

thedirect+diffuseradia&on).

Solarradia&onoutsidetheEarthsatmosphere

iscalledAM0.


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AirMass

Alterna&vemethodtodeterminetheairmassisfromtheshadowofaver&calpole.

Airmassisthelengthofthehypotenuse

dividedbytheobjectheighth.

Enter the object height, h = 1 units

Enter the shadow length, s = 1 unitsAir Mass, AM = 1.4142 units

Theabovecalcula&onforairmassassumesthattheatmosphereisaflathorizontallayer,

butbecauseofthecurvatureoftheatmosphere,theairmassisnotquiteequaltothe

atmosphericpathlengthwhenthesunisclosetothehorizon.Atsunrise,theangleofthesunfromthever&calposi&onis90andtheairmassisinfinite,whereasthepath

lengthclearlyisnot.Anequa&onwhichincorporatesthecurvatureoftheearthis:

hYp://pvcdrom.pveduca&on.org/Source:

11


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OrbitEllip7city

TheEarthsorbitaroundthesunisslightlyellip&cal

Resul&nginaslight

periodicChangein

theradiantsolarpowerdensityasa

func&onofseason.



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AngularDependenceofSolarRadia7on

TheanglebetweenthesunandtheEarthssurfaceischangingcon&nuallyduetotheEarthsrota&onaroundsunanditsrota&onalongitsaxis.

Thesunsposi&onischaracterizedbythreeangles:-Declina&onangle-Eleva&onangle-AzimuthangleCalcula&onofthesolaranglescanbeexpressedmoreconvenientlythedayoftheyear(d)asthenumberofdegrees,whichtheearthhasrotatedaroundthesunis

representedby.

ThedayoftheyearisreferencedfromtheSpringEquinox(d=81).


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EarthRota7on

Theangulardependenceofsolarradia&onpossesbothdailyandseasonalcomponent.

Both&ltoftheEarthsaxisandrota&onaroundthesuncausevaria&onintheangleatwhichthesunstrikestheEarth.

The&ltoftheearthsaxiswithrespecttothesuniscalleddeclina&onangle.

Themaximum&ltoftheearthsaxis(23.45)isequaltothemaximumvalueofthedeclina&onangle.


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SolarRadia7on

Atanygiven&methedeclina&onangleonearthateveryplaceissame.However,

thedeclina&onanglevarieswithseasons.


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EarthRota7on

Declina&onanglerepresentedby,isdependentoftheseasonsasdescribedearlier.

Thedeclina&onanglechangewithrespecttoseasonsisshowninthefigurebelowwith

respecttoequinox.

Themaximumdeclina&onangleoccursinsummerandminimumoccursinwinter.

wheredisthespecificnumberdayoftheyear


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SunsPath

Throughoutthedaythenetpathofsundependsontheazimuthandal&tudeangle.


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AzimuthandEleva7on


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Eleva7onAngle

Eleva&onangleand/oral&tudeareusedtorepresentthesunsangularheightinskyfromahorizontallevelontheearth.Note:Donotgetconfusedwithabovethesealevelmeasurementsofal&tudeandeleva&on.

Thesunsanglemeasuredfromthever&callevelontheearthgivestheenithangle.

Assunsposi&onchangesduringthedaytheeleva&onandzenithanglesalsovariesaccordingly.Themaximumeleva&onangleoccursatnoonatSummerSols&ceplusmaximumdeclina&on(23.45).

Theminimumeleva&onangleoccursatWinterSols&ceminusmaximumdeclina&on(23.45).

Forla&tudeshigherthan23onewillnotfindthesundirectlyinoverheadposi&on.


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Eleva7onAngle

Theeleva&onanglesmaximumandminimumvaluesareimpactedbybothla&tudeandseasons.Furthermore,themaximumvalueoftheal&tudeanglecanbedetermined

usingdeclina&onandla&tude.


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AzimuthAngle

Inthecaseofasolarmoduleorsolarcollector,

theazimuthangleisthehorizontalangle

betweenexactsouthandthedirec&onthe

surfaceofthedeviceisfacing.Theother

importantangleinvolvedinse{ngupasolar

moduleorcollectoristhe&ltangletheangle

atwhichthedeviceis&ltedawayfromthe

horizontal.

Theazimuthangledisplayedisthehorizontaldirec&onoftheSunatsunriseorsunset&mes.

Asonacompass,theazimuthismeasuredin

degrees,with360inafullcircle,countedina

clockwisedirec&onfromnorth.Northhasan

azimuthvalueof0degrees,eastis90degrees,

southis180degrees,andwestis270degrees.


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SolarRadia7on

Bothazimuthandeleva&onanglevaluesneedtobemonitoredduringtheday&metoperformhourlycalcula&ons.


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PoweronTiltedSurface

Theamountofradia&onreceivedbya&ltedsolarmodulesurfaceisequaltotheincidentsolarradia&oncomponentintheperpendiculardirec&ontothemodulesurface.

However,insolarradia&ondatathenormalcomponenttoahorizontalsurface,

Shorizontal.


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Summary

TheconceptswaveandenergyareexplainedusingElectromagne&cSpectrum.Radia&onpowerdensityiscalculatedbymul&plyingthephotonfluxwithphotonenergy.Typicallylightsourceischaracterizedbythespectralirradianceatagivenwavelength.Theblackbodyradia&onprovideswithasetofpreciseworkingequa&onsthatrelatethetemperatureofanobjecttothelightitemits.TypicallyStefan-Boltzmannlawisusedto

predictspectraldistribu&onatagiventemperature.

Theblackbodyisatheore&calobjectthatabsorbs100%ofincidentradia&on.However,thereisnomaterialthatreplicatestheblackbody100%.

Theprimarysourceofenergyfromthesuncomesfromthermonuclearreac&onsinthesunscore.Theenergyfromthesunscoreradiatesthroughsunsmiddlelayerbubblesandboilstothesurfacethroughconvec&onprocess.

TheElectromagne&cradia&onemiYedfromsunpassesthroughtheatmosphereandisreflectedinvaryingdegreesbyEarth'ssurfaceandatmosphereandisdetectableonly

duringDaylight.


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Summary

Theterrestrialradia&onistheenergyemiYedfromtheEarthandatmosphereandcanbedetectedbothdayandnight.TheAirMassquan&fiesthereduc&oninpoweroflightasitpassesthroughtheEarthsatmosphere.

Thesolarradia&onhasangulardependenceduetotheEarthsrota&onaroundsunandalongitsownaxiscausesvaria&oninsunsradia&onreachingtheEarth.

Theposi&onofthesunischaracterizedbythreeangles;declina&on,eleva&onandazimuth.Thehourlycalcula&onsofthedaycanbeperformedbasedontheseangles.

alt 410 lecture 2 solar radiation 2010

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