optical radiation measurments for photo vol tic applatest

8/6/2019 Optical Radiation Measurments for Photo Vol Tic Applatest

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OPTICAL RADIATION

MEASURMENTS FORPHOTOVOLTAIC APPLICATIONS:

INSTRUMENTATION

UNCERTAINITY ANDPERFORMANCE

Solar Standards & Testing Workshop


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OVERVIEW

The basic calibration and measurement

uncertainty associated with this instrumentation

are based on guidelines described in ISO and

BIPM guide.

ThereThere areare twotwo typestypes ofof radiometricradiometric instrumentationinstrumentation usedused forforcharacterizingcharacterizing broadbandbroadband andand spectralspectral irradianceirradiance forfor PVPVapplicationsapplications

1.1. Spectral radiometric measurementSpectral radiometric measurementsystems(systems( spectroradiometersspectroradiometers))

2.2. Broadband radiometers (Broadband radiometers ( pyranometerspyranometers andandpyrheliometerspyrheliometers).).



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1.1. Spectro radiometers: Used toSpectro radiometers: Used to

measure spectral distributionmeasure spectral distribution

of solar simulators and naturalof solar simulators and natural

sunlight.sunlight.

Spectroradiometers & Broadband Radiometers

2.2. Broadband radiometers: Used toBroadband radiometers: Used to

access solar recourses foraccess solar recourses for

renewable applications andrenewable applications and

develop and validate broadbanddevelop and validate broadband

solar radiation models forsolar radiation models for

estimating system performanceestimating system performance

outdoorsoutdoors



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SpectroradiometerSpectroradiometer includeinclude classicclassic scanningscanning gratinggrating monchromatormonchromator andand diodediode arrayarrayspectroradiometerspectroradiometer.. NISTNIST opticaloptical techtech.. divisondivison providesprovides aa calibratedcalibrated 10001000WW incandescentincandescent

tungstentungsten halogenhalogen lamplamp withwith tabulatedtabulated spectralspectral irradianceirradiance data(data( 3030 wavelength)wavelength) forfor thesethese

radiometersradiometers..

Spectroradiometer Calibration & Measurement

Fig. shows calibration geometry for diode array spectrometer.Fig. shows calibration geometry for diode array spectrometer.

500 mm



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Graph: Response curve

Wavelength

(nm)

Typical

Value

(W/cm*-3)

Relative expanded

uncertainty in %,k=2

250 0.2 1.8

350 7 1.1

655 170 0.9

900 215 1.1

1600 115 1.4

2400 40 4.4

Table: Statement of uncertainty withTable: Statement of uncertainty withspectral calibration (NIST)spectral calibration (NIST)

Spectroradiometer Calibration & Measurement



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Historically total uncertainty) was computed asHistorically total uncertainty) was computed as

U*2=U*2= (bias)*2 +(bias)*2 + (2 x random)*2(2 x random)*2

Random values were related to variance of measured data sets.Random values were related to variance of measured data sets.

Biases were estimates of deviations from a true value.Biases were estimates of deviations from a true value.

For uncertainty measurement GUM defines two types of values:For uncertainty measurement GUM defines two types of values:

Type A values derived from statistical methodType A values derived from statistical method

Type B values derived by other means such as scientific judgment, experience,Type B values derived by other means such as scientific judgment, experience,

specs. ,comparisons.specs. ,comparisons.

GUM replaces historical factor of 2 with coverage factorGUM replaces historical factor of 2 with coverage factorkk andand

U*2=U*2= (Type B)*2+(Type B)*2+ (kx Type A)*2(kx Type A)*2

U is expanded uncertainty and kis in the range 2 to 3 for confidence intervals ofU is expanded uncertainty and kis in the range 2 to 3 for confidence intervals of

95% and 99% respectively.95% and 99% respectively.

Uncertainty Analysis



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For each parameter Type A and Type B estimates are based on specs. OfFor each parameter Type A and Type B estimates are based on specs. Of spectroradiometerspectroradiometer,,

previous measurements or educated estimates.previous measurements or educated estimates.

Lab Spectral Calibration Uncertainly

Table: Uncertainties for 95% confidence interval, Spectroradiometer calibration 250 nm to 1600 nm

Type A (Statistical) UNC(%) STDUNC(%)

distance(2/500mm) 0.8 0.4

wavelength precision 0.01 0.005

power current (Irr dl/di%)* 0.2 0.2

NIST lamp precision 1.13 0.565

detector sig/noise 1.00E-04 5.00E-04

sig detection system 1 0.5

temp sensitivity 1 0.5

observed noise 3 1.5

Type B UNC(%) STDUNC(%)

NIST transfer 1.82 0.91

distance 0.8 0.4

Stray Light 1.00E-04 0.00005

Lamp

Alignment 0.1 0.05

Power Current* 0.2 0.2

shunt bias(-

.000002) 0.04 0.02

wavelength 0.01 0.005

Effective Degree of freedom >100

Coverage Factor (k) 2

Confidence Interval 95%

Expanded Uncertainty 4.15%

TOTAL UNCERT(%) STD UNCERT(%)

TYPE A 3.6 1.808

TYPE B 2.001 1.015

Combined 4.154 2.077



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An analysis similar to that in previous table can be conducted on wavelength by wavelengthAn analysis similar to that in previous table can be conducted on wavelength by wavelengthbasis.basis.

For exFor ex : Fig. compares the measurement of 7 NIST spectral irradiance standards as unknown: Fig. compares the measurement of 7 NIST spectral irradiance standards as unknownsources, using a system calibrated using 8sources, using a system calibrated using 8thth lamp.lamp.

Envelope of estimated standard uncertainties is shown by thickhatched lines.

Lab Spectral Calibration Uncertainty



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Experience has showed that 1% error produces a 9% irradiance error at 300nmExperience has showed that 1% error produces a 9% irradiance error at 300nm

and a 4% irradiance error at 1000nmand a 4% irradiance error at 1000nm

Combined uncertainties are the root sum square (RSS) of type A and type BCombined uncertainties are the root sum square (RSS) of type A and type B

standard uncertainties.standard uncertainties.

Expanded uncertainties is the RSS of type A and type B standard uncertaintiesExpanded uncertainties is the RSS of type A and type B standard uncertaintieswith the coverage factorkapplied to achieve the desired confidence intervals.with the coverage factorkapplied to achieve the desired confidence intervals.

Lab Spectral Calibration Uncertainty



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BROADBAND RADIOMETER

CALIBRATION AND MEASURMENTS

Broadband solar radiation measurements are important inBroadband solar radiation measurements are important in pvpv module and arraymodule and arrayperformance monitoring and evaluation.performance monitoring and evaluation.

The basis for the calibration of these instruments is the group of 7 absoluteThe basis for the calibration of these instruments is the group of 7 absolute

cavity radiometers (ACR) denoted as World Standard Group the mean ofcavity radiometers (ACR) denoted as World Standard Group the mean ofwhich establish the world radiometric ref. (WRR)which establish the world radiometric ref. (WRR)

World radiometric ref. and calibration techniques:World radiometric ref. and calibration techniques:



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ResponsivityResponsivity of a diffusedof a diffused pyranometerpyranometer

Rsd=(U-S) /[B * cos (Z)]

UU unshadedunshaded o/p voltage from sensorso/p voltage from sensors

SS shaded o/p voltage from sensorsshaded o/p voltage from sensors

ZZ Zenith angle (45 deg.)Zenith angle (45 deg.)

BB is measured by ACRis measured by ACR

PyrheliometerPyrheliometer reponsivitiesreponsivities ((Rs,ORs,O/P/P signalsignal perper stimulusstimulus unit)unit) areare derivedderived byby directdirectcomparisonscomparisons withwith refref.. ACRsACRs traceabletraceable toto WRRWRR..

PryanometrerPryanometrer responsivitiesresponsivities are derived from the component summation technique by usingare derived from the component summation technique by using

G=B cos (Z)+D

GG ref. global irradianceref. global irradianceBB BeamBeam measurmentmeasurment of cavity radiometerof cavity radiometer

DD shadedshaded pyranometerpyranometer ( diffuse) measurement( diffuse) measurement

Broadband Radiometer Calibration And

Measurements



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Several types of detectors such as silicon cells and thermal detectors such asSeveral types of detectors such as silicon cells and thermal detectors such as

resistance thermometers, thermopiles are used forresistance thermometers, thermopiles are used for pyrheliometerspyrheliometers andand

pyranometerspyranometers..

Radiometer Uncertainty SourcesRadiometer Uncertainty Sources

Thermal OffsetsThermal Offsets

BSRN have characterized thermal zeroBSRN have characterized thermal zerooffsets in thermopileoffsets in thermopile pyranometrespyranometres with allwith all--

blacksensors measuring diffuse radiation.blacksensors measuring diffuse radiation.

The offsets in the shaded andThe offsets in the shaded and unshadedunshaded

states are different and are a source ofstates are different and are a source of

uncertainty in the shadeuncertainty in the shade unshadeunshade

calibrations.calibrations.

MODTRAN atmospheric spectralMODTRAN atmospheric spectral radiativeradiative

transfer code is used to compute short wavetransfer code is used to compute short wave

and long wave direct beam and skyand long wave direct beam and sky

radiationradiation



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As shown in previous graph, direct beam has significant energy in short waveregion from 1000nm to 2800nm,therefore for several different water vapour

conc. and direct normal irradiances same shaded signal is possible from the

pyranometer. by varying total precipitable water vapour from 0.5 atm-cm to 3.5

atm-cm,MODTRAN modeling result in differences of about 0.5% in Rs.

Geometric, Enviormental And Equipment Uncertainty

Additional well known contributors to radiometer calibration and measurment

uncertanity includes: accuracy of zenith angle calculation,non-Lambertian

cosine response of detector surface, temp. coffecients, linearity andelectromagnetis interfearence

Other Spectral Effects



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UshadeUshade computed from the propagation of error formula for shadecomputed from the propagation of error formula for shade--unshadeunshade calibration eq. is:calibration eq. is:

U*2(shade)=[U*2(shade)=[(U)(U)Rs.eRs.e(U)]*2 + [(U)]*2 + [(S)(S)Rs.eRs.e(S)]*2 + [(S)]*2 + [(B)(B)Rs.eRs.e(B)]*2 + [(B)]*2 + [(Z)(Z)Rs.eRs.e(Z)]*2(Z)]*2

For component summation, propagation of error formula becomes:For component summation, propagation of error formula becomes:

U*2(SUM)=[U*2(SUM)=[(U)(U)Rs.eRs.e(U)]*2 + [(U)]*2 + [(D)(D)Rs.eRs.e(D)]*2 + [(D)]*2 + [(B)(B)Rs.eRs.e(B)]*2 + [(B)]*2 + [(Z)(Z)Rs.eRs.e(Z)]*2(Z)]*2

Sensitivity Functions

Shade unshadesensitivityfunctions

Summationsensitivityfunctions

Shade -unshade totaluncertainty

Summationtotaluncertainty


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ResponsivityResponsivity functionsfunctions derivedderived fromfrom calibrationcalibration datadata withwith thethe offsetsoffsets embeddedembedded inin

thethe resultresult shouldshould bebe usedused toto retrieveretrieve thethe mostmost accurateaccurate irradianceirradiance fromfrom aa

pyranometerpyranometer..GraphGraph showsshows pyranometerpyranometer responsivityresponsivity VsVs ZenithZenith angleangle.. UncertanityUncertanity

inin eacheach pyranometerpyranometer calibrationcalibration isis summarizedsummarized inin tabletable..

Responsivity Functions

Type A(Statistical) UNC(%) STD UNC(%)

WRR Transfer 0.2 0.2

Cos(z)(2Z bin) 0.01 0.005

Dif(2.5%d=>0.25%ref.) 0.125 0.063

Temp(2Z bin) 0.1 0.05

Data Logger Precision 5.00E-03 2.50E-03

ACR(wind,T) 0.025 0.013

Temp Chg(10C) 0.25 0.125

Diff Offset B&W 0.125 0.063

UUT IR OFFSET 0.25 0.125

EMI/Thermal EMF 0.01 0.005

Type B UNC(%) STD UNC(%)

Logger Bias 0.09 0.09

WRR Std U95 0.3 0.3

Cos(z);Z100

Coverage Factor(k) 2

Expanded Uncertainty 1.84%



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TheThe responsivityresponsivity for a given zenithfor a given zenithangle at the time of measurement,angle at the time of measurement,Rs(m),can be obtained from a fit of theRs(m),can be obtained from a fit of theform:form:

Rs(z)am/pm=Rs(z)am/pm= ai.cos*ai.cos*ii(z)(z)

{{ii=0 to 46 and=0 to 46 and aiai are 46 coefficients forare 46 coefficients foreach morning and afternoon set of Z}each morning and afternoon set of Z}

With this approach, uncertainty of aboutWith this approach, uncertainty of about++-- 1.8% in measured1.8% in measured pyranometerpyranometer datadatacan be achieved.can be achieved.



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When ACR andWhen ACR and pyrheliometerspyrheliometers are pointed at the sun, tracking errors may arise.are pointed at the sun, tracking errors may arise.The final tally of theThe final tally of the pyrheliometerpyrheliometer uncertainty components is shown in table. Withuncertainty components is shown in table. Withdeployment to the fielddeployment to the field pyrheliometerpyrheliometer data becomes subject to additional trackingdata becomes subject to additional trackingand window issues deferring data logger specifications, etc. These requires anand window issues deferring data logger specifications, etc. These requires anadditional analysis specific to the deployment for estimating total uncertainty in theadditional analysis specific to the deployment for estimating total uncertainty in thefield measurement.field measurement.

Pyrheliometer Uncertainities

Type A(Statistical) UNC(%) STD UNC(%)

WRR Transfer 0.2 0.2

Temp Response UUT 0.500 0.050

Data Logger Precision 0.005 0.0025

Linearity(empirical) 0.200 0.100

ACR(wind,T) .025 .013

Tracking Variations 0.125 0.250

Spectral(window) 0.500 0.500

EMI/Thermal EMF 0.010 0.005

Type B UNC(%) STD UNC(%)

Logger Bias 0.09 0.09WRR Std U95 0.3 0.3

Temp Response UUT 5.00E-01 0.25

ACR Bias(M,wind,T) 0.025 0.013

Temp B(event to

event)10C 0.25 0.125

Spectral Error 0.5 0.5

Tracking Bias 0.25 0.0125EMI/Thermal EMF 0.5 0.005

TOTAL UNCERT(%) STD UNCERT(%)

Type A 0.802 0.615

Type B 0.851 0.504

Combined 1.169 0.918

Effective Degree of freedom >100

Coverage Factor(k) 2

CONFIDENCE INTERVAL 95.00%

EXPANDED UNCERTAINTY 1.59%



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Sensitivity functions derived from the functional form of the shade unshadeand components summation pyranometer calibaration techniques show thatuncertanities in ignal voltages including thermal offset voltages,affectcalibration result the most.

Finally empirical comparisions of several solar radiometer instrumentation setsillustrate that the best measurment accuracy for broadband radiation is of the

order of 3%,and spectrally dependent ncertanity for spectro radiometersystems range from 4% in the visible to 8% to 10% in the ultra violet andinfrared regions.

CONCLUSIONS


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