ASTM E1125-99
(Test Method)Standard Test Method for Calibration of Primary Non-Concentrator Terrestrial Photovoltaic Reference Cells Using a Tabular Spectrum
Standard Test Method for Calibration of Primary Non-Concentrator Terrestrial Photovoltaic Reference Cells Using a Tabular Spectrum
SCOPE
1.1 This test method is intended to be used for calibration and characterization of primary terrestrial photovoltaic reference cells to a desired reference spectral irradiance distribution, such as Tables E891 or E892. The recommended physical requirements for these reference cells are described in Specification E1040. Reference cells are principally used in the determination of the electrical performance of photovoltaic devices.
1.2 Primary photovoltaic reference cells are calibrated in natural sunlight using the relative spectral response of the cell, the relative spectral distribution of the sunlight, and a tabulated reference spectral irradiance distribution.
1.3 This test method is a technique that may be used instead of the procedures found in Test Methods E1039 and Test Method E1362. This test method offers convenience in its ability to characterize a reference cell under any spectrum for which tabulated data are available. The selection of the specific reference spectrum is left to the user.
1.4 This test method applies only to the calibration of a photovoltaic cell that shows a linear dependence of its short-circuit current on irradiance over its intended range of use, as defined in Test Method E1143.
1.5 This test method applies only to the calibration of a reference cell fabricated with a single photovoltaic junction.
1.6 There is no similar or equivalent ISO standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:E1125–99
Standard Test Method for
Calibration of Primary Non-Concentrator Terrestrial
Photovoltaic Reference Cells Using a Tabular Spectrum
This standard is issued under the fixed designation E1125; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This test method is intended to be used for calibration 2.1 ASTM Standards:
and characterization of primary terrestrial photovoltaic refer- E772 Terminology Relating to Solar Energy Conversion
ence cells to a desired reference spectral irradiance distribu- E816 Method for Calibration of Secondary Reference
tion, such as Tables E891 or E892. The recommended Pyrheliometers and Pyrheliometers for Field Use
physicalrequirementsforthesereferencecellsaredescribedin E891 Tables for Terrestrial Direct Normal Solar Spectral
Specification E1040. Reference cells are principally used in Irradiance for Air Mass 1.5
the determination of the electrical performance of photovoltaic E892 Tables for Terrestrial Solar Spectral Irradiance atAir
devices. Mass 1.5 for a 37° Tilted Surface
1.2 Primary photovoltaic reference cells are calibrated in E948 Test Methods for Electrical Performance of Non-
natural sunlight using the relative spectral response of the cell, Concentrator Terrestrial Photovoltaic Cells Using Refer-
therelativespectraldistributionofthesunlight,andatabulated ence Cells
reference spectral irradiance distribution. E973 Test Method for Determination of the Spectral Mis-
1.3 This test method requires the use of a pyranometer that match Between a Photovoltaic Device and a Photovoltaic
is calibrated according to Test Method E 816, which requires Reference Cell
the use of a pyrheliometer that is traceable to the World E1021 Test Methods for Measuring the Spectral Response
Radiometric Reference (WRR). Therefore, reference cells of Photovoltaic Cells
calibrated according to this test method are traceable to the E1039 TestMethodforCalibrationandCharacterizationof
WRR. Non-ConcentratorTerrestrial Photovoltaic Reference Cells
1.4 Thistestmethodisatechniquethatmaybeusedinstead Under Global Irradiation
of the procedures found in Test Methods E1039 and Test E1040 Specification for Physical Characteristics of Non-
Method E1362. This test method offers convenience in its Concentrator Terrestrial Photovoltaic Reference Cells
ability to characterize a reference cell under any spectrum for E1328 Terminology Relating to Photovoltaic Solar Energy
whichtabulateddataareavailable.Theselectionofthespecific Conversion
reference spectrum is left to the user. E1362 Test Method for Calibration of Non-Concentrator
1.5 This test method applies only to the calibration of a Photovoltaic Secondary Reference Cells
photovoltaic cell that shows a linear dependence of its short-
3. Terminology
circuit current on irradiance over its intended range of use, as
defined in Test Method E1143. 3.1 Definitions—Definitions of terms used in this test
method may be found in Terminology E772 and Terminology
1.6 This test method applies only to the calibration of a
reference cell fabricated with a single photovoltaic junction. E1328.
3.2 Symbols:
1.7 There is no similar or equivalent ISO standard.
1.8 This standard does not purport to address all of the 3.2.1 The following symbols and units are used in this test
method:
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- l—Wavelength, nm or µm,
I —Short-circuit current, A,
priate safety and health practices and determine the applica-
sc
−2
bility of regulatory limitations prior to use. E—Irradiance, Wm ,
−2
E—Total irradiance, Wm ,
t
−2 −1
E(l)—Spectral irradiance, Wm µm ,
This specification is under the jurisdiction ofASTM Committee E-44 on Solar,
Geothermal,andOtherAlternativeEnergySourcesandisthedirectresponsibilityof
Subcommittee E44.09 on Photovoltaic Electric Power Conversion.
Current edition approved Oct. 10, 1999. Published November 1999. Originally Annual Book of ASTM Standards, Vol 12.02.
e1 3
published as E1125–86. Last previous edition E1125–86 (1993) . Annual Book of ASTM Standards, Vol 14.04.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1125
−1
R(l)—Spectral response, AW , ity of the user to specify the applicable irradiance distribution,
−1
R (l)—Reference cell spectral response, AW , for example Tables E891 or E892. This test method allows
r
T—Temperature, °C, calibration with respect to any tabular spectrum.
−1
a—Temperature coefficient of reference cell I ,°C , 5.3 A reference cell should be recalibrated at yearly inter-
sc
n—Total number of data points, vals, or every six months if the cell is in continuous use
2 −1
C—Calibration constant,Am W , outdoors.
M—Spectral mismatch parameter, 5.4 Recommended physical characteristics of reference
F—Spectral correction factor, and cells can be found in Specification E1040.
S—Standard deviation.
6. Apparatus
4. Summary of Test Method
6.1 Pyrheliometer— A secondary reference pyrheliometer
that is calibrated in accordance with Method E816. An
4.1 The calibration of a primary photovoltaic reference cell
absolute cavity radiometer may also be used. Because second-
consists of measuring the short-circuit current of the cell when
ary reference pyrheliometers are calibrated against an absolute
illuminated with natural sunlight, along with the total solar
cavityradiometer,thetotaluncertaintyintheprimaryreference
irradiance using a pyrheliometer. The ratio of the short-circuit
cell calibration constant will be reduced if an absolute cavity
current of the cell to the irradiance, divided by a correction
radiometer is used.
factor similar to the spectral mismatch parameter defined in
6.2 Collimator—A collimator fitted to the reference cell
TestMethodE973,isthecalibrationconstantforthereference
during calibration that has the same field-of-view as the
cell. Also, if the temperature of the cell is not 25 6 1°C, the
pyrheliometer.An acceptable collimator design is described in
short-circuit current must be corrected to 25°C.
Annex A1.
4.1.1 The relative spectral irradiance of the sunlight is
6.3 Spectral Irradiance Measurement Equipment,asre-
measured using a spectral irradiance measurement instrument
quired by Test Method E973.
as specified in Test Method E973.
4.2 The following is a list of measurements that are used to 6.3.1 The spectral range of the spectral irradiance measure-
ment shall be wide enough to include the spectral response of
characterize reference cells and are reported with the calibra-
tion data: the cell to be calibrated.
6.3.2 The spectral range of the spectral irradiance measure-
4.2.1 The spectral response of the cell is determined in
ment shall include 98% of the total irradiance to which the
accordance with Test Methods E1021.
pyrheliometer is sensitive.
4.2.2 The cell’s short-circuit current temperature coefficient
6.3.3 If the spectral irradiance measurement is unable to
is determined experimentally by measuring the short-circuit
measure the entire wavelength range required by 6.3.2, it is
current at various temperatures and computing the temperature
acceptabletouseareferencespectrum,suchasTableE891,to
coefficient (see 7.2.2).
supply the missing wavelengths. The reference spectrum is
4.2.3 Linearity of short-circuit current versus irradiance is
scaled to match the measured spectral irradiance data over a
determined in accordance with Test Method E1143.
convenientwavelengthintervalwithinthewavelengthrangeof
4.2.4 Thefillfactorofthereferencecellisdeterminedusing
the spectral irradiance measurement equipment. It is also
Test Method E948. Providing the fill factor with the calibra-
acceptable to calculate the missing spectral irradiance data
tion data allows the reference cell to be checked in the future
using a numerical model.
for electrical degradation or damage.
6.3.4 The spectral irradiance measurement equipment shall
5. Significance and Use
have the same field-of-view as the pyrheliometer and the
5.1 The electrical output of a photovoltaic device is depen- reference cell collimator.
dent on the spectral content of the illumination source, its 6.4 Normal Incidence Tracking Platforms—Tracking plat-
intensity, and the device temperature. To make standardized, forms used to follow the sun during the calibration and to hold
accurate measurements of the performance of photovoltaic the reference cell to be calibrated, the pyrheliometer, the
devices under a variety of light sources, it is necessary to collimator, and spectral irradiance measurement equipment.
account for the error in the short-circuit current that occurs if The pyrheliometer and the collimator must be parallel within
the relative spectral response of the reference cell is not 60.25°. The platforms shall be able to track the sun within
identical to the spectral response of the device to be tested. A 60.5° during the calibration procedure.
similarerroroccursifthespectralirradiancedistributionofthe 6.5 Temperature Measurement Equipment—An instrument
testlightsourceisnotidenticaltothedesiredreferencespectral orinstrumentsusedtomeasurethetemperatureofthereference
irradiance distribution. These errors are accounted for by the cell to be calibrated, that has a resolution of at least 0.1°C, and
spectralmismatchparameter(describedinTestMethodE973), a total error of less than 61°C of reading.
a quantitative measure of the error in the short-circuit current 6.5.1 Sensorssuchasthermocouplesorthermistorsusedfor
measurement. It is the intent of this test method to provide a the temperature measurements must be located in a position
recognized procedure for calibrating, characterizing, and re- that minimizes any temperature gradients between the sensor
porting the calibration data for primary photovoltaic reference and the photovoltaic device junction.
cells using a tabular reference spectrum. 6.6 Electrical Measurement Equipment—Voltmeters, am-
5.2 The calibration of a reference cell is specific to a meters, or other suitable electrical measurement instruments,
particular spectral irradiance distribution. It is the responsibil- used to measure the I of the cell to be calibrated and the
sc
E1125
pyrheliometer output, that have a resolution of at least 0.02% 8.2.2 Measure the short-circuit current of the reference cell,
of the maximum current or voltage encountered, and a total I .
sc
error of less than 0.1% of the maximum current or voltage 8.2.3 Measure the reference cell temperature, T.
encountered. 8.2.4 Repeat 8.2.1 and 8.2.2 at least four times. These
repetitions must be distributed in time during the spectral
6.7 Spectral Response Measurement Equipment,asrequired
by Test Method E1021. irradiance measurement. To assure temporal stability, the
short-circuitcurrentofthereferencecellshallnotvarybymore
6.7.1 The wavelength interval between spectral response
data points shall be a maximum of 50 nm. than 60.2% during the repetitions.
8.2.5 Average the short-circuit current and total irradiance
6.8 Temperature Control Block (Optional)—A device to
valuesfrom8.2.4toobtainthe I and E thatcorrespondstothe
maintain the temperature of the reference cell at 25 6 1°C for
sc t
spectral irradiance measurement.
the duration of the calibration.
8.3 Perform a minimum of five replications of 8.2.
8.3.1 The five replications must be performed on at least
7. Characterization
three separate days. Therefore, five replications all performed
7.1 Prior to the characterization measurements, illuminate
−2 on the same day would not be an acceptable data set for the
the reference cell to be calibrated at 1000 Wm for 2 h. This
calibration.
is necessary to stabilize any light-induced degradation of the
8.3.2 In order to reduce precision errors through averaging,
cell prior to calibration.
it is recommended that at least 30 replications of 8.2 be
7.2 Characterize the reference cell being calibrated by the
performed.
following methods:
7.2.1 Spectral Response—Determine the relative spectral
9. Calculation of Results
response, R(l), (optionally the absolute spectral response) of
9.1 Each spectral irradiance measurement obtained in 8.2
the cell to be calibrated in accordance with Test Methods
defines one data point.The total number of these data points is
E1021.
denoted as n.
7.2.2 Temperature Coeffıcient—Determine the temperature
9.1.1 For each data point, calculate the spectral correction
coefficient, a, of the cell to be calibrated as follows:
factor, F, using the spectral mismatch parameter calculation,
7.2.2.1 Using the electrical measurement equipment, mea-
8.1,ofTestMethodE973.Toperformthiscalculation,replace
sure I at four or more temperatures over at least a 50°C
sc
the reference cell spectral response R (l) with unity (this
r
temperature range centered around 35°C. The irradiance shall
represents the spectral response of the pyrheliometer), and
−2 −2
be at least 750 Wm and less than 1100 Wm , as measured
replace M with F.
with a second reference cell. Measure the temperature of the
9.2 Calculate the calibration constant for each data point,
being calibrated at the same time.
using the following:
7.2.2.2 Divide each value of I by the normalized instanta-
sc
I 1 1
sc
neous irradiance level at the time of each measurement.
C 5 (1)
i
E F 12a 25 2 T!
~
t
NOTE 1—The normalized instantaneous irradiance can be determined
NOTE 2—The temperature correction term may be deleted if the
by dividing the second reference cell’s I by its calibration constant.
sc
measured reference cell temperature is 25 6 1°C.
7.2.2.3 Determinethetemperaturecoefficientbyperforming
9.3 Calculate the average calibration constant with the
a least-squares fit of the I versus T data to a straight line.The
sc
following:
slope of the line divided by the value of the current from the
n
least-squares fit at 25°C is the temperature coefficient, a.
C 5 C (2)
(
i
n
i 51
7.2.3 Linearity—Determine the short-circuit current versus
irradiance linearity of the cell being calibrated in accordance 9.4 Compute the standard deviation of the calibration con-
stant using the following:
with Test Method E1143 for the irradiance range 750 to 1100
−2
Wm .
n 1/2
2 2
@~C ! # 2 nC
7.2.4 Fill Factor—Determinethefillfactorofthecelltobe (
i
i 51
F G
S 5 (3)
calibrated from the I-V curve of the device, as measured in
n 21
accordance with Test Methods E948.
9.4.1 The value of S shall be 1% or less of the calibration
constant
...
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