T.L. Stoffel, A.A. Andreas, S.M. Wilcox,
I. Reda, M. Anderberg, P. Gotseff, and B. Kay
Technical Report
NREL/TP-560-37954
September 2005 FY 2005 Midyear Progress Report
on Solar Radiometry and
Metrology Task PVC57301
October 1, 2004 to March 15, 2005
D.R. Myers, T.L. Stoffel, A.A. Andreas, S.M. Wilcox,
I. Reda, M. Anderberg, P. Gotseff, and B. Kay
Prepared under Task No. PVC57301
Technical Report
NREL/TP-560-37954
September 2005
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Preface

This report documents technical details for work performed in the Solar Radiometry and
Metrology Task PVC57301 in the period from October 1 2004 to March 15 2005. The
Task has conducted improved broadband and spectral calibrations, measurements, and
modeling activities supporting NRELs Photovoltaic Systems Engineering Project, and
Photovoltaic Measurements and Characterization Projects. The task calibrated nine
spectroradiometer systems used to classify and characterize continuous and pulsed solar
simulators for NREL and PV industry partners. These instruments were used to acquire
and use data to correct for deviations from standard reporting condition reference spectra.
Tests were conducted to evaluate the suitability of replacing an obsolescent group of
spectroradiometers with new diode array and scanning spectrometers. We conducted
detailed studies thermal offsets, a significant source of measurement error in broadband
radiometers measuring total hemispherical solar radiation, and published peer reviewed
papers describing the research and correction schemes. An intercomparison of absolute
cavity radiometers was conducted to assure the stable maintenance of the World
Radiometric Reference at NREL. Preparations are underway for travel to the World
Radiation Data Center, Davos, Switzerland, to accomplish the transfer of the World
Radiometric Reference to NREL reference absolute cavity radiometers, in compliance
with International Standards Organization ISO standard 17025 requirements for
traceability of NREL's solar measurements. A new, more accurate data acquisition
system was purchased to upgrade broadband radiometer calibrations, and is undergoing
testing and development of software to integrate with existing Radiometer
Characterization and Calibrations software. Improvements have been made to the Solar
Radiation Research Laboratory web-accessible data ensemble, permitting hourly
summary data to be generated from any data collection station accessible through the site.
We have documented over 135 technical exchanges with photovoltaic industry,
academic, other national laboratories, and individuals.

iii Contents

1.0 Introduction
................................................................................................................1

1.1 Background and Objectives ...............................................................................1
1.2 Major FY 2005 Milestones and Deliverables ....................................................2

2.0 Spectroradiometric
Calibrations and Measurements ..................................................3
2.1
Spectrometer
Calibrations..................................................................................4

2.2 Spectral Calibration Uncertainty Analysis ........................................................8

2.3 Laboratory Spectral Calibration Uncertainty ....................................................9

2.4 Example Spectral Measurement Results .........................................................11

2.5 Updating NREL Spectroradiometer Systems .................................................15

3.0 Broadband Calibrations and Research......................................................................16
3.1 WRR Traceability: NPC 2004 .........................................................................17

3.2 Radiometer Uncertainty Sources ....................................................................20
3.2.1
Thermal
Offsets ......................................................................................20


3.2.2 Other Spectral Errors ..............................................................................21
3.2.3 Characterizing Longwave Infrared Thermal Offset as
Shortwave Error Signals .........................................................................23


3.2.4 Geometric, Environmental and Equipment Uncertainty.........................24
3.2.5
Responsivity
Functions...........................................................................25
3.2.6 Pyranometer Corrections from Field Data..............................................26
3.2.7
Pyrheliometer Uncertainties ...................................................................30
3.2.8 Revisions to ASTM Radiometer Calibration Standards ........................32
3.3 Upgrade of NREL Radiometer Calibration System ..........................................36

4.0 Upgrades to the Measurement and Instrumentation Team Website .........................37

5.0 Solar Radiometric Metrology PV Industry Interactions 1st half FY 05...................41

6.0 Major
Publications....................................................................................................43

7.0 Conclusion ..............................................................................................................43

8.0 References ..............................................................................................................44

iv List of Figures

2.1.
Large (left) and small (right) scale solar simulators for evaluating PV
performance in the laboratory..................................................................................4
2.2.
Scanning grating monochromator with integrating sphere input.............................4
2.3.
Example of spectral irradiance calibration configuration using NIST Standard of
Spectral Irradiance (at right)....................................................................................5
2.4.
NIST standard lamp spectral irradiance (filled circles with NIST error bars;
(linear axis) and typical spectrometer response function (log axis, step at 1100 nm
and 1950 nm due to detector change) ......................................................................6
2.5.
Wavelength test scan (peaks) of known emission sources (vertical lines) to
establish spectroradiometer wavelength accuracy...................................................6
2.6.
Ratio of check measurement to NIST lamp data after spectral calibration (center
line). Top and bottom lines are uncertainty limits for the measurement. Noise at
short wavelengths (left) is due to poor signal to noise ratio for this unit ................7
2.7
Ratio (center line) of new to previous calibration showing changes in responsivity
of the test spectrometer. Top and bottom envelopes are uncertainty limits for
measured data...........................................................................................................7
2.8. AIM database entry for spectroradiometer calibrations. ASCII versions of
calibration files and report documents are accessed though links at the bottom of
the frame. .................................................................................................................8
2.9.
Percent differences between NIST and Measured spectral irradiance at NIST data
wavelengths for seven NIST spectral irradiance standard lamps (symbols on lines)
measured with a spectrometer system calibrated using an 8th lamp. The envelope
of estimated standard unce