Dissecting the Differences Between Pyranometer and Reference Cell Irradience Measurements

1. Dissecting the Differences between
Pyranometer and Reference Cell
Irradiance Measurements
Anton Driesse
PV Performance Labs, Freiburg, Germany
October 24, 2016
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2. ηη
Acknowledgements
European Joint Research Center (JRC)
Sandia National Laboratories (Sandia)
National Renewable Energy Laboratory (NREL)
Fraunhofer Institute for Solar Energy Systems (ISE)

3. ηηIntroduction
Output: ± 1% or better Input: ± 3% or worse

4. ηηIntroduction
Output: ± 1% or better
± 0.5%
Input: ± 3% or worse
± 5.0%

5. ηη42 Sensors
1. Secondary standard thermopile
pyranometers
2. Second class thermopile
pyranometers
3. Photodiode pyranometers
4. Reference cells

6. ηηTest Facilities

7. ηη42 Sensors

8. ηη42 Irradiances

9. ηη“Recalibration” at AM1.5

10. ηηSimilarities
Pyranometer
• output signal mainly
proportional to irradiance
Reference cell
• output signal mainly
proportional to irradiance

11. ηηDifferences
Pyranometer
• circular
• dome (usually)
• thermoelectric principle
Reference cell
• square
• flat (usually)
• photovoltaic principle

12. ηηAngular Response

13. ηηSpectral Response

14. ηηTemperature Response

15. ηηToday’s Question
• How much do each of these effects contribute to
differences in reported irradiance (and by extension
to reported PV system performance)?
Procedure
• Using measured spectra, component irradiances and
other weather parameters, for one year, calculate
the expected output signal of each type of sensor.
• Calculate and illustrate the differences from a
reference value both in absolute and relative terms.

16. ηηLocations Details
• Irradiance measurements from NREL-Solar Radiation
Research Laboratory
– DNI, GHI, DHI, global tilted at 40°
– ambient temperature
• Spectral measurements from NREL Outdoor Test
Facility
– Eko MS-700
– global tilted at 40°
• “Latitude tilt”, therefore high irradiance levels in
winter as well as summer

17. ηηResults

18. ηηDaily Profile for a Reference Cell

19. ηηDaily Profile for a Reference Cell

20. ηηDaily Profile for a Pyranometer

21. ηηDaily Profile for a Pyranometer

22. ηηAnnual Profile for a Reference Cell

23. ηηAnnual Profile for a Reference Cell

24. ηηAnnual Profile for a Pyranometer

25. ηηAnnual Profile for a Pyranometer

26. ηηReference Cell vs Pyranometer

27. ηηAdditional Factors
Pyranometer
• delayed response
• thermal offsets
• alignment accuracy
• non-linearity
Reference cell
• spectral resp. vs temperture
• spectral resp. vs. angle
• alignment accuracy
• non-linearity

28. ηηConclusions
• Understanding of the systematic difference between
sensor types gives insight into when and why
different instruments readings are obtained
• Precise quantification of those systematic
differences allows instrument readings to be
corrected, or adapted to the analysis needs
• Temperature and angular effects can be measured
and processed with reasonable effort
• Spectral effects are challenging because spectral
measurements have limited wavelength range

29. ηη