Version 1003

State of the art of indoor calibration of pyranometers and pyrheliometers

2Indoor calibration

Main points

• Most field pyranometers are calibrated indoors

• Many procedures for indoor calibration

• Not all optimally connected to ISO 98-3 GUM

• Industry requires straightforward approach

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Industry

• Meteorology - Solar renewable energy • Site assessment• Installation performance• Professionalisation / IEC

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Future

• A few high accuracy outdoor calibrations

• A lot of indoor facilities• Accredited labs

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Conclusion

• Points for discussion• Normal Incidence NI calibration is

preferred (Diffuse Sphere Source DSS not)

• Uncertainty & accuracy of reference can be optimised

• Indoor calibration complies with GUM• Pyrheliometer indoor calibration must

be allowed by ISO

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Myself

• Kees VAN DEN BOS• Director / owner Hukseflux Thermal

Sensors• Last 20 years sensor design

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Hukseflux DR01 pyrheliometer

• Founded 1993

• Thermal sensors

• 15 employees

• 5 radiometry

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9Hukseflux 2010

10Reolith thermal properties on moon rover

11Snow thermal conductivity

My interest

• Hukseflux company cannot work with outdoor calibration

• Our customers want a understandable accuracy statement

• Feedback• More questions than answers

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Background

• Most pyranometers and pyrheliometers have indoor calibration

• Exception: highest accuracy (BSRN, outdoor)

• Exceptions on national level: Japan, China, … (outdoor)

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Background

• Cost, time, weather; outdoor calibration is unacceptable to industry

•DISADVANTAGE: Indoor methods only work with reference type = field type

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Present status (excerpt)

• Eppley, US Weather Bureau: indoor integrating diffuse source

• Kipp, Hukseflux: indoor normal incidence

• EKO: outdoor tracker with collimation tube

• KNMI: indoor (network) and outdoor (BSRN)

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16ISO 9060

17ISO 9060

Background

• Measurement uncertianty is a function of:

• Characterisation / class • Calibration (+characterisation and

class)• Measurement & maintenance

conditions• Environmental conditions

(+characterisation and class)

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Background

• Indoor calibration covered by ISO 9847

• Present ASME: “Indoor Transfer of Calibration from Reference to Field Pyranometers”

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20ISO 9846

21ISO 9847 also indoor

22ISO 98-3 GUM

Hierarchy of Traceability

• A: Reference calibration (uncertainty)• B: Correction of reference to indoor

conditions (uncertainty)• C: Indoor calibration of field

instrument (uncertainty)• Indoor calibration uncertainty

estimate (A+B+C)• Field measurement uncertainty

estimate

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Hierarchy of Traceability

• A: Reference calibration (uncertainty)• B: Correction of reference to indoor

conditions (uncertainty)• C: Indoor calibration of field

instrument (uncertainty)• Indoor calibration uncertainty

estimate (A+B+C)

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25ISO 98-3 GUM

26Hierarchy of traceability

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28Indoor calibration Normal Incidence NI

29ISO 98-3 GUM

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Hierarchy of Traceability

• KNMI TR 235 "uncertainty in pyranometer and pyrheliometer measurements at KNMI in De Bilt".

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Hierarchy of Traceability

• A: Reference calibration (uncertainty)• B: Correction of reference to indoor

conditions (uncertainty)• C: Indoor calibration of field

instrument (uncertainty)• Indoor calibration uncertainty

estimate (A+B+C)• Field measurement uncertainty

estimate

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34ISO 98-3 GUM

NI Hierarchy of Traceability

• A: Reference calibration (uncertainty) (conditions and class)

• B: Correction of reference to indoor conditions (uncertainty)

• C: Indoor calibration of field instrument (uncertainty)

• Indoor calibration uncertainty estimate (A+B+C)

• Field measurement uncertainty estimate (conditions & class)

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Strange…

• Errors in reference calibration re-appear in measurement errors

• Counted double• At least systematic errors (Zero offset

A and directional errors) can be avoided.

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One step back

• Calibration with restricted conditions results in lower uncertainty

• See yesterday’s presentation by Ibrahim Reda

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One step back

• Present reference works well if calibrated pyranometers are used:

• Outdoor / unventilated• At same latitude

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One step back

• Present approach does NOT work well calibrated if instruments are used:

• As indoor reference• At different latitudes• Ventilated

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Typical secondary standard calibration• Irradiance 800 W/m2

• 40 to 60 degrees angle of incidence, + / - 30 degrees azimuth• Zero offset A: -9 +/- 3 W/m2 (larger

than ISO9060)• Directional: +/- 10 W/m2 @ 1000

W/m2 , now estimated +/- 5 W/m2

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Typical calibration

• PMOD specified uncertainty +/- 1.3%• Systematic error -1%? Type B.

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NI reference improved

• Restricted conditions• Zero offset A: -9 +/- 3 W/m2 (larger

than ISO9060)• Directional: +/- 10 W/m2

• Solution 1: ventilation• Solution 2: single angle of incidence

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For consideration

• Japanese collimated tube with tilt correction and ventilation

• Tilted sun-shade method

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Diffuse Sphere Source DSS

• Uniformity of sphere top-edge (experimental -13%)

• Weighing for non uniform source requires weighing of reference with source

• Diffuse sphere: weighing requires weiging of field instrument with source. Complicated!

• Normal incidence: weighing of field instrument is not necessary

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DSS Hierarchy of Traceability

• A: Reference calibration (uncertainty) (conditions and class)

• B: Correction of reference to indoor conditions (uncertainty)

• C: Indoor calibration of field instrument (uncertainty) (conditions and class)

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DSS Hierarchy of Traceability

• Indoor calibration uncertainty estimate (A+B+C)

• Field measurement uncertainty estimate (conditions & class)

• Additional uncertainty under C compared to NI calibration

• Bottom line: DSS has less restricted conditions than NI

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Conclusion

• Indoor calibration offers only acceptable solution for manufacturers and “general users” in solar industry

• Indoor calibration fits within ISO 98-3 GUM

• detailed statements about field measurement still need to be agreed upon

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Conclusion

• Indoor calibration: Normal Incidence calibration is preferred (Diffuse Sphere Source is not)

• Accuracy and precision of reference can be optimised to serve as indoor calibration reference (restricted: single angle, ventilated)

• Pyrheliometer indoor calibration must be added /allowed by ISO

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