Version 1003 State of the art of indoor calibration of pyranometers and pyrheliometers

2 Indoor 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 3

Industry Meteorology - Solar renewable energy Site assessment Installation performance Professionalisation / IEC 4

Future A few high accuracy outdoor calibrations A lot of indoor facilities Accredited labs 5

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 6

Myself Kees VAN DEN BOS Director / owner Hukseflux Thermal Sensors Last 20 years sensor design 7

Hukseflux DR01 pyrheliometer Founded 1993 Thermal sensors 15 employees 5 radiometry 8

9 Hukseflux 2010

10 Reolith thermal properties on moon rover

11 Snow thermal conductivity

My interest Hukseflux company cannot work with outdoor calibration Our customers want a understandable accuracy statement Feedback More questions than answers 12

Background Most pyranometers and pyrheliometers have indoor calibration Exception: highest accuracy (BSRN, outdoor) Exceptions on national level: Japan, China, … (outdoor) 13

Background Cost, time, weather; outdoor calibration is unacceptable to industry DISADVANTAGE: Indoor methods only work with reference type = field type 14

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) 15

16 ISO 9060

17 ISO 9060

Background Measurement uncertianty is a function of: Characterisation / class Calibration (+characterisation and class) Measurement & maintenance conditions Environmental conditions (+characterisation and class) 18

Background Indoor calibration covered by ISO 9847 Present ASME: “Indoor Transfer of Calibration from Reference to Field Pyranometers” 19

20 ISO 9846

21 ISO 9847 also indoor

22 ISO 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 23

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) 24

25 ISO 98-3 GUM

26 Hierarchy of traceability

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28 Indoor calibration Normal Incidence NI

29 ISO 98-3 GUM

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Hierarchy of Traceability KNMI TR 235 "uncertainty in pyranometer and pyrheliometer measurements at KNMI in De Bilt". 31

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 32

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34 ISO 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) 35

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. 36

One step back Calibration with restricted conditions results in lower uncertainty See yesterday’s presentation by Ibrahim Reda 37

One step back Present reference works well if calibrated pyranometers are used: Outdoor / unventilated At same latitude 38

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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 40

Typical secondary standard calibration Irradiance 800 W/m 2 40 to 60 degrees angle of incidence, + / - 30 degrees azimuth Zero offset A: -9 +/- 3 W/m 2 (larger than ISO9060) Directional: +/- 10 W/m 1000 W/m 2, now estimated +/- 5 W/m 2 41

Typical calibration PMOD specified uncertainty +/- 1.3% Systematic error -1%? Type B. 42

NI reference improved Restricted conditions Zero offset A: -9 +/- 3 W/m 2 (larger than ISO9060) Directional: +/- 10 W/m 2 Solution 1: ventilation Solution 2: single angle of incidence 43

For consideration Japanese collimated tube with tilt correction and ventilation Tilted sun-shade method 44

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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 46

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) 47

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 48

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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 50

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 51