Based on an article that will be printed in the Oct. issue (Vol. 22, No. 10) in

1. Determination of Longwave Response of Shortwave Solar Radiometers to Correct for their Thermal Offset Errors Based on an article that will be printed in the Oct. issue (Vol. 22, No. 10) in Journal of Atmospheric and Oceanic Technology (JTECH) by I. Reda1, J. Hickey2, D. Myers1, T. Stoffel1, S. Wilcox1, C. Long3,E. G. Dutton4, D. Nelson4, and J. J. Michalsky4 1National Renewable Energy Laboratory Golden CO2The Eppley Laboratory Newport RI3Pacific Northwest Laboratory Richland WA4National Oceanic and Atmospheric Administration, Boulder CO

2. Outline Introduction • Pyranometer Thermal Offset evidence • Thermopile Detector Configurations • How it is determined and show results for 14 pyranometers • Explain how to correct for the thermal offset errors • Explain effect of thermal offset error on two calibration methods and show correction results • Conclusions and acknowledgements

3. Eppley Model PSP & 8-48 Pyranometers

4. Broadband Shortwave: Clear Sky Night ≤ 0? Really?

5. Shaded PSP Zero-Offsets

6. Nighttime Offsets

7. Additional Evidence… ARM Program scientists (e.g., Dr. Robert Cess) found several examples of clear-sky diffuse measurements that were less than predicted for pure atmospheric Rayleigh scattering: Expected Minimum Diffuse = 100 Wm-2 Measured Diffuse = 90 Wm-2

8. Still More Evidence… Comparison of calibration results: Manufacturer’s pyranometer responsivity (Rs) typically 1.5% - 2% HIGHER than Outdoor calibration results at NREL. Rs(indoor) > Rs(outdoor)

9. Heat Budget of Generic Pyranometer with Single Black Detector SW tg(1-) + FgdAggTg4 - FdgAddTd4 - (Td - Ts)k ± Con = 0 xx a b c d e SW • SW = shortwave solar t = transmission of glass dome(s) = SW reflectance • F= config factor (area=A) T = temperature (°K)  = emissivity • k = heat conduction coeff. Con = convection &unwanted conduction g = glass dome • d = detector surface V = pyranometer output voltage (mV) s = thermopile heat sink

10. The Key Concept Gordon Conference June 14-19, 1998 Solar Radiation & Climate John Hickey describes concept of characterizing a Model PSP pyranometer in a Blackbody Pyrgeometer Calibration System currently under development for ARM…

11. ARM Radiometers: Longwave Calibration Traceability

12. The Approach To calculate the thermal offset error of pyranometers when calibrated or deployed outdoors Calibrated 14 pyranometers in NREL-Pyrgeometer Blackbody Calibration System: EPLAB PSP & 8-48 (BW) Kipp & Zonen CM22 SpectroSun SR-75 to calculate their Indoor NET-IR Responsivity (RSnet) Error: - 0% to 2% error in Responsivity (Rs) from outdoor calibrations - 0 W/m2 to 20 W/m2 error in measured solar irradiance outdoors Then used RSnet to correct for the thermal offset error…

13. Simplified Diagram for NREL Blackbodywith a Pyranometer

14. Blackbody Temperature Plateaus Simulate the possible sky and ambient temperatures at NREL

15. WNET (W/m2) Thermopile Output (uV) Plots of the thermopile output versus the net longwave radiation for 14 pyranometers The Net IR (WNET)=  [(Tblackbody)4 - (Tcase)4] WNET is Negative → TP output is Negative

16. Black Body Calibration Results for Pyranometers in April, 2004 ** has case thermistor

17. RSbb = RSmfr Shortwave/NET-IR Equivalence V / RSmfr where, V = thermopile output voltage during BB-Cal (uV) RSbb = Longwave (blackbody) responsivity (uV/W/m-2) RSmfr = Shortwave (manufacturer) responsivity (uV/W/m-2) E = V / RSbb

18. Blackbody Calibration Results for Pyranometers in April, 2004

19. Thermal Offset Voltage • RsSW-Corr = VTP - ∆Vcorr / (Cavity * Cos(Z) + Diffuse) • where, • ∆ Vcorr = WNET * RSbb * E • = WNET * RSNET • WNET is measured using a collocated pyrgeometer • WNET = TPpyrg * K1 • K1 = pyrgeometer thermopile sensitivity

20. Calibrated 28403F3 on 3/30/2004 RSsumm ~ 1.9%↓ Two pyranometer calibration methods: Summation and Shade/Unshade

21. Thermal Offset Effect on Calibration Methods Summation (Component Sum) is effected by Net IR: + ∆u U RS = B * Cos z + D • Where: • RS = Responsivity [uV/(W/m2)] • U = Unshaded thermopile output (uV) • B = Beam irradiance, measured by a cavity (W/m2) • z = Zenith angle (°) • D = Diffuse irradiance (W/m2). - ∆u = unshaded thermal offset error signal (uV). Negative for clear sky conditions.

22. + (∆s) ] - S U = B * Cos z Thermal Offset Effect on Calibration Methods • Shade/Unshade is NOT effected by Net IR: [ - S + (∆u) U RS = B * Cos z • Where: • S = Shaded thermopile output (uV) - ∆s = shaded thermal offset error signal (uV). Assuming the Net IR is stable during the shade/unshade period, then ∆u = ∆s.

23. Why Not Use the Shade/Unshade? We calibrate up to 100 pyranometers at each calibration session, from sunrise to sundown The shade/unshade will require more trackers with shading disks which will increase cost, labor, maintenance, space, etc... Compared to The component sum, one tracker for the diffuse reference, and all pyranometers are installed on horizontal tables.

24. Correcting Summation Results for Thermal Offsets Brings Agreement with Shade/Unshade Method

25. Summation Calibration with Correction on 4/17/2004

26. Results: Shade/Unshade (SU), Uncorrected Component Sum (UCS), and Corrected Component Sum (CCS)

27. Conclusions & Recommendations • We developed a method to characterize/correct most of the thermal offset error of all-black thermopile pyranometers. • The method resulted in reducing the difference between the component sum and shade/unshade method from 1.47% to 0.18% for model PSP, and 0.42% to 0.25% for CM22. The 8-48 difference increased from 0.09% to 0.15%, but this is negligible. • Small differences between corrected component and shade/unshade responsivities could be attributed to calculating the NET-IR as the difference between the sky and pyranometer case radiation, rather than the difference between the sky and detector surface radiation. ! needs further research ! ! More accurate WNET will result in a minor thermal offset correction for the shade/unshade method ! - Routine field measurements must be corrected for coincident thermal offset errors (actual WNET) regardless of the calibration method.

28. Acknowledgements We thank Peter Gotseff for his patience and expertise in the blackbody and outdoor calibrations, and Bev Kay for her continuous administrative support. Research funds for this work were provided by the Atmospheric Radiation Measurement (ARM) Program, NREL-National Center for Photovoltaics, and NREL-Metrology Laboratory.