Determination of Longwave Response of Shortwave Solar Radiometers to Correct for their Thermal Offset Errors 1 National Renewable Energy Laboratory Golden CO 2 The Eppley Laboratory Newport RI 3 Pacific Northwest Laboratory Richland WA 4 National Oceanic and Atmospheric Administration, Boulder CO 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. Reda 1, J. Hickey 2, D. Myers 1, T. Stoffel 1, S. Wilcox 1, C. Long 3, E. G. Dutton 4, D. Nelson 4, and J. J. Michalsky 4

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

Eppley Model PSP & 8-48 Pyranometers

Broadband Shortwave: Clear Sky Night ≤ 0? Really?

Shaded PSP Zero-Offsets

Nighttime Offsets

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

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)

Heat Budget of Generic Pyranometer with Single Black Detector SW t g (1-  ) + F gd A g  g  T g 4 - F dg A d  d  T d 4 - (T d - T s )k ± Con = 0 xx SW = shortwave solart = transmission of glass dome(s)  = SW reflectance F  = config factor (area=A)T = temperature (°K)  = emissivity k = heat conduction coeff.Con = convection &unwanted conductiong = glass dome d = detector surfaceV = pyranometer output voltage (  V) s = thermopile heat sink abcde SW

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…

ARM Radiometers: Longwave Calibration Traceability

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 ( RS net ) The Approach To calculate the thermal offset error of pyranometers when calibrated or deployed outdoors Error: - 0% to 2% error in Responsivity (Rs) from outdoor calibrations - 0 W/m 2 to 20 W/m 2 error in measured solar irradiance outdoors Then used RS net to correct for the thermal offset error…

Simplified Diagram for NREL Blackbody with a Pyranometer

Blackbody Temperature Plateaus Blackbody Temperature ( °C) Case Temperature ( °C) T BB - T Case ( °C) Simulate the possible sky and ambient temperatures at NREL

Plots of the thermopile output versus the net longwave radiation for 14 pyranometers W NET (W/m 2 ) Thermopile Output (uV) The Net IR (W NET )=  [(T blackbody ) 4 - (T case ) 4 ] W NET is Negative → TP output is Negative

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

Shortwave/NET-IR Equivalence where, V = thermopile output voltage during BB-Cal (uV) RS bb = Longwave (blackbody) responsivity (uV/W/m -2 ) RS mfr = Shortwave (manufacturer) responsivity (uV/W/m -2 ) E = V / RS mfr V / RS bb = RS bb RS mfr

Blackbody Calibration Results for Pyranometers in April, 2004

Thermal Offset Voltage Rs SW-Corr = V TP - ∆V corr / (Cavity * Cos(Z) + Diffuse) where, ∆ V corr = W NET * RS bb * E = W NET * RS NET W NET is measured using a collocated pyrgeometer W NET = TP pyrg * K 1 K 1 = pyrgeometer thermopile sensitivity

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

Thermal Offset Effect on Calibration Methods Summation (Component Sum) is effected by Net IR: RS = U B * Cos z + D + ∆ u Where: - RS = Responsivity [uV/(W/m 2 )] - U = Unshaded thermopile output (uV) - B = Beam irradiance, measured by a cavity (W/m 2 ) - z = Zenith angle (°) - D = Diffuse irradiance (W/m 2 ). - ∆ u = unshaded thermal offset error signal (uV). Negative for clear sky conditions.

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

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.

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

Summation Calibration with Correction on 4/17/2004

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

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 W NET 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 W NET ) regardless of the calibration method.

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.