Results of first outdoor comparison between Absolute Cavity Pyrgeometer (ACP) and Infrared Integrating Sphere (IRIS) Radiometer at PMOD Atmospheric System Research Science Team Meeting (March 18 ‐ 21, 2013) by Ibrahim Reda*, Julian Gröbner**, Stefan Wacker**, and Tom Stoffel* * = NREL , ** = PMOD/WRC NREL/PR ‐ 3B10 ‐ 58121 NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Outline The ACP and IRIS are developed to establish a world reference for calibrating pyrgeometers with traceability to SI units. The two radiometers are unwindowed with negligible spectral dependence, and traceable to SI units through the temperature scale (ITS ‐ 90). The first outdoor comparison between the two designs was held from January 28 to February 8, 2013 at the Physikalisch ‐ Metorologisches Observatorium Davos (PMOD). The difference between the irradiance measured by ACP and that of IRIS was within 1 W/m 2 . A difference of 5 W/m2 was observed between the irradiance measured by ACP&IRIS and that of the interim World Infrared Standard Group (WISG). 2 2
Absolute Cavity Pyrgeometer (ACP) ‐ ACP Net irradiance: K * V * W ( 1 )* W ( 2 )* K * W 1 tp atm c 2 r ‐ By cooling the ACP case temperature, and since W atm is stable, then, ( 1 )* W ( 2 )* K * W c 2 r K 1 V tp ‐ Then the atmospheric longwave irradiance is, K * V ( 2 ) * K * W ( 1 ) * W 1 tp 2 r c W atm Where, K 1 , V tp , ϵ , K 2 , W r , W c and τ are the reciprocal of ACP’s responsivity, thermopile voltage, gold emittance, detector’s emittance, receiver irradiance, CPC irradiance, and throughput (NIST characterization), consecutively. Reference: Reda, I.; Zeng, J.; Schulch, J.; Hanssen, L.; Wilthen, B.; Myers, D.; Stoffel, T. Dec. 2011. “An absolute cavity pyrgeometer to measure the absolute outdoor longwave irradiance with traceability to International System of Units, SI”. Journal of Atmospheric and Solar-Terrestrial Physics, 77 (2012) 132-143. http://dx.doi.org/10.1016/j.jastp.2011.12.011 3
The Infrared Integrating Sphere (IRIS) Radiometer Key features of the IRIS Radiometer • Windowless • Irradiance measurement by using a 60 mm gold ‐ plated integrating sphere as input optic • High sensitivity from a windowless Aperture Detector pyroelectric detector • Flat spectral response • Measurement frequency 0.1 Hz • Automatic unattended operation Shutter • Nighttime measurements only IRIS Uncertainty Reference 1.8 Wm ‐ 2 summer (+15°C) U95%= surface 2.4 Wm ‐ 2 winter ( ‐ 15°C) Reference: Gröbner, J., A Transfer Standard Radiometer for atmospheric longwave irradiance measurements, Metrologia, 49, S105-S111,2012. 4
ACP versus PMOD ‐ BB on Jan 29 to Feb 2, 2013 ACP Net irradinace vs Vtp during cooling ACP Net irradinace vs Vtp during cooling Vtp (uV) Vtp (uV) ‐ 250 ‐ 265 ‐ 1400 ‐ 1200 ‐ 1000 ‐ 800 ‐ 600 ‐ 400 ‐ 200 0 ‐ 350 ‐ 300 ‐ 250 ‐ 200 ‐ 150 ‐ 100 ‐ 50 0 2 ‐ 270 2 /m /m ‐ 300 W ‐ 275 W y = 0.0842x ‐ 255.2 ‐ 350 ‐ 280 y = 0.0834x ‐ 254.62 R 2 = 0.9978 R 2 = 1 ‐ 400 ‐ 285 Residuals of fitting Net irradiance vs V tp Residuals of fitting Net irradiance vs V tp 0.3 0.4 0.2 0.1 2 2 /m /m 0 W W ‐ 0.1 1.00 11.00 13.16 13.24 13.33 13.41 13.49 ‐ 0.2 ‐ 0.4 ‐ 0.3 UTC UTC W ACP ‐ W BB W ACP ‐ W BB 3 3 2 2 /m 2 /m 2 W W 1 1 0 0 13.16 13.24 13.33 13.41 13.49 1.00 11.00 UTC UTC ACP Net irradinace vs Vtp during cooling ACP Net irradinace vs Vtp during cooling Vtp (uV) Vtp (uV) ‐ 280 ‐ 320 ‐ 1400 ‐ 1200 ‐ 1000 ‐ 800 ‐ 600 ‐ 400 ‐ 200 0 ‐ 300 ‐ 1200 ‐ 1000 ‐ 800 ‐ 600 ‐ 400 ‐ 200 0 W/m 2 2 W/m ‐ 340 ‐ 320 y = 0.0827x ‐ 254.7 y = 0.0822x ‐ 273.94 ‐ 360 ‐ 340 R 2 = 0.9992 R 2 = 0.9996 ‐ 360 ‐ 380 Residuals of fitting Net irradiance vs V tp Residuals of fitting Net irradiance vs V tp 1.5 1.5 0.5 W/m 2 0.5 2 W/m ‐ 0.5 ‐ 0.5 11.62 11.71 11.79 11.87 17.66 17.74 17.82 ‐ 1.5 ‐ 1.5 UTC UTC W ACP ‐ W BB W ACP ‐ W BB 1.5 3 1 W/m 2 2 2 W/m 0.5 1 0 0 11.62 11.71 11.79 11.87 17.66 17.74 17.82 UTC UTC 5
Transient vs steady state in BB, Jan 29 ‐ Feb 2, 2013 W ACP ‐ W BB W ACP ‐ W BB 3.5 3.5 Transient during cooling Transient during cooling Steady State before cooling Steady State before cooling 3 3 2.5 2.5 2 2 W / m 2 W / m 2 1.5 1.5 1 1 0.5 0.5 0 0 12.85 12.94 13.02 13.10 13.19 13.27 13.35 13.44 23.38 23.47 23.55 23.63 23.72 23.80 UTC UTC W ACP ‐ W BB W ACP ‐ W BB 3.5 2 Transient during cooling Transient during cooling Steady State before cooling Steady State before cooling 1.8 3 1.6 2.5 1.4 1.2 2 W / m 2 W / m 2 1 1.5 0.8 0.6 1 0.4 0.5 0.2 0 0 17.35 17.43 17.51 17.60 17.68 17.76 17.85 11.32 11.41 11.49 11.57 11.66 11.74 11.82 11.91 UTC UTC 6
Outdoor ACP&IRIS at night Feb. 5, 2013 7
Outdoor ACP&IRIS at night Feb. 5, 2013 8
Outdoor ACP, IRIS & WISG at night Feb. 5, 2013 9
Irradiance difference (WISG minus IRIS) at PMOD From Julian’s presentation, IRS2012-Germany (Data from 180 nights) WISG 1 Night averages WISG 2 WISG 3 IRIS u 95 Results WISG 4 ‐ 4.1 ± 1.5 Wm ‐ 2 Average Offset (IWV>10) Gradient (IWV<10) ‐ 0.45 ± 0.1 Wm ‐ 2 mm ‐ 1 IWV 10
Irradiance difference (ACP minus WISG) at NREL Three cooling cycles on November 18 and 21, 2012 with 40% RH at SRRL Consistent with Julian’s observation with high water vapor* * Algebra is reversed for consistency with NREL’s historical files 11
Preliminary Conclusions • Special set ‐ up of ACP in BB due to unknown gradient in CPC • Outdoor agreement between ACP & IRIS to within 1 W/m 2 • Irradiance measured by WISG is ~4 W/m 2 lower than that measured by ACP&IRIS. Is Consistent with a Water Vapor Column of 8 mm. This was also observed at NREL/SRRL at RH = 40% (on November 18, 2012 at NREL/SRRL: Water Vapor Column from 7 mm to 9 mm during cooling cycles) • Future comparison with higher/lower water vapor to resolve observed spectral effect on outdoor pyrgeometer calibrations • A 3 rd design might increase confidence in establishing a consensus reference with traceability to SI units. 12
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