Results of first outdoor comparison between Absolute Cavity - - PowerPoint PPT Presentation

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.

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

2

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/m2. 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

3

Absolute Cavity Pyrgeometer (ACP)

‐ ACP Net irradiance: ‐ By cooling the ACP case temperature, and since Watm is stable, then, ‐ Then the atmospheric longwave irradiance is,

Where, K1, Vtp, ϵ , K2, Wr , Wc 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.

K V W W K W

tp atm c r 1 2

1 2 * * ( )* ( )* *

    

  

W K V K W W

atm tp r c

    

1 2

2 1 * ( ) * * ( ) *

  

K W K W V

c r tp 1 2

1 2

   

( )* ( )* *

    

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

4

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

• ptic
• High sensitivity from a windowless

pyroelectric detector

• Flat spectral response
• Measurement frequency 0.1 Hz
• Automatic unattended operation
• Nighttime measurements only

Reference surface Detector Aperture Shutter

IRIS Uncertainty U95%= 1.8 Wm‐2 summer (+15°C) 2.4 Wm‐2 winter (‐15°C)

Reference: Gröbner, J., A Transfer Standard Radiometer for atmospheric longwave irradiance measurements, Metrologia, 49, S105-S111,2012.

5

ACP versus PMOD‐BB on Jan 29 to Feb 2, 2013

ACP Net irradinace vs Vtp during cooling

y = 0.0842x ‐ 255.2 R2 = 0.9978 ‐285 ‐280 ‐275 ‐270 ‐265 ‐350 ‐300 ‐250 ‐200 ‐150 ‐100 ‐50 Vtp (uV) W /m

WACP‐WBB

1 2 3 1.00 11.00 UTC W /m

Residuals of fitting Net irradiance vs Vtp

‐0.4 ‐0.2 0.2 0.4 1.00 11.00 UTC W /m

ACP Net irradinace vs Vtp during cooling

y = 0.0834x ‐ 254.62 R2 = 1 ‐400 ‐350 ‐300 ‐250 ‐1400 ‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 Vtp (uV) W /m

WACP‐WBB

1 2 3 13.16 13.24 13.33 13.41 13.49 UTC W /m

Residuals of fitting Net irradiance vs Vtp

‐0.3 ‐0.1 0.1 0.3 13.16 13.24 13.33 13.41 13.49 UTC W /m

ACP Net irradinace vs Vtp during cooling

y = 0.0827x ‐ 254.7 R2 = 0.9992 ‐360 ‐340 ‐320 ‐300 ‐280 ‐1400 ‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 Vtp (uV) W/m2

WACP‐WBB

1 2 3 11.62 11.71 11.79 11.87 UTC W/m2

Residuals of fitting Net irradiance vs Vtp

‐1.5 ‐0.5 0.5 1.5 11.62 11.71 11.79 11.87 UTC W/m2

ACP Net irradinace vs Vtp during cooling

y = 0.0822x ‐ 273.94 R2 = 0.9996 ‐380 ‐360 ‐340 ‐320 ‐1200 ‐1000 ‐800 ‐600 ‐400 ‐200 Vtp (uV) W/m

2

WACP‐WBB

0.5 1 1.5 17.66 17.74 17.82 UTC W/m

2

Residuals of fitting Net irradiance vs Vtp

‐1.5 ‐0.5 0.5 1.5 17.66 17.74 17.82 UTC W/m

2

6

Transient vs steady state in BB, Jan 29‐Feb 2, 2013

WACP‐WBB

0.5 1 1.5 2 2.5 3 3.5 12.85 12.94 13.02 13.10 13.19 13.27 13.35 13.44 UTC W / m 2

Transient during cooling Steady State before cooling

WACP‐WBB

0.5 1 1.5 2 2.5 3 3.5 23.38 23.47 23.55 23.63 23.72 23.80 UTC W / m 2

Transient during cooling Steady State before cooling

WACP‐WBB

0.5 1 1.5 2 2.5 3 3.5 11.32 11.41 11.49 11.57 11.66 11.74 11.82 11.91 UTC W / m 2

Transient during cooling Steady State before cooling

WACP‐WBB

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 17.35 17.43 17.51 17.60 17.68 17.76 17.85 UTC W / m 2

Transient during cooling Steady State before cooling

7

Outdoor ACP&IRIS at night Feb. 5, 2013

8

Outdoor ACP&IRIS at night Feb. 5, 2013

9

Outdoor ACP, IRIS & WISG at night Feb. 5, 2013

10

Irradiance difference (WISG minus IRIS) at PMOD

Results Average Offset (IWV>10) ‐4.1 ± 1.5 Wm‐2 Gradient (IWV<10)‐0.45 ± 0.1 Wm‐2mm‐1

IWV

WISG 1 WISG 2 WISG 3 WISG 4 Night averages IRIS u95

From Julian’s presentation, IRS2012-Germany

(Data from 180 nights)

11

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

12

Preliminary Conclusions

• Special set‐up of ACP in BB due to unknown gradient in CPC
• Outdoor agreement between ACP & IRIS to within 1 W/m2
• Irradiance measured by WISG is ~4 W/m2 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 3rd design might increase confidence in establishing a

consensus reference with traceability to SI units.