An Update on Reducing the Uncertainty in Solar Radiometric Measurements

Transcription

1 An Update on Reducing the Uncertainty in Solar Radiometric Measurements Daryl Myers, Ibrahim Reda, Stephen Wilcox National Renewable Energy Laboratory Golden Co Alisha Lester Smith College, Northampton, MA May 26-27, 2005 Athens, Greece NREL/PR

2 Solar Radiation Component Equation

3 Solar Radiometer Responsivity Issues Pyranometer Thermopile Offset IR voltage Corrections at calibration time Post-hoc correction schemes based on cosine response though DAY and Year IR radiation exchange error voltage Pyrheliometer Environmental Influences Correction at calibration time α Wind speed,dtemperature/dt,, Irradiance Post-hoc correction based on Ws,dT,, I

4 Shade/Unshade Response Beam x Cos(z) Z Beam Reda, I., T. Stoffel, D. Myers, A Method to Calibrate a Solar Pyranometer for Measuring Reference Diffuse Irradiance. Solar Energy, : p. p

5 Shade/Unshade Response Z Beam Δ z α B cos Reda, I., T. Stoffel, D. Myers, A Method to Calibrate a Solar Pyranometer for Measuring Reference Diffuse Irradiance. Solar Energy, : p. p

6 U Rs = [B Cos(z) + D ] Component Sum Method U = Test Pyranometer signal volts; D= Diffuse B = Beam radiation; Z = Zenith Angle Diffuse: Shade/Unshade calibrated pyranometer (vs Beam) Shade B from Working Reference Absolute Cavity Pyrheliometers

7 Empirical Reference Irradiance Uncertainties Cavity Reference (World Radiometric Reference) Transfer Uncertainty ±0.35% Diffuse Pyranometer Calibration Uncertainty

8 Pyranometer Offset Error Signal All Black thermopile detectors with reference junctions in instrument body are never in thermal equilibrium; suffer 5 W/m 2 to 20 W/m 2 thermal offset. Offset produced by INFRARED exchange between detector & Sky/domes. Black & White reference and hot junctions in same thermal conditions, low thermal offsets. All-black unshaded units posses offset! Dutton, E. G., J. J. Michalsky, T. Stoffel, B. W. Forgan, J. Hickey, T. L. Alberta, I. Reda, Measurement of Broadband Diffuse Solar Irradiance Using Current Commercial Instrumentation with a Correction for Thermal Offset Errors. Journal of Atmospheric and Oceanic Technology, (3): p

9 Pyranometer Offset Error Signal All Black thermopile detectors with reference junctions in instrument body are never in thermal equilibrium; suffer 5 W/m 2 to 20 W/m 2 thermal offset. Offset produced by INFRARED exchange between detector & Sky/domes. Black & White reference and hot junctions in same thermal conditions, low thermal offsets. All-black unshaded units posses offset! Dutton, E. G., J. J. Michalsky, T. Stoffel, B. W. Forgan, J. Hickey, T. L. Alberta, I. Reda, Measurement of Broadband Diffuse Solar Irradiance Using Current Commercial Instrumentation with a Correction for Thermal Offset Errors. Journal of Atmospheric and Oceanic Technology, (3): p

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12 Characterize shortwave pyranometer net-ir response using Blackbody IR system W NET W bb -W c (Wm -2 ) Pyran Signal (uv) NET IR Rs(bb) uv W -1 m Reda, I., J. Hickey, C. Long, D. Myers, T. Stoffel, S. Wilcox, J.J. Michalsky, E.G. Dutton, D. Nelson, Using a Blackbody to Calculate Net-Longwave Responsivity of Shortwave Solar Pyranometers to Correct for Their Thermal Offset Error During Outdoor Calibration Using the Component Sum Method. Journal of Atmospheric and Oceanic Technology, In Press.

13 Shortwave pyranometer signals in response to net infrared (longwave) radiation Pyranometer Model # Tested RS bb (µv/wm -2 ) RS MFR (µv/wm -2 ) EPLAB K&Z CM EPLAB PSP Spectrosun SR

14 Pyranometer Responsivity Calibration Results Pyranometer Rs U95 = +2% -5% ± 1.0%

15 Pyranometer Calibration IR Corrections Pyranometer Rs( offset corrected)

16 Pyranometer Calibration IR Corrections Pyranometer Rs( offset corrected)

17 Daily Calibration & Characterization Pyranometer Rs through the Year Lester, A., D. Myers, A Method for Improving Global Pyranometer Measurements by Modeling Responsivity Functions. Solar Energy, In Press.

18 Daily Calibration & Characterization Pyranometer Rs through the Year RS u8-48 (Z,D,I)= ln[cos(z)] COS[(D 2π 360)/365] I Lester, A., D. Myers, A Method for Improving Global Pyranometer Measurements by Modeling Responsivity Functions. Solar Energy, In Press.

19 RS 8-48U (Z, DA, IR) Applied to Pyranometer mv One year of data +60 to -40 W m -2 ± 20 W m -2

20 Pyrheliometer Rs Calibration Results Pyrheliometer Rs U95 = ± 1.8% ± 1.0%

21 Pyrheliometer Rs environmental influences (flange, window, instrument) Flange Shading Effect vs Wind Speed (by Irradiance) Flange Shading Effect (mv) (Unshaded - Shaded) Irradiance (W/m 2 ) Wind Speed (m/s) Environmental Thermal Effects on the Eppley Normal Incidence Pyrheliometer Stephen Wilcox, John Hickey,Daryl Myers Draft research summary; April 5, 2005

22 Pyrheliometer Rs environmental influences (flange, window, instrument) Flange Shading Effect vs Wind Speed (by Irradiance) Flange Shading Effect (mv) (Unshaded - Shaded) Flange Shading Effect (mv) (Unshaded - Shaded) Flange Shading Effect vs Irradiance Irradiance (by Wind Speed) (W/m 2 ) Wind Speed (m/s) Direct Beam Irradiance (W/m 2 ) Wind Speed (m/s) < Environmental Thermal Effects on the Eppley Normal Incidence Pyrheliometer Stephen Wilcox, John Hickey,Daryl Myers Draft research summary; April 5, 2005

23 Pyrheliometer Rs environmental influences (flange, window, instrument) Flange Shading Effect vs Wind Speed (by Irradiance) Flange Shading Effect (mv) (Unshaded - Shaded) Flange Shading Effect (mv) (Unshaded - Shaded) Wind Speed (m/s) Thermopile Output (mv) Flange Shading Effect vs Irradiance Wind Speed Irradiance (m/s) (by Wind Speed) (W/m 2 ) < Effective of Temperature Change on the Flange-shaded Instrument y = x R 2 = Direct Beam Irradiance (W/m 2 ) minute Delta Temperature (degrees C) Environmental Thermal Effects on the Eppley Normal Incidence Pyrheliometer Stephen Wilcox, John Hickey,Daryl Myers Draft research summary; April 5, 2005

24 Shade window Instrument Calibration & Characterizaton Minus Shade Flange DNI Pyrheliometer Responsivity variations from environmental influences (on flange, instrument)

25 Pyrheliometer Calibration Corrections Pyrheliometer Rs(-Environment corrected-)

26 Solar Radiometer Responsivity Issues Pyranometer Thermopile Offset IR voltage Corrections at calibration time Monitor IR; reduce U95 by ½ to 1% Z Post-hoc correction schemes based on cosine response though DAY and Year Global data uncertainty 60 Wm - 2 -> > 20 Wm -2 Pyrheliometer Environmental Influences Correction at calibration time α Ws,dT/dt,, DNI Reduce U95 offset in Rs ~ 1/3; 0.6% to 0.2% Post-hoc correction based on Ws,dT/dt,, I Research continues!