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The “ M onitor to measure the I ntegral TRA nsmittance” ( MITRA )

The “ M onitor to measure the I ntegral TRA nsmittance” ( MITRA ) and the “ C ryogenic S olar A bsolute R adiometer” ( CSAR ) IPC-XII Seminar, Davos, 12 October 2015 Benjamin Walter, Wolfgang Finsterle, Andre Fehlmann, Rainer Winkler, Ricco Soder, Markus Suter, Werner Schmutz.

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The “ M onitor to measure the I ntegral TRA nsmittance” ( MITRA )

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  1. The “Monitor to measure the Integral TRAnsmittance” (MITRA) and the “Cryogenic Solar Absolute Radiometer” (CSAR) IPC-XII Seminar, Davos, 12 October 2015 • Benjamin Walter, Wolfgang Finsterle, Andre Fehlmann, Rainer Winkler, Ricco Soder, Markus Suter, Werner Schmutz

  2. MOTIVATION Direct Solar Irradiance measurements [W m-2] traceable WRR scale difference 0.34 % ± 0.09 % (k=1) internal stray light Intercomparisons: SI (Fehlmann et al. 2012)

  3. MOTIVATION Direct Solar Irradiance measurements [W m-2] traceable WRR CSAR = LINK directly traceable Intercomparisons SI SI - cryogenic radiometers

  4. The MITRAInstrument Transmittance depends on solar spectrum which changes with air mass and atmospheric conditions World Standard Group MITRA WSG Monitoring of transmittance entrance window CSAR = Cryogenic Solar Absolut Radiometer entrance window

  5. Operation method & target uncertainty: window IPC-XI uncertainty ≈ 1 % Previous uncertainty > 1 % sensing reference cavity Measuring temperature rise Δt at both cavities • Target uncertainty for the CSAR: • 0.01% (= 100 ppm) spectrallyintegrated transmittance  • MITRA requirement: • uncertainty of tint < 100 ppm

  6. Measurements in front of the sun WSG tracker  outdoor conditions HELIOSTAT  Laboratory conditions

  7. Measurements in front of the sun Laboratory: - instrument stability = 150 ppm = 0.015% - heliostat windows: wavelength dependent reflectivity Outdoor: - instrument stability = 900 ppm ≈ 0.1% - Measurement influenced by: wind / temperature variations Two questions: 1. How accurate is the absolute value? 2. How to reduce wind and temperature variation problems in outdoor measurements?

  8. How accurate is the absolute value? no long term stability of parameters Temperature differences must be very accurate: Δt measurement uncertainty < 0.1 mk no generally valid thermometer calibration possible  Periodical recalibration !

  9. How accurate is the absolute value? Measured offset to validated simulations of Fehlmann: 1334 ± 300 ppm (k = 1) Calculated offset: 1290 ppm • Good agreement between measured and calculated offset to simulations •  Periodical recalibration ± 150 - 300 ppm (k = 1) depending on measurement conditions • 2. wind and heat accumulation problem for outdoor measurements

  10. WIND and HEAT ACCUMULATION: 10°C warmer relative to ambient air temperature

  11. WIND and HEAT ACCUMULATION: wind cannot enter the cavity covered by the window wind influences both cavities similarly • Thermally-stable environment • wind shelter

  12. Thermally-stable environment No wind influences and Heat accumulation problems anymore electrical and thermal insulation • Indirect cooling • of MITRA - 6 Peltier elements - CPU fans - PID - controller • Constant backplate • temperature ≈ ± 0.1 K

  13. Thermally-stable environment Measurement on the WSG sun tracking platform ≈ 0.6 K Calibration of thermometers < ± 1 mK

  14. Thermally-stable environment Measurement on the WSG sun tracking platform ≈ ± 100 ppm ≈ 0.6 K

  15. Thermally-stable environment Measurement on the WSG sun tracking platform ≈ ± 100 ppm ≈ ± 100 ppm ≈ 0.6 K uncertainty =115 ppm mean(tint) = 0.9269

  16. Thermally-stable environment Measurement on the WSG sun tracking platform ≈ ± 100 ppm ≈ ± 100 ppm uncertainty =165 ppm mean(tint) = 0.9269

  17. Summary MITRA improvements: ≈ ± 150 ppm • Thermally-stable environment (Peltier cooling system) •  Outdoor instrument stability ≈ 150 ppm (k = 1) • Periodical recalibration of thermometers •  Absolute value uncertainty about 150 - 300 ppm (k = 1) • 3. Overall uncertainty: • 250 - 350 ppm (k = 1)

  18. Summary MITRA improvements: ≈ ± 150 ppm • Promising results • Future improvements to reach the 100 ppm goal: • Three cavity design to monitor the temperature drift • Liquid cooling of the instrument • Larger heatsink to provide more stable conditions

  19. Cryogenic Solar Absolute Radiometer (CSAR) Cryocooler (Helium) 20 K reference block quartz window 10 mm View limiting aperture apertures cavity

  20. Cryogenic Solar Absolute Radiometer Measurement procedure: Cleaning the windows One day for cooling CSAR to 20K Cavity characterization Electrical measurements every 10-25 min Sampling rate = 3 s MITRA Helium pipes

  21. Cryogenic Solar Absolute Radiometer IPC-XII DAY: 2015-09-30 CSAR measures about 0.2 - 0.3% lower than WRR

  22. Cryogenic Solar Absolute Radiometer ( k = 2 ) IPC-XII Good agreement with previous findings of difference between the WRR and the SI scales. Detailed values and uncertainties will follow soon ...

  23. SUMMARY • MITRA: Thermally-stable environment (Peltier system) • a.) No wind influences / convection / heat accumulation • b.) Strongly reduce temperature drift • c.) Remaining drift can empirically be corrected for • d.) Periodical recalibration of thermometers • d.) MITRA uncertainty ≈ 250 - 350 ppm (k = 1) • CSAR: a.) Overal stable performance • b.) Preliminary results are in good agreement with previous • findings that the SI scale is about 0.2% - 0.3% lower than WRR Thank you for your attention!

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