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Characterization of selective solar absorber under high vacuum

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Abstract

Total absorption and emission coefficients of selective solar absorbers are measured under high vacuum conditions from room temperature up to stagnation temperature. The sample under investigation is illuminated under vacuum @1000W/m2 and the sample temperature is recorded during heat up, equilibrium and cool down. During stagnation, the absorber temperature exceeds 300°C without concentration. Data analysis allows evaluating the solar absorptance and thermal emittance at different temperatures. These in turn are useful to predict evacuated solar panel performances at operating conditions.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (4)

Fig. 1
Fig. 1 Experimental data of temperatures and pressure versus time. A stagnation temperature of about 320°C is reached when the absorber is illuminated with 1000W/m2. The internal pressure remains well below 1x10−5 mbar throughout the experiment.
Fig. 2
Fig. 2 Left: absorber temperature derivative (black dots) vs absorber temperature is fitted (red line) using Eq. (2), assuming Pin = 0 (cool down). Rigth: Mirotherm emittance versus temperature: black dots are obtained assuming emissivity constant in a 20 degrees interval, blue line is obtained fitting the whole set of cool down data using Eq. (2) with Pin = 0 and assuming a quadratic temperature dependence for the emittance. The red line, which overlaps with the blue line, is the quadratic fit of the black dots.
Fig. 3
Fig. 3 Left: Absorber temperature derivative under 1000W/m2 illumination fitted using Eq. (2). Black dots: experimental data. Continuous red line: best fit from 45°C to 320°C with ε a ( T a ) and α as free fitting parameters. Blue dashed curve: ε a ( T a ) from cool down and only α as free fitting parameter. Right: absorptivity obtained by hemispherical reflectivity measurement using an integrating sphere. The Spectrally averaged absorptivity α ¯ is 0.95 when averaged on the solar standard ASTM G173 and 0.96 when averaged on our white LED spectrum.
Fig. 4
Fig. 4 Left: stationary simulation of the temperature of the absorber, with the bolt and nut to keep the thermocouple in thermal contact. The presence of the thermocouple assembly introduces a small error of about 0.5°C in the stagnation temperature. Right: time dependent simulation of the temperature derivative with (blue dots) and without (black squares) thermocouple.

Equations (3)

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m c p dT dt = αA P in P losses  
(m+ m th )  c p d T a dt = αA P in   ε a ( T a )σA( T a 4 T g 4 )  ε sub ( T a )σA( T a 4 T v 4 )
ε a ( T a )= 0.0597+3.4* 10 5 * T a +2.66* 10 7 * T a 2
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