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Proceedings Paper

Influence of the steepness of absorbance on high-temperature thermal conversion of nonconcentrated solar radiation
Author(s): Wolfgang Spirkl
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Paper Abstract

Thermal conversion of non-concentrated solar radiation at high temperatures can be achieved by a selective absorber with high absorptance in the energy regions where solar irradiance exceeds thermal absorber radiance and with low absorptance (emittance) in the other regions. In the simplest case optimum absorptance (emittance) is one above a certain threshold energy and zero below. While this optimum design requires infinitely steep transitions, it is shown that the loss due to a broadened transition is of second order with regard to the transition width, at least for sufficiently smooth solar spectra and for a given type of transition being a function of the ration of the energy deviation from the threshold to the width. Hence comparatively large widths can be used without significantly deteriorating performance. It is shown that the slope at the threshold is not a sufficient measure to quantify losses by a non- ideal transition, since radiative losses increase with the distance form the threshold. For converting 800 Wm-2 solar irradiance at 500 degree(s)C, a width in the order of the thermal energy can be accepted. This is confirmed for different types of transitions. For thermal conversion at high temperatures it is more important to choose the right threshold and to achieve low emittance away from the threshold than to pursue a steep transition.

Paper Details

Date Published: 9 September 1994
PDF: 6 pages
Proc. SPIE 2255, Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XIII, (9 September 1994); doi: 10.1117/12.185394
Show Author Affiliations
Wolfgang Spirkl, Ludwig-Maximilians-Univ. Muenchen (Germany)

Published in SPIE Proceedings Vol. 2255:
Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XIII
Volker Wittwer; Claes G. Granqvist; Carl M. Lampert, Editor(s)

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