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Research Papers

Beam-Down Mirror: Thermal and Stress Analyses

[+] Author and Article Information
Rami Ben-Zvi1

Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israelrami.ben-zvi@weizmann.ac.il

Akiba Segal, Michael Epstein

Solar Research Facilities Unit, Weizmann Institute of Science, P.O. Box 26, Rehovot 76100, Israel

1

Corresponding author.

J. Sol. Energy Eng 131(4), 041003 (Sep 17, 2009) (12 pages) doi:10.1115/1.3197536 History: Received May 31, 2007; Revised February 27, 2008; Published September 17, 2009

The “beam-down” optics or solar tower reflector, developed and demonstrated at the Weizmann Institute of Science during the past 9 years, could be a useful modification of the classic solar tower technology, especially for solar applications where reacting solids are involved or heavy equipment has to be placed on top of a conventional tower. The theory of this optics has been thoroughly studied and reported elsewhere. This paper details the development and experience gained with the mirror facets of the tower reflector. Thermal and stress analyses are presented here, validated by temperature measurements and calculated incident flux map. The projection for a large scale solar plant of about 100 MW at the aperture of the receiver is illustrated. The current basic design of the facet made of a sandwich of mirrors glued back-to-back seems to be a feasible solution for future applications. Aluminum-glass facets failed, and cracks in the glass were observed in the course of time. Years of experience proved that using only natural cooling to the surrounding, for the glass/glass facets, which can reach 130140°C during operation under average incident solar flux of about 30kW/m2, is a viable design. Maximum working temperatures of 160°C were experienced without any degradation of the reflectivity and the performance of these facets after several hundreds of operation hours.

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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

The beam-down system (schematic)

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Figure 2

Top view of the WIS tower reflector. Thermocouples locations are marked by “●.”

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Figure 3

Calculated TR flux contours (in kW/m2) for the 100 MW plant (the case of the 1×1 m2 facet; see Table 1) for Equinox, March, 21st, Noon, latitude 32°N. The abscissa is the azimuth and the ordinate is the facet row number, starting at the minor radius.

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Figure 4

Schematic beam-down mirror segment—sandwich glass layout

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Figure 5

Schematic beam-down mirror segment—finned metal plate: layout (top) and repetitive layout and nomenclature (bottom)

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Figure 6

Flux-contours (in kW/m2) for the WIS hyperboloid mirror calculated for the experiment of 31 August 2005, 12:52–12:59; 38 heliostats; power incident on the hyperboloid mirror: 1163 kW; insolation: 752 W/m2; mirror side edges: −35.58°W, +43.95°E. The abscissa is the azimuth and the ordinate is the facet row number, starting at the minor radius.

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Figure 7

The temperature at the bottom mirror silver surface versus the bottom incident flux from the heliostats for 0 m/s and 5 m/s wind. Glass sandwich (solid) and finned Al-glass (dashed) layouts. Length scales of 1 m are used in the convection correlations.

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Figure 8

The temperature at the bottom mirror silver surface versus the wind velocity for 20 kW/m2 and 30 kW/m2 fluxes from the heliostats. Glass sandwich (solid) and finned Al-glass (dashed) layouts. Length scales of 1 m are used in the convection correlations.

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Figure 9

Thermal stress in an aluminum-glass cantilever beam—total strain (top) and stress (bottom) components versus the thickness coordinate, z

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Figure 10

Thermal stress in a simply supported sandwich glass-aluminum facet: deformation (maximum displacement (DMX), m) and maximum principal stress contours (Pa) in the glass

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Figure 11

Thermal stress in a simply supported finned-aluminum facet: deformation (DMX (m)) and maximum principal stress contours (Pa)

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Figure 12

Thermal stress in a finned aluminum-glass cantilever beam: solid elements (top) and shell elements (bottom) models (DMX (m)) and axial stress component contours (Pa)

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Figure 13

Wind load stress analysis in a simply supported sandwich glass facet: deformation (DMX (m)) and maximum principal stress contours (Pa)

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