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Research Papers

Optical Design of a Novel Two-Stage Solar Trough Concentrator Based on Pneumatic Polymeric Structures

[+] Author and Article Information
R. Bader, P. Haueter

Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zürich, Switzerland

A. Pedretti

 ALE Airlight Energy SA, 6710 Biasca, Switzerland

A. Steinfeld1

Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zürich, Switzerland; and Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerlandaldo.steinfeld@ethz.ch

1

Corresponding author.

J. Sol. Energy Eng 131(3), 031007 (Jun 29, 2009) (9 pages) doi:10.1115/1.3142824 History: Received January 04, 2009; Revised April 07, 2009; Published June 29, 2009

An innovative concept for fabricating solar trough concentrators based on pneumatic polymer mirrors supported on precast concrete frames is presented. Optical aberration is corrected by means of a secondary specular reflector in tandem with a primary cylindrical concentrator. The optimal design is formulated for maximum solar flux concentration. The Monte Carlo ray-tracing technique is applied to determine the effect of reflective surface errors and structural beam deformations on the performance of the combined primary and secondary concentrating system. The numerical results are validated with field measurements on a 49.4 m length, 7.9 m width sun-tracking prototype system. Theoretical maximum solar concentration ratio is 151 suns; the measured one with a flat secondary reflector was 55 suns.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Scheme of conceptual design of solar trough concentrator based on pneumatic polymer mirrors supported on precast concrete frames

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Figure 2

Photograph of the 49.4 m length and 7.9 m width sun-tracking solar trough concentrating prototype: (a) front view and (b) rear view

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Figure 3

Ray paths after reflection on (a) perfect parabolic trough concentrator and (b) perfect circular trough concentrator; parameters are wPM=8 m and ϕrim=53.1 deg

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Figure 4

Optical components of the solar concentrator system comprising two symmetric primary trough mirrors—denoted PM—and two symmetric secondary mirrors—denoted SM—on axis with PM; indicated is the target area of width wtarget and length ltarget that matches the tubular solar receiver at z=Fz

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Figure 5

Scheme of PM mounted on the two longitudinal beams

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Figure 7

Variation in PM as a consequence of the deformation of the longitudinal beams caused by gravitational forces; the dashed lines indicate the undistorted system and ĝ indicates the direction of gravitation

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Figure 8

Derivation of SM designed to correct the optical aberration by directing the sunrays reflected by PM onto the focal line parallel to the y-axis described by F+q⋅ĵ

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Figure 9

Right-hand side SM profile for the baseline parameters of Table 1 (solid line) and linear regression z=0.1273x+2.3915 (dashed line); the average distance between the SM profile and the regression line is 2.09 mm

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Figure 10

Distribution of the solar concentration ratio on the target plane along the x-axis; the parameter is the PM’s surface error mode σPM in the range 0–10 mrad

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Figure 23

Measured C-distribution for case 1 (solid line) and for a parabolic trough concentrator with σPM=2.5 mrad (dashed line)

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Figure 22

C-distribution for case 1 (Table 3) with σPM=2 mrad (dashed line: measurement, solid line: simulation)

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Figure 21

Numerically simulated (solid curve) and experimentally measured (dashed curve) local C-distributions at (a) y=18.88 m and (b) y=19.03 m; the geometrical parameters and the insolation are given in Table 2

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Figure 20

Portion of incident solar radiation transmitted through the transparent envelope as a function of the skew angle θskew

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Figure 19

Measured spectral normal transmittance Tλ,n and reflectance Rλ,n of the transparent envelope

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Figure 18

(a) Measured spectral normal reflectivity ρλ,n of PM and SM and (b) ASTM G173-03 reference solar spectrum used to calculate the total mirror reflectivity ρ(9)

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Figure 17

System’s optical efficiency as a function of the target’s width for various maximum beam deformations umax (in millimeters, averaged over the concentrator length)

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Figure 16

Distribution of the solar concentration ratio on the target along the x-axis for various maximum beam deformations umax (in millimeters, averaged over the concentrator length)

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Figure 15

Distribution of the solar concentration ratio on the target along the x-axis for a system with a maximum beam deformation umax=3 mm (at β=30 deg); the parameter is the axial position in the range y=18.53–24.70 m (half of one PM section).

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Figure 14

Contour map of the solar concentration ratio at the focal plane for a system with a maximum beam deformation umax=3 mm (at β=30 deg)

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Figure 13

Optical efficiency as a function of the target’s width; the parameter is the SM’s surface error mode σSM in the range 0–10 mrad

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Figure 12

Distribution of the solar concentration ratio on the target plane along the x-axis; the parameter is the SM’s surface error mode σPM in the range 0–10 mrad

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Figure 11

Optical efficiency as a function of the target’s width; the parameter is the PM’s surface error mode σPM in the range 0–10 mrad

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Figure 6

(a) Forces and moments acting on one of the prestressed beam sections cut free at y and (b) gravitational force acting on the longitudinal beam and resulting bending moment; β is the concentrator slope

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