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High performance Fresnel-based photovoltaic concentrator

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Abstract

In order to achieve competitive system costs in mass-production, it is essential that CPV concentrators incorporate sufficient manufacturing tolerances. This paper presents an advanced concentrator optic comprising a Fresnel lens and a refractive secondary element, both with broken rotational symmetry, an optic producing both the desired light concentration with high tolerance (high acceptance angle) as well as an excellent light homogenization by Köhler integration. This concentrator compares well with conventional Fresnel-based CPV concentrators.

©2010 Optical Society of America

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Figures (16)

Fig. 1
Fig. 1 (Left) 3D view of the LPI’s four-fold Fresnel-Kohler (FK) concentrator: maroon rays show how on-axis rays uniformly illuminate the cell while green rays illustrate how a point of the primary is imaged on the cell, and. (Right) 2D schematic drawing of the edge-ray mapping in an ideal FK concentrator.
Fig. 2
Fig. 2 Phase space representation of the Kohler integration in 2D geometry (the actual design is in 3D) of (a) & (b) the Fresnel Kohler concentrator, (b) & (c) a classical imaging concentrator. The sun is represented by the yellow bar.
Fig. 3
Fig. 3 (Left) Close-up rendering of the center of the LPI’s FK Fresnel lens, showing the four sectors. (Right) Photograph of an LPI’s FK secondary made by glass molding with a ring to be used as holder.
Fig. 4
Fig. 4 (a) Lossless spectral photocurrent densities and spectral optical efficiency of an LPI’s FK concentrator with Cg = 625x and f/1, (b) Monochromatic (at 555 nm) and polychromatic optical efficiencies as a function of the f-number of the FK concentrator, considering no AR coating on the SOE and perfect AR coating (i.e. no Fresnel reflection on the SOE).
Fig. 5
Fig. 5 For an LPI’s FK concentrator with Cg = 625x and f/1: (a) monochromatic (@ 555 nm) power impinging the solar cell as a function of the incidence angle of the parallel rays on the entry aperture, (b) Cell photocurrent as a function of the concentrator tracking error angle for AM1.5d ASMT G173 spectrum (therefore, it accounts for the finite size of the sun and the EQE of the three subcells to find which one is limiting). Both SOE’s, without AR coating and with perfect AR coating, are considered. All curves are relative to normal incidence.
Fig. 6
Fig. 6 Concentration acceptance angle products CAP and CAP* for the LPI’s FK concentrator with no AR coating and with a perfect AR coating on the SOE versus the POE f-number (f/#).
Fig. 7
Fig. 7 Irradiance distribution on the cell for the LPI’s FK concentrator with Cg = 625x, f/1, no AR coating on SOE, when the sun is on axis and the solar spectrum is restricted to: (a) the top-subcell range (360-690 nm), and (b) the middle-subcell range (690-900 nm).
Fig. 8
Fig. 8 Cumulative distribution of light power received on the cell with off–normal angle for the LPI’s FK concentrator with parameters Cg = 625x, f/1, when the sun is perfectly tracked and when the tracking error is 1° (this design has α = ± 1.43 deg).
Fig. 9
Fig. 9 Diagonal cross-section of the Fresnel-based concentrators considered in the comparison. SOE’s and cells are not to scale, but the concentrator depth to POE diagonal ratio is. From left to right: Fresnel (no SOE), Spherical dome, SILO, XTP, RTP, FK concentrator.
Fig. 10
Fig. 10 Cross section of the Secondary Optical Elements of the Fresnel-based concentrators of Fig. 8. All these concentrators have the same POE entry aperture area (625 cm2) and the same acceptance angle (α = ± 1 deg). The cross section of their corresponding cells, which should be centered at the origin, are shown displaced downward to make them visible.
Fig. 11
Fig. 11 Effective concentration acceptance angle product (CAP*) for the concentrators under comparison when the SOE has no AR coating SOE (left) and when a perfect AR coating is assumed (right). The curves of the LPI’s FK concentrator are the same as those shown in Fig. 6.
Fig. 12
Fig. 12 Estimated SOE cost as a function of its height for the RTP SOE and for the Dome-type SOE’s (FK, SILO and spherical dome). The dots correspond to the SOE’s of Fig. 14.
Fig. 13
Fig. 13 SOE and solar cell cost per Fresnel lens unit area as a function of the effective acceptance angle α*. The SOE’s have no AR coating.
Fig. 14
Fig. 14 Cost comparison (per POE unit area) of the set formed by the SOE (green) and solar cell (blue squares) between the RTP and FK with the same POE area (625 cm2) and the same effective acceptance angle α* = ± 1°. The cost advantage of the FK resides on the smaller cell active area needed: 92 mm2 in the RTP versus 59 mm2 in the FK concentrator. The costs of these two SOE’s are highlighted with spots in Fig. 12.
Fig. 15
Fig. 15 Irradiance distributions, in suns (1 sun = 1kW/m2), on the cell for the (a) spherical dome with f/1.5 and the (b) f/1 FK concentrator (this is the average of the graphs in Fig. 7). Both have geometrical Cg = 625x; and α* = ± 0.61° for the spherical dome and α* = ± 1.30° for the LPI’s FK concentrator.
Fig. 16
Fig. 16 Practical aspects of the R-TP secondary versus the LPI’s FK secondary.

Tables (3)

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Table 1 Monochromatic acceptance angle α and effective acceptance angle α* of the LPI’s FK concentrators at different geometrical concentration factors.

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Table 2 Ratio of the distance between the cell and Fresnel lens to the Fresnel lens diagonal (i.e., f-number) of the selected Fresnel-based concentrators under comparison

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Table 3 Geometrical concentration at which the concentrators under comparison are tolerance-equivalent (α* = ± 1°). For this table the f-number of the FK and SILO has been fixed to 1.0 and 1.2, respectively.

Equations (2)

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C A P = C g sin α
η o p t , p o l y c h r o m = I s c c o n c C g I s c 1 s u n = min { I s c , t o p c o n c , I s c , m i d d l e c o n c , I s c , b o t t o m c o n c } C g min { I s c , t o p 1 s u n , I s c , m i d d l e 1 s u n , I s c , b o t t o m 1 s u n }
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