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Free-form optics for Fresnel-lens-based photovoltaic concentrators

Juan C. Miñano, Pablo Benítez, Pablo Zamora, Marina Buljan, Rubén Mohedano, and Asunción Santamaría

Open Access Open Access

Abstract

The Concentrated Photovoltaics (CPV) promise relies upon the use of high-efficiency triple-junction solar cells (with proven efficiencies of over 44%) and upon high-performance optics that allow for high concentration concurrent with relaxed manufacturing tolerances (all key elements for low-cost mass production). Additionally, uniform illumination is highly desirable for efficiency and reliability reasons. All of these features have to be achieved with inexpensive optics containing only a few (in general no more than 2) optical elements. In this paper we show that the degrees of freedom using free-forms allow the introduction of multiple functionalities required for CPV with just 2 optical elements, one of which is a Fresnel lens.

©2013 Optical Society of America

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Figures (9)
Fig. 1
Fig. 1 A 4-sector FK concentrator during measurements (a) and its glass molded SOE (b) showing the second part of the four sectors of the Köhler array.

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Fig. 2
Fig. 2 Simulated irradiance distribution on the cell for the FK concentrator with parameters Cg = 625x, f/0.85, 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). The vertical axis denotes the irradiance gain of the respective wavelength ranges relative to the 1-sun case.

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Fig. 3
Fig. 3 Rendered view of a dome-shaped FK concentrator (DFK). At bottom left, a detailed view of the SOE, where the 4 sectors can be distinguished, each one receiving light from one of the POE sectors.

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Fig. 4
Fig. 4 DFK concentrator: Irradiance distribution on the cell, when illuminating the DFK at normal incidence with 1 sun irradiance.

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Fig. 5
Fig. 5 Simulated and measured transmission curves (a) with αsim = ± 1.25° and αmeasured = ± 1.24°. Illuminated SOE back surface (b) where the cell has been substituted by a transmissive diffuser.

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Fig. 6
Fig. 6 (a) Scheme of the normal-incidence rays passing through one quarter of the POE (Fresnel lens) and focusing on the corresponding surface of the SOE (RXI). Light is focused by each Fresnel lens quarter on four different foci (b), corresponding to the four SOE parts.

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Fig. 7
Fig. 7 Simulation showing the relative optical efficiency versus incidence angle on a 2,300x free-form F-RXI Köhler concentrator (a). The resulting acceptance angle is α = ± 1.02°. The irradiance distribution on the cell when the sun’s center is at normal incidence (@DNI = 1 sun) is shown in (b).

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Fig. 8
Fig. 8 Cross section of the SOE of the concentrators being compared. All these concentrators have the same POE entry aperture area (625 cm2) and the same acceptance angle (α = ± 1°). The cross sections of their corresponding cells, which should be centered at the origin, are shown displaced downward to make them visible. f-numbers and geometrical concentrations are given in the left hand side.

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Fig. 9
Fig. 9 Concentration acceptance angle product (CAP) for the concentrators under comparison.

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Equations (1)

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CAP= C g sin(α)
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