Pablo Benitez

Design in 3D geometry with the simultaneous multiple surface design method of nonimaging optics

The Simultaneous Multiple Surfaces of Nonimaging Optics has been used successfully in the past for the synthesis of concentrators in two dimensions. In this paper we present a first approach to extend this design method to 3D geometry. As a first result, an aspheric lens without rotational symmetry that focus sharply two plane wavefronts at two points in 3D geometry is found, feature which cannot be obtained with an axisymmetric optical system with a finite number of optical surfaces.

TIR-R concentrator: a new compact high-gain SMS design

In this work it is presented a new design of a TIR lens-mushrooms lens device developed with the Simultaneous Multiple Surfaces (SMS) method. In SMS nomenclature, it is named TIR-R. In contrast to previous TIR-mushroom designs, application of the SMS method to this configuration consists in the simultaneous design of both TIR (total internal reflection) and R (refraction) optical surfaces using extended ray-bundles and the edge-ray theorem. In this paper is presented a basic approach to do the design. In this basic approach, first it is considered the TIR lens as a microstructured surface with infinitesimal flat facets. Afterwards, it is generated a TIR lens with finite size facets from the already designed one. In an advanced approach could be considered the TIR lens with finite facet size and designed simultaneously each facet with a portion of the outer surface of the mushroom lens. With respect to others SMS high-gain devices (as the RXI), the TIR-R concentrator has the following advantages: is a mirror-less device, there is not shadowing elements, and the receiver/emitter elements placement is more favorable for encapsulation and electrical connection. As it is common in the SMS devices, the TIR-R concentrator achieves wide acceptance angle and high efficiency with a low aspect ratio (thickness to entry aperture diameter ratio). For example, a 1256X concentration device has a theoretical efficiency of 100 percent (without optical losses) with an acceptance angle of +/- 1.7 decgrees, and an aspect ratio of 0.34.

RXI concentrator for 1000X photovoltaic energy conversion

The purpose of this work is to present the measurements of the main characteristics of a series of RXI concentrators developed: angular transmission, acceptable angle, optical efficiency, and optical concentration. The RXI concentrator has been designed with the Simultaneous Multiple Surfaces method developed by Minano et al. at the Instituto de Energia Solar--Universidad Politecnica de Madrid. The design characteristics are: geometric concentration 1256X, acceptance angle 1.8 degree(s).

Application of Lorentz geometry to nonimaging optics: new three-dimensional ideal concentrators

A new family of three-dimensional ideal nonimaging concentrators with rotational symmetry is presented. The flow-line concentrator and the cone concentrator are particular cases of this family. First, we looked for elliptic bundles of rays, i.e., bundles such that the subset of rays passing through any point of the space forms a cone with an elliptic base (this search was done with the Lorentz geometry formalism.) Second, the concentrators, defined by their reflectors and receiver shapes, were derived from these elliptic bundles with the flow-line design method.

The free form XR photovoltaic concentrator: a high performance SMS3D design

A novel photovoltaic concentrator is presented. The goal is to achieve high concentration design with high efficiency and high acceptance angle that in the same time is compact and convenient for thermal and mechanical management. This photovoltaic system is based on 1 cm2 multi-junction tandem solar cells and an XR concentrator. The XR concentrator in this system is an SMS 3D design formed by one reflective (X) and one refractive (R) free-form surfaces (i.e., without rotational or linear symmetry) and has been chosen for its excellent aspect ratio and for its ability to perform near the thermodynamic limit. It is a mirror-lens device that has no shadowing elements and has square entry aperture (the whole system aperture area is used for collecting light). This large acceptance angle relaxes the manufacturing tolerances of all the optical and mechanical components of the system included the concentrator itself and is one of the keys to get a cost competitive photovoltaic generator. For the geometrical concentration of 1000x the simulation results show the acceptance angle of ±1.8 deg. The irradiance distribution on the cell is achieved with ultra-short homogenizing prism, whose size is optimised to keep the maximum values under the ones that the cell can accept. The application of the XR optics to high-concentration is being developed in a consortium leaded by The Boeing Company, which has been awarded a project by US DOE in the framework of the Solar America Initiative.

New nonimaging static concentrators for bifacial photovoltaic solar cells

Two new static nonimaging designs for bifacial solar cells are presented. These concentrators have been obtained with the Simultaneous Multiple Surface design method of Nonimaging Optics. The main characteristics of these concentrators are: (1) high compactness, (2) linear symmetry (in order to be made by low cost extrusion), (3) performance close to the thermodynamic limit, and (4) a non-shading sizable gap between at least one of the cell edges and the optically active surfaces. This last feature is interesting because this gap can be used to allocate the interconnections between cells, with no additional optical losses. As an example of the results, one design for an acceptable angle of +/- 30 degrees gets a geometrical concentration of 5.5X, with an average thickness to entry aperture width ratio of 0.24. The 3D ray-tracing analysis of the concentrators is also presented.

Design, construction, and measurement of a dielectric single-mirror two-stage (DSMTS) photovoltaic concentrator

The 30 X DSMTS is a trough-like photovoltaic concentrator, meant to track the sun in one axis, which has a mirror allocating two concentration stages and a secondary lens that increases its acceptance up to +/- 2.3 degrees. Provided that the sun subtends an angle of +/- 0.256 degrees, such acceptance seems excessive. However, thanks to it, we can relax requirements that often demand accuracy in systems of the kind. For instance, the shape of the mirror can be achieved by simply bending an aluminum sheet. To foresee the results we may expect of this strategy, we carried out some mechanical calculations, whose results are the boundary conditions that lead to a minimum standard deviation on the local slope of the elastic mirror with regards to the theoretical value. We checked by ray tracing that such an error actually provoked a small decrease on the acceptance. This fact persuaded us to carry out the manufacture of two elastic prototypes. In the test that have been performed so far with them we achieved an acceptance angle of +/- 1.63 degrees and a collection efficiency of 98 percent at a geometrical concentration of 30 times, results that can be considered as outstanding in the photovoltaics framework.

New nonimaging designs: the RX and RXI concentrators

Two new nonimaging concentrators called RX and RXI are presented. Both of them have been designed with a method of design recently developed. The concentrators, formed from a single dielectric piece, achieve the optimum relation between concentration and acceptance angle in 2D. Their performance in 3D is very good when the acceptance angle of the concentrators is small (less than 5 degrees for a source at infinity): total transmissions above 95% are usually achieved (total transmission is the ratio of the etendue of those rays impinging on the entry aperture, within the acceptance angle, which reach the receiver, over the etendue of the bundle formed by all the rays impinging the receiver). These values are similar or even higher than those achieved by an equivalent CPC (same acceptance angle and same refractive index). The RX shown here have been designed for a finite source and the RXI for a source at infinity.

Photovoltaic performance of the dome-shaped Fresnel-Köhler concentrator

In order to have a cost-effective CPV system, two key issues must be ensured: high concentration factor and high tolerance. The novel concentrator we are presenting, the dome-shaped Fresnel-Köhler, can widely fulfill these two and other essential issues in a CPV module. This concentrator is based on two previous successful CPV designs: the FK concentrator with a flat Fresnel lens and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. The concentrator has shown outstanding simulation results, achieving an effective concentration-acceptance product (CAP) value of 0.72, and an optical efficiency of 85% on-axis (no anti-reflective coating has been used). Moreover, Köhler integration provides good irradiance uniformity on the cell surface and low spectral aberration of this irradiance. This ensures an optimal performance of the solar cell, maximizing its efficiency. Besides, the domeshaped FK shows optimal results for very compact designs, especially in the f/0.7-1.0 range. The dome-shaped Fresnel- Köhler concentrator, natural and enhanced evolution of the flat FK concentrator, is a cost-effective CPV optical design, mainly due to its high tolerances. Daido Steel advanced technique for demolding injected plastic pieces will allow for easy manufacture of the dome-shaped POE of DFK concentrator.

Novel nonimaging lens for photodiode receivers with a prescribed angular response and maximum integrated sensitivity

12 In this work we present a novel optical lens that can be designed to provide any specified angular sensitivity to a receiver, illuminating the sensitive area almost isotropically. This lens, which has been designed in the framework of nonimaging optics, consists in a single dielectric piece that encapsulates the receiver (as conventional epoxy packages of photodiodes), whose interface with air is an aspheric refractive surface. Several trial products have been manufactured with different angular sensitivities (linear response, cos-1(theta) and cos-2(theta) ). The experimental results have shown that the trial devices have the specified angular sensitivity with +/- 5% accuracy.