Rubén Mohedano

Etendue-preserving mixing and projection optics for high-luminance LEDs, applied to automotive headlamps

By having a single optical element combine the light of several high luminance LEDs, a high luminance light source can be formed, with shape and emission characteristics adaptable to nearly all illumination problems. The illuminance distribution of this virtual source facilitates the generation of the desired intensity pattern via its imaging-stye projection into the far field. This projection is achieved by one refractive and one reflective freeform surface, both calculated simultaneously by the 3D SMS method, which is herein demonstrated for an LED automotive headlamp.

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

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.

Triple-junction solar cell performance under Fresnel-based concentrators taking into account chromatic aberration and off-axis operation

Concentration photovoltaic (CPV) systems might produce quite uneven irradiance distributions (both on their level and on their spectral distribution) on the solar cell. This effect can be even more evident when the CPV system is slightly off-axis, since they are often designed to assure good uniformity only at normal incidence. The non-uniformities both in absolute irradiance and spectral content produced by the CPV systems, can originate electrical losses in multi-junction solar cells (MJSC). This works is focused on the integration of ray-tracing methods for simulating the irradiance and spectrum maps produced by different optic systems throughout the solar cell surface, with a 3D fully distributed circuit model which simulates the electrical behavior of a state-of-the-art triple-junction solar cell under the different light distributions obtained with ray-tracing. In this study four different CPV system (SILO, XTP, RTP, and FK) comprising Fresnel lenses concentrating sunlight onto the same solar cell a...

Free-form integrator array optics

A new design method of free-form Kohler integrator array optics is presented. Only few solutions to the integrator design problem are known, which apply for specific and simple source and targets (for instance, flat integrator lenslet arrays when the source and target are squares located at infinity). The method presented here find more general solutions and the resulting optics is formed by two arrays of free-form optical surfaces (which can be either reflective of refractive). The contour curves of the array units are also obtained from the design. Two types of Kholer integrators will be defined, depending if they integrate only along one direction across the source (one-directional integrators) or in the two directions (two-directional integrators). This design method has been applied for an ultra-compact high efficiency LED low beam head lamp producing a legal pattern independently of the chip luminance variation and permitting LED position tolerances of ±200 microns. The ray tracing proves that the high gradient (0.32) and its vertical position in the pattern remain invariable when chip is moved.

SMS design method in 3D geometry: examples and applications

The Simultaneous Multiple Surface (SMS) method in 3D geometry is presented. Giving two orthotomic input ray bundles and other two orthotomic output ray bundles, the method provides an optical system with two free-form surfaces that deflects the rays of the input bundles into the rays of the corresponding output bundles and vice versa. In nonimaging applications, the method allows controlling the light emitted by an extended light source much better than single free-form surfaces designs, and also enables the optics contour to be shaped without efficiency losses. The method is expected to find also applications in imaging optics

Experimental characterization of Fresnel-Köhler concentrators

Abstract. Most cost-effective concentrated photovoltaics (CPV) systems are based on an optical train comprising two stages, the first being a Fresnel lens. Among them, the Fresnel-Köhler (FK) concentrator stands out owing to both performance and practical reasons. We describe the experimental measurements procedure for FK concentrator modules. This procedure includes three main types of measurements: electrical efficiency, acceptance angle, and irradiance uniformity at the solar cell plane. We have collected here the performance features of two different FK prototypes (ranging different f-numbers, concentration ratios, and cell sizes). The electrical efficiencies measured in both prototypes are high and fit well with the models, achieving values up to 32.7% (temperature corrected, and with no antireflective coating on SOE or POE surfaces) in the best case. The measured angular transmission curves show large acceptance angles, again perfectly matching the expected values [measured concentration acceptance product (CAP) values over 0.56]. The irradiance pattern on the cell (obtained with a digital camera) shows an almost perfectly uniform distribution, as predicted by raytrace simulations. All these excellent on-sun results confirm the FK concentrator as a potentially cost-effective solution for the CPV market.

Optimum irradiance distribution of concentrated sunlight for photovoltaic energy conversion

The irradiance distribution on a concentration photovoltaic cell that produces maximum conversion efficiency has been found with the tools of Variational Calculus. The analysis is two dimensional and can be applied to a comb-like double busbar solar cell illuminated by a line-focus concentrator. The optimum distribution is, in general, nonuniform, and depends on the internal parameters of the solar cell: the higher the contribution of the grid to the global cell series resistance, the lower the uniformity of the optimum irradiance distribution. In practical cases, the efficiency for uniform illumination is close to that of the optimum, but in the latter the irradiance close to the busbar may be noticeable higher than the average.

The XR nonimaging photovoltaic concentrator

The performance of the XR solar concentrator, using a high efficiency multi-junction solar cell developed recently by Spectrolab, is presented. The XR concentrator is an ultra-compact Nonimaging optical design composed of a primary mirror and a secondary lens, which can perform close to the thermodynamic limit of concentration (maximum acceptance angle for a given geometrical concentration). The expected acceptance angle of the concentrator is about ±2 deg for a geometrical concentration of 800x (a Fresnel lens and secondary system typically has ±0.6 deg of acceptance for 300x of geometrical concentration). This concentrator is optimized to improve the irradiance distribution on the solar cell keeping it under the maximum values the cell can accept. The XR concentrator has high manufacturing tolerance to errors and can be produced using low cost manufacturing techniques. The XR is designed with the Simultaneous Multiple Surface (SMS) design method of Nonimaging Optics. Its application to high-concentration photovoltaics is now being developed in a consortium led by The Boeing Company, which has recently been awarded a project by the US DOE in the framework of the Solar America Initiative.

Improving performances of Fresnel CPV systems: Fresnel-RXI Köhler concentrator.

The optical design presented here has been done in order to achieve superior optical performance in comparison with the state-of-the-art Fresnel CPV systems. The design consists of a Photovoltaic Concentrator (CPV) comprising a Fresnel lens (F) as a Primary Optical Element (POE) and a dielectric solid RXI as a Secondary Optical Element (SOE), both with free-form surfaces (i.e. neither rotational nor linearly symmetric). It is the first time the RXI-type geometry has been applied to a CPV secondary. This concentrator has ultra-high CAP value ready to accommodate more efficient cells eventually to be developed and used commercially in future.

Performance Modeling of Fresnel‐Based CPV Systems: Effects of Deformations under Real Operation Conditions

Getting a lower energy cost has always been a challenge for concentrated photovoltaic. The FK concentrator enhances the performance (efficiency, acceptance angle and manufacturing tolerances) of the conventional CPV system based on a Fresnel primary stage and a secondary lens, while keeping its simplicity and potentially low‐cost manufacturing. At the same time F‐XTP (Fresnel lens+reflective prism), at the first glance has better cost potential but significantly higher sensitivity to manufacturing errors. This work presents comparison of these two approaches applied to two main technologies of Fresnel lens production (PMMA and Silicone on Glass) and effect of standard deformations that occur under real operation conditions.