João Mendes-Lopes

Recent trends in concentrated photovoltaics concentrators’ architecture

Abstract. The field of concentrated photovoltaics (CPV) has met some remarkable advances in recent years. The continuous increase in conversion efficiency of multijunction solar cells and new advancements in optics have led to new demands and opportunities for optical design in CPV. This paper is a mini-review on current requirements for CPV optical design, and it presents some of the main trends in recent years on CPV systems architecture.

9-fold Fresnel–Köhler concentrator with Fresnel lens of variable focal point

Non-uniform irradiance patterns over Multi-Junction Cells gives rise to power losses, especially when considering spectral irradiance distributions over different junctions. Thermal effects on Silicone-on-Glass lenses affect spectral irradiance distributions. A new Photovoltaic Concentrator (CPV), formed by nine optical channels, each one with a Köhler configuration, has been designed to overcome these effects at high concentrations for a large acceptance angle. A Fresnel Lens with a Variable Focal Point is proposed to prevent optical crosstalk in multichannel systems. When integrated into the concentrator, improves the acceptance angle. These designs are designed to fulfill the expected requirements of four junction CPV systems.

The dome-shaped Fresnel-Köhler concentrator

Manufacturing tolerances, along with a high concentration ratio, are key issues in order to obtain cheap CPV systems for mass production. Consequently, this manuscript presents a novel tolerant and cost effective concentrator optic: the domed-shaped Fresnel-Kohler, presenting a curved Fresnel lens as Primary Optical Element (POE). This concentrator is based on two previous successful CPV designs: the FK concentrator, based on a flat Fresnel lens, and the dome-shaped Fresnel lens system developed by Daido Steel, resulting on a superior concentrator. The manuscript shows outstanding simulation results for geometrical concentration factor of Cg = 1,230x: high tolerance and high optical efficiency, achieving acceptance angles of 1.18° (dealing to a CAP*=0.72) and efficiencies over 85% (without any anti-reflective coating). Moreover, Kohler integration provides good irradiance uniformity on the cell surface without increasing system complexity by means of any extra element. Daido Steel advanced technique for d...

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.

9-fold Fresnel-Köhler concentrator for increased uniform irradiance on high concentrations

Non-uniform irradiance patterns created by Concentrated Photovoltaics (CPV) concentrators over Multi-Junction Cells (MJC) can originate significant power losses, especially when there are different spectral irradiance distributions over the different MJC junctions. This fact has an increased importance considering the recent advances in 4 and 5 junction cells. The spectral irradiance distributions are especially affected with thermal effects on Silicone-on-Glass (SoG) CPV systems. This work presents a new CPV optical design, the 9-fold Fresnel Köhler concentrator, prepared to overcome these effects at high concentrations while maintaining a large acceptance angle, paving the way for a future generation of high efficiency CPV systems of 4 and 5 junction cells.

Simultaneous multiple surface design method for diffractive surfaces

The simultaneous multiple surface (SMS) design method is extended to include design of diffractive optical surfaces besides refractive and reflective ones. This method involves the simultaneous and direct (no optimization) calculation of diffractive and refractive/reflective surfaces. Using the phase-shift properties of diffractive elements as an extra degree of freedom, two rays for each point on each diffractive surface are controlled. Representative diffractive systems designed by the SMS method are shown.

Design of compact optical systems using multichannel configurations

Compacting devices is an increasingly demanding requirement for many applications in both nonimaging and imaging optics. “Compacting” means here decreasing the volume of the space between the entry and the exit aperture without decreasing the optical performance. For nonimaging optical systems, compact optics is mainly important for reducing cost. Its small volume means less material is needed for mass-production and small size and light weight save cost in transportation. For imaging optical systems, in addition to the mentioned advantages, compact optics increases portability of devices as well, which contributes a lot to wearable display technologies such as Head Mounted Displays (HMD). After reviewing the different techniques to design compact systems, we analyze here the multichannel strategies. These type of designs split the incoming bundle of rays in different sub-bundles that are optically processed (independently) and then recombined in a single outgoing bundle. The optics volume decreases rapidly with the number of sub-bundles. These designs usually need to be combined with freeform optics in order to get optimum performance.

New Fresnel Köhler optical design: 9-fold photovoltaic concentrator

Non-uniform irradiance patterns created by Concentrated Photovoltaics (CPV) concentrators over Multi-Junction Cells (MJC) can originate significant power losses, especially when there are different spectral irradiance distributions over the different MJC junctions. This fact has an increased importance considering the recent advances in 4 and 5 junction cells. This work presents a new CPV optical design, the 9-fold Fresnel Köhler concentrator, prepared to overcome these effects at high concentrations while maintaining a large acceptance angle, paving the way for a future generation of high efficiency CPV systems of 4 and 5 junction cells.

Recent advances in the SMS method: diffraction and 3D aplanatism

Recent advances in the Simultaneous Multiple Surfaces (SMS) design method are reviewed in this paper. In particular, we review the design of diffractive surfaces using the SMS method and the concept of freeform aplanatism as a limit case of a 3D SMS design.

Design of diffractive optical surfaces within the nonimaging SMS design method

The Simultaneous Multiple Surface (SMS) method was initially developed as a design method in Nonimaging Optics and later, the method was extended for designing Imaging Optics. We show an extension of the SMS method to diffractive surfaces. Using this method, diffractive kinoform surfaces are calculated simultaneously and through a direct method, i. e. it is not based in multi-parametric optimization techniques. Using the phase-shift properties of diffractive surfaces as an extra degree of freedom, only N/2 surfaces are needed to perfectly couple N one parameter wavefronts. Wavefronts of different wavelengths can also be coupled, hence chromatic aberration can be corrected in SMS-based systems. This method can be used by combining and calculating simultaneously both reflective, refractive and diffractive surfaces, through direct calculation of phase and refractive/reflective profiles. Representative diffractive systems designed by the SMS method are presented.