This PDF file contains the front matter associated with SPIE Proceedings Volume 8468, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

In this paper we analyze the effects of neighbor shadowing of tracking solar photovoltaic arrays when they are set out in solar farms for large scale generation. Closer tracker spacing yields more power per unit area of land, but less power per tracking unit because of shadowing. A model has been developed to quantify and compare efficiencies for different tracker aspect ratios and field layouts, on an hourly, daily and annual basis. The model accounts for atmospheric absorption as well as neighbor shadowing at low solar altitude angles. We have focused on the case of CPV arrays which are oriented normal to the rays from the sun. The field layout is best characterized by the ratio of total array area to land area (the ground cover ratio or GCR). We explore as a function of GCR both the fraction of all the direct sunlight energy that is intercepted by the arrays (the irradiance collection efficiency) and the energy lost by each array because of shadowing.Examples are worked out for rectangular arrays on dual axis trackers at 33° latitude. We find that for a ground cover ratio of 30% the annual irradiance collection efficiency is 50%, almost independent of the layout pattern or the array aspect ratio. For a ground cover ratio of 40%, the irradiance collection efficiency rises to 65%. The corresponding shadowing losses do depend on aspect ratio, thus for 30% GCR the annual average of shadowing loss is 7.2% for 3:1 aspect ratio, rising to 7.8% for 2:1 aspect ratio. High GCR is not realizable for higher aspect ratios, which lead to large swing radius, but for 2:1 aspect ratio 40% GCR results in shadowing loss of 11.5%. One conclusion is that a solar farm with arrays of 2:1 aspect ratio set out with 40% GCR is good compromise when land is scarce: 64% of all the direct sunlight energy incident on the land is harvested by the arrays, with only 11.5% shadowing loss. We have compared these efficiencies with those for trough CSP systems, which also harvest direct sunlight but with reflectors turning about a single, horizontal N-S axis. For given GCR, the shadowing loss is slightly less (0.5%) than for the above dual-axis arrays, however the irradiance collection efficiency is worse in winter, leading to a lower annual average for a given GCR. For example, at 40% GCR, a single-axis system realizes a 56% irradiance collection efficiency compared to 64% for the dual axis systems.

Ensuring 25-year reliability of a CPV system requires knowledge of potential failure modes and material deficiencies. While Emcore’s CPV system conforms to all IEC 62108 tests, additional tests to eliminate potential long term reliability concerns have been performed. Performance is evaluated through all levels of integration, from cell to module. Tests at the cell level include IEC 62108 tests where feasible, as well as several other tests to establish the ability of the cell to survive additional integration and perform well throughout the lifetime of the CPV module. At a receiver assembly and module level, potential reliability concerns are addressed through targeted testing, which consists of accelerated stress tests which are used to quickly evaluate material performance and designed stress tests which allow the determination of activation energies. With this information, expected lifetime can be assessed and reliability concerns mitigated. Test methodologies and results from cell, receiver assembly and full module are presented demonstrating that targeted stress testing at each level of integration is a viable approach to assessing potential CPV failure modes.

In this work, a concentrating photovoltaic (CPV) design methodology is proposed which aims to maximize system efficiency for a given irradiance condition. In this technique, the acceptance angle of the system is radiometrically matched to the angular spread of the site’s average irradiance conditions using a simple geometric ratio. The optical efficiency of CPV systems from flat-plate to high-concentration is plotted at all irradiance conditions. Concentrator systems are measured outdoors in various irradiance conditions to test the methodology. This modeling technique is valuable at the design stage to determine the ideal level of concentration for a CPV module. It requires only two inputs: the acceptance angle profile of the system and the site’s average direct and diffuse irradiance fractions. Acceptance angle can be determined by raytracing or testing a fabricated prototype in the lab with a solar simulator. The average irradiance conditions can be found in the Typical Metrological Year (TMY3) database. Additionally, the information gained from this technique can be used to determine tracking tolerance, quantify power loss during an isolated weather event, and do more sophisticated analysis such as I-V curve simulation.

In this paper we investigate the use of holographic filters in solar spectrum splitting applications. Photovoltaic (PV) systems utilizing spectrum splitting have higher theoretical conversion efficiency than single bandgap cell modules. Dichroic band-rejection filters have been used for spectrum splitting applications with some success however these filters are limited to spectral control at fixed reflection angles. Reflection holographic filters are fabricated by recording interference pattern of two coherent beams at arbitrary construction angles. This feature can be used to control the angles over which spectral selectivity is obtained. In addition focusing wavefronts can also be used to increase functionality in the filter. Holograms fabricated in dichromated gelatin (DCG) have the benefit of light weight, low scattering and absorption losses. In addition, reflection holograms recorded in the Lippmann configuration have been shown to produce strong chirping as a result of wet processing. Chirping broadens the filter rejection bandwidth both spectrally and angularly. It can be tuned to achieve spectral bandwidth suitable for spectrum splitting applications. We explore different DCG film fabrication and processing parameters to improve the optical performance of the filter. The diffraction efficiency bandwidth and scattering losses are optimized by changing the exposure energy, isopropanol dehydration bath temperature and hardening bath duration. A holographic spectrum-splitting PV module is proposed with Gallium Arsenide (GaAs) and silicon (Si) PV cells with efficiency of 25.1% and 19.7% respectively. The calculated conversion efficiency with a prototype hologram is 27.94% which is 93.94% compared to the ideal spectrum-splitting efficiency of 29.74%.

A design is presented for a planar spectrum-splitting photovoltaic (PV) module using Holographic Optical Elements (HOEs). A repeating array of HOEs diffracts portions of the solar spectrum onto different PV materials arranged in alternating strips. Several combinations of candidate PV materials are explored, and theoretical power conversion efficiency is quantified and compared for each case. The holograms are recorded in dichromated gelatin (DCG) film, an inexpensive material which is easily encapsulated directly into the panel. If desired, the holograms can focus the light to achieve concentration. The side-by-side split spectrum layout has advantages compared to a stacked tandem cell approach: since the cells are electrically isolated, current matching constraints are eliminated. Combinations of dissimilar types of cells are also possible: including crystalline, thin film, and organic PV cells. Configurations which yield significant efficiency gain using relatively inexpensive PV materials are of particular interest. A method used to optimize HOE design to work with a different candidate cells and different package aspect ratios is developed and presented. (Aspect ratio is width of the cell strips vs. the thickness of the panel) The relationship between aspect ratio and HOE performance properties is demonstrated. These properties include diffraction efficiency, spectral selectivity, tracking alignment sensitivity, and uniformity of cell illumination.

Luminescent solar concentrators (LSCs) generally consist of transparent polymer sheets doped with luminescent species. Incident sunlight is absorbed by the luminescent species and emitted with high quantum efficiency, so that the emitted light is trapped in the sheets and travels to the edges where it can be collected by solar cells. Unlike regular solar spectrum, the emission spectrum of LSCs based on Lumogen Red dye red shifts and concentrates to a small range of wavelengths (600nm to 700nm). Therefore, hydrogenated amorphous silicon (a-Si:H), whose bandgap is around 750nm, can absorb the emission light without many thermalization losses. Due to the low diffusion lengths in a-Si:H, thin absorbing layer should be applied, causing insufficient light absorbance. In this letter, we propose a structure that coupling nanostructured plasmonic back contact to LSC solar cell. After optimization, numerical results show that the photocurrent intensity increases by a factor of 1.30 compared with LSC solar cells with randomly textured back contacts. In contrast, when illuminated by one Sun, the photocurrent for textured cell compares to that for nanostructured cell. The remarkable photocurrent enhancement in LSC cells is attributed to two main reasons. First, the wavelengths, where nanostructured cell shows higher absorbance compared with textured one, are identical with the emission peak of LSC. Second, the light interferences constructed in flat cells, which cause the absorbance curve to red shift and match with the emission spectrum, are depressed in textured cell, but are maintained in nanostructured cell. The second reason is described in detail.

A new design for a photovoltaic concentrator, the most recent advance based on the Kohler concept, is presented. The system is mirror-based, and with geometry that guaranties a maximum sunlight collection area (without shadows, like those caused by secondary stages or receivers and heat-sinks in other mirror-based systems). Designed for a concentration of 1000x, this off axis system combines both good acceptance angle and good irradiance uniformity on the solar cell. The advanced performance features (concentration-acceptance products –CAP- about 0.73 and affordable peak and average irradiances) are achieved through the combination of four reflective folds combined with four refractive surfaces, all of them free-form, performing Köhler integration ². In Köhler devices, the irradiance uniformity is not achieved through additional optical stages (TIR prisms), thus no complex/expensive elements to manufacture are required. The rim angle and geometry are such that the secondary stage and receivers are hidden below the primary mirrors, so maximum collection is assured. The entire system was designed to allow loose assembly/alignment tolerances (through high acceptance angle) and to be manufactured using already well-developed methods for mass production, with high potential for low cost. The optical surfaces for Köhler integration, although with a quite different optical behavior, have approximately the same dimensions and can be manufactured with the same techniques as the more traditional secondary optical elements used for concentration (typically plastic injection molding or glass molding). This paper will show the main design features, along with realistic performance simulations considering all spectral characteristics of the elements involved.

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.

Using illumination modeling, we provide a comparison of glass Total Internal Reflection Concentrators (TIRC) and metal Hollow Reflective Concentrators (HRC) used as secondary concentrator elements in a dish-based highconcentration photovoltaic (CPV) system. Comparisons of optical efficiency, flux uniformity, off-axis acceptance angle, and cost are vital to choosing an ideal secondary concentrator element for a CPV system employing multi-junction (MJ) cells. In many CPV systems, a free-form optic or sharp-cornered rectangular TIRC composed of glass is used to increase the geometrical flux concentration at the surface of the MJ cell, and may also serve as homogenizers to mix the light to increase flux uniformity. We have demonstrated in on-sun testing that an electroformed metal HRC can be used in place of a glass TIRC of the same geometry, eliminating the need for polymeric bonding to the MJ cell surface, and providing a side-contact surface pathway for active cooling. Although geometrically equivalent, we show that glass TIRC’s achieve superior off-axis performance (higher etendue from surface refraction) and are generally acknowledged to have less degradation than optics with over-coated silver, yet metal HRC’s employing over-coated silver are superior in spectral absorption characteristics under high solar flux (no losses from glass absorption or Fresnel surface reflections) and don't require accurate glass pressing into many shapes. To better understand the trade-offs between optical efficiency, off-axis performance, mechanical tolerances, cost and reliability, metal (HRC) and glass (TIRC) tapered funnels are analyzed at the surface of equal irradiance in a Kohler-Illumination concentrator system, and a trade study is presented.

The University of Arizona has developed a new dish-based High Concentration Photovoltaic (HCPV) system which is in the process of being commercialized by REhnu, Inc. The basic unit uses a paraboloidal glass reflector 3.1 m x 3.1 m square to bring sunlight to a high power point focus at a concentration of ~20,000x. A unique optical system at the focus reformats the concentrated sunlight so as to uniformly illuminate 36 triple junction cells at 1200x geometric concentration¹. The relay optics and cells are integrated with an active cooling system in a self-contained Power Conversion Unit (PCU) suspended above the dish reflector. Only electrical connections are made to the PCU as the active cooling system within is completely sealed. Eight of these reflector/PCU units can be mounted on a single two axis tracking structure². Our 1st generation prototype reflector/PCU unit consistently generated 2.2 kW of power normalized to 1kW/m² DNI in over 200 hours of on-sun testing in 2011³. Here, we present on-sun performance results for our 2^nd generation prototype reflector/PCU unit, which has been in operation since June 2012. This improved system consistently generates 2.7 kW of power normalized to 1kW/m² DNI and has logged over 100 hours of on-sun testing. This system is currently operating at28% DC net system efficiency with an operating cell temperature of only 20°C above ambient. Having proven this system concept, work on our 3^rd generation prototype is underway with a focus on manufacturability, lower cost, and DC efficiency target of 32% or better.

EMCORE's Concentrator Photovoltaic (CPV) systems use large-format Fresnel lenses to achieve 1090X concentration onto high-efficiency multi-junction solar cells. The use of Fresnel lenses is common in CPV systems due to their thin profile and light weight. EMCORE uses silicone-on-glass (SOG) lens technology, which provides a high-reliability, high-durability alternative to acrylic lenses. This paper describes performance variations of these lenses based on the Fresnel groove depth. Both the optical efficiency and temperature dependence of the optical system are evaluated as a function of groove depth.

In 2011, the Amonix Advanced Technology Group was awarded DOE SunShot funding in the amount of $4.5M to design a new Balance of System (BOS) architecture utilizing Amonix MegaModules™ focused on reaching the SunShot goal of $0.06-$0.08/kWhr LCOE. The project proposal presented a comprehensive re-evaluation of the cost components of a utility scale CPV plant and identified critical areas of focus where innovation is needed to achieve cost reduction. As the world's premier manufacturer and most experienced installer of CPV power plants, Amonix is uniquely qualified to lead a rethinking of BOS architecture for CPV. The presentation will focus on the structure of the BOS-X approach, which looks for the next wave of cost reduction in CPV through evaluation of non-module subsystems and the interaction between subsystems during the lifecycle of a solar power plant. Innovation around nonmodule components is minimal to date because CPV companies are just now getting enough practice through completion of large projects to create ideas and tests on how to improve baseline designs and processes. As CPV companies increase their installed capacity, they can utilize an approach similar to the methodology of BOS-X to increase the competitiveness of their product. Through partnership with DOE, this holistic approach is expected to define a path for CPV well aligned with the goals of the SunShot Initiative.

SST is developing a new Dish CPV dense array system that overcomes the flux uniformity requirement of previous designs. The ability to operate without flux uniformity relaxes the precision requirements of primary collector optics and eliminates homogenizing optics previously required for dense array CPV. Array design can be configured for dish and tower/heliostat systems developed for thermal CSP applications. The design uses industry standard CPV cells and manufacturing materials and methods for minimum cost and high reliability. Nominal input flux to the array for full power is about 250 suns. Internal array optics increase flux to the cells to about 1200 suns. Linear optics provide additional concentration, permit novel use of commercial glass production methods and facilitate power collection design that is integrated with dynamic power conversion and maximum power point tracking (MPPT). Efficient power hybrid packaging methods are used along with advanced liquid cooling “cold-plate” thermal management. Byproduct “waste heat” can be provided for on-site CHP use. We report on the design approach and status of development with the beginning of on-sun alpha testing of the first of 50 kW of CPV modules being produced.

We propose a concentrator photovoltaic system based on a planar waveguide. Here, the waveguide has one stem at one end and the other end is divided into multiple branches. A right-angle prism is attached to each end of the branches. A lens-array is stacked on the waveguide such that each prism is placed near a focal point of a corresponding lens. Its 45-degree slope leads the focused sun light into the waveguide via total internal reflection. The light propagates inside the waveguide and its intensity increases at each branching point. A solar cell is coupled to the end of the stem for photoelectric conversion. The branched portion can be either straight or curved. In both cases, according to our ray tracing simulations, the light loss inside the waveguide becomes negligible when we set the focal length of the lens larger than a certain value. For example, this value is 300mm for a 5mm-thick, 150mm-long straight waveguide coupled to a lens-array with a lens diameter of 90mm. This number is reduced to 220mm for a curved waveguide. It is further reduced to 100mm when we assume 100% reflection at the 45-degree slope. In these cases, the efficiency defined as the ratio of the optical power exiting the waveguide to one entering the lens-array is close to 87%. The major loss mechanism is the Fresnel reflections at the lens surfaces (8%) and the prism surfaces (3%). The rest is mostly due to the absorption by the material assumed for the waveguide (PMMA) (1-2%).