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Preface: 7th International Conference on Concentrating Photovoltaic Systems

Frank Dimroth, Sarah Kurtz, Gabriel Sala, and Andreas Bett

AIP Conf. Proc. 1407, pp. 1-1; doi:http://dx.doi.org/10.1063/1.3658281 (1 page)

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88.40.hj	Efficiency and performance of solar cells
88.40.fr	Concentrating collectors
88.05.Bc	Energy efficiency; definitions and standards
88.05.Ec	Renewable energy targets

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III‐V Multijunction Solar Cells—New Lattice‐Matched Products And Development Of Upright Metamorphic 3J Cells

W. Guter, R. Kern, W. Köstler, T. Kubera, R. Löckenhoff, M. Meusel, M. Shirnow, and G. Strobl

AIP Conf. Proc. 1407, pp. 5-8; doi:http://dx.doi.org/10.1063/1.3658282 (4 pages)

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AZUR SPACE's current CPV product 3C40C is an advanced lattice‐matched In_0.50Ga_0.50P/In_0.01Ga_0.99As/Ge triple junction (3J) solar cell. Recently, efficiencies up to 41.2% (450–600×AM1.5d) have been confirmed for this 40%‐class product. This kind of solar cell structure has now reached its practical efficiency limit and went into production in 2010. AZUR offers customized cell structures, regarding size and grid design, as well as anti‐reflection coatings adapted to the individual CPV system. Integration grades from diced wafers up to assemblies, such as dense arrays, are available. Special features and production results for the 3C40C structure are presented in this work. In order to further push efficiencies beyond 42% AZUR has successfully transferred the upright metamorphic cell from Fraunhofer ISE and advances this concept now for production. First results on metamorphic 3J solar cells at AZUR SPACE and the actual potential of this concept will be discussed. A target efficiency of 42% seems to be realistic.

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88.40.fh	Advanced materials development
88.40.fr	Concentrating collectors
88.40.hj	Efficiency and performance of solar cells
42.25.Gy	Edge and boundary effects; reflection and refraction

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42% 500X Bi‐Facial Growth Concentrator Cells

S. Wojtczuk, P. Chiu, X. Zhang, D. Pulver, C. Harris, and B. Siskavich

AIP Conf. Proc. 1407, pp. 9-12; doi:http://dx.doi.org/10.1063/1.3658283 (4 pages)

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Data are presented from three‐junction concentrator photovoltaic cells using a new cell architecture (1.9 eV InGaP top cell lattice‐matched to a 1.42 eV GaAs middle cells on one side of a infrared‐transparent GaAs wafer with a lattice‐mismatched 0.95 eV InGaAs bottom cell grown isolated on the wafer backside). The cell uses a new epitaxial bifacial growth (BFG) technique. The impetus is to replace the 0.67 eV Ge bottom cell in the standard three junction InGaP∕GaAs∕Ge tandems with a higher bandgap 0.95 eV InGaAs cell that boosts the bottom cell voltage by about 40% while maintaining a simple high‐yield cell process without use of complex large area epitaxial liftoff or wafer bonding steps used to make similar cell stacks. Efficiency was independently‐verified by NREL for a 1 cm×1 cm cell (42.3% at 406 suns, with Voc 3.452V, 87.1% FF and 1xJsc of 14.07 mA/cm², at 25 °C AM1.5D, 100 mW/cm²), which was the world record at the time of the CPV‐7 conference. No degradation was seen during concentrated solar operation after a 2000 hr 165C burn‐in and PbSn solder tests. Average efficiency of 1 cm² cells designed for 500 suns at 1018 suns was 40.5% (Spire test, 25 °C, spectrally corrected flash simulator). Measured efficiency temperature coefficient for gen2 cells is −0.06%/°C, similar to InGaP∕GaAs∕Ge tandems.

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88.40.fr	Concentrating collectors
81.15.Kk	Vapor phase epitaxy; growth from vapor phase
88.40.ff	Performance testing
88.05.Gh	Energy conservation; electricity demand reduction

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Extended Triple‐Junction Solar Cell 3D Distributed Model: Application to Chromatic Aberration‐Related Losses

I. Garcia, P. Espinet‐González, I. Rey‐Stolle, E. Barrigón, and C. Algora

AIP Conf. Proc. 1407, pp. 13-16; doi:http://dx.doi.org/10.1063/1.3658284 (4 pages)

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An extended 3D distributed model based on distributed circuit units for the simulation of triple‐junction solar cells under realistic conditions for the light distribution has been developed. A special emphasis has been put in the capability of the model to accurately account for current mismatch and chromatic aberration effects. This model has been validated, as shown by the good agreement between experimental and simulation results, for different light spot characteristics including spectral mismatch and irradiance non‐uniformities. This model is then used for the prediction of the performance of a triple‐junction solar cell for a light spot corresponding to a real optical architecture in order to illustrate its suitability in assisting concentrator system analysis and design process.

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42.15.Fr	Aberrations
84.37.+q	Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)
88.40.hj	Efficiency and performance of solar cells
88.05.Bc	Energy efficiency; definitions and standards

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CPV Solar Cell Metal Adhesion: How Good Is Good Enough?

Ian Aeby, Michael Winter, Amos Gutierrez, James Foresi, Greg Flynn, Pravin Patel, Tansen Vargese, and Ben Cho

AIP Conf. Proc. 1407, pp. 17-20; doi:http://dx.doi.org/10.1063/1.3658285 (4 pages)

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The CPV application places severe demands on the mechanical and electrical performance characteristics of the materials employed in the solar cell structure. This is especially true for the interfaces between the active semiconductor and the metal layers that are needed for electrical connection to the external circuits. At these interfaces TCE mismatch can result in high levels of stress during temperature cycling that is a natural component of the CPV operating environment. As the industry moves from R&D into production, methods are needed for quickly assessing the mechanical and electrical integrity of the metal∕semiconductor interface during CPV solar cell fabrication. To provide a level playing field for cell suppliers and CPV module manufactures, such a technique should be established as part of an industry accepted cell qualification or specification standard.

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88.40.jm	Thin film III-V and II-VI based solar cells
85.40.Ls	Metallization, contacts, interconnects; device isolation
68.35.Np	Adhesion
81.05.Bx	Metals, semimetals, and alloys
81.40.Jj	Elasticity and anelasticity, stress-strain relations

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Energy Harvest Predictions for a Spectrally Tuned Multiple Quantum Well Device Utilising Measured and Modelled Solar Spectra

Alison Dobbin, Matthew Norton, George E. Georghiou, Matthew Lumb, and Tom N. D. Tibbits

AIP Conf. Proc. 1407, pp. 21-24; doi:http://dx.doi.org/10.1063/1.3658286 (4 pages)

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We present a comparison between modelled and measured solar spectra in Nicosia, Cyprus. The modelled spectra were generated using the SMARTS model, driven by various sources of measured atmospheric data, updated every 30 minutes or less. A comparison with measured spectra reveals the most reliable source of data for that location and period. The spectral simulations demonstrate that both aerosol optical depth and precipitable water content must be accurately known at the location of interest in order to realistically recreate the shape and power of measured spectra accurately. Energy harvest calculations of four triple junction (3J) solar cell designs were performed using the simulated spectra. The model predicts a 5.7% increase in energy harvest when multiple quantum wells are included in a standard 3J device. Simple modifications made to the quantum well device to ‘tune’ the cell to the incident spectra result in a 6.3% increase in predicted energy production over the standard 3J device.

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42.68.Jg	Effects of aerosols
85.35.Be	Quantum well devices (quantum dots, quantum wires, etc.)
88.05.Bc	Energy efficiency; definitions and standards
88.40.jm	Thin film III-V and II-VI based solar cells

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Considerations for How to Rate CPV

Sarah Kurtz, Matthew Muller, Bill Marion, Keith Emery, Robert McConnell, Sandheep Surendran, and Adrianne Kimber

AIP Conf. Proc. 1407, pp. 25-29; doi:http://dx.doi.org/10.1063/1.3658287 (5 pages)

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The concentrator photovoltaic (CPV) industry is introducing multiple products into the marketplace, but, as yet, the community has not embraced a unified method for assessing a nameplate rating. The choices of whether to use 850, 900, or 1000 W/m² for the direct‐normal irradiance and whether to link the rating to ambient or cell temperature will affect how CPV modules are rated and compared with other technologies. This paper explores the qualitative and quantitative ramifications of these choices using data from two multi‐junction CPV modules and two flat‐plate modules.

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88.40.jp	Multijunction solar cells
88.40.fr	Concentrating collectors
88.40.fh	Advanced materials development
88.05.Bc	Energy efficiency; definitions and standards

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Estimating Saturation Current Based on Junction Temperature and Bandgap

John R. Wilcox, Alexander W. Haas, Jeffery L. Gray, and Richard J. Schwartz

AIP Conf. Proc. 1407, pp. 30-33; doi:http://dx.doi.org/10.1063/1.3658288 (4 pages)

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This paper describes a method for estimating a solar cell's reverse saturation current density, J_O(T,E_G(300 K)), based upon the bandgap energy at 300 K and the junction operating temperature. In an easy to use functional form, the solar cell performance can be calculated without knowing material specific parameters. This method can be used to optimize the bandgaps for highest conversion efficiency, during the initial design phase of a multijunction system.

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88.40.ff	Performance testing
88.40.jp	Multijunction solar cells
88.05.Bc	Energy efficiency; definitions and standards
42.79.Ek	Solar collectors and concentrators

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Development and On‐Sun Performance of Dish‐Based HCPV

Roger Angel, Tom Connors, Warren Davison, Matt Rademacher, Blake Coughenour, Guillaume Butel, and David Lesser

AIP Conf. Proc. 1407, pp. 34-37; doi:http://dx.doi.org/10.1063/1.3658289 (4 pages)

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The paper describes a new system architecture optimized for utility‐scale generation with concentrating photovoltaic cells (CPV). The system concept is optimized to use predominantly low‐cost materials manufactured by methods proven for high volume production. Triple‐junction cells are used to convert 1000x concentrated sunlight into electricity. Compared to silicon panels, these commercially available cells convert at least twice as much of the incident sunlight energy into electricity, and at 1000x optical concentration, they cost one‐tenth as much per watt of power output. The architecture combines three novel elements: large (3.1 m×3.1 m square) paraboloidal glass dish reflectors to collect and concentrate the sunlight; compact receivers at each dish focus, each one incorporating multiple, actively cooled cells; and a lightweight steel spaceframe structure to hold multiple dish∕receiver units in co‐alignment and oriented to the sun. A manufacturing process for replicating the reflector dishes is well advanced in development at the Steward Observatory Mirror Lab. A lightweight steel spaceframe structure to hold and track eight dish∕receiver units to generate 20 kW has been completed. Tests over several months showed that for 99% of the time the tracking error was less than 0.1 degree. A test receiver populated with 8 out of 36 cells and attached to the tracker was operated for two months, yielding consistently over 500 W for much of each day. The receiver also maintained 95% of full power for mispointing of 1∕2°. The receiver is now being completed with its full 36 cell complement, for >2 kW output. This technology is being commercialized by REhnu, LLC, under an exclusive license from the University of Arizona.

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88.40.hj	Efficiency and performance of solar cells
88.40.jj	Silicon solar cells
42.79.Ek	Solar collectors and concentrators
42.79.Fm	Reflectors, beam splitters, and deflectors

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Wafer Processing Aspects of High Efficiency Multi‐junction Solar Cells

Dhananjay Bhusari, Anna Ly, Jack Serra, Adam Crouch, Scott Diamond, Hoon Lee, Maggy Lau, and Richard King

AIP Conf. Proc. 1407, pp. 38-41; doi:http://dx.doi.org/10.1063/1.3658290 (4 pages)

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Solar cell efficiency is extremely leveraging for reducing the levelized cost of energy (LCOE) in CPV systems. Over the past three years, Spectrolab has developed and introduced four generations of III‐V multi‐junction solar cells for terrestrial concentrator applications, with continuous improvement in the conversion efficiency from 37% for C1MJ to 40% for C4MJ. This continuous increase in conversion efficiency has been achieved by improvements in the cell design and quality of epitaxial layers, as well as by improvements in wafer fabrication processes.

Design and fabrication of metal gridlines have large impact on the solar cell performance. Optimization of the gridline architecture is extremely important to achieve high efficiency. Modeling studies were carried out to find optimum gridline parameters (viz. width, thickness and spacing) as a function of cell size and level of light concentration. Wafer fabrication processes were continuously modified to increase the aspect ratio of the gridlines, which is critical to minimize the metal obscuration while simultaneously maintaining the high electrical conductance of the grid structure. The results of modeling studies are compared with experimental results for various gridline designs. These improvements in the gridline design have yielded ∼1.2% absolute increase in solar cell efficiency between C1MJ and C4MJ cell generations. Another model was developed to estimate the effects of non‐normal angles of light incidence on cell performance, that included the effects of gridline shadowing and light reflection from the gridline sidewalls. In addition, new all‐chemical mesa etch processes have been developed for cell isolation, the results of which will be presented and implications from the wafer fabrication as well as cell performance perspectives will be discussed.

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88.40.hj	Efficiency and performance of solar cells
88.40.jp	Multijunction solar cells
88.40.fr	Concentrating collectors
88.40.ff	Performance testing

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Thermal Test Platform for Solar Cell Modules in Concentrated Photovoltaics

Louis‐Michel Collin, Osvaldo Arenas, Luc G. Fréchette, and Richard Arès

AIP Conf. Proc. 1407, pp. 42-45; doi:http://dx.doi.org/10.1063/1.3658291 (4 pages)

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Under high concentration, solar cells tend to increase in temperature, directly impacting their efficiency. The carrier that supports the cell plays a key role in extracting this high heat flux, but no standardized means to characterize the carrier's thermal performances is currently available. This work presents a test platform and the basis of a standardized experimental procedure to characterize the thermal performance of concentrated photovoltaic solar cell carriers. The platform design, based on heat injection by conduction, is presented with its experimental characterization. Different carrier structures have been tested to validate the bench concept.

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88.40.ff	Performance testing
88.40.hj	Efficiency and performance of solar cells
88.05.Bc	Energy efficiency; definitions and standards
84.37.+q	Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)

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Silicon Based Photovoltaic Cells For Concentration–Research And Development Progress In Laser Grooved Buried Contact Cell Technology

A. Cole, I. Baistow, L. Brown, S. Devenport, K. Drew, K. C. Heasman, D. Morrison, T. M. Bruton, L. Serenelli, S. De Iuliis, M. Izzi, M. Tucci, E. Salza, and L. Pirozzi

AIP Conf. Proc. 1407, pp. 46-49; doi:http://dx.doi.org/10.1063/1.3658292 (4 pages)

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The Laser grooved buried contact silicon solar cell (LGBC) process employed by Narec currently produces LGBC cells designed to operate at concentrations ranging from 1–100 suns and has demonstrated efficiencies at 50X of over 19% and at 100X of over 18.2% using 300 μm CZ silicon[1] wafers. As part of the LAB2LINE[1], APOLLON[2] and ASPIS[3] projects funded under the European Commission Framework Programs (FP6 and FP7) we have made improvements to the LGBC process to improve efficiency or make the cell technology more suitable for industrial CPV receiver manufacturing processes. We describe a process which hybridizes LGBC and more standard screen printing technologies which yields at least a 6% relative improvement at concentration when using more readily available 200 μm thick CZ wafers. We describe a pioneering front dicing technique (FDT). The FDT process is important in small cells where edge recombination effects are detrimental to the performance. We show that by using this new technique we can produce cells that perform better at concentration and improve the positioning of the front contact of the cell. We also describe a busbar technology that uses laser processing and electroless chemical plating to allow not only soldering to the front contact of the cell but also wire bonding. The advances in research and development of LGBC cells leading to improved cell performance may provide significant reductions in levilised cost of energy (LCOE) for low to medium CPV systems.

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88.40.hj	Efficiency and performance of solar cells
88.40.ff	Performance testing
73.40.Cg	Contact resistance, contact potential
88.40.fc	Modeling and analysis
88.40.hm	Cost of production of solar cells

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Temperature‐Dependent Quantum Efficiency of Quantum Dot Enhanced Multi‐Junction Solar Cells

Olivier Thériault, Jeffrey F. Wheeldon, Alex Walker, Paul Bitar, Mark D. Yandt, Christopher E. Valdivia, and Karin Hinzer

AIP Conf. Proc. 1407, pp. 50-53; doi:http://dx.doi.org/10.1063/1.3658293 (4 pages)

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The external quantum efficiency of a commercial quantum dot enhanced multi‐junction solar cell is measured over a range of temperatures (15 °C to 75 °C). A complete numerical model of the cell is built and calibrated based on the experimental data. The short circuit current density is calculated over different temperatures under standard AM1.5D illumination; the measurements compare well to simulated results. The current ratio between the top and middle sub‐cell is studied over temperature and air mass. It is shown that the current ratio and hence the optimal AM value for which the cell should be designed increase with increasing temperature.

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85.35.Be	Quantum well devices (quantum dots, quantum wires, etc.)
88.05.Bc	Energy efficiency; definitions and standards
88.05.Ec	Renewable energy targets
88.40.fc	Modeling and analysis

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Effects of Temperature on Hybrid Lens Performance

Steve Askins, Marta Victoria, Rebeca Herrero, César Domínguez, Ignacio Antón, and Gabriel Sala

AIP Conf. Proc. 1407, pp. 57-60; doi:http://dx.doi.org/10.1063/1.3658294 (4 pages)

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In hybrid Silicone‐on‐glass Fresnel lenses, an optical silicone is molded onto a glass substrate and forms the Fresnel structure. These lenses offer a cost effective solution as a primary optical element in point‐focus concentrator photovoltaic modules, as well as performance advantages. However, these lenses have a high performance variation with temperature due both to the change in index of refraction of silicone as well as to deformations in the facets caused by coefficient of thermal expansion (CTE) mismatch. In this study we perform measurements of the light flux at the focal plane of a family of SOG lenses, varying temperature and lens‐to‐receiver distances. The effect of varying silicone cure temperature and the depth of the silicone between the lens and the glass substrate on temperature dependence was investigated. A preliminary computer model of this behavior is presented.

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88.40.hj	Efficiency and performance of solar cells
88.40.jj	Silicon solar cells
42.79.Bh	Lenses, prisms and mirrors
65.60.+a	Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

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Concentrating Optics: From Giant Astronomical Telescopes To Low‐Cost HCPV

Roger Angel

AIP Conf. Proc. 1407, pp. 61-65; doi:http://dx.doi.org/10.1063/1.3658295 (5 pages)

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Triple‐junction PV cells used at 1000x concentration are both highly efficient and inexpensive, per watt of electricity produced. A power system based on telescope design principles uses these cells to make utility‐scale solar electricity at cost parity with fossil fuel. First, sunlight is concentrated by an array of large square dish reflectors, co‐aligned in a mechanically‐efficient, open spaceframe structure with built‐in elevation tracking axis and drive. Second, the concentrated sunlight at each focus is converted into electricity by many cells packaged in a small receiver, with a ball lens and optical funnels to ensure even distribution between cells. This architecture is optimized for minimum cost in high volume production. The large steel and glass elements and the small integrated receiver and radiator are separately manufactured and shipped for assembly in a facility near the solar plant.

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88.40.hj	Efficiency and performance of solar cells
88.40.jp	Multijunction solar cells
88.40.fm	Dish/engine systems
42.79.Fm	Reflectors, beam splitters, and deflectors

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Fresnel Lens Concentrator with Improved Thermal Behavior

Thorsten Hornung, Martin Neubauer, Andreas Gombert, and Peter Nitz

AIP Conf. Proc. 1407, pp. 66-69; doi:http://dx.doi.org/10.1063/1.3658296 (4 pages)

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Recent investigations of silicone‐on‐glass (SoG) Fresnel lens concentrator optics have shown a dependence of the optical efficiency and module performance on lens temperature. It was shown that the temperature dependence is dominated by a reduction of the refractive index with increasing temperature and a deformation of the lens structure due to thermal expansion of the lens material. We succeeded in modeling these effects on a computer by simulating thermal deformations of the lens structure using the finite element method (FEM) and analyzing the resulting deformed structure using ray tracing simulations. In former work, we were able to demonstrate a very good match to high precision optical measurements of Fresnel lenses at varying temperatures. The detailed insight we gained through our measurements and computer simulations was used to develop and manufacture a lens with improved optical performance at the temperature interval which is relevant for operation within a concentrator photovoltaic module. For these optimized SoG lenses, detailed computer simulations predict a significant increase in optical efficiency when compared to non‐optimized lenses. High precision measurements of the optical efficiency allowed us to verify our expectations and compare experimental results of a Fresnel lens optimized for operation under varying ambient temperatures to results obtained for non‐optimized Fresnel lenses.

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88.40.hj	Efficiency and performance of solar cells
88.40.jj	Silicon solar cells
02.60.Pn	Numerical optimization
02.70.Dh	Finite-element and Galerkin methods
88.05.Bc	Energy efficiency; definitions and standards

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High Performance Concentrating Photovoltaic Module Designs Employing Reflective Lens Optics

Sergey V. Vasylyev and Viktor P. Vasylyev

AIP Conf. Proc. 1407, pp. 70-73; doi:http://dx.doi.org/10.1063/1.3658297 (4 pages)

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The present study is aimed at advancing the optical component as well as optimizing the design of concentrating photovoltaic (CPV) modules in order to increase the conversion efficiency and improve the utility of CPV while obtaining the prescribed concentration ratio. In this work, we turn to non‐traditional concentrating optics, namely Reflective Lenses™ (RL), first introduced in early 2000s. The optical configuration of RLs is unique since it combines the very low F∕D number (hence resulting in a very low profile of the unit) of mirrors with a rear‐focus of lenses and uses only a single‐stage reflection. A liner‐focus version of RLs, the Slat‐Array Concentrator (SAC), is a capable alternative to the parabolic troughs for mid‐concentration CPV. A point‐focus version called the Ring‐Array Concentrator (RAC) is deemed suitable for high concentration photovoltaics.

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88.05.Bc	Energy efficiency; definitions and standards
02.60.Pn	Numerical optimization
84.60.Bk	Performance characteristics of energy conversion systems; figure of merit
42.25.Gy	Edge and boundary effects; reflection and refraction

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

A. Cvetkovic, R. Mohedano, O. Gonzalez, P. Zamora, P. Benitez, P. M. Fernandez, A. Ibarreche, M. Hernandez, J. Chaves, and J. C. Miñano

AIP Conf. Proc. 1407, pp. 74-78; doi:http://dx.doi.org/10.1063/1.3658298 (5 pages)

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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.

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88.40.ff	Performance testing
88.40.fr	Concentrating collectors
88.40.hj	Efficiency and performance of solar cells
88.40.jp	Multijunction solar cells

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Choosing a Silicone Encapsulant for Photovoltaic Applications

Michelle Velderrain

AIP Conf. Proc. 1407, pp. 79-83; doi:http://dx.doi.org/10.1063/1.3658299 (5 pages)

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Growth in the solar industry has resulted in newer technologies, specifically concentrator photovoltaic (CPV) modules, to explore using new types of materials such as silicone encapsulants. CPV and LCPV module designs are to achieve the most efficient energy conversion possible however it is equally important to demonstrate long term reliability. Silicone is a material of interest due to its thermal stability and ability to absorb stresses incurred during thermal cycling. The refractive index of clear silicone adhesives is advantageous because it can be optimized using phenyl groups to match BK7 glass and other substrates to minimize light loss at the interfaces but it is relatively unknown how the optical properties change over time possibly yellowing in such a harsh environment. A 1.41 silicone encapsulant is compared to a 1.52 refractive index silicone. Optical Absorption (300 nm–1300 nm), Water Vapor Permeability, Moisture Absorption and effects of oxidation at elevated temperatures will be compared of these materials to aid the engineer in choosing a silicone for their CPV application. Non‐phenyl containing 1.41 RI silicones have been used for several years for bonding solar arrays in the satellite industry. Phenyl groups on the siloxane polymer can change various properties of the silicone. Understanding how phenyl affects these properties allows the engineer to understand the benefits and risks when using a RI matching silicone to minimize light loss versus a non‐phenyl containing silicone.

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88.40.fc	Modeling and analysis
88.40.hj	Efficiency and performance of solar cells
88.40.jj	Silicon solar cells
78.20.Ci	Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
84.60.Jt	Photoelectric conversion

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Experimental Set‐Up to Evaluate the Degradation of the Optical Components of a CPV Module

Jaione Bengoechea, Mikel Ezquer, Iñigo Petrina, and Ana Rosa Lagunas

AIP Conf. Proc. 1407, pp. 84-87; doi:http://dx.doi.org/10.1063/1.3658300 (4 pages)

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The efficiency of CPV modules strongly depends on the characteristics and performance of the optical components. Therefore, the complete characterization of the optical components and their evolution during the module lifetime becomes an important issue to correctly estimate the energy supplied by the CPV systems along the time. Due to the series electrical connection of the multi‐junction solar cells, the spectral distribution of the light after the optical components plays a key role in the energy production. A spectrally resolved characterization of the light is hence desirable. In this paper, an indoor set‐up to study the degradation of optical components of CPV modules is presented. The first part of the experimental set‐up consists of a collimated Xe lamp, a diaphragm, and a spectroradiometer. With this set‐up information about the variation of the light spectrum spatial distribution due to the CPV optical component is obtained. The second part of the experimental set‐up is based on a combination of Deuterium and Halogen lamps and an integrating sphere. This set‐up allows the detection of changes in the optical properties of the material through the measurement of the global spectral transmittance and reflectance.

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42.25.Bs	Wave propagation, transmission and absorption
42.25.Gy	Edge and boundary effects; reflection and refraction
88.40.fc	Modeling and analysis
88.40.hj	Efficiency and performance of solar cells

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Ultra‐High Efficiency, High‐Concentration PV System Based On Spectral Division Between GaInP∕GaInAs∕Ge And BPC Silicon Cells

P. Benítez, R. Mohedano, M. Buljan, J. C. Miñano, Y. Sun, W. Falicoff, J. Vilaplana, J. Chaves, G. Biot, and J. López

AIP Conf. Proc. 1407, pp. 88-92; doi:http://dx.doi.org/10.1063/1.3658301 (5 pages)

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A novel HCPV nonimaging concentrator concept with high concentration (>500×) is presented. It uses the combination of a commercial concentration GaInP∕GaInAs∕Ge 3J cell and a concentration Back‐Point‐Contact (BPC) concentration silicon cell for efficient spectral utilization, and external confinement techniques for recovering the 3J cell′s reflection. The primary optical element (POE) is a flat Fresnel lens and the secondary optical element (SOE) is a free‐form RXI‐type concentrator with a band‐pass filter embedded it, both POE and SOE performing Köhler integration to produce light homogenization. The band‐pass filter sends the IR photons in the 900–1200 nm band to the silicon cell. Computer simulations predict that four‐terminal terminal designs could achieve ∼46% added cell efficiencies using commercial 39% 3J and 26% Si cells. A first proof‐of concept receiver prototype has been manufactured using a simpler optical architecture (with a lower concentration, ∼ 100× and lower simulated added efficiency), and experimental measurements have shown up to 39.8% 4J receiver efficiency using a 3J with peak efficiency of 36.9%.

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88.40.hj	Efficiency and performance of solar cells
88.40.fc	Modeling and analysis
07.05.Tp	Computer modeling and simulation
42.79.Bh	Lenses, prisms and mirrors

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Analytical Investigation of Diffraction in Fresnel Lens Concentrators

Thorsten Hornung and Peter Nitz

AIP Conf. Proc. 1407, pp. 93-96; doi:http://dx.doi.org/10.1063/1.3658302 (4 pages)

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When simulating the optical behavior of primary optics for concentrator photovoltaic systems, one usually utilizes ray tracing techniques. This approach considers geometrical optics only, whereas diffraction effects are generally assumed to be small enough to be neglected. When comparing our high precision optical measurements to computer simulation results the match between measurement and simulation data can be improved when diffraction is included. An analytical treatment of this problem is possible using the universal geometrical theory of diffraction. These formulas are evaluated for a miniature Fresnel lens concentrator. The influences of Fresnel prism size and of the divergence of the incident light are analyzed. Finally, the results are compared to ray tracing calculations that have been enhanced by an approximate treatment of diffraction.

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88.40.ff	Performance testing
88.40.hj	Efficiency and performance of solar cells
42.15.Dp	Wave fronts and ray tracing
78.20.Ci	Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

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Estimation of the Influence of Fresnel Lens Temperature on Energy Generation of a Concentrator Photovoltaic System

Thorsten Hornung, Marc Steiner, and Peter Nitz

AIP Conf. Proc. 1407, pp. 97-100; doi:http://dx.doi.org/10.1063/1.3658303 (4 pages)

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Recent investigations of Fresnel lens concentrator optics have shown that optical efficiency and module performance depend on lens temperature. In former works we succeeded in modeling these effects at a computer and demonstrated a very good match to measurements for SoG Fresnel lenses. In the work presented here, we combined these tools with irradiation spectra and ambient temperature data at typical sites for concentrator photovoltaic power plants for a whole year of operation. We appended a model of a solar cell based on the detailed balance limit introduced by Shockley and Queisser. Thereby the temperature dependence of concentrator photovoltaic systems consisting of a triple junction solar cell and a Fresnel lens concentrator can be evaluated including changes in spectrum over daytime and during all seasons. In our analysis polymethylmethacrylate (PMMA) Fresnel lenses and silicone on glass (SoG) Fresnel lenses show comparable energy harvesting efficiency at close to market locations.

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88.40.hj	Efficiency and performance of solar cells
88.40.jp	Multijunction solar cells
88.05.Bc	Energy efficiency; definitions and standards
65.40.De	Thermal expansion; thermomechanical effects

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Optical Design and Manufacturing of Fresnel Lenses for The First Korean High Concentration Solar PV System

Kwangsun Ryu, Goo‐Hwan Shin, Wonho Cha, Seongwon Kang, Youngsik Kim, and Gi‐Hwan Kang

AIP Conf. Proc. 1407, pp. 101-104; doi:http://dx.doi.org/10.1063/1.3658304 (4 pages)

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In this study, we designed and optimized flat Fresnel lens and the light pipe to develop 500X concentrated solar PV system. In the process, we compare the transmission efficiencies according to groove types. We performed rigorous ray tracing simulation of the flat Fresnel lenses. The computer aided simulation showed the ‘grooves in’ case has the better efficiency than that of ‘grooves out’ case. Based on the ray‐trace results, we designed and manufactured sample Fresnel lenses. The optical performance were measured and compared with ray‐trace results. Finally, the optical efficiency was measured to be above 75%. All the design and manufacturing were performed based on that InGaP∕InGaAs∕Ge triple junction solar cell is used to convert the photon energy to electrical power. Field test will be made and analyzed in the near future.

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42.15.Dp	Wave fronts and ray tracing
42.79.Bh	Lenses, prisms and mirrors
88.40.hj	Efficiency and performance of solar cells
07.05.Tp	Computer modeling and simulation

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Simple Köhler Homogenizers for Image‐forming Solar Concentrators

Roland Winston and Weiya Zhang

AIP Conf. Proc. 1407, pp. 105-108; doi:http://dx.doi.org/10.1063/1.3658305 (4 pages)

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We demonstrate that the Köhler illumination technique can be applied to the image‐forming solar concentrators to solve the problem of “hot” spot and to generate the square irradiance pattern. The Köhler homogenizer can be simply a single aspheric lens optimized following a few guidelines. Two examples are given including a Fresnel lens based concentrator and a two‐mirror aplanatic system.

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88.40.fr	Concentrating collectors
88.40.fj	Parabolic-trough mirrors
88.40.ff	Performance testing
02.60.Pn	Numerical optimization

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