This PDF file contains the front matter associated with SPIE Proceedings Volume 7407, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

This paper discusses the concentrating solar technologies used for utility-scale solar power. Specifically, it explains the rationale behind the use of concentration for photovoltaic modules, and how concentrating photovoltaic technologies compare with concentrating solar thermal technologies. It describes innovations in both areas, and how they are addressing the cost challenges to meet the needs of the utility scale market.

After twenty years of commercial deployments of concentrator photovoltaic systems using silicon cells, Amonix has built a new generation of systems designed for III-V multijunction cells. The resulting 7th-generation systems yield a considerable performance dividend in the field-proven system design. The first systems, operating in Las Vegas, NV, achieve AC efficiencies in excess of 25%. Detailed modeling of the cell and system parameters provides a prediction of energy generation that is within 3% of the measured energy after seven months of operation. The predicted annual yield in this location is over 2600 kW-hr/kW.

The Boeing Company Phantom Works has developed three different prototype photovoltaic concentrator arrays since March 2007. Identified as Prototype A, B and C, the experimentally proven technical characteristics of each design are presented. The concentrator designs utilize a 1 cm² multi-junction solar cell assembly in conjunction with SMS non-imaging optical designs [1, 2] manufactured with low-cost mass-producible technologies. Prototype A is an on-axis XR optical concentrator with a 733x geometrical concentration demonstrating a ± 1.73° acceptance angle and 23.7% conversion efficiency. Prototype B is an off-axis free-form XR optical concentrator with a 810x geometrical concentration demonstrating a ± 1.32° acceptance angle and 25.3% conversion efficiency. Prototype C is the most recent off-axis free-form XR optical concentrator with a 801x geometrical concentration and a theoretical ±1.80° acceptance angle demonstrating a conversion efficiency greater than 27.0%. Prototype C is also the basis for the Boeing Proof of Design (POD) module, demonstrating an acceptance angle of ±1.48° and a conversion efficiency of 29.4% (as of May 8, 2009). Manufacturability has been paramount during the design process, resulting in high performance concentrating photovoltaic modules using production quality components.

Soliant Energy, Inc. is producing 500X CPV systems for commercial rooftop applications. Our unique application requires an accurate method for comparing CPV panel performance to silicon and thin-film flat panel performance. The true metric for comparing rooftop CPV to flat panel can only be kWh/m²/yr. It is possible to calculate module level temperature coefficients for the output power and calculate a power rating comparable to an STC rating for this module. However, the best comparison is to measure outside, side by side with a flat panel. This paper presents performance data collected at Sandia National Laboratories from November 2008 to January 2009, and at Soliant's Monrovia facility in March and April 2009, showing the enhanced performance of our CPV system under real world conditions.

In order to generate key knowledge on CPV technology, ISFOC has already installed 1,4MW of CPV and is executing 3MW of power plants incorporating seven different technologies which will be finished in 2009. The objective of these pilot plants is to assist the industries in the setting up of pilot production lines and to obtain very valuable information such as reliability, suitability and production. In collaboration with the various suppliers, ISFOC has followed in detail all the qualification tests and their results. Therefore a great body of knowledge and experience is being built up. After the completion of the plants, ISFOC has started the campaign of measurements, following its own methodology. It is based on the equations of the Shockley model and only one measurement is needed to establish the nominal power of the CPV system. Heat-sink temperature to calculate the cell temperature through the thermal resistance, DNI with a pyrheliometer and the I-V Curve are measured in this procedure. But, ISFOC will also test other rating procedures, like the ASTM 2527-E or the IEC draft for CPV modules. The results will be shown in this paper.

Skyline Solar Inc. has developed a novel silicon-based PV system to simultaneously reduce energy cost and improve scalability of solar energy. The system achieves high gain through a combination of high capacity factor and optical concentration. The design approach drives innovation not only into the details of the system hardware, but also into manufacturing and deployment-related costs and bottlenecks. The result of this philosophy is a modular PV system whose manufacturing strategy relies only on currently existing silicon solar cell, module, reflector and aluminum parts supply chains, as well as turnkey PV module production lines and metal fabrication industries that already exist at enormous scale. Furthermore, with a high gain system design, the generating capacity of all components is multiplied, leading to a rapidly scalable system. The product design and commercialization strategy cooperate synergistically to promise dramatically lower LCOE with substantially lower risk relative to materials-intensive innovations. In this paper, we will present the key design aspects of Skyline's system, including aspects of the optical, mechanical and thermal components, revealing the ease of scalability, low cost and high performance. Additionally, we will present performance and reliability results on modules and the system, using ASTM and UL/IEC methodologies.

The results of thermal analysis and experiments are presented for a 1-kW brand new medium-level (8X) concentration photovoltaic (CPV) unit that is cooled by evaporation and built as an elongated floating solar unit. The unit keeps the silicon PV elements at low and stable temperature around the clock, significantly outperforms competitors' systems in terms of the power output and the life span of identical PV elements. It is demonstrated theoretically and experimentally that the PV element temperature level exceeds the temperature level of water in the water basin (used as a heat sink) by just a few degrees.

High-efficiency, low-cost InGaP/GaAs dual-junction epitaxial liftoff (ELO) solar cells have been fabricated on full 4" GaAs substrates. These dual-junction solar cells exhibited an efficiency of 28.69% at AM1.5D, one-sun illumination. This is the highest reported efficiency for dual-junction ELO solar cells to date. After application of antireflection coating, the dual-junction ELO cells also exhibited fill factor >85%, open circuit voltage = 2.37 V, and short circuit current density = 13 mA/cm². An external quantum efficiency >85% was measured for both the GaAs and InGaP sub-cells. An ELO dual-junction solar cell wafer typically weighs less than 1.7 g and has a total semiconductor thickness <5 μm. Reclaim and reuse of the GaAs substrate after the ELO process has been successfully demonstrated.

Tunnel diodes constitute an essential part of multi-junction concentrator photovoltaics. These tunnel junctions exhibit a transition from low-resistance tunneling to high-resistance thermal diffusion, commonly at current densities of the order of 10²-10³ mA/mm². Experimental evidence of a fundamentally new effect is reported and confirmed in distinct cell architectures: the dependence of the threshold current density on the extent of localized irradiation. It is also shown that photovoltaic cells with a non-uniform metal grid can possess an additional spatial dependence to the threshold current density. These new phenomena should be observable in all solar cell tunnel diodes subjected to inhomogeneous illumination, and are posited to stem from the lateral spreading of excess majority carriers (similar to current spreading in LEDs). The implications for concentrator solar cells are also addressed.

Conventional CPV systems focus sunlight directly onto a PV cell, usually through a non-imaging optic to avoid hot spots. In practice, many systems use a shared tracking platform to mount multiple smaller aperture lenses, each concentrating light into an associated PV cell. Scaling this approach to the limit would result in a thin sheet-like geometry. This would be ideal in terms of minimizing the tracking system payload, especially since such thin sheets can be arranged into louvered strips to minimize wind-force loading. However, simply miniaturizing results in a large number of individual PV cells, each needed to be packaged, aligned, and electrically connected. Here we describe for the first time a different optical system approach to solar concentrators, where a thin lens array is combined with a shared multimode waveguide. The benefits of a thin optical design can therefore be achieved with an optimum spacing of the PV cells. The guiding structure is geometrically similar to luminescent solar concentrators, however, in micro-optic waveguide concentrators sunlight is coupled directly into the waveguide without absorption or wavelength conversion. This opens a new design space for high-efficiency CPV systems with the potential for cost reduction in both optics and tracking mechanics. In this paper, we provide optical design and preliminary experimental results of one implementation specifically intended to be compatible with large-scale roll processing. Here the waveguide is a uniform glass sheet, held between the lens array and a corresponding array of micro-mirrors self-aligned to each lens focus during fabrication.

Holographic elements have several unique features that make them attractive for solar collector and concentrator systems. These properties include the ability to diffract light at large deflection angles, Bragg selectivity, grating multiplexing, and angle-wavelength matching. In this presentation we review how these properties can be applied to solar collection and concentrator systems. An algorithm is presented for analyzing the energy collection properties of holographic concentrators in specific geometries and is applied to a planar collection format. Holographic elements are shown to have advantages for low concentration ratio solar concentrator systems.

A spectrally selective reflector (SSR) can be fabricated by depositing a transparent conducting oxide on a reflective substrate. SSRs can be incorporated in concentrating solar cells in order to minimize the heating. We deposited TiO₂:Nb thin films on glass by DC magnetron sputtering, extracted optical constants, and used those to model an optimized SSR. Corresponding films were then successfully produced. The best wavelength-integrated reflectance values were 79 % and 31 % in the ranges 300 < λ < 1100 nm and 1100 < λ < 2500 nm, respectively. These data are better than those previously achieved using SnO₂:F.

Concentrated photovoltaics (CPV) has recently gained interest based on its scalability and expected low levelized cost of electricity. The reliability of materials used in CPV systems, however, is not well established. The current qualification test for photodegradation of CPV modules includes only real-time ultraviolet (UV) exposure, i.e. methods for accelerated UV testing have not been developed. Therefore, the UV and infrared (IR) spectra transmitted through representative optical systems is evaluated in this paper. The measurements of concentrating optics are used to assess expected optical performance as well as to understand how to quantify damaging optical exposure. Optical properties (transmittance, refractive index, reflectance, and absorptance) of candidate materials are identified. The dose and flux analysis here identifies the increased significance of IR (as opposed to UV) exposure for CPV systems, particularly for the most concentrating systems. For these, the UV dose may not greatly exceed the unconcentrated global solar condition, but the thermal load scales nearly directly with the geometric concentration.

Skyline Solar has developed a novel Concentrated Photovoltaic (CPV) architecture design called High Gain Solar based on a reflective trough design with optimized Si panels. This design provides a distinct separation between the functionality of key components enabling parallel development and optimization as well as very rapid deployment. A predictive tool has been developed to link component characteristics to overall energy production to accurately predict the performance and degradation of the system in time, as a function of weather patterns and system architecture. This predictive tool is based on empirical and analytical models combining accelerated stress tests, on-sun performance tests, finite element analysis and manufacturing variability analyses.

The emergence of high efficiency photovoltaic cells is leading the industry into using solar concentrators in order to reduce costs by decreasing the number of cells used. In this paper Optics department of Universidad Complutense de Madrid has designed a multifocal Fresnel lens of PMMA and has studied the main parameters that have influence on its final function. This has been done by taking into account its manufacturing tolerances. The lens is square shaped with sides measuring 270 mm and it is composed of three different zones based on three different criteria: The central zone has been designed by using paraxial formulation, the intermediate one has been designed based on Fresnel classical formula while the marginal zone's purpose is to deflect the light by total internal reflection on prism faces. All three zones have different focal areas and different optical axis so the energy distribution will be more uniform whilst avoiding cell damage caused by hot spots. The design stage is feedback through simulations using a ray tracer software. In order to characterize the lens operation a measure of optical concentration was first taken on different lens areas using an integrating sphere. Finally, the lens performance in terms of concentration and in terms of uniformity at the focal spot was studied by processing the images taken with a CCD camera on a screen placed at the focal plane of the lens.

A new reflector surface and geometry using low-concentration mirror boosting of flat-plate photo voltaic devices is described. The overheating effects that have previously been seen using non-uniform, high reflectivity side mirrors have been reduced. The new high-stability reflector material has lower UV reflectivity that reduces panel ageing and over heating. A moderate reflectivity in the violet wavelength further cuts the level of overheating while sacrificing only minimally in electrical power output efficiency. The new surface maintains high, uniform reflectivity at green, yellow, red, and IR wavelengths. Mass-produced panels are undergoing tests, and some preliminary results are presented. Surface self-cleaning of hydrophilic and hydrophobic coating over the reflecting surface is also discussed. Other applications of the same mirror in the solar thermal field are briefly discussed. Some improved tracking PV geometry versions using the new material are presented.