This PDF file contains the front matter associated with SPIE Proceedings Volume 8108, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

The Boeing CPV system has been developed as a jointly funded effort between the Boeing companies, its industrial partners and the Department of Energy. As with all commercial solar systems the key driver of success is a production cost which is competitive with existing power sources. In this paper we describe an approach for driving the near term cost and LCOE (levelized cost of energy) of less than $0.10 per kWh with growth opportunities down to $0.07 per kWh. This objective is achievable through a combination of existing high performance optics, the future availability of +40% to 50% conversion efficient multi-junction cells, low cost design tailored to high speed and low cost manufacturing, field assembly, and low cost trackers.

The Amonix 7700 CPV power system is a massive pedestal mounted, dual axis tracking photovoltaic generator and truly a sight to see. The form factor of the system is one of a kind when compared to other products in the PV marketplace, but the characteristics of the system are no accident. Discussed is the reasoning behind the profile of the Amonix 7700 CPV power system, why the Amonix 7700 product is the lowest cost CPV solution of today and the lowest cost renewable energy solution of tomorrow.

The most common triple-junction solar cell design which has been commercially available to date utilizes a germanium bottom cell with an (In)GaAs and InGaP middle and top cell respectively. This type of device has a well-known efficiency limitation somewhere around 40% at 500 suns. Higher efficiencies can be obtained by changing the effective bandgaps of the three junctions, but the choice of materials and approaches to do so is very limited. We at Solar Junction have adopted the dilute nitride material system to obtain these new bandgaps, and break through the 40% efficiency barrier. The unique advantage of the dilute nitrides is that the bandgap and lattice constant can be tuned independently, allowing bulk material lattice matched to Germanium or GaAs over a wide range of bandgaps. The dilute nitride technology in our first commercial product has enabled us to maximize the efficiency of a triple junction solar cell by using the optimal set of bandgaps (including one around 1eV). Commercial Solar Junction concentrator cells with efficiencies of 43.5% have been independently verified by NREL and Fraunhofer. These higher efficiencies are generally the result of higher output voltage, not higher current, which keeps system-level resistive wiring losses in check.

Semprius' two-stage pupil imaging concentrated photovoltaic (CPV) module design incorporates extremely costeffective glass ball secondary lenses in addition to plano-convex primary lens arrays. Optimization of the optical concentrator design involves modeling the illumination uniformity of the primary aperture (the 'pupil') on the multijunction solar cell in response to the secondary lens index, diameter, surface quality, location, and tolerance offsets. We reconcile our theoretical model with experimental results from a single fully adjustable 'concentrating unit cell', and we thereby create a robust model for design updates, for tolerance and sensitivity modeling, and for prediction of full module and on-sun tracker performance based on receiver placement relative to our primary lens array. In this paper, we discuss the rationale behind our optics approach, our criteria for optimizing our optics, and our tolerancing approach. Then we discuss our experimental approach, including our universally adjustable 'concentrating unit' fixture, our light source, and our primary and secondary optics. We show sensitivity curves of our 'concentrating unit' performance to receiver placement, and ball lens size. We reconcile these with our ray-traced model, and, finally, we show predicted module performance based on receiver tolerance data and receiver wiring in the module using a parameter driven high level circuit model.

Concentrix modules are based on III-V triple junction cells, a Fresnel lens array with a relatively small single lens aperture of 5 square inch, and a cover and bottom plate made out of glass. The first installations were conducted in 2008 in Europe, later installations followed in the US, in East Asia, in the Arabian Peninsula, and in Africa. This paper gives an overview of the performance of Soitec´s CPV systems with special focus on reliability, the different climatic conditions and their impact on the system performance. The seasonal distribution of the direct normal irradiation at the mentioned locations was found to be very different which enabled us to perform studies on the system performance depending on irradiation and ambient temperature. The first generation modules which were installed in 2008 had an average efficiency of 25%, resulting in a peak solar-to-grid system efficiency of 23% and an average AC system energy efficiency of > 20%. The system peak efficiencies of the second and third module generations reach maximum values of > 25% and average AC system efficiencies of > 22%. A detailed analysis of the reliability and the performance of the different system generations is presented.

A primary failure mode for glass failure in reflective CPV systems is the mechanical stress caused by a thermal gradient. To establish the necessary reflector specifications, it is essential to have both economic techniques to measure glass strength and an insight into the failure mechanisms. Due to the highly stochastic nature of glass fracture, large data sets are necessary for statistical validity and to provide meaningful estimates of field failure rates. This paper discusses experimental measurement techniques for both value-added reflectors and for non-value surrogate substrates which are generated as waste during the manufacturing process. Specialized tooling enables measurement by commercially available stress-strain equipment (e.g., "Instron" testers). The glass strength is calculated from the force-to-break data, sample thickness and a substrate shape dependence. These strength data are regressed using a two-parameter Weibull model, enabling calculation of the Weibull modulus, which is a measure of the distribution of flaws of a brittle material. Using a finite element analysis model of the thermal-mechanical stress to determine the critical stress, the Weibull analysis enables extrapolation of the data to predict field failure rates. The test and regression now comprise an On-going Reliability Test (ORT) that is inherently low-cost and appropriate for high-volume manufacture. For fracture, the initiating flaws are the result of glass cutting and trimming operations. There can be low-strength outliers which result from bulk glass defects, though such flawed product should be culled during the manufacturing and inspection processes. As expected and commonly known, the glass strength is very sensitive to the cutting method and resulting quality.

Concentrating Photovoltaic technology garnered significant attention during the polysilicon shortage of 2007-2009. Between 70-80 companies disclosed plans to produce photovoltaic equipment, but only a few actually deployed systems. This paper reviews the candidates in this market who might be able to deploy 50MW systems by 2012, and discusses these companies and their technologies. Some analysis of the prospects for the CPV market is also detailed. This paper draws from a survey of the CPV market performed by the Electric Power Research Institute (EPRI) for the Sacramento Municipal Utilities District¹, a recent NREL² study and the International Electronics Manufacturing Initiative (iNEMI) Advance Manufacturing Technology Roadmap3.

The viability of a concentrator technology is determined by five interrelated factors: economic benefit, cell performance under concentration, thermal management, optical performance and manufacturability. Considering these factors, the 5- 10x concentration range is ideal for silicon-based receivers because this level of concentration captures the bulk of available economic gains while mitigating technical risk. Significant gains in capital efficiency are forsaken below the 5x concentration level. Above the 10x level of concentration, marginal improvements to economic benefit are achieved, but threats to reliability emerge and tend to erode the available economic benefit. Furthermore, optic solutions that provide for concentration above 10x tend to force a departure from low-profile flat-plate designs that are most adoptable. For silicon based receivers, a 5-10x level of concentration within a traditional module form factor is optimal.

Replex Plastics aims to develop a low-cost, low-concentration photovoltaic module using a metallized acrylic reflector designed for use with an inclined single-axis tracker. An asymmetric compound parabolic concentrator is developed and analyzed optimizing the many factors impacting the design, such as tracking strategy, manufacturing process, and cell size. Ray tracing is used to improve the design as well as predict the performance. Results of the simulation closely match the tested performance of the prototype. The final design is an asymmetric compound parabolic concentrator mounted to an encapsulated silicon cell receiver with a system optical efficiency of 60%. The prototype concentrator achieves ~7x increase in power output over an encapsulated receiver with no reflector.

The design of cost-efficient medium-concentration PV (MCPV) systems requires the analysis of dozens of engineering, manufacturing and financial trade-offs. In 2007, Skyline Solar designed its first system, the HGS1000, based on a 7x concentration factor and 0.5 m aperture width. At the then-current cost of components, this was the optimum design. However, as the cost of silicon cells has fallen and the efficiency of cells has improved, the least-cost point has moved. This paper explains how we used the combination of field data and advanced performance modeling to re-optimize the design for lowest levelized cost of energy (LCOE) and achieve 40% cost reduction.x=14&up

For the last eight years, the solar industry has experienced annual growth of over 40%. Concerns over the rising costs of conventional energy, as well as its environmentally damaging effects, have been driving this vigorous investment in solar energy. Now, with grid parity almost upon us, the solar industry is reassessing what it will look like as a commoditized, mainstream power source. Do the best opportunities for solar lie with a centralized or decentralized model of production? What are the most bankable PV technologies? How can costs be cut and efficiencies improved to take things to the next level and make that last push to reach grid parity? In this paper, we will examine Solaria's LCPV technology and approach to the future of solar power.

A comparison of static and single-axis tracking holographic planar concentrator systems is made. Tracking is used as a design parameter that can provide additional degrees of freedom in the spectrum and uniformity of the beam illuminating the photovoltaic cell surface. These parameters impact the energy yield of the system. An overview of these factors and an estimate of the cost differences for the two systems will be presented.

In a Luminescent Solar Concentrator (LSC), short-wavelength light is converted by a luminescent material into longwavelength light, which is guided towards a photovoltaic cell. In principle, an LSC allows for high concentration, but in practice this is prevented by loss mechanisms like limited sunlight absorption, limited quantum efficiency and high self absorption. To tackle these problems, a suitable luminescent material is needed. Another important loss mechanism is the escape of luminescent radiation into directions that do not stay inside the light guide. To reduce this amount, wavelengthselective filters can be applied that reflect the luminescent radiation back into the light guide while transmitting the incident sunlight. In this paper, we discuss experiments and simulations of new luminescent and filter materials. We will introduce a phosphor with close-to-optimal luminescent properties. A problem for use in an LSC is the large scattering of this material; we will discuss possible solutions for this. Furthermore, we will discuss the use of broad-band cholesteric filters in combination with this phosphor.

We demonstrate a novel hybrid solar photovoltaic electrical and thermal energy cogeneration system with high efficiency, at potentially reduced overall weight and size compared with current solar energy systems. The new system is based on highly efficient photovoltaic solar cells and tubular water thermal receivers, incorporating holographic spectral beam light guide concentrators resulting in a more cost-effective solution. Details of fabrication and preliminary experimental testing results are presented.

Solar power is a supplying inexhaustibly, causing no pollution, requiring slightly maintenance economic energy source. Currently, very high efficiency solar cells based on III-V semiconductors are available on the market. Since these solar cells are more expensive than silicon or thin film ones, they need to be used at high concentration ratio to reduce III-V multi-junction solar cell area. Concentrated photovoltaic (CPV) system is to collect light by using low-cost optical elements so that the total cost can be reduced. This paper presents a new design of solar concentrator for III-V concentrated photovoltaic. Minilens array focus sunlight to the triangular prisms causing traveling wave by total internal refraction in the waveguide. High efficiency solar cell is placed on the sidewall of the waveguide. The light and thin optical system could be fabricated by low-cost process and have the linear property for the tilt of the sun light. As a result, the sun tracker works only nine times a day accompanying by linear-moving waveguide. It also reduces the cost by decreasing the electricity demand for tracking system.

This paper reports methods developed to maximize the overall reflectance second-surface silvered glass. The reflectance at shorter wavelengths is increased with the aid of a dielectric enhancing layer between the silver and the glass, while at longer wavelengths it is enhanced by use of glass with negligible iron content. The calculated enhancement of reflectance, compared to unenhanced silver on standard low-iron float glass, corresponds to a 4.4% increase in reflectance averaged across the full solar spectrum, appropriate for CSP, and 2.7% for CPV systems using triple junction cells. An experimental reflector incorporating these improvements, of drawn crown glass and a silvered second-surface with dielectric boost, was measured at NREL to have 95.4% solar weighted reflectance. For comparison, non-enhanced, wetsilvered reflectors of the same 4 mm thickness show reflectance ranging from 91.6 - 94.6%, depending on iron content. A potential drawback of using iron-free drawn glass is reduced concentration in high concentration systems because of the inherent surface errors. This effect is largely mitigated for glass shaped by slumping into a concave mold, rather than by bending.

In a Power Tower solar thermal power plant, alignment and control of the heliostats constitutes one of the largest costs of both time and money. This is especially the case in systems where individual heliostats are small (~1m2). I describe a closed-loop control system that generates the required feedback by inducing small mechanical vibrations in the heliostat reflector surface using piezoelectric actuators. These vibrations induce time-dependent changes in the reflected wavefront that can be detected by photosensors surrounding the thermal receiver target. Time and frequency encoding of the vibrations allows identification of a misaligned heliostat from among the thousands in the system. Corrections can then be applied to bring the reflected beam onto the receiver target. This technique can, in principle, control thousands of heliostats simultaneously.Outdoor testing of a small-scale model of this system has confirmed that such a system is effective and can achieve milliradian tracking accuracy. If such a system were implemented in a commercial plant, it could relax the accuracy specification required of the heliostats as well as provide an automated alignment and calibration system. This could significantly reduce the installed cost of the heliostat field.