This PDF file contains the front matter associated with SPIE Proceedings Volume 9175, including the Title Page, Copyright information, Table of Contents, Author list, and Conference Committee listing.

The past decade has seen concentrator photovoltaics (CPV) technology transition from a few backyard experiments to an installed global capacity of well over 100 MW. With several utility scale power plants in commercial operation, the presentation will first focus on history and quality of performance since commissioning and overall customer satisfaction. Critical lessons learned from the process of designing, manufacturing and installing product for the utility scale renewable generation market are discussed. The presentation will reflect on how lessons thus far will guide CPV innovators through the flowering of Next Generation CPV technologies, from spectrum splitting optics to five junction solar cells, currently underway throughout the industry. Overall, CPV continues to serve as a robust platform for the new ideas and research needed to mature the higher performance and lower cost technologies critical for continued competitiveness of CPV in the renewable generation marketplace.

A solar simulator capable of producing an irradiance of 300 suns is reported. Technical challenges were not limited to optical design; developing a methodology to measuring 300 suns was difficult. This document reports on the design of the custom fabricated solar simulator, the measurement methodology for high-powered solar concentrator measurements and empirical results validating the desired simulated power density as well as the irradiance stability, spectral accuracy, beam uniformity and irradiance beam size.

A solar concentrator with a highly asymmetric acceptance cone is investigated. Concentrating photovoltaic systems require dual-axis sun tracking to maintain nominal concentration throughout the day. In addition to collecting direct rays from the solar disk, which subtends ~0.53 degrees, concentrating optics must allow for in-field tracking errors due to mechanical misalignment of the module, wind loading, and control loop biases. The angular range over which the concentrator maintains <90% of on-axis throughput is defined as the optical acceptance angle. Concentrators with substantial rotational symmetry likewise exhibit rotationally symmetric acceptance angles. In the field, this is sometimes a poor match with azimuth-elevation trackers, which have inherently asymmetric tracking performance. Pedestal-mounted trackers with low torsional stiffness about the vertical axis have better elevation tracking than azimuthal tracking. Conversely, trackers which rotate on large-footprint circular tracks are often limited by elevation tracking performance. We show that a line-focus concentrator, composed of a parabolic trough primary reflector and freeform refractive secondary, can be tailored to have a highly asymmetric acceptance angle. The design is suitable for a tracker with excellent tracking accuracy in the elevation direction, and poor accuracy in the azimuthal direction. In the 1000X design given, when trough optical errors (2mrad rms slope deviation) are accounted for, the azimuthal acceptance angle is +/- 1.65°, while the elevation acceptance angle is only +/-0.29°. This acceptance angle does not include the angular width of the sun, which consumes nearly all of the elevation tolerance at this concentration level. By decreasing the average concentration, the elevation acceptance angle can be increased. This is well-suited for a pedestal alt-azimuth tracker with a low cost slew bearing (without anti-backlash features).

Solar thermal receivers absorb concentrated sunlight and can operate at high temperatures exceeding 600°C for production of heat and electricity. New fractal-like designs employing light-trapping structures and geometries at multiple length scales are proposed to increase the effective solar absorptance and efficiency of these receivers. Radial and linear structures at the micro (surface coatings and depositions), meso (tube shape and geometry), and macro (total receiver geometry and configuration) scales redirect reflected solar radiation toward the interior of the receiver for increased absorptance. Hotter regions within the interior of the receiver also reduce thermal emittance due to reduced local view factors in the interior regions, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs. Results show that fractal-like structures and geometries can reduce total radiative losses by up to 50% and increase the thermal efficiency by up to 10%. The impact was more pronounced for materials with lower inherent solar absorptances (< 0.9). Meso-scale tests were conducted and confirmed model results that showed increased light-trapping from corrugated surfaces relative to flat surfaces.

The initial prototype design and characterization of a small-scale concentrating solar thermal system will be presented. A large Fresnel lens, with an area of 0.77 square meters on a two-axis tracking mount, is used to concentrate the solar energy and focus it to an area of a few square centimeters. The system design and the first prototype testing will be discussed. By leveraging concentration to achieve the maximum solar-to-thermal energy efficiency and low cost, the system is unique compared to currently available residential hot water systems.

We propose a design for a concentrating PV/T collector utilizing plasmonic nanoparticles directly suspended in the working fluid to spectrally filter the incoming solar flux. This liquid filter serves two purposes: the direct capture of thermal energy as well as filtering off of key portions of the spectrum before transmission to the PV cell. Our device builds upon the current Cogenra T14 system with a two-pass architecture: the first pass on the back side of the PV cell pre-heating the fluid from any thermalization losses, and the second pass in front of the PV cell to achieve the spectral filtering. Here we present details on the selection of plasmonic nanoparticles for a given cell bandgap as well as the impact to the overall system pumping power and cost.

Concentrated solar energy has proven to be an efficient approach for both solar thermal energy applications and photovoltaics. Here, we propose a passive optical device, the Adiabatic Optical Coupler (AOC), that efficiently couples concentrated solar light from a primary solar concentrator into an optical fiber, enabling light collection and energy conversion/storage to be geographically separated, thus maximizing the overall system efficiency. The AOC offers secondary concentration of concentrated solar light through an adiabatic optical mode conversion process. Solar light, highly focused by this two stage concentrator, is delivered by optical fiber to either be subsequently converted to electricity or thermally stored. The ability to transport high energy light flux eliminates the need for high temperature working fluids in solar-thermal systems. In order to design the AOC and related peripherals, we used various modeling tools to cover different optical regimes at macroscopic and microscopic scales. We demonstrated a set of optical thin films with spatially varied refractive index up to 3 and negligible optical absorption by using proprietary sputtering technique to fabricate the AOC. We further studied the films using experimental measurements and theoretical analysis to optimize their optical properties. Preliminary cost analysis suggests that solar thermal power generation systems that employ our S2F concept could offer the cost and efficiency required to achieve the 2020 SunShot initiative levelized cost of electricity (LCOE) target. Success of this endeavor could change the energy conversion paradigm, and allow massively scalable concentrated solar energy utilization.

Here we report a newly developed method for gravity sag molding of large glass solar reflectors, 1.65 m x 1.65 m square, with either line or point focus, and short focal length. The method is designed for high volume manufacture when incorporated into a production line with separate pre-heating and cooling. The tests reported here have been made in a custom batch furnace, with high power radiative heating to soften the glass for slumping. The mold surface is machined to the required shape as grooves which intersect the glass at cusps, reducing the mold contact area to <1%. Optical metrology of replicas made with the system has been carried out with a novel test using a linear array of coaligned lasers translated in a perpendicular direction across the reflector while the deviation of each beam from perfect focus is measured. Slopes measured over an array of 4000 points show an absolute accuracy of <0.3 mrad rms in sx and sy. The most accurate replicas we have made are from a 2.6 m² point focus mold, showing slope errors in x and y of 1.0 mrad rms. The slump cycle, starting with rigid flat glass at 500C, uses a 350 kW burst of radiative heating for 200 seconds, followed by radiative and convective cooling.

Concentration photovoltaic systems uses free-space optics to concentrate sunlight onto photovoltaic cells, using mechanical trackers to accurately track the sun’s position and keep the focal spot on the PV cell. We recently proposed and demonstrated a proof-of concept of a self-tracking mechanism using a phase-change material to greatly extend the acceptance angle of the concentration device. The light responsive mechanism allows for efficient waveguide coupling and light concentration independent of the angle of incidence inside its angular range of acceptance. The system uses a lens pair to achieve a flat Petzval field curvature over the acceptance angle range. A waveguide slab acts as the concentrating device. The use of a dichroic prism membrane separates the solar spectrum into two parts, lower wavelengths (<750nm) are coupled into the waveguide, while the energy of higher wavelengths (>750nm) is used to power the self-tracking mechanism. The energy in this part of the spectrum is absorbed by a carbon black paraffin wax mixture that undergoes a phase change and subsequently creates a coupling feature due to thermal expansion which allows the lower wavelength part to be coupled into the waveguide. We show the extension of our proof-of concept to a device-like demonstrator, featuring the extension to a lens array and much larger dimensions. All parts of the proof-of-concept device have been upscaled to meet the requirements of a larger scale demonstrator. The demonstrator has an acceptance angle of +/- 16 degrees and can achieve an effective concentration factor of 20x. We present experimental and simulation results of the demonstration device.

Multijunction photovoltaics enable significantly improved efficiency over their single junction analogues by mitigating unabsorbed sub-bandgap photons and voltage loss to carrier thermalization. Lateral spectrum-splitting configurations promise further increased efficiency through relaxation of the lattice- and current-matching requirements of monolithic stacks, albeit at the cost of increased optical and electrical complexity. Consequently, in order to achieve an effective spectrum-splitting photovoltaic configuration it is essential that all optical losses and photon misallocation be characterized and subsequently minimized. We have developed a characterization system that enables us to map the spatial, spectral, and angular distribution of illumination incident on the subcell reception plane or emerging from any subset of the concentrating and splitting optics. This positional irradiance measurement system (PIMS) comprises four motorized stages assembled in an X-Z-RY configuration with three linear degrees of freedom and one rotational degree of freedom, on which we mount an optical fiber connected to a set of spectrometers covering the solar spectrum from 280-1700 nm. In combination with a xenon arc lamp solar simulator with a divergence half angle of 1.3 degrees, we are able to characterize our optics across the full spectrum of our photovoltaic subcells with close agreement to outdoor conditions. We have used this tool to spectrally characterize holographic diffraction efficiency versus diffraction angle; multilayer dielectric filter transmission and reflection efficiency versus filter incidence angle; and aspheric lens chromatic aberration versus optic-to-receiver separation distance. These examples illustrate the versatility of the PIMS in characterizing optical performance relevant to both spectrum-splitting and traditional multijunction photovoltaics.

Spectrum-splitting is a beneficial technique to increase the efficiency and reduce the cost of photovoltaic (PV) systems. This method divides the incident solar spectrum into spectral components that are spatially separated and directed to PV cells with matching spectral responsivity characteristics. This approach eliminates problems associated with current and lattice matching that must be maintained in tandem multi-junction systems. In this paper, a two-junction holographic spectrum-splitting photovoltaic system is demonstrated with a folded PV geometry. The system is designed to use both direct and diffuse solar irradiation. It consists of holographic elements, a wedge-shaped optical guide, and PV substrates with back reflectors. The holographic elements and back reflectors spatially separate the incident solar spectrum and project spectral components onto matching PV cell types. In addition, the wedge-shaped optical guide traps diffuse illumination inside the system to increase absorption. In this paper, the wedge spectrum splitting system is analyzed using tabulated data for InGaP2/GaAs cells with direct illumination combined with experimental data for reflection volume holograms. A system efficiency of 31.42% is obtained with experimental reflection hologram data. This efficiency is a 21.42% improvement over a similar system that uses one PV cell with the highest efficiency (GaAs). Simulation results show large acceptance angle for both in-plane and out-of plane directions. Simulation of the output power of the system with different configurations at different times of the year are also presented.

Shockley and Queisser have shown that systems based on single junction PV cells are limited to a system efficiency of 33%. This restriction results from the mismatch between the photon energy of the incident sunlight and the inability of a single junction device to optimally convert the broad incident spectrum. One approach to overcome this difficulty is to incorporate multiple PV cells with different bandgaps that are optimized to convert different parts of the incident spectrum to electrical power. Spectrum splitting configurations distribute incident photons onto several single bandgap PV cells that are spatially separated. Although, systems with different methods and geometries have been proposed, optical systems relying on reflective filters have not been compared to transmissive ones. Since reflection-type films are primarily based on the interference of reflected waves from optical interfaces, systems based on these filters do not have dispersion losses. Dispersive spectrum splitting systems rely on optical elements that use diffraction or refraction for spectral separation. The dispersion from a single broad band optical element can be used for spectral separation. The geometrical relationship between focusing power, the degree of dispersion, the system aperture, and the PV cell aperture and position can be used to tailor the spectral shape of the incident spectrum into each of the PV cells comprising the system. In this paper, the effects of dispersion introduced by transmission type filters are presented compared to reflective filters.

High efficiency concentrator photovoltaic systems are currently based on costly III/V cells and, to offset the high cell capital cost, elevated optical concentrations are used, with consequent reduction in acceptance angles and tight tolerance optics. While this allows for spectacular conversion efficiencies, it does not provide cost effectiveness in a market dominated by low efficiency/low cost technologies. An alternative approach, well known in literature, is based on the combined use of an optical concentrator and a spectral splitting element allowing for the use of separate cells with different spectral responses and, thus, opening the way to a much wider range of possible materials and technologies. While many configurations have been presented during the years, optical efficiency has often been an issue due to the separate action of the concentrating and splitting element. We propose here, as substantial evolution of a previous design [1], a single injection molded plastic non-imaging optical element embodying both two axes concentration and spectral splitting functions. Based on the specific dispersion characteristics of polycarbonate and on a constructive analytical design procedure, this element allows for optical efficiencies exceeding 80%. Theory, simulations and preliminary experimental results will be presented.

Most concentrating solar power (CSP) facilities in the USA are located in the desert southwest where open land and sunshine are abundant, but airborne dust is prevalent. The accumulation of dust, sand and other natural pollutants on collector mirrors and heliostats presents a significant operational problem and M&O cost for the CSP facilities in this region. The optical performance of the CSP collectors is key to achieving low electricity costs, where a 1% decrease in reflectance directly leads to a 1% increase in the levelized cost of electricity (LCOE) generated by these facilities. In this paper we describe the development of low cost, easy to apply anti-soiling coatings based on superhydrophobic (SH) functionalized nano silica materials and polymer binders that possess the key requirements necessary to inhibit particulate deposition on, and adhesion to, CSP mirror surfaces, and thereby significantly reducing mirror cleaning costs and facility downtime. The key requirements for these coatings are excellent optical clarity with minimal diffuse reflectance, and coating mechanical and exposure durability in harsh desert environments while maintaining SH and dirt shedding properties. The coatings developed to date have excellent SH properties with water contact angles >165° and rolling angles <5°. The solar weighted optical reflectance of the anti-soiling coating over the wavelength range 250 nm to 3μm is >99% that of uncoated mirror surfaces with coating diffuse reflectance being <1% over this wavelength range. Ongoing mechanical and accelerated solar UVA exposures also indicate these coatings will meet the required durability goals.

Large-scale solar plants are mostly installed in semi-arid and desert areas. In those areas, dust layer buildup on solar collectors becomes a major cause for energy yield loss. Development of transparent electrodynamic screens (EDS) and their applications for self-cleaning operation of solar mirrors are presented with a primary focus on the removal dust particles smaller than 30 µm in diameter while maintaining specular reflection efficiency < 90%. An EDS consists of thin rectangular array of parallel transparent conducting electrodes deposited on a transparent dielectric surface. The electrodes are insulated from each other and are embedded within a thin transparent dielectric film. The electrodes are activated using three-phase high-voltage pulses at low current (< 1 mA/m² ). The three-phase electric field charges the deposited particles, lifts them form the substrate by electrostatic forces and propels the dust layer off of the collector’s surface by a traveling wave. The cleaning process takes less than 2 minutes; needs energy less than 1 Wh/m² without requiring any water or manual labor. The reflection efficiency can be restored > 95% of the original clean-mirror efficiency. We briefly present (1) loss of specular reflection efficiency as a function of particle size distribution of deposited dust, and (2) the effects of the electrode design and materials used for minimizing initial loss of specular reflectivity in producing EDS-integrated solar mirrors. Optimization of EDS by using a figure of merit defined by the ratio of dust removal efficiency to the initial loss of specular reflection efficiency is discussed.

In order to address the metal oxidation issue in cermet solar thermal absorbers at high working temperatures, we developed solution-processed plasmonic Ni nanochain-SiO_x (x≤2) selective solar thermal absorbers that exhibit high solar absorption, low thermal emittance, and strong anti-oxidation behavior up to 600 °C in air. The thermal stability is far superior to more conventional Ni nanoparticle-Al₂O₃ selective solar thermal absorbers, which readily oxidize at 450 °C. Ni nanochains were embedded in SiO_x and SiO₂ matrices which are derived from hydrogen silsesquioxane (HSQ) and tetraethyl orthosilicate (TEOS) precursors, respectively. Fourier transform infrared spectroscopy (FTIR) shows that the dissociation of Si-O cage-like structures into Si-O networks helped to retard the oxidation process of Ni, possibly by facilitating the formation of chemical bonding between Si in the matrix and the Ni nanochains. X-ray photoelectron spectroscopy (XPS) further shows that the excess Si from the dissociation of HSQ formed silicide-like chemical bonds with Ni that are robust to high temperature oxidation and protect the Ni nanostructures. Besides, the Ni-SiO_x system showed 90% solar absorptance and a low thermal emissivity of 20% at 300 °C in air, compared to ~30% emittance of conventional coating at the same temperature. This technology helps to eliminate the problem of vacuum breaching and further reduces the fabrication cost of the solar selective coating. With a high solar absorptance, a low thermal emittance in the infrared region, and excellent anti-oxidation property, this type of selective solar thermal absorber is promising for applications in future generations of CSP systems.

The first experimental demonstration results will be presented for a novel, two-dimensional waveguiding solar concentrator consisting of a primary concentrator (a microlens array) and a secondary concentrator (tapered multimode waveguides). The microlens array collects the incident sun light and focuses it onto a turning mirror. The turning mirror couples the light into a tapered multimode waveguide, which alleviates connection, cooling and uniformity issues associated with conventional solar concentrating systems. Therefore, a large area of light can be efficiently concentrated to a small waveguide cross-section and guided to an array of co-located photovoltaic cells with high optical efficiency. To achieve the maximum coupling efficiency of the light to the waveguide, the design of the turning mirror and waveguides are optimized to avoid any inherent decoupling loss in the subsequent waveguide propagation. Experimental results indicate that a 38 mm diameter lens with a multimode waveguide that is 3 mm x 3 mm x 10 cm, using only total internal reflection surfaces, can achieve 126x concentration with 62.8% optical efficiency. We will present details on the experimental device characterization. A critical requirement for this design is maintaining low waveguide propagation losses, which as we demonstrate can be less than 0.1 dB/cm. Considering 100% TIR coupling and the use of antireflection layers, the theoretical efficiency limit for this particular system is ~88%.

An electroluminescence test for a Concentrated PV system is presented with the objective of capturing high resolution pseudo-efficiency maps that highlight optical defects in the concentrator system. Key parameters of the experimental setup and imaging system are presented. Image processing is discussed, including comparison of experimental to nominal results and the quantitative estimation of optical efficiency. Efficiency estimates are validated using measurements under a collimated solar simulator and ray-tracing software. Further validation is performed by comparison of the electroluminescence technique to direct mapping of the optical efficiency. Initial results indicate the mean estimation error for I_sc is -2.4% with a standard deviation is 6.9% and a combined measurement and analysis time of less than 5 seconds per optic. An extension of this approach to in-line quality control is discussed.

In this work luminescent optical epoxy samples with micropatterning were fabricated. Five different photoluminescent materials were used as substance in the curing of epoxy resin to form luminescent epoxies of different colors. The absorbance spectra of the unpatterned epoxy samples were measured with spectrometer and the luminescence intensities of all fabricated luminescent epoxy samples were measured using custom made bispectrometer. The micropatterns were copied adequately in the epoxy surface, which would also suggest that nanopatterning would also be possible. Furthermore, the ability to pattern the surface allows fabrication of for example anti-reflective, hydrophobic and selfcleaning structures. Further investigation in different luminescent materials suitable for solar concentrator applications is required in order to develop current systems.