The High-Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL) has a faceted primary concentrator and a long focal-length-to-diameter ratio (due to its off-axis design). Each primary facet can be aimed individually to produce different flux distributions at the target plane. Two different types of attenuators are used depending on the flux distribution. A sliding­ plate attenuator is used primarily when the facets are aimed at the same target point. The alternate attenuator resembles a venetian blind. Both attenuators are located between the concentrator and the focal point. The venetian-blind attenuator is primarily used to control the levels ;of sunlight falling on a target when the primary concentrators are not focused to a single point. This paper will demonstrate the problem of using the sliding­ plate attenuator with a faceted concentrator when the facets are not aimed at the same target point. We will show that although the alternate attenuator necessarily blocks a certain amount of incoming sunlight, even when fully open, it provides a more even attenuation of the flux for alternate aiming strategies.

The outdoor lifetime of ReflecTech® mirror film has increased as it has advanced through various commercial versions. Quantifying its lifetime is important, and use in utility-scale power generation requires extensive testing and validation to ensure compliance with the demands of extreme outdoor service environments. A test matrix was developed and implemented with NREL; a variety of ReflecTech  samples were subjected to many accelerated exposure tests. The tests spanned several years, and include several versions of ReflecTech  , from the early versions through the newest version (ReflecTech ® PLUS) that incorporates an abrasion resistant coating. This paper describes the various tests, how they were used to determine reflector degradation rates, and how these tests lead to the quantification of outdoor service lifetimes. The analysis of accelerated test results permits the prediction of the useful lifetime of ReflecTech  under various conditions that are characteristic of Europe, China, Brazil, India, and the United States. For all these locations, the test results indicate outdoor lifetimes above 35 years.

Solar-thermal reactors make use of concentrated solar irradiation to drive endothermic reactions. One example of processes that can be carried out in solar-thermal reactors is the solar-thermal conversion of biomass into synthesis gas (H2, CO and CO2). The operation of these reactors is dependent on available sunlight and is affected by the presence of clouds, which act as disturbances. During a partly cloudy day, operation of solar-thermal reactors is still possible but flow rate adjustment is required in order to maintain consistent operation. Thus, a robust control system that will allow continuous high performance operation of the reactor is required in order to make the process financially viable. A model predictive control system (MPC) is proposed, which uses a model of the process to determine the required control signal. The first step in the development of the control system is the formulation of a dynamic mathematical model that describes process behavior adequately yet can be solved in real time. A simplified dynamic model for a solar-thermal transport-tube reactor has been developed based on unsteady mass and energy balances. The model was solved using MATLAB™ and validated with experimental data. Model validation was carried out at the High Flux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL) in Golden, CO, using different solar power level inputs. In this work, a description of the model and a comparison with the experimental results are presented. Simulations of the model were performed for a reflective and an absorbing cavity. The former allows for faster heating during experimental runs, while the latter is more representative of an industrial setting.

Alumina thin film structures were produced by coating high surface area polymer particles via atomic layer deposition (ALD), using the polymer as a sacrificial template. Burnout of the polymer material left high surface area, high pore volume structures, with 15 nm wall thickness. Further deposition of up to 27 mol% Co and Fe was performed via ALD to produce high surface area CoFe2O4 particles for thermochemical water splitting. The ALD particles were thermally cycled in electrically heated lab reactors and on-sun using a concentrated solar, reflective cavity reactor. Surface area measurements of cycled ALD particles showed improved surface area retention as compared to bulk Fe2O3 nanopowders. Reaction rates as high as 15.2 and 9.8 mu mol/s/g were observed, on-sun, for H2O and CO2 splitting respectively. Thermochemical cycling in a concentrated solar cavity reactor showed an order of magnitude increase in solar utilization efficiency between ALD particles and bulk Fe2O3 nanopowders. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

A three-dimensional, steady state computational model coupling radiative transfer with fluid flow, heat transfer, mass transfer, and chemical reaction kinetics is developed to describe a solar receiver consisting of an array of five tubes enclosed within a specularly reflective cylindrical cavity with a windowed aperture. Radiation heat transfer is incorporated via a combination of ray tracing, Monte Carlo, and finite volume techniques. Steam gasification of entrained 42 nm acetylene black particles is considered and particle transport is described by an aerosol population balance featuring convection, Brownian motion, and thermophoretic diffusion. Maximum temperatures of 1813 K, 1343 K and 1546 K are predicted for the center, front and back tubes respectively, with corresponding reaction conversion of 40%, 2.5% and 9.2% for a solar power input of 6 kW and a carbon feed rate of 0.5 g/min. Temperature of the fluid/particle mixture tracks closely with that of the surrounding tube walls owing to radiative absorption by the particulate phase. Estimated solar-to-chemical receiver efficiency is limited by comparatively low temperatures achieved in outlying tubes and ranges between 1% and 4% with up to 9 kW solar power. Experimentally measured carbon conversion is compared with predictions from the theoretical model utilizing various sets of kinetic parameters available in the literature for steam gasification of petcoke, graphite, activated carbon and low ash coal char.

A three-dimensional, steady state computational model coupling radiative transfer with convective and conductive heat transfer is developed to describe a solar receiver consisting of an array of five tubes enclosed within a specularly reflective cylindrical cavity with a windowed aperture. Ray trace modeling of the concentrating system provides the magnitude and direction of solar energy incident on the aperture. Transport of solar radiation in the cavity space is decoupled from all other transport processes occurring in the receiver and profiles of the absorbed solar energy are determined via a Monte Carlo technique requiring only the receiver geometry, solar profile at the aperture, and spectral directional optical properties. A finite volume method is utilized to account for thermal radiation emitted by heated surfaces and implemented in conjunction with a computational fluid dynamics model. Maximum temperatures of 1820K, 1355K and 1536K are predicted for the center, front, and back tubes, respectively for a solar power input of 6kW though temperature gradients as high as 340K develop between the front and back sides of the center tube. More than 79% of the solar energy is absorbed by tube surfaces. Emission losses account for 11–25% of the solar input whereas conductive heat losses account for 55–69% of the solar input and arise predominantly from conduction along the tube length to cooled cavity walls. Average discrepancies between theoretically predicted and experimentally measured temperatures are 44K (4%) for silicon carbide tubes and 21K (2%) for Inconel tubes over temperature ranges of, respectively, 600–1700K and 700–1400K.

The objective of this work is to identify economically feasible concepts for the production of hydrogen from water using solar energy. The ultimate project objective was to select one or more competitive concepts for pilot-scale demonstration using concentrated solar energy. Results of pilot scale plant performance would be used as foundation for seeking public and private resources for full-scale plant development and testing. Economical success in this venture would afford the public with a renewable and limitless source of energy carrier for use in electric power load-leveling and as a carbon-free transportation fuel. The Solar Hydrogen Generation Research (SHGR) project embraces technologies relevant to hydrogen research under the Office of Hydrogen Fuel Cells and Infrastructure Technology (HFCIT) as well as concentrated solar power under the Office of Solar Energy Technologies (SET). Although the photoelectrochemical work is aligned with HFCIT, some of the technologies in this effort are also consistent with the skills and technologies found in concentrated solar power and photovoltaic technology under the Office of Solar Energy Technologies (SET). Hydrogen production by thermo-chemical water-splitting is a chemical process that accomplishes the decomposition of water into hydrogen and oxygen using only heat or a combination of heat and electrolysis instead of pure electrolysis and meets the goals for hydrogen production using only water and renewable solar energy as feed-stocks. Photoelectrochemical hydrogen production also meets these goals by implementing photo-electrolysis at the surface of a semiconductor in contact with an electrolyte with bias provided by a photovoltaic source. Here, water splitting is a photo-electrolytic process in which hydrogen is produced using only solar photons and water as feed-stocks. The thermochemical hydrogen task engendered formal collaborations among two universities, three national laboratories and two private sector entities. The photoelectrochemical hydrogen task included formal collaborations with three universities and one national laboratory. The formal participants in these two tasks are listed above. Informal collaborations in both projects included one additional university (the University of Nevada, Reno) and two additional national laboratories (Lawrence Livermore National Laboratory and Lawrence Berkeley National Laboratory).

One of the most promising developments for lowering the cost of utility scale concentrating solar power (CSP) is the emergence of durable reflective polymer films as an alternative to conventional curved glass mirrors. The broad adoption of wide web polymer film reflectors has been slowed by the lack of long-term weathering data. With the advent of the Ultra Accelerated Weathering System (UAWS), testing and development can proceed at a faster pace, and ReflecTech® Mirror Film has recently exceeded the equivalent terrestrial UV cumulative dosage of 25 years. ReflecTech® Mirror Film was developed through earlier collaborations between ReflecTech, Inc. (a subsidiary of SkyFuel, Inc.) and the National Renewable Energy Laboratory (NREL). More recently, through a Cooperative Research and Development Agreement (CRADA), ReflecTech, Inc. and NREL have developed an abrasion resistant coating (ARC) appropriate for application to polymer based mirror film. This hardcoat was developed to address the need for reflectors that are low in cost, high in performance and durable to mechanical cleaning methods sometimes used in the CSP environment. The combined impact of these two developments has the potential to significantly change the preferred supply choice for solar reflectors in new utility scale CSP projects. ReflecTech® Mirror Film samples prepared with and without the ARC hardcoat were subjected to accelerated exposure conditions more extreme than actual conditions. Both the uncoated and ARC films exhibit excellent weatherability with no loss in reflectance after highly accelerated exposure of over 25 years equivalent terrestrial UV. The ARC coated samples also exhibited outstanding initial abrasion resistance and adhesion to ReflecTech® Mirror Film, properties that were retained after exposure to various accelerated stress conditions including condensation cycling, thermal cycling, water immersion, and accelerated exposure to UV light. ReflecTech® Mirror Film is a commercially available product. The ARC-coated ReflecTech® Mirror Film has been successfully manufactured as a 1.5 m (5 ft) wide roll-to-roll construction in a commercial production environment and after further testing is expected to be commercially available in late 2011.

This paper describes the structure and design rationale for a new ultra accelerated weathering system. The system allows a 63 year (approximate) equivalent South Florida UV radiant exposure within a single year of ultra accelerated exposure. The system provides high fidelity to natural solar UV spectral power distributions while attenuating visible and IR wavelengths to maintain acceptable specimen exposure temperatures. The paper includes data showing correlation between ultra accelerated exposure and real world exposure. This paper is the first in a series describing the system and exposure results and reviews accomplishments of a cooperative development effort between Atlas Material Testing Technology, the National Renewable Energy Laboratory and the Institute for Laser Optical Technology under the Department of Energy’s Global Initiative for Proliferation Prevention program.

NREL's Ultra-Accelerated Weathering System (UAWS) selectively reflects and concentrates natural sunlight ultraviolet irradiance below 475 rim onto exposed samples to provide accelerated weathering of materials while keeping samples within realistic temperature limits. This paper will explain the design and Implementation of the UAWS which allow it to simulate the effect of years of weathering in weeks of exposure. Exposure chamber design and instrumentation will be discussed for both a prototype UAWS used to test glazing samples as well as a commercial version of UAWS. Candidate polymeric glazing materials have been subjected to accelerated exposure testing at a light intensity level of up to 50 UV suns for an equivalent outdoor exposure in Miami, FL exceeding 15 years Samples include an Impact modified acrylic, fiberglass, and polycarbonate having several thin UV-screening coatings. Concurrent exposure is carried out for identical sample sets at two different temperatures to allow thermal effects to be quantified along with resistance to UV.

High temperature biomass gasification has been performed in a prototype concentrated solar reactor. Gasification of biomass at high temperatures has many advantages compared with historical methods of producing fuels. Enhancements in overall conversion, product composition ratios, and tar reduction are achievable at temperatures greater than 1000°C. Furthermore, the utilization of concentrated solar energy to drive these reactions eliminates the need to consume a portion of the product stream for heating and some of the solar energy is stored as chemical energy in the product stream. Experiments to determine the effects of temperature, gas flow rate, and feed type were conducted at the high flux solar furnace at the National Renewable Energy Laboratory, Golden, CO. These experiments were conducted in a reflective cavity multitube prototype reactor. Biomass type was found to be the only significant factor within a 95% confidence interval. Biomass conversion as high as 68% was achieved on sun. Construction and design considerations of the prototype reactor are discussed as well as initial performance results.

Current research into hydrogen production through high temperature metal oxide water splitting cycles has created a need for robust high temperature materials. Such cycles are further enhanced by the use of concentrated solar energy as a power source. However, samples subjected to concentrated solar radiation exhibited lifetimes much shorter than expected. Characterization of the power and flux distributions representative of the High Flux Solar Furnace(HFSF) at the National Renewable Energy Laboratory(NREL) were compared to ray trace modeling of the facility. In addition, samples of candidate reactor materials were thermally cycled at the HFSF and tensile failure testing was performed to quantify material degradation. Thermal cycling tests have been completed on super alloy Haynes 214 samples and results indicate that maximum temperature plays a significant role in reduction of strength. The number of cycles was too small to establish long term failure trends for this material due to the high ductility of the material.

Parabolic trough receivers, or heat collection elements (HCEs), absorb sunlight focused by the mirrors and transfer that thermal energy to a fluid flowing within them. Thje absorbing tube of these receivers typically operates around 400 C (752 F). HCE manufacturers prevent thermal loss from the absorbing tube to the environment by using sputtered selective Cermet coatings on the absorber and by surrounding the absorber with a glass-enclosed evacuated annulus. This work quantifies the heat loss of the Solel UVAC2 and Schott PTR70 HCEs. At 400 C, the HCEs perform similarly, losing about 400 W/m of HCE length. To put this in perspective, the incident beam radiation on a 5 m mirror aperture is about 4500 W/m, with about 75% of that energy ({approx} 3400 W/m) reaching the absorber surface. Of the 3400 W/m on the absorber, about 3000 W/m is absorbed into the working fluid while 400 W/m is lost to the environment.

A graphite fluid-wall aerosol flow reactor heated with concentrated sunlight has been developed over the past five years for the solar-thermal decarbonization of methane. The fluid-wall is provided by an inert or compatible gas that prevents contact of reactants and products of reaction with a graphite reaction tube. The reactor provides for a low thermal mass that is compatible with intermittent sunlight and the graphite construction allows rapid heating/cooling rates and ultra-high temperatures. The decarbonization of methane has been demonstrated at over 90% for residence times on the order of 10 milliseconds at a reactor wall temperature near 2000 K. The carbon black resulting from the dissociation of methane is nanosized, amorphous, and ash-free and can be used for industrial rubber production. The hydrogen can be supplied to a pipeline and used for chemical processing or to supply fuel cell vehicles.

Computational fluid dynamics simulations of a "fluid-wall" solar reactor for the dissociation of ZnO were performed to determine the effectiveness of the fluid-wall strategy at preventing oxygen from reacting with the reactor wall. An axial boundary layer near the porous wall was found to exist where ZnO concentrations were essentially zero, demonstrating that the concept was effective at preventing particle contact with the reactor wall. ZnO particles were found to heat nearly instantaneously (> 10(5) K/s), and ZnO conversions were found to be significant (> 50%) in the short residence times employed. Conditions for high levels of oxidation of the tube wall coincided with those for high levels of ZnO conversion, and oxidation levels were high (> 50%). This was confirmed in solar experiments, where all oxygen products reacted with the tube wall. Although an effective concept for keeping particles from contacting the wall, the fluid-wall strategy was not effective for preventing oxidation. (c) 2007 American Institute of Chemical Engineers.