[Show abstract][Hide abstract] ABSTRACT: We present a design space exploration of a 5 MWth Small Particle Solar Receiver for solar tower power plants. This new solar receiver, developed under the support of the U.S. Department of Energy's SunShot Program, aims to volumetrically absorb concentrated solar irradiation using an air-particle mixture to drive a gas turbine or a combined cycle at much higher temperature than the state-of-the-art molten salt receivers. Among other advantages, the thermodynamic efficiency of the power block and the overall efficiency of the plant would considerably increase with this technology. The design space consists of the wall angle of the receiver, the geometry of the window (necessary to allow the solar irradiation to enter into the receiver) and the radiative properties of the walls. The constraints are based on material limits, ensuring the mechanical integrity of the quartz window, and other technical issues; though some of them are imposed via a penalty method. The design space is explored through parametric studies and a multidisciplinary approach is adopted. The aluminum oxide walls, the 45° spherical-cap window and the 45° wall- angle receiver are preferred due to their best compromise between thermal efficiency and wall temperature.
Energy Procedia 12/2014; 49:344–353. DOI:10.1016/j.egypro.2014.03.037
[Show abstract][Hide abstract] ABSTRACT: We present a design space exploration of a 5 MWth Small Particle Solar Receiver for solar tower power plants. This new solar receiver, developed under the support of the U.S. Department of Energy’s SunShot Program, aims to volumetrically absorb concentrated solar irradiation using an air-particle mixture to drive a gas turbine or a combined cycle at much higher temperature than the state-of-the-art molten salt receivers. Among other advantages, the thermodynamic efficiency of the power block and the overall efficiency of the plant would considerably increase with this technology. The design space consists of the wall angle of the receiver, the geometry of the window (necessary to allow the solar irradiation to enter into the receiver) and the radiative properties of the walls. The constraints are based on material limits, ensuring the mechanical integrity of the quartz window, and other technical issues; though some of them are imposed via a penalty method. The design space is explored through parametric studies and a multidisciplinary approach is adopted. The aluminum oxide walls, the 45º spherical-cap window and the 45º wall-angle receiver are preferred due to their best compromise between thermal efficiency and wall temperature.
19th SolarPACES International Conference, Las Vegas, NV, USA; 09/2013
[Show abstract][Hide abstract] ABSTRACT: The concept of absorbing concentrated solar radiation volumetrically, rather than on a surface, is being researched by several groups with differing designs for high temperature solar receivers. The Small Particle Heat Exchange Receiver (SPHER), one such design, is a gas-cooled central receiver capable of producing pressurized air in excess of 1100 C designed to be directly integrated into a Brayton-cycle power block to generate electricity from solar thermal power. The unique heat transfer fluid used in the SPHER is a low-density suspension of carbon nano-particles (diameter ∼ 200 nm) to absorb highly concentrated solar radiation directly in a gas stream, rather than on a fixed absorber like a tube or ceramic foam. The nano-particles are created on-demand by pyrolyzing a small flow of natural gas in an inert carrier gas just upstream of the receiver, and the particle stream is mixed with air prior to injection into the receiver. The receiver features a window (or multiple windows, depending on scale) on one end to allow concentrated sunlight into the receiver where it is absorbed by the gas-particle suspension prior to reaching the receiver walls. As they pass through the receiver the carbon nano-particles oxidize to CO2 resulting a clear gas stream ready to enter a downstream combustor or directly into the turbine. The amount of natural gas consumed or CO2 produced is miniscule (1–2%) compared to what would be produced if the natural gas were burned directly to power a gas turbine.The idea of a SPHER, first proposed many years ago, has been tested on a kW scale by two different groups. In the new work, the engineering for a multi-MW SPHER is reported. An in-house Monte Carlo model of the radiation heat transfer in the gas-particle mixture has been developed and is coupled to FLUENT to perform the fluid dynamic calculations in the receiver. Particle properties (size distribution and complex index of refraction) are obtained experimentally from angular scattering and extinction measurements of natural gas pyrolysis in a lab-scale generator, and these are corroborated using image analysis of Scanning Electron Microscope (SEM) pictures of particles captured on a filter. A numerical model of the particle generator has been created to allow for scale-up for a large receiver. We have also designed a new window for the receiver that will allow pressurized operation up to 10 bar with a 2 m diameter window. Recent progress on overcoming the engineering challenges in developing this receiver for a prototype test is reported.
ASME 2012 6th International Conference on Energy Sustainability collocated with the ASME 2012 10th International Conference on Fuel Cell Science, Engineering and Technology; 07/2012
[Show abstract][Hide abstract] ABSTRACT: The Mie model is widely used to analyze light scattering from particulate aerosols. The Diesel particle scatterometer, for example, determines the size and optical properties of Diesel exhaust particles that are characterized by the measurement of three angle-dependent elements of the Mueller scattering matrix. These elements are then fitted by Mie calculations with a Levenburg-Marquardt optimization program. This approach has achieved good fits for most experimental data. However, in many cases, the predicted complex index of refraction was smaller than that for solid carbon. To understand this result and explain the experimental data, we present an assessment of the Mie model by use of a light-scattering model based on the coupled-dipole approximation. The results indicate that the Mie calculation can be used to determine the largest dimension of irregularly shaped particles at sizes characteristic of Diesel soot and, for particles of known refractive index, tables can be constructed to determine the average porosity of the particles from the predicted index of refraction.
[Show abstract][Hide abstract] ABSTRACT: Aerogels containing both Al2O3 and Cr2O3 were prepared by the reduction, by alcohols, of a precursor salt solution derived from Al(OH)3 and CrO3, followed by supercritical drying in either CO2 or ethanol. TEM analyses showed a microstructure typical of aerogels, with a connected matrix of ∼10-nm diameter particles and an open pore network. Subsequent thermal processing converts the initial aerogels to a high surface-area material comprised of Al2O3 and Cr2C3. Addition of ∼6% SiO2, relative to Al results in an increased retention of surface area at high temperatures. Surface areas of the aerogels after supercritical drying ranged from 240 to 700 m2/g, while after treatment at 1000 °C values ranged from 110 to 170 m2/g. The composition which showed the greatest temperature stability was 2(0.94Al2O3 · 0.06SiO2)Cr2O3. After treatment at 1000 °C, all a samples contained a large number of crystallites of the Cr2O3 phase, eskolaite, with diameters ranging from 0.5 to 1.0 μm. An additional unidentified phase may also be present. The presence of these larger crystallites leads to a lower transmittance in the near-IR due to increased scattering.
[Show abstract][Hide abstract] ABSTRACT: We report on a new optical method to observe the onset of aggregation in alcoholic tetraethoxysilane (TEOS) sols using laser speckle contrast measurements. The contrast in a speckle image produced by coherent light provides information about the internal contrast of the medium being studied. For silica sols, changing the amount of acid or base catalyst was the most important factor in determining the aggregation behavior of the sol. We investigated this effect by varying the TEOS/base ratio by a factor of 6. This shifted the onset of aggregation as determined by speckle contrast from ∼46% of the gel time for the lowest amount of base to ∼74% for the highest. Conversely, varying TEOS/acid ratio by a factor of 3, shifted the onset of aggregation from ∼74% of the gel time for the lowest amount of acid to ∼64% for the highest. Measurements of this type provide information that can be used to test models of sol aggregation and gel formation.
[Show abstract][Hide abstract] ABSTRACT: Chitosan, a polymer that is soluble in dilute aqueous acid, is derived from chitin, a natural polyglucosamide. Aquagels, where the solid phase consists of both chitosan and silica, can be easily prepared by using an acidic solution of chitosan to catalyze the hydrolysis and condensation of tetraethylorthosilicate. Gels with chitosan/tetraethoxysilane (TEOS) mass ratios of 0.1–1.1 have been prepared by this method. Standard drying processes using CO2 give the corresponding aerogels. The amount of chitosan in the gel plays a role in the shrinkage of the aerogel during drying. Gels with the lowest chitosan/silica ratios show the most linear shrinkage, up to 24%, while those with the highest ratios show only a 7% linear shrinkage. Pyrolysis at 700°C under nitrogen produces a darkened aerogel due to the thermal decomposition of the chitosan, however, the aerogel retains its monolithic form. The pyrolyzed aerogels absorb slightly more infrared radiation in the 2–5 μm region than the original aerogels. B.E.T. surface areas of these aerogels range from 470 to 750 m2/g. Biocompatibility screening of this material shows a very high value for hemolysis, but a low value for cytotoxicity.
[Show abstract][Hide abstract] ABSTRACT: Light scattering methods have previously been used to monitor the formation of gels. In this report we present new light scattering techniques to study the properties of silica alcogels during the aging process. Monitoring one particular polarization transformation of scattered light with time reveals a clear increase in internal strain in standing alcogels with time. The stress birefringence coefficient of an acid-catalyzed SiO2 gel was found to be 1.3 Brewsters. Additionally, the evolution of the stiffness of alcogels was investigated using laser speckle methods. Specifically, image analysis of specklegrams obtained during multi-frequency acoustic excitation of aging gels was used to non-destructively measure the hardening of alcogels. For an acid-catalyzed gel with a theoretical density of ∼0.05 g/cm3 SiO2, the rate of hardening is found to be greatest between gelation and 2×the gel time, and drops considerably thereafter. The Young's modulus of the gel can be monitored over time with this method and was found to range from 6.2×103N/m2 after 6 h to 2.2×105N/m2 after 24 h for acid-catalyzed silica gels.
[Show abstract][Hide abstract] ABSTRACT: Light scattering is a useful tool to evaluate aerogel clarity, study its structure, pore size, mechanical strain, and examine the modes of sol–gel evolution that determine its microstructure. Ultraviolet–visible transmission spectroscopy can be used to study the wavelength dependent scattering to readily compare aerogels of differing origins, thickness, and to evaluate effects of residual contaminants. Infrared reflectance measurements can be used to determine the effective real and imaginary indices of refraction of porous aerogel materials for material property and radiant heat transfer studies. Measurements of scattering at a fixed angle can be used for quality control, to evaluate sources of scattering, and study inhomogeneities. Measurement of the Mueller matrix (describing the 16-element angle-dependent transformation of intensity and polarization of incident to scattered light) provides information about the anisotropy, large pore fraction, induced stresses, microstructure and inhomogeneities in the aerogel. The time evolution of scattering before and after gel formation gives information.
[Show abstract][Hide abstract] ABSTRACT: Photoluminescent silica aerogel acts as the active element of an optical sensor for molecular oxygen. The luminescent aerogel is prepared by the action of energized reducing gases on a standard silica aerogel. Intensity of aerogel photoluminescence decreases as the collision frequency between oxygen molecules and the luminescent carriers in the aerogel matrix increases. This behavior is a characteristic of many photoluminescent materials and arises from a transfer of energy from the aerogel to surrounding oxygen molecules. A sensor for oxygen concentration or air pressure can therefore be simply constructed utilizing an ultraviolet source for excitation and a suitable detector for the emitted visible signal. Stern–Volmer quenching constants for the aerogel sensing element are 1.55×10−2 Torr−1 for hydrophilic aerogel and 2.4×10−3 Torr−1 for hydrophobic aerogel.
[Show abstract][Hide abstract] ABSTRACT: The skeletal structure of aerogel is determined before, during, and after the gel is formed. Supercritical drying of aerogel largely preserves the pore structure that is determined near the time of gelation. To better understand these gel formation mechanisms we carried out measurements of the time evolution of light scattering in a series of gels prepared without conventional acid or base catalysis. Instead, ultraviolet light was used to catalyze the formation of silica gels made from the hydrolysis of tetraethylorthosilicate and partly prehydrolyzed tetraethylorthosilicate in ethanol. Time evolution of light scattering provides information regarding the rate and geometrical nature of the assembly of the primary silica particles formed in the sol. UV-catalyzed gels show volumetric growth typical of acid-catalyzed gels, except when UV exposure is discontinued at the gel point, where gels then show linear chain formation typical of base-catalyzed gels. Long term UV exposure leads to coarsening of the pore network, a decrease in the clarity of the aerogel, and an increase in the surface area of the aerogel. Additionally, UV exposure up to the gel point leads to increased crystallinity in the final aerogel.
[Show abstract][Hide abstract] ABSTRACT: Photoluminescent silica aerogel acts as the active element of an optical sensor for molecular oxygen. The luminescent aerogel is prepared by the action of energized reducing gases on a standard silica aerogel. Intensity of aerogel photoluminescence decreases as the collision frequency between oxygen molecules and the luminescent carriers in the aerogel matrix increases. This behavior is a characteristic of many photoluminescent materials and arises from a transfer of energy from the aerogel to surrounding oxygen molecules. A sensor for oxygen concentration or air pressure can therefore be simply constructed utilizing an ultraviolet source for excitation and a suitable detector for the emitted visible signal. Stern-Volmer quenching constants for the aerogel sensing element are 1.55 × 10-2 Torr-1 for hydrophilic aerogel and 2.4 × 10-3 Torr-1 for hydrophobic aerogel.
Journal of Non-Crystalline Solids 04/1998; 225(1):343-347. · 1.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The reaction of hydrogen peroxide with excess titanium metal produces rigid titanium oxide aquagels. Subsequent solvent exchanges with ethanol and carbon dioxide, and supercritical drying produces the corresponding aerogels. The aerogels are translucent yellow in appearance, are amorphous to X-rays, and have a BET surface area of 350 m2/g. The empirical formula of the material, as prepared, is TiO3H2.7C0.35. Infrared spectroscopy indicates the presence of peroxide and carbonate groups. The microstructure of the aerogel consists of a network of elongated particles 2–5 nm in diameter and tens of nm in length. Thermal treatment under argon at 473 K causes rapid decomposition of the aerogel, forming a blue-gray powder consisting of a mixture of rutile and anatase with a surface area of 80 m2/g. Additional thermal treatment at 973 K under air forms predominantly rutile, with a surface area of 20 m2/g.
[Show abstract][Hide abstract] ABSTRACT: The intensity and polarization of light scattered from marine aerosols affect visibility and contrast in the marine atmospheric boundary layer (MABL). The polarization properties of scattered light in the MABL vary with size, refractive index, number distributions, and environmental conditions. Laboratory measurements were used to determine the characteristics and variability of the polarization of light scattered by aerosols similar to those in the MABL. Scattering from laboratory-generated sea-salt-containing (SSC) [NaCl, (NH(4))(2) SO(4), and seawater] components of marine aerosols was measured with a scanning polarization-modulated nephelometer. Mie theory with Gaussian and log normal size distributions of spheres was used to calculate the polarized light scattering from various aerosol composition models and from experimentally determined distributions of aerosols in the marine boundary layer. The modeling was verified by comparison with scattering from distilled water aerosols. The study suggests that polarimetric techniques can be used to enhance techniques for improving visibility and remote imaging for various aerosol types, Sun angles, and viewing conditions.
[Show abstract][Hide abstract] ABSTRACT: The angle- and polarization-dependent light scattering were measured for oriented first-year and multiyear sea ice taken from the Chukchi Sea near Pt. Barrow, Alaska. The entire Mueller matrix for these samples was determined at 532 nm. Mueller matrices were also determined for artificially grown saline ice samples and melted samples of the respective ice types. Phase functions for thin-slab samples are qualitatively consistent with calculations for scattering from brine inclusions in a solid ice medium and depend strongly on the shape of the scattering sample. Small orientation-dependent effects are observed for scattering from oriented sea ice. A simple model is used to describe qualitatively some features of the measured sea ice Mueller matrices. This model combines the effects of scattering from spherical inhomogeneities and the intrinsic birefringence of pure water ice. A set of Mueller matrix inequalities is presented and used to obtain physical insight into the measurement results.
[Show abstract][Hide abstract] ABSTRACT: An apparatus was developed for the measurement of thermal conductivity under steady conditions. The apparatus uses a 10 nm thick gold film heater that is coated on a polyester sheet. The important advantages of this heater are spatial uniformity of the heat generation and a small thickness. The apparatus is compact and has only a small edge loss of heat. The apparatus was used to measure the thermal conductivity of silica aerogel. The heat transfer modes in silica aerogel include gas and solid conduction and thermal radiation. The experiments provide results for the total heat transfer and hence, total (apparent) thermal conductivity. The separate solid and radiative contributions were obtained from analyses of the energy transfer. The solid conductivity of silica aerogel varies with temperature more slowly than the radiative conductivity does. The Rosseland absorption coefficient of silica aerogel decreases with temperature. Assuming one-dimensional heat transfer, the uncertainty of the results for the thermal conductivity is about 5.5%. A three-dimensional analysis is used to evaluate the validity of the assumption of one-dimensional heat transfer in the apparatus. The results shows that the assumption of one-dimensional heat transfer in the apparatus results in a reduction of the thermal conductivity of 1.7% at 20°C and a 7.3% reduction at 90°C.
International Journal of Heat and Mass Transfer 07/1996; 39(11):2311–2317. DOI:10.1016/0017-9310(95)00307-X · 2.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this work the normal reflectance, R, at a planar silica aerogel interface and the normal transmittance, T, of a silica aerogel slab were measured using a Fourier Transform Infrared Spectrometer. Two procedures were used to obtain the effective optical constants, i.e., the refractive index n and the absorption index κ, of silica aerogel. One procedure determined κ from the measured transmittance T and then determined n from the results for κ and from the measured reflectance R using the Kramers–Kronig relation; the other procedure determined n and κ of silica aerogel from n and κ of fully dense silica glass by using the Clausius–Mossotti equation, Maxwell Garnett formula, and Bruggeman formula. The first procedure has a relatively large error due to the inaccuracy of the transmission and reflection measurements. The second procedure, especially the Clausius–Mossotti equation, yields values of n that are consistent with experiments and may be used for the calculation of the effective optical constants and the extinction coefficient of silica aerogel.
[Show abstract][Hide abstract] ABSTRACT: Nanocomposite materials based on silica aerogel hosts have been produced using chemical vapour infiltration/decomposition methods and characterized by X-ray diffraction and electron microscopy. Amorphous tungsten in SiO2 aerogel was formed by the decomposition of W(CO)6 at 250 C. Alternatively, reaction of this material with sulphur at 700 C produced needle-shaped WS2 crystals with lengths ranging from 25–230 nm. Reaction of the W/SiO2 composite with anhydrous NH3 formed crystals of -WN with diameters of 1–5 nm. Fe(CO)5 is readily absorbed into the silica aerogel, forming an amorphous iron oxide/SiO2 composite after slow oxidation in air. Treatment of this material with additional Fe(CO)5 produced an Fe3O4/SiO2 aerogel composite. Fe3O4 particle sizes were 20–55 nm. After additional heat treatment, this composite exhibited soft ferromagnetic behaviour with a coercivity of 170 Oe. Fe9S10 crystals with diameters of 30–90 nm were formed by the reaction of the amorphous iron oxide/SiO2 composite with H2S at 900 C.