Co‐Al2O3 composite films were produced by electron‐beam coevaporation in a system with elaborate process control. The deposits were analyzed by transmission electron microscopy, electron diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering, field ion microscopy, mechanical stylus measurements, and electrical dc and ac measurements. A uniform separated‐grain structure with regular hcp Co particles embedded in Al2O3 was found for Co contents (fCos’) up to ∼30 vol %. The complex dielectric permeability e was evaluated in the 0.3≲λ≲40‐μm wavelength range for samples with 0.11≲ fCo ≲0.60 by carefully selected combinations of spectrophotometric transmittance and reflectance data. Numerical accuracy and internal consistency were investigated. Effective medium theories for e were derived by applying classical scattering theory to spherical random unit cells defined so as to properly represent a number of typical microgeometries. The formulations due to Maxwell Garnett, Bruggeman, and others were thus rederived in a unified way. Large‐size limits of validity and extensions to nonspherical particles were treated. It was found that the Maxwell Garnett theory could reproduce the experimental e’s in detail at low fCo provided that some Co was taken to be dispersed in the insulating matrix. At larger fCo’s we found discrepancies which are likely to be due mainly to dipole‐dipole coupling among adjacent particles. A comparison of the experimental e’s and the rigorous Bergman–Milton bounds, which hold irrespective of detailed microgeometry, gave several interesting results: the e’s varied monotonically along one of the bounds for isotropic materials as fCo was increased, at large fCo we found evidence for anisotropy, and at λ≊12.5 μm we noted certain cases of disagreement with even the most generous bounds. The empirical e’s were used to construct surfaces which combine a high solar absorptance as with a low hemispherical thermal emittance eH. From a computer optimization study we found that as =0.95 and eH =0.07 could be obtained with 0.07 μm of Co‐Al2O3 ( fCo≊0.6) antireflected with 0.07 μm of Al2O3 and laid on Ni. These results were verified by measurements on samples which approximate the ideal design.

Optical properties and solar selectivity of coevaporated Co‐Al2O3 composite films

Heat transfer analysis of parabolic trough solar receiver

Concentrated solar power plants (CSPs) are gaining momentum due to their potential of power generation throughout the day for base load applications in the desert regions with extremely high direct normal irradiance (DNI). Among various types of the CSPs, solar tower power technologies are becoming the front runners especially in the United States and around the world with the possibility to compete with traditional power generation technologies in terms of efficiency and levelized cost of electricity (LCOE). A bibliometric analysis of the publications on the CSP systems and components since 1990 shows a total of 6400+ publications and reveals an exponential growth due to reasons that CSP systems promises a lot of potential as the future large scale power source for varied applications. This review consolidates the benefits and challenges of the CSP technologies particularly in the desert regions. Thorough literature analysis as well as the meteorological data projects the trend that the CSP systems would become a reality in the Middle East and North Africa (MENA), Australia, Southwestern region of the United States, Southwestern part of China and China/Mongolia border with high direct normal irradiance. However, enormous amount of support and capital investments are needed for making these CSP systems realistic as there is not much power grid network in existence. It is evident that there are multiple challenges specifically in water consumption, materials design and development for the optimum heat transfer fluid, thermal energy storage and receiver subsystems in addition to commercial viability and environmental impacts. Each of the challenges is discussed in detail and suggestions are made to address the challenges.

Prospects and problems of concentrating solar power technologies for power generation in the desert regions

A comprehensive classified list of annotated references on surfaces which combine high solar absorptance with low thermal emittance is presented, covering the period 1955–1981. The list embraces papers printed in scientific journals and in published conference proceedings. An index of authors and on surface coatings studied is appended.

Surfaces for selective absorption of solar energy: an annotated bibliography 1955–1981

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Selective solar absorbers

Free molecule thermal conduction in concentric tubular solar collectors

A graded surface coating is reactively sputtered onto a tubular substrate by advancing the substrate in an axial direction through a cylindrical sputtering chamber in the presence of a sputter supporting gas. The sputtering chamber includes a cathode liner from which metal is sputtered onto the substrate. A reactive gas is directed into the sputtering chamber from a feed point outside of the chamber, whereby reactive sputtering occurs within the chamber. The reactive gas is induced to flow into the chamber in a direction counter to the direction of advancement of the substrate whereby, as the substrate is progressively advanced through the chamber and the applied coating gradually increases in thickness, the proportion of the reactive gas constituent in the coating increases relative to the metal constituent proportion with increasing thickness of the coating.

Brian Window

Papers

Free molecule thermal conduction in concentric tubular solar collectors

Method of and apparatus for reactively sputtering a graded surface coating onto a substrate

Thermal conduction in evacuated concentric tubular solar energy collectors degraded by low pressure gas

Selective absorber design

Rapid simulated solar absorptance measurements on flat or curved surfaces