Solar flux is inherently of very high thermodynamic quality (low entropy). However, the terrestrial use of solar heat at high temperature can only be achieved by means of solar concentrators of the reflecting or refracting type. It is the purpose of this paper to describe the three major design variables of concentrators - optical efficiency, heat loss coefficient and heat removal factor. The optical efficiency no embodies many important concentrator properties including mirror surface reflectance and slope, tracking accuracy, receiver transmittance and absorptance and solar beam incidence angle effects. The heat loss coefficient Uc of a solar concentrator represents its thermal performance and depends upon the relative magnitudes of convection and conduction flux rates, surface emittances of receiver components and operating temperature relative to the environment. The third design parameter - the heat removal factor FR - is a measure of the efficiency of the receiver when viewed as a heat exchanger. Its value is governed by working fluid properties and flow rates as well as the thermal properties of the receiver material. The principal factors which determine the value of no, Uc and FR are described in the paper.
This paper explores recent work in low cost - low ratio concentrating solar collectors. To keep cost low, stationary systems are assumed, a concentration of at least two to three is desired. Relatively even energy distribution at the receiver, minimal loss of diffuse radiation and simple low cost surfaces are important additional criteria. A brief discussion of stationary imaging and non-imaging collectors is presented. Examples of non-imaging collectors incorporating booster and trapping designs indicates features of potentially cost effective system designs.
This paper describes some new configurations for linear incremental or Fresnel refractors and reflectors for solar energy concentration. The first system, a reflex type lens, uses crossed linear echelon elements and has convergence power in two dimensions. This refractor/reflector concentrates solar radiation to a spot focus. Large area solar concentrators can be sectionally constructed to provide high power solar flux concentration. The second type of echelon reflector forms a linear focus of incident solar radiation. The reflector is selectively tilted with respect to incoming solar radiation, such that the design eliminates all riser step blockage of radiation at the reflecting echelons. This results in a higher efficiency concentrator than is achievable with a normally oriented linear focus concentrator. Design parameters and a ray trace analysis are presented for both concentrator systems.
This paper describes a flat plate multiple pass solar collector which is believed to be new to the subject and gaps the need between concentrators (which typically are associated with high cost of manu-facture and vulnerability to the elements) and the conventional flat plate collectors (which typically reradiate as much energy as received on reaching a temperature level less than suitable for air conditioning. This new multiple pass flat plate collector performs optically by taking advantage of two fortuitous characteristics of plain water; one, high transparency in the visible spectrum, and two, near total opacity in the infrared. As solar energy heats the water, its potential radiative escape is via the infrared. A simple multiple pass system impacts the solar energy sequentially as it is blocked by opacity of the water in the earlier passages. The design is also such that it minimizes air convection losses by virtue of the layers of the multipass system. The function of the design has been confirmed by computer analysis. A model has been built and tests are in progress.
The fabrication of reflective parabolic surfaces for solar concentrating collector systems is fraught with the produceability problems of cost and accuracy. In the considerable experience at American Science and Engineering, Inc., and in the wide persual of the subject literature and pertinent company brochures, producing a reasonably accurate, stable trough type parabolic collector is aptly described by the buzz words "labor-intensive". In order to overcome this obstacle to the progress of this type of solar concen-trating collectors, a number of iterations at AS&E has led to the design and fabrication of an engineering model that is not only reasonably accurate, but is assembled from a few simple parts in moments. The design is the result of a continuous effort in solar energy at American Science and Engineering, Inc., and the present intention is to use the design towards fulfillment of the AS &E contract which is part of the DOE Solar Collector Research and Development Program.
Solar thermal energy collection at seven sites is calculated for air and vacuum flat plate collectors and a vacuum collector using cylindrical Winston concentration. The hourly intensity and angular distribution of solar radiation is predicted for the calculations using insolation measurements and an insolation model. Tracking configurations include two-axis, vertical axis, polar axis and east-west axis tracking. Fixed collector arrays are all tilted towards the equator at the latitude angle. These calculations indicate energy collection varies linearly with cloudiness index KT. Diffuse radiation is ≈ 15% of energy collected by concentrating collector arrays at low operating temperatures where large acceptance angles maximize energy collection. Smaller acceptance angles, required to maximize collection at higher operating temperatures, eliminate the advantage of Winston concentration for tracking arrays. Absorbers with high selectivity are necessary for high efficiency of fixed evacuated collectors at higher operating temperatures. Ratios of tracking to fixed collector energy collection increase rapidly between ≈ 200°C and ≈ 300°C making it likely that tracking becomes economically viable in this temperature range. These results are easily extended to other cylindrical concentrating collectors.
A solar furnace consisting of a paraboloid of revolution which tracks the sun and reflects radiation to a hyperboloid of revolution having a common focus with the paraboloid is analyzed to determine the concentration available, using various eccentricities. The hyperboloid, in turn, reflects radiation to a focal plane placed at various distances from the vertex of the paraboloid. The ideal concentration is determined using the largest radius of all the rays from the sun falling on the paraboloid, this radius being the dis-tance from the pierce point of the ray, in the focal plane, to the hyperboloid focal point. The concentrations can be augmented using a compound paraboloidal concentrator, and the ideal augmented concentration is developed for various combinations of eccentricity and vertex distances. The effect of scattering angles for the paraboloid and hyperboloid on the ideal concentration is shown separately and jointly.
High infrared reflectance, coupled with high solar absorptance, is required for efficient photothermal conversion. Converters can be fabricated by depositing an absorber on a highly reflecting metal. The absorber functions in the visible, yet becomes transparent in the near infrared, allowing the metal to suppress the thermal emittance. Economic considerations demand the use of thin films, rather than bulk materials. The thin film reflector must be capable of withstanding high temperatures of operation. Compatibility of the re-flector with the substrate below, and the absorber above, is required for long-time service. Highly reflective silver films suffer reflectance losses by agglomeration, and require stabilization layers. Refractory materials such as molybdenum avoid agglomeration at temperatures of operation of photothermal converters. Unlike other deposition methods, chemical vapor deposition (CVD) can produce molybdenum films with an infrared reflectance rivaling that of bulk molybdenum. CVD is a non-vacuum based technology with potential for sequential throughput fabrication. Studies are being undertaken to determine how sensitively the reflectance reacts to inclusions of impurities into the molybdenum. Thin film passivators deposited on the molybdenum prevent reflectance losses induced by oxidation, and insure high temperature survival of optimal reflectance. Complete converter stacks have been annealed at 550°C for over 1000 hours in air.
Efficient photothermal conversion requires surfaces of high solar absorptance and low thermal emittance. This can be accomplished by the tandem action of a good infrared reflector overlaid by a film of sufficient solar absorptance that is transparent in the infrared. Crystalline silicon is a suitable candidate for the absorber layer. Its indirect band gap, however, results in a shallow absorption edge that extends too far into the visible. In contrast, the absorption edge of amorphous silicon is steeper and located farther into the infrared, resulting in a larger solar absorptance. We report on the fabrication of amorphous silicon absorbers by chemical vapor deposition (CVD). Their optical and structural properties are determined as a function of the deposition temperature. We describe the effects of a progressive crystallization during anneal above 650 C and report the performance of converter stacks that are identical "twins" except for the use of a polycrystalline silicon absorber in one and an amorphous absorber in the other.
We report reflectance measurements on stainless steel colored in an acid solution. We have obtained a-values between 0.62 and 0.91 together with e = 0.1. The steel deteriorates at temperatures over 200°C; the deterioration mechanisms are dehydration and oxidation. We suggest that the colored steel should be stabilized at higher temperatures by coating it with a thin film such as Cr2O3, SiO2, Al2O3 or Si3N4.
The microstructure and reflective properties of "CHROM-ONYX" type of black chrome/metal selective absorber was investigated to obtain a better understanding of their influence upon the mechanism of wavelength selectivity. Hemispherical reflectance measurements were performed on seven samples. In this research, the best selectivity a/e ratio was found by these measurements to be 0.7 micron of black chrome on nickel and 1.0 micron of black chrome on copper. Both scanning and transmission electron microscopy were employed to study micro-structure and chemical composition. As a result of the combined studies, some consequences of black chrome thickness and the metallic substrate were determined. It was determined that black chrome consisted of a very fine metallic distribution of particles of chromium in the 100A range, suspended within a matrix of a chromium oxide phase. This assembly was, in turn, agglomerated into larger particles within the 0.05-0.3 micron size range. These larger particles formed a continuous network which constituted the surface coating.
The proper evaluation of selective surfaces requires that their optical properties and durability be determined under simulated operating conditions. The Measurement Laboratory at the University of Arizona's Optical Sciences Center has been involved in this area for the last six years. Recent developments include: (1) A transmission cell for the high temperature reflectometer is being designed and built to measure samples at high temperatures--800°C--under vacuum or controlled atmospheres. Such samples include thin film refractory coatings suitable for concentrating collector receivers and laser mirrors deposited on transparent substrates for transmission measurements from 0.38 to about 8 μm. (2) A new data processing system has been interfaced with the existing integrating sphere reflectometer. Sample-and-hold electronics process the "triple-beamed"--sample, reference, and background--signal and pass it to a M6800 based microcomputer. The latter averages the signal until a sufficient signal-to-noise level is reached and advances the dispersive optics to the next wavelength. (3) A cylindrical vacuum emissometer for the measurement of the total hemispherical radiative power loss from a heated sample is under consideration. Coupled with existing, specially designed furnaces for lifetime testing, the Measurement Lab will provide adequate simulation capabilities for testing selective surfaces operating in most standard collector configurations. (4) A Gaertner L119 ellipsometer is being extended into the near infrared to provide measurements of optical constants of absorber materials through the solar spectrum.
A technique using Fourier transform analysis which is suitable for measuring the specularity of solar glass components in the mrad and sub-mrad is discussed and demonstrated. A brief mathematical background as well as illustrative examples are included. A number of methods for image analysis are discussed with particular emphasis given to electronic integrating detectors. Typical Fourier plane image distributions are given for a few common solar utilization materials and details of the instrument used to produce the images are considered. The limitations and capabilities of various instruments are outlined along with methods for further enhancing the utility and sensitivity of the technique.
The inclination dependence of the sensitivity of Eppley Model 8-48 pyranometers has been measured indoor tests conducted at levels of illumination corresponding to roughly 500 and 1000 w.m. The sensitivity is found to decrease as the pyranometer is tilted from the horizontal, the magnitude of the decrease depending on the level of illumination as well as the angle of inclination. Outdoor comparisons of Eppley Model 8-48 and Model PSP pyranometers, although less well controlled, generally confirm the measured inclination dependence.
Accurate determinations of the widely varying spectral extinction of the direct solar beam by the earth's atmosphere has applications in several fields; these include solar energy, optical communication and remote sensing of atmospheric parameters. A system used to make extinction measurements at nine wavelengths in the 0.3 to-10 μm range is described and sample results are presented. Plans to use the data to study remotely the characteristics of clouds and aerosols, particularly the size distributions of the particles along the transmission path, are discussed.