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Segregated tandem filter for enhanced conversion efficiency in a thermophotovoltaic energy conversion system

Description: A filter system to transmit short wavelength radiation and reflect long wavelength radiation for a thermophotovoltaic energy conversion cell comprises an optically transparent substrate segregation layer with at least one coherent wavelength in optical thickness; a dielectric interference filter deposited on one side of the substrate segregation layer, the interference filter being disposed toward the source of radiation, the interference filter including a plurality of alternating layers of high and low optical index materials adapted to change from transmitting to reflecting at a nominal wavelength {lambda}{sub IF} approximately equal to the bandgap wavelength {lambda}{sub g} of the thermophotovoltaic cell, the interference filter being adapted to transmit incident radiation from about 0.5{lambda}{sub IF} to {lambda}{sub IF} and reflect from {lambda}{sub IF} to about 2{lambda}{sub IF}; and a high mobility plasma filter deposited on the opposite side of the substrate segregation layer, the plasma filter being adapted to start to become reflecting at a wavelength of about 1.5{lambda}{sub IF}.
Date: December 31, 1996
Creator: Brown, E.J.; Baldasaro, P.F. & Dziendziel, R.J.
Partner: UNT Libraries Government Documents Department

Spectrally selective glazings

Description: Spectrally selective glazing is window glass that permits some portions of the solar spectrum to enter a building while blocking others. This high-performance glazing admits as much daylight as possible while preventing transmission of as much solar heat as possible. By controlling solar heat gains in summer, preventing loss of interior heat in winter, and allowing occupants to reduce electric lighting use by making maximum use of daylight, spectrally selective glazing significantly reduces building energy consumption and peak demand. Because new spectrally selective glazings can have a virtually clear appearance, they admit more daylight and permit much brighter, more open views to the outside while still providing the solar control of the dark, reflective energy-efficient glass of the past. This Federal Technology Alert provides detailed information and procedures for Federal energy managers to consider spectrally selective glazings. The principle of spectrally selective glazings is explained. Benefits related to energy efficiency and other architectural criteria are delineated. Guidelines are provided for appropriate application of spectrally selective glazing, and step-by-step instructions are given for estimating energy savings. Case studies are also presented to illustrate actual costs and energy savings. Current manufacturers, technology users, and references for further reading are included for users who have questions not fully addressed here.
Date: August 1, 1998
Partner: UNT Libraries Government Documents Department

Characteristics of indium oxide plasma filters deposited by atmospheric pressure CVD

Description: Thin films of undoped and tin-doped In{sub 2}O{sub 3} are being investigated for use as plasma filters in spectral control applications for thermal photovoltaic cells. These films are required to exhibit high reflectance at wavelengths longer than the plasma wavelength {lambda}{sub p}, high transmittance at wavelengths shorter than {lambda}{sub p} and low absorption throughout the spectrum. Both types of films were grown via atmospheric pressure chemical vapor deposition (APCVD) on Si (100) and fused silica substrates using trimethylindium (TMI), tetraethyltin (TET), and oxygen as the precursors. Fourier Transform InfraRed (FTIR) spectroscopy was used to measure the filter transmittance and reflectance between 1.8--20 {micro}m. Nominal conditions used during the growth of undoped In{sub 2}O{sub 3} were a substrate temperature of 450 C and partial pressures of 1.4 {times} 10{sup {minus}4} atm. and 1 {times} 10{sup {minus}3} atm. for TMI and O{sub 2} respectively. The O{sub 2}/TMI partial pressure ratio and substrate temperature were systematically varied to control the filter characteristics. The plasma wavelength {lambda}{sub p} was found to be a sensitive function of these parameters. Post-growth annealing of the films was done in inert as well as air ambient at elevated temperatures, but was found to have no beneficial effect. Tin-doped In{sub 2}O{sub 3} was grown under similar conditions as above, with a typical TET partial pressure of 4 {times} 10{sup {minus}6} atm. Here also, the material properties and consequently the optical response were found to be strongly dependent on growth conditions such as O{sub 2} and TET partial pressures. Both undoped and tin-doped In{sub 2}O{sub 3} grown on fused silica exhibited enhanced transmittance due to the close matching of refractive indices of In{sub 2}O{sub 3} and silica. X-ray diffractometer measurements indicated that all these films were polycrystalline and highly textured towards the (111) direction. The best undoped and tin-doped In{sub ...
Date: July 1, 1995
Creator: Langlois, E.; Murthy, S.D.; Bhat, I.; Gutmann, R.; Brown, E.; Dziendziel, R. et al.
Partner: UNT Libraries Government Documents Department

High performance anti-reflection coatings for broadband multi-junction solar cells

Description: The success of bandgap engineering has made high efficiency broadband multi-junction solar cells possible with photo-response out to the band edge of Ge. Modeling has been conducted which suggests that current double layer anti-reflection coating technology is not adequate for these devices in certain cases. Approaches for the development of higher performance anti-reflection coatings are examined. A new AR coating structure based on the use of Herpin equivalent layers is presented. Optical modeling suggests a decrease in the solar weighted reflectance of over 2.5{percent} absolute as a result. This structure requires no additional optical material development and characterization because no new optical materials are necessary. Experimental results and a sensitivity analysis are presented.
Date: February 23, 2000
Creator: AIKEN,DANIEL J.
Partner: UNT Libraries Government Documents Department

Iterative retrieval of surface emissivity and temperature for a hyperspectral sensor

Description: The central problem of temperature-emissivity separation is that we obtain N spectral measurements of radiance and need to find N + 1 unknowns (N emissivities and one temperature). To solve this problem in the presence of the atmosphere we need to find even more unknowns: N spectral transmissions {tau}{sub atmo}({lambda}) up-welling path radiances L{sub path}{up_arrow}({lambda}) and N down-welling path radiances L{sub path}{down_arrow}({lambda}). Fortunately there are radiative transfer codes such as MODTRAN 3 and FASCODE available to get good estimates of {tau}{sub atmo}({lambda}), L{sub path}{up_arrow}({lambda}) and L{sub path}{down_arrow}({lambda}) in the order of a few percent. With the growing use of hyperspectral imagers, e.g. AVIRIS in the visible and short-wave infrared there is hope of using such instruments in the mid-wave and thermal IR (TIR) some day. We believe that this will enable us to get around using the present temperature - emissivity separation (TES) algorithms using methods which take advantage of the many channels available in hyperspectral imagers. The first idea we had is to take advantage of the simple fact that a typical surface emissivity spectrum is rather smooth compared to spectral features introduced by the atmosphere. Thus iterative solution techniques can be devised which retrieve emissivity spectra {epsilon} based on spectral smoothness. To make the emissivities realistic, atmospheric parameters are varied using approximations, look-up tables derived from a radiative transfer code and spectral libraries. By varying the surface temperature over a small range a series of emissivity spectra are calculated. The one with the smoothest characteristic is chosen. The algorithm was tested on synthetic data using MODTRAN and the Salisbury emissivity database.
Date: November 1, 1997
Creator: Borel, C.C.
Partner: UNT Libraries Government Documents Department

Dependence of the solar absorptance of selective absorber coatings on the angle of incidence

Description: The directional solar absorptances ..cap alpha../sub s/(theta) of samples of a number of selective absorber coatings have been determined. The spectral directional hemispherical reflectances plambda(theta;2..pi..) of each sample was measured over the wavelength range 0.3..mu.. to 2.5..mu.. at angles of incidence theta between 10/sup 0/ and 80/sup 0/. The quantity (1-plambda(theta;2..pi..)) was convoluted over an AM2 solar spectrum to obtain ..cap alpha../sub s/(theta) at each angle of incidence. The solar absorptance at near normal incidence varied from sample to sample and from coating to coating, as expected, given the present state of the art. All the absorptances show similar angular dependences, however. When normalized to unity at normal incidence, the data nearly describe a single curve, for which a power series in theta was found. For comparison, the solar absorptance was also determined for freshly prepared lamp black.
Date: January 1, 1977
Creator: Reed, K A
Partner: UNT Libraries Government Documents Department

Development of radiative-cooling materials. Final technical report: FY 1980-1981

Description: Work on research and development on glazing and selective emitter materials that will enhance day and night sky radiative cooling is described. The emphasis is on glazing development with a secondary interest in the appropriate selective emitter. The testing focused on the individual material properties. (MHR)
Date: January 1, 1981
Partner: UNT Libraries Government Documents Department

Development of surfaces optically suitable for flat solar panels. Final report

Description: This final report contains three principal items. The first describes a simple and novel reflectometer which can separately evaluate the spectral and diffuse reflectivities of surfaces. A phase locked detection system for the reflectometer is also described. The second item is a selective coating on aluminium potentially useful for flat-plate solar collector applications. The coating is composed of strongly bound copper oxide (divalent) and is formed by an etching process performed on an aluminium alloy with high copper content. Because of this one step fabrication process, fabrication costs are expected to be small. Process parameters, however, need further definition. The third item contains conclusions gleaned from the literature as to the required optical properties of flat plate solar collectors.
Date: August 1, 1979
Partner: UNT Libraries Government Documents Department

Durability testing of antireflection coatings for solar applications

Description: Antireflection (AR) coatings can be incorporated into highly transmitting glazings that, depending on their cost, performance, and durability of optical properties, can be economically viable in solar collectors, agricultural greenhouses, and PV systems. A number of AR-coated glazings have been prepared under the auspices of the International Energy Agency (IEA) Working Group on Durability of Materials for Solar Thermal Collectors. The AR coatings are of two types, including (1) various sol-gels applied to glass and (2) an embossed treatment of sheet acrylic. Typically, for unweathered glazings, a 4%--5% increase in solar-weighted transmittance has been achieved. For AR-coated glass, reflectance values as low as 0.5%--0.7% at selected wavelengths (680--720 nm) were obtained. To determine the durability of the hemispherical transmittance, several collaborating countries are testing these materials both outdoors and in accelerated weathering chambers. All materials exposed outdoors are affixed to mini-collector boxes to simulate flat-plate collector conditions. Results for candidate AR coatings weathered at geographically disperse outdoor test sites exhibit changes in spectral transmittance primarily in the high visible range (600--700 nm). Accelerated testing at measured levels of simulated solar irradiance and at different constant levels of temperature and relative humidity have been performed in different countries. Parallel testing with different levels of laboratory-controlled relevant stress factors permits the time-dependent performance of these materials to be compared with measured results from in-service outdoor exposure conditions. Coating adhesion and performance loss resulting from dirt and dust retention are also discussed.
Date: January 5, 2000
Creator: Jorgensen, G.; Brunold, S.; Koehl, M.; Nostell, P.; Roos, A. & Oversloot, H.
Partner: UNT Libraries Government Documents Department

Spectral selectivity of electrochromic windows with color state for all-sky conditions

Description: The optical performance of an electrochromic window is studied for the visible, ultraviolet, and near infrared spectral regions. The performance is found to deviate strongly with window color state and for clear or cloudy skies. A new spectral cloud model is applied to an electrochromic window recently developed at NREL. A spectral comparison is made between the electrochromic window and spectrally selective standard windows. Two series of double-glazed window sections, including the electrochromic window with color state and a series of low-E windows, were measured for transmittance and reflectance (300-2500nm), With these spectral data, a new near-infrared blocking (reflection + absorption) factor is developed for window application in warm climates for cooling load reduction. A chromaticity analysis is presented for both the daylight spectra and the transmitted electrochromic window spectra with color state, Computed daylight correlated color temperatures show a wide range, with values of 5660K for clear global irradiation, 6210K for clouds, and 13,250K for a zenith blue sky. Chromatic trajectories with color state for transmitted radiation extend further toward the blue to 8180K for the global and 28,990K for zenith sky irradiation.
Date: July 1, 1995
Creator: Soule, D.E.; Zhang, J.G. & Benson, D.K.
Partner: UNT Libraries Government Documents Department

Evaluation of an all-glass, evacuated, tubular, nonfocusing, nontracking solar collector array. First annual progress report, July 1, 1976--August 31, 1977

Description: A spaced, evacuated tubular radiation receiver employing a highly selective absorber surface and light enhancement features has been subject to extensive test and analysis. Air was used as the heat transfer fluid. The design, installation, test and analysis of the air cooled collector was conducted. The thermal performance of the air cooled collector was found to be better than predicted when rated on an all day operating basis. The high performance characteristic is enhanced by the features of ease of installation and freedom from maintenance and operational problems. High quality air or liquid may be provided (the latter by heat exchange) for a variety of load applications.
Date: December 1, 1977
Creator: Moan, K L
Partner: UNT Libraries Government Documents Department

Method of sputter etching a surface

Description: A surface is textured by co-sputter etching with a target and a seed material with the surface at a pre-selected temperature. By pre-selecting the temperature of the surface while sputter etching, it is possible to predetermine the reflectance properties of the etched surface. The surface may be textured to absorb sunlight efficiently and have minimal emittance in the infrared region so as to be well-suited for use as a solar absorber for photothermal energy conversion.
Date: February 26, 1982
Creator: Henager, C.H. Jr.
Partner: UNT Libraries Government Documents Department

Near-term improvements in parabolic troughs: an economic and performance assessment

Description: Improved parabolic-trough concentrating collectors will result from better design, improved fabrication techniques, and the development and utilization of improved materials. This analysis qualifies the performance potential of various parabolic-trough component improvements from a systems viewpoint and uses these performance data to determine the worth of each improvement on an economic basis. The improvements considered are evacuated receivers, silvered-glass reflectors, improved receiver, selective coatings, higher optical accuracy concentrations, and higher transmittance receiver glazings. Upper-bound costs for each improvement are provided as well as estimates of the increased solar system rates of return that are made possible by these improvements. The performance and economic potential of some of these improvements are shown to be substantial, especially at higher collector operating temperatures.
Date: August 1, 1981
Creator: Gee, R. & Murphy, L.M.
Partner: UNT Libraries Government Documents Department

Methods for reducing heat losses from flat plate solar collectors: Phase III. Final report, May 1, 1977-January 31, 1979

Description: The present study extends earlier studies by examining the effect of the emissivities of both the absorber plate and the glass cover on the honeycomb's ability to suppress free convection, and on the radiative and conductive heat transfer which takes place across the honeycomb in the absence of free convection. By means of heat transfer measurements on ten Mylar honeycombs, this study has shown that the effect of the above emissivities on the suppression of convection by the honeycomb is slight, and can probably be ignored. On the other hand, the measurements also showed that the effect of these emissivities on the non-convective heat transfer is substantial, but not nearly so large as would have been predicted by existing theories. To explain this latter result theories were developed which take into account the important coupling (ignored by previous theories) between the conductive and radiative modes of heat transfer. These new theories predict the measured heat transfer rates to with about +-15%, depending on the emissivities. The results of this study have direct application to evaluating solar collectors which combine a selective surface with a honeycomb.
Date: March 1, 1979
Creator: Hollands, K.G.T.; Raithby, G.D.; Russell, F.B. & Wilkinson, R.G.
Partner: UNT Libraries Government Documents Department

Black germanium selective absorber surfaces. Semi-annual progress report, September 1, 1978-March 1, 1979

Description: It was shown that by controlling the sputtering preparation conditions of noncrystalline Ge films the surface microstructure can be drastically altered by simply etching in 30% H/sub 2/O/sub 2/. The resulting surface has a total reflectance of less than 3% in the solar spectrum. This flat-black surface appearance is shown to be due to a dense array of aligned, needle-like protrusions which have an extremely high aspect ratio and both a cross-sectional area and a separation between needles on the order of the wavelength of solar radiation. A systematic series of thin films were prepared by rf-sputtering with rf-power and Ar gas pressure as the primary preparation parameters and characterizing the various films.
Date: April 23, 1979
Creator: Messier, R F & Vedam, K
Partner: UNT Libraries Government Documents Department

Solar-absorber-selective paint research

Description: Research and development on thickness-sensitive and thickness-insensitive solar paints are discussed. The thickness-sensitive paints include reverse roll coated, gravure printed, and spray coated paints. The coating methods and optical properties of the thickness-sensitive paints are discussed. The thickness-insensitive solar paints include a low emittance flake such as aluminium-flake, and pigment. Durability tests are discussed, including accelerated weathering and humidity durability tests, for the thickness-sensitive coatings. (LEW)
Date: January 1, 1982
Creator: Moore, S.W.
Partner: UNT Libraries Government Documents Department

Selective emitter. Task II topical report for radiative-cooling materials

Description: The theoretical basis of the selective emitter is that, if a material emits only in the 8-13 micron interval, it will absorb only in the same interval. After a survey of several candidate materials, the silicon oxynitride system was found to have better optical properties than previously used materials as well as economic feasibility, and efforts to acquire a sample are described. Other promising materials are described: polyvinylidene fluoride (Tedlar), silicon monoxide, silicon dioxide, and silicon nitride. (LEW)
Date: September 20, 1981
Partner: UNT Libraries Government Documents Department

Measurement of the total hemispherical emittance of copper and of spectrally selective surfaces. Report No. 2992

Description: The transient calorimetric technique is used to measure the temperature dependence of the total hemispherical emittance of polished copper, solar selective thin films on copper and a solar selective thin film on a metal film on quartz. Experimental results on copper do not agree with the predictions of the Davisson and Weeks theory. A new theory of the emissivity of metals based on the Drude Free Electron model is developed and compared with the expermental data on copper. With no adjustable parameters, good agreement between this theory and experiment is obtained.
Date: May 1, 1978
Creator: Smalley, R.
Partner: UNT Libraries Government Documents Department

Optimization of processing and modeling issues for thin film solar cell devices: Final report, February 3, 1997--September 1, 1998

Description: This final report describes results achieved under a 20-month NREL subcontract to develop and understand thin-film solar cell technology associated to CuInSe{sub 2} and related alloys, a-Si and its alloys, and CdTe. Modules based on all these thin films are promising candidates to meet DOE's long-range efficiency, reliability and manufacturing cost goals. The critical issues being addressed under this program are intended to provide the science and engineering basis for the development of viable commercial processes and to improve module performance. The generic research issues addressed are: (1) quantitative analysis of processing steps to provide information for efficient commercial-scale equipment design and operation; (2) device characterization relating the device performance to materials properties and process conditions; (3) development of alloy materials with different bandgaps to allow improved device structures for stability and compatibility with module design; (4) development and improved window/heterojunction layers and contacts to improve device performance and reliability; and (5) evaluation of cell stability with respect to device structure and module encapsulation.
Date: February 28, 2000
Creator: Birkmire, R. W.; Phillips, J. E.; Shafarman, W. N.; Hegedus, S. S. & McCandless, B. E.
Partner: UNT Libraries Government Documents Department

Chemical vapor deposition of refractory metal reflectors for spectrally selective solar absorbers. Annual report, May 1, 1978-April 30, 1979

Description: Three methods are described to fabricate by chemical vapor deposition (CVD) molybdenum films which rival conventional mirror materials in their infrared reflectance. With an absolute reflectance of 97.4% at 10 ..mu..m, these Super Molybdenum films reflect better than any other molybdenum film previously reported. When deposited in the presence of oxygen, films of a reduced reflectance over the solar emission range result. If passivated and antireflected by a Si/sub 3/N/sub 4/ layer, such Black Molybdenum films are characterized by a solar absorptance value in excess of 90% and a thermal emittance value of 11% at 500/sup 0/C. No deterioration of these values has yet been observed in ongoing tests after 1000 hours exposure to 500/sup 0/C in a 1 Torr vacuum. Stacks containing Super Molybdenum as the reflector and amorphous silicon as the absorber have thus far survived 1000 hours in open air at 500/sup 0/C.
Date: May 1, 1979
Creator: Seraphin, B.O. & Carver, G.E.
Partner: UNT Libraries Government Documents Department

A solar test collector for evaluation of both selective and non-selective absorbers

Description: A solar test collector was designed for the testing of thermally absorbing coatings under controlled conditions. The design consisted of a collector fed by a controlled temperature fluid within the range of 25 to 90/sup 0/C (77 to 194/sup 0/F). This temperature was maintained by a custom electronic controller. A small variable flow pump circulated water through three collector pipes at selected flow rates. Strip heaters coupled with a differential temperature controller compensated for edge losses associated with small collectors. Detailed design and operation data were presented and three black chrome and one non-selective absorber were analyzed in detail by test collector measurements. Results showed Efficiencies as high as 77% and 75% (..delta..T = 0) were obtained respectively for 1.0 ..mu..m black chrome on copper and nickel plated steel. The lowest loss coefficients were about 3.8 W/m/sup 2/ /sup 0/C for all black chrome/metal surfaces with the highest being 8.4 W/m/sup 2/ /sup 0/C for the black paint/metal sample.
Date: August 1, 1978
Creator: Lampert, C.M. & Washburn, J.
Partner: UNT Libraries Government Documents Department

Solar collectors. Part II. Recent developments and future performance data and economic analysis

Description: The concepts, materials, and collector types indicative of recent developments in solar collector research for increasing thermal performance are listed by category. The categories are: selective surfaces, honeycombs, all-glass evacuated tube collectors, non-tracking concentrating collectors, tracking concentrating collectors, and heat pipe collectors. (MHR)
Date: January 1, 1978
Creator: Collier, R.K.
Partner: UNT Libraries Government Documents Department