Following a program definition phase of 2 years, Task 18 of the International Energy Agency Solar Heating & Cooling program commenced a 5 year research phase in April 1992. Task 18 investigates a wide range of advanced glazing materials and glazing systems which include monolithic and granular aerogels, transparent insulation materials, chromogenic materials, evacuated glazings, low-emittance coatings, solar collector covers, angular selective transmittance thin films, holographic and light guide materials, and frame and edge seal technology. In addition to materials-centered research, Task 18 concentrates on measurement of key glazing properties such as total energy transmittance, U-value, and spectral directional optical properties, and through the use of building energy analysis software tools the identification of appropriate applications, control strategies, and energy and environmental benefits to be derived from advanced glazing products. Fifteen OECD countries are participating in Task 18 which is led by the United Kingdom.

Selective solar absorbers have been prepared for industrial applications with an original treatment -- chemical conversion of the surface -- of ferritic stainless steels. They present a high solar absorbance ((alpha) s approximately equals 0.95), a low emittance ((xi) approximately equals 0.20), a high thermal stability, and a high resistance to corrosion by atmospheric agents. Characteristics of the material have been thoroughly analyzed. Results are given and discussed. Physical chemical composition and texture of the coatings are very complex. The coating is a thin, microporous and rough medium presenting a fractal structure. The main component is Cr3+ substituted magnetite. Optical properties have been studied with the help of a simple model. In spite of the complexity of the material, the spectral selectivity can be explained and predicted taking chemical composition, texture, and depth of the coating into account.

TiNxOy films were prepared by activated reactive evaporation (ARE) on copper to produce a tandem solar absorber exhibiting a low emissivity (epsilon) (250 degree(s)C) 2/PO2) gas pressure ratios, show that maximum (epsilon) is obtained when the ratio PN2/PO2 is held approximately equals eight. The chemical composition of these TiNxOy films was determined by x-ray photoelectron spectroscopy (XPS) and related to solar selective properties. The investigated samples consist of about 38 - 42 atomic % titanium, 35 - 37 at% oxygen and 21 - 23 at% nitrogen (bulk values), depending on preparation conditions. A depth profile indicates an oxygen enrichment at the surface, partially due to the exposure of the samples to atmosphere. After sputtering the photoelectron spectra show a marked peak at the TiO and TiN positions and a weaker one at the TiO2 position. Therefore, with regard to the known composition, the film is mainly a TiO(-N) compound with a small admixture of TiO2. The surface layer is almost entirely consisting of TiO2. Additionally, there might also be a small amount of TiC (3 at% carbon impurity was detected).

Electrodeposited black nickel coating plated from chloride baths on metallic substrates has high solar absorptance coefficient (> 0.92) and low thermal emittance (< 0.15). The black nickel selective coating was formed of two layers of different porosity with non- overlapping pores. This coating is very stable under thermal implication and humid conditions.

TSSS paint coatings were prepared by using FeMnCuOx pigment in combinations with phenoxy and silicone resins. The spectral selectivity achieved was 0.92/0.14 for the first and 0.87/0.18 for the second type of the paint. The surface of phenoxy resin based coatings is characterized by pigment particles protruding from the surface, while silicone based paints exhibit a much smoother surface. Abrasion resistance and adhesion of both types of coatings were enhanced by cross linking the resin binder with diisocyanate hardener. Cured coatings withstand temperatures up to 150 degree(s)C but show degradation in 20 days when aged at 180 degree(s)C. Cross-linking and degradation mechanisms of both types of coatings were studied by using FT-IR spectroscopy and termogravimetric measurements, cycling and aging tests.

Most of the commercially available selective solar absorber coatings are produced by electroplating. Often the reproducibility or the durability of their optical properties is not very satisfying. Good reproducibility can be achieved by sputtering, the technique for the production of low-(epsilon) coatings for windows. The suitability of this kind of deposition technique for flat-plate solar absorber coatings based on the principle of ceramic/metal composites was investigated for different material combinations, and prototype collectors were manufactured. The optical characterization of the coatings is based on spectral measurements of the near-normal/hemispherical and the angle-dependent reflectance in the wavelength-range 0.38 micrometers - 17 micrometers . The durability assessment was carried out by temperature tests in ovens and climatic chambers.

A batch coater for depositing selective absorbing surfaces on the outside of 34 inner tubes was set up using cylindrical magnetron sputtering technique. The appropriate process has been carried out to produce graded Al-N/Al selective absorbing surfaces with optimum optical properties. The average solar absorptance of the surface was found to be about 0. 923 and emittance around 0. 06 ( lOOt),after evacuation. Three evacuation systems of 5 0 tubes each were designed and constructed with automatic evacuation process.The as-produced pressure in the tube in lower than 1 X 1O3Pa.

Heat mirrors based on thin silver and dielectric films for thermal solar energy applications at elevated temperatures (200 - 300 degree(s)C) are studied numerically. The well known three layer design dielectric/silver/dielectric can be improved by additional anti-reflective films. Thicknesses and selection of dielectrics of these more complex designs are optimized with a genetic algorithm. At absorber temperatures above 200 degree(s)C and for bulk optical constants the optimal sequence with five layers contains two silver films: dielectric/silver/dielectric/silver/dielectric. This design is compared to optimal designs with four and five layers, but only one silver layer. The five layer design with two silver layers allows a reduction in hemispherical emittance in comparison to the design with only one silver layer. An improvement of 14% in the figure of merit at a solar radiant flux of 500 W/m² and an absorber temperature of 300 degree(s)C is possible. However, optical constants of thin silver films differ substantially from bulk optical constants. Thickness dependent optical constants modeling experimental data were evaluated within a Drude model. The figure of merit is generally smaller than for bulk optical constants. For the design with two silver layers, the figure of merit is smaller than for designs with one silver layer, except at the highest investigated absorber temperature of 300 degree(s)C.

Triple layer structures of TiO₂/TiN/TiO₂ and quadruple layer structures of TiO₂/Al/TiN/TiO₂ have been sputtered on glass substrates at temperatures ranging from room temperature to 300 degree(s)C. The reflectance and transmittance were measured in the visible and the near infrared wavelength regions. Accelerated degradation tests with respect to high temperature and acid exposure have been performed with these laboratory samples of low-e coatings and the degradation has been compared with that of commercial silver based window coatings. As expected the durability of the nitride based coatings is far superior to the stability of those based on noble metals. Furthermore, the nitride coatings with an aluminum sacrificial layer have been found to resist aging at elevated temperatures (as high as 350 - 400 degree(s)C) far better than similar coatings without the aluminum. It has also been shown that the aluminum layer protects the nitride film during deposition of the top oxide layer. The effects of high temperature annealing have been modeled with optical multilayer calculations. Comparison of two degradation mechanisms demonstrates that the silver layers fail by agglomeration while the nitride suffers successive oxidation. This explains the effectiveness of the aluminum layer which forms a dense oxide during the initial stages of TiO₂- deposition.

When appropriately coated, suspended polymeric films can substantially improve the energy efficiency of insulated glass units or IGUs. For example, when an induced-transmission low- emissivity film, such as PET coated with a dielectric/metal/dielectric or DMD stack, it suspended within an IGU, the thermal conductivity of the unit is reduced considerably. This is a consequence of both the attenuation of heat radiation from the low-emissivity surface and the reduction of convective heat transfer due to the additional partition created by the suspended film. If an IGU with more solar screening is desired, the low-=emissivity DMD coating can be tuned to decrease solar and visible transmission while maintaining relatively constant coloration. An alternative approach for increasing solar screening which affects visible optics less, is to replace the DMD coating with a Fabry Perot or DMDMD filter. As a final example of the utility of coated suspended films, an IGU with an exceptionally high energy rating can be obtained by combining an antireflected film with a pyrolytic low-emissivity glass. The antireflected film serves as a light weight `invisible' partition which decreases thermal conductivity with an insignificant attenuation of solar radiation.

Angle dependent light control film (LCF) is a new film which scatters only incident lights from particular angles and transmits incident lights from all other angles. This unique light control function of LCF is caused by the microstructure in itself. The optical performance and properties described in this paper support that the principle of an angle dependent light control function can be explained mainly by diffraction due to the microstructure like a stack of various transmission volume phase gratings.

We have studied the degradation of cermet selectivity solar absorbing coatings produced by magnetron sputtering. The samples consisted of a reflector layer of molybdenum, a graded Mo-Al₂O₃ cermet layer, and an antireflection layer of aluminum oxide. The coatings were exposed to temperatures between 673 K and 1123 K in an atmosphere where the pressure was kept below 10^-4 Torr. The degradation of the solar absorptance was determined as a function of time and temperature. A remarkable stability at high temperatures in vacuum is reported. The optical properties could be interpreted within a multilayer optical model employing the Bruggeman effective medium theory for the cermet layer. It was found that the metal content of the cermet layer continuously decreased as the degradation proceeded. We assume that oxygen ions move through the amorphous aluminum oxide in microchannels and subsequently oxidize the molybdenum particles in the cermet layer. Theories for oxidation by anion movement are reviewed and used in order to interpret the experimental degradation kinetics.

Solar and visual light transmittance, color appearance, thermal emissivity, and other optical properties of architectural glazing are in general angular dependent. Realistic computation of solar properties, therefore, requires the angular behavior to be known. Determination of these properties for normal or near-normal incidence according to the standards is part of a standard procedure for most laboratories, while experimental data on their angular behavior can, in general, only be obtained with great difficulty. An alternative way of determining angular properties is by calculation. This is possible using a model based upon Fresnel's equations and the optical constants of all optical media involved (glass and coatings). The optical constants (spectral complex refractive index) can be derived from the transmittance and reflectance spectra which are measured for normal or near-normal incidence. The paper discusses in detail both the measurement methods and the computer model for the determination of directional and hemispherical solar energy transmittance of single and double glazing. Results are given for single and double glazing with and without spectrally selective coating (SnO₂).

An apparatus is presented for measuring the hemispherical emittance (epsilon) in a temperature range of 100 - 400 degree(s)C. It is designed especially for samples with low values of (epsilon) between 0.015 and 0.15. The error margin lies around 6% of the measured value. A polished stainless steel sample (PTB, Braunschweig, Germany) serves as reference reproducing the PTB-results within 2%. Measurements of solar selective absorbers, a TiN_xO_y-Cu tandem absorber, a multilayer cermet absorber, and an Al₂O₃-Ni-Al absorber have been carried out. The values of (epsilon) range between 0.02 and 0.15 for 100 degree(s)C and between 0.035 and 0.3 for 400 degree(s)C. TiN_xO_y-Cu achieves the lowest values in both cases. The effects of temperature on the optical constants in the near IR of TiN_xO_y-Cu tandem layers are discussed. The results of measurements indicate a shift from dielectric to metallic behavior with rising temperature.

Plastic foils containing nonabsorbing pigments can display a high reflectance of solar radiation combined with a high transmittance in the atmospheric window region in the thermal infrared. Such foils can be applied as selective covers enabling radiative cooling of an underlying material during the night and avoiding heating in direct sunlight during the day. The foils could be used for condensing water or cooling food, buildings, etc. We have prepared ZnS pigmented polyethylene foils with various thicknesses and volume fractions of pigments. The optical properties of the foils were compared with theory, and good agreement was achieved for pigment volume fractions up to 0.1. The optimum solar reflectance of the foil is 0.825 for the available pigment powder; but should be 0.88 if heating were to be avoided at noon with the sun in its zenith. The cooling power for a radiator covered with the best sample is 52 Wm^-2 at night, and the equilibrium temperature of a radiator is 12 K below the ambient temperature. Heating of the radiator is avoided 18 hours per day, and the radiator temperature at noon is 8 K above the ambience.

Smooth TiN_xO_y-Cu absorbers show low hemispherical emittancies (epsilon) < 0.05 at 250 degree(s)C and poor absorptances of < 0.08, when choosing optimal thickness and composition. Thin TiN_xO_y films were deposited on rough copper substrates in order to increase the absorptance. Specular and diffuse reflectance measurements of these coatings are compared with measurements on smooth films. An increase of 0.05 ... 0.1 in absorptance can be achieved. With moderate roughnesses (delta) ₀₁ approximately equals 50 ... 60 nm, the absorptance can be increased to values above 0.85 without increasing the emittance significantly. Thin films of approximately equals 30 nm and moderate roughness (delta) ₀₁ equals 55 nm should be favored against thick films because the emittance is lower at high temperature. A theoretical model is presented, explaining the experiments for not too rough interfaces. This model is suitable to be used for optimizations, provided the temperature dependence of optical constants is taken into account. Coatings on very rough substrates (delta) > 150 nm can achieve absorptances of more than 0.9 but accompanied with emittancies greater than 0.1. The interference peak at short wavelengths, typical for TiN_xO_y-Cu, cannot be suppressed completely; (alpha) is limited by this interference peak. For high temperature applications roughnesses greater than 60 to 80 nm are of no use.

Spectrally selective absorption coatings for solar thermal energy conversion applications should exhibit a high absorptance in the 0.4 - 1 micrometers wavelength range of the electro magnetic spectrum. Additionally, a low emittance in the thermal radiation range has to be attained. Niobium films displaying such optical properties were obtained by sputtering onto Si(100) and Mo substrates and suitable variation of the preparation parameters. The sputtered films were characterized by in situ photoelectron spectroscopy, and not in situ optical reflectometry and scanning electron microscopy (SEM). Emittance and absorptance were determined as function of temperature by the `dynamical vacuum technique.' It is found in particular that the as prepared films show a dendritic surface structure as revealed by SEM. In addition, the wavelength of transition from highly absorbing to reflecting character is shown to correlate with the inter-dendrite distance.

This paper describes the development of electro-deposited selective cobalt-cadmium coating for solar thermal energy conversion. Electrolyte composition and operating conditions were standardized by the use of Hull Cell. Ammonium thiocyanate has been used as a complexing agent to produce quality black coating with good optical properties. Cetyl trimethyl ammonium bromide (CTAB) has been used as an addition agent to improve the physical properties of the deposit. The present system has been studied in the current density range 1.6 to 8 A/dm². Solar absorptance ((alpha) ) of 0.94 and thermal emittance ((epsilon) ) of 0.1 were achieved. A thermal cycling test was also carried out. It showed decrease in the thermal emittance ((epsilon) ) and increase in the solar absorptance ((alpha) ) and thereby shows its ability for use in high temperature solar thermal energy conversion systems.

CuO thin solid coatings on glass and on stainless steel substrates have been prepared by the dip-coating technique. Sols have been made from a cupric acetate precursor using citric acid as a chelating agent. Scanning electron microscopy (SEM) was used for the determination of the structure of the coatings. These consist of particles with the narrow size distribution. The size of particles is around 0.14 micrometers . Specularly reflecting CuO films on stainless steel and glass substrates were prepared in the thickness range from 0.04 micrometers to 0.7 micrometers . The spectral selectivity of the CuO/stainless steel tandem coating was in the range a_s < 0.90 - 0.85 and e_T > 0.10 -0.30. Due to the high refractive index of the CuO film the reflectance of the coatings was diminished by the introduction of fumed silica particles in the initial sols which gave a mat appearance to the coatings and increases their solar absorptance for 5%. The CuO coatings produced on glass exhibit a reddish-purple color with chromaticity coordinates which values change as a function of the film thickness. The structural features of the gel and xerogel from which the final oxide coating has been made are discussed on the basis of their FT-IR spectra.

The optical selectivity of nickel pigmented anodized aluminum with different surface roughness of the aluminum substrate has been compared. The hemispherical emittance at 100 degree(s)C has been measured and normal emittance calculated from spectral reflectance for aluminum oxide films of different thickness, pigmented and unpigmented. The discrepancy between normal and hemispherical emittance was large especially for unpigmented aluminum oxide. One reason for this is found from non normal reflectance measurements in the infrared wavelength range. The absorption at the wavelengths of the optical longitudinal phonon of aluminum oxide is strong for oblique incidence and, therefore, enhances the thermal hemispherical emittance at this temperature. We, therefore, stress the importance of measuring the hemispherical emittance when characterizing a solar absorber surface of this material.

Sb doped and undoped SnO₂ thin solid coatings were prepared by the dip-coating technique via the sol-gel route using a SnCl₄ precursor. Aqueous gels of undoped compound were made according to Hiratsuka and doping was achieved through the addition of SbCl₃ in the concentration range of 1 - 10 mol%. FT-IR spectroscopy was utilized to determine the influence of Sb dopant on the vibrational modes of oxohydroxo skeleton of the gel. It was found that oxo bridges between Sb dopants and tinoxohydroxo polymeric species were favored in doped gels. Doping brings about a broadening of the x-ray diffraction peaks and increases the infrared reflectivity of the samples (approximately 50%) in the spectral range 4000 - 600 cm^-1, due to the formation of the plasma mode. The dip-coating process gives 0.03 - 0.10 micrometers thick films per dipping, depending on the viscosity of the gel and the dopant concentration. The lowest electrical resistivity of the doped SnO₂ coating is 1.2.10^-2 (Omega) cm and is comparable to the electrical resistivity of the alkoxide derived Sb doped SnO₂².

SnO_x films doped with Sb and/or F were made by reactive rf magnetron sputtering of Sn and Sn/Sb alloys in Ar+O₂(+CF₄) onto glass. Electrical dc resistivity, mobility, free electron concentration, spectral optical properties, and structural properties were investigated as a function of sputtering parameters. Optimized deposition parameters gave SnO_x:(Sb,F) films with high luminous transmittance, low luminous absorptance, high IR reflectance and dc resistivity down to 9.1 (DOT)10^-4 (Omega) cm. X- ray diffraction investigations showed that the sputtered SnO_x films had a preferred direction of growth, dependent on the oxygen amount as well as specific kind of doping. The optical and electrical properties were quantitatively calculated from theories embodying ionized impurity scattering and Drude free-electron theory and compared to experimental measurements.

The performance of solar water heater varies depending upon many parameters, such as operating temperature, mass flow rate, solar insolation, collector orientations, solar time, wind conditions, ambient temperature, selectivity of the absorber surface, etc. A Linear Fresnel reflecting concentrator in a planar configuration using commercially available mirror strips of reflectivity approximately 0.7 in conjunction with a reverse flat plate selective and non-selective absorber surface has been designed and developed. The performance studies of the solar water heater-cum-steam cooker have been carried out. The efficiencies of the devices were found to be about 60% and 55% for selective and non-selective surfaces, respectively. This solar energy device can be made useful in the domestic sector by operating it year around in our solar insolation condition.

Directional emittance plays an important role in the calculation of radiative heat exchange. It partly determines the thermal insulation of single and multiple glazing and the efficiency of solar collectors. An emissiometer has been designed and built, capable for measurements of the directional emittance at angles up to 85 degree(s). The emissiometer can be used for absolute measurements, with a black body radiator as reference, and for relative measurements using a known reference sample. In the case of low-emissivity materials the directional emittance usually has a maximum at an angle between 80 degree(s) and 90 degree(s) (pseudo Brewster angle). The emissiometer is, therefore, especially useful for the accurate characterization of low-emissivity materials like spectral selective coated glazing and solar absorbers. The paper gives a description of the instrument and results are discussed for three coated glass samples with low, medium, and high emissivity, respectively, which were measured for a temperature of 283 K. Results are given of the calculation of the thermal transmittance (or U-value) and of three types of double glazing in which the low, medium, and high emissivity coating was applied.

Broadband antireflection coatings can be made from multi-layer stacks or by microstructuring the surface. One approach for microstructuring is to prepare random rough surfaces by sputtering or evaporation technology. A second is the production of periodic holographic double gratings, so-called `moth eyes' with spatial periods of less than 1 micrometers . The effect of these two possibilities on the solar absorptance is demonstrated by the example of cermet solar absorber coatings.

The arcing problem in reactive sputter processes for deposition of highly insulating materials and its possible sources are discussed. It is argued that most of the arcs are initiated by electrical breakdown processes of the insulating layer on the magnetron cathode. The methods to avoid arcs are considered. The theoretical studies lead to a new set up, using a mid- frequency ac power supply and a twin-cathode arrangement. With this set up the charging up of the insulating layer, which leads to the process instabilities, is reduced to an acceptable limit. Using this arrangement transparent layers of SiO₂ were produced without arcing over the entire lifetime of the target (more than one week). The deposition rate obtained for these films was 8 nm/sec which is nearly 10 times higher than for the conventional magnetron sputter arrangement. The refractive index of the transparent layers was controlled by oxygen amount in the sputter chamber. Values obtained for n range from 1.45 to 1.6. Results of chemical and mechanical durability tests have shown that the prepared layers are suitable for industrial application.

Several recent patents have described Hewlett Packard Optoelectronics Division light emitting diode, or LED, lamp designs. These designs have led to several innovative, mass produced products that are now on the market. These designs can be directly traced to the known principles of non-imaging optics, and can thus serve as a guide to solar optics fabrication.

An asymmetric CPC is developed to provide concentrated solar radiation to a multi-effect type still. A brief description of the still is provided and the design of the CPC is explained. Thermal performance results are interpreted in terms of the optical performance of the CPC, showing that the expected behavior was obtained.

A 2-D ray tracing model incorporating light trapping effect to compute the optical generation rate (OGR) for light generated minority carriers in high efficiency micro-groove ((mu) G) silicon solar cells is presented. Based on the principles of geometrical optics, six major types of rays are identified which mainly contribute to light trapping in the (mu) G structure. Assuming .1 and 1.0 reflectances, respectively, on the front and back surfaces, our model computes light generated current density J_L to be 53.79 mA/cm² at AMO for a 100 micrometers thick cell. This value exhibits better agreement with ideal values as compared to those of other 1-D models existing in the literature. On the basis of OGR data from the model, a closed form 2-D OGR formula is developed using fast Fourier transform (FFT) technique. This formula facilitates the apply method of separation of variables to obtain closed form solutions of the 2-D minority carrier transport equations and satisfies the requirements for the boundary conditions therein for device modeling and analysis of (mu) G cells. Performance parameters of (mu) G cells predicted by our computer simulation agree fairly well with recent experimental results.

Light trapping by means of external cavities theoretically offers the same potential to improve the efficiency of solar converters as does concentration. The ideal efficiency of 86% could be reached by any combination of concentration and light trapping, provided all the etendue of the cell is coupled either to itself (giving light trapping) or to the sun (giving concentration). But with real solar cells, there are optimum conditions of concentration and light trapping which maximize the efficiency. The possibility of achieving these conditions is compared between two optical systems: (1) axisymmetric Fresnel lens with plane upper surface, and ellipsoidal cavity, (2) similar Fresnel lens plus secondary lens using total internal reflections to trap light. With regard to a deep-emitter cell, of moderate technology, either system can attain near optimum conditions, giving an efficiency about 4% points above 1-sun efficiency. With a more speculative cell model assuming back mirroring and cell thinning to reduce series resistance, the maximum efficiency (predicted 8% points above 1-sun efficiency) cannot be obtained due to limitations of the optical system, and about 5% points above 1-sun efficiency appears feasible.

The main requirements for luminescent solar concentrators (LSC) are their efficiency, photostability, and ease of fabrication. This has been achieved here by deposition of organically modified sol-gel films doped by photostable perylimide dyes on glass substrate. The absorption spectra of these dyes extends from 420 to 620 nm covering the visible part of the solar spectrum and the emission is between 550 and 750 nm, close to the optimum response of silicon and germanium arsenide solar cells. The efficiency of such a type of collector was calculated from the absorption coefficients, quantum efficiency of the fluorescence, and the overlap between emission and absorption spectra by using the method of Monte-Carlo and found to be close to 20%. Optimum concentrations are shown to be strongly dependent on the extent of overlap between the absorption and the emission spectra, which also appears to be the limiting factor in respect to the efficiency of the concentrator.

Glass is a well established material in the area of solar reflectors for the excellent match between its basic properties and the requirements for such applications. When using 4 mm glass with low iron content in combination with silver as a reflecting layer and the appropriate protection layers to guarantee its environmental stability, the initial reflectance of 94% (averaged over the solar spectrum) can be maintained during its useful life even under harsh conditions as they are prevalent in such applications. The FLACHGLAS Group has manufactured solar reflectors since 1978 and the accumulated output exceeds 2 million m². This paper gives information about the experience gained during the past decade in terms of optical performance, soiling and cleaning history, and overall durability of the mirrors. When using glass as a reflector material, the mirror is also an important structural element of the solar collector as a whole and this paper, therefore, gives some basic design considerations. Using examples from other fields where glass is used for structural elements it shows that the mechanical performance of glass is often underestimated. Most of the reflector surface operating in solar applications today serves to concentrate the incident solar radiation onto heat collecting elements. Optical quality is then described by the solar averaged reflectivity and the image quality via the contour accuracy of the formed mirror. The solar R&D community has currently put an emphasis on `Solar Chemistry.' Here, an additional specification for solar concentrators is a high reflectance in the UV-range of wavelengths, which conventional silver reflectors cannot render. The paper describes a recent development of a durable glass mirror with enhanced UV-reflectance while maintaining a solar averaged reflectance of over 94%.

Aluminum first surface mirrors have some advantages over second surface mirrors as has been discussed. At this stage of development some advantages are obtained: the first advantage was using two electron guns, one for aluminum evaporation permitting us to eliminate or to minimize the pinholes and the other to allow the evaporation of SiO without any mirror contamination as it was before due to the air when the chamber was opened to introduce the SiO, despite having only one e-gun in the laboratory. The second advantage was a better adherence between the aluminum film and the Si₂O₃, this last substance obtained with an oxidation of SiO with some oxygen inside the evaporation chamber (10^-4 Torr). This improvement was due to the use of two e-guns that permit us not to open the chamber. These mirrors are actually under test in the environmental chamber for accelerated weather evaluations. One important aspect is the cleaning of the glass substrate. The chromic mixture cleaning is one of the most effective.

We proposed an experimental method for the establishing of optimum efficiency for PMMA doped with ORACET 6 GF. We determined absorption spectra, emission spectra, and quantum efficiency for different concentrations. Spectral distribution of solar radiation for different air masses, spectral response of solar cells, and fluorescent characteristics allow determination of dye optimum concentration and of concentrator efficiency.

Transparent insulation materials are a new class of materials, which need new measurement techniques for their correct characterization. These measurement methods are summarized. Besides the data of some newly developed materials, a survey of the most important available materials is given. In addition, the potential of granular aerogel windows is shown. All these data are compared with the state of the art of conventional glazed window units including vacuum windows, which might be possible in the future. The importance of the ratio between the total energy transmittance and the U-value of a window under different climatic conditions is discussed, as a possibility to rate a window.

Elastic light scattering has been used to study structural properties of different transparent aerogels, which may be used as filling materials in super-windows. With a goniometer having an angular resolution better than 0.6 degree(s) and a He-Ne laser as the light source we investigated the angular distribution of scattered intensity from transparent silica aerogels and one xerogel. The densities ranged between 0.11 and 0.60 gcm^-3. An exponential correlation function for the density fluctuations of a random porous medium has been utilized to analyze the large-angle scattering, which is dominated by bulk scattering, for different polarization of the incident light. The determination of correlation lengths in the nanometer range was possible, because the absolute scattering intensities were determined. For relative angular dependence measurements, this range would have been accessible only to small angle x-ray scattering (SAXS). The resulting mean pore sizes between 8 nm and 50 nm and specific surface areas between 500 and 700 m²/g agree well with nitrogen-porosimetry data from the literature. The data compare quite well with correlation lengths calculated from specular transmittance data from an ordinary spectrophotometer. This method, which is not sensitive to the angular distribution of superposed forward scattering with large correlation lengths, has also been applied to a series of base-catalyzed TMOS aerogels with different catalyst concentrations. The forward scattering peak of the signal may be attributed to correlation lengths in the micrometer range. Experimental results for aerogel surfaces with evaporated aluminum indicate that this might be due to the surface properties. A quantitative analysis, however, is not possible yet.

Simulation studies have shown that windows with total U-values under 0.8 W/m²-C and solar heat gain coefficients greater than 0.5 will admit more useful solar heat gain than they will loose by conduction/convection and radiation in virtually all locations in the continental United States, independent of orientation. Such fenestration products, when used in typical homes, thus become net energy gainers. Laboratory and field testing/simulations conducted as part of LBL's superwindow research program have proven that glazing systems with three glazing layers, two low-emissivity coatings, and the appropriate low-conductivity gas-fill can achieve this performance level. Beginning in 1990 several U.S. manufacturers started to offer such products commercially. However, laboratory and field testing, as well as computer simulations, have also shown that existing frame/edge designs and materials significantly reduce the total performance of windows using such superglazings. Current research focuses on the use of simulation tools and a high resolution laboratory infrared thermography imaging system to work with manufacturers to develop highly insulating frames and edges.

Monolithic silica aerogel is a transparent material with very low thermal conductivity. These properties make the material interesting for use as insulation in, for example, windows, solar collectors, and solar walls. To produce silica aerogel it is necessary to circumvent the high capillary forces working when the solvent is being removed from the gel structure during drying. Supercritical drying has successfully achieved this. However, supercritical drying with an alcohol might be a dangerous and expensive way to produce the aerogel material. In this work we have studied a new type of monolithic silica xerogels made without supercritical drying. The xerogels are produced by strengthening the gel structure before drying, and low densities in the range 0.42 - 0.73 g/cm³ have been obtained. Properties of this new type of silica xerogels have been compared to the properties of silica aerogel made by supercritical drying. Density, pore size, surface area, thermal conductivity, and optical transmittance are reported in this work and some application advantages are discussed.

Evacuated glazing consists of two plane sheets of glass, joined together around the edges, with the internal volume evacuated. An array of support pillars maintains the separation of the sheets against atmospheric pressure forces. The design of evacuated glazing involves trade- offs between heat transport through the pillars, and mechanical stresses. The state-of-the-art of the design and construction of evacuated glazing is reviewed.

Daylight can be processed by a smart window in a transmission, reflective, refractive, and diffractive mode. In the future an optimization will be realized by a mixing of these approaches depending on the applied cases. Non-imaging diffractive optics has its roots in the work done in holographic diffractive coating for head up displays (HUD) and helmet mounted displays. For having globally good results on smart window with diffractive coating, a very high diffraction efficiency must be reached close to 100% without having a too important lowering of the control of other parameters of the light processed by a smart window (direction and frequency control essentially). We propose a method for designing, realizing, and using diffractive coating for a smart window that is based on a new organic material and diffractive model that were already validated in HUD. Potential low cost is possible for mass production on a large surface with an adapted investment. We describe the present technology and its limits and the ones that can be reached in the future. In this work, we present a holographic way to modify the slant of sun rays through a window, and to filter infrared radiations by using dichromated gelatin material. In this way it would be able to ensure a more uniform lighting and a more pleasant temperature inside buildings or vehicles, without using dye or photochromics glasses.

Improvement in energy efficiency in the glass part of the window has been remarkable during the last few years. Introduction of low-emissivity glasses, argon and krypton gas filling and the chromogenic glass materials among others, have brought the U-values of the glazing down to 0.7 W/m² degree(s)C. Even 0.55 W/m² degree(s)C is possible with the quadruple glazing. However, these super glazings combined with the ordinary frames and sashes are not so effective anymore, because the U-values of the frames are much higher than those of the new glazing. Thus, developing of the thermal performance of the frames and wall connections should be carried out as well parallel to the more fascinating and hi-tech categorized glass research.

MnO₂ thin films have been prepared by thermal evaporation technique onto glass substrate at varying deposition pressure. Electrical studies of the films deposited at about 1.5 X 10^-5 Torr show a positive TCR indicating metallic characteristics, whereas a negative TCR indicating semiconducting behavior exhibit on the films deposited at a pressure of about 3.0 X 10^-6 Torr. Thickness dependent electrical conductivity, activation energy, and aging effect were also carried out. Optical measurements in the wavelength range 0.3 < (lambda) < 2.5 micrometers show the films highly transparent in the visible. These studies may be of importance for the application of this material in surface coating for energy efficient devices.

A Fresnel reflecting solar concentrator fabricated from mirror strips of equal width and almost equi-spaced from one another could possibly lead to an easy design. The axes of rotation of mirror strips are parallel and lying in the same plane. The reflectivity of the mirror strips was about 0.7. Optical and time constant performance of this concentrator-receiver assembly employing mirror strips of 50 mm widths and at a 1 m focal distance in conjunction with a reverse flat plate absorber with black-board paint (non-selective coating) was studied. Employing provisions for suppressing heat loss and reducing absorber area, a time constant of 9 minutes and an overall efficiency of 46% was achieved.

Research in materials science and solar energy at the physics department of Makerere University supported by the International Program in the Physical Sciences started in 1989. The research has been mainly on selective surfaces for solar energy applications. This poster paper presents the planned research activities of the group. For the short-term research is mainly on normally and obliquely deposited metal films in the presence of magnetic fields. In the long-term the area of research will be widened to include amorphous and other photovoltaic materials and devices.

A collaborative research program is engaged to produce `super window' elements with monolithic silica aerogel used as a spacer between two glass panes. The first improvement is a new precursor called polyethoxysiloxane, for an easier and shorter process. This modification is followed by optical measurements, introducing a new concept: the `transparency ratio.' And we show how the diffuse part of the transmitted light may be corrected by the chemical composition and process. With the two further improvements (`in situ' production of aerogel and CO₂ substitution method) we anticipate progress toward a high transparency of the final product.