Solar energy transmittance of translucent samples: A comparison between large and small integrating sphere measurements

Optical transmittance and reflectance of a translucent plastic PTFE film have been measured over the solar wavelength range using different integrating spheres. The same sample has been measured with small and large spheres and the total solar transmittance has been obtained from both broad band measurements and from integration of spectral data. The fact that the sum of reflectance and transmittance often exceeds 100% shows that all types of spheres tend to overestimate the transmittance of this highly scattering sample. This error can be attributed to the sphere geometry in combination with the light scattering properties of the sample, and unless proper correction of recorded data is carried out the error may be as large as 5–10%. Some specific errors are presented and an approximate correction procedure is suggested.These results show that there is a need for a transmittance standard which can be used to calibrate integrating spheres. Such a standard with negligible thickness would be especially useful for measurements with large, broadband integrating spheres, but would also be helpful for the correct handling of data from small spectral instruments.     

Structure and optical properties of electrochromic copper oxide films prepared by reactive and conventional evaporation techniques

Copper oxide films (Cu_xO) are deposited by thermal evaporation techniques using copper oxide (CuO) or copper (Cu) as starting material. By varying the deposition parameters, two main types of Cu_xO film exhibiting different optical properties form. These are reddish gray and colorless films. The samples are characterised optically and morphologically. X-ray diffraction spectra reveal that evaporated Cu_xO films are amorphous. Fourier-transform infrared spectra of the samples were studied to evaluate chemical identification. The refractive index, the extinction coefficient and the thickness of the films are evluated from transmittance characteristics in the ultraviolet, visible and near-infrared regions. The refractive indices of the samples are between 2.9 and 3.1. The values determined for the optical constants are in aggreement with the results found in the literature. We report for the first time that Cu_xO films show reversible optical switching from the colored to bleached state. Optical transmittance measurements of the copper oxide film relative to indium tin oxide coated glass varied during coloring from spectral transmittance T_s = 85−40%     

Transmittance from photovoltaic materials under diffuse light

The optical transmittance properties, under diffuse light, of semitransparent materials used to fabricate photovoltaic devices have been investigated by using an apparatus provided with two “coupled” integrating spheres, one for producing the incident diffuse light and the other for collecting the transmitted light. A detailed analysis of the measurement conditions has been performed taking into account the perturbing effect each sphere produces on the other, and a final expression for the correct transmittance of the sample is derived.     

Infrared flame radiationRayonnement infrarouge des flammesInfrarot-flammenstrahlungИнфpaкpacнoe излyчeниe плaмeни

Gaseous and particulate absorption for a non-homogeneous medium is considered both analytically and experimentally. A simple peak partial pressure and peak temperature scaling method applicable to combustion zones is successfully employed for non-homogeneous intensity calculations. Experimentally, natural convection controlled diffusion flame radiation is investigated. Spectral measurements of the radiance and transmittance are reported for polystyrene, Delrin, and Plexiglas fuels. The homogeneous gray model is shown to predict the experimental results reasonably well, yet is useful only when experimental data exist. The homogeneous nongray approximation is found to be inapplicable to highly non-homogeneous pathlengths. Non-homogeneous nongray calculations are shown to be accurate and with the peak partial pressure and peak temperature scaling method, simple calculations are possible from fundamental properties of the flames.     

Performance simulation of a parabolic trough solar collector

A new analytical model for optical performance and a modified integration algorithm are proposed and applied to simulate the performance of a parabolic trough solar collector with vacuum tube receiver. The analytical equation for optical efficiency of each point at reflector is derived first, then the optical efficiency of the system is simulated by numerical integration algorithm. The cosine factor, receiver efficiency, heat loss and efficiency of conversion of solar energy into net heat energy at any time can be calculated with the program. The annual average efficiency is also simulated considering discard loss. The effects of optical error, tracking error, position error from installation of receiver, optical properties of reflector, transmittance and absorptivity of vacuum tube receiver on efficiencies of the trough system are simulated and analyzed as well as optical parameter.     

The total infrared transmittance of polymerised vinyl fluoride films for a wide range of radiant source temperature

The current growing interest on the use of polymer films is mainly attributed to their suitability as glazing and construction materials in various fields of contemporary technology. Polymerised vinyl fluoride (Tedlar) films are nowdays considered as very suitable for construction and energy applications. Most polymer films are partly transparent, showing a strongly selective absorbing behaviour at the infrared spectrum. Their total hemispherical infrared transmittance which depends on radiant source temperature and film thickness, is a very useful fundamental property for energy balance and heat transfer calculations. However it is not always possible to locate information about this property in the literature. In the present investigation the calculation of the total infrared transmittance of Tedlar films is presented, based on recent spectral transmittance measurements of commercial tedlar films for the wavelength band between 2.5–42.5 μm and for a broad range of radiant source temperatures between –20 to 500 C. Results from the present analysis were found to be in reasonable agreement with earlier data, derived for a specific film thickness and for a narrow range of radiant source temperatures by Willier.     

Spectral transmittance of water and sodium chloride water solution as working substance for a solar pond

The transmission properties of water and sodium chloride water solutions with various salt concentrations were investigated. This paper concerns the measurement of the spectral transmittance and the calculation of the total transmittance for water and NaCl water solutions. The spectral transmittance of the NaCl water solutions over the infrared and nearby regions increased with increasing salt concentration for constant specimen thickness and air mass. The spectral transmittance was found to be useful for precisely calculating the thermal performance of a solar pond, and by using an effective absorption coefficient, a five-partition method dividing the wavelengths into five bands by which the total transmittance up to a water depth of 3 m could be calculated, was found to be important as a practical method of solar pond technology. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 26(1): 1–15, 1997     

用分光光度方法测定建筑材料表面的太阳光-热参数

利用分光光度计对建筑材料表面,对紫外到近红外线范围(240～15000nm)内的正分光透射比、正分光反射比进行测定.另外,还利用积分球对紫外到红外线范围(240～2200nm)内的漫射比谱进行测定,同时将测定结果与正分光透射比、反射比进行比较,并计算出太阳吸收比及热放射比.     

Radiation characteristics of honeycomb solar collectors

A simple closed-form expression for the infrared emittance and the solar absorptance of honeycomb solar collectors has been obtained in terms of the passage transmittance function. The predicted results agree well with the existing data of infrared emittance for thin-walled square-cell honeycomb collectors in vacuum. A new concept of double-honeycomb structure is also introduced and analyzed. This concept provides considerable flexibility in various designs of honeycomb collectors.     

灰色预测自适应内模控制在电厂过热汽温控制中的应用

针对电厂过热汽温的大惯性、大滞后和参数时变等特性,提出了基于灰色预测的自适应内模控制方法.使用Adaline神经网络实时辨识对象的增益和时滞,克服参数时变影响.在误差反馈通道上增加灰色预测模块,根据辨识出的对象时滞的大小,动态确定误差反馈灰色预测的长度;通过超前调节,可以有效克服模型失配和外扰的影响.     

Transparent heat mirrors based on tungsten oxide-silver multilayer structures

Transparent heat mirrors based on tungsten oxide/silver three-layer structures were fabricated using thermal evaporation. The optical and morphological properties of the single layers were first investigated to serve as a basis for the fabrication of the heat mirrors. Only silver films with a thickness higher than 18 nm were found to be continuous. Subsequently, WO₃/Ag/WO₃ multilayers were deposited, where the WO₃ layers thickness was fixed at 35 nm, and the thickness of the silver layer was varied from 18 to 39 nm. The optical properties of the multilayers were measured over the visible and near infrared ranges. These multilayers exhibited the desired heat mirror behavior, namely the transmittance was largely confined to the visible range and the reflectance was diminished in that range. The maximum visible transmittance was 88.3% at 554 nm. Increasing the thickness of the silver films resulted in a decrease of the visible transmittance, with a corresponding increase in the infrared reflectance. Optimization of these two opposing trends was evaluated using a figure of merit, from which the best performance was obtained for multilayers with a silver layer of thickness of 24 nm.     

Point Thermal Transmittance of Rib Intersections in Concrete Sandwich Wall Panels

Concrete sandwich panels are building elements made by two concrete wythes separated by a layer of lightweight material: the central layer is inhomogeneous due to the presence of concrete ribs which tie the external wythes and act as thermal bridges. This paper deals with the problem of determining point thermal transmittance associated with rib intersections. Together with previous results by the authors, it allows accurate calculation of thermal transmittance of sandwich panels according to current International Standards. A dataset of 1080 point transmittance values is obtained upon use of a spectral element method, varying systematically material conductivities and thickness of panel layers, for the most common pairs of rib widths in current panel production. To limit the computational cost, a solution strategy based on the use of low-order polynomials on three grids of increasing refinement, coupled with Richardson extrapolation is adopted. Finally, a power law correlation is proposed that allows to estimate point transmittance within a relative error of 10%.     

Estimation of atmospheric transmittance from upper-air humidity

A very simple model of atmospheric transmittance uses as its input regularly observed and archived measures of upper air humidity. The five model parameters were determined empirically to optimize the agreement with radiation measured at six sites in the northwestern contiguous U.S. in summer and winter 1990. The model uses the relative humidity at 500 hPa, the relative humidity at either 950 hPa or 800 hPa depending on the altitude of the station, and the precipitable water.Although the model does not use cloud observations explicitly, it is partitioned into a low-humidity stage and a high-humidity stage with markedly different dependences on relative humidity. In the low-humidity stage the transmittance is approximated as the product of factors for dry air and for water vapor. In the high-humidity stage the transmittance is the product of the transmittances of each of the two layers used; the product is a strongly decreasing function of relative humidity, as a representation of the effect of cloud.A split-sample test using measurements for spring and autumn indicated that the model parameters were not badly distorted by conditions unique to the summer and winter seasons. In terms of the average daily global shortwave radiation received at the surface, the rms errors are 31 W m⁻² in spring, 33 in summer, 30 in autumn, and 18 in winter. The corresponding relative errors are 0.25, 0.17, 0.20, and 0.31. Because of high-frequency temporal variation in the model residuals, the error declines to about half of the daily error for 7-day averages and to about a quarter of the daily error for 30-day averages.     

Recent developments on transparent insulation materials and light switching devices for passive solar energy usage

This paper gives an overview on new developments in the field of transparent insulation (TI) materials and systems. Monolithic or granular SiO₂-aerogels, PMMA, polycarbonate or glass capillaries as well as light switching layers are discussed.Monolithic aerogels prepared under basic conditions show only weak Rayleigh- scattering resulting in high transmittance values. Thus a solar averaged hemispherical transmission of for 20 mm thick specimens is achieved. In the visible spectral region directional-directional transmittance values ranging up to 90 % are measured. Beside the granular waterglass derived SiO2-aerogels produced by BASF also the lightscattering properties of newly developed aerogel pellets using TMOS as precursor are investigated. In comparison with both aerogel types the thermal and optical properties of capillary structures are discussed.A newly developed thermochromic gel layer allows to switch the solar input from 85 % down to 10 % via radiation backscattering. To achieve this transmission reduction a gel thickness of only 1 mm is necessary. With an optimized polymer dispersed liquid crystal system a transmission reduction of 30 % is also obtained solely by scattering. The electrical power necessary to maintain the transparent state was reduced to a value of 5 W/m² by decreasing the frequency of the applied ac-voltage. An electrochronic switching levice exhibits a switching level of 40 % ensuring a high visual image transmission by changing the absorption coefficients mainly in the red and near infrared spectral region.     

Solar transmittance characteristics of evacuated tubular collectors with diffuse back reflectors

The partition matrix between light sources and light sinks is introduced to evaluate the transmittance properties of optically asymmetric collectors constructed with evacuated tubes. A ray tracing model is used to compute its elements. Curve fits to the ray tracing results are provided. The effects of polarization are discussed. Comparisons with previously published results are presented in order to check the accuracy of the proposed model. The beam and diffuse transmittance of a tubular cover are calculated and their dependency on the tube orientation investigated. The biaxial incidence angle modifier product is extended to evacuated tube collector arrays with a diffuse back reflector, providing a simple interpolation process for the beam transmittance. An effective beam direction for diffuse radiation is computed so that transmittances for ground reflected radiation and sky diffuse radiation can be calculated from biaxial incidence angle modifier measurements.     

Mixed Convection and Non-Gray Radiation in a Horizontal Rectangular Duct

Numerical studies for fluid flow and heat transfer in a horizontal rectangular duct are carried out. The flow is considered to be laminar, hydrodynamically and thermally developing. Heat transfer by both forced and natural convection is taken into account. The radiation from the gas is modeled with weighted sum of gray gases (WSGG) model. While considering non-gray radiation with WSGG, the fluid is considered to be a mixture of CO₂ and H₂O. Simulations are carried out with lower wall temperature than the inlet temperature of the gas. The effect of buoyancy and radiation on bulk mean temperature and Nusselt number are studied. The effects of temperature dependent properties are discussed. Comparative studies are carried out among forced convection, mixed convection, gray and non-gray gas radiation. It is found from the simulations that the assumption of gray gas can produce an error of ±10% over a non-gray model with WSGG for the cases studied.     

Models for the angle-dependent optical properties of coated glazing materials

Optical transmittance and reflectance of window materials can be measured accurately at normal incidence using standard equipment. Sunlight often strikes at angles for which the transmittance and reflectance are significantly different from their values at normal incidence. A reliable procedure for extrapolating from normal properties to oblique properties is thus needed for accurate annual energy performance calculations and product comparisons. The structural models for the materials are greatly constrained by the limited amount of data that is usually available. For monolithic materials such as uncoated glass or plastic substrates it is possible to solve directly for the optical indices and then apply Fresnel’s equation to obtain the oblique properties. For coated glass, the situation is more complex, but a numerical solution is often possible. First, detailed optical models were constructed and accurate angle-dependent data were generated for a wide selection of coated glazing materials. Then, a set of very simple thin-film models were chosen that would converge given a limited amount of data. At 60° incidence, the monolithic model was often accurate to within 2% but frequently deviated farther up to 8%. The single-layer thin-film model fared little better. Highly constrained multilayer models often deviated less than 1% although convergence became increasingly specific to similar coating types.     

Effects of deposition variables on spray-deposited MnO2 thin films prepared from Mn(C2H3O2)2·4H2O

Undoped MnO₂ thin films have been prepared by a modified spray pyrolysis technique under various deposition conditions and the effects of different variables on electrical and optical properties have been studied in detail. It is found that substrate temperature, spray rate, solution concentration, carrier air pressure and post-deposition heat-treatment, spray outlet to substrate distance play important role in obtaining optimum films.Electrical conductivity study shows an anomaly in conductivity at a temperature 323 K and its thickness dependent resistivity follows Fuchs–Sondheimer theory. The Hall effect and thermoelectric studies indicate that the deposited sample is an n-type semiconductor. Optical study in the entire wavelength 0.3–2.5 μm range exhibits a high transmittance in the visible as well as in the near infrared. Calculation from optical data, the sample exhibits a band gap at 0.28 eV, which also supports the value obtained from the Hall effect study. These studies may be of importance for the applications of this material in energy efficient surface coating devices.     

Antireflection of glazings for solar energy applications

Several antireflection treatments such as Teflon dipping, dipping in silica saturated fluosilic acid, have previously been tested. The films made with the fluosilicic acid showed an increased solar transmittance of up to 5%. Outdoor testing for more than 7 years indicated an excellent long-term stability. The problem with this process is the involved chemicals, which are too harmful to permit a large-scale production in Sweden. Therefore, a dip-coating process has been investigated, which uses silica sols. Two sols were investigated, one polydisperse and one monodisperse. The highest attained solar transmittance was achieved with the monodisperse sol. It is believed that the increase in the solar transmittance is connected to the size of the silica particles, the larger the smallest particles the higher the solar transmittance. A decrease by up to 5.2% in the solar reflectance has been noted. A problem is the mechanical properties and especially the adhesion tends to be low. It is, however, possible to improve these properties by baking the film at approximately 550°C during 30 min. The solar transmittance decreases when baking the film since it collapses partially, but the mechanical properties seemed to improve. During baking the temperature is close to that of commercial tempering. It should therefore be possible to bake and temper at the same time. The deposited film does not introduce any absorption. It is concluded that this process should be suitable for large-scale production, since the involved chemicals are harmless and the process is fairly rapid. Films were made at withdrawal rates of up to 1 cm/s.     

Solar transmittance measurements using an integrating sphere with broad area irradiation

This paper examines the broad area irradiation method for measuring solar transmittance of advanced glazing materials. It shows that the apparent transmittance measured by this method is the product of the true transmittance, and two other factors related to the reflectance of the sample. Not accounting for these other factors may lead to errors. Methods for reducing and correcting these errors are discussed and measurements are presented that illustrate these errors. It is shown that the broad area irradiation technique can be used with appropriate corrections to accurately measure transmittance.