Evaluation and optimization of the optical performance of low-concentrating dielectric compound parabolic concentrator using ray-tracing methods

We present a detailed design concept and optical performance evaluation of stationary dielectric asymmetric compound parabolic concentrators (DiACPCs) using ray-tracing methods. Three DiACPC designs, DiACPC-55, DiACPC-66, and DiACPC-77, of acceptance half-angles (0° and 55°), (0° and 66°), and (0° and 77°), respectively, are designed in order to optimize the concentrator for building fa?ade photovoltaic applications in northern latitudes (>55 °N). The dielectric concentrator profiles have been realized via truncation of the complete compound parabolic concentrator profiles to achieve a geometric concentration ratio of 2.82. Ray-tracing simulation results show that all rays entering the designed concentrators within the acceptance half-angle range can be collected without escaping from the parabolic sides and aperture. The maximum optical efficiency of the designed concentrators is found to be 83%, which tends to decrease with the increase in incidence angle. The intensity is found to be distributed at the receiver (solar cell) area in an inhomogeneous pattern for a wide range of incident angles of direct solar irradiance with high-intensity peaks at certain points of the receiver. However, peaks become more intense for the irradiation incident close to the extreme acceptance angles, shifting the peaks to the edge of the receiver. Energy flux distribution at the receiver for diffuse radiation is found to be homogeneous within ±12% with an average intensity of 520 W/m2.     

Optical design of a solar flux homogenizer for concentrator photovoltaics

An optical solution is described for the redistribution of the light reflected from a 400-m2 paraboloidal solar concentrating dish as uniformly as possible over an approximately 1-m2 plane. Concentrator photovoltaic cells will be mounted at this plane, and they require a uniform light distribution for high efficiency. It is proposed that the solar cells will be mounted at the output of a rectangular receiver box with reflective sidewalls (i.e., a kaleidoscope), which will redistribute the light. I discuss the receiver box properties that influence the light distribution reaching the solar cells.     

Monitoring of concentrated radiation beam for photovoltaic and thermal solar energy conversion applications

Methods for evaluating the light intensity distribution on receivers of concentrated solar radiation systems are described. They are based on the use of Lambertian diffusers in place of the illuminated receiver and on the acquisition of the scattered light, in reflection or transmission mode, by a CCD camera. The spatial distribution of intensity radiation is then numerically derived from the recorded images via a proprietary code. The details of the method are presented and a short survey of the main applications of the method in the photovoltaic and thermal solar energy conversion field is proposed. Methods for investigating the Lambertian character of commercial diffusers are also discussed.     

Theoretical Models of Surface Heat Treatment of Products in Solar Furnaces. 2. Nonuniform Radiant-Energy Flux

The nonlinear and two-dimensional problem of heating of a plane plate in a solar furnace with a Gaussian distribution of the radiant heat flux over the surface has been reduced to the one-dimensional problem by introducing a nondifferentiable parameter dependent on the radial coordinate. Using the modified Goodman heat-balance integral method, we have obtained estimates of the minimum heat flux necessary for fusion of the surface of the plate and the limiting diameter of the melt hole. An example of calculating the dynamics of fusion of the surface of a ceramic plate is presented.     

The Sun's global magnetic field

Our present-day understanding of solar and stellar magnetic fields is discussed from both an observational and theoretical viewpoint. To begin with, observations of the Sun's large-scale magnetic field are described, along with recent advances in measuring the spatial distribution of magnetic fields on other stars. Following this, magnetic flux transport models used to simulate photospheric magnetic fields and the wide variety of techniques used to deduce global coronal magnetic fields are considered. The application and comparison of these models to the Sun's open flux, hemispheric pattern of solar filaments and coronal mass ejections are then discussed. Finally, recent developments in the construction of steady-state global magnetohydrodynamic models are considered, along with key areas of future research.     

Magnetic flux emergence and associated dynamic phenomena in the Sun

We present a review of the process of magnetic flux emergence in the Sun. We focus on observations and numerical experiments that explore the dynamical rise of magnetic fields from the solar interior to the corona. We describe the response of the highly stratified solar atmosphere on flux emergence and, consequently, we present a comprehensive picture of the coupling between solar dynamic events and flux emergence. We discuss potential applications of this process in other astrophysical environments.     

卫星红外特性及空间热流对其的影响

建立了完善的卫星温度场与红外特性的理论计算模型.首先根据卫星、地球和太阳的位置关系,计算了卫星接收的空间热流;其次利用有限元法求解瞬态热平衡方程,得到了卫星的表面温度分布;然后在3～6 μm和6～16μm波段分别计算了卫星作为点源时的红外辐射强度空间分布;最后比较了卫星、卫星本体和卫星太阳翼在红外双波段的辐射强度,并详细分析了空间热流对卫星红外特性的影响.文章的研究结果对于空间目标的红外探测与识别具有参考价值.     

基于多孔陶瓷的体吸收太阳能集热器性能分析

体吸收型太阳能集热器是3种主要的太阳能集热器之一,具有结构简单,效率高等优点,并且出口空气温度可达800℃以上,应用前景广阔。在体吸收太阳能集热器中,采用多孔材料而不是管路来加热工质,入射辐射可以从外到内逐步进行吸收。该文基于多孔泡沫陶瓷材料构建了一个一维体吸收太阳能集热器计算模型,在此基础上,计算分析了不同参数下的温度分布情况及热辐射吸收效率。研究结果表明,随着多孔陶瓷孔隙率的降低,出口空气温度及陶瓷内部气体与固体达到稳定的深度均逐渐降低,而接收器前表面温度逐渐提高;随着孔径的下降接收器尾部气体出口温度逐渐下降,内部气体和固体达到稳定的深度逐渐减小,但由于孔径的减小使得对流换热系数显著上升,因此其前表面的温度变化并不明显。太阳能吸收效率随着空气流速的降低和入口空气温度的增大而明显提高,但随着入射辐射强度的提高而降低。     

Toward clean environment: evaluation of solar electric power technologies using fuzzy logic

The rapid expansion of the use of solar energy power plants worldwide is a subject that is being followed with interest. Fuzzy logic methodology is used for evaluating the solar thermal power technology, it compresses huge amount of data into smaller sets, and it has the ability to decide between different solar technologies on the basis of their benefits and costs. The most often considered solar technologies were parabolic trough, central receiver, dish sterling engine, compact linear Fresnel reflector (CLFR), solar chimney, photovoltaic (PV), and solar pond. The aim of our research is to provide the needed information to make a judgment or a decision of adopting the most preferred solar technology in terms of installation and development using fuzzy set methodology. The criteria of the evaluation were based on different parameters, i.e., power capacity, efficiency, availability, capacity factor, storage capability, cost, maturity, water usage, land usage, and safety. The key barriers and features for each technology on the basis of benefit-to-cost ratios are addressed. The results showed that CLFR was found to be the best choice in terms of research, development, and implementation, followed by parabolic trough technology, then the central receiver technology, dish sterling engine, solar chimney, PV, and solar pond, according to the order of preference.     

Fabrication and optimisation of highly efficient cermet-based spectrally selective coatings for high operating temperature

In terms of both high photo-thermal efficiency and high stability, multi-layer structures based on metal-dielectric composites (cermet) can be considered the most attractive selective solar absorbers for receiver tubes operating at medium-high temperatures in the field of solar thermodynamic plants. The double cermet layer approach represents a very simple fabrication method and can give high performances in terms of high solar absorptance and low hemispherical emittance. Optimised solar coatings based on cermet layers were fabricated in our laboratories by sputtering technique following the double layer approach. The joined employment of ellipsometric measurements and optical simulation is proposed as an effective method to optimise and fabricate coatings showing the best performances at a fixed operating temperature of the receiver tube. Interesting results concerning an optimised spectrally selective coating are shown. Solar absorptance higher than 0.94 and hemispherical emittance at 580 °C lower than 0.13 were obtained.     

塔式太阳能热发电吸热器模拟计算

本文建立了塔式太阳能热发电高温吸热器传热性能数值计算模型,采用有限元分析方法进行模拟计算,编制了预测塔式太阳能热发电吸热器热工模拟程序。本文在综合考虑多场耦合特性的情况下,研究了吸热器单模块周向非均匀热流边界条件下吸热介质对流换热系数、吸热管外壁温度、光热转化效率的数值及分布。数值模拟为工程设计提供可靠的科学依据,为塔式太阳能热发电整体控制策略的制定提供参考。     

旅客列车硬座车厢内气流模拟与浓度场分析

采用稳态不可压缩雷诺时均N-S方程、k-ε湍流模型,对旅客列车空调硬座车厢内气流场和浓度场进行了数值计算。采用立方体代表旅客,以人体呼出的CO2作为代表性污染物,研究了非空载下车厢内气流和浓度分布。计算结果表明：现有的送风方式除车厢两端外,车厢内沿长度方向气流分布比较均匀;人体散热和太阳辐射对车厢内流场温度场影响较大,非空载时车厢内流场分布与空载时有较大差别,太阳照射和人体产生的热气流使车厢内存在较大的温度梯度;车厢内过道区浓度较低,但座位区由于人员集中,人体呼吸区污染物积聚,浓度偏高,且车厢中部断面污染     

Determination of the solar calibration constant for a sun-sky radiometer: proposal of an in-situ procedure

A new procedure is presented for determining in situ the solar calibration constant, i.e., the Sun-sky radiometer counts for a direct normal solar flux extrapolated to the top of the atmosphere. The method makes use of a modified version of the Langley plot based on the use of an inversion code of column-integrated aerosol size distribution, and it is ordinarily applied to calibrate Prede Sun-sky radiometers. To analyze how such an in situ method can work accurately, the technique has been applied to a five-month dataset obtained from measurements taken in Rome, Italy, by a Prede Sun-sky radiometer from 22 April to 5 November 2001. The precision of the in situ method has been estimated to within 1-2.5%, depending on the wavelength.     

The analysis of residual stress in glass-to-metal seals for solar receiver tube

The solar receiver tube is a key component in the parabolic trough solar thermal power system. Breakage of glass-to-metal sealing is main cause for damages of receivers in existing power plants. The residual stresses that are generated during the cooling process of the seal can decrease the seal strength and induce the breakage of the glass-to-metal sealing. Residual stresses were simulated by finite element software ANSYS and measured experimentally by photoelastic techniques. In order to lower the seal failure probability, the effects of the component dimensions were analyzed using finite element method. The simulation results agreed with the measurement results. The dangerous tensile stresses occur not only at the glass–metal interface but also on the outer surface of glass tube near the sealing area. When the depth of metal embedded into glass increases, the magnitude of residual stress decreases and the sealing strength will increase. The results of this study have important implications on the optimization of seal configuration in the solar receiver tubes.     

Materials for photothermal solar energy conversion

Commencally or potentially available selective and non-selective absorber surfaces for solar heat collectors are reviewed and the state-of-the-art of solar collector corrosion processes is outlined. The review of available published literature has indicated that a lack of quantitative information exists, relative to corrosion of collector surfaces. Available information (mostly qualitative) on durability aspects and corrosion of solar receiver surfaces is described to indicate potential corrosion problem areas and corrosion prevention possibilities. An outline of appropriate durability tests is presented.     

Contactless efficient two-stage solar concentrator for tubular absorber

The design of a new type of two-mirror solar concentrator for a tubular receiver, the XX concentrator, is presented. The main feature of the XX is that it has a sizable gap between the secondary mirror and the absorber and it still achieves concentrations close to the thermodynamic limit with high collection efficiencies. This characteristic makes the XX unique and, contrary to current two-stage designs, allows for the location of the secondary outside the evacuated tube. One of the XX concentrators presented achieves an average flux concentration within +/-0.73 deg of 91.1% of the thermodynamic limit with a collection efficiency of 96.8% (i.e., 3.2% of the rays incident on the primary mirror within +/-0.73 deg are rejected). Another XX design is 92.5% efficient and receives 95.1% of the maximum concentration. These values are the highest reported for practical concentrators, to our knowledge. The gap between the absorber and the secondary mirror is 6.8 and 10.5 times the absorber radius for each concentrator. Moreover the rim angle of the primary mirror is 98.8 and 104.4 deg in each case, which is of interest for the collector's good mechanical stability.     

Optical properties of an optically rough coating from inversion of diffuse reflectance measurements

A method is developed for determining the optical properties of an optically rough coating on an opaque substrate from reflectance measurements. A modified Kubelka-Munk two- flux model is used to calculate the reflectance of the coating as a function of the refractive index, absorption coefficient, scattering coefficient, and thickness. The calculated reflectance is then fitted to measurements using a spectral projected gradient algorithm, allowing the optical properties to be obtained. The technique is applied to titanium dioxide coatings on a titanium substrate. Realistic values of refractive index and absorption coefficients are generally obtained. Quantities that are useful for solar water-splitting applications are calculated, including the depth profile of absorption and the proportion of the incident photon flux absorbed in the coating under solar illumination.     

Removal of fluoride from water using anion-exchange membrane under Donnan dialysis condition

The transport of fluoride through Neosepta-ACM anion-exchange membrane has been studied as a function of feed phase and receiver phase concentration and co-existence anions under Donnan dialysis condition. It was observed that the transport of fluoride was maximum at pH 6 of feed phase and at pH 1 of receiver phase. Moreover, transport of fluoride increased with increase of feed and receiver phase concentration and decreased in the presence of other co-existence anions in the feed phase. The transport of fluoride was also correlated with the flux data and explained according to structure of membrane.     

Sputtering deposition and characterization of tandem absorbers for photo-thermal systems operating at mid temperature

In this paper the first results of a work concerned with the development of TiAlON selective solar absorbers with high solar absorptance and low thermal emittance are presented. These absorbers are thought for solar collectors application at operational temperature of about 300 °C. Solar absorber tandems, consisting of an absorbing TiAlON compound sputtered on a TiC buffer layer deposited on a metal (Cu) substrate, have been produced in two ways: by changing the nitrogen flux percentage in (Ar + N₂ + O₂) mixture at fixed thickness (300 nm) and by varying the absorber thickness at fixed nitrogen percentage (5%) in the sputtering gas. The optical response in the UV-IR range is discussed. The tuning of the nitrogen flux allows to change the metal elements ratio (Ti/Al) in the oxynitride layer, varying the reflectance intensity both in the IR and visible range. The thickness variation permits to shift the cut-off wavelength of the transition from short wavelength high absorptance to long wavelength low emittance. The tuning of these two parameters (nitrogen flux and absorber thickness) enables to control the optical response of the samples to make them suitable for photo-thermal device application.     

Radiative properties of snow for clear sky solar radiation

Spectral and integrated radiative properties (reflection, transmission, and the rate of heating) of finegrained wind-packed snow typical of subpolar regions are studied through a model taking into account surface reflection and volumetric multiple scattering. The surface reflection is modeled by a bidirectional reflectance distribution function applicable to powdered dielectric material. For the volumetric multiple scattering, the radiative transfer equation designed for strongly asymmetric scattering is solved. All multiple scattering parameters (single scattering albedo, various moments of the scattering phase function, and optical depth) are related to measurable physical characteristics (density, grain size, and the absorption coefficient of pure ice).Parameterized atmospheric spectral transmission coefficients for scattering and absorption by aerosols and gases are used to obtain the direct and diffuse components of solar flux, incident on the snow-cover. Calculated values of spectral and integrated visible and near infrared reflection and flux attenuation coefficients of snow are compared with observations. The rate of radiative heating at different depths within the snowcover is calculated from the net flux divergence. It is shown that the conventional method of calculating this rate using measured bulk extinction coefficients grossly underestimates the amount of heating within the top few millimeters. This study provides a better overall understanding of the radiative properties of snow under clear sky conditions in terms of the physical characteristics of the snowcover.