点聚焦太阳炉设计方法与研制实践

从聚光原理、光学计算、硬件设计,到安装与调试,详细介绍了点聚焦太阳炉的研制过程。根据当地用户的需求和太阳能资源,通过自行设计的光学软件,计算出典型日一次反光定日镜在二次聚光器截光口平面上的照射情况、二次聚光器的最佳焦距以及焦平面焦点处的峰值热功率。模块化设计的一次反光定日镜,不仅降低了制造成本,提高了生产效率,简化了一次反光定日镜的整体支架,同时给运输和安装带来了方便。单元反光镜反光面采用12点支撑,为定日镜的单元镜面调平创造了条件。同样采用模块化设计的二次聚光器,有利于制造成本的降低,方便运输和现场安装。反光镜单元面型可调,大大增强了点聚焦太阳炉参数的适应性。通过样机研制,验证了设计方法的可行性。     

An optical analysis of a static 3-D solar concentrator

Concentrating technology is long established in the field of solar thermal applications. However, there is still scope for improvement due to innovation in design, materials and manufacturing methods. The optical efficiency of a solar concentrator depends largely on the geometry of the concentrator profile. This paper evaluates the optical performance of a static 3-D Elliptical Hyperboloid Concentrator (EHC) using ray tracing software. Ray tracing has been used extensively to calculate the optical efficiency of the static 3-D EHC. Performance parameters such as effective concentration ratio, optical efficiency and geometric concentration ratio are also evaluated for different aspect ratios of the elliptical profile. Optimization of the concentrator profile and geometry is also carried out to improve the overall performance; this parametric study includes the concentrator height, solar incidence angle and aspect ratio of the ellipse. The overall performance of the concentrator was assessed based on the acceptance angle, effective concentration ratio and optical efficiency. Finally, the flux distribution on the receiver area for different concentrator heights is also presented.     

Design and fabrication of low concentrating second generation PRIDE concentrator

Prototype first generation Photovoltaic Facades of Reduced Costs Incorporating Devices with Optically Concentrating Elements (PRIDE) technology incorporating 3 and 9 mm wide single crystal silicon solar cells showed excellent power output compared to a similar non-concentrating system when it was characterized both indoors using a flash and continuous solar simulator. However, durability and instability of the dielectric material occurred in long-term characterisation when the concentrator was made by using casting technology. For large scale manufacturing process, durability, and to reduce the weight of the concentrator, second generation PRIDE design incorporated 6 mm wide “Saturn” solar cells at the absorber of dielectric concentrators. Injection moulding was used to manufacture 3 kWp of such PV concentrator module for building façade integration in Europe. Special design techniques and cost implications are implemented in this paper. A randomly selected PV concentrator was characterised at outdoors from twenty-four (≈3 kWp) 2nd-G PRIDE manufactured concentrators. The initial PV concentrators achieved a power ratio of 2.01 when compared to a similar non-concentrating system. The solar to electrical conversion efficiency achieved for the PV panel was 10.2% when characterised outdoors. In large scale manufacturing process, cost reduction of 40% is achievable using this concentrator manufacturing technology.     

Stretched tape design of fixed mirror solar concentrator with curved mirror elements

The optical design of a fixed mirror line-focus solar concentrator, using curved mirror elements whose radius of curvature is matched to the radius of the reference cylinder of the concentrator, is presented. It is shown that this design leads to a considerable decrease in the transverse width of the focal intensity profile as compared with a fixed mirror solar concentrator of similar design made of flat mirror elements, and thus enables reduction in the cost of the heat receiver assembly. The development of a stretched tape construction of a 12 m × 3 m fixed mirror solar concentrator, conforming to the above design by using cold rolled steel tapes with constant levels of curvature across their width as substrates for the curved mirror elements, is briefly reported. Results from optical tests on the concentrator, which confirm the predictions from the theoretical model of the optics of the concentrator, are presented.     

Theoretical analysis of error transfer from surface slope to refractive ray and their application to the solar concentrated collector

This paper discusses the error transfer from the slope of optical surface to the focus ray. It presents a general equation to calculate the standard deviation of the refractive ray error from that of slope error of the optical surface through geometric optics analysis, applying the equation to calculate the standard deviation of the focus ray error in 6 kinds of solar concentrator, and providing typical results. The results indicate that the slope errors in two directions are transferred to any one direction of the focus ray when the incidence angle is more than 0; for a point focus Fresnel lens, a point focus parabolic glass mirror, and a line focus parabolic glass mirror, the error transferring coefficient from the optical surface to the focus ray will increase when the rim angle or distance of reflection or refraction point to the axis increases; for a TIR-R concentrator, it will decrease; for a glass heliostat, it relates to the incidence angle and azimuth of the reflecting point. The results show that the slope error of the optical surface may be enlarged more than ten folds to the focus ray to decrease the optical efficiency of the solar concentrator greatly.     

Computations of the optical properties of metal/insulator-composites for solar selective absorbers

Solar selective absorbers are very useful for photo thermal energy conversion. The absorbers normally consist of thin films (mostly composite), sandwiched between the antireflection layer and (base layer on) a metallic substrate, selectively absorbing in the solar spectrum and reflecting in the thermal spectrum. The optical performance of the absorbers depends on the thin film design, thickness, surface roughness and optical constants of the constituents. The reflectivity of the underlying metal and porosity of the antireflection coating plays important roles in the selectivity behavior of the coatings. Computer simulations, applying effective medium theories, have been used to investigate the simplest possible design for composite solar selective coatings. A very high solar absorption is achieved when the coating has a non-uniform composition in the sense that the refractive index is highest closest to the metal substrate and then gradually decreases towards the air interface. The destructive interference created in the visible spectrum has increased the solar absorption to 98%. This paper also addresses the optical performance of several metals/dielectric composites like Sm, Ru, Tm, Ti, Re, W, V, Tb, Er in alumina or quartz on the basis of their refractive indices. The antireflection coating porosity and surface roughness has been analyzed to achieve maximum solar absorption without increasing the thermal emittance. Antireflection layer porosity is a function of dielectric refractive index and has nominal effect on the performance of the coating. While, up to the roughness of 1×10⁻⁷ m RMS, the solar absorption increases and for higher roughness, the thermal emittance increases only.     

太阳能会聚器的优化设计

讨论了用光学设计软件CODE V对太阳能会聚器进行优化设计的问题。在描述了太阳能会聚器的主要设计参数后，我们分析了抛物型会聚器的特性。利用光学设计软件，设计出一类新的高次非球面会聚器。实验结果表明，此类会聚器可实行间断跟踪太阳，且接收面光能量分布均匀，提高了系统转换效率，还具有节约能量、降低成本等优点。     

Experimental comparison of alternative convection suppression arrangements for concentrating integral collector storage solar water heaters

An experimental investigation of an inverted absorber integrated collector storage solar water heater mounted in the tertiary cavity of a compound parabolic concentrator with a secondary cylindrical reflector has been performed under simulated solar conditions. The solar water heaters performance was determined with the aperture parallel to the simulator for a range of transparent baffles positioned at different locations within the collector cavity. Results indicate that glass baffles located at the upper portion of the exit aperture of the CPC can reduce thermal losses through convection suppression without significantly increasing optical losses.     

A stationary evacuated collector with integrated concentrator

A comprehensive set of experimental tests and detailed optical and thermal models are presented for a newly developed solar thermal collector. The new collector has an optical efficiency of 65 per cent and achieves thermal efficiencies of better than 50 per cent at fluid temperatures of 200°C without tracking the sun. The simultaneous features of high temperature operation and a fully stationary mount are made possible by combining vacuum insulation, spectrally selective coatings, and nonimaging concentration in a novel way. These 3 design elements are “integrated” together in a self contained unit by shaping the outer glass envelope of a conventional evacuated tube into the profile of a nonimaging CRC-type concentrator. This permits the use of a first surface mirror and eliminates the need for a second cover glazing. The new collector has been given the name “Integrated Stationary Evacuated Concentrator”, or ISEC collector. Not only is the peak thermal efficiency of the ISEC comparable to that of commercial tracking parabolic troughs, but projections of the average yearly energy delivery also show competitive performance with a net gain for temperatures below 200°C. In addition, the ISEC is less subject to exposure induced degradation and could be mass produced with assembly methods similar to those used with fluorescent lamps. Since no tracking or tilt adjustments are ever required and because its sensitive optical surfaces are protected from the environment, the ISEC collector provides a simple, easily maintained solar thermal collector for the range 100–300°C which is suitable for most climates and atmospheric conditions. Potential applications include space heating, air conditioning, and industrial process heat.     

Effects of geometrical parameters of a dish concentrator on the optical performance of a cavity receiver in a solar dish‐Stirling system

Dish‐Stirling concentrated solar power (DS‐CSP) system is a complex system for solar energy‐thermal‐electric conversion. The dish concentrator and cavity receiver are optical devices for collecting the solar energy in DS‐CSP system; to determine the geometric parameters of dish concentrator is one of the important steps for design and development of DS‐CSP system, because it directly affects the optical performance of the cavity receiver. In this paper, the effects of the geometric parameters of a dish concentrator including aperture radius, focal length, unfilled radius, and fan‐shaped unfilled angle on optical performance (ie, optical efficiency and flux distribution) of a cavity receiver were studied. Furthermore, the influence of the receiver‐window radius of the cavity receiver and solar direct normal irradiance is also investigated. The cavity receiver is a novel structure that is equipped with a reflecting cone at bottom of the cavity to increases the optical efficiency of the cavity receiver. Moreover, a 2‐dimensional ray‐tracking program is developed to simulate the sunlight transmission path in DS‐CSP system, for helping understanding the effects mechanism of above parameters on optical performance of the cavity receiver. The analysis indicates that the optical efficiency of the cavity receiver with and without the reflecting cone is 89.88% and 85.70%, respectively, and former significantly increased 4.18% for 38 kW XEM‐Dish system. The uniformity factor of the flux distribution on the absorber surface decreases with the decreases of the rim angle of the dish concentrator, but the optical efficiency of the cavity receiver increases with the decreases of the rim angle and the increase amplitude becomes smaller and smaller when the rim angle range from 30° to 75°, So the optical efficiency and uniformity factor are conflicting performance index. Moreover, the unfilled radius has small effect on the optical efficiency, while the fan‐shaped unfilled angle and direct normal irradiance both not affect the optical efficiency. In addition, reducing the receiver‐window radius can improve the optical efficiency, but the effect is limited. This work could provide reference for design and optimization of the dish concentrator and establishing the foundation for further research on optical‐to‐thermal energy conversion.     

Practical design considerations for CPC solar collectors

Several practical problems are addressed which arise in the design of solar collectors with compound parabolic concentrators (CPC's). They deal with the selection of a receiver type, the optimum method for introducing a gap between receiver and reflector to minimize optical and thermal losses, and the effect of a glass envelope around the receiver. This paper also deals with the effect of mirror errors and receiver misalignment, and the effect of the temperature difference between fluid and absorber plate. The merits of a CPC as a second stage concentrator are analyzed.     

Optical performance analysis of an innovative linear focus secondary trough solar concentrating system

The parabolic trough solar concentrating system has been well developed and widely used in commercial solar thermal power plants. However, the conventional system has its drawbacks when connecting receiver tube parts and enhancing the concentration ratio. To overcome those inherent disadvantages, in this paper, an innovative concept of linear focus secondary trough concentrating system was proposed, which consists of a fixed parabolic trough concentrator, one or more heliostats, and a fixed tube receiver. The proposed system not only avoids the end loss and connection problem on the receiver during the tracking process but also opens up the possibility to increase the concentration ratio by enlarging aperture. The design scheme of the proposed system was elaborated in detail in this paper. Besides, the optical performance of the semi and the whole secondary solar trough concentrator was evaluated by using the ray tracing method. This innovative solar concentrating system shows a high application value as a solar energy experimental device.     

Performance evaluation of v-trough solar concentrator for water desalination applications

The working principle and thermal performance of a new v-trough solar concentrator are presented in this paper. Compared with the common parabolic trough solar concentrators, the new concentrator has two parabolic troughs which form a V-shape with the focal line at the bottom of the troughs. This is beneficial for the installation and insulation of the receiver, and the shadow on the reflective surface is avoided. The new v-trough collector does not require high precision tracking devices and reflective material. And therefore the cost of the system could be significantly reduced. Various experimental tests were carried out both outdoor and indoor using different types of receiver tubes. The results show that the collector system can have thermal efficiency up to 38% at 100 °C operating temperature. System modelling was used to predict the rate of fresh water produced by four different solar collector systems which include both static and one-axis solar tracking technologies. Comparison of the solar collectors at different temperature ranges for humidification/dehumidification desalination process using specific air flow rate were considered. At each temperature range, suitable solar collectors were compared in the aspect of fresh water production and area of solar collector required. Results showed that the new v-trough solar collector is the most promising technology for small to medium scale solar powered water desalination.     

All electrochemical layer deposition for crystalline silicon solar cell manufacturing: Experiments and interpretation

A manufacturing process for crystalline silicon solar cells is presented which consists mainly of electrochemical steps. The deposition of doping glass layers for the front side emitter as well as the back surface field is performed anodically onto the etched and cleaned wafers. The doping atoms, phosphorus or boron, are diffused into the silicon crystal in a furnace at 950 °C in an atmosphere of simply clean air. After the diffusion process the front side doping glass has a blue colour and is suitable to serve as an antireflection coating with a very low surface recombination velocity. For this reason, the doping glass is not etched away on the sun exposed regions of the solar cell. The masking technology for all electrochemical processes provides inherently an edge exclusion and, therefore, no additional processing for preventing shorts on the wafer edge is necessary. For the metallization a reusable rubber mask defines the pattern. First, the mask is used for the doping glass patterning by wet chemical etching. Then, on both sides first nickel is deposited electrolytically directly onto silicon, and in a second step copper electroplating onto the nickel barrier is performed. All three steps, etching, nickel and copper deposition are self adjusting through said rubber mask. A short forming gas anneal finishes the solar cell processing. During all electrochemical processing the wafer is electrically contacted on the opposite surface on a stainless steel plate by the force of vacuum clamping. With this low cost processing 12.5% cell efficiency has been achieved on multi-crystalline 156 mm wafers, which originally have a minority carrier lifetime of 4 μs measured after damage etch and thermal oxidation. In this paper, experiments, surface analysis and physical interpretations are presented.     

Thermal Load of Direct Steam-Generating Absorber Tubes with Large Diameter in Horizontal Linear Fresnel Collectors

Solar thermal power plants are presently the cheapest technology for solar electricity production. Today, level electricity costs of 15 ct/kWh are achievable at good sites with high levels of direct normal irradiation. Nevertheless, further cost reductions are necessary to make solar thermal power plants economically feasible. One possibility for a further cost reduction might be the use of so-called Linear Fresnel Collectors in solar thermal power plants. Preliminary cost estimates of this option show that this is a promising potential for cost reduction. The technical feasibility of the linear Fresnel collector has to be checked in theoretical studies as well as during the operation of a life-sized prototype under real solar conditions. This paper presents results of preliminary theoretical studies regarding the thermal load of the absorber tube used in a linear Fresnel collector with a secondary concentrator.     

Performance of an array of compound parabolic concentrations with plain tubular receivers

Operational aspects for an array of four 1.27X-CPC troughs, connected in series, were evaluated using solar radiation to establish the characteristic performance curve. To produce the limbs of each concentrator, a carefully prepared master mold was fabricated from which castings of polyester resin/fiber glass were produced and made reflective with a film of aluminized acrylic laminate. Four different cases investigated included the array with and without an acrylic cover plate and provided with copper receiver tubes which were either oxidized or made black using a special treatment which increased the optical efficiency 7–10%. The experimental results obtained were used to generate efficiency vs Δt/G relationships. For each case, these relationships could be represented linearly with intercepts to yield optical efficiencies that ranged from 0.545 up to 0.695. The introduction of the cover plate, although decreasing the optical efficiency by 15%, served to suppress heat losses by a factor greater than two. This type of solar collector is suited for applications such as hot water generation, space conditioning, and moderate temperature process heat. Even though CPC collectors have been studied for more than 10 years, many new potential applications remain.     

内聚光型太阳能光电-光热复合管的光学设计

根据非跟踪内聚光型太阳能光电-光热复合管的结构形式及其受光体的特殊需求,采用Winston的CPC设计原理,对圆管内具有特殊形状的半圆柱接收器的复合抛物面聚光器进行光学设计,得到了较为理想的聚光器设计曲线,并对结果进行数值仿真,给出各入射角的光线分布图和光强分布图。     

Comparative assessment of solar concentrator materials

This paper reports results from long-term durability tests of reflector materials to be used for solar concentrating systems. The studies have been conducted under the auspices of an IEA–SolarPACES collaboration between the National Renewable Energy Laboratory (NREL, USA), the Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT, Spain) and Deutsches Zentrum für Luft- und Raumfahrt (DLR, Germany). In this co-operative effort, accelerated ageing tests as well as outdoor exposures at a number of test sites having various climatic conditions have been carried out since 1995. In addition to materials already in use at solar power stations, newer materials offering the chance of a significant cost reduction in solar electricity and process heat generation are being investigated. Comparative optical tests are carried out to assess the efficiency as a function of exposure/service time in a solar concentrator. Among the materials showing promise for long-term outdoor applications are various silvered glass mirrors, a silvered polymer film, and an anodized sheet aluminium having an additional protective polymer coating. In addition to durability tests of reflector material samples, practical results are also reported for experiences with field applications of silvered thin glass and anodized sheet aluminium mirrors.     

具有二次聚光器的新型大开口槽式太阳集热器设计与光学性能分析

针对大开口和更高运行温度的槽式太阳能热发电系统,提出一种可实现高聚光比、低辐射热损及能流密度均匀的新型槽式太阳集热器,即在集热管内放置外壁具有太阳选择吸收膜层和内壁具有反射膜层二次聚光器的大开口槽式太阳集热器。建立圆弧为微元段的自适应设计新方法,提出3种典型的二次聚光器面型,利用蒙特卡洛光线追迹方法仿真新型集热器的能流密度分布特性,验证该光学仿真方法,分析影响集热器光学性能的各种因素。结果表明,该集热器可显著提升集热效率。     

Optical and thermal characterization procedure for a variable geometry concentrator: A standard approach

Concentrating solar collectors are mentioned in the International Standards, but the general testing methods for solar collectors mentioned cannot easily be applied to such unusual collector designs. In this study, the best optical and thermal model for a variable geometry solar concentrator has been investigated. In the particular case of a collector with a fixed mirror concentrator, the relative position of the receiver with respect to the reflector is not constant during the day, and this variable geometry is not taken into account in the current testing Standards. An optical characterization of the prototype using a ray-tracing program has been performed, and the results have been used as an initial hypothesis to define two thermal models adapted from the European Standard. Those two different models have been compared. The optical results obtained from experiments have been compared to ray-tracing simulation results, and they have been found to be quite similar, considering the measurement uncertainties. This validation procedure of the optical simulation could be an important point to be taken into account in a future Standard revision for variable geometry collector types for which the normal incidence is not easy to obtain.