The Fresnel lens concept for solar control of buildings

Fresnel lenses are optical devices for solar radiation concentration and are of lower volume and weight, smaller focal length and lower cost, compared to the thick ordinary lenses. The advantage to separate the direct from the diffuse solar radiation makes Fresnel lenses suitable for illumination control of building interior space, providing light of suitable intensity level and without sharp contrasts. In this paper, the Fresnel lens concept is suggested for solar control of the buildings to keep the illumination and the interior temperature at the comfort level. Laboratory scale experimental results are presented, giving an idea about the application of this new optical system. The collection of 60–80% of the transmitted solar radiation through the Fresnel lenses on linear absorbers leaves the rest amount to be distributed in the interior space for the illumination and thermal building needs. In low intensity solar radiation, the absorber can be out of focus, leaving all light to come in the interior space and to keep the illumination at an acceptable level. The Fresnel lenses can be combined with thermal, photovoltaic, or hybrid type photovoltaic/thermal absorbers to collect and extract the concentrated solar radiation in the form of heat, electricity or both. By using thermal absorbers and for low operating temperature, efficiency of about 50% can be achieved, while considering photovoltaics, satisfactory electrical output can be obtained. Regarding the effect of the suggested system to building space cooling, the results showed a satisfactory temperature reduction, exceeding 10 °C for cold water circulation through the absorber.     

A novel application of a Fresnel lens for a solar stove and solar heating

This paper presents a novel design and the prototyped solar cooking stove which uses a large Fresnel lens for the concentration of sunlight. The technology demonstrates high safety and efficiency of solar cooking and heating using Fresnel lenses which are low cost and available from off-the-shelf. The stove has a fixed heat-receiving area located at the focal point of the lens. The sunlight tracking system rotates the Fresnel lens about its focal point in both zenith and azimuth angles. The tracking is accomplished through a revolving motion of two rotation arms that hold the lens and a horizontal rotation of a platform that the lens system stands on. The rotation of the arms tracks the sunlight in zenith plane, while the rotation of the platform tracks in the azimuth plane. Since the solar tracking allows the Fresnel lens to concentrate sunlight to a fixed small heat-receiving area, relatively low heat loss and high energy efficiency is made possible. The heat is used to maintain a stovetop surface at temperatures around as high as 300 °C, which is practical for cooking applications in a very safe, user-friendly, and convenient manner. The system also demonstrates the possibility of transferring heat using a working fluid for indoor heating and cooking. Wider applications using the system for solar thermal collection and utilization are also undergoing development.     

Design of Fresnel lens with spherical facets for concentrated solar power applications

In this paper, a ray-tracing model is developed using MATLAB based on mathematical formulations used in the design of Fresnel lens with spherical facets. Further, a design chart for Fresnel lens with spherical facets is developed to determine aperture radius and design angle for a given focal length and focus size. Concentration ratio and spherical aberration for Fresnel lens with spherical facets are also compared with those of plano-convex lens and conventional Fresnel lens with triangular facets. Furthermore, the present mathematical model is validated with SolTrace model and with the experimental study conducted on a prototype of Fresnel lens with spherical facets fabricated using CO₂ laser cutting machine. Focal length for the proposed Fresnel lens with spherical facets is found to be the same. However, concentration ratio computed from the present ray-tracing model is in fair agreement with SolTrace model and experimental data with percentage deviations at focal length 5.4% and 12.4%, respectively.     

基于菲涅尔聚光的太阳能砷化镓电池热特性分析

以菲涅尔三级聚光器和三结砷化镓太阳电池芯片为研究对象,利用Trace Pro模拟菲涅尔高倍聚光条件下电池芯片表面的能流密度分布,并将结果导入ANSYS中作为三结砷化镓太阳电池芯片的边界条件。通过有限元模拟了电池芯片的温度和热流分布,并利用热-结构耦合分析法,得到了电池芯片的热应力分布。结果表明:三级聚光器能有效提高聚焦光斑能量均匀性、增大系统接收角,从而降低热应力,提高光伏系统的整体性能。     

Concept and design of modular Fresnel lenses for concentration solar PV system

In this paper, we propose a new configuration of solar concentration optics utilizing modularly faceted Fresnel lenses to achieve a uniform intensity on the absorber plane with a moderate concentration ratio. The uniform illumination is obtained by the superposition of flux distributions resulted from modularly faceted Fresnel lenses. Based on the concept of modularly faceted Fresnel lenses, the cost effective 3-D concentration solar PV system is designed for future applications. Mathematical treatments for deriving the flux distribution and the concentration efficiency at the absorber plane are introduced. As an example, the distribution of the solar flux, at the cell position, is simulated using ray-trace technique for 9, 25, 49, 81, and 121 suns concentration systems. The irradiance distributions at the cell plane are estimated to be uniform within 20%, with a transmission efficiency larger than 70% for low and medium concentration ratios (less than 50 suns).     

Solar-radiation concentration in the Fresnel lens: an optical medium system

One of the tasks of Fresnel lenses (FLs), which are used in solar micromodules jointly with cascade photocells, is to increase the concentration capability. This paper considers the solution of this task at the expense of using optical media (L). A program for the numerical simulation of the concentration characteristics of the FL-L system has been developed with allowance for inaccuracies in FLs and Fresnel losses at the interface of media. It is shown that the average concentration can be increased by 36% in the FL-L system for the case in which there is 95% receiver capture of a flow going from a medium and, as a whole, with allowance for losses at the medium-photocell boundary (they can be regulated), the growth in an average concentration for a silicon solar cell without coating will be 20–25%. At a smaller acceptable percentage of flow capture (if there is a goal to increase concentration), the efficiency of optical media in the FL-L system grows and can reach the values obtained in the paraboloid-optical medium system.     

Linear Fresnel lens concentrators

The use of the linear Fresnel lens as a seasonally adjusted or one-axis tracked solar concentrator is investigated. Ray tracing techniques are used to show that this type of concentrator is very good in the period of time within 1 hr of solar noon. However, increased refraction due to increased incident angles of the solar rays causes a sharp drop-off in energy delivered to the absorber at other hours of the day. Daily collection efficiencies are typically 50 per cent at concentration ratios of near 5. Effects of absorber width, distance from the lens to the absorber and alignment accuracy are considered in detail. Energy collected over a year's time is calculated assuming cloudless skies. Qualitative experimental results are presented that confirm the analytical results.     

Investigation of solar hybrid system with concentrating Fresnel lens,photovoltaic and thermoelectric generators

An experimental model of a solar hybrid system including photovoltaic (PV) module, concentrating Fresnel lens, thermoelectric generator (TEG), and running water heat extracting unit was created and studied. The PV module used was of c‐Si and TEG of Bi₂Te₃; the Fresnel lens (solar concentrator) and TEG share an optical train, whereas PV module was illuminated separately with non‐concentrated light. Heat extracting unit operated in thermo siphon mode. In climatic conditions of Mexico (Queretaro, 20^o of North latitude, summer time), the Fresnel lens accepted 120 W of solar radiation power, and the system generated 7.0 W of electric power and 30 W of thermal one. The discussion is made of the possible characteristics of a hypothetical hybrid system where all its elements share the same optical train. Copyright © 2016 John Wiley & Sons, Ltd.     

对菲涅耳透镜的工艺改进--塑料透射式太阳能聚光器

180多年前，数学家菲涅耳将平凸透镜拉平．制成了由系列小棱镜构成的平板薄片透镜，这就是著名的“菲涅耳透镜”（见图1）。由于薄而轻，用材少，已在多学科、多领域得到应用，如眼科用薄片透镜、齿科照明灯、灯塔、教学仪器、太阳灶等等。菲涅耳透镜的缺点在于：其一，由于棱镜二面全部为平面，焦点为聚焦斑（如图2甲），聚集比远不如球面透镜高（如图2乙）；     

A fixed Fresnel lens with tracking collector

The application of a wide angle concentrating Fresnel lens to a linear solar energy system, in which the optical concentration is stationary while the absorber follows the locus of best foci, is investigated. The two substantial direction possibilities of the linear axis, east-west and polar, are compared to each other. It is shown that such a concentrator may operate about six hours a day throughout the year with an average effective concentration exceeding 10. Specifically, a polar installation, including a fixed lens and a fixed assembly of separate absorbers behind it, may enable sufficient concentration for residential heating and airconditioning without any moving parts.     

Experimental analysis and exergy efficiency of a conventional solar still with Fresnel lens and energy storage material

This study focuses on the experimental investigation and exergy analysis of a modified solar still (MSS) with convex lenses on glass cover to collect the solar radiation at the focus on surface water. A comparative analysis of the performance and yield of the MSS with convex lenses and the conventional single slope SS were carried out for the same climatic condition of Tanta (Egypt). Similarly, the effect of modification in the SS using convex lenses, with or without black stones, on the freshwater yield is experimentally investigated. The results indicated that the lenses focus the solar radiation to the water placed in the basin and increase the water‐glass temperature difference (T _w – T _g). The yield of freshwater from the MSS with the convex lenses is comparatively higher than that of the conventional SS (26.64%). In addition to convex lenses in the inner cover surface, freshwater yield improved by 35.55% by adding blue stones as energy material inside the basin under constant water mass of 30 kg. The maximum exergy efficiency of the SS with lenses and blue stones was 11.7%, while the SS with lenses alone was 4%. The quality of freshwater produced after desalination was well within the World Health Organization standards. The total dissolved solids and pH after desalination were 22 mg/L and 8.08, respectively.     

Analysis of textured solar cells at various angles of incidence: Fresnel concentration to 500 suns

Textured solar cells are becoming the norm in terrestrial applications. In most applications of textured solar cells the light will be incident at various angles, i.e., time of day for flat plate collectors or radial position for a concentrating lens system. However, a detailed analysis of the performance of light trapping geometries at various angles of incidence has not been reported. A quantitative analysis of the light trapping of several texturing geometriesas a function of angle of incidence, in the context of a concentrating Fresnel lens, is presented for the first time. For normally incident light at one sun intensity the improvement in collection efficiency for the textured cells compared to a planar cell is 4.84%, 7.74%, and 12.72% for the slats, inverted pyramids, and perpendicular slats geometries, respectively. At 500 suns concentration under a Fresnel lens the collection efficiency compared to the planar cell is 5.26%, 10.9% and 12.28% for the same three texturing geometries. This implies that a linear response to concentration does not apply for textured surfaces. Collection efficiency as a function of angle of incidence is also used to provide guidelines for cell orientation within flat plate arrays to provide maximum collection efficiency at several hours of the day.     

Compact Linear Fresnel Reflector solar thermal powerplants

This paper evaluates Compact Linear Fresnel Reflector (CLFR) concepts suitable for large scale solar thermal electricity generation plants. In the CLFR, it is assumed that there will be many parallel linear receivers elevated on tower structures that are close enough for individual mirror rows to have the option of directing reflected solar radiation to two alternative linear receivers on separate towers. This additional variable in reflector orientation provides the means for much more densely packed arrays. Patterns of alternating mirror inclination can be set up such that shading and blocking are almost eliminated while ground coverage is maximised. Preferred designs would also use secondary optics which will reduce tower height requirements. The avoidance of large mirror row spacings and receiver heights is an important cost issue in determining the cost of ground preparation, array substructure cost, tower structure cost, steam line thermal losses, and steam line cost. The improved ability to use the Fresnel approach delivers the traditional benefits of such a system, namely small reflector size, low structural cost, fixed receiver position without moving joints, and non-cylindrical receiver geometry. The modelled array also uses low emittance all-glass evacuated Dewar tubes as the receiver elements. Alternative versions of the basic CLFR concept that are evaluated include absorber orientation, absorber structure, the use of secondary reflectors adjacent to the absorbers, reflector field configurations, mirror packing densities, and receiver heights. A necessary requirement in this activity was the development of specific raytrace and thermal models to simulate the new concepts.     

Optical geometry approach for elliptical Fresnel lens design and chromatic aberration

This research formulates an elliptical-based Fresnel lens concentrator system using optical geometry and ray tracing technique. The author incorporates solar spectrum with the refractive indices of lens materials to form different color mixes on the target plane. The model illustrates the solar spectrum distributions under the Fresnel lens. It can be used to investigate each spectral segment's distribution patterns and helps to match the concentration patterns of different wavelengths to different solar energy applications.     

Experimental characterisation of a Fresnel lens photovoltaic concentrating system

An extensive indoor experimental characterisation program to investigate the heat loss from a point focus Fresnel lens PV Concentrator (FPVC) with a concentration ratio of 100× was performed for a range of simulated solar radiation intensities between 200 and 1000 W/m², different ambient air temperatures, and natural and forced convection. From the experimental program it was found that the solar cell temperature increased proportionally with the increase in simulated solar radiation for all experimental tests, indicating that conductive and convective heat transfer were significantly larger than the long wave radiative heat transfer within and from the FPVC system. For the simulated worst case scenario, in which the FPVC system was tested under a simulated solar radiation intensity of 1000 W/m² and ambient air temperature of 50 °C with no forced convection, the predicted silicon solar cell efficiency in the FPVC system was reduced to approximately half that at standard test conditions.     

Theoretical and experimental analysis on efficiency factors and heat removal factors of Fresnel lens solar collector using different cavity receivers

The efficiency and heat removal factors are useful parameters for evaluating the thermal performance of concentrating solar collectors. In this work, the efficiency factors and heat removal factors of Fresnel lens solar collectors using different kinds of point-focus cavity receivers were obtained both theoretically and experimentally. An experimental unit was built, whereby eight kinds of cavity receivers, namely: conical, spherical, cylindrical, hemispherical, positive cone frustum, reverse cone frustum, heteroconical and domical, were tested and analyzed. It is found that the theoretical results agree well with the test results. For the point-focus Fresnel lens solar collector, the conical cavity receiver showed the best thermal performance, with a highest theoretical heat removal factor of 0.868. Effect of conical cavity parameters on the efficiency and heat removal factors were studied. Results showed that under given operation conditions, the optimum aperture diameter of the cavity, the optimum inside diameter of the receiver tube and the optimum vertex angle of the cross section through the symmetric axis of the receiver are 80 mm, 15 mm and 60°, respectively. For better thermal performance, the geometrical concentration ratio of the studied Fresnel lens solar collector should be more than 500.     

High temperature solar collector with optimal concentration: Non-focusing fresnel lens with secondary concentrator

A non-evacuated collector consisting of a linear Fresnel lens and a second stage concentrator of the CPC type is described and tested in detail. Use of a Fresnel lens accomplishes two different objectives simultaneously: it allows for the design of a nearly ideal light collector (of the CPC type) of high concentration and height-to-aperture ratio close to 1 and plays the role of a cover, making the collector less sensitive to the environment than one with exposed reflector surface. The geometric concentration is 15.56 and the acceptance half angle is 3°. The optical efficiency measured with an Active Cavity Radiometer (ACR) is 65.6 per cent and the efficiency at of 0.235 is 48 per cent (ΔT = T_avfluid − T_amb = 200°C, I_ACR = 850 W/m²). Heat loss measurements for double glazed configurations are reported and the resulting efficiency at of 0.3 is predicted to be 48 per cent. These numbers are expected to be raised by 3 percentage points for a next generation of lenses. The collector is mounted with its tracking axis oriented oriented NS since EW tracking axis orientation is impractical for a linear Fresnel lens, but its wide acceptance angle permits tracking by a simple clock mechanism at constant speed. Two different strategies are considered (i) polar mount, (ii) two adjustments of the tracking axis a year (summer and winter); the predicted yearly performance is calculated for four locations and four working fluid temperatures.The projected cost is estimated to be $70.00/m² (1976 dollars), possible because the construction of the collector lends itself to the use of inexpensive materials such as plastic and glass.     

Luminescent spectral splitting: Efficient spatial division of solar spectrum at low concentration

The purpose of this study was to investigate the potential performance of a novel concept for dividing solar radiation into spectral components that separately illuminate photovoltaic (PV) cells of different band gaps using an optical design that (1) is simple, easily manufactured, and extensible to many spectral channels and (2) does not achieve high geometric concentration factors. The concept that we explore leverages the approach of stacked luminescent solar concentrators (LSCs) for dividing the solar spectrum using fluorophores that are tuned to different spectral bands. However, whereas multicolor LSCs must perform two functions using the same optical component—spectral division and concentration—we consider the performance of a similar design when only one demand—spectral division—is placed on it. We find that the optical quantum and power efficiencies can be quite high (QE>90%, PE>80%) compared to what one might intuitively expect. When we couple the light output to a simple detailed balance model of a solar cell using experimental performance parameters we find that solar-to-electric conversion could exceed 30% with four junctions, using existing PV materials. While this does not exceed what can be achieved by HCPV designs on multijunction epitaxially grown stacks, the concept presented here has the major advantage of being easily extensible to an arbitrarily large number of spectral channels. Because of this extensibility, the number of junctions in the system is limited only by the availability of PV cells with appropriate band gaps, so significantly higher system efficiencies should be accessible without major revision to the basic design presented here.     

菲涅耳反射式太阳能聚光系统的三运动复合跟踪

提出了线性菲涅耳式太阳能反射聚光系统新的跟踪方式:置于接收器上方的二次聚光器配合镜场平移的同时进行旋转,与平面镜的转动组成三运动复合形式。通过镜场的整体平移减小余弦损失,提高系统整体的聚光集热效率。从理论分析和计算机模拟两个方面对这种三运动复合形式进行的研究发现,三运动系统的总余弦损失在一天中基本不变。对一个24 m2镜场的测试分析表明,在夏季晴天条件下,三运动复合系统的有效能量增加率约为5.8%,每天的能量增益达到38 MJ左右。