Impact of heliostat curvature on optical performance of Linear Fresnel solar concentrators

The present paper gives a numerical investigation of the effect of mirror curvature on optical performance of a Linear Fresnel Reflector solar field installed recently in Morocco. The objective is to highlight and discuss the effect of mirror curvature on the flux density distribution over the receiver and the system optical efficiency. For this purpose, a Monte Carlo-ray tracing simulation tool is developed and used to optimize the optical design taking into account the curvature degree of the heliostat field. In order to assess the accuracy of the numerical code developed and the validity of simulation results, a set of verification tests were developed and detailed within this article. Then, the optical performance of the system is evaluated as a function of mirror curvature and receiver height. The major challenge of this study is to find a trade-off between heliostat curvature and receiver height since lower and smaller receivers may reduce the system cost. It has been found that the flux distribution over the receiver and the optical efficiency of the system are relatively sensitive to the mirror curvature. We have demonstrated quantitatively how the use of curved mirrors can enhance the optical performance and reduce the required receiver size.     

Comparison of Fresnel concentrators for building integrated photovoltaics

To develop concentrating photovoltaic systems for building integration applications, two optical devices are proposed. The concentrators are based in stationary linear Fresnel lenses and secondary CPC. The moving focal area is ten times smaller than the Fresnel lens aperture. Concentrator characteristics are studied in detail: shadowing effect, placement of the focal area and optical concentration efficiency. The main contribution of this paper is the three-dimensional optical analysis of the non-imaging concentrating systems. In terms of solar radiation, photovoltaic moving modules placed in the focal area of stationary concentrators are compared with simply fixed photovoltaic modules. In favourable weather locations, the beam radiation incident on the concentrating modules would be a large percentage, more than 50%, of the global radiation received by the fixed photovoltaic devices.     

Efficient Fresnel lens for solar concentration

The development effort in optical components for optimally concentrating solar energy has thus far emphasized reflecting elements, such as cylindrical and compound parabolic mirrors. In this paper we consider transmission elements, particularly a new design for an efficient linear Fresnel lens capable of high concentration for a given acceptance angle. The predicted performance of the lens is comparable to that of the “ideal” reflector, while providing greater reliability at a lower cost.     

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）；     

线性菲涅耳太阳能聚光系统

线性菲涅耳聚光反射装置是对连续抛物面聚光镜的一种离散化近似,因由法国工程师Fresnel发明而得名。线性菲涅耳太阳能热发电技术在太阳能热发电系统中非常有用,尤其是在需要安装大面积的镜场时,对成熟的槽式太阳能聚光热发电系统形成强有力的竞争。简要介绍了线性菲涅耳聚光反射装置的技术特点及发展现状。     

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.     

Linear Fresnel lenses, their design and use

A linear Fresnel lens (LFL) designed according to Fermat's principle is slightly modified with respect to used technology for mass production from glass. Manufactured Fresnel lenses are used in a fully tracking concentrating collector with aperture about 36 m² and in a collector with stationary concentrator and movable absorber, which may serve as solar collector with temperature and illumination control. A combination of linear Fresnel lenses with PV cells may reduce cost of autonomous solar installations.     

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.     

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.     

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。     

Comparison of Linear Fresnel and Parabolic Trough Collector power plants

The Linear Fresnel Collector (LFC) technology is currently being commercialised by several companies for the application in solar thermal power plants. This study compares the electricity generation costs for LFC and Parabolic Trough Collector (PTC). PTC is the most commercial CSP technology to date and is therefore regarded as the benchmark. For reasons of comparability, direct steam generation is assumed for both LFC and PTC.For the LFC, cost data comparable to typical CSP plant sizes are hardly available. Therefore, the break even cost – referring to aperture-specific collector investment – is determined, where cost-parity of the electricity generation with a PTC reference plant is reached.This study varies the assumptions on collector performance and operation and maintenance costs to reflect different designs of LFC technologies. The calculations were carried out using cost and hourly simulation performance models. Depending on the assumptions, the costs for a linear Fresnel collector solar field should range between 78 and 216 €/m² to reach cost-parity at assumed reference solar field costs of 275 €/m² for the PTC.The LFC principle of arranging the mirrors horizontally leads to lower aperture-related optical efficiency which must be compensated by lower cost per m² of aperture compared to PTC. The LFC is a collector with significant cost reduction potential, mainly due to cheaper mirrors and structural advantages.The presented cost and performance targets shown in this study must be met by LFC technology developers to reach the PTC benchmark.     

线性菲涅尔集热器光学特性实例分析与模拟

对线性菲涅尔集热器的聚光性能和光学效率进行了模拟与计算.根据几何光学原理,对集热器镜场各项光学损失(如余弦损失、阴影与遮挡损失)建立数学模型,计算出每项光学损失对应的光学效率.再用TracePro光学软件建立集热器的几何模型,利用光线追踪的方法,模拟入射到镜场的光线在模型空间的传播.光线在模型表面发生吸收、反射和散射等过程,追踪每束光线的光通量,计算得到集热器的光学效率和聚光比等性能参数.结果表明:通过数学模型和光学软件模拟得出的集热器光学效率一致,2种方法分别从细节与整体上剖析了影响集热器光学性能的因素,在集热器设计中可以结合使用,互相补充.     

大型曲面菲涅耳薄板透镜的制造与应用

一引言 传统的平面菲涅耳透镜质轻价廉,用材料少,已在多学科得到应用.其难点是直径难以造大,限制了其在阳光集热方面的使用,许多专家试图设计大型精密机床来制造大尺寸整体菲涅耳透镜用于阳光集热,其技术难度之大、成本不菲可以想见.笔者创新提出区块分割法,发明新型太阳能聚光集热器--塑料透射式太阳能聚光器(专利申请号:CN200410020974.2,公开号:CN595011A),核心技术之一是其巨型的薄板聚光透镜由一系列曲面聚光瓦拼接搭建而成,从此人们可以利用聚光瓦像用砖瓦盖房一样轻易地构筑自己所需要的聚光集热装置,从太阳光中获得所需的高温能量.     

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.     

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.     

Theoretical analysis of the Fresnel lens as a function of design parameters

The author has studied the theoretical performance of the Fresnel lens as a function of the design parameters—i.e. the radius to the centre of the steps, the focal distance from the back of the lens to the plane of the image with the object (i.e. the sun) at infinity, the thickness of the lens plate, the step width and the refractive index of the material (with respect to air)—used in its fabrication. Numerical calculations have also been carried out for a Fresnel lens of perspex sheet.     

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.     

Influence of Dust Accumulation on the Solar Reflectivity of a Linear Fresnel Reflector

Solar reflecting and heat collecting systems are generally operated outdoors year-round, and the optical performance of the reflector is reduced by dust accumulation on the surface. In this study, the dust accumulation on a linear Fresnel reflector was investigated. The relative reflectivity of the mirror before and after dust accumulation and the physical and chemical characteristics of the dust were measured using an ultraviolet spectrophotometer, scanning electron microscope, and X-ray diffractometer. The results showed that the dust density on the mirror increased, and the relative reflectivity decreased with an increase in the dust accumulation time. During 48 days of dust accumulation, the average relative reflectivity decreased 9.4% for a 1 g/m~2 increase in the dust density. Additionally, the dust density on the mirror increased while the relative reflectivity decreased with a decrease in the mirror tilt angle. The rate of decrease of the relative reflectivity was higher for the aluminum mirror than that of the silver mirror after dust accumulation. The main component of the dust particles in the test area was Si O_2, and the particle size range of the dust was 0.9 μm to 87 μm. According to the physical and chemical properties of the dust and the shielding effect of the dust on the mirror, a model to predict the influence of natural dust accumulation on the relative reflectivity of the linear Fresnel reflector was proposed. The predicted results deviated about 1% from the test results.     

Nontracking solar concentrators

We derive the theoretical upper limit for concentration of direct solar radiation at low latitudes with nontracking concentrators from the projected solid angle sampled by the apparent motion of the sun, for the case where the energy efficiency is referred to the energy incident on the entrance aperture. Based on the fact that the solar radiation is not uniformly distributed within this projected solid angle and that the apparent solar motion is known, we derive the optimal acceptance as a function of direction and time, which means rejecting the lower density radiation and switching off the device when losses would be higher than gains. Just as a device may gain concentration by rejecting radiation from certain directions, it can also gain by not operating at all, thus avoiding losses at certain times. Trough-type systems, which have translational symmetry, cannot be ideal nontracking concentrators, but for low losses they perform only slightly worse than general three-dimensional concentrators.