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).     

Two stage linear Fresnel lenses

A new design of a second stage reflective element, which is closely related to the earlier trumpet configuration, is presented. The implementation for linear Fresnel lenses is derived, leading to high solar concentration with both convex and flat lenses.     

A method to calculate Fresnel lenses

In solar engineering, in contrast to image optics, Fresnel lenses are intended for securing the required concentrations of solar radiation and its distribution over a receiver’s surface. It is also important to secure a high use coefficient of the concentrated flux. In particular, this defines the features of calculation of Fresnel lenses: it is necessary to take into account inaccuracies in fabrication of Fresnel lenses and solar radiation redistribution by means of selecting the respective parameters of Fresnel lens belts. In the present work, we examine the procedure for the calculating geometrical parameters of Fresnel lenses on a flat base by considering the mentioned requirements. A corresponding software for calculating the geometrical parameters and concentrating characteristics of the Fresnel lenses is developed, and examples of calculation are given. For a constant refractive index of Fresnel lens material, it is shown that the Fresnel lens can secure a concentration of about 1000, but in this case the optical efficiency of the Fresnel lens will not be higher than 70%. The procedure that has been developed may be the basic one for determining the parameters and concentrating characteristics of Fresnel lenses by considering refractive index variance.     

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.     

Design of one-axis tracked linear Fresnel lenses

The most frequently used criterion in the design of linear Fresnel lenses consists of minimizing the radiation spread at the collector when the lens is illuminated by rays that are contained in the full lens acceptance angle and are incident on the lens in the plane perpendicular to the lens axis. This paper analyzes this topic and concludes that when the lenses are one-axis-tracked this criterion can be improved by replacing the perpendicular plane with another forming a certain angle to it. When calculated for a specific lens, at the location of Madrid, the new criterion was found to yield an increase of more than 7% in the total annual energy collected.     

Analysis of spectrum distribution and optical losses under Fresnel lenses

During late 1970s and early 1980s, Fresnel lenses have received more attention in the field of solar energy application. This paper briefly examines the Fresnel lens development since 1970s and investigates the losses inherent in the linear Fresnel lenses. The research develops the formulation that helps to quantify the linear lens’ transmittance loss and prism-tip scattering loss and then derive a realistic model to simulate the measured data. In addition, the research has identified an elliptical-based lens that comes closest to the condition of minimum deviation. Such lens closely duplicates the curvature needed for maximum transmission. The researcher applies different design wavelengths to study the contribution of each wavelength interval on the receiving plane.     

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.     

Sandblasting durability of acrylic and glass Fresnel lenses for concentrator photovoltaic modules

The impact of sandblasting on Fresnel lenses for concentrator photovoltaic (CPV) modules must be carefully considered when CPV systems are installed in desert areas. In this study, the transmittance after sandblasting of acrylic polymethylmethacrylate (PMMA) and glass substrates was evaluated, and their durability was assessed. The transmittance decreased as the momentum of the blown sand increased. The conversion efficiency of the CPV modules was estimated using equivalent circuit calculations, and the influence of sandblasting on the output of the CPV modules was quantitatively predicted. The coefficients of degradation for the PMMA and glass CPV modules were 0.17 and 0.08 points per unit of momentum, respectively. The durability of the CPV module with the glass Fresnel lens is approximately twice that with the PMMA lens.     

Design of dome-shaped non-imaging Fresnel lenses taking chromatic aberration into account

Concentration PV system is a technology for providing solar-based electricity at very high conversion efficiency of 40%. It needs solar concentration of 500 suns or more, for which the authors developed dome-shaped non-imaging Fresnel lenses with a certain acceptance half angle. As conventional design method uses only one wave length, the performance suffers from chromatic aberration. In this paper, a new design method is proposed. One of the points is that it uses two kinds of design wave length which covers a given range of solar spectrum for the concentration. The other is that new design points are located on the corners of prisms while the conventional point is at the center of prisms. Numerical examples with the concentration ratio of 500 were designed and optical efficiency was examined by ray tracing simulation. The results indicate that the lens based on the conventional way has dish-like shape and the lenses designed by the proposed method have relatively deep dome shape in contrast. The optical efficiency of the new design is better than that of the conventional one at the incident angle equal to the acceptance half angle. It was concluded that the proposed method could produce more effective solar concentrator with a certain tolerance of solar incident angle.     

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.     

Performance of a concentrated photovoltaic energy system with static linear Fresnel lenses

A new type of greenhouse with linear Fresnel lenses in the cover performing as a concentrated photovoltaic (CPV) system is presented. The CPV system retains all direct solar radiation, while diffuse solar radiation passes through and enters into the greenhouse cultivation system. The removal of all direct radiation will block up to 77% of the solar energy from entering the greenhouse in summer, reducing the required cooling capacity by about a factor 4. This drastically reduce the need for cooling in the summer and reduce the use of screens or lime coating to reflect or block radiation.All of the direct radiation is concentrated by a factor of 25 on a photovoltaic/thermal (PV/T) module and converted to electrical and thermal (hot water) energy. The PV/T module is kept in position by a tracking system based on two electric motors and steel cables. The energy consumption of the tracking system, ca. 0.51 W m⁻², is less than 2% of the generated electric power yield. A peak power of 38 W m⁻² electrical output was measured at 792 W m⁻² incoming radiation and a peak power of 170 W m⁻² thermal output was measured at 630 W m⁻² incoming radiation of. Incoming direct radiation resulted in a thermal yield of 56% and an electric yield of 11%: a combined efficiency of 67%. The annual electrical energy production of the prototype system is estimated to be 29 kW h m⁻² and the thermal yield at 518 MJ m⁻². The collected thermal energy can be stored and used for winter heating. The generated electrical energy can be supplied to the grid, extra cooling with a pad and fan system and/or a desalination system. The obtained results show a promising system for the lighting and temperature control of a greenhouse system and building roofs, providing simultaneous electricity and heat. It is shown that the energy contribution is sufficient for the heating demand of well-isolated greenhouses located in north European countries.     

线性菲涅尔反射式太阳能集热系统的设计与试验研究

本文研究了线性菲涅尔反射式太阳能集热系统,基于几何光学原理计算模拟了线性菲涅尔反射镜镜场和复合抛物面的接收系统,在减少末端损失的基础上设计了系统的机械结构,制作了线性菲涅尔反射式太阳能集热系统的装置,并进行了集热性能的实验测试。测试结果表明,该系统在9倍聚焦倍率下,导热油的最高温度可达176.2℃,系统的平均瞬时热效率约为53%,很好地实现了其集热性能。     

Durability of Fresnel lenses: A review specific to the concentrating photovoltaic application

The durability of Fresnel lenses used in the concentrating photovoltaic (CPV) application is reviewed from the literature. The examination here primarily concerns monolithic lenses constructed of poly(methyl methacrylate) (PMMA), with supplemental examination of silicone-on-glass (SOG) composite lenses. For PMMA, the review includes the topics of: optical durability (loss of transmittance with age); discoloration (the wavelength-specific loss of transmittance); microcrazing and hazing; fracture and mechanical fatigue; physical aging, creep, shape change, buckling, and warping; and solid erosion. Soiling, or the accumulation of particulate matter, is examined in the following contexts: its magnitude of reduction in transmittance; variation with time, module tilt, and wavelength; the processes of adhesion and accumulation; particle size, distribution, composition, and morphology; and its prevention. Photodegradation and thermal decomposition, mechanisms enabling aging, are examined relative to the CPV-specific environment. Aspects specific to SOG lenses include: solarization of the glass superstrate; corrosion of glass; delamination of the silicone/glass interface; change in focus due to thermal misfit between the laminate layers; and the chemical stability of poly(dimethylsiloxane) (PDMS). Recommendations for future research are provided, based on the most important and the least explored topics.     

线性菲涅耳集热器接收器对镜场的遮挡研究

带有二次反射器的线性菲涅耳聚光集热器,虽然其接收器表面的能量分布更均匀,但也增加了接收器对反射镜场的遮挡。本文采用理论计算和模拟两种方式对已设计的线性菲涅耳集热器二次反射器对镜场的遮挡情况进行分析,先通过理论计算得出二次反射器对西边三块初级反射镜遮挡角θni的范围,然后利用光线追踪软件模拟出安装和不安装二次反射器两种情况下到达初级反射镜的光线数目,根据光线数目变化情况得出西边三块反射镜的遮挡角变化范围分别为70º ~ 90º、54º ~ 72º和42º ~ 58º,同时引入光线损失率来衡量二次反射镜对镜元遮挡的影响,结果得出安装二次反射器后镜元光线损失率最大值达到23.53%。     

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.     

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.     

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.     

Modeling and Performance Simulation of an Innovative Concept of Linear Fresnel Reflector based CSP System

Concentrating solar power technology is one of the most promising alternative energy technologies. In recent past, Linear Fresnel Reflector systems have received great attention and novel designs have been proposed keeping in view the objective to enhance its functionality and performance. For achieving the same objective, this study presents a novel concept where a conventional LFR is enclosed in a greenhouse called greenhouse-LFR. It was expected that such an enclosure can:(1) increase the incoming solar radiation,(2) further improve the overall efficiency due to simplified cleaning process and(3) reduce the capital cost for the construction of LFR support system. A complete thermal and optical analysis was presented for modeling and performance evaluation of the solar field of both conventional-LFR and novel greenhouse-LFR. Sets of non-linear equations for each system were solved using Newton-Raphson method. More detailed optical analysis was further performed for conventional-LFR considering the seasonal variations. The results concluded that the greenhouse-LFR is better than the conventional-LFR as it had higher efficiency and useful heat with lesser heat losses. For greenhouse-LFR, the maximum thermal efficiency was 73.2% whereas for conventional-LFR it was 37.2%. Also, there was an average increase of useful heat by 3 times in the month of February and 4.7 times in the month of September. Sol Trace~(TM) analysis indicated significant spillage loss when a conventional-LFR was used without a secondary reflector or slight curvature of the mirrors.