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。     

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。     

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.     

Analysis of heat losses from a trapezoidal cavity used for Linear Fresnel Reflector system

A Computational study to investigate the heat loss due to radiation and steady laminar natural convection flow in a trapezoidal cavity having eight absorber tubes for a Linear Fresnel Reflector (LFR) solar thermal system with uniformly heated tubes and adiabatic top wall and side walls has been performed. The losses due to convection and radiation were considered from the bottom glass cover. The results are validated with experimental data. Radiative component of losses from the cavity was found to be dominant which contributes around 80–90%. Heat loss characteristics have been studied for cavities of different depths. Simulations have been carried out for various values of heat transfer coefficient based on the wind speed below the glass surface. Effect of emissivities of the tubes on the heat loss has also been simulated. Flow pattern and isotherms inside the cavity for various depths have been analyzed. Finally, the correlation between the total average Nusselt number and its influencing parameters has been obtained for the proposed cavity.     

Concentrations of the Linear Fresnel Reflector with the Facets Orientated to the Immovable Receiver

A numerical model of a linear Frensel reflector (LFR) with oriented facets is suggested. The concentrating performances of linear Frensel reflectors are determined by considering the errors of manufacturing and facet positioning relative to the Sun. Rational optical—geometrical parameters of linear Frensel reflectors are revealed. It is shown that an LFR can operate efficiently for eight hours during the day.     

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.     

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.     

Neural-net based real-time economic dispatch for thermal powerplants

This paper proposes the application of artificial neural networks to real-time optimal generation dispatch of thermal power plant units. The approach can take into account operational requirements and power network losses. The proposed economic dispatch uses an artificial neural network (ANN) for the generation of penalty factors, depending on the input generator powers and identified system load change. Then, a few additional iterations are performed within an iterative computation procedure for the solution of coordination equations, by using reference-bus penalty-factors derived from Newton-Raphson load flow. A coordination technique for environmental and economic dispatch of pure thermal power systems, based on neural net theory for simplified solution algorithms and an improved man-machine interface is introduced. Numerical results on two test examples show that the proposed algorithm can efficiently and accurately develop optimal and feasible generator output trajectories by applying neural net forecasts of power system load patterns     

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.     

Adaptive optimal control of steam temperatures for thermal powerplants

An adaptive optimal control method using a process control computer was developed for boiler steam temperature control for thermal power plants. A process dynamics model, which describes the dynamic plant behavior, is used in the calculations for adaptive optimal control. The mode used is a multi-input and output ARMA (autoregressive moving-average) with parameters identified online by the Kalman filter method. The adaptive optimal control method was applied to a coal-fired thermal power plant simulator. Errors in predictions of the present model were one-third to one-half those of a model using parameters identified offline by the LWR (Levinson-Wiggins-Robinson) method. The fluctuation of the temperatures from the set points was reduced to half of that obtained by a conventional control method which uses PI (proportional+integral) controllers     

太阳能中高温热发电反射式线性菲涅尔技术简介

<正>一引言随着常规化石能源的日渐枯竭,世界上兴起了新能源利用的热潮。太阳能作为新能源的典型代表,得到世界各国的重视。尤其是太阳能中高温热发电技术得到了各国政府的财政支持,也是科研工作者研究的热点。太阳能反射聚光技术作为太阳能中高温热发电的核心技术,目前主要分为塔式、碟式、槽式和线     

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.     

Comprehensive Review of Line-Focus Concentrating Solar Thermal Technologies: Parabolic Trough Collector (PTC) vs Linear Fresnel Reflector (LFR)

In the present review,parabolic trough collector(PTC) and linear Fresnel reflector(LFR) are comprehensively and comparatively reviewed in terms of historical background,technological features,recent advancement,economic analysis and application areas.It is found that although PTC and LFR are both classified as mainstream line-focus concentrating solar thermal(CST) technologies,they are now standing at different stages of development and facing their individual opportunities and challenges.For PT...     

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.     

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 comprehensive model for optical and thermal characterization of a linear Fresnel solar reflector with a trapezoidal cavity receiver

In this paper, a comprehensive numerical model was developed by coupling Monte Carlo Ray Tracing (MCRT) and Finite Volume Method (FVM) for simulating the energy conversion process in the linear Fresnel reflector (LFR) with a Trapezoidal Cavity Receiver (TCR). Based on the model, firstly, the optical performance of a typical LFR was studied, followed by analyzing its heat transfer characteristics and thermal performance at various conditions. Then, the effects of key parameters were investigated. Finally, a LFR prototype was simulated to illustrate the application of the model. The results indicate that the solar fluxes on the absorber tubes exhibit non-uniform characteristics which would result in the non-uniform temperatures. The annual optical efficiency of 60.1%–44.7% from the equator to N50° and the collector efficiency of 48.3%–72.0% for the superheating section at normal incidence can be achieved, respectively. Moreover, the heat transfer characteristic study reveals that the radiation loss from the tubes is the dominant mode and contributes around 81%–87% at typical conditions. Parameter studies indicate that the energy absorbed by the glass which influences the heat loss obviously should be considered in the heat loss study of TCR. And the heat loss from the tubes increases rapidly with the coating emissivity, so the coating with low emissivity should be recommended for the TCR. In addition, the application in the realistic LFR indicates that the present model is an exercisable and useful tool for the LFR.     

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.     

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。     

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.