基于三角腔体吸收器的透射式菲涅尔定焦线系统聚光特性分析

为解决线性菲涅尔太阳能集热系统单轴跟踪过程中出现的聚光焦线偏移以及降低系统跟踪能耗等问题,提出一种透射式菲涅尔定焦线太阳能聚光器.该聚光器采用极轴跟踪方式与线性菲涅尔透镜定期滑移调节方式相结合,可实现固定焦线聚光.将该聚光器与三角腔体吸收器所组成的太阳能集热系统,利用基于蒙特卡罗光线追迹法的TracePro光学软件分析...     

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.     

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.     

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.     

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.     

一种新型极轴跟踪式定日镜的研究

提出一种新型采用轮胎面聚光镜极轴跟踪式定日镜。分析了极轴跟踪式定日镜的跟踪原理,设计了用于该定日镜的轮胎面并分析其聚光性能。结果表明:该定日镜聚光性能较高,能够满足生活热水、太阳炉等应用领域的要求。     

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.     

小型菲涅尔定焦线聚光系统关键参数影响特性研究

基于线性菲涅尔透镜聚光特性和极轴式跟踪原理,提出一种采用圆弧腔体吸收器的小型菲涅尔定焦线聚光系统。采用蒙特卡洛光线追迹方法与数理统计原理,详细研究太阳赤纬角、太阳时角和腔体内表面吸收率等关键参数对聚光系统光学性能的影响。结果表明,腔体内表面吸收率对光学效率因子的影响最显著,其次为太阳赤纬角、太阳时角。腔体内表面吸收率分别为1.00、0.85、0.75时,系统平均光学效率因子分别为0.950、0.865、0.799。太阳赤纬角对能流均匀性影响最显著,其次为太阳时角、腔体内表面吸收率。在太阳赤纬角分别为0°、8°、16°、23.45°时的平均均匀因子分别为0.507、0.519、0.561、0.612。该系统可减少余弦损失、降低焦线偏移对端部损失的影响。     

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.     

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.     

Numerical modeling of tunable optofluidics lens based on combined effect of hydrodynamics and electroosmosis

A numerical model of liquid-core liquid-cladding optofluidics lens under the combined effect of hydrodynamics and electroosmosis are presented in this paper. In the numerical simulation, a combined formulation using only one set of conservation equations to treat both fluids are employed. The coupled electric potential equation and Navier–Stokes equation are solved using the finite volume method. The level-set method is used to capture the interface between the fluids. To overcome a weakness in the level-set method, the localized mass correction scheme is applied to ensure mass conservation. The validity of the numerical scheme is evaluated by comparing with the experimental results; numerical results highlight the electroosmotic effect; the combined effect of pressure driven and electroosmosis can form optically smooth interfaces with arc-shape between the cladding fluids and the core fluid. Under fixed cladding flow rates, the same electric field forms symmetric biconvex lens only. Different electric fields can form biconvex lens, plane-convex lens, and meniscus lens. The results also present the velocity profiles and flow fields of micro lens. There is a good agreement between numerical and experimental results.     

Simple tracking strategies for solar concentrations

Consideration is given to the validity of single axis tracking systems for solar concentrators of low to medium concentration ratios having moderate acceptance angles. If the misalignment between the sun and reflector normals is within the acceptance angle perfect tracking can be assumed. Rotation about a fixed polar axis gives a constant misalignment equal ot the sun's declination angle on that day. Rotation about a declination axis gives perfect alignment at noon, but increasing misalignment towards each end of the day varying with the time from the equinoxes. Data is also given for monthly adjustment of the declination axis. All the results are independant of latitude.     

Two-stage concentrator permitting concentration factors up to 300x with one-axis tracking

Economic operation of high-efficiency concentrator solar cells requires solar concentration ratios which up to now can only be achieved with two-axis tracking. In this paper we present a two-stage concentrator approaching concentration ratios up to 300X while being tracked around only one polar axis. Its principle is as follows: a parabolic trough focuses the direct solar radiation onto a line parallel to the polar tracking axis. The half rim angle of this first concentrating stage is chosen to be equal to the sun's maximum declination of 23.5°. The second stage consists of a row of dielectric, nonimaging 3-D-concentrators, which couple the concentrated light directly into square solar cells. In contrast to linear secondaries the 3-D-secondaries make use of the limited divergence of ± 23.5° in the NS-direction to achieve additional concentration. The performance of the system depends sensitively on how well the angular acceptance characteristic of the second stage matches with the square-shaped angular irradiance distribution in the focal line of the parabolic trough. A new concentrator profile has been found that exhibits an almost ideal square acceptance characteristic with a very sharp cut-off. A prototype two-stage concentrator has been constructed with a total geometrical concentration of 214X. In outdoor measurements a total optical efficiency of 77.5% was obtained.     

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.     

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.     

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.     

Effect of concentration distribution on cell performance for low-concentrators with a three-dimensional lens

The effect of concentration distribution on the performance of a static concentrator cell with a new three-dimensional lens was investigated to design an optimum cell structure. A concentration distribution of this lens was simulated using a ray-tracing method. Based on these results, fill factors were calculated as a function of series resistance and irradiance to clarify the effect of concentration distribution. The effect of concentration distribution appeared when series resistance is larger than 0.2 Ω cm² and irradiance is higher than 30 mW/cm².     

DESIGN OF A NONIMAGING FRESNEL LENS FOR SOLAR CONCENTRATORS

An optimum convex shaped nonimaging Fresnel lens is designed following the edge ray principle. The lens is evaluated by tracing rays and calculating a projective optical concentration ratio. This Fresnel lens is intended for use in evacuated tube type solar concentrators, generating mid-temperature heat to drive sorption cycles, or provide industrial process heat. It can also be used along with a secondary concentrator in photovoltaic applications.     

The study of the performance and efficiency of flat linear Fresnel lens collector with sun tracking system in Iraq

Two flat linear Fresnel lenses and two absorbers connected in series. Tracking system is constructed so that it tracks the sun in two directions. Thermal and optical losses are introduced. The thermal efficiency of the first lens is higher than the second lens and reaches 0.65. The FLFL all-day collector efficiency reaches 0.58 and it varies depending on weather condition.     

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.