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.     

基于菲涅尔透镜聚光的遮阳伞温差发电系统

为了使太阳能可以得到更加充分的利用,设计了一种应用在大型遮阳伞上的半导体温差发电系统。本设计运用半导体材料的塞贝克效应,利用半导体温差发电片进行发电,通过菲涅尔透镜的聚光作用来为半导体温差发电片的热端加热,采用水冷的方式为其冷端散热,并用蓄电池储存半导体温差发电片所产生的电能。通过实验,得到了菲涅尔透镜与温差发电片的相对布置距离以及冷却水流速对温差发电片输出功率的影响。     

Efficiency of fresnel lenses

This paper discusses the efficiency of Fresnel lenses with respect to optical (reflection and transmission) losses. The efficiencies of lenses of different step widths—i.e. 2, 3, 4 and 5 mm—are the same (91·9%) in the case of reflection losses and the maximum efficiency of a lens of 5 mm step width is 95·8% with respect to transmission losses.     

Efficiency of fresnel lenses with respect to thermal losses

Experimental observations and heat loss calculations for a Fresnel lens under forced and natural convection have both been obtained. A lens with a 2·0 mm step width and 325·99 cm² area when exposed to solar radiation and kept normal to solar radiation with an intensity of 43·116 cal/cm²/h has given a steady-state temperature of 187·5°C at its focus.     

Experimental update to fresnel diffraction theory

Diffraction of coherent radiation by obstructions is discussed in the context of camera obscura employment to align the facets of the Big Solar Furnace concentrator.     

Performance testing of a linear fresnel reflector

Results of optical and thermal performance evaluation tests conducted on a prototype linear Fresnel reflector system fabricated using locally available material are presented in this paper.     

A fresnel strip reflector-concentrator for tubular solar-energy collectors

A linear Fresnel reflector-concentrator employing commercial flat, front-reflecting mirror strips (of reflectivity ϱ = 0·6), arranged in a planar configuration, has been fabricated and analyzed. An average concentration ratio of about 18 has been obtained for two-thirds of the periphery of a tubular receiver of 0·025 m diameter. Working in conjunction with a partially evacuated (10⁻² Torr) cylindrical collector employing a selective cobalt oxide coating (with α = 0·88, ε_250°C = 0·25 and stable up to 600°C), a stagnation temperature of ∼ 385°C has been obtained, corresponding to a direct solar flux of ∼ 600 W/m².     

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.     

Performance of an inclined solar still with rectangular grooves and ridges

This work investigates the experimental performance of a new type inclined solar still with rectangular grooves and ridges in absorber plate. The still was fabricated and tested for various inclination angles of 25°, 30° and 35° facing south with absorber plate. Performances of the still were compared with different wick materials (Black cotton cloth, Jute cloth, and Waste cotton pieces) on the absorber plate. The effect of placing porous material (Clay pot) and energy storing material (Mild steel pieces) in the grooves were studied. The results demonstrate that 30° inclination is optimum which yielded 3.77 L/day production. Compared to different wick materials, black cotton cloth helps to achieve maximum productivity of 4.21 L/day. The addition of permeable materials and energy absorbing materials also enhances the distillate output to 4.27 L/day.     

Some geometrical design aspects of a linear fresnel reflector concentrator

A somewhat new approach to the design of solar concentrators of Fresnel reflector geometry is outlined. the constituent mirror elements of the concentrator surface are characterised by three parameters, shift, tilt and width. the evaluation of these parameters and the concentration characteristics are investigated on the basis of a simple ray optical model.     

Evaporation heat transfer characteristics of a grooved heat pipe with micro-trapezoidal grooves

A detailed mathematical model predicting the effect of contact angle on the meniscus radius, thin film profile and heat flux distribution occurring in the micro-trapezoidal grooves of a heat pipe has been presented. The model can be used to determine the maximum evaporating heat transfer rate in the evaporator including the effects of disjoining pressure and surface tension. The equation of meniscus radii calculation in the evaporator at given heat load based on the liquid wicks configuration has been put forward. The numerical results show that while the capillary limitation governs the maximum heat transport capability in a grooved heat pipe, the thin film evaporation determines the effective thermal conductivity in a grooved heat pipe. The ratio of the heat transfer through the thin film region to the total heat transfer through the wall to the vapor phase decreases when the contact angle increases. The superheat effects on the heat flux distribution in the thin film region also have been conducted and the results show that the disjoining pressure plays an important role in this region. The current investigation will result in a better understanding of thin film evaporation and its effect on the effective thermal conductivity in a grooved heat pipe.     

Geometrical designs and performance analysis of a linear fresnel reflector solar concentrator with a flat horizontal absorber

Two different approaches for designing a linear Fresnel reflector solar concentrator (LFRSC) with a flat horizontal absorber are described. The performance characteristics of both the designs are studied in detail. The distribution of local concentration ratio on the surface of the absorber, for each design, is investigated using the ray trace technique. Results of some typical numerical calculations are presented graphically and discussed.     

Sweep‐out flow of film condensation on vertical plates with horizontal grooves

We have investigated fluid flow characteristics of film‐wise condensation on vertical plates with horizontal periodic grooves. Condensate stays at the edge of the grooves due to the surface tension. The condensate starts to flow, however, when the balance between the surface tension and the condensate's own weight is broken. It is found that the condensate flows downward successively and periodically from the top part of the plate as a group. In addition, we have obtained the relation between the frequencies of the periodic flow and the degree of sub‐cooling for two different pitches of the grooves. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20249     

Numerical simulation and multiobjective optimization of a microchannel heat sink with arc-shaped grooves and ribs

ABSTRACT

In order to obtain the optimal structure size of a microchannel heat sink (MCHS) with arc-shaped grooves and ribs according to the actual demand, multiple parameters that influence the performance of the microchannel are analyzed by combining the multi-objective evolutionary algorithm (MOEA) with computational fluid dynamics (CFD). The design variables include the relative groove height, relative rib height and relative rib width, and the two objective functions are to minimize the total thermal resistance and pumping power in constant volume flow rate. The influences of the design variables on the two objective functions are analyzed by CFD firstly. The results show that each design variable has a different impact on the two functions. The competitive relationship between the two objective functions is depicted in plots of the Pareto front obtained by MOEA. Pareto sensitivity analysis is carried out to find that the relative rib height has the most significant impact on the two objective functions.     

Intensity distribution in the collector plane from structured booster reflectors with rolling grooves and corrugations

While testing different reflector materials for external reflectors for solar collector arrays, it was found that standard rolled aluminium and corrugated aluminium materials could perform almost as well as mirror-like materials. A ray tracing model was developed to calculate the intensity in the collector plane for solar radiation from reflector materials with grooves or corrugations. Laboratory measurements, for reflector samples, with a specially designed spectral scatterometer were used to determine the angular intensity distribution of the reflected radiation. Calculations with the model using measured intensity distributions show that the scatter from aluminium materials with rolling grooves will be directed close to the specular direction and along an almost circular arc in the collector plane. The intensity in the collector plane will be redistributed slightly upward or downward depending on the season and time of day; therefore, both an increase and decrease in average intensity can occur during the year relative to a mirror-like material with the same total reflectance. For rolled aluminium, a small performance improvement can be achieved compared to a mirror reflector with equal total reflectance. Corrugated surfaces will yield a significant increase in average intensity onto the collector aperture at times when the radiation from a mirror-like reflector would otherwise be lost above the collector.     

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.     

Fluidic lens by using thermal lens effect

This research presents a novel fluidic lens based on thermal lens effect. Effects of the pump power and the pump beam intensity distribution to the probe beam profile in the dual thermal lens system are investigated experimentally and theoretically. A model, which accounts for heat conduction, natural heat convection and ray tracing in inhomogeneous medium is developed to predict the characteristics of the thermal lens system. Numerical results show the advantage of the uniform pump beam in reducing the spherical aberration compared with the Gaussian pump beam. An experiment with the uniform pump beam is carried out to confirm the numerical prediction. Experimental results show a good agreement with the calculated results. Finally, the pump power is varied to adjust the focal length of the system.     

Numerical study on heat transfer of turbulent channel flow over periodic grooves

A numerical investigation of turbulent forced convection in a two-dimensional channel with periodic transverse grooves on the lower channel wall is conducted. The lower wall is subjected to a uniform heat flux condition while the upper wall is insulated. To investigate turbulence model effects, computations based on a finite volume method, are carried out by utilizing four turbulence models: the standard k − ε, the Renormalized Group (RNG) k − ε, the standard k − ω, and the shear stress transport (SST) k − ω turbulence models. Parametric runs are made for Reynolds numbers ranging from 6000 to 18,000 with the groove-width to channel-height ratio (B/H) of 0.5 to 1.75 while the groove pitch ratio of 2 and the depth ratio of 0.5 are fixed throughout. The predicted results from using several turbulence models reveal that the RNG and the k − ε turbulence models generally provide better agreement with available measurements than others. Therefore, the k − ε model is selected to use in prediction of this complex flow. In addition, the results of the heat transfer coefficient, friction factor, skin friction coefficient and thermal enhancement factor are also examined. It is found that the grooved channel provides a considerable increase in heat transfer at about 158% over the smooth channel and a maximum gain of 1.33 on thermal performance factor is obtained for the case of B/H = 0.75. This indicates that the reverse/re-circulation flow in a channel with transverse grooves can improve the heat transfer rate.