A three-dimensional performance analysis of all-glass vacuum tubes with coaxial fluid conduit

The thermal performance of a solar system composed of parallel, all-glass (double skin) vacuum tubes has been investigated by using a three-dimensional analytical model. Each vacuum tube is equipped with a coaxial fluid conduit for water to flow through and collect the sun's thermal energy. The space between the exterior of the fluid conduit and the glass tube is filled with antifreeze solution to facilitate heat transfer from the solar heated absorber surface to water and to prevent the functional problems due to freezing in frigid weather conditions. Different from one-dimensional analytical models, the three-dimensional model considered in the present analysis enables the prediction of spatial variation of water temperatures as it flows through the coaxial conduit. This is quite useful in extracting major variables for the operation of the solar system using all-glass vacuum tubes as considered in the present investigation.     

太阳能热水器传热传质和强化传热研究

在低温太阳能光热光伏联合应用试验台的基础上,结合GB/T 17049—2005,利用Gambit、Ansys Fluent和Tecplot软件,对全玻璃真空管太阳能热水器进行传热传质和强化传热分析。结果表明:所建立的二维数值计算模型,能准确反映同一条件下,全玻璃真空管太阳能热水器的变化趋势;在数值模拟基础上,确定了单面受热时的最佳安装角度为51°,加装反光板类似双面受热的最佳安装角度为38°;在粗略估算和细化分析的基础上,确定了不同真空管结构的最佳导流板长度及安装位置;通过实验和数值模拟,确定了58mm×1 800mm为优化的全玻璃真空管结构。     

简化CPC式全真空玻璃集热管太阳能高温空气集热器的传热模型研究

对一种新型简化CPC(非追踪式复合抛物线聚光板)式全真空玻璃集热管太阳能高温空气集热系统的传热过程进行了理论分析和数值模拟计算,通过实验数据对该传热模型进行了验证分析。该系统由多个集热单元组成,每个集热单元包括一个简化CPC集热板,一根全真空玻璃集热管,在玻璃集热管内安装一个U形铜管。流动空气在各级U形铜管内被逐级加热。计算研究表明:系统空气最大出口温度可达到200℃,系统平均集热效率达到0.3以上,整个系统表现了良好的高温集热特性。同时,计算也表明当系统工质流量增加时,只要系统增加更多的集热管以增加系统总功率即可满足工质温度达到200℃的设计要求。研究提出的新型简化CPC式全真空玻璃集热管太阳能高温空气集热系统是一种有工业实用前途的太阳能集热器;研究提出的传热模型模拟效果也可以满足一般性工程计算需求。     

全玻璃真空管太阳能热水器影响因素的数值模拟研究

利用热能工程专业基础知识和场协同原理,结合计算流体力学配套商业软件,对反光板、倾斜角度、辐照强度、集热管尺寸和水箱大小等因素,影响全玻璃真空管太阳能热水器的传热传质特性进行了数值模拟研究和可视化分析,通过数值模拟研究给出了全玻璃真空管太阳能热水器的的最佳条件。     

全玻璃真空太阳集热管光—热性能

从能量平衡议程导出全玻璃真空太阳集热管的3个光－热性能参数,即热损系数UTL、闷晒太阳曝辐量H和空晒性能参数Y与真空热管理的罩管玻璃的太阳透射比τ、选择性吸收表面的太阳吸收比a、发射比ε、和集热管内的气体压强p,太阳辐照度、环境温度和真空太阳集热管的几何尺寸等函数关系。理论计算的真空太阳集热管的3个热性能参数与实验的结果比较一致。上述全玻璃真空太阳集热管的光－热性能参数已经被国家标准《全玻璃真空太阳集热管》（GB／T17049－1997）采用。     

Water-in-glass evacuated tube solar water heaters

Evacuated tube solar collectors have better performance than flat-plate solar collectors, in particular for high temperature operations. A number of heat extraction methods from all-glass evacuated tubes have been developed and the water-in-glass concept has been found to be the most successful due to its simplicity and low manufacturing cost. In this paper, the performance of a water-in-glass evacuated tube solar pre-heater is investigated using the International Standard test method ISO 9459-2 for a range of locations. Factors influencing the operation of water-in-glass collector tubes are discussed and a numerical study of water circulation through long single-ended thermosyphon tubes is presented. Preliminary numerical simulations have shown the existence of inactive region near the sealed end of the tube which might influence the performance of the collector.     

Investigation of evacuated tube heated by solar trough concentrating system

Two types of solar evacuated tube have been used to measure their heating efficiency and temperature with fluids of water and N₂ respectively with a parabolic trough concentrator. Experiments demonstrate that both evacuated tubes present a good heat transfer with the fluid of water, the heating efficiency is about 70–80%, and the water is easy to boil when liquid rate is less than 0.0046 kg/s. However, the efficiency of solar concentrating system with evacuated tube for heating N₂ gas is less than 40% when the temperature of N₂ gas reaches 320–460 °C. A model for evacuated tube heated by solar trough concentrating system has been built in order to further analyze the characteristics of fluid which flow evacuated tube. It is found that the model agrees with the experiments to within 5.2% accuracy. The characteristics of fluid via evacuated tube heated by solar concentrated system are analyzed under the varying conditions of solar radiation and trough aperture area. This study supports research work on using a solar trough concentrating system to perform ammonia thermo-chemical energy storage for 24 h power generation. The current research work also has application to solar refrigeration.     

Heat extraction from single ended glass absorber tubes

The efficiencies of solar thermal collectors are degraded if there are appreciable temperature differences between the solar absorbing surface and the heat extraction fluid. This problem has been well researched for flat plate collectors, but little has been done on the many possible heat extraction methods for all-glass tubular evacuated collectors. Measurements are reported here for assorted heat removal schemes in single-ended glass tubes with the non-uniform solar input around the tube simulated electrically. The results for two schemes with the flowing water in direct contact with the inside of the glass tube are consistent with a simple laminar flow model, with the heat input assumed to be uniform around the tube. Results for more complex schemes where the water is contained in a metal pipe are also given, and some are shown to possess unacceptably high temperature penalties. It is also shown that a simple fluid-in-glass scheme where the natural buoyancy forces alone keep the cold ingoing and hot outgoing streams apart should give acceptable performance.     

Optimal tilt-angles of all-glass evacuated tube solar collectors

In this paper, a detailed mathematical procedure is developed to estimate daily collectible radiation on single tube of all-glass evacuated tube solar collectors based on solar geometry, knowledge of two-dimensional radiation transfer. Results shows that the annual collectible radiation on a tube is affected by many factors such as collector type, central distance between tubes, size of solar tubes, tilt and azimuth angles, use of diffuse flat reflector (DFR, in short); For collectors with identical parameters, T-type collectors (collectors with solar tubes tilt-arranged) annually collect slightly more radiation than H-type collectors (those with solar tubes horizontally arranged) do. The use of DFR can significantly improve the energy collection of collectors. Unlike the flat-plate collectors, all-glass evacuated tube solar collectors should be generally mounted with a tilt-angle less than the site latitude in order to maximize the annual energy collection. For most areas with the site latitude larger than 30° in China, T-type collectors should be installed with a tilt-angle about 10° less than the site latitude, whereas for H-type collectors without DFR, the reasonable tilt-angle should be about 20° less than the site latitude. Effects of some parameters on the annual collectible radiation on the collectors are also presented.     

开放式热管中温太阳能空气集热装置的试验研究

设计一种使用简化CPC(非追踪式复合抛物线聚光板)集热板和新型开放式热管组合的全真空玻璃集热管中温太阳能空气集热装置。每个集热单元包括一个简化CPC集热板,一根全真空玻璃集热管,在玻璃集热管内安装一个铜管和外部的一个蒸汽包连接构成一个开放式热管结构。蒸汽包内安装螺旋换热管加热通过换热管的流动空气工质。分别使用水和CuO纳米流体作为热管工质,以空气作为集热工质,对热管式中温空气集热器的传热特性进行了实验研究。分析了不同工作压力、不同工质及纳米流体质量分数对热管集热传热特性的影响,详细比较了热管水工质和纳米流体工质在集热传热性能上的优劣。试验结果表明:本系统只使用2根玻璃集热管构成集热器,空气最大出口温度在夏天可达到200℃,在冬天可接近160℃,系统平均集热效率达到0.4以上,整个系统表现了良好的中温集热特性。以纳米流体为工质的热管热阻比以水为工质时平均降低了20%左右     

槽式聚光集热系统加热真空管的特性及应用研究

建立了槽式聚光集热真空管装置的数学模型,并与实验结果进行了对比。理论计算结果与实验结果吻合较好,误差在5.2％以内,表明可用所建立的模型对聚光式真空管加热流体的性能进行分析。在太阳辐射强度及聚光面积变化的情况下,分析了通过聚光真空管内流体的性能特性。还给出了利用槽式聚光装置驱动太阳能吸附制冷的应用实例。     

A novel integrated solar desalination system with multi-stage evaporation/heat recovery processes

A novel small-sized integrated solar desalination system with multi-stage evaporation/heat recovery processes is designed and tested in this study. The system consists of four linked collecting units and operates under barotropic and atmospheric pressure. Each of the four units contains a seawater tank and at least one solar collecting/desalination panel mainly comprising a simplified CPC (Compound Parabolic Concentrator) and an all-glass evacuated tube collector. In the last three units, heat exchangers made of copper tubes are inserted concentrically into the all-glass evacuated tubes to recover heat. In each unit, an independent desalination process including solar collecting, heat recovery (no heat recovered in the first unit) and seawater evaporation can be carried out completely. The experimental results show that the freshwater field of the designed system can reach as high as 1.25 kg/(h m²) in the autumn and the system total efficiency is close to 0.9. Both experimental results provide a striking demonstration that the designed solar desalination system has outstanding performance in solar collecting, heat recovery and seawater evaporation.     

Buoyancy effects and the manifolding of single ended absorber tubes

Measurements are reported on some manifolds of the fluid in glass type for evacuated all-glass tubular collectors. Temperatures at the closed end of glass tubes were measured while heating the tubes electrically to simulate solar energy input. Both parallel and series connection of the tubes are considered. The results show that thermosiphon effects can be used to achieve balanced flows in the parallel configuration, and for both parallel and series configurations efficient heat extraction can be achieved from the tubes with no partitioning of the inner volume of the tubes.     

Thermal performances comparisons of the glass evacuated tube solar collectors with shapes of absorber tube

The thermal performance of a glass evacuated tube solar collector is numerically and experimentally investigated. The solar collector considered in this paper consists of a two-layered glass tube and an absorber tube. Air is used as the working fluid. The length and diameter of this glass tube are 1200 and 37 mm, respectively. Four different shapes of absorber tubes are considered, and the performances of the solar collectors are studied to find the best shape of the absorber tube for the solar collector. Beam irradiation, diffuse irradiation, and shade due to adjacent tubes are taken into account for a collector model to obtain a realistic estimation. In addition, a single collector tube with only beam irradiation is studied as a simplified model, and the results of the simplified model are compared to those of the collector model to identify the difference between these two models. The performance of a solar collector is affected by the shape of the absorber, incidence angle of solar irradiation, and arrangement of collector tubes. The results obtained from the simplified model are very different from those from the collector model, which considered not only beam and diffuse irradiation but also shade due to adjacent tubes.     

Measurement and simulation of flow rate in a water-in-glass evacuated tube solar water heater

This paper evaluates the characteristics of water-in-glass evacuated tube solar water heaters including assessment of the circulation rate through single ended tubes. A numerical model of the heat transfer and fluid flow inside a single-ended evacuated tube has been developed assuming no interaction between adjacent tubes in the collector array. Flow measurement using Particle Image Velocimetry (PIV) has been undertaken to validate the numerical model. The experimental rig consists of a single full-scale tube coupled to a storage tank. A non-dimensional correlation has been developed of the circulation rate through a single evacuated tube mounted at 45° inclination over a diffuse reflector. Simulation results show that the natural convection flow rate in the tube is high enough to disturb the tank’s stratification and that the tank temperature strongly affects the circulation flow rate through the tubes. Circumferential heat distribution was found to be an important parameter influencing the flow structure and circulation rate through the tube, hence a separate correlation needs to be developed if a concentrating reflector is incorporated into the collector.     

Effect of tube inclination on laminar convection in uniformly heated tubes for flat-plate solar collectors

A numerical study using a combination of boundary vorticity method and line iterative relaxation method is carried out to determine the free convection effects on fully developed upward laminar forced flow in uniformly heated inclined tubes. The combined free and forced laminar convection for water with the inclined tube configuration in the low Reynolds number flow regime has practical application in flat-plate solar collectors for water heating. The tube inclination or gravitational force orientation effects on flow and heat transfer characteristics are clarified and show that in high Rayleigh number regime the tube orientation effect has considerable influence on the results, particularly in the neighborhood of horizontal direction. The numerical results show that the perturbation analysis in terms of power series of Rayleigh number is invalid for the present problem and reveal further that a maximum value for Nusselt number does not exist for any tube inclination angle with given values of the dimensionless parameters which is clearly contrary to the result from perturbation solution.     

两种太阳能集热器的瞬时效率对比分析

对强制循环全玻璃真空管太阳能热水系统和强制循环热管式真空管太阳能热水系统的瞬时效率进行了对比分析,发现全玻璃真空管热水系统的效率曲线的斜率大于热管式真空管热水系统,这说明两种热水系统在运行温度相同时,热管式真空管的热损失较小,瞬时效率较高,有较好的高温特性和保温特性,并且热管式真空管太阳集热器的集热效率波动较小,能稳定在较高的水平。     

Numerical simulation and experiment investigation on unit heat exchange tube for solar heat receiver

In this paper, numerical simulation and experimental investigation on unit heat exchange tube for solar heat receiver are reported. Based on enthalpy method, a physical and numerical model of the unit heat exchange tube was developed. An experimental system of solar simulator test rig was also set up with high temperature LiF–CaF₂ eutectic mixture as the PCM and dry air as the working fluid. The hardware, test procedures, and test results from these experiments are also discussed. The simulating orbit data was numerically analyzed and compared with test data. Canister thermal performance can be predicted well by numerical canister analyses. The results show that the output temperature of the gas of the working fluid tubes meets the expected demand during the sunlight and eclipse period and the maximum temperature and average temperature of the PCM container were all under the safe range.     

太阳能吸热器换热管蓄热数值模拟与试验研究

对以高温共晶盐LiF—CaF2为相变材料(PCM)和以干空气为工质的相变蓄热系统，采用焓方法建立了以控制体单元为对象的单管相变蓄热模型，并对系统进行了数值分析，得到了循环工质气体出口温度、相变材料容器最高温度和平均壁温等参数的瞬态变化曲线，实验研究了吸热器换热管的蓄傲热性能，分析了工质进口温度、输入热流级工质流量对工质出口温度、PCM容器平均壁温及最高壁温的影响。计算结果和试验表明单元换热管的蓄傲热性能达到了设计要求，试验结果与数值计算吻合良好。     

Heat losses from parabolic trough solar collectors

Parabolic trough solar collector usually consists of a parabolic solar energy concentrator, which reflects solar energy into an absorber. The absorber is a tube, painted with solar radiation absorbing material, located at the focal length of the concentrator, usually covered with a totally or partially vacuumed glass tube to minimize the heat losses. Typically, the concentration ratio ranges from 30 to 80, depending on the radius of the parabolic solar energy concentrator. The working fluid can reach a temperature up to 400°C, depending on the concentration ratio, solar intensity, working fluid flow rate and other parameters. Hence, such collectors are an ideal device for power generation and/or water desalination applications. However, as the length of the collector increases and/or the fluid flow rate decreases, the rate of heat losses increases. The length of the collector may reach a point that heat gain becomes equal to the heat losses; therefore, additional length will be passive. The current work introduces an analysis for the mentioned collector for single and double glass tubes. The main objectives of this work are to understand the thermal performance of the collector and identify the heat losses from the collector. The working fluid, tube and glass temperature's variation along the collector is calculated, and variations of the heat losses along the heated tube are estimated. It should be mentioned that the working fluid may experience a phase change as it flows through the tube. Hence, the heat transfer correlation for each phase is different and depends on the void fraction and flow characteristics. However, as a first approximation, the effect of phase change is neglected. Copyright © 2013 John Wiley & Sons, Ltd.