真空热管式集热器储水温度对集热效率影响研究

在太阳能生活热水供应和供暖系统中,高频使用的是真空热管式集热器。水箱与集热器的精确匹配成为影响热管冷却段的进水温度的关键因素,进而影响真空热管式集热器集热效率。基于平板真空热管式集热管物理结构,建立平板热管式真空集热管能量平衡模型,并在此基础上归纳整理出平板热管式真空集热管瞬时效率方程。分析热效率方程提出集热器集热效率的优化措施,控制水箱中加热流体的温度,及时将水箱中的热量换热储存出去,有利于保持集热管处在持续高效的工作环境中。     

基于动态传热分析的平板型太阳能集热器设计软件开发

平板型太阳能集热器的热性能受到环境因素、运行条件和设计因素的共同影响。为提升太阳能热利用工程的应用效果,需要针对环境和运行条件,对平板型太阳能集热器进行优化设计。本研究通过分析平板型太阳能集热器内部以及与环境间导热、对流、热辐射等动态传热过程,建立了计算集热量、集热效率以及集热器瞬时效率曲线的平板型太阳能集热器三维动态传热模型与差分求解算法,开发了具有传热过程分析和运行性能预测的平板型太阳能集热器设计软件,提高了不同环境因素、设计因素以及运行参数影响下集热器热性能分析速度和准确度。     

高性能平板太阳能集热器集热性能优化研究

为了提高平板太阳能集热器的集热性能,通过增大透明盖板和吸热板之间的间距、在透明盖板上镀增透膜、激光焊接、采用聚氨酯板、铝箔反光面朝向吸热板等措施,对平板集热器进行优化。通过试验测试了平板太阳能集热器的归一化温差与瞬时效率曲线,优化后的平板集热器相较普通平板集热器的瞬时截距效率提高了15.1%,热损系数减少了20.3%。     

槽式太阳能集热器传热模型及性能分析

槽式太阳能集热器一维和二维传热数学模型是一组非线性代数方程,为改进求解的稳定性和计算精度,将槽式太阳能集热器一维和二维传热模型的求解看作有约束优化问题,建立了集热器传热过程求解的有约束优化数学模型,应用MATLAB软件优化函数fmincon进行求解。分析了传热流体入口温度及太阳能辐射热流密度变化对集热器性能的影响。采用fmincon函数求解集热器传热过程,计算速度快,计算过程稳定。分析表明,传热流体温度变化对集热器效率的影响大于太阳能辐射热流密度对集热器效率的影响。     

平板型太阳能集热器三维动态传热计算模型开发与应用

平板型太阳能集热器热性能受到环境、运行条件和设计因素的共同影响.本文在分析辐照、温度、风速、入射角度等多因素影响的基础上,建立平板型太阳能集热器部件与环境间导热、对流、热辐射等三维动态传热过程计算模型,开发快速差分求解算法并形成软件,应用于平板型集热器的瞬时传热过程分析和全年运行性能预测.通过在人工环境实验系统中进行不同环境条件多工况实验,计算集热效率与实测最大偏差为0.023,相对偏差为4.9％,集热器热性能分析速度和准确度大大提高.软件在考虑风速、入射角度、热惰性等因素后,全年单位面积辐照量4 941 MJ/m2条件下,平板型太阳能集热器的单位面积集热量比依据瞬时效率曲线计算得到的集热量降低11.0％,可更准确地计算集热器在实际环境和运行条件下的热性能,为系统优化设计提供依据.     

L型翅片式太阳能平板双效集热器集热性能数值模拟研究

为提高太阳能双效集热器的集热效率,本文对传统平板式太阳能双效集热器和L型翅片式集热器进行数值模拟,对比分析空气集热模式和空气-水复合集热模式的传热特性,并研究空气和水的流量以及翅片高度对L型翅片式双效集热器瞬时集热效率的影响。结果表明:当空气质量流量为0. 016kg/s,水质量流量为0. 018 kg/s时,L型翅片式双效集热器集热效率比传统平板式集热效率提高了9%;集热器中水瞬时集热效率和空气的瞬时集热效率存在相互制约的关系,但随着空气质量流量或者水质量流量的增加,集热器瞬时效率均有所提高;增加翅片高度可以有效的提高集热器的集热效率。     

屋顶太阳能集热器对顶层房间冷热负荷的影响研究

屋顶太阳能集热器的存在改变了屋顶的太阳辐射得热,在一定程度上影响建筑的冷热负荷,特别是对于顶层房间,屋顶是受室外热作用影响最大的外围护结构。本文通过实验测量有太阳能集热器的屋顶和无太阳能集热器的屋顶的外表面温度,建立一维非稳态传热模型,对屋顶的内表面温度及传热量进行数值求解,获得屋顶的热流量数据并对实验结果进行分析。实验结果表明:太阳能集热器的存在会减少屋顶的辐射得热,降低建筑物的夏季冷负荷,增大建筑物的冬季热负荷。     

真空管太阳能空气集热系统阻力影响因素研究

通过对真空管太阳能空气集热系统内部流动过程的分析,建立了集热系统阻力计算模型,分析了不同设计参数下真空管太阳能空气集热器、串联集热系统阻力变化规律。结果表明:随着集热工质温度升高,集热器阻抗呈指数减小趋势;各影响因素中,串联集热器数量和工质流量对系统阻力影响最大,集热系统各组集热器阻力随串联集热器数量增加呈增长率减小的增大趋势,随质量流量增大呈指数增大趋势,最大值为最小值的3.6倍。此外,系统阻力随环境温度、太阳辐照度、进口温度升高呈线性增大趋势,其中环境温度对系统阻力的影响可忽略不计。通过该研究建立的真空管太阳能空气集热系统阻力计算模型及总结得到的单位面积集热器阻力计算表,可为该集热器、集热系统阻力理论计算提供便捷可靠的依据。     

严寒地区太阳能槽式集热器影响因素研究

以大庆地区为研究背景,分析了LS-2型太阳能槽式集热器光热转化过程中的能量传递,综合利用Sol Trace软件与Fluent软件计算了集热器的光学效率及瞬时效率。研究内容包括聚光面积、环境温度、流量以及管长对集热器瞬时效率的影响。结果表明:针对该模型而言,太阳能槽式集热器的瞬时效率随着聚光面积的增加而增大;环境温度在-20~20℃的变化范围内,瞬时效率有小幅度的提高,约为1%;且对于流量及管长来说,均存在一个使瞬时效率趋于稳定的值。本文的研究结果可以为严寒地区利用太阳能光热转换技术提供支撑和技术参考。     

平板型太阳能集热器性能影响因素模拟分析

建立平板型太阳能集热器的热力学模型,利用MATLAB软件模拟分析环境温度、集热器安装倾角、循环水质量流量对集热器循环水进出口温差、热效率的影响。对于张家口地区,集热器最佳安装倾角为55°,循环水质量流量为0.02 kg/s。此时,集热器的进出口温差为8.41 K,热效率为47.46%。     

建筑一体化电热冷联产光伏组件能量特性研究

传统太阳能光伏或光热建筑一体化只能为建筑提供单一电能或热能。通过研究一种集成发电、集热、制冷3种功能的建筑一体化电热冷联产光伏组件,对其夏季工况下能量特性进行了实际检测。结果表明:白天,组件集热同时能有效降低光伏电池温度,组件工作温度高于环境温度约8~16℃,发电和集热效率分别为14.1%~13.7%和40.1%~15.7%;晴朗夜间,组件通过对流和辐射两种传热方式进行散热制冷,总制冷功率为26.0~268.5 W/m~2。电热冷联产光伏组件适合与热泵结合,为建筑提供所需能源。     

槽式太阳能集热器聚光面能流分布及传热特性

本文运用蒙特卡罗光线追踪法模拟了LS-2型槽式太阳能集热系统的聚光特性,并以此为边界条件,进一步研究传热工质为THERMINOL55合成导热油时该集热器内管壁和管内流体的温度分布特性和传热特性。结果表明,管壁和管内流体温度分布十分不均匀。并考察了不同导热油以及导热油的流速对传热效率的影响工质流速对管壁温度分布影响较大,当太阳直射辐照为1000 W/m^2,导热油入口温度为160℃,流速为0.05 m/s时,吸热管圆周方向最大温差为235℃左右,当流速增加到0.05 m/s时,最大温差减小到142℃左右。     

Thermal performance of the solar parabolic trough collector at different flow rates: an experimental study

Currently, the most capable thermal systems based on the solar energy are the concentrating collectors, which are essentially finding applications in power generation and process industries. In the present study, thermal performance of the parabolic tough collector (PTC) is investigated experimentally at different flow rates of working fluid. Mass flow rate is one of the key parameters influencing its performance. Here, PTC is constructed as a simple structure having a non-evacuated tube and tested in tracking and south-facing modes utilising water as the working fluid. The performances in terms of water temperature rise, useful heat gain, collector efficiency are evaluated with and without utilising glazing on the receiver. Results revealed that performance of collector chiefly depends upon the mass flow rate and no considerable change is found when the flow rate of water is more than 0.024?kg/s. Furthermore, small-sized PTC offers slight better performance in the south-facing than the tracking mode.     

太阳能辅助地源热泵供热运行特性研究

介绍了太阳能辅助地源热泵,对其各装置性能进行了研究。分析了地下埋管换热器进出口水温及有、无蓄热水箱对太阳能辅助地源热泵性能的影响。太阳能辅助地源热泵制热性能系数随地下埋管换热器进口水温的升高呈下降趋势,随其出口水温的升高呈上升趋势。随地下埋管换热器出口水温升高,蒸发器传热量增大。当太阳能辅助地源热泵中无蓄热水箱时太阳能集热器的瞬时集热效率高于有蓄热水箱时的瞬时集热效率。就总体效果而言,有蓄热水箱要优于无蓄热水箱,这样可使地源热泵运行更加稳定。     

Dynamic performances of solar heat storage system with packed bed using myristic acid as phase change material

This paper is aimed at analyzing the thermal characteristics of packed bed containing spherical capsules, used in a latent heat thermal storage system with a solar heating collector. Myristic acid is selected as phase change material (PCM), and water is used as heat transfer fluid (HTF). The mathematical model based on the energy balance of HTF and PCM is developed to calculate the temperatures of PCM and HTF, solid fraction and heat release rate during the solidifying process. The latent efficiency, which is defined as the ratio between the instantaneous released latent heat and the maximum released heat, is introduced to indicate the thermal performances of the system. The inlet temperature of HTF (50 °C), flow rate of HTF (10 kg/min) and initial temperature of HTF (66 °C) were chosen for studying thermal performances in solar heat storage system. The influences of inlet temperature of HTF, flow rate of HTF and initial temperatures of HTF and PCM on the latent efficiency and heat release rate are also analyzed and discussed.     

Thermal solar water heater with H2O-Al2O3 nano-fluid in forced convection: experimental investigation

This paper describes laboratory experiments with a thermal solar water heater consisting of a flat-plate solar collector and helical coil heat exchanger using Al₂O₃ nano-particles dispersed in water as a working. The experiments were carried out for various nano-particle concentrations, from 0% to 3% (by volume), through forced convection cooling. The experiments were carried out under the climatic conditions of Tanta University, Egypt. The laboratory work has been carried out in actual thermal environment in August 2013. The experiments have an emphasis on the main parameters with impact on the water production temperature. These parameters include the solar radiation, the feed water mass flow rate and the nano-particle volume fraction. The main conclusion is that considerable improvement in the daily solar collector efficiency is obtained with increasing the nano-particle concentration up to 11% for concentration 3% with; this increase in efficiency is bounded by ±10% uncertainty. The outlet water temperature is increased with increasing of nano-particle concentration by 5.46% for concentration 2%. The helical heat exchanger effectiveness is increased by 4.25% for a concentration of 1% with ±13% uncertainty. The helical heat exchanger effectiveness and solar collector efficiency are increased with decreasing the working fluid mass flow rate.     

Experimental study and analysis of air heating system using a parabolic trough solar collector

An experimental study of air heating system was carried out using a parabolic trough collector with a U-tube aluminium heat exchanger. An evacuated tube placed at the focal length of the parabolic trough collected the solar radiations reflected from the surface of parabolic trough. The air was used as a working fluid, which was heated by passing it through a U-shaped aluminium heat exchanger placed inside the evacuated tube. It was found that efficiency of the parabolic trough collector depends on the mass flow rate, solar intensity and use of fins. It was observed that by using fins at a high mass flow rate of 4.557?kg/h, the maximum temperature of 126°C was achieved which is 13.27% more than the maximum temperature obtained without fins. Furthermore, for a low-mass-flow rate of 1.69?kg/h, the maximum temperature obtained was 149°C.     

Experimental analysis of convective heat transfer on tubes using twisted tape inserts,louvered strip inserts and surface treated tube

This paper analyses and compares the effects on outlet water temperature, heat transfer rate and its thermal efficiency of three different types of tubes such as twisted tape inserted tube, louvered strip inserted tube and a surface treated (laser peened) tube are dealt with different operating parameters, viz., solar insolation, water temperature at the inlet and ambient temperature. Non-concentrate collector (flat plate) is used. A plain or normal tube (non-surface treated) was also tested for comparison. Polystyrene and acrylic glasses were used for reduced heat loss. It is observed that the heat transfer rate and its thermal efficiency tend to increase with the increased ambient temperature and solar insolation with the louvered strip inserts had given the best thermal performance in comparison with the other types.     

Thermal performance analysis of the glass evacuated tube solar collector with U-tube

In this paper, based on the energy balance for the glass evacuated tube solar collector with U-tube, the thermal performance of the individual glass evacuated tube solar collector is investigated by analytical method. The solar collector considered in this study is a two-layered glass evacuated tube, and the absorber film is deposited in the outer surface of the absorber tube. The heat loss coefficient and heat efficiency factor are analyzed using one-dimensional analytical solution. And the influence of air layer between the absorber tube and the copper fin on the heat efficiency is also studied. The results show that the function relation of the heat loss coefficient of the glass evacuated tube solar collector with temperature difference between the absorbing coating surface and the ambient air is nonlinear. In the different ambient temperatures, the heat loss coefficient of the solar collector should be calculated by different expressions. The heat efficiency factor will be subject to influence of air layer between absorber tube and the copper fin. Specially, the influence is remarkable when the heat loss coefficient of the collector is large. When the synthetical conductance amounts to 5 W/m K, the solar collector efficiency decreases 10%, and the outlet fluid temperature decreases 16% compared with the case which the air thermal resistance is neglected. And the surface temperature of the absorbing coating increases 30 °C due to the effect of air thermal resistance. So the surface temperature of the absorbing coating is an important parameter to evaluate the thermal performance of the glass evacuated tube solar collector.