一种新型太阳能集热器循环工质的制备

采用直流碳弧法制备了平均粒径为25 nm的碳包铜纳米颗粒,该颗粒适合用作直接吸收式太阳能集热器的循环工质——碳包铜纳米流体。通过X射线衍射(XRD)、透射电镜(TEM)等分析手段,对所制备的纳米颗粒的成分、形貌等进行了表征。采用球磨分散法对碳包铜纳米颗粒进行了分散,制得了碳包铜纳米流体。通过分光光度法和沉降法研究了分散剂含量、球磨时间以及球磨转速等因素对碳包铜纳米流体分散性能的影响,对其分散机理进行了初步探讨。研究结果表明:当阿拉伯树胶的质量分数为0.1%、球磨时间为2 h、球磨转速为250 r/min时,碳包铜纳米流体的分散效果最佳。     

纳米流体在直接吸收式太阳能集热器上的应用研究

综述了提高集热器集热效率的措施及目前的研究进展,说明了直接吸收式太阳能集热器的优越性。对纳米流体稳定性的影响因素进行了总结,并从太阳辐射吸收、导热性能及对流换热性能三个方面阐述了纳米流体作为集热工质的优越性。表明了纳米流体作为集热工质在直接吸收式太阳能上应用的可行性,并展望了纳米流体作为传热工质在在供暖上的应用。     

Cu-H2O纳米流体平板太阳集热器集热性能研究

采用两步法配制Cu-H2O纳米流体,对不同质量分数、粒径的Cu-H2O纳米流体和水作为平板太阳热水器的集热工质,测试其导热系数,并在相同太阳辐照下进行集热性能实验,研究平板集热器的集热效率、水箱中的水温与得热量。实验结果表明:纳米流体可明显提高水的导热性能。粒径为25 nm,质量分数为0.10%的Cu-H2O纳米流体集热效率比水的提高了23.83%,质量分数为0.20%的Cu-H2O纳米流体的集热效率反而低于0.10%的。粒径为50 nm的Cu-H2O纳米流体集热效率低于粒径为25 nm的。粒径为25 nm、质量分数为0.10%的纳米流体循环系统中最高水温与最高得热量相对于水作为工质分别提高了12.24%和24.52%。     

油基CuO和HgS纳米流体的光热转换特性研究

以氙灯作为模拟太阳光源,实验研究CuO和HgS纳米流体的光热转换特性。与透明导热油相比,浓度为0.1 mg/m L的CuO纳米流体和浓度为1.0 mg/m L的HgS纳米流体的光热转换效率可分别提升35%和27%。随着浓度的增大,由于表面效应,CuO纳米流体的温升出现饱和现象,而HgS纳米流体则未出现类似现象。分析表明:对于CuO和HgS纳米流体而言,光热转换效率的提高分别是由于纳米颗粒的吸收和散射2种效应所导致的。该研究揭示出具有不同光谱吸收特性的纳米流体其提高光热转换效率的机制有所不同,有助于在太阳能光热转换应用中纳米流体的合理选择。     

基于CNT/Ag纳米流体的分光谱太阳能光伏光热系统实验研究

传统太阳能光伏光热(PV/T)系统的光电转换和光热转换过程耦合在一起，对太阳能全光谱能量的利用率较低，为使得光电、光热过程解耦，该文探究Ag、CNT、CNT/Ag纳米流体作为分光谱PV/T系统媒介时的光谱及能量性能。首先对不同浓度纳米流体的光谱性能进行测试，然后通过实验研究不同浓度的CNT/Ag纳米流体对系统电效率和热效率的影响。结果发现相比于Ag纳米流体，浓度为1×10⁶、3×10⁶、5×10⁶、1×107μg/m³的CNT/Ag纳米流体在太阳电池光谱响应区的透过率分别上升了8.6%、9.3%、8.5%、9.2%，响应区外波段的吸收率增加了30.4%、44.5%、58.4%、56.7%。系统电效率最高为8.2%、热效率最高为45%，当CNT/Ag纳米流体浓度为5×10⁶μg/m³时，分光谱容器效率最高为18.3%时热效率达到了43%，电效率为7%。     

一种槽式太阳能空气集热器热性能的试验研究

为了研究日光温室用槽式太阳能空气集热器的热性能,基于TracePro光学模拟软件设计了一种槽式太阳能空气集热器,对其进行试验研究,分析了不同因素对集热性能的影响规律。实验结果表明,管中空气流速的变化对集热器集热效率和集热量的影响规律是相同的,在不同的流速下,存在最佳空气流速约为4.4 m/s,使得集热器的集热量和集热效率最大,集热量达到373.2 W,集热效率约为25%,此时集热性能最好。对于不同太阳辐照度,正午时刻之前,太阳辐照度越大,集热器的集热效率越大,正午时刻之后,集热器的集热效率基本保持不变,15:40之后集热器集热效率逐渐减小。当太阳辐照度和管中流速相同时,室外温度越高,集热器集热效率越大,集热性能越好。集热管中空气温度沿着集热管出口方向不断增大,太阳辐照度越大,集热管相同位置空气温度越高。该研究结果可为此种槽式太阳能空气集热器在日光温室的应用中提供参考。     

纳米流体为工质的直吸式太阳集热器性能研究

搭建具有自动跟踪功能的直吸式太阳集热器实验系统,以Ti N-EG纳米流体为工质,采用菲涅尔透镜将太阳光聚焦到集热管上,集热管采用双层真空管。测量不同流量、辐照度时集热管内部及进出口处的流体温度;使用CFD软件得出集热管内的温度分布。通过分析管内的温度分布、进出口温差及集热器的热效率,对集热器的工作性能进行评价。结果表明:数值模拟与实验结果较吻合,集热器的热效率可达70%以上,出口处工质温度达到120℃以上,该集热器可工作于中温区域。     

水基碳纳米管纳米流体在室外自然条件下的光热性能研究

通过两步法制备碳纳米管水基纳米流体,进行不同浓度纳米流体的光吸收特性和光热转换特性研究。结果表明:在去离子水中添加少量碳纳米管能显著提高对光的吸收性能;当纳米流体质量分数达到0.1%时,对光基本表现出不可透过性;纳米流体的光热转换效率随着质量分数的增加而降低,最高为37.31%。     

纳米颗粒对盐梯度太阳池热性能影响的实验研究

为了增强太阳池的储热温度以及热效率,采用了在太阳池下对流层添加纳米颗粒的方法.通过光照实验与沉降实验选取质量分数为0.010％的碳纳米管溶液添加到太阳池的下对流层,然后与普通盐梯度太阳池进行对比实验,并对实验数据进行分析、计算.实验结果表明:在相同模拟光源下,含纳米颗粒小型太阳池的下对流层平均温度提高了 1.7℃,效率...     

槽式太阳能集热器热性能实验测试

针对天津市槽式太阳能集热系统性能测试平台,对不同太阳辐射强度、入口流体温度以及不同工质流量状况下集热效率和集热管压降变化规律进行实验测试,通过测试数据对槽式太阳能集热器热性能进行分析.试验结果表明:在天津地区槽式太阳能集热器集热效率可以达到66.1％;太阳辐射强度的增强,会提高集热效率,并且集热器进出口的压降会随之降低...     

An innovative,high-efficiency silver/silica nanocomposites for direct absorption concentrating solar thermal power

Using of nanofluids in the concentrating direct absorption solar collectors has the potential of reducing thermal losses because of the excessive temperature of the absorbing surface in the conventional solar collectors. However, increasing the concentration ratio of solar radiation must be followed by increasing the volume fraction of the nanoparticles, which, in turn, has the drawbacks of increasing the settlement and agglomeration rates of the nanoparticles. In this study, we have suggested using the plasmonic nanofluids for volumetric absorption in the concentrated solar power applications because of the less volume fraction of the plasmonic nanoparticles that are required to harvest the concentrated solar radiation. The interaction of concentrated solar radiation with different morphologies of silver nanoparticles coated by silica shell has been computationally studied. Then, the finite element method has been implemented to determine the photo-thermal conversion efficiency for silver nanosphere and nanoplates with a silica shell. Silver nanoparticles coated by silica exhibit a promising potential because of their distinct characteristics. The silica shell is transparent to the visible and near-infrared radiation bands; it also consolidates the intensity of the localized plasmon resonance and so the absorption characteristics, besides its protective role. A high-efficiency low concentration nanofluid has been designed using blended morphologies of Ag nanospheres and nanoprisms with silica-coating–based nanofluid for full-spectrum absorption characteristics. The suggested nanofluid exhibits a promising performance at a volume fraction of 0.0075 wt% where the volumetric solar collector efficiency exceeds 75% under the solar concentration ratio of 50.     

Recent advances on nanofluids for low to medium temperature solar collectors: energy,exergy, economic analysis and environmental impact

The efficient exploitation of solar irradiation is one of the most encouraging ways of handling numerous environmental concerns. Solar collectors are suitable devices that capture solar irradiation and convert it into thermal energy and electricity. In the last years, the nanofluids used in solar thermal systems have been studied as a useful technique for enhancing the solar collectors’ performance and establishing them as viable and highly efficient systems. The present review paper aims to summarize and discuss the most important numerical and experimental studies in nanofluid-based solar systems for application at low and medium temperature levels, while the emphasis on the fundamental physical phenomena that occur. In the first part, numerous numerical models and the principal physical phenomena affecting the heat transfer rate in the nanofluid have been analyzed. More specifically, the importance of different forces in nanofluid flows that exist in particulate flows such as drag, lift (Magnus and Saffman), Brownian, thermophoretic, Van der Waals, electrostatic double-layer forces are considered. Moreover, an overview of the thermophysical properties, physical models, heat transfer models, and evaluation criteria of nanofluids are included in this work. In the second part, which is the main part of this work, a comprehensive review is performed to gather and discuss the new advantages in the nanofluid-based solar collectors that operate at low and medium temperatures. More specifically, the examined solar systems are the flat plate collectors, the evacuated tube collectors, the direct absorption collectors, and the thermal photovoltaic systems, while the investigated applications are space-heating, space-cooling, household hot water production, desalination, industrial activities, and power generation. The aforementioned collectors and applications are the most usual in the real systems, indicating the importance of the present work. Moreover, the emphasis is given in the thermal, exergy, economic, and environmental evaluation of the studied systems, as well as in the discussion of the possible limitations of the use of nanofluids like the lack of long-term stability, the agglomeration of nanoparticles, and the increased pumping work due to the increased pressure drop. Finally, it is found that the nanofluid utilization usually enhances the collector efficiency up to 5%, while higher enhancements can be found in thermal photovoltaics. Moreover, it is concluded that there is a need to emphasize issues such as stability and the use of eco-friendly solar systems. Lastly, the field's future trends are highlighted, and a clear image of the present situation and the next steps in the field are given.     

Performance enhancement of a direct absorption solar collector using copper oxide porous foam and nanofluid

The current research proposes the idea of using water-saturated metal oxide foams and water-based nanofluids as solar absorber in the direct absorption solar collectors (DASCs). Specifically, the novel solar collector design utilizes copper oxide (CuO) porous foam and nanoparticle with high optical properties and is expected to have enhanced thermal performance than the conventional collectors utilizing pure water. The finite volume technique is used to solve the governing equations of flow and heat transfer in the radiative participating media. Also, to establish the reliability and accuracy of numerical solutions, the obtained results are compared with the corresponding numerical and experimental data. The computations are carried out for different nanoparticle volume fractions, foam pore sizes, working fluid mass flow rates, and both porous layer thicknesses and positions (inserted at the lower or upper wall of the collector). It is found that the efficiency of DASC partially/fully filled with metal oxide foam is maximized when the collector is completely filled with it. Compared with the water flow, the numerical results show that the collector efficiency using CuO nanofluid and metal oxide foam is improved by up to 26.8% and 23.8%, respectively. Moreover, considering the second law of thermodynamics, the use of CuO nanofluids in the DASC seems to be more effective than the use of CuO porous foam.     

Photo-thermal conversion and heat storage characteristics of multi-walled carbon nanotubes dispersed magnetic phase change microcapsules slurry

The development of high-efficient working fluids with excellent photo-thermal conversion and heat storage properties is an important factor to solar thermal utilization. In this work, magnetic phase change microcapsules (MPCMs) were prepared via in situ polymerization, where melamine-formaldehyde (MF) resin and octadecane containing oleic acid-coated magnetic nanoparticles (OA-MNs) were used as the shell and core, respectively. The microstructure, magnetic and thermal properties of MPCMs were investigated, and the photo-thermal conversion and heat storage properties of slurries prepared by dispersing MPCMs into multi-walled carbon nanotubes (MWCNTs) nanofluids were explored. The encapsulation efficiency of MPCMs with superparamagnetic nature achieved 63.51%, while the thermal conductivity of MPCMs was slightly increased with regard to phase change microcapsules (PCMs) and the thermal stability of octadecane was enhanced after being encapsulated. Moreover, the slurry with 0.01 wt% MWCNTs and 15 wt% MPCMs had the optimal photo-thermal conversion properties and thermal storage capacity. Beyond that, the recycled MPCMs represented excellent recyclability. Our research demonstrated that the MWCNTs-dispersed MPCMs slurry is one of ideal working fluids with excellent photo-thermal conversion and heat storage characteristics for direct absorption solar collector.     

An Experimental Investigation on the Effect of Ferrofluids on the Efficiency of Novel Parabolic Trough Solar Collector Under Laminar Flow Conditions

ABSTRACT

The paper is related to the use of magnetic nanofluids (ferrofluids) in a direct absorption solar parabolic trough collector, which enhances thermal efficiency compared to conventional solar collectors. By applying the right magnetic intensity and magnetic field direction, the thermal conductivity of the fluid increased higher than typical nanofluids. Moreover, the ferrofluids exhibit excellent optical properties. The external magnetic source is installed to alter the thermo-physical properties of the fluid, and the absorber tube does not have selective surface allowing ferrofluids to absorb the incoming solar irradiance directly. In this paper, an experimental investigation of the performance of small scale direct absorption solar collector using ferrofluids as an absorber was conducted. Nanoparticle concentrations of 0.05 vol% at the operational temperatures between 19°C and 40°C were used in the current study. The results show that using ferrofluids as a heat transfer fluid increases the efficiency of solar collectors. In the presence of the external magnetic field, the solar collector efficiency increases to the maximum, 25% higher than the conventional parabolic trough. At higher temperatures, the ferrofluids show much better efficiency than conventional heat transfer fluid. The study indicated that nanofluids, even of low-content, have good absorption of solar radiation, and can improve the outlet temperatures and system efficiencies. The study shows the potential of using ferrofluids in the direct absorption solar collector.     

Optimization of nanofluid volumetric receivers for solar thermal energy conversion

Improvements in solar-to-thermal energy conversion will accelerate the development of efficient concentrated solar power systems. Nanofluid volumetric receivers, where nanoparticles in a liquid medium directly absorb solar radiation, promise increased performance over surface receivers by minimizing temperature differences between the absorber and the fluid, which consequently reduces emissive losses. We present a combined modeling and experimental study to optimize the efficiency of liquid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A one-dimensional transient heat transfer model was developed to investigate the effect of solar concentration, nanofluid height, and optical thickness on receiver performance. Simultaneously, we experimentally investigated a cylindrical nanofluid volumetric receiver, and showed good agreement with the model for varying optical thicknesses of the nanofluid. Based on the model, the efficiency of nanofluid volumetric receivers increases with increasing solar concentration and nanofluid height. Receiver-side efficiencies are predicted to exceed 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness and solar exposure time. This work provides insights as to how nanofluids can be best utilized as volumetric receivers in solar applications, such as receivers with integrated storage for beam-down CSP and future high concentration solar thermal energy conversion systems.     

ZrC nanoparticle-modified microencapsulated phase change materials with enhanced thermal conductivity and photo-thermal conversion performance

A photo-thermal energy storage microcapsule modified by KH550-treated ZrC nanoparticles was prepared via in situ polymerization. ZrC nanoparticles were treated by KH550 to stabilize the paraffin-ZrC emulsion, which could ensure ZrC nanoparticles were incorporated into the microcapsule shell. In this article, microcapsules modified by different mass fractions of modified ZrC nanoparticles (paraffin@MUF-ZrC) were prepared to investigate the influence of ZrC nanoparticles on microstructure, thermal storage, thermal stability, thermal conductivity, and photo-thermal conversion behavior. According to the SEM images, the obtained paraffin@MUF-ZrC microcapsules exhibited good morphology with regular spherical shape and uniform particle size distribution. DSC, TGA, and thermal conductivity analysis were used to characterize the thermal storage properties, thermal stability, and thermal conductivity of paraffin@MUF-ZrC microcapsules, respectively. It showed that these three thermal performance indices improved as the mass fraction of modified ZrC nanoparticles increased. Specifically, the thermal properties of paraffin@MUF-4%ZrC microcapsules were better than paraffin@MUF microcapsules. As a result, specific enthalpy was slightly increased by 14.92% (121.74 J/g) and thermal conductivity increased by 225.16% (0.4962 W/m·K). Moreover, the optical absorption capability also increased. The paraffin@MUF-4%ZrC microcapsules showed remarkable optical absorption capability of 70.23%, which was a 247.67% improvement compared to the 20.20% of the paraffin@MUF microcapsules. Moreover, it was found that paraffin@MUF-4%ZrC microcapsules could heat up from 30°C to 78°C under simulated solar radiation, showing excellent photo-thermal conversion behavior. Based on good thermal storage properties, thermal conductivity, and light absorption capability over the full spectrum, the new photo-thermal energy storage microcapsules have good application prospects in solar thermal conversion and energy storage systems.     

Evaluation of the effect of nanofluid-based absorbers on direct solar collector

As conventional energy sources like fossil fuels are getting rare, cost of energy production has become higher as well as the concern of environmental pollution by burning of fossil fuels among the developed and developing nations. Solar energy is the most vastly available energy and very effective in terms of energy conversion. The most common solar thermal collector used is the black surface as radiant absorber but the thermal energy efficiency is low. In this study, the effect of nanofluid has been analyzed by using as working fluid for direct solar collector. The extinction coefficient of water based aluminum nanofluid has been investigated and evaluated by varying nanoparticle size and volume fraction. The particle size has minimal influence on the optical properties of nanofluid. On the other hand, the extinction coefficient is linearly proportionate to volume fraction. The improvement is promising within 1.0% volume fraction and the nanofluid is almost opaque to light wave.     

无盖板渗透型集热器热性能的对比试验

利用太阳能空气加热系统实验台,对黑、深绿和深蓝3种颜色无盖板渗透型集热器的热性能进行了户外瞬态对比试验。试验结果表明:太阳辐射照度和风量是影响系统热性能的重要因素。在高档和低档两种风量下,黑色集热器的瞬时平均热效率分别为76.04%和67.50%,高于普通平板太阳能空气集热器;集热器表面颜色对其热性能有一定影响,在高档和低档两种风量下,深绿色和深蓝色集热器的瞬时平均热效率比黑色集热器低15%～22%,空气温升低3～4℃,但仍然优于普通平板空气集热器。从保持建筑立面美观考虑,无盖板渗透型集热器的集热板可以采用颜色较深的彩色,不会对系统热性能造成较大影响。     

Development of a simple low-cost solar simulator for indoor collector testing

Indoor test methods for solar collectors are widely accepted and recommended by ASHRAE. The development of a solar simulator for indoor testing and the certification procedure for photo-thermal solar devices is discussed in this paper. The system consists of 14 quartz halogen lamps and provides a testing area of about 1 × 1 m. The irradiance can be varied from 400 to 1500 W/m². This is achieved by varying the input power supplied to individual lamps. The radiation output characteristics of a single lamp have been studied. The distribution of intensity over the covered area of 1·20 × 1·20 m has been recorded using a precision Eppley pyranometer.A variety of collectors—evacuated tubular collectors and flat-plate collectors of both liquid and air heating types—has been tested under sufficiently stable insolation conditions. Results for a conventional solar air heater are presented together with suggestions for further improvements.