塔式太阳能热发电吸热器技术进展

文中介绍了现有塔式太阳能热发电吸热器技术,针对不同吸热器结构和传热介质,结合国内外现有主要塔式太阳能热发电站,对吸热器性能进行比较研究。结果表明,外露管式吸热器结构简单、造价低,应用广泛;而熔盐作为传热和蓄热介质具有较好的性能,系统无压运行且能承受高热流密度,是将来的研究方向。     

基于能量流的塔式太阳能热发电吸热器动态建模

塔式太阳能热发电空气吸热器的最大热应力与其温度变化率成正比,吸热器出口空气温度的动态特性影响塔式光热系统的功率特性。结合热电比拟理论,采用对流换热系数和Rosseland辐射传递方程描述传热过程,建立塔式太阳能热发电系统中碳化硅泡沫陶瓷吸热体的能量流模型。通过剖析空气吸热器工作过程的传热特性,得出平均能流密度、吸热体厚度、平均孔径对出口空气温度、吸热体温度的影响,为该类空气吸热器的设计提供了理论依据。     

槽式太阳能新型腔式吸热器的热性能研究

提出一种适用于抛物槽集热器的新型太阳能腔式吸热器,该装置具有较高的集热效率,同时连接安装和日常运行维护也相对便利。对其建立一套三维传热模型,并搭建采用新型腔式吸热器的抛物槽集热器实验系统,通过实验测试对比吸热器瞬时效率,验证模型的准确性。此外,定量分析不同环境参数与工作参数对新型腔式吸热器热性能的影响,结果表明:集热效率随着法向直接日射辐照度、环境温度的升高而增加,随着环境风速和吸热器入口传热流体温度的升高而降低,而受传热流体质量流量的影响较小。     

高温熔盐吸热器的传热研究和系统设计

本文对非均匀辐射热流密度太阳能熔盐吸热器传热过程进行了模拟研究,得到了熔盐吸热器内部的温度、传热性能等特征参数。结果表明在轴向和径向上熔盐流体温度和管壁的温度都非常不均匀,同时其综合传热性能要高于按照Sieder-Tate公式的计算值。并对10 MW塔式太阳能热发电的熔盐吸热器进行了设计和分析。     

多孔介质太阳能吸热器性能分析

采用4种多孔骨架中辐射传输模型,包括:忽略多孔骨架内部辐射模型(模型A)、Rosseland模型(模型B)、均匀内热源模型(模型C)与吸热器中辐射传输满足Beer定律的模型(模型D),推导得到了局部非热平衡条件下4种模型所对应的吸热器中多孔骨架温度、空气温度和吸热器热效率的解析解,分析了多孔骨架孔隙率、导热系数和孔隙直径对吸热器性能的影响。结果表明,对模型A和模型B,吸热器中最高温度位于吸热器进口处;对模型C,吸热器中最高温度位于吸热器出口处;而在模型D中,吸热器中吸热器内部或吸热器的出口处温度最高。吸热器效率取决于多孔骨架导热系数、孔隙率和孔隙直径等参数,当吸热器中内热源均匀分布时,吸热器效率是最高的。     

碳化硅泡沫陶瓷空气吸热器的动态仿真研究

在Modelica语言和Dymola软件平台的基础上,建立塔式太阳能热发电系统中碳化硅泡沫陶瓷空气吸热器的一维非稳态仿真模型。仿真模型中采用体积对流换热系数和Rosseland辐射传递方程描述对流换热和辐射传热过程,空气热物性参数随温度和压力变化。该模型的仿真结果正确性得到空气吸热器实验平台的实测验证,可用于以碳化硅泡沫陶瓷为吸热体的空气吸热器动态特性预测。     

两段式塔式太阳能腔式吸热器设计及性能分析

针对一种新型两段式塔式太阳能热发电的吸热器进行几何设计,建立了呈高斯分布热流密度的条件下吸热器辐射和对流换热以及流动模型,确定了吸热器I和吸热器II受热面蛇形管管道布置方式和几何尺寸,获得了吸热器内部不同位置受热面的热流密度分布情况.结合气液两相传热和流动特点确定了吸热器典型管道内部工质温度、干度、压降和沿管道流程的壁温分布规律.得出两段式塔式太阳能腔式吸热器几何结构的系统化设计流程,并对吸热器进行了热力性能分析.结果表明:两段式塔式太阳能腔式吸热器能够有效减小预热蒸发吸热器的几何尺寸,提高平均辐射热负荷的同时降低吸热器的平均温度,有效提高吸热器的热效率;多管程蛇形管道布置可使出口参数分布更加均匀,避免受热严重不均等安全问题.     

玻璃套管双层吸热芯的热辐射特性研究

为降低多孔介质高温吸热器的辐射散热损失,设计一种玻璃套管-多孔介质双层吸热芯。采用实验测量和蒙特卡洛数值模拟方法,定量分析双层吸热芯的多光谱热辐射传输特性。结果表明,绝大多数太阳辐射外热源在双层吸热芯的内部被吸收（多孔介质入口段）。当工作温度为1000 K时,双层吸热芯的太阳光吸收比与自身红外辐射发射比之比达到约2.0,其辐射热效率较单层多孔介质吸热芯高约30%。     

多孔介质辅助平板式太阳能集热器热性能强化的数值仿真研究

多孔介质材料具有良好的传热和蓄热性能。设计了新型多孔介质辅助平板式太阳能集热器的二维数值仿真模型，对其内部热性能进行了数值模拟，研究多孔介质块的形状(矩形、梯形、三角形结构)、布置数量和渗透率(达西数Da=10^-5～10^-2)等因素对平板式太阳能集热器热性能强化的影响；然后考虑到插入多孔介质伴随的压降和摩擦阻力损失，提出了评价集热器传热性能与阻力损失的性能评估标准。研究结果表明：在平板式太阳能集热器换热通道插入4种不同形状的多孔介质块，矩形多孔介质块背部附近区域更易产生涡区，集热器内传热性能最强，但通道内流动阻力系数大，从而导致阻力损失大。当多孔介质区域总长度一定时，随着多孔介质块布置数量的增加，涡区数量相应增加，集热器内传热性能加强，且通道内流动阻力损失呈现先增加后降低的规律。多孔介质块渗透率对集热器传热性能的影响显著，当Da=10^-2时，集热器传热性能最强。集热器内多孔介质块布置任意数量、高渗透率(Da=10^-2)条件下，矩形多孔介质块的性能评估标准最佳；在多孔介质块布置数量(N=6)较多、低渗...     

超临界二氧化碳太阳能腔式吸热器光热特性研究

针对太阳能碟式聚光器,设计了一种工质为超临界二氧化碳(sCO2)的圆台形腔式吸热器,建立了吸热器的光热模型。采用蒙特卡洛光线追踪法分析了腔式吸热器的光学特性,并基于腔式吸热器的相关理论将热边界条件导入Ansys Fluent软件中,对吸热器的流动传热特性进行了计算流体力学（CFD）仿真模拟。研究了工质进口温度为150 ℃、太阳光辐射强度为800 W/m2时,吸热器不同采光口直径、倾斜角和辐射发射率对其光热特性影响的规律。研究结果表明：吸热器采光口直径对其光热效率的影响较大,采光口直径增加会降低吸热器光学效率,采光口直径过大或过小都会降低吸热器的热效率;随着吸热器倾斜角的增大,采光口内部热空气和外部冷空气之间的自然对流传热明显增加;辐射发射率对吸热器热效率的影响较小。     

Heat transfer analysis of solar-driven high-temperature thermochemical reactor using NiFe-Aluminate RPCs

Converting solar energy efficiently into hydrogen is a promising way for renewable fuels technology. However, high-temperature heat transfer enhancement of solar thermochemical process is still a pertinent challenge for solar energy conversion into fuels. In this paper, high-temperature heat transfer enhancement accounting for radiation, conduction, and convection heat transfer in porous-medium reactor filled with application in hydrogen generation has been investigated. NiFe-Aluminate porous media is synthesized and used as solar radiant absorber and redox material. Experiments combined with numerical models are performed for analyzing thermal characteristics and chemical changes in solar receiver. The reacting medium is most heated by radiation heat transfer and higher temperature distribution is observed in the region exposed to high radiation heat flux. Heat distribution, O₂ and H₂ yield in the reacting medium are facilitated by convective reactive gas moving through the medium's pores. The temperature gradient caused by thermal transition at fluid-solid interface could be more decreased as much as the reaction chamber can store the transferred high-temperature heat flux. However, thermal losses due to radiation flux lost at the quartz glass are obviously inevitable.     

Numerical Investigation of Energy-Efficient Receiver for Solar Parabolic Trough Concentrator

In this paper, a thermal analysis of an energy-efficient receiver for solar parabolic trough concentrator is presented. Various porous receiver geometries are considered for the performance evaluation of a solar parabolic trough concentrator. Numerical models are proposed for a porous energy-efficient receiver for internal heat gain characteristics and heat loss due to natural convection. The internal flow and heat transfer analysis is carried out based on a RNG k-? turbulent model, whereas external heat losses are treated as a laminar natural convection model. The numerical models have been solved using the commercial engineering package, FLUENT. The thermal analysis of the receiver is carried out for various geometrical parameters, such as fin aspect ratio, thickness, and porosity, for different heat flux conditions. The inclusion of porous inserts in tubular receiver of solar trough concentrator enhanced the heat transfer about 17.5% with a pressure penalty of 2 kPa. The Nusselt number correlation is proposed based on the extensive numerical data for internal heat transfer inside the receiver. The proposed model is compared with more well-known natural convection models. A comparative study is carried out with different porous geometries to evolve an optimum configuration of energy-efficient receivers.     

Enhanced Evaporation in a Multilayer Porous Media Under Heat Localization: A Numerical Study

Evaporation and steam generation are two of the most vital processes in industry. A new method to advance the efficiency of evaporation involves localizing heat at the water surface where the vapor escapes into the air to minimize energy loss. In this research, we numerically investigate the improvement of a novel evaporation process via solar heat localization in a porous medium. A layer of carbon foam with a combination of interconnected and dead-end pores with a high hydrophilicity surface adjacent to a layer of expanded graphite with known porosity and properties were modeled numerically using a finite volume method. The hydrophilic porous media facilitates the capillary forces for better transportation of the bulk water through the porous media to the top surface of the porous media where the absorbed solar energy is delivered to the water inside the pores for evaporation. Continuity, momentum, heat and mass transfer equations were solved in this modeling effort. The modeling results were validated with the experimental data available in the literature. The findings in this numerical study can shed light on the complex interplay between the fluid dynamics and heat and mass transfer across the porous medium, which are important for efficient evaporation processes.     

Numerical investigation on porous media heat transfer in a solar tower receiver

In order to investigate the steady heat transfer characteristics of a porous media solar tower receiver developed in China, this paper applies the steady heat and mass transfer models of the porous media to solar receivers, chooses the preferable volume convection heat transfer coefficient model, solves these equations by using the numerical method, and analyzes the typical influences of the porosity, average particle diameter, air inlet velocity, and thickness on the temperature distribution. The following conclusions have been drawn: in the same position or relative position along the downstream, the bigger the average particle diameter is, the higher the solid matrix dimensionless temperature is, the higher the air dimensionless temperature is. The bigger the porosity is, the lower the solid matrix dimensionless temperature is, the bigger the porosity is, the higher the air dimensionless temperature is. The bigger the thickness is, the lower the solid matrix dimensionless temperature is, the higher the air dimensionless temperature is. In a certain depth, the bigger the air inlet velocity is, the higher the solid matrix dimensionless temperature is. After a certain depth, the bigger the air inlet velocity is, the lower the solid matrix dimensionless temperature is, and the bigger the air inlet velocity is, the higher the air dimensionless temperature is. The paper can provide a reference for this type of receiver design and reconstruction.     

Effects of porous media on thermal and salt diffusion of solar pond

Laboratory and field experiments were carried out along with numerical simulations in this paper to study the effects of porous media on thermal and salt diffusion of the solar ponds. From our laboratory experiments simulating heat transfer inside a solar pond, it is shown that the addition of porous media to the bottom of a solar pond could help enhance its heat insulation effect. The experiment on salt diffusion indicates that the upward diffusion of the salt is slowed down when the porous media are added, which helps maintain the salt gradient. Our field experiments on two small-scaled solar ponds indicate that when porous media are added, the temperature in the lower convective zone (LCZ) of the solar pond is increased. It is also found that the increase in turbidity is repressed by porous media during the replenishment of the salt to the LCZ. Thermal diffusivities and conductivities of brine layers with porous media such as pebble and slag were also respectively measured in this paper based on the unsteady heat conducting principles of a semi-infinite body. These measured thermal properties were then used in our numerical simulations on the effect of porous media on thermal performance of a solar pond. Our simulation results show that brine layer with porous media plays more positive role in heat insulation effect when thermal conductivity of the ground is big. On the other hand, when the ground has a very small thermal conductivity, the performance of solar pond might be deteriorated and total heat storage quantity of solar pond might be reduced by brine layer with porous media.     

Thermal Analysis of a Volumetric Solar Receiver

The volumetric receiver has received wide attention due to its high thermal efficiency. This paper studied a new type of a solid-liquid composite volumetric receiver. The heat transfer in a solid-liquid composite volumetric solar receiver was analyzed using a one-dimensional unsteady simulation model of the solid-liquid receiver. The model included absorption of the incident solar radiation by the glass window, the silicon carbide porous ceramic heat absorber panel and the water. The results were verified against experimental data for a volumetric receiver and the error did not exceed 10%. It can be used to predict the heat transfer in solid-liquid composite volumetric receivers.     

Heat Transfer Enhancement in Shell-and-Tube Heat Exchangers Using Porous Media

A numerical simulation has been carried out to investigate the heat transfer enhancement in a shell-and-tube heat exchanger using a porous medium inside its shell and tubes, separately. A three-dimensional geometry with k-? turbulent model is used to predict the heat transfer and pressure drop characteristics of the flow. The effects of porosity and dimensions of these media on the heat exchanger's thermal performance and pressure drop are analyzed. Inside the shell, the entire tube bundle is wrapped by the porous medium, whereas inside the tubes the porous media are located in two different ways: (1) at the center of the tubes, and (2) attached to the inner wall of the tubes. The results showed that this method can improve the heat transfer at the expense of higher pressure drop. Evaluating the method showed that using porous media inside the shell, with particular dimension and porosity can increase the heat transfer rate better than pressure drop. Using this method inside the tubes leads to two diverse results: In the first configuration, pressure loss prevails over the heat transfer augmentation and it causes energy loss, whereas in the second configuration a great performance enhancement is observed.     

Thermal analysis of solar parabolic trough with porous disc receiver

In this paper, 3-D numerical analysis of the porous disc line receiver for solar parabolic trough collector is presented. The influence of thermic fluid properties, receiver design and solar radiation concentration on overall heat collection is investigated. The analysis is carried out based on renormalization-group (RNG) k–ε turbulent model by using Therminol-VP1 as working fluid. The thermal analysis of the receiver is carried out for various geometrical parameters such as angle (θ), orientation, height of the disc (H) and distance between the discs (w) and for different heat flux conditions. The receiver showed better heat transfer characteristics; the top porous disc configuration having w = d_i, H = 0.5d_i and θ = 30°. The heat transfer characteristic enhances about 64.3% in terms of Nusselt number with a pressure drop of 457 Pa against the tubular receiver. The use of porous medium in tubular solar receiver enhances the system performance significantly.     

Heat transfer analysis of parabolic trough solar receiver

Solar Parabolic Trough Collectors (PTCs) are currently used for the production of electricity and applications with relatively higher temperatures. A heat transfer fluid circulates through a metal tube (receiver) with an external selective surface that absorbs solar radiation reflected from the mirror surfaces of the PTC. In order to reduce the heat losses, the receiver is covered by an envelope and the enclosure is usually kept under vacuum pressure. The heat transfer and optical analysis of the PTC is essential to optimize and understand its performance under different operating conditions. In this paper a detailed one dimensional numerical heat transfer analysis of a PTC is performed. The receiver and envelope were divided into several segments and mass and energy balance were applied in each segment. Improvements either in the heat transfer correlations or radiative heat transfer analysis are presented as well. The partial differential equations were discretized and the nonlinear algebraic equations were solved simultaneously. Finally, to validate the numerical results, the model was compared with experimental data obtained from Sandia National Laboratory (SNL) and other one dimensional heat transfer models. Our results showed a better agreement with experimental data compared to other models.     

Evaluation of thermal efficiency of double-pass solar collector with porous–nonporous media

The double-pass solar collector with porous media in the lower channel provides a higher outlet temperature compared to the conventional single-pass collector. Therefore, the thermal efficiency of the solar collector is higher. A theoretical model has been developed for the double-pass solar collector. An experimental setup has been designed and constructed. The porous media has been arranged in different porosities to increase heat transfer, area density and the total heat transfer rate. Comparisons of the theoretical and the experimental results have been conducted. Such comparisons include the outlet temperatures and thermal efficiencies of the solar collector for various design and operating conditions. The relationships include the effect of changes in upper and lower channel depth on the thermal efficiency with and without porous media. Moreover, the effects of mass flow rate, solar radiation, and temperature rises on the thermal efficiency of the double-pass solar collector have been studied. In addition, heat transfer and pressure drop relationships have been developed for airflow through the porous media. Close agreement has been obtained between the theoretical and experimental results. The study concluded that the presence of porous media in the second channel increases the outlet temperature, therefore increases the thermal efficiency of the systems.