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。     

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。     

基于CSO算法优化的LSSVM熔盐温度预测

吸热器的安全高效运行在太阳能热发电系统中起到了至关重要的作用。吸热管中熔盐出口最高温度和平均温度为太阳能热发电系统中吸热器的设计、材料的选择和运行控制提供了重要的科学依据。文章提出了基于最小二乘支持向量机(Least squares support vector machines,LSSVM)的快速预测方法,预测不同工况下吸热管中熔盐出口最高温度和平均温度。为了提高预测精度,利用猫群优化(Cat Swarm Optimization,CSO)算法求解LSSVM方法中的超参数。数值计算结果证实了LSSVM方法是可行的,从而为研究塔式太阳能吸热器的热特性以及运行控制提供一种新的有效方法。     

基于分布参数模型的塔式熔盐吸热器换热过程动态特性研究

文章以塔式太阳能热发电站中的圆柱形外露管式吸热器为研究对象,利用分布参数方法建立了对流-辐射传热吸热器的非稳态传热模型。基于此模型,分别研究了当辐射热流密度、熔盐流量、熔盐进口温度和环境风速等参数发生阶跃变化时,圆柱形外露管式吸热器的动态特性。研究结果表明:当上述参数发生阶跃变化时,熔盐出口温度的动态变化过程存在不同的时间常数;辐射热流密度、熔盐进口温度和熔盐流量是影响熔盐出口温度的主要因素,瞬时风对熔盐出口温度影响较小。     

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。     

一种多孔介质太阳能吸热器传热研究

为了研究塔式太阳能多孔介质吸热器的传热传质特性,建立吸热器稳态传热模型,选择适合多孔介质太阳能吸热器的体积对流换热系数模型,采用数值方法求解,并分别分析孔隙密度、孔隙率和入口空气速度对温度场的影响。文中技术可以为同类型太阳能吸热器的设计和改造提供参考。     

一种太阳能吸热器过热蒸汽温度控制系统

介绍了一种应用于塔式太阳能热发电系统的水/蒸汽吸热器过热蒸汽温度控制系统。受到太阳辐射能间歇性和不确定性的影响,吸热器产生的过热蒸汽温度难于控制。控制系统根据吸热器在蒸汽流量变化、光功率变化和减温水流量变化等3种主要扰动下的过热汽温度动态响应特性,以减温水流量作为控制量,光功率和蒸汽负荷作为前馈信号,设计和研制了两段式过热蒸汽温度控制系统,使吸热器过热区出口汽温维持在允许的范围内。     

塔式太阳能热发电技术的发展

塔式太阳能热发电系统具有聚光比大、投射到塔顶吸热器上的平均热流密度高、工作温度高、电站规模大和年度发电效率高等特点,提高效率和降低成本的潜力较大。文章阐述了塔式太阳能热发电技术的工作原理和系统组成;介绍和分析了国内外已建成的塔式太阳能电站;指出了塔式太阳能热发电技术的发展现状、技术难点和未来发展方向;探讨了我国规模化发展塔式太阳能热发电技术须解决的问题。     

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.     

塔式太阳能热发电系统性能模型及其仿真研究

结合塔式太阳能热发电系统中主要设备的机理模型和变工况下的运行方式,建立一套能完整反映电站工作过程的性能模型。发电系统在太阳法向直射辐照度扰动下的动态过程研究结果表明：吸热器出口熔盐温度、表面最高温度、散热功率的过渡过程较平缓,效率在瞬时突变后会逐渐恢复到接近扰动发生前的水平;吸热器出口熔盐温度、表面最高温度、散热功率与太阳法向直射辐照度的变化基本呈正比关系,效率随太阳法向直射辐照度的增大而升高但斜率逐渐减小。发电系统对电网负荷特性的适应能力研究结果表明：机组在热态启动方式下更适合带早高峰,而在温态启动方式下更适合带晚高峰,高峰时段附近的天气状况对机组带早晚高峰负荷的能力具有明显影响。     

One dimensional thermal analysis of silicon carbide ceramic foam used for solar air receiver

Using air as heat transfer fluid for electricity generation offers some significant advantages for the development of Concentrated Solar Power (CSP): high conversion efficiency, low environmental impact and being used in deserts or other areas scarce of water resources. Silicon carbide ceramic foams have the characteristics of light weight, high strength, large specific surface areas, high porosity and excellent thermal shock resistance performance which make them particularly fit for absorber material in CSP. In this paper, thermal performance of silicon carbide ceramic foam as solar air receiver is investigated analytically based on the one dimensional physical model. The analytical results show that the air flow resistance increases obviously with increasing air outlet temperature, the air flow resistance while the air outlet temperature is equal to 1000 °C is nearly 3 times the one while the air outlet temperature is equal to 20 °C with air velocity range is between one and six meters per second. The results of one dimensional analysis of flow and heat transfer process of ceramic foams suggest that there exists an input solar energy flux limit for the unpressurized system, which will lead to limit the power capacity and the outlet air temperature enhancement.     

Numerical simulation of convective heat transfer between air flow and ceramic foams to optimise volumetric solar air receiver performances

Porous ceramic foams are used to achieve high performance in solar heat recovery systems. Understanding the convective heat transfer between the air flow and the ceramic foam is of great importance when optimising the volumetric air receiver. In this work, the convective heat transfer was numerically studied. The present approach was designed to compute the local convective heat transfer coefficient between the air flow and a porous ceramic foam. For that purpose, the energy balance and the flow inside the porous ceramic foam were solved. In addition, a detailed geometry of the porous ceramic foam was considered. The ceramic foams were represented by idealised packed tetrakaidecahedron structures. The numerical simulations were based on the three dimensional Reynolds-averaged Navier–Stokes (RANS) equations. A sensitivity study on the heat transfer coefficient was conducted with the porosity, velocity and mean cell size as parameters. Based on the numerical simulation results, a correlation for the volumetric local convective heat transfer coefficient between air and ceramic foams was developed. The resulting correlation covers a wide range of porosities, velocities, cell sizes and temperatures. The correlation results were compared with experimental data from the literature, and the comparison shows good agreement. The correlation is intended to be used in the design of volumetric solar air receivers.     

Coupled radiation and flow modeling in ceramic foam volumetric solar air receivers

Ceramic foams are promising materials for the absorber of volumetric solar air receivers in concentrated solar thermal power (CSP) receivers. The macroscopic temperature distribution in the volumetric solar air receiver is crucial to guarantee that volumetric solar air receivers work steadily, safely and above all, efficiently. This study analyzes the temperature distribution of the fluid and solid phases in volumetric solar air receivers. The pressure drop in the ceramic foams and the interfacial heat transfer between the flowing fluid and solid are included in the model. The radiative heat transfers due to concentrated solar radiation absorption by the ceramic foam and the radiation transport in the media were modeled with the P₁ approximation. The energy fields of the fluid and solid phases were obtained using the local thermal non-equilibrium model (LTNE). Comparison of the macroscopic model with experimental results shows that the macroscopic model can be used to predict the performance of solar air receivers. Sensitivity studies were conducted to analyze the effects of velocity, porosity, mean cell size and the thermal conductivity of the solid phase on the temperature fields. The results illustrate that the thermal non-equilibrium phenomena are locally important, and the mean cell size has a dominant effect on the temperature field.     

腔式太阳能高温空气吸热器传热过程数值模拟

介绍了一种应用于塔式太阳能热发电站的腔式高温空气吸热器,建立了吸热器内部空气流动及传热过程模拟数学模型,并通过数值方法,模拟了吸热器内部的空气流场和温度场。结果得知:空气进入吸热器后,沿内壁面轴向高速流动,随着深度的增加,速度越来越小,到达底部时速度最小;在压差的作用下,进入吸热器内部的空气会不断流向和冲刷针肋及壁面,而主流方向的流量不断减少;空气通过冲刷高温针肋及壁面不断吸收热量,温度不断升高;由于吸热器底部空气速度较小,对流换热系数较小和热流密度较大,因此该处温度较高,是整个吸热器的最脆弱部位;在高辐照强度情况下,虽然加大空气流量可降低吸热器壁面的温度,但由于其对流换热系数与空气流速不成正比例,壁面温度一般还会有所升高。     

抛物面聚光太阳能PV/T系统的热电性能分析

建立了带有散热翅片的聚光太阳能PV/T热电联产系统内部传热过程的一维稳态数学模型,对传热过程进行了数值模拟,分析了空气质量流速、入射光强度、聚光比、环境温度、上部通道高度及翅片参数对系统的空气温度、电池板温度及系统热、电效率的影响.结果表明:随着入射光强、聚光比的增加,空气出口温度和电池板温度都会增加,系统热电总效率增加;通过增空气流量可以有效降低电池温度,提高电池的光电转换效率和系统的总能量利用效率;吸热板背面的翅片可以强化通道内空气的传热过程,降低电池板的温度,系统效率可增加约2%;在相同的光照条件下,人口空气温度越低,上部通道越窄,系统热效率越高.研究结果为聚光太阳能PV/T热电联产系统的设计和运行提供了理论依据.     

太阳辐射强度和温度对增强型冷却塔换热性能影响的研究

太阳能增强型自然通风冷却塔(SENDDCT)作为一种新型空气冷却系统,可利用太阳能提高自然通风冷却塔的效率。利用FLUENT软件建立太阳能增强型冷却塔的三维模型,研究太阳辐射强度和环境温度对其性能的影响;同时设计并搭建太阳能增强型冷却塔的实验系统,在实际天气条件下研究太阳辐射强度对其换热性能的影响。模拟结果表明太阳辐射强度一定时,通过换热器的空气流速以及换热率都会随着环境温度升高而降低。太阳辐射强度为500 W时,环境温度由10℃升到40℃,换热率由1.106 kW下降到0.281 kW。实验结果表明有辐射时冷却塔入口处的平均空气温度要比无辐射时的高5℃;实验期间无辐射时的平均换热率为0.682 kW,有辐射时的平均换热率为0.794 kW,即利用太阳能将平均换热率提高了0.112 kW。     

平板式太阳能空气集热器流道改进的试验研究和数值模拟

对传统平板式太阳能空气集热器的流道进行了改进,把对角型进出口流道改为多进出口式流道。对改进的集热器的性能进行了试验测试。新的进出口流道消除了吸热板和空气换热不均的现象,出口温度提升明显。在相同条件下,集热器的瞬时效率增加约20%。用CFD方法对集热器内部的流场结构和传热进行的数值模拟对比表明,传统集热器内部存在流动死区,中心截面温度分布不均匀,吸热板上有局部的高温区域,改进后的集热器流场和温度场分布得较均匀。     

Heat transfer analysis of receiver for large aperture parabolic trough solar collector

Parabolic trough solar collector (PTSC) is one of the most proven technologies for large‐scale solar thermal power generation. Currently, the cost of power generation from PTSC is expensive as compared with conventional power generation. The capital/power generation cost can be reduced by increasing aperture sizes of the collector. However, increase in aperture of the collector leads to higher heat flux on the absorber surface and results in higher thermal gradient. Hence, the analysis of heat distribution from the absorber to heat transfer fluid (HTF) and within the absorber is essential to identify the possibilities of failure of the receiver. In this article, extensive heat transfer analysis (HTA) of the receiver is performed for various aperture diameter of a PTSC using commercially available computational fluid dynamics (CFD) software ANSYS Fluent 19.0. The numerical simulations of the receiver are performed to analyze the temperature distribution around the circumference of the absorber tube as well as along the length of tube, the rate of heat transfer from the absorber tube to the HTF, and heat losses from the receiver for various geometric and operating conditions such as collector aperture diameter, mass flow rate, heat loss coefficient (HLC), HTF, and its inlet temperature. It is observed that temperature gradient around the circumference of the absorber and heat losses from the receiver increases with collector aperture. The temperature gradient around the circumference of the absorber tube wall at 2 m length from the inlet are observed as 11, 37, 48, 74, and 129 K, respectively, for 2.5‐, 5‐, 5.77‐, 7.5‐, and 10‐m aperture diameter of PTSC at mass flow rate of 1.25 kg/s and inlet temperature of 300 K for therminol oil as HTF. To minimize the thermal gradient around the absorber circumference, HTFs with better heat transfer characteristics are explored such as molten salt, liquid sodium, and NaK78. Liquid sodium offers a significant reduction in temperature gradient as compared of other HTFs for all the aperture sizes of the collector. It is found that the temperature gradient around the circumference of the absorber tube wall at a length of 2 m is reduced to 4, 8, 10, 13, and 18 K, respectively, for the above‐mentioned mass flow rate with liquid sodium as HTF. The analyses are also performed for different HTF inlet temperature in order to study the behavior of the receiver. Based on the HTA, it is desired to have larger aperture parabolic trough collector to generate higher temperature from the solar field and reduce the capital cost. To achieve higher temperature and better performance of the receiver, HTF with good thermophysical properties may be preferable to minimize the heat losses and thermal gradient around the circumference of the absorber tube.     

Exergy‐based performance analysis of packed‐bed solar air heaters

This paper presents an experimental investigation of the thermal performance of a solar air heater having its flow channel packed with Raschig rings. The packing improves the heat transfer from the plate to the air flow underneath. The dimensions of the heater are 0.9 m wide and 1.9 m long. The aluminium‐based absorber plate was coated with ordinary black paint. The characteristic diameter of the Raschig rings, made of black polyvinyl chloride (PVC) tube, is 50 mm and the depth of the packed‐bed in flow channel is 60 mm. Energy and exergy analyses were applied for evaluating the efficiency of the packed‐bed solar air heater. The rate of heat recovered from the packed‐bed solar air heater varied between 9.3 and 151.5 W m^?2, while the rate of thermal exergy recovered from the packed‐bed solar air heater varied between 0.04 and 8.77 W m^?2 during the charging period. The net energy efficiency varied from 2.05 to 33.78%, whereas the net exergy efficiency ranged from 0.01 to 2.16%. It was found that the average daily net energy and exergy efficiencies were 17.51 and 0.91%, respectively. The energy and exergy efficiencies of the packed‐bed solar air heater increased as the outlet temperature of heat transfer fluid increased. Copyright © 2004 John Wiley & Sons, Ltd.