球形腔式太阳能吸热器性能的数值研究

利用蒙特卡洛光线追踪法分析了6种不同开口比(D/d)的球形腔式吸热器的光学性能,并以光学模拟所得壁面能流作为热分析的边界条件导入CFD软件中,运用CFD软件对6种不同开口比的球形腔式吸热器进行流固耦合传热计算,获得了球形腔式吸热器和内部流体的温度场分布。通过计算球形腔式吸热器的反射光损失、对流热损失和热辐射损失,得到聚光器/球形腔式吸热器系统的光热转化效率为81.9%～84.4%,球形腔式吸热器的最佳开口比1相似文献   

太阳能热发电系统中腔式吸热器的光学性能

建立了球形、圆柱形、圆锥形和平顶圆锥形4种典型腔式吸热器与抛物面聚光器的三维模型,利用蒙特卡洛光线追踪法预测了4种典型腔式吸热器内部辐射能流的分布,其中球形吸热器内部的辐射能流分布均匀性最好,且辐射峰值最小,具有较好的光学性能。通过统计逸出腔口的反射光计算出这4种腔式吸热器的反射光损,其中球形吸热器的反射光损最小。在聚光器反射率为0.9,腔体内壁吸收率为0.9时,球形吸热器反射光损仅为0.66%,聚光器/球形吸热器的光学效率为88.9%。     

A novel integrated simulation approach couples MCRT and Gebhart methods to simulate solar radiation transfer in a solar power tower system with a cavity receiver

An integrated simulation approach, which couples Monte Carlo ray tracing (MCRT) and Gebhart methods, is proposed to simulate solar radiation transfer in a solar power tower system with a cavity receiver. The MCRT method is used to simulate the solar radiation transfer process from the heliostat field to interior surfaces of the cavity receiver, and the Gebhart method is used to simulate the multiple reflections process of solar radiation within the cavity. This integrated simulation method not only reveals the cavity effect on receiver performance but also provides real-time simulation results. Based on this method, the reflection loss of the cavity receiver and solar flux distributions are discussed in detail. The results indicate that the cavity effect can significantly reduce the reflection loss and homogenize the concentrated solar energy distributed on interior surfaces to some extent. Moreover, the surface absorptivity has less effect on the reflection loss when cavity effect is considered. The cavity effect on homogenizing solar flux distributions is greater with lower surface absorptivity. In addition, although the concentrated solar energy is distributed on the cavity aperture with similar shapes at different times, the shape of the solar flux distribution on interior surfaces varies greatly with time.     

Numerical simulation of the heat flux distribution in a solar cavity receiver

In the solar tower power plant, the receiver is one of the main components of efficient concentrating solar collector systems. In the design of the receiver, the heat flux distribution in the cavity should be considered first. In this study, a numerical simulation using the Monte Carlo Method has been conducted on the heat flux distribution in the cavity receiver, which consists of six lateral faces and floor and roof planes, with an aperture of 2.0 m×2.0 m on the front face. The mathematics and physical models of a single solar ray’s launching, reflection, and absorption were proposed. By tracing every solar ray, the distribution of heat flux density in the cavity receiver was obtained. The numerical results show that the solar flux distribution on the absorbing panels is similar to that of CESA-I’s. When the reradiation from walls was considered, the detailed heat flux distributions were issued, in which 49.10% of the total incident energy was absorbed by the central panels, 47.02% by the side panels, and 3.88% was overflowed from the aperture. Regarding the peak heat flux, the value of up to 1196.406 kW/m² was obtained in the center of absorbing panels. These results provide necessary data for the structure design of cavity receiver and the local thermal stress analysis for boiling and superheated panels.     

Thermal performance simulation of a solar cavity receiver under windy conditions

Solar cavity receiver plays a dominant role in the light-heat conversion. Its performance can directly affect the efficiency of the whole power generation system. A combined calculation method for evaluating the thermal performance of the solar cavity receiver is raised in this paper. This method couples the Monte-Carlo method, the correlations of the flow boiling heat transfer, and the calculation of air flow field. And this method can ultimately figure out the surface heat flux inside the cavity, the wall temperature of the boiling tubes, and the heat loss of the solar receiver with an iterative solution. With this method, the thermal performance of a solar cavity receiver, a saturated steam receiver, is simulated under different wind environments. The highest wall temperature of the boiling tubes is about 150 °C higher than the water saturation temperature. And it appears in the upper middle parts of the absorbing panels. Changing the wind angle or velocity can obviously affect the air velocity inside the receiver. The air velocity reaches the maximum value when the wind comes from the side of the receiver (flow angle α = 90°). The heat loss of the solar cavity receiver also reaches a maximum for the side-on wind.     

非均匀热流下球形腔式吸热器热性能的数值研究

建立了球形腔式吸热器三维模型;以基于蒙特卡罗光线追迹法(MCRT)进行光学模拟得到的能流分布,作为吸热管壁的热边界条件;通过数值模拟研究球形吸热器的耦合传热问题;探讨吸热流体入口参数对热性能的影响。研究表明:吸热管壁的辐射能流密度分布不均匀;在相同条件下,下入口吸热器的热性能优于上入口吸热器;在吸热流体的入口速度为0.2~0.4 m/s,提高流速可明显增大吸热器热效率,入口速度大于0.6 m/s时,热效率的增大速率变得平缓;随入口温度升高,热效率几乎呈线性下降。基于非均匀热流边界条件下的吸热器三维数值模拟结果更符合实际情况,为吸热器的优化设计与推广应用提供依据和参考。     

Numerical investigation of flow and heat transfer in a volumetric solar receiver

Volumetric solar receivers are used in solar power plants to convert concentrated solar radiation into high temperature heat to operate a thermal engine. In general, porous high temperature materials are used for this purpose. Since the pore geometry is important for the efficiency performance of the receiver, current R&D activities focus on the optimization of this quantity. In this study, the influence of slight geometry changes of this component on its temperature distribution and efficiency has been investigated with the objective of an overall improvement. A numerical analysis of the mass and heat transfer through the receiver has been performed. The investigated receiver was an extruded honeycomb structure made out of Silicon Carbide. Additionally, experimental tests have been performed. In these tests, selected receiver samples have been exposed to concentrated radiation. From these tests solar-to-thermal efficiency data have been derived, which could be compared with the calculated data. Two numerical models have been developed. One makes use of the real geometry of the channel (single channel model), the other one considers the receiver to be “porous continuum”, which is described by homogenized properties such as permeability and effective heat conductivity. The experimental parameters such as the average solar heat flux and the mass flow were taken into account in the models as boundary conditions. Various quantities such as the average air outlet temperatures, the temperature distributions and the solar-to-thermal efficiency were used for the comparison. The correspondence between the experimental and numerical results of both numerical models confirms the capability of the approaches for further studies.     

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。     

Three-dimensional numerical study of heat transfer characteristics in the receiver tube of parabolic trough solar collector

The solar energy flux distribution on the outer wall of the inner absorber tube of a parabolic solar collector receiver is calculated successfully by adopting the Monte Carlo Ray-Trace Method (MCRT Method). It is revealed that the non-uniformity of the solar energy flux distribution is very large. Three-dimensional numerical simulation of coupled heat transfer characteristics in the receiver tube is calculated and analyzed by combining the MCRT Method and the FLUENT software, in which the heat transfer fluid and physical model are Syltherm 800 liquid oil and LS2 parabolic solar collector from the testing experiment of Dudley et al., respectively. Temperature-dependent properties of the oil and thermal radiation between the inner absorber tube and the outer glass cover tube are also taken into account. Comparing with test results from three typical testing conditions, the average difference is within 2%. And then the mechanism of the coupled heat transfer in the receiver tube is further studied.     

Allowable flux density on a solar central receiver

The allowable flux density on a solar central receiver is a significant receiver parameter and is related to the receiver life span and economics. The allowable flux density has gradually increased as receiver technologies have developed and is related to various factors, such as the material characteristics, tube sizes, and internal tube flow conditions. A mathematical model was developed to calculate the allowable flux density for the Solar Two receiver which agrees well with published data. The model was then used to show that a higher allowable flux density can be obtained by increasing the allowable strain of the tube material and the fluid velocity and decreasing the tube thermal resistance, the convective thermal resistance, and the tube diameter and wall thickness. A sensitive analysis shows that the most important influence is the wall thickness, followed by the tube diameter and fluid velocity. Finally, a molten salt receiver gives a much higher allowable flux density than water/steam receivers and is even better than a supercritical steam receiver.     

An improved model for natural convection heat loss from modified cavity receiver of solar dish concentrator

A 2-D model has been proposed to investigate the approximate estimation of the natural convection heat loss from modified cavity receiver of without insulation (WOI) and with insulation (WI) at the bottom of the aperture plane in our previous article. In this paper, a 3-D numerical model is presented to investigate the accurate estimation of natural convection heat loss from modified cavity receiver (WOI) of fuzzy focal solar dish concentrator. A comparison of 2-D and 3-D natural convection heat loss from a modified cavity receiver is carried out. A parametric study is carried out to develop separate Nusselt number correlations for 2-D and 3-D geometries of modified cavity receiver for estimation of convective heat loss from the receiver. The results show that the 2-D and 3-D are comparable only at higher angle of inclinations (60° ? β ? 90°) of the receiver. The present 3-D numerical model is compared with other well known cavity receiver models. The 3-D model can be used for accurate estimation of heat losses from solar dish collector, when compared with other well known models.     

Experimental performance investigation of modified cavity receiver with fuzzy focal solar dish concentrator

In this paper, thermal performance analysis of 20 m² prototype fuzzy focal solar dish collector is presented. The focal image characteristics of the solar dish are determined to propose the suitable design of absorber/receiver. First, theoretical thermal performance analysis of the fuzzy focal solar parabolic dish concentrator with modified cavity receiver is carried out for different operating conditions. Based on the theoretical performance analysis, the total heat loss (conduction, convection and radiation heat losses) from the modified cavity receiver is estimated. It is observed that the maximum theoretical efficiencies of solar dish collector are found to be as 79.2% for no wind conditions and 78.2% and 77.8% for side-on and head-on winds speed of 5 m/s respectively. Latter, real time analysis of parabolic dish collector with modified cavity receiver is carried out in terms of stagnation test, time constant test and daily performance test. From stagnation test, the overall heat loss coefficient is found to be 356 W/m² K. The time constant test is carried out to determine the influence of sudden change in solar radiation at steady state conditions. The daily performance tests are conducted for different flow rates. It is found that the efficiency of the collector increases with the increase of volume flow rates. The average thermal efficiencies of the parabolic dish collector for the volume flow rate of 100 L/h and 250 L/h are found to be 69% and 74% for the average beam radiation (I_bn) of 532 W/m² and 641 W/m² respectively.     

空间站太阳能热动力发电系统吸热器研究

系统介绍了国际上对空间站太阳能热动力发电系统中吸热器的研究成果。详细地介绍了目前国际上研究设计的两种吸热器的结构、尺寸以及工作原理,对蓄热相变材料及其和吸热管材料间的腐蚀、相容性进行了讨论。分析了吸热器相变材料储罐壁面的热松脱问题及其对策。研究结果对促进我国空间站事业的发展具有指导意义。     

Investigations on heat losses from a solar cavity receiver

Thermal as well as optical losses affect the performance of a solar parabolic dish-cavity receiver system. Convective and radiative heat losses form the major constituents of the thermal losses. In this paper, an experimental and numerical study of the steady state convective losses occurring from a downward facing cylindrical cavity receiver of length 0.5 m, internal diameter of 0.3 m and a wind skirt diameter of 0.5 m is carried out. The experiments are conducted for fluid inlet temperatures between 50 °C and 75 °C and for receiver inclination angles of 0° (side ways facing cavity), 30°, 45°, 60° and 90° (vertically downward facing receiver). The numerical study is performed for fluid inlet temperatures between 50 °C and 300 °C and receiver inclinations of 0°, 45° and 90° using the Fluent CFD software. The experimental and the numerical convective loss estimations agree reasonably well with a maximum deviation of about 14%. It is found that the convective loss increases with mean receiver temperature and decreases with increase in receiver inclination. Nusselt number correlations are proposed for two receiver fluid inlet temperature ranges, 50-75 °C and 100-300 °C, based on the experimental and predicted data respectively. Besides no-wind tests, investigations are also carried out to study the effects of external wind at two different velocities in two directions (head-on and side-on). The wind induced convective losses are generally higher than the no-wind convective loss (varying between 22% and 75% for 1 m/s wind speed and between 30% and 140% for the 3 m/s wind speed) at all receiver inclination angles, the only exception being the loss due to side-on wind at 0° receiver inclination angle. This is because the wind acts as a barrier at the aperture preventing the hot air to flow out of the receiver. The head-on wind causes higher convective loss than the side-on wind. Nusselt number correlations proposed in this work are compared with the existing correlations in the literature. It is found that the correlations available in literature under-predict the convective losses at mean receiver temperatures between 100 °C and 300 °C. This is due to the fact that the correlations are developed for certain receiver geometries having the ratio of aperture diameter to receiver diameter equal to or lesser than one.     

Convection heat loss from cavity receiver in parabolic dish solar thermal power system: A review

The convection heat loss from cavity receiver in parabolic dish solar thermal power system can significantly reduce the efficiency and consequently the cost effectiveness of the system. It is important to assess this heat loss and subsequently improve the thermal performance of the receiver. This paper aims to present a comprehensive review and systematic summarization of the state of the art in the research and progress in this area. The efforts include the convection heat loss mechanism, experimental and numerical investigations on the cavity receivers with varied shapes that have been considered up to date, and the Nusselt number correlations developed for convection heat loss prediction as well as the wind effect. One of the most important features of this paper is that it has covered numerous cavity literatures encountered in various other engineering systems, such as those in electronic cooling devices and buildings. The studies related to those applications may provide valuable information for the solar receiver design, which may otherwise be ignored by a solar system designer. Finally, future development directions and the issues that need to be further investigated are also suggested. It is believed that this comprehensive review will be beneficial to the design, simulation, performance assessment and applications of the solar parabolic dish cavity receivers.     

Numerical modelling and optimisation of natural convection heat loss suppression in a solar cavity receiver with plate fins

This study details the numerical modelling and optimization of natural convection heat suppression in a solar cavity receiver with plate fins. The use of plate fins attached to the inner aperture surface is presented as a possible low cost means of suppressing natural convection heat loss in a cavity receiver. In the first part of the study a three-dimensional numerical model that captures the heat transfer and flow processes in the cavity receiver is analyzed, and the possibilities of optimization were then established. The model is laminar in the range of Rayleigh number, inclination angle, plate height and thickness considered. In the second part of the study, the geometric parameters considered were optimized using optimization programme with search algorithm. The results indicate that significant reduction on the natural convection heat loss can be achieved from cavity receivers by using plate fins, and an optimal plate fins configuration exit for minimal natural convection heat loss for a given range of Rayleigh number. Reduction of up to a maximum of 20% at 0° receiver inclination was observed. The results obtained provide a novel approach for improving design of cavity receiver for optimal performance.     

Design optimization of a tubular solar receiver with a porous medium

The main objective of this research is to find the optimal design point of the proposed solar receiver concept to heat up compressed air. Within a tubular receiver made of stainless steel, a porous medium is filled to enhance the heat transfer via the large contact area and thereby to increase the system efficiency. Due to the low melting point associated with the selected material, a numerical simulation is conducted to pre-evaluate the effects of various controlling parameters on the maximum temperature and pressure loss of the system. The design factors expected to influence the system performance were the length, porosity, and thermal conductivity of the porous medium as well as the number of inlet pipes. The effect of each variable on the maximum temperature and pressure drop of the system is numerically investigated and the optimal design point is selected. The results of this study offer a valuable design guideline for future manufacturing processes.     

热流计测量精度影响因素的数值分析

本文通过建立二维非稳态导热的数学模型 ,模拟了瞬态热流计测量低温金属表面与高温流体换热时影响测量精度的五种主要因素。计算得出 ,由于热流计测头的出现 ,对流换热系数的变化会给热流测量带来较大误差。其次 ,热流计测头自身因素的变化也会给测量带来误差。测头越薄 ,测量误差越小 ,稳定越迅速 ;测头边长越长 ,测量误差越小。但当被测物表面积较大时 ,测头边长存在一个最优值 ,能达到测量精度与测头尺寸的最佳结合 ,这个值约为 2 0mm ;测头与被测物粘贴越紧密 ,误差越小 ,稳定越迅速 ;被测物导热系数越小 ,则测量误差越小。模拟计算的结果能够为热流计的设计制作以及实际应用提供指导和参考     

Monte Carlo radiative transfer simulation of a cavity solar reactor for the reduction of cerium oxide

Radiative heat transfer in a solar thermochemical reactor for the thermal reduction of cerium oxide is simulated with the Monte Carlo method. The directional characteristics and the power distribution of the concentrated solar radiation that enters the cavity is obtained by carrying out a Monte Carlo ray tracing of a paraboloidal concentrator. It is considered that the reactor contains a gas/particle suspension directly exposed to concentrated solar radiation. The suspension is treated as a non-isothermal, non-gray, absorbing, emitting, and anisotropically scattering medium. The transport coefficients of the particles are obtained from Mie-scattering theory by using the optical properties of cerium oxide. From the simulations, the aperture radius and the particle concentration were optimized to match the characteristics of the considered concentrator.     

Experimental studies on the thermal performance of a parabolic dish solar receiver with the heat transfer fluids SiC + water nano fluid and water

An experimental investigation has been carried out with aa point focusing dish reflector of 12 square meters aperture area,exposed to the average direct normal irradiations of 810 W/m2.This work focuses on enhancinge the energy and exergy efficiencies of the cavity receiver by minimizing the temperature difference between the wall and heat transfer fluids.Two heat transfer fluids Water and SiC + water nano fluid have been prepared from 50 nm particle size and 1％ of volume fraction,and experimented separately for the flow rates of 0.2 lpm to 0.6 lpm with an interval of 0.1 lpm.The enhanced thermal conductivity of nano fluid is 0.800115 W/mK with the keff/kb ratio of 1.1759 determined by using the Koo and Kleinstreuer correlation.The maximum attained energy and exergy efficiencies are 29.14％ and 24.82％ for water,and 32.91％ and 39.83％ for SiC+water nano fluid.The nano fluid exhibits enhanced energy and exergy efficiency of 12.94％ and 60.48％ than that of water at the flow rate of 0.5 lpm.The result shows that the system with SiC+Water produces higher exergy efficiency as compared to energy efficiency;in the case ofwater alone,the energy efficiency is higher than exergy efficiency.