槽式集热器真空集热管传热特性研究

通过分析太阳能槽式集热器真空集热管的热性能,建立了槽式集热器真空集热管稳态传热模型,通过和典型实验数据对比,验证该模型的适用性和准确性。在此基础上,分析在无光照条件下,吸热管内壁温度、环境温度、风速和集热管残存气体种类等因素对集热管热损失的影响。结果表明：吸热管内壁温度的升高会增大玻璃管外壁温度和集热管热损失;环境温度和风速对玻璃管外壁温度有显著影响,但对热损失的影响甚微;吸热管与玻璃管之间以辐射传热为主,对流换热受环形区域残存气体种类和压力的影响。     

太阳能真空集热管集热过程的动态方程

基于太阳能真空集热管结构,应用传热学模型、气象参数模型以及流体工质物理模型分析其传热特性,并在此基础上归纳方程,得到真空集热管的吸热管温度、热损失以及被加热流体工质温度等与室外气象条件的关系。最后,利用该方程模拟真空集热管在冬季白天气象条件下的动态响应特征,并给出模拟结果。结果表明:吸热管温度与室外温度关联度小;热损失在较高辐照度时较大,呈"瘦高"型;同时,对热损失以及玻璃管外壁温度进行试验验证。试验表明:吸热管温度在320℃以下,模拟结果与试验符合较好。     

抛物槽式太阳能吸热管热损失研究

利用稳态平衡的方法测出了抛物槽式太阳能吸热管的热损,并拟合出了公式,为吸热管热性能提供了重要参数。分析计算了吸热管端部保温部分热损失以及金属管的膨胀量、风速和环境温度对热损的影响,结果表明:吸热管端部热损比较大,占总热损7.3%~9.1%,在槽式太阳能热发电站运行时也应该做好端部的保温;吸热管温度越高,热损越大;风速越大,环境温度越高,热损越大,但风速和环境温度影响较小;热损随着金属管的膨胀量的增大而减小,但是热损变化微乎其微,可以不加考虑。     

平板型太阳能集热器的热损失分析

对平板型太阳能集热器的热损失进行理论分析,建立热损失的数学模型,由于平板型集热器的热损主要是顶部热损,所以本文对顶部热损进行详细的理论推导。通过迭代法对平板集热器顶部热损进行估算,并和Klein提出的热损系数经验公式进行验证,两种方法估算顶部热损的结果和变化规律一致,通过对比验证分析,得到了影响集热器效率的众多因素中的两因素——盖板层数和吸热板的热辐射性质对集热器顶部热损失的影响。     

碱金属热电直接发电装置的热辐射损失

寄生热辐射损失，特别是BASE管外表面的热辐射是影响碱金属热电转换器(AMTEC)高效运行的主要因素之一。为了量化BASE管外表面的热辐射对碱金属热电转换器热电转换效率的影响程度，文章用一简化模型，计算了采用不同层数遮热屏的碱金属热电转换器中的BASE管外表面的净热辐射量。结果表明，恰当设计AMTEC装置的结构，并在BASE管外加数层遮热屏，可使BASE管外单位电极表面的净热辐射损失在其工作温度范围内控制在1W／cm^2以下。     

抛物槽集热器的能量效率

对以镜面不锈钢板为反光材料的抛物槽集热器的能量效率进行了研究,分析了光学效率、热损失对能量效率的影响,计算出有空气夹层的接收器的总热损系数。试验测得的能量效率为21.1%,光学效率为23.9%,总热损系数为10.45W/(m2.℃)。通过对试验数据的分析得知,光学效率是影响能量效率的主要因素,吸收管的传导热损失较辐射热损失大,若将接收器内的空气夹层抽成真空,会大大地减少热损失。     

主动太阳能采暖供水管流量规律研究

通过对集热器有效集热量、建筑热负荷、蓄热系统蓄热量、蓄热水箱损失以及蓄热水箱温度的理论分析,建立了太阳能采暖供水管流量数学模型,且分别在不同太阳辐射强度与不同建筑热负荷波动规律下进行模拟分析,得出蓄热水箱温度变化规律以及各种情况下采暖供水管流量变化规律.结果表明:蓄热水箱温度受太阳辐射强度波动规律影响较大,受建筑热负荷波动规律影响较小,其中蓄热水箱温度基本在30～70℃之间;太阳辐射强度相同,建筑热负荷越大所需采暖管流量也越大,流量最大、最小值分别可达到0.810、0.008kg/s;随着太阳辐射强度波动规律的增大,采暖管流量波动规律亦相应增大.     

用于槽式聚光集热器的真空集热管的热损失分析

在分析了国内外真空集热管发展现状和最新动态的基础上,对槽式聚光集热器真空集热管建立了热损失模型,运用理论公式分析方法对热损失产生的原因进行了深入分析,并通过实验验证了热损失经验公式,得出结论:影响热损失的主要因素在于工质与环境的温差,以及真空集热管的真空度。     

抛物面槽式太阳能集热器场热损失分析

在已有的计算集热器场吸收有用能量模型的基础上,加入影响集热器场效率的热学因素,优化了集热器场效率计算模型,并验证了优化模型的精确性。利用优化模型对抛物面槽式太阳能集热器场热损失机理进行了研究。结果表明,集热器集热元件热效率、入射角以及由入射角引起的端部损失是影响集热器场效率的主要因素。在太阳辐射强度一定的情况下,入射角越小、集热器热收集元件的热效率越高时,集热器场效率越高。     

汽阳极多管AMTEC辐射与导热模型的研究

针对研制并改进新一代汽阳极多管碱金属热电转换装置(AMTEC)需要准确计算汽阳极多管AMTEC内的辐射和导热损失,以PX-3A型装置为分析对象,利用RadCAD软件计算了装置内各辐射面之间角系数,角系数计算结果满足完整性要求;以此角系数作为输入数据建立了装置辐射与导热模型,计算了不同负载电阻下装置的输出功率、负载电压和BASE管冷端温度等参数,结果表明计算值与文献实验值相吻合,从而验证了模型的正确性和合理性;最后对不同热端温度对转换效率的影响进行了分析,推荐装置热端运行温度为1123 K。     

Theoretical approach of a flat-plate solar collector taking into account the absorption and emission within glass cover layer

A rigorous theoretical approach of a flat-plate solar collector with a black absorber considering the glass cover as an absorbing–emitting media is presented. The glass material is analyzed as a non-gray plane-parallel medium subjected to solar and thermal irradiations in one-dimensional case using the Radiation Element Method by Ray Emission Model (REM²). The optical constants of a clear glass window proposed by Rubin have been used. These optical constants, 160 values of real part n and imaginary part k of the complex refractive index of a clear glass, cover the range of interest for calculating the solar and thermal radiative transfer through the glass cover. The computational time for predicting the thermal behavior of solar collector was found to be prohibitively long for the non-gray calculation using 160 values of n and k. Therefore a suitable semi-gray model is proposed for rapid calculation. The profile of the efficiency curve obtained in the present study was found to be not linear in shape. Indeed, the heat loss from the collector is a combination of convection and radiation and highly non linear. The effect of the outside convective heat transfer on the efficiency curve is also studied. In fact, when the convection is the dominant heat transfer mode compared with the radiation one, the profile of the efficiency curve is more or less straight line. Consequently, the heat loss coefficient could be calculated using Klein model. It has been also shown that the effect of the wind speed on the glass cover mean temperature is very important. This effect increases with the increase of the mean absorber temperature.     

Analysis of heat losses from a trapezoidal cavity used for Linear Fresnel Reflector system

A Computational study to investigate the heat loss due to radiation and steady laminar natural convection flow in a trapezoidal cavity having eight absorber tubes for a Linear Fresnel Reflector (LFR) solar thermal system with uniformly heated tubes and adiabatic top wall and side walls has been performed. The losses due to convection and radiation were considered from the bottom glass cover. The results are validated with experimental data. Radiative component of losses from the cavity was found to be dominant which contributes around 80–90%. Heat loss characteristics have been studied for cavities of different depths. Simulations have been carried out for various values of heat transfer coefficient based on the wind speed below the glass surface. Effect of emissivities of the tubes on the heat loss has also been simulated. Flow pattern and isotherms inside the cavity for various depths have been analyzed. Finally, the correlation between the total average Nusselt number and its influencing parameters has been obtained for the proposed cavity.     

Optimization of solar-powered Stirling heat engine with finite-time thermodynamics

A mathematical model for the overall thermal efficiency of the solar-powered high temperature differential dish-Stirling engine with finite-rate heat transfer, regenerative heat losses, conductive thermal bridging losses and finite regeneration processes time is developed. The model takes into consideration the effect of the absorber temperature and the concentrating ratio on the thermal efficiency; radiation and convection heat transfer between the absorber and the working fluid as well as convection heat transfer between the heat sink and the working fluid. The results show that the optimized absorber temperature and concentrating ratio are at about 1100 K and 1300, respectively. The thermal efficiency at optimized condition is about 34%, which is not far away from the corresponding Carnot efficiency at about 50%. Hence, the present analysis provides a new theoretical guidance for designing dish collectors and operating the Stirling heat engine system.     

Gas heat conduction in an evacuated tube solar collector

We investigated experimentally the pressure dependency of the gas heat conduction in an evacuated plate-in-tube solar collector. A stationary heat loss experiment was built up with an electrically heated real-size collector model. The gas pressure was varied from 10⁻³ to 10⁴ Pa, the temperatures of the absorber and the casing were held at 150°C (electrical heaters) and 30°C (water cooling), respectively. Losses by radiation and solid conduction were determined experimentally at pressures below 0.1 Pa. At higher pressures these background losses were subtracted from the total heat losses, to receive the heat losses by gas heat conduction. The experimental results were compared with approximative theoretical models. The onset of convection is in agreement with the usual theories for parallel plates, taking the largest distance between the absorber and the glass tube as the plate distance. As a first approximation the pressure dependency of the gas heat conduction is described by the usual theory for parallel plates, taking the smallest distance between the absorber and the glass tube as the plate distance.     

Thermal performance of a novel rooftop solar micro-concentrating collector

Concentrating solar thermal systems offer a promising method for large scale solar energy collection. Although concentrating collectors are generally thought of as large-scale stand-alone systems, there is a huge opportunity to use novel concentrating solar thermal systems for rooftop applications such as domestic hot water, industrial process heat and solar air conditioning for commercial, industrial and institutional buildings. This paper describes the thermal performance of a new low-cost solar thermal micro-concentrating collector (MCT), which uses linear Fresnel reflectors, and is designed to operate at temperatures up to 220 °C. The modules of this collector system are approximately 3 m long by 1 m wide and 0.3 m high. The objective of the study is to optimise the design to maximise the overall thermal efficiency. The absorber is contained in a sealed enclosure to minimise convective losses. The main heat losses are due to natural convection inside the enclosure and radiation heat transfer from the absorber tube. In this paper we present the results of a computational and experimental investigation of radiation and convection heat transfer in order to understand the heat loss mechanisms. A computational model for the prototype collector has been developed using ANSYS–CFX, a commercial computational fluid dynamics software package. The numerical results are compared to experimental measurements of the heat loss from the absorber, and flow visualisation within the cavity. This paper also presents new correlations for the Nusselt number as a function of Rayleigh number.     

CFD analysis of the thermal losses on the upper part of a flat solar collector

To investigate the reduction of heat losses on the upper part of a flat solar collector, a two‐dimensional study was carried out by CFD analysis using Fluent. For this purpose, the heat transfer behavior in the air gap over a wide range of thicknesses of the latter (1‐20 mm) and the addition of a second glass cover fixed at midheight of the air gap spacing have been investigated. For small thicknesses of the air gap (1‐8 mm), the heat transfer is essentially conductive. An increase in the thickness leads to the intensification of the natural convection which induces high thermal losses. The simulation results have shown that the addition of a second cover glazing leads to the weakening of the natural convection and thus to an average enhancement of the solar collector temperature over the range of thicknesses studied of approximately 17%. The overall thermal losses coefficient is then reduced by an average of 26% compared with the single‐glazed solar collector. They have also shown that the thickness of the air gaps resulting in the minimum overall heat losses is 8 mm and that the thickness of the second glass cover has no significant effect on these results. In addition, this study has highlighted the importance of taking into account the radiation heat transfer in establishing the thermal balance of a flat solar collector. Indeed, this consideration leads to an average decrease of the absorber temperature of about 30%.     

Book Review Corner

A thermal circuit model has been used to examine the heat transfer characteristics of an idealized station-keeping near-space platform. Forced convection, natural convection, and thermal radiation heat transfer mechanisms have been included in the model. The contributions of forced and natural convection were incorporated using previously published correlations. Thermal radiation heat transfer was assumed to be the result of direct solar flux, planetary infrared, and planetary albedo thermal radiation contributions. Model closure was obtained by using data from the 1976 standard atmosphere model, an empirical model for wind speed and models for variable thermophysical properties. The results of the scaling analysis indicate that, as expected, natural convection may be neglected above certain altitudes. The results also show the relative importance of thermal radiation and forced convection as the dominant heat transfer mechanisms with respect to altitude. Results from parametric studies of albedo and surface material effects are also included.     

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.     

A comprehensive model for optical and thermal characterization of a linear Fresnel solar reflector with a trapezoidal cavity receiver

In this paper, a comprehensive numerical model was developed by coupling Monte Carlo Ray Tracing (MCRT) and Finite Volume Method (FVM) for simulating the energy conversion process in the linear Fresnel reflector (LFR) with a Trapezoidal Cavity Receiver (TCR). Based on the model, firstly, the optical performance of a typical LFR was studied, followed by analyzing its heat transfer characteristics and thermal performance at various conditions. Then, the effects of key parameters were investigated. Finally, a LFR prototype was simulated to illustrate the application of the model. The results indicate that the solar fluxes on the absorber tubes exhibit non-uniform characteristics which would result in the non-uniform temperatures. The annual optical efficiency of 60.1%–44.7% from the equator to N50° and the collector efficiency of 48.3%–72.0% for the superheating section at normal incidence can be achieved, respectively. Moreover, the heat transfer characteristic study reveals that the radiation loss from the tubes is the dominant mode and contributes around 81%–87% at typical conditions. Parameter studies indicate that the energy absorbed by the glass which influences the heat loss obviously should be considered in the heat loss study of TCR. And the heat loss from the tubes increases rapidly with the coating emissivity, so the coating with low emissivity should be recommended for the TCR. In addition, the application in the realistic LFR indicates that the present model is an exercisable and useful tool for the LFR.     

Consistent heat transfer analysis for performance evaluation of multichannel solar absorbers

The numerical evaluation of solar absorber performance must be based on the coupling between solar flux modeling and heat transfer modeling. We have developed a ray-tracing method to model the solar flux distribution absorbed at SiSiC multichannel absorber surfaces under a solar furnace, and solved one-dimensional heat transfer governing equations that import the solar flux modeling results. By consistently dealing with changes in properties or dimensions of absorbers for the two modeling processes, we are able to evaluate absorber performance with the balance of radiation loss. It turns out that the diffuse irradiation assumption is applicable for performance evaluation on multichannel absorbers although it may differ from a real solar flux distribution. A sensitivity analysis demonstrates that the increase of absorptivity is most effective to performance enhancement because the increase of reflection loss dominates the decrease of emission loss. The proposed consistent approach provides a better understanding of heat transfer in volumetric solar absorbers and thus helps the improvement of their performance.