多孔介质对太阳池传热传质影响的研究

通过实验与数值模拟相结合的方法研究在太阳池底部增设多孔介质水层对太阳池热盐双扩散及储热量、稳定性等的影响.在实验室研究了多孔介质对盐扩散的抑制作用.在海边沙滩建造了两个小型太阳池,测量了有无多孔介质太阳池的温度分布和盐度分布并进行数值模拟,模拟值与实验值吻合较好.计算了有无多孔介质太阳池盐梯度的分布、池子稳定性情况以及多孔介质对太阳池的储热量的影响.结果表明,在太阳池底部加设多孔介质水层可以提高太阳池LCZ温度.多孔介质水层有利于提高太阳池的储热量,有利于抑制盐分向上扩散,可以节省太阳池的盐资源消耗,并有利于提高非对流层的盐梯度和稳定性.     

Soret效应和Dufour效应对盐梯度太阳池热盐双扩散的影响研究

该文建立盐梯度太阳池数学及物理模型,考虑Soret效应和Dufour效应对热盐双扩散的作用,采用格子Boltzmann方法,以C++为平台分别对速度场、温度场和浓度场进行描述。选取不同的Soret数和Dufour数,得到速度、温度和浓度的微观变化情况,讨论Soret效应和Dufour效应对盐梯度太阳池热盐双扩散的影响。结果表明:Soret效应对传热和传质均具有促进作用;Dufour效应对传热具有一定的抑制作用,对传质具有促进作用,Soret效应比Dufour效应对盐梯度太阳池热盐双扩散的影响作用更强烈。     

梯形盐水太阳池的实验与模拟研究

以梯形盐水太阳池为研究对象,搭建表面积2.4 m×2.4 m,底面积1.0 m×1.0 m的小型梯形盐水太阳池,基于实验数据对梯形盐水太阳池进行一维数值模拟,建立热盐双扩散离散模型,改进池底反射模型、辐射透射模型和包括表面热损、壁面热损和土壤层热损在内的详细热损失模型,通过模拟与实验结果的对比验证模型的有效性。结果表明:太阳池梯形结构可增强太阳辐射利用率,减少下对流层散热量,提高太阳池储热能力,从而提升太阳池的供热温度。     

基于二维数值模拟的梯形太阳池特性分析

以梯形太阳池为研究对象,以Fluent 13.0为平台,建立太阳池热盐双扩散模型,对梯形太阳池进行二维非稳态数值模拟研究,从温度场和流场2个角度验证模型有效性,分析太阳池的温度变化、速度分布及稳定性等特性。结果表明:梯形池温度整体高于矩形池,最大温差可达5℃;同一水平面内温度以两侧低、中间高规律分布;速度环流主要发生在上层对流(UCZ)和下层对流(LCZ),稳定运行后LCZ内出现2个反向大涡旋;上、下交界面存在明显滑移,UCZ与LCZ的厚度存在增厚趋势,故对太阳池进行盐梯度维护是十分必要的。     

动态太阳池的yong分析

对动态盐梯度太阳池的热性能进行了ｙｏｎｇ分析,对太阳池的ｙｏｎｇ系数、ｙｏｎｇ效率及其对外供热时整个系统的ｙｏｎｇ效率进行了理论探讨,并根据热效率与ｙｏｎｇ效率之间的制约关系探讨了太阳池的最佳运行温度。     

太阳池热能利用技术

回顾了太阳池的发展历史,介绍了太阳池的结构和工作原理,论述了从太阳池中提取热量的方法及太阳池的应用.太阳池发电可为不发达地区提供部分电能,是一种很有竞争力和发展前景的太阳能收集和储存系统.     

表层结冰对太阳池性能的影响研究

在对现有太阳池进行计算研究的基础上,针对目前北方太阳池冬季要经历结冰期这一现状,提出了表层结冰后太阳池辐射透射模型及热传导模型,将结冰情况考虑到太阳池的运行中,所提出的太阳池模拟计算中考虑了太阳池表面结冰对其整体性能的影响,为太阳池的长期连续性运行提供了研究基础.根据实际温度资料及气象条件,对结冰情况中太阳池的运行进行...     

太阳池的热盐双扩散模型及性能分析

建立了太阳池热盐双扩散数值模型,模拟结果与实验值对比,吻合较好。讨论了提热过程对太阳池热盐特性的影响规律,在提热速率不破坏NCZ稳定性的前提下,随着提热量的增大,热损失和盐损失都减小。热损降低,太阳池热效率提高;而盐损失降低,则利于NCZ内盐梯度的保持。模拟了太阳池受池底热损影响的热盐扩散规律。Soret通量所占总通量的比值在提热量和池底热损较小的情况下最大超过10%,因此Soret效应引起的盐扩散对总盐扩散起着不可忽视的作用。     

不同非对流层盐梯度层层数对太阳池性能影响

研究不同非对流层层数对太阳池稳定性、热性能的影响,以实现太阳池性能的优化。以工业NaCl为工质配制不同盐度溶液制备太阳池。以浊度、盐度和温度为考核指标,对太阳池的稳定性、吸热性和储热性进行研究。结果表明:当上对流层、非对流层、下对流层所占太阳池池体体积比率不变,非对流层各层厚度均匀设置时,太阳池降浊速度、盐梯度层稳定性、热性能均随太阳池非对流层盐梯度层层数的增加而增加。增加非对流层盐梯度层层数有利于改善太阳池的性能,太阳池的最佳非对流层盐梯度层层数为4。     

芒硝太阳池的研究与应用

该本对芒硝太阳池梯度区的浓度分布、温度分布以及芒硝太阳池的能量利用和应用进行了研究，并与实验结果进行比较，两者基本吻合，研究认为芒硝太阳池梯度区(非对流层)内温度分布，浓度分布是非线形的，利用太阳池技术为开发芒硝资源提供了一种新技术。     

Fertilizer solar ponds as a clean source of energy: Some observations from small scale experiments

Commercially available NH₂CONH₂ is used to establish a salinity gradient solar pond in a small 1 m² outdoor tank. With a salinity difference of 35% between the upper and lower zone, a temperature difference of 23°C was obtained without any instabilities in the gradient zone. The difference in concentration of solution required to sustain a temperature difference of 40°C across the gradient zone is 520 kg/m³. By economically using runoff into the fertilizer cycle of an agricultural system the estimated cost of fertilizer solar pond generated heat is Rs. 1.10/kWh.     

Dynamic effects in a salinity-gradient solar-pond heating system

Numerical computer models have been developed to study the dynamics of a salt-gradient solar-pond heating system in a northern cold climate. The models are applicable for predicting the temperature and salinity profiles in a pond. Special emphasis is laid on the behaviour of the upper convective layer. In the calculations, the solar pond is considered as a part of a community-scale residential heating system and the effects of the pond's dynamics on the overall system performance are assessed. All calculations were made with 1-h time steps for a hypothetical pond in Helsinki (60° N). The results indicate that the consideration of the dynamics of the salinity profile may reduce the pond's bottom temperature by 10°C in comparison with a static salt distribution. The maintenance of the salinity gradient would allow a maximum surface washing interval of 5 weeks without severely affecting the pond's performance. Then the daily salt consumption would be about 40 g per square metre. For regions with cold winters, the surface should be washed with fresh water, just before surface freezing takes place, to prevent shrinking of the non-convective stabilizing gradient zone. It was also observed that a solar-pond heating system may reach considerable solar fractions in a northern climate.     

Transient natural convection in an enclosure with vertical solutal gradients

Transient natural convection in an enclosure with vertical solutal gradients has been studied in this paper. Transfers in a rectangular cavity configuration translating hydrodynamic and thermal phenomena are numerically predicted by means of computational fluid dynamics (CFD) in transient regime.The objective of this numerical study is to give a fine knowledge of the hydrodynamic and thermal characteristics during energy storage in an enclosure filled with water stratified by downward salinity gradient. The enclosure is divided into three zones with different salinity level such as salt gradient pond (SGP). Water is heated by a heating device at the bottom of the cavity.The Navier–Stokes, energy and mass equations are discretized using finite-volume method, and a two-dimensional analysis of the hydrodynamic and thermal behaviors generated in transient regime in the cavity are performed. The mathematical modelling has allowed the prediction of the storage performances by developing parametrical study in view to search the convective heat transfer coefficient at the bottom of the enclosure. Velocity vector fields show the presence of recirculation zones caused only in the lower region and permit to explain the increase of the temperature in the lower convective zone (LCZ).This study shows also the importance of the salinity in the preservation of the high temperature in the bottom of the cavity, and the important reduction of the phenomenon of thermal transfer across the non-convective zone (NCZ).     

Theoretical and experimental comparison of conventional and advanced solar pond performance

Numerical and physical experiments were carried out to compare the performance of two solar pond systems: (a) a conventional salt gradient solar pond (CSP) and (b) a salt gradient pond operated as an “advanced solar pond” (ASP). The main differences in the ASP, as originally proposed by Osdor[1], are an increase in overall salinity and the introduction of a stratified flowing layer near the bottom of the gradient zone. The increased salinity is meant to reduce evaporative heat loss and make up water requirements, while the additional flowing layer allows extra heat extraction and possibly higher temperatures to develop in the lower convective zone. A numerical study was performed to evaluate the salinity effect and the results show only a minor effect of increased salinity on heat collection efficiency. However, slightly higher collection temperatures are obtained, which may provide some benefit for heat engine efficiency. Physical experiments were performed to test the feasibility of constructing and maintaining the necessary flow system for the ASP and also to compare the performance of the ASP and the CSP under similar laboratory conditions. These tests showed that a stable stratified flowing layer could be maintained and that the ASP configuration produced higher efficiencies.     

Double diffusive zone-boundary migration in solar ponds

The problem of gradient zone-boundary migration in solar ponds has been investigated. Models based on thermal burst stability theory and microconvection theory of boundary undulation and temperature modulation have been considered. Computational results of solutions for stability problem of the double diffusive interface are presented for an extended range of salinity gradient (1-10000 kg m⁻⁴) and examined with empirical correlations as well as experimental data. The results indicate that the erosion or growth of the gradient zone at the interface is a strong function of the ratio of diffusivities τ and density stability ratio R_ρ.     

Modeling the dynamics of the mixed layer in solar ponds

A new model is proposed to explain the Nielsen's equilibrium condition. This model takes into account the effects of both turbulent entrainment and diffusion on the growth/erosion of the gradient zone. The existing turbulent entrainment model is modified to make it applicable near the equilibrium condition. The new model predictions indicate the dependence of the equilibrium condition on the mixed layer depth, apart from the salinity and temperature gradients in the gradient zone.     

Radiation transmission through a composite pure water layer of uniform and depth-dependent extinction characteristics

The paper describes the calculation of the beam solar radiation transmission across the uniform salinity upper convecting zone (UCZ) and the salinity stratified gradient zone (GZ), when the extinction coefficient is not only wavelength but also salinity-concentration dependent, assuming that the effect of scattering to be negligible. The results are useful in determining the effects of dissolved salt ions on pure water transmission, at least according to the existing measured data in the literature.     

Stability analysis of double-diffusive solar ponds with non-constant temperature and salinity gradients

In this communication, the stability of the double-diffusive solar ponds with non-uniform temperature and salinity gradients has been investigated. This is a further generalization of our approach to this problem initiated in Ref. [7]. Using a stochastic approach, the linearized system of basic equations of motion is reduced to a single integro-differential equation. For convective motion, this equation reduces to a time-independent Schrödinger equation for a particle moving in a potential field ƒ(Z) characterized by the non-uniform temperature and salinity gradients. This equation can, in general, be solved (exactly or approximately depending on the form of the gradient profile) by methods commonly used in quantum mechanics.

In the Appendix, we show that, for a quadratic gradient profile, the above equation has an analytical solution similar to that obtained by Zangrando using numerical computations.     

Control of a solar pond

Solar ponds hold the promise of providing an alternative to diesel generation of electricity at remote locations in Australia where fuel costs are high. However, to reliably generate electricity with a solar pond requires high temperatures to be maintained throughout the year; this goal had eluded the Alice Springs solar pond prior to 1989 because of double-diffusive convection within the gradient zone. This paper presents control strategies designed to provide successful high temperature operation of a solar pond year-round. The strategies, which consist mainly of manipulating upper surface layer salinity and extracting heat from the storage zone are well suited to automation. They were tested at the Alice Springs solar pond during the summer of 1989 and maintained temperatures in excess of 85°C for several months without any gradient stability problems.     

Simulation of the control of a salt gradient solar pond in the south of Tunisia

We are interested in the modeling and control of a salt gradient solar pond (SGSP) in the south of Tunisia. We developed a model of a closed cycle salt gradient solar pond (CCSGSP) that ensures successful year round operation. This model was used to study the response of the solar pond (SP) to various control techniques. It takes into account heat and salt diffusion within the pond and simulates the transient behavior of a SGSP. Furthermore, we investigated the dynamic process, which involves internal gradient stability, boundary behavior between the gradient zone and the convective zones. We thus incorporated the double diffusive processes into the SP model by using the one dimensional stability criterion produced by linear theory. The governing differential equations are solved numerically by using a control-volume scheme.The results show that successful operation of a SP requires three things: the maintenance of the storage zone temperature through heat extraction and brine injection, the use of surface washing to control the deepening of the upper mixed layer and a well designed initial salt stratification to prevent the formation of instability within the gradient. Using linear salinity profile as an initial condition, three round year simulations were run using average meteorological data with the result that adequate stability (Rρ2 throughout the gradient and Rρ10 at the interfaces) was maintained. Numerical results show also that 10–30% efficiency could have been reached if heat extraction is performed routinely especially when one considers that the storage temperature is within 40–80 °C. The model is validated against data taken from the operation of the UTEP SP. Close correlation between computed and measured data was obtained.