Dependence of speed of sound on salinity and temperature in concentrated NaCl solutions

An ultrasonic velocimeter for salinity measurements in a solar pond has been developed and tested. Calibration of the probe is accomplished by comparing speed of sound measurements in distilled water with accepted values. Speed of sound in NaCl solutions with salinities from 0%–21% by weight has been measured over a temperature range of 7°–88°C. An equation has been developed to compute salinity from the measured speed of sound and the temperature for NaCl solutions. Accuracies better than 0.2% by weight have been demonstrated. The method shows good potential for in situ salinity measurements in salt-gradient solar ponds.     

Solar ponds using ammonium salts

Ammonium salts have good potential for use in salt gradient solar ponds. The environmental problems associated with NaCl are eliminated by incorporating the salt discharge from the solar pond into the fertilizer cycle of an agricultural system. Thermophysical and optical properties of several ammonium salts are sufficiently close to those of NaCl to suggest that the thermal efficiency of solar ponds using ammonium salts should be equivalent to that of a NaCl pond. Ammonia outgassing is minimal, and algae growth is curtailed by precipitating soluble phosphates. Because fertilizer is purchased for the agricultural system, the cost of salt for the solar pond is determined by the real interest for holding the fertilizer in inventory. These economics make feasible several desirable maintenance schedules for the solar pond.     

Experimental study of natural brine solar ponds in Tibet

A salinity gradient solar pond (SGSP) is a simple and effective way of capturing and storing solar energy. The Qinghai-Tibet Plateau has very good solar energy resources and very rich salt lake brine resources. It lacks energy for its mineral processes and is therefore an ideal location for the development and operation of solar ponds. In China, solar ponds have been widely applied for aquaculture, in the production of Glauber’s salt and in the production of lithium carbonate from salt lake. As part of an experimental study, a SGSP using the natural brine of Zabuye salt lake in the Tibet plateau has been constructed. The pond has an area of 2500 m² and is 1.9 m deep. The solar pond started operation in spring when the ambient temperature was very low and has operated steadily for 105 days, with the LCZ temperature varying between 20 and 40 °C. During the experimental study, the lower convective zone (LCZ) of the pond reached a maximum temperature of 39.1 °C. The results show that solar ponds can be operated successfully at the Qinghai-Tibet plateau and can be applied to the production of minerals.     

四种太阳池工质的浓度与折射率和温度的关系

选取可用作太阳池工质的ＮａＣｌ、ＣａＣｌ２、ＭｇＣｌ２和Ｎａ２ＣＯ３四种水溶液，对浓度Ｓ－折射率ｎ－温度Ｔ之间的关系进行测定，利用计算机对实验曲线进行拟合，给出相应的多项式回归方程，获得了较好的结果。它可用于分析、确定上述四种盐水溶液在太阳池内的浓度分布和变化，便于保持太阳池的稳定性。     

Comparative performance evaluation of fertiliser solar pond under simulated conditions

An experimental test rig for solar pond simulation was developed to study the chosen fertiliser salt, Muriate of Potash (MOP) for use in a solar pond under simulated conditions with provisions to vary the heating input and maintain a particular lower convective zone temperature. The performance, in terms of temperature and density profiles, was studied for MOP and was compared with that of sodium chloride and saltless solar ponds for different heating regimes and lower convective zone temperatures. The formation of three zones viz., upper convective zone, nonconvective zone, and lower convective zone was distinct at all heating combinations for both MOP and sodium chloride salts under simulated conditions. The temperature and density gradients were not affected significantly by intermittent no-heating spells of the solar ponds. Maintaining lower convective zone temperature of 70 °C and above led to the initiation of minor internal convective zone under simulated conditions. The temperature decay of lower convective zone (LCZ) was at lesser rate for different LCZ temperatures associated with both the heating regimes, for a MOP pond over a 24 h period of cessation of heating as compared to sodium chloride and saltless ponds.     

Effect of sloping walls on salt concentration profile in a solar pond

The effect of sloping walls on the salt concentration profile in solar ponds is studied. The variation of the area of the pond at different depths is expressed in terms of the top surface area and a single non-dimensional parameter defined in terms of the geometrical characteristics of the pond. This variation is then introduced into the differential equation governing the upward salt diffusion. The dependence of the molecular diffusivity of salt on temperature and the resulting vertical variation of the molecular diffusivity in solar ponds with sloping walls is also considered. The differential equation is then solved and the general solution for the salt concentration as a function of depth is obtained. Results for different pond configurations and also different top and bottom salt concentrations are presented and discussed. It is shown that as a result of sloping walls the density gradient in the top region assumes a smaller value than at the bottom of the solar ponds. If this effect is not considered in the design of solar ponds the density gradient in the top region may decrease well below the stability limit which can then result in an undesired growth of the top mixed layer.     

Introduction of a passive method for salt replenishment in the operation of solar ponds

A passive method for the replenishment of salt in solar ponds is suggested, based on the natural circulation of water caused by density differences. Water, from a selected depth in the solar pond, is passed through a salt bed in an adjacent tank, where its salinity is increased before it is returned to the bottom region of the solar pond. The difference between the densities, at the points of intake and outlet, provide the driving force for the natural circulation. Careful system design ensures that this circulation will transport enough salt to the bottom region of the pond to compensate for its upward diffusion in salt gradient solar ponds. This method negates the need for the pumping installation normally required for salt replenishment; and it provides a simple, self-regulating, and reliable method for density control in the bottom region of solar ponds.     

Examining potential benefits of combining a chimney with a salinity gradient solar pond for production of power in salt affected areas

The concept of combining a salinity gradient solar pond with a chimney to produce power in salt affected areas is examined. Firstly the causes of salinity in salt affected areas of northern Victoria, Australia are discussed. Existing salinity mitigation schemes are introduced and the integration of solar ponds with those schemes is discussed. Later it is shown how a solar pond can be combined with a chimney incorporating an air turbine for the production of power. Following the introduction of this concept the preliminary design is presented for a demonstration power plant incorporating a solar pond of area 6 hectares and depth 3 m with a 200 m tall chimney of 10 m diameter. The performance, including output power and efficiency of the proposed plant operating in northern Victoria is analysed and the results are discussed. The paper also discusses the overall advantages of using a solar pond with a chimney for production of power including the use of the large thermal mass of a solar pond as a practical and efficient method of storing collected solar energy.     

Effects of climatic conditions on the performance of carnallite solar ponds

This paper reports an experimental investigation on the effects of climatic conditions as well as the size of plastic rings on the performance and stability of two Carnallite solar ponds each of 5 m² surface area and 1 m in depth. Measurements of salinity and the temperature histories of the ponds are included. The ponds are tested under the climatic conditions of Jordan. It is found that the effectiveness of plastic rings as wind suppressors for Carnallite solar ponds is very much dependent on their size. The smaller the size the better the performance and stability of the pond. In general, it was found that the overall effect of rain in Jordan on Carnallite solar pond performance and stability is negligible.     

Prospects and scopes of solar pond: A detailed review

Solar pond is an artificially constructed pond in which significant temperature rises are caused to occur in the lower regions by preventing convection. To prevent convection, salt water is used in the pond. Those ponds are called “salt gradient solar pond”. In the last 15 years, many salt gradient solar ponds varying in size from a few hundred to a few thousand square meters of surface area have been built in a number of countries. Nowadays, mini solar ponds are also being constructed for various thermal applications. In this work, various design of solar pond, prospects to improve performance, factors affecting performance, mode of heat extraction, theoretical simulation, measurement of parameters, economic analysis and its applications are reviewed.     

Saturated solar ponds: 2. Parametric studies

The effects of following parameters on the performance of saturated solar ponds are studied: thickness of upper convective zone, nonconvective zone, and lower convective zone; starting time of the pond; water table depth below the pond; ground thermal conductivity; transmissivity of salt solution; incident radiation; ambient air temperature, humidity, and velocity; thermophysical properties of salt solution; pond bottom reflectivity; convection, evaporation, radiation, and ground heat losses; temperature and rate of heat removal; type of salt. Magnesium chloride and potassium nitrate salt ponds located at Madras (India) are considered for the parametric study. A comparison is also made with an unsaturated solar pond.     

Computer simulation of solar pond thermal behavior

A computer model of salt gradient solar pond thermal behavior has been developed and used to verify the validity of assuming constant salt solution physical parameters and long term averaging schemes for ambient temperature and insolation in previous solar pond analytical models. A theoretical limit for pond transparency is calculated which is significantly higher than that previously assumed. It is suggested that a transparent membrane be placed just below the air/water interface of solar ponds to maintain pond solution purity and approach the theoretical limit for transparency. A means of estimating the diffuse insolation input into a solar pond is given which utilizes sky color temperatures for different values of the clearness index (K_T). A single sky color temperature is calculated for each average clearness index value ( ).     

全饱和型太阳池的热稳定性条件

对太阳池的一般特征及共工作原理、双扩散系统所遵循的流体力学普遍方程组、具有非恒定的温度和盐浓度梯度的双扩散系统进行介绍，并对全饱和型太阳池的热稳定性条件进行讨论。     

Thermal characteristics and performance of salt gradient solar ponds

A mathematical model with various parameters such as effective absorptivity-transmitivity product and total heat loss factor, including ground losses and angle of refraction, which are related to the physical properties and dimensions of the pond, is developed to study the thermal behaviour of salt gradient solar ponds at different operational conditions. A linear relation is found between the efficiency of the solar pond and the function (ΔT/H ). The convective heat loss, the heat loss to the atmosphere due to evaporation through the surface of the pond and ground heat losses have been accounted for in finding out the efficiency of the pond. The dependence of the thermal performance of the solar pond on the ground heat losses is investigated and minimized using low cost loose and insulating building materials such as dry dunes and, Mica powder and loose asbestos at the bottom of the pond. The ground heat losses are considerably reduced with the asbestos (loose) and the retention power of solar thermal energy of the pond increases.     

Numerical and experimental analysis of a salt gradient solar pond performance with or without reflective covered surface

An experimental salt gradient solar pond having a surface area of 3.5 × 3.5 m² and depth of 2 m has been built. Two covers, which are collapsible, have been used for reducing the thermal energy loses from the surface of the solar pond during the night and increasing the thermal efficiency of the pond solar energy harvesting during daytime. These covers having reflective properties can be rotated between 0° and 180° by an electric motor and they can be fixed at any angle automatically. A mathematical formulation which calculates the amount of the solar energy harvested by the covers has been developed and it is adapted into a mathematical model capable of giving the temporal temperature variation at any point inside or outside the pond at any time. From these calculations, hourly air and daily soil temperature values calculated from analytical functions are used. These analytic functions are derived by using the average hourly and daily temperature values for air and soil data obtained from the local meteorological station in Isparta region. The computational modeling has been carried out for the determination of the performance of insulated and uninsulated solar ponds having different sizes with or without covers and reflectors. Reflectors increase the performance of the solar ponds by about 25%. Finally, this model has been employed for the prediction of temperature variations of an experimental salt gradient solar pond. Numerical results are in good agreement with the experiments.     

A RECTANGULAR SOLAR POND MODEL INCORPORATING EMPIRICAL FUNCTIONS FOR AIR AND SOIL TEMPERATURES

A novel theoretical model, capable of giving the temporal temperature variation at any point inside or outside a non-insulated rectangular solar pond at any time, is presented. Incorporating the finite difference approach, the model makes use of one- and two-dimensional heat balances written on discrete regions in the brine and in the soil adjacent to the pond. These simultaneous equations are solved for the local temperatures, using a computer program. Values of hourly averaged air temperature and daily averaged soil temperature for the site were used as input parameters, and empirical functions for the time-dependence of these variables were incorporated into the theoretical model. It was found necessary to use this level of detail of the meteorological data for reliable predictions on the solar ponds. The model results are compared with measured results on an actual solar pond built in Cukurova, Turkey. The modelled and experimental temperature profiles are found to be in a very good agreement. The results indicate that the thickness of the salt gradient region of a solar pond should not be less than 1.3 m. Heat losses form the pond side-walls was found not to effect the performance of solar ponds when the surface area is greater than 100 m².     

磁悬浮液体密度计及其标定

改进了一种利用磁悬浮和霍耳效应的液体密度计及其标定方法，描述了该密度计的结构，给出了分段拟合二变量实验曲线的标定方法。这种密度计适用于在太阳池中连续地测量不同深度处盐水的浓度和密度。计算表明，在太阳池中可能出现的温度和盐浓度的范围内，标定结果与实验数据精确相符。     

Gradient maintenance using discrete brine injections in a salt gradient solar pond

A method for the maintenance of the stratification in the gradient zone of a salt gradient solar pond is presented. The method is unique for solar ponds in that it involves the injection of highly turbulent colummar jets into homogeneous convective zones. This contrasts with the more common practice of traversing the gradient zone with a disk-shaped diffuser while injecting fluid at low exit Froude numbers. Using turbulent jet theory which is well understood for columnar buoyant jets, the method allows a priori determination of the resulting salinity gradient with a reasonable level of confidence. The simple injector is easily constructed and deployed. Field data collected at the Alice Springs solar pond show that the technique can quickly remove internal convective zones as well as extend the top of the gradient into the surface layer, providing a valuable tool for the operators of solar ponds.     

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.     

Transient performance of a two-dimensional salt gradient solar pond—A numerical study

Solar ponds have recently become an important source of energy that is used in many different applications. The technology of the solar pond is based on storing solar energy in salt-gradient stratified zones. Many experimental and numerical investigations concerning the optimum operational conditions and economical feasibility of solar ponds have been published in the last few decades. In the present study, a novel two-dimensional mathematical model that uses derived variables is developed and presented. This model utilizes vorticity, dilatation, density, temperature and concentration as dependent variables. The resulting governing partial differential equations are solved numerically in order to predict the transient performance of a solar pond in the two-dimensional domain. The boundary conditions are based on measured ambient and ground temperatures at Kuwait city. Based on the present formulation, a computer code has been developed to solve the problem at different operating conditions. The results are compared with the available experimental data and one-dimensional numerical results. Two-dimensionality effects are found to depend mainly on the aspect ratio of the pond. A parametric study is conducted to determine the optimum pond dimensions and operating conditions.