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.     

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.     

A one-dimensional numerical study of the salt diffusion in a salinity-gradient solar pond

A one-dimensional transient mathematical model is used for the study of the salt diffusion and stability of the density gradient in a solar pond. A finite difference method with a diffusion coefficient dependent on both temperature and salt concentration is used to solve the salt diffusion equation. On the basis of simple considerations we analyze the influence of the salinity-gradient thickness on the useful energy which can be withdrawn from the bottom layer of the solar pond. Finally some considerations on the effect of the velocity of injected brine in rising solar ponds are presented, making use of the Rayleigh analysis of the small perturbations in order to study the stability of the system.     

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.     

Optical analysis of the behaviour of a salt-gradient solar pond

The three-zone salt-gradient solar pond is a body of saline water that collects solar radiation and stores it in the water as thermal energy. The performance of solar ponds largely depends on the portion of solar radiation which reaches the bottom region (LCZ) and from which heat is extracted subsequently. An analysis is made to determine the form of the attenuation of the solar rays inside the pond as a function of wavelength and depth, taking into consideration that each zone has its extinction coefficient due to its salt concentration. Insertion of partitions between zones (between the UCZ and NCZ and between the NCZ and LCZ) has also been discussed. Equations describing the transmissions and reflection coefficients in the presence of partitions were derived. The portion of the solar energy that is absorbed by the different depths of NCZ has been calculated for Cairo. About 20% of the incident radiation is absorbed by the NCZ, and with the presence of transparent partitions this quantity decreases by about 20%.     

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.     

Development of a simple instrument for determination of salt concentration profile in the operation of solar ponds

The development and construction details of an instrument to measure densities of the brine at different depths in a solar pond are presented and discussed. Using a vertically moving sampling tube, brine is drawn from the pond and then is fed to the bottom of a conical container in which a sealed conical glass vessel is fully submerged. The bouyancy force exerted by the water on the sealed glass vessel is measured by a system of cantilever beam and strain gauges. The output of the strain gauge bridge is proportional to the density of the fluid passing through the conical container. It is shown that this instrument can be effectively used to determine the salt concentration profile in salt gradient solar ponds.     

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.     

浊度和池底反射率对太阳池热性能的影响

提出了同时考虑浊度和池底漫反射的太阳池辐射透射模型和热效率模型,计算分析了浊度和池底反射 率对太阳池热性能的综合影响。模拟计算表明,浊度一定时,热性能总是随着池底反射率的增大而降低。计算 还表明,存在一个临界池底反射率,当池底反射率小于临界值时,浊度的增大导致太阳池热性能的下降;当池 底反射率超过临界值时,一定范围内浊度的增加反而有利于太阳池热性能的提高;当池底反射率在临界值附近 时,浊度的变化对太阳池热性能的影响很小。通过模拟计算得出了典型情况下的临界池底反射率,在太阳池的 实际研究和设计中可供参考。     

Salt concentration profiles for marginal stability in solar ponds

In salt-stabilized solar ponds, the establishment and maintenance of salt gradients forms one of the significant factors controlling the economics of energy conversion. Hence, considerable attention is directed towards minimizing the expenditure. Recently, Elwell's analysis of the marginal stability criterion has highlighted the interesting possibility of stabilizing solar ponds with much smaller concentrations than the conventionally adopted saturation concentration at the pond bottom. A general treatment for the use of the stability criterion for evaluating concentration profiles has been given. The stabilities of different layers of pond fluids with marginal stability conditions have been discussed.     

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.     

An investigation of a surface drift current return pipe system for application on salt gradient solar ponds

Based on the premise that wind-induced mixing could be the factor that limits the size of salt gradient solar ponds, this paper explores the notion of returning surface drift currents in pipes from the downwind end to the upwind end of the pond. An unstratified constant density pond subjected to a steady wind is assumed. A mathematical model is developed to predict the effect of such a pipe return system on drift and circulation currents. In an experimental study conducted in Utah State University's wind tunnel, set-up was measured on a shallow unstratified body of water to exposed wind at various speeds. The tests confirm that set-up may persist even when waves are suppressed. Circulation currents could be virtually eliminated with the return pipe system, but surface drift currents are increased. Larger scale tests on stratified ponds are needed.     

Modeling the performance of a solar pond as a source of thermal energy

The use of solar ponds is becoming more attractive in today's energy scene. A major advantage of solar ponds over other collectors is the ability to store thermal energy for long periods of time. The solar pond comprises a hydraulic system subject to processes of heat and mass transfer. The design of this system and the related equipment requires a thorough knowledge of the pond heating-up process and expected thermohaline structure within the pond. The current study considers that convection currents in the pond are inhibited by the salinity distribution, and applies a finite difference implicit model in order to investigate the interaction among physical variables represented by various dimensionless parameters. Variables which are included in the analysis comprise the solar radiation input and absorption as it passes through the pond; diffusion and dispersion of heat within the pond; absorption of heat at the bottom of the pond; and withdrawal of heat from layers within the pond. The physical variables generate 3 dimensionless variables associated with the pond's heating-up process. A 4 dimensionless variable is associated with the heat utilization. The analysis represented in this paper concerns the interaction between these dimensionless parameters and its implications.     

An investigation of rain and wind effects on thermal stability of large-area saltpan solar ponds

An investigation of the thermal stability of large area saltpan solar ponds under different climatic conditions is presented. The study focuses on time taken by the pond to reach its stable conditions with heavy rainfall and the effect of wind-mixing process for the stability of the pond. Investigations were carried out over a period of 60 days on a large-area solar pond of 90 cm deep. The temperature and density profiles obtained 34 days after filling showed that the pond had attained its stability with a bottom temperature of 63 °C. Results reveal that heavy rainfall is the prime cause for the pond to reach stability in a time period of about 30 days. Strong wind-induced mixing prevailed during the second half of the investigation, which contributed to the erosion of the nonconvecting zone is the cause for observed destabilization of the pond. The estimated critical wind speed for complete destruction of the nonconvecting zone is about 25 km/h.     

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.     

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.     

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.     

A study on the transient behaviour of solar ponds

In this paper, the transient behaviour of solar ponds has been investigated using a finite difference formulation. The performance of solar ponds can be successfully analysed and the effects of various parameters studied. The thickness of the layer with density gradient has a profound effect on the performance of a solar pond and an increase of the thickness of this layer from 1 to 2m increases substantially the operating temperature for the same overall efficiency.

The effect of the pattern of heat extraction is discussed. Heat extraction at a constant rate will result in large temperature fluctuations from summer to winter. But, if the heat is extracted at a varying rate proportional to the monthly average solar radiation, then the temperature fluctuation is considerably decreased as compared to the case of constant heat removal for the same yearly efficiency. It is possible to operate a solar pond in the Melbourne area with a yearly efficiency of 15% having a high temperature of 67 °C in summer and a low of 40 °C in winter.

The performances of solar ponds in Alice Springs and Darwin have been studied. It is found that the maximum bottom temperature does not go beyond 80 °C for a pond efficiency of 20%. Since these two cities have the most favourable locations for solar ponds, the indicated maximum temperature casts doubt on the prevalent view that a solar pond can operate at a temperature above 90 °C with an efficiency of 20%.     

Experimental study of the salt gradient solar pond stability

Many natural systems such as oceans, lakes, etc.…, are influenced by the effect of double-diffusive convection. This phenomenon, which is a combination of heat and mass transfer, can destroy the stability of system-flows.In the case of solar ponds the middle layer, that is linearly stratified, acts as a thermal and mass insulator for the lower layer. This middle layer, called the Non-Convective Zone (NCZ), needs special care to avoid convection and to maintain its stability. In fact, due to an excess of heat stored, a thermal gradient occurs within the NCZ. A convective movement appears at the bottom of the stratified-layers and then grows to a double-diffusive convection movement. This movement transforms the stratified-layers into a well mixed layer, reducing the storage capacity of the pond.Laboratory small-scale pond and middle-scale outdoor solar ponds were designed and built to provide both quantitative data and to study the dynamic processes in solar ponds, including the behavior of the gradient zone.Particle Image Velocimetry (PIV) visualization-experiments carried out in the mechanical and energetic laboratory in the engineering school of Tunisia and experiments in the field showed that the instability of solar ponds could be limited by using porous media placed in the lower layer of the stratification.