Experimental and numerical analysis of sodium-carbonate salt gradient solar-pond performance under simulated solar-radiation

The objective of this study is to investigate experimentally and theoretically whether sodium carbonate (Na₂CO₃) salt is suitable for establishing a salinity gradient in a salt-gradient solar-pond (SGSP). For this purpose, a small-scale prismatic solar-pond was constructed. Experiments were conducted in the laboratory under the incident radiation from two halogen-lamps acting as a solar simulator. Furthermore, a one-dimensional transient mathematical model that describes the heat and mass transfer behaviour of the SGSP was developed. The differential equations obtained were solved numerically using a finite-difference method. It was found from the experiments that the density gradient, achieved using sodium carbonate salt, can suppress convection from the bottom to the surface of the pond.     

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.     

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.     

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.     

Modeling and testing a salt gradient solar pond in northeast Ohio

A dynamic computer model of a salt gradient solar pond as an annual-cycle solar energy collection and storage system was developed. The model was validated using experimental results of a solar pond located at the Ohio Agricultural Research and Development Center (OARDC), Wooster, Ohio. The model was then used to analyze the transient energy phenomena which occurred within the storage zone of the pond. Generalized daily weather functions used were the incident solar radiation upon a horizontal surface, the daylight length and the daily maximum and minimum ambient air temperatures.Various simulations were performed to evaluate the OARDC solar pond and to improve its overall effective capacity of heat storage. It was found that 4–6 weeks variation in start-up time and 5–10°C variation in start-up temperature had no effect on late summer peak storage temperature. The pond operated at a 20 per cent collection efficiency with a 1.5-m deep gradient. Insulating the pond in the winter would be beneficial if no heat was removed during the fall. Reducing the gradient zone thickness to 1 m and enlarging the storage zone could improve the performance of a 3-m deep pond. The model could be used to predict and analyze the transient thermal response of large storages associated with solar heating system for a variety of purposes and climatic conditions.     

Computer simulation of salt gradient solar pond’s thermal behaviour

This paper investigates a few mathematical aspects of computer simulation of salt gradient solar pond’s thermal behavior. The basic equation governing heat flow in the non-convective zone of solar pond is solved by finite difference approach using the Crank–Nicholsen method. Stability and convergence of the method, specifically for the case of solar pond, is examined over a wide range of depth difference (Δx) and time difference (Δt). It is observed that the mesh ratio parameter ( ) which is used to define the stability and convergence of the method does not have an absolute value, rather its value varies with Δx. While using an actual set of Δx and Δt, the stability must be tested with reference to the set being used. Few other mathematical aspects pertaining to the actual application of the method are also investigated. Also, the effect of fineness of ambient input data on long term performance of the pond is investigated. It is observed that the diurnal variation of ambient input data yields the same accuracy as the hourly variation. Different approaches of calculating the heat losses from upper convective zone are compared for long term performance of the pond. A simple method is suggested to calculate the radiation flux at a depth which results due to multiple reflections between bottom and surface of the pond. The method saves computational time when used for simulation and is also suitable for hand calculations.     

The philosophy of design and construction of the Kuwait 1700 m2 solar pond

This paper discusses the design and construction philosophy of the Kuwait solar-pond/multistage-flash system. The present work came about to study the performance of a solar pond as a main source for energy collection and storage and to use the collected heat in producing fresh water, which is difficult to obtain in remote areas. The pond is built with 1700 m² of surface area and 45° sloping sides. Taking the natural surroundings and the nature of the climate into account, the pond is estimated to perform at 18% efficiency. The collected energy, which is estimated at 1800 kWh/day, will be used to produce 25 m³ of fresh water daily.     

太阳池的研究与应用

自从1902年Kalecsinsky首次发现了天然太阳池现象以后，经过长期的研究和发展，太阳池技术已被广泛应用于发电、取暖、海水淡化．矿物加工等领域，太阳池成为近期内进行大规模太阳能热利用的最有前景的低温热源装置。主要综述了太阳池的集热原理及建造方法、太阳池中热量的贮存及提取方式、太阳池的应用以及研究动向等，并指出目前我国太阳池技术还处于实验研究的阶段，而我国具有丰富的太阳能和盐资源，大力开发太阳池技术将为发展地方经济起到重要的作用。     

Solar ponds for space heating

Solar ponds are shallow bodies of water in which an artificially maintained salt concentration gradient prevents convection. They combine heat collection with long-term storage and can provide sufficient heat for the entire year. We consider the absorption of radiation as it passes through the water, and we derive equations for the resulting temperature range of the pond during year round operation, taking into account the heat that can be stored in the ground underneath the pond. Assuming a heating demand of 25000 Btu/degree day (Fahrenheit), characteristic of a 2000 ft² house with fair insulation, and using records of the U.S. Weather Bureau, we carry out detailed calculations for several different locations and climates. We find that solar ponds can supply adequate heating, even in regions near the arctic circle. In midlatitudes the pond should be, roughly speaking, comparable in surface area and volume to the space it is to heat. Under some circumstances, the most economical system will employ a heat pump in conjunction with the solar pond. Cost estimates based on present technology and construction methods indicate that solar ponds may be competitive with conventional heating.     

Feasibility of solar pond heating for northern cold climates

A computational performance and feasibility study of the salt-gradient solar pond for residential heating in high-latitude and cold climates has been made using computer simulations with meteorological data characteristic to southern Finland (60°N). Freezing, snow coverage and surface insulation for wintertime are also considered in the computations. For an individual house with 100 m² living area, a solar pond of 3.5 m depth and 400 m² area would provide a solar fraction of 50 per cent without surface insulation and 75 per cent with surface insulation. A large pond for district heating would require about one third of the pond area per house as compared with an individual house. A heat pump would still reduce the pond size requirement.     

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.     

A wind-mixed layer model for solar ponds

A computer model is described which may be used for predicting transient salinity and temperature profiles in a salt gradient solar pond. It is intended for use in modeling large surface area ponds where wind-mixing would be expected to play an important part in the dynamics of the upper layer. The formulation predicts the depth of the upper convecting zone using a mixed-layer model which incorporates the wind-mixing algorithm described by Bloss and Harleman [1,2]. This is in contrast to earlier solar pond models which have generally assumed a constant value for this layer depth. Results have been obtained for a number of 1-yr simulations of a large hypothetical pond in Richmond, Virginia, and these have been used in testing the sensitivity of the model to several of the input parameters, including the radiation term and the form of the wind-mixing algorithm. The model output is also compared with field data from an operating solar pond and good agreement is found. Results have indicated that some measures will have to be taken to counteract the mixing action due to wind stress, if the upper mixed layer depth is to be maintained at an acceptable level. The model is expected to be useful in large-scale pond design.     

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.     

A comparison of gradient zone pumping and rising pond maintenance techniques for gradient zone stabilization

Two gradient zone maintenance methods for solar ponds are examined relative to stability constraints in the internal gradient zone region and at the convection interface region. The results show that both “rising pond” and gradient zone pumping methods allow similar overall gradient zone thicknesses to be maintained. Gradient zone pumping allows one to adjust the upper gradient zone strength in a more controlled manner. The rising pond method tends to maintain stronger stability conditions throughout the interior region of the gradient zone. Salt transport rates across the gradient zone region are also similar for both gradient zone maintenance methods.     

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.     

Theoretical study of solar ponds for space heating in diverse climatic conditions of Turkey

The thermal performance of a solar pond operating under steady-state conditions is analysed theoretically for use in space heating in four diverse Turkish locations having widely different climates. The average pond temperature for each month is obtained for maximum heat extraction, with the thickness of the pond-insulating layers as a fixed parameter. For a fixed pond-insulating layer, the useful heat withdrawn during the month is calculated for these four locations. Space-heating loads and the solar heating fractions are estimated as functions of pond area for four different types of buildings. The necessary pond area is calculated for supplying the heat requirements for these prototype buildings. Finally, the economic feasibility of constructing solar-pond systems in Turkey is discussed.     

A transient model of a laboratory-scale carnalite salt gradient solar pond

The thermal performance of a laboratory-scale salt gradient solar pond has been modeled as a one-dimensional unsteady conduction heat transfer problem with heat generation. The pond is assumed to be cut into horizontal slices and finite difference heat balance equations are solved simultaneously to predict the temperature of each slice at any time. The initial conditions were the temperature profile data. The boundary conditions were determined by studying the heat balance at the bottom of the pond and by assuming the pond surface temperature to be equal to the ambient temperature. Solar radiation attenuation is calculated by the Bryant and Colbeck formula. A computer program is constructed to perform the calculations. In addition, Kooi's model was compared with our model. Similarly the salinity behavior was studied by writing the one-dimensional differential mass balance equation over a small slice with the appropriate boundary and initial conditions. The resultant set of linear equations was solved simultaneously for the unknown new concentrations. A computer program has been constructed to perform the calculations. Fair agreement between experimental and predicted profiles was obtained.     

Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond’s temperature

Absorption thermal systems are attractive for using waste heat energy from industrial processes and renewable energy such as geothermal energy, solar energy, etc. The Absorption Heat Transformer (AHT) is a promising system for recovering low-level waste heat. The thermal processes in the absorption system release a large amount of heat to the environment. This heat is evolved considerably at temperature, the ambient temperature results in a major irreversible loss in the absorption system components. Exergy analysis emphasises that both losses and irreversibility have an impact on system performance. Therefore, evaluating of the AHT in exergy basis is a much more suitable approach. In this study, a mathematical model of AHTs operating with the aqua/ammonia was developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. A heat source at temperatures not higher than 100 °C was used to simulate the heat input to an AHT from a solar pond. In this paper, exergy analysis of the AHT were performed and effects of exergy losses of the system components on performance of the AHT used to increase solar pond’s temperature were investigated. The maximum upgrading of solar pond’s temperature by the AHT, is obtained at 51.5 °C and gross temperature lift at 93.5 °C with coefficients of performance of about 0.4. The maximum temperature of the useful heat produced by the AHT was ˜150 °C. As a result, determining of exergy losses for the system components show that the absorber and the generator need to be improved thermally. If the exergy losses are reduced, use of the AHT to increase the temperature of the heat used from solar ponds will be more feasable.     

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.