Salt Gradient Solar Pond Research Articles

Artificial neural network approach for evaluation of temperature and density profiles of salt gradient solar pond

AbstractThe purpose of this study is to evaluate temperature and density profiles of an experimentally investigated salt gradient solar pond (SGSP) by using artificial neural network (ANN). The input parameters of the ANN are solar pond depth, ambient temperature, radiation absorption coefficient of salty solution in the pond, initial density values of the pond and time of day. The output parameters of the ANN are temperature and density profiles in the pond. The experimental data set consists of 168 values. These divided into two groups, of which the 134 values were used for training/learning of the network and the rest of data (34 values) for testing/validation of the network performance. According to the ANN predicted results compared to the experimental results, the mean relative error (MRE) is 2·30% for temperature and 0·63% for density. The correlation coefficients (R2) between the experimentally measured and the ANN predicted results are 0·9632 for temperature and 0·9855 for density in the test/val...

Simulation and model validation of a horizontal shallow basin solar concentrator

Salt removal from drainage water is becoming increasingly important for sustainable irrigated arid land agriculture, where inadequate drainage infrastructure exists. Solar evaporation and concentration systems are currently in development in California for this purpose. The thermal behavior and evaporation rates of a horizontal shallow basin solar concentrator were modeled for design purposes and investigated experimentally in order to validate the model. Three different evaporation rate models were evaluated and compared. Measured and predicted peak brine temperatures differed by as much as 5 °C when using prescribed literature coefficients without calibration. Model prediction was improved by calibration so that peak brine temperature deviated less than 3 °C when tested against independent data sets. Minimum root mean square error was used to calibrate the mass transfer coefficient and absorptance of the collector surface for solar radiation, which are the main factors affecting the heat transfer associated with the solar concentrator. Calibrated collector surface absorptance for solar radiation declined while mass transfer coefficients were increased from reported literature values. Under calibration, the absorptance of the collector surface was adjusted from 0.8 to 0.61, and mass transfer coefficients estimated by Newell et al. [Newell, T.A., Smith, M.K., Cowie, R.G., Upper, J.M., Cler, C.L., 1994. Characteristics of a solar pond brine reconcentration system. Journal of Solar Energy Engineering 116 (2), 69–73] from 1.36 × 10 −6(1.9 + 1.065 V) to 1.70 × 10 −6(1.84 + 1.0 V) kg m −2 s −1 mm Hg −1, by Manganaro and Schwartz [Manganaro, J.L., Schwartz, J.C., 1985. Simulation of an evaporative solar salt pond. Industrial & Engineering Chemistry Process Design and Development 24, 1245–1251] from 0.0208(1 + 0.224 V) to 0.0233(1 + 0.214 V) kg m −2 h −1 mm Hg −1, and by Alagao et al. [Alagao, F.B., Akbarzadeh, A., Johnson, P.W., 1994. The design, construction, and initial operation of a closed-cycle, salt-gradient solar pond. Solar Energy 53 (4), 343–351] from 2.8 + 3.0 V to 3.0 + 3.33 V W m −2 °C −1. The calibrated models were tested using an independent data set. Maximum deviation between measured and predicted brine temperatures differed by less than 3 °C. The measured and predicted peak evaporation rates were between 1.2 and 1.4 kg m −2 h −1. The calibrated Newell model was used to predict the monthly productivity and daily maximum evaporation rates at Five Points, California for the year 2004. The productivity from April to September and from March to October was 80.7% and 94.3% of the total annual productivity, respectively.

Performance evaluation of a small-scale sodium carbonate salt gradient solar pond

In this study, thermal performance of the salt gradient solar pond (SGSP), which of density gradient is artificially with sodium carbonate solution, was tested under Karabuk prevailing weather conditions in Turkey. A small-scale prismatic glass tank was constructed with an area of 0.45 × 0.20 m2 and a depth of 0.25 m as solar pond. A series of experiments with four different density levels were conducted in July–August 2004. The variations of the temperature and density profiles were observed for each of experiment for a week. It was found that the maximum temperature difference between the bottom and surface of the pond is 21°C and maximum temperature in the lower convective zone (LCZ) has been measured as 49°C at the first experiment. The efficiency of the pond was evaluated 13.33% weekly mean radiation intensity of 524 W m−2 for the first experiment. Copyright © 2006 John Wiley & Sons, Ltd.

Transient heat and mass transfer and long-term stability of a salt-gradient solar pond

In this work, the problem of transient heat and mass transfer and long-term stability of a SGSP has been numerically investigated using a 2D-transient-variable properties model and a finite-control-volume numerical method. The pond, which was assumed initially stabilized with linear temperature and salinity profiles, has been subject to real weather conditions. The numerical model has been satisfactorily validated against measured temperature data. Numerical results have clearly shown that the solar heating effect appears considerably more pronounced during the hot seasons (spring and summer) than during the cold ones (winter and autumn). The existence of two critical zones, one beneath the water surface and the other one located near the pond bottom, has clearly been established at a very early time of operation. It has been found that such critical zones have progressively become more vulnerable in time. Also, the solar heating effect, the heat losses through the free surface as well as the water transparency have an important influence on the pond stability characteristics and its temporal evolution. The presence of a heat extraction with its cooling effect tends to stabilize the pond. Such a beneficial effect, which is mainly observed in the bottom region of the pond, has been found to be more pronounced during the summer than during the winter time. Results have also shown that the pond with good transparency water would likely be more susceptible to develop instabilities than the one with poorer transparency water. Such an effect appears to be more important inside the lower critical zone.

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 (Na 2CO 3) 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.

05/00246 Maintenance strategy for a salt gradient solar pond coupled with an evaporation pond: Agha, K. R. et al. Solar Energy, 2004, 77, (1), 95–104

Process simulation has become an accepted tool for the performance, design, and optimization of thermal processes. Solving the mathematical models representing solar heating process units and systems is one of the most tedious and repetitive problems. Nested iterative procedures are usually needed to solve these models. To tackle these problems, several researchers have developed different methods, techniques, and computer programs for the simulation of very wide verity of solar heating process units and systems.It is of interest in this work to characterize and classify these methods, techniques, and programs in order to better understand their relations, types, structures, and procedures.The simulation problems are outlined; the simulation programs are grouped into two main types; special purpose, and general-purpose programs. Sequential and simultaneous computational sequences are illustrated. Simulator structure, program evaluation, and numerical techniques are summarized.By considering the unit and/or system entropy generation as well as the energy and material balances equations, more realistic models can be obtained. Also, rapid development of computer hardware and software will suggest new techniques and programs to be considered. These progress directions are noted.

NUMERICAL SIMULATION OF SALT GRADIENT SOLAR PONDS

The salt-gradient solar ponds are considered in this work in order to study its performance numerically. Governing equations of the upper convective zone, nonconvective zone, and lower convective zone of a salt gradient solar pond are deduced as a set of non-linear partial differential equations. The equations are solved numerically to predict the thermal performance of the solar pond over a long time. The meteorological data of Egypt such as incident solar radiation, ambient temperature, air velocity, and relative humidity are taken into considerations in the model. Heat transfer modes considered between the upper convective zone and the ambient are convection, evaporation, and radiation. The present model is used to study the development of temperature inside the three zones of salt gradient solar pond. The optimum thickness of each layer is obtained with close agreement of previous results.

Numerical study of transient heat and mass transfer and stability in a salt-gradient solar pond

In this work, the problem of the transient heat and mass transfer and stability within a salt-gradient solar pond has been numerically investigated by mean of a 3D-model with all properties variable as function of temperature and salt concentration. The pond has been subject to real external perturbations resulting from the variations of the local weather conditions. The initial conditions correspond to an ‘artificially stabilized’ pond with the linear temperature and salinity profiles in the gradient zone. Numerical results as obtained for a Tunisian experimental test-site have been satisfactorily compared and validated against measured temperature data. They have clearly shown that after a relatively short period of operation, say only a few days, the changes in the weather conditions have produced important effects not only on the internal temperature field of the pond, but also on its stability as well. In general, one may expect that some instability could develop and growth from within two critical zones: the first one beneath the water free surface and the second one in the region immediate to the bottom surface. The transient behaviours of the pond, in particular its stability, have also been investigated for a very short period of time, say 24 hours of operation. It has been clearly observed that the overnight cooling at the water free surface may rend unstable the immediate region beneath that surface. On the other hand, the solar heating effect during the daytime may have adverse effect on the stability near the bottom surface. Finally, results have also shown that the water transparency has an important effect on the pond stability itself. A pond with good transparency water has been found to be more susceptible to instabilities than a poor transparency one.

04/01368 Combined effect of bottom reflectivity and water turbidity on steady state thermal efficiency of salt gradient solar pond: Husain, M. et al. Energy Conversion and Management, 2004, 45, (1), 73–81

04/01368 Combined effect of bottom reflectivity and water turbidity on steady state thermal efficiency of salt gradient solar pond: Husain, M. et al. Energy Conversion and Management, 2004, 45, (1), 73–81

223 表画集熱式ソーラーポンドを用いた温度差発電と温室暖房の複合システム解析(OS-5 環境負荷ミニマイズのための熱流体・環境工学の展開(3))

Performance of complex system included power generation by temperature difference and greenhouse heating using floating collector type solar pond with two-layer was investigated for simulation models. It was also compared with salt-gradient solar pond. In thermal collector part, thermal storage layer and soil, some equations of heat transfer were drawn to establish optimum conditions for design, respectively. They were given by analysis of complex heat transfer using simulation models. They were concluded as being the following: (1) Flow rate of the water circulation on the collectors was 50(kg/(m^2・h)). (2) Minimum incident solar radiation for starting of pumping was 500(W/m^2). (3) Depth of thermal storage layer is 3〜5(m). (4) Thickness of the insulation was more than 10(cm), (5) Surface area of pond is more than 100(m^2). Although the maximum water temperature in thermal storage layer was lower than salt-gradient solar pond, annual temperature hardly changed. Consequently, this system has advantage of use during winter was shown for this system. In addition, it was clearly presented that it is possible to use also for power generation and greenhouse heating.

Maintenance strategy for a salt gradient solar pond coupled with an evaporation pond

In a previous study, the authors presented a simple mathematical model for predicting the ratio of the evaporation pond area to that of a salt gradient solar pond area. The evaporation pond idea provides a very attractive method of salt recycling by evaporation, especially in areas of high evaporation and low rates of rain as it is the case for North Africa. In this paper, the model was elaborated upon and applied to two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli, Libya (latitude=32.86°N). All the results presented were predicted for the first three years of operation. The daily variations of brine concentration in the of Tajoura's Experimental Solar pond and those based on different designs were predicted and discussed under different scenarios. The quantities of brine provided by the evaporation pond and that required by were predicted for both cases of surface water flushing (fresh water and seawater) under the different design conditions. It was predicted that the can provide 20–40% during the first year and 45–95% during the third year depending on the design selected.

Helio-hydro and helio-thermal production of hydrogen

Fossil fuels account for about 80% of the world annual energy demands. Renewables contribute 14% and nuclear some 6%. These numbers will soon change as the world's population grows, energy demand rises, cheap oil and gas deplete, global warming effects continue rising and city pollution worsens the living conditions. The development of energy sources and devices will emerge more aggressively to address the world's energy and environmental situation. A concept of using hydrogen as an energy carrier or storage as a fuel, a replacement of burning fluid fossil fuels is presented. Sources of energy from which hydrogen can be produced in a massive quantity and at a low cost are briefly surveyed. A short account of devices to be employed for hydrogen production is given. Primarily the sun, sea, runoff waters, winds and fissionable materials are to be utilized. The discussion on the inexhaustibility of naturally occurring sources utilized and/or harnessed in this process will lead to the low cost for hydrogen production. Some hydrogen rich products including hydrogen sulfide and methane accompany the oil, gas and brine, when they are pumped out of the ground. While methane is used sometimes as fuel; the hydrogen sulfide is disposed off invariably. In principle, hydrogen can be extracted from these waste products. We discuss here to produce hydrogen in economically feasible manner. The use of brine as a means of usable solar energy in the form of heat and electricity was discussed earlier. Here, we aim at discussing the production of hydrogen from the brine and hydrogen sulfide gas. The brine is proposed to be utilized for two purposes: one for salt gradient solar pond to produce usable heat and electricity, and the other as an electrolyte to produce hydrogen out of itself. The hydrogen from hydrogen sulfide can chemically be extracted.

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 ( Rρ≻2 throughout the gradient and Rρ≅10 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.

Combined effect of bottom reflectivity and water turbidity on steady state thermal efficiency of salt gradient solar pond

In salt gradient solar ponds, the clarity of water and absorptivity of the bottom are important concerns. However, both are practically difficult to maintain beyond a certain limit. The reflectivity of the bottom causes the loss of a fraction of the incident radiation flux, resulting in lower absorption of flux in the pond. Turbidity hinders the propagation of radiation. Thereby it decreases the flux reaching the storage zone. Both these factors lower the efficiency of the pond significantly. However, the same turbidity also prevents the loss of radiation reflected from the bottom. Hence, the combined effect is compensatory to some extent. The present work is an analysis of the combined effect of the bottom’s reflectivity and water turbidity on the steady state efficiency of solar ponds. It is found that in the case of a reflective bottom, turbidity, within certain limits, improves the efficiency of pond. This is apparently contradictory to the conventional beliefs about the pond. Nevertheless, this conclusion is of practical importance for design and maintenance of solar ponds.

Optimum size of non-convective zone for improved thermal performance of salt gradient solar pond

The salt gradient solar pond is a long-term heat storage system with a considerable warm-up time. A pond is efficient when it reaches the desired temperature quickly and maximum heat is subsequently retrieved at steady state. This requires optimum sizing of the non-convective zone. In the present work, the optimum size of the non-convective zone for fast warm-up is determined. This is found to differ considerably from the optimum size of the steady state criterion. The possibility of achieving both performance parameters, i.e. fast warm-up and maximum heat collection later on, is analyzed. It is suggested that when commissioning a pond, the size of the non-convective zone should at first be the optimum value from the warm-up rate criterion, but may later be changed to the optimum size from the steady state criterion.

Design methodology for a salt gradient solar pond coupled with an evaporation pond

This paper presents the results of a simple mathematical model for predicting the ratio of the evaporation pond (EP) area to that of a Salt Gradient Solar Pond (SGSP) area. The EP idea provides a very attractive method of salt recycling by evaporation, especially in areas of high rates of evaporation and low rates of rain as it is the case for North Africa. The model is applied for two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli-Libya (Lat.=32.68°N) and for measured evaporation rates. Under the summer conditions and for the case of surface flushing by fresh water, the area ratio was estimated at about 0.17. While for the case of using seawater this ratio increases enormously to about 14.4. The time required for the salt concentration to increase from seawater concentration to a high concentrated brine, which can be injected at the bottom of the solar pond, is also presented. It was estimated that the time required to increase the salt concentration from 3.5 to 35% is about 120 to 250 days during the summer months and about 200 to 220 days during the winter months.

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 (Δ x) and time difference (Δ t). It is observed that the mesh ratio parameter ( r = K Δt / ρ C p Δx 2 ) which is used to define the stability and convergence of the method does not have an absolute value, rather its value varies with Δ x. While using an actual set of Δ x and Δ t, 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.

Experimental study about erosion in salt gradient solar pond

An experimental study was executed using a small model pond to examine the erosion phenomenon on the gradient zone of a solar pond. By means of observing the flow pattern of the heat reserve zone, it was made clear that the erosion was caused by naturally occurring convection, which is based on the vertical temperature difference in the heat reserve zone. In other words, the ruling factor in erosion is the vertical temperature difference in the heat reserve zone. With regards to a real solar pond, a policy was proposed which suppresses erosion velocity by controlling the pond's operating conditions while keeping the temperature gradient of the non-convection zone (NCZ) as large as possible and making the vertical temperature difference in the heat reserve zone small. On analysis, the rational modification was effected by the Grashof number Gr, which is a dimensionless number and shows the influence of the flotage on the speed field and the temperature field. As a result, it was found that the modified dimensionless Grashof number explains the erosion velocity of the gradient zone well. Moreover, the function of the correlation between the erosion velocity and the modified Grashof number was obtained by regression calculation and could estimate the erosion condition of the gradient zone quantitatively.

Natural brine solar pond: an experimental study

The South East of Tunisia is a sunny region. This area contains many mineral reserves of natural brine, which currently are not well exploited. These reserves are estimated to several millions of cubic metres. The abundance of solar energy and salt has led us to test the collection and storage of solar energy by a salt gradient solar pond. A small laboratory solar pond of dimensions 2 m×2 m×1 m, excavated from the ground, has been constructed at ENIT (National School of Engineers of Tunis). The salt utilised in the pond is a natural brine which comes from south of Tunisia. Temperature and solar radiation measurements have been carried out over 8 weeks to evaluate the efficiency of this pond.

THE THERMAL CHARACTERISTICS AND ECONOMIC ANALYSIS OF A SOLAR POND COUPLED LOW TEMPERATURE MULTI STAGE DESALINATION PLANT PART I: THERMAL CHARACTERISTICS

Salt Gradient Solar Ponds (SGSP) have the potential of providing low grade energy with the advantage of an annual thermal energy storage cycle. The development of Multi-Stage Flash (MSF) distillation plants operating below 100°C allows SGSP to be considered as the heat source for these systems. In this paper, two schemes of matching the SGSP with the MSF distillation plant are presented. The first scheme is based on the assumption that the solar pond is to be used as the sole heat source for the distillation plant (i.e. all the plant's thermal energy requirements are provided by the solar pond). The second scheme considers a hybrid system (solar + fuel), where a 20,000 m2 solar pond is linked to an otherwise stand alone, fuel driven desalination plant. Both options are simulated with the same daily product water output of 1000m3/day. The thermal simulation of the MSF desalination process was predicted by using a mathematical model based on stage by stage calculations taking into account the variations in fluid properties and flow conditions. The generated simultaneous equations of the mass and energy balances were combined and arranged in a matrix form and then translated into algorithm to predict process variables such as temperature and flash evaporation rates.