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.     

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.     

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.     

Salt gradient stabilized solar pond collector

This paper presents a theoretical analysis of a salt gradient solar pond as a steady state flat plate solar energy collector. We explicitly take into account the convective heat and mass flux through the pond surface and evaluate the temperature and heat fluxes at various levels in the pond by solving the Fourier heat conduction equation with internal heat generation resulting from the absorption of solar radiation as it passes through the pond water. These evaluations, in combination with energy balance considerations, enable the derivation of the expressions for solar pond efficiency of heat collection as well as the efficiency of heat removal. The efficiency expressions are Hottel-Whillier-Bliss type, prevalent for flat plate collectors. Numerical computations are made to investigate the optimization of geometrical and operational parameters of the solar pond. For given atmospheric air temperature, solar insolation and heat collection temperature, there is an optimum thickness of nonconvective zone for which the heat collection efficiency is maximum. The heat removal factor is also similar to that of a flat plate collector and the maximum efficiency of heat removal depends on both the flow rate and the temperature in the nonconvective zone.     

Linear stability analysis of double-diffusive solar pond

In this communication, the stability of the double-diffusive solar ponds has been investigated in the linear approximation. The corresponding linearized system of equations of motion is reduced to a single integro-differential equation using the Green-function technique. In contrast to the conclusions of Veronis that, initially, the instability occurs as an oscillatory mode and at a value of R_T (Rayleigh number for temperature) greater than R_S the motion becomes steady, the present analysis shows that, initially, as R_T increases from zero but remains considerably less than R_S, exponentially growing and decaying modes (steady motion) occur first; for a value of R_T more than a critical value R_T^c, the motion becomes oscillatory. This oscillatory motion may, due to the basic non-linear dynamics of the system, even become aperiodic. Further, for R_S → ∞, the minimum value of R_T for which steady motions can occur tends to $\text{K}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{·R}{}_{\mathrm{S}}$, where $\text{K}\text{= K}{}_{\mathrm{S}}\text{/K}{}_{\mathrm{T}}$ where K_S and K_T are diffusivity coefficients for salt and temperature, respectively; as a contrast, according to Veronis, $\text{R}{}_{\mathrm{T}}\text{a}\text{?}\text{σ}{}^{\mathrm{?1}}\text{R}{}_{\mathrm{S}}\text{; σ = v/K}{}_{\mathrm{T}}\text{, v}$ being the kinematic viscosity.     

Mathematical modelling of salt gradient solar pond performance

This paper presents a mathematical model of the performance of the salt gradient solar pond. A lumped parameter model of the upper convective zone, non-convective zone and lower convective zone is used. This model enables the temperatures of the upper-convective zone and the lower convective zone of the solar pond to be predicted. The experimental results agree well with theoretically predicted values. The major error in the theoretical results is due to the difference between the theoretical value of the solar radiation inside the water and that observed experimentally. It is found that the experimental value of the solar radiation at a depth of 90 cm is approximately 26 per cent of the total solar radiation falling on the solar pond surface, whereas the corresponding theoretical value is found to be 33 per cent. The results conclude that the lumped parameter model can be used as a simple model to predict the performance of the solar pond.     

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.     

A computational study of salt diffusion and heat extraction in solar pond plants

The problem of the development of salt concentration profiles in a solar pond is investigated, the thermodiffusion contribution is also taken into account, using a one-dimensional mathematical model and a finite-difference approach. A novel scheme of heat extraction from the solar pond is presented, along with preliminary two-dimensional computational fluid dynamics (CFD) simulations.     

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.     

盐梯度太阳池Kalina循环发电系统性能研究

Kalina循环发电系统是一种典型的低温热源发电系统,具有广阔的应用前景。盐梯度太阳池能够实现连续聚热和跨季节蓄热,可广泛应用于光热发电系统和光热供热系统。文章提出了一种以太阳池储热量为热源的盐梯度太阳池Kalina循环发电系统,并利用Aspen Hysys软件对该系统进行建模。而后根据模拟结果,研究了提热温度、运行压力和氨水浓度对该系统各项性能的影响。此外,还分析了典型工况下,该系统的热力性能。分析结果表明:随着提热温度逐渐升高,盐梯度太阳池Kalina循环发电系统的发电功率、热效率和效率均逐渐增加;随着运行压力逐渐升高,该系统的热效率和效率逐渐升高,并且存在最佳的运行压力1.75 MPa,使得该系统获得最大发电功率;随着氨水浓度逐渐增大,该系统的发电功率也会逐渐增大,但热效率和效率却逐渐降低;当氨水浓度为85%、运行压力为1.75 MPa、提热温度为90℃时,该系统的热效率和效率分别为7.93%,57.59%。     

Performance investigation of a salt gradient solar pond coupled with desalination facility near the Dead Sea

Solar ponds provide the most convenient and least expensive option for heat storage for daily and seasonal cycles. This is particularly important for a desalination facility, if steady and constant water production is required. If, in addition to high storage capacity, other favorable conditions exist, the salt gradient solar ponds (SGSPs) are expected to be able to carry the entire load of a large-scale flash desalination plants without dependence upon supplementary sources. This paper presents a performance investigation of a SGSP coupled with desalination plant under Jordanian climatic conditions. This is particularly convenient in the Dead Sea region characterized by high solar radiation intensities, high ambient temperature most of the year, and by the availability of high concentration brine. It was found that a 3000 m² solar pond installed near the Dead Sea is able to provide an annual average production rate of 4.3 L min⁻¹ distilled water compared with 3.3 L min⁻¹ that would be produced by El Paso solar pond, which has the same surface area. Based on this study, solar ponds appear to be a feasible and an appropriate technology for water desalination near the Dead Sea in Jordan.     

The stability of an unsustained salt gradient solar pond

A solar pond of area 5.712 m² was constructed. It was filled with water to a height of 87.5 cm. The stability of density and temperature profile, variation of salt flux due to temperature and depth, temperature loss during night time, and the evaporation losses at the surface were analyzed.     

Investigation of solar pond surface zone temperature assumptions

This paper presents the results of tests concerning two assumptions about the surface zone of a solar pond. The first assumption is that the surface zone temperature of the solar pond is equal to the air dry bulb temperature, and the second one is that it is equal to the air wet bulb temperature. The surface zone temperature and the storage zone temperature are predicted by using a lumped-parameter model. The experimental results of the surface zone conform well with theoretical values. The results indicate that the air dry bulb temperature is more accurate in winter-time but that the errors generated by both assumptions are almost equal in the summer-time.     

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.     

Nonlinear dynamic behavior of non-convective zone in salt gradient solar pond

Non-Convective Zone (NCZ) of salt gradient solar pond is a typical double diffusive system of salinity and temperature, and it is subjected to instable effects of adverse temperature gradient. The onset of instability may occur as an oscillatory motion because of the stabilizing effect of the salinity. In this paper, the marginal state between the steady state and the convection of the NCZ is studied. The stability of the Boussinesq approximation of the Navier-Stokes equations is analyzed by a perturbation approach. The marginal states for the onset of convection are obtained by analytical method, which is based on the linearization of the ordinary differential equations, and then numerical method is used to solve the nonlinear ordinary differential equations. Numerical results provide the trajectories of the temperature and velocity coefficients in the three-dimensional phase space, as well as the two-dimensional temperature, salinity and velocity fields in NCZ. The results demonstrate that the numerical study is in agreement with the marginal stability and the critical Rayleigh number derived from linear stability analysis. Both the linear and nonlinear studies indicate that oscillation is a narrow region above the stable region; however, the nonlinear numerical results indicate that the linear stability analysis leans to a larger upper boundary in the oscillatory regions.     

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.     

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.     

Performance investigation of salt gradient cylindrical solar pond integrated and nonintegrated with evacuated tube solar collectors

In this study, the energetic and exergetic efficiencies of a salt gradient cylindrical solar pond (SGCSP) that integrated and nonintegrated evacuated tube solar collectors (ETSCs) are investigated to improve daily heat preservation performance of the heat storage zone (HSZ). The integrated system is consisted of an SGCSP and four ETSCs. The SGCSP has a surface area of 2 m², a depth of 1.65 m, salty water layers at different densities, and HSZ in which the cylindrical serpentine type heat exchanger (CSHE) is located. Thus, the daily effects of the heat storage performance of both the ETSCs and the SGCSP in the winter season was determined experimentally. The analysis of the data regarding the efficiencies of the system is investigated separately by means of experimental studies where the SGCSP is integrated and nonintegrated with the ETSCs. The number of ETSCs integrated with SGCSP is increased to 1, 2, 3, and 4, respectively, and each of the five different experimental systems is performed separately. The temperature distributions of the integrated system are measured by a data acquisition system on 11 different points per hour. The efficiencies are calculated using the data obtained from these studies. Consequently, the energetic and exergetic efficiencies of the SGCSP are obtained without collectors as 10.4% and 4.3% and with one collector as 12.83% and 6.15%, with two collectors 14.88% and 8.25%, with three collectors 16% and 9.35%, and finally with four collectors 16.94% and 10.3%, respectively. Furthermore, the theoretical efficiencies are found to be consistent with the experimental results obtained by increasing the collector numbers.     

Turbidity study of solar ponds utilizing seawater as salt source

A series of experiments were conducted to study the turbidity reduction in solar ponds utilizing seawater as salt source. The experiment on the turbidity reduction efficiency with chemicals indicates that alum (KAl(SO₄)₂·12H₂O) has a better turbidity control property because of its strongly flocculating and also well depressing the growing of algae and bacteria in the seawater. In comparison with bittern and seawater, our experiment shows that the residual brine after desalination can keep limpidity for a long time even without any chemical in it. Experiments were also conducted on the diffusion of turbidity and salinity, which show that the turbidity did not diffuse upwards in the solution. In the experiment on subsidence of soil in the bittern and saline with the same salinity, it was found that soil subsided quite quickly in the pure saline water, but very slowly in the bittern. In this paper we also proposed an economical method to protect the solar pond from the damage of rain. Finally, thermal performance of a solar pond was simulated in the conditions of different turbidities using a thermal diffusion model.