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.     

The steady state salt gradient solar pond

The three-zone salt gradient solar pond is analyzed as a steady-state flat-plate solar energy collector. The resultant efficiency equation is of the Hottel-Whillier-Bliss type commonly used for flat-plate collectors. The quantities that occur in this equation—the effective absorptivity-transmissivity product ατ, the loss factor U_L, the heat removal factor F_R, and the incident angle modifier θ(i)—are related to the physical properties and dimensions of the pond. For a given ΔT/H [(fluid inlet temperature—surface temperature)/insolation], the thickness of the nonconvective zone can be adjusted for maximum efficiency. U_L and ατ are smaller than the equivalent quantities for flat-plate collectors, while θ(i) and F_R are close to unity. As a consequence, steady-state salt-gradient solar ponds are less efficient than common flat-plate collectors at low ΔT/H, but they are more efficient at high ΔT/H.     

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.     

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.     

Meteorologically efficient commissioning of salt gradient solar pond

Salt gradient solar pond has thermal performance parameters as rate of warm-up, highest achievable temperature, and cumulative heat collection. All these are strongly influenced by the meteorology. Consequently, specific to the meteorology of a geographic location, there is a best starting day for the as pond defined by Singh et al. [1]. The present work has done rigorous analysis of influence of meteorology on pond??s thermal performance. It is found that the starting day has strong influence in initial stage of pond warm-up; however the effect diminishes in long-term. Finally pond started on any day of the year acquires same highest temperature. It is also found that in order to retrieve maximum heat, waiting for the best starting day to commission a pond is not judicious, rather it is always more beneficial commission the pond at the earliest possible day. This finding is of practical significance while planning to put a pond in operation.     

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.     

On the performance of a salt gradient solar pond

The performance of a laboratory-scale salt-gradient solar pond is described in this paper. Different methods of saline injection to the bottom layer and corresponding temperature and concentration profiles as a function of depth are reviewed and compared with experimental results. A time history of the development of temperatures, salinities and elevations of the lower and upper layers at various climatological situations is reported. The ‘dynamic stability’ and ‘equilibrium boundary criterion’ are discussed and verified experimentally for the lower and upper gradient interfaces.     

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.     

Performance analysis of an in-pond heat exchanger of a salt gradient solar pond

The Salt gradient solar pond (SGSP) is used for process heating, power generation and to achieve refrigeration. In this paper the performance of an in-pond heat exchanger of a laboratory model SGSP was analyzed both experimentally and computationally. A laboratory model solar pond was fabricated using GI sheet of 1.5 mm thick for the dimension 600 × 500 × 500 mm and the performance of the pond was experimentally determined under solar irradiated condition. The experiment was conducted for a period of 10 days and the hourly variations of the temperatures of inlet, outlet, storage zones and ambient were measured and analyzed. To computationally analyze the performance of the SGSP, a model was developed using software tools CATIA and HYPERMESH and the performance of the modeled in-pond heat exchanger of the solar pond was determined using FLUENT software. The performance of the SGSP was analyzed for laminar and turbulent flow conditions. The experimental results were compared with computational results and close agreement was observed.     

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.     

Experiment and analysis of practical-scale solar pond stabilized with salt gradient

A large-scale solar pond with salty water was constructed in the suburbs of Kitami in 1985. Its performance has been measured and analyzed by the authors after that. The solar pond body is circular of 44 m diameter, and the pond water is of 3 min total depth. After, 15 months, the depth of the salt gradient zone (S.G.Z.) was thinned by 10 cm in the top and by 20 cm in the bottom due to convection of the top and bottom zones. The temperature in the convective storage zone (C.S.Z.) reached 70°C, its maximum, at the beginning of September in 1985, however, it was not as high in 1986 due to contamination of the pond water. The temperature of the storage zone was reduced from November to April due to ice covering on the pond surface. The collected heat yielded largely and the collection efficiency reached more than 30% in summer, but decreased to negative values in winter. The thermal performance of the solar pond was predicted by a simulation calculation, and the calculated result compared well with the measurements.     

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.     

Thermal analysis and measurement of a solar pond prototype to study the non-convective zone salt gradient stability

Solar ponds combine solar energy collection with long-term storage and can provide reliable thermal energy at temperature ranges from 50 to 90 °C. A solar pond consists of three distinct zones. The first zone, which is located at the top of the pond and contains the less dense saltwater mixture, is the absorption and transmission region, also known as the upper convective zone (UCZ). The second zone, which contains a variation of saltwater densities increasing with depth, is the gradient zone or non-convective zone (NCZ). The last zone is the storage zone or lower convective zone (LCZ). In this region, the density is uniform and near saturation. The stability of a solar pond prototype was experimentally performed. The setup is composed of an acrylic tube with a hot plate emulating the solar thermal energy input. A study of various salinity gradients was performed based on the Stability Margin Number (SMN) criterion, which is used to satisfy the dynamic stability criterion. It was observed that erosion of the NCZ was accelerated due to mass diffusion and convection in the LCZ. It can be determined that for this prototype the density of the NCZ is greatly affected as the SMN reaches 1.5.     

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.     

The effect of thermodiffusion on the stability of a salinity gradient solar pond

A one-dimensional mathematical model is used for the study of the NaCl diffusion in a salinity-gradient solar pond. The model takes into account the effect of the thermodiffusion, or Soret effect, and also the possibility of injection of concentrated brine at the bottom of the salinity-gradient zone of the solar pond. The model results show that the thermodiffusion moves in the same direction of the molecular diffusion process, thus contributing to destabilize the salinity-gradient layer. This effect can be a significant contribution to the salt diffusion (over 10%), when the temperature gradient and salt concentration are high, like at the bottom of the salinity-gradient zone.     

The process of loss of internal stability in salt gradient solar ponds

Loss of the bulk stability in salt gradient solar ponds is a rather rare, short duration phenomenon, which can lead to complete mixing of the gradient zone. Laboratory investigations have allowed close observation of this phenomenon and comparison of the derived data with theory. It is shown that there is little or no probability of such instability occurring with the maximum salt concentration normally used (about 20% at the bottom). Boundary erosion of the gradient zone is an entirely unrelated matter.     

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.     

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%。