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.     

Control of a solar pond

Solar ponds hold the promise of providing an alternative to diesel generation of electricity at remote locations in Australia where fuel costs are high. However, to reliably generate electricity with a solar pond requires high temperatures to be maintained throughout the year; this goal had eluded the Alice Springs solar pond prior to 1989 because of double-diffusive convection within the gradient zone. This paper presents control strategies designed to provide successful high temperature operation of a solar pond year-round. The strategies, which consist mainly of manipulating upper surface layer salinity and extracting heat from the storage zone are well suited to automation. They were tested at the Alice Springs solar pond during the summer of 1989 and maintained temperatures in excess of 85°C for several months without any gradient stability problems.     

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.     

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 parametric design study of solar ponds

The salt stratified solar pond is found to be a reliable solar collector and storage system. This paper discusses the effect of varying certain design parameters on pond steady-state temperatures. These significant parameters are sizing parameters—pond surface area and depth of the pond; operating parameters—storage volume and the heat extraction fraction; and geo-climatic parameters3s?olar radiation, water table depth and upper convective zone thickness. Studies indicate that there is an optimum depth and storage volume of the pond for each application in terms of temperature and heat load desired.     

Dynamic effects in a salinity-gradient solar-pond heating system

Numerical computer models have been developed to study the dynamics of a salt-gradient solar-pond heating system in a northern cold climate. The models are applicable for predicting the temperature and salinity profiles in a pond. Special emphasis is laid on the behaviour of the upper convective layer. In the calculations, the solar pond is considered as a part of a community-scale residential heating system and the effects of the pond's dynamics on the overall system performance are assessed. All calculations were made with 1-h time steps for a hypothetical pond in Helsinki (60° N). The results indicate that the consideration of the dynamics of the salinity profile may reduce the pond's bottom temperature by 10°C in comparison with a static salt distribution. The maintenance of the salinity gradient would allow a maximum surface washing interval of 5 weeks without severely affecting the pond's performance. Then the daily salt consumption would be about 40 g per square metre. For regions with cold winters, the surface should be washed with fresh water, just before surface freezing takes place, to prevent shrinking of the non-convective stabilizing gradient zone. It was also observed that a solar-pond heating system may reach considerable solar fractions in a northern climate.     

Salt gradient solar pond with reflective bottom: Application to the “saturated” pond

The “saturated” salt gradient solar pond operates near the solubility limit. Consequently, temperature fluctuations may cause precipitation of the salt, which can increase the reflectivity of the bottom. It is shown that this can reduce the width of the nonconvective zone and seriously degrade the performance of the pond. The temperature distribution, efficiency and optimum operating conditions are calculated, taking account of diffusely reflected light from the bottom of the pond. The mechanism for narrowing the nonconvective zone is described. A semiquantitative analysis is made of a known case of simultaneous salt precipitation and nonconvective zone destruction. It is argued that the boundary between the nonconvective zone and the lower convective zone will move to its maximum temperature position if the solubility is a sufficiently strongly increasing function of temperature.     

Heat extraction methods from salinity-gradient solar ponds and introduction of a novel system of heat extraction for improved efficiency

Heat has generally been successfully extracted from the lower convective zone (LCZ) of solar ponds by two main methods. In the first, hot brine from the LCZ is circulated through an external heat exchanger, as tested and demonstrated in El Paso and elsewhere. In the second method, a heat transfer fluid circulates in a closed cycle through an in-pond heat exchanger, as used in the Pyramid Hill solar pond, in Victoria, Australia. Based on the experiences at the El Paso and Pyramid Hill solar ponds, the technical specifications, material selection, stability control, clarity maintenance, salt management and operating strategies are presented. A novel method of extracting heat from a solar pond is to draw the heat from the gradient layer. This method is analysed theoretically and results of an experimental investigation at Bundoora East, RMIT, are presented. An in-pond heat exchanger made of polyethylene pipe has been used to extract heat for over 2 months. Results indicate that heat extraction from the gradient layer increases the overall energy efficiency of the solar pond by up to 55%, compared with conventional method of heat extraction solely from the LCZ. The experimental results are compared with the theoretical analysis. A close agreement has been found. From this small-scale experimental study, convection currents were found to be localised only and the density profiles were unaffected. An experimental study using an external heat exchanger for brine extraction and re-injection at different levels within the gradient layer still needs to be conducted to determine the effect of the heat extraction from the non-convective zone (NCZ) on the stability of the salinity gradient (both vertically and horizontally) and an economic analysis needs to be conducted to determine the economic gains from increased thermal efficiency.     

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.     

Thermal analysis of three zone solar pond

This paper presents a periodic analysis of a three zone solar pond as a solar energy collector and long term storage system. 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 during its year round operation by solving the time dependent Fourier heat conduction equation with internal heat generation resulting from the absorption of solar radiation in the pond water. Eventually, an expression, for the transient rate at which heat can be retrieved from the solar pond to keep the temperature of the zone of heat extraction as constant, is derived. Heat retrieval efficiencies of 40.0 per cent, 32.1 per cent, 28.3 per cent and 25.5 per cent are predicted at collection temperatures of 40, 60, 80 and 100°C, respectively. the retrieved heat flux exhibits a phase difference of about 30 to 45 days with the incident solar flux; the load levelling in the retrieved heat flux improves as the thickness of the non-convective zone increases. the efficiency of the solar pond system for conversion of solar energy into mechanical work is also studied. This efficiency is found to increase with collection temperature and it tends to level around 5 per cent at collection temperatures about 90°C.     

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.     

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.     

Fertilizer solar ponds as a clean source of energy: Some observations from small scale experiments

Commercially available NH₂CONH₂ is used to establish a salinity gradient solar pond in a small 1 m² outdoor tank. With a salinity difference of 35% between the upper and lower zone, a temperature difference of 23°C was obtained without any instabilities in the gradient zone. The difference in concentration of solution required to sustain a temperature difference of 40°C across the gradient zone is 520 kg/m³. By economically using runoff into the fertilizer cycle of an agricultural system the estimated cost of fertilizer solar pond generated heat is Rs. 1.10/kWh.     

Membrane stratified solar ponds

The membrane stratified solar pond is a body of liquid utilizing closely spaced transparent membranes to quench convective heat transfer in the top part of the pond. Membranes may be configured as horizontal sheets, vertical sheets or vertical tubes. Several suitable liquids and membrane materials are discussed. Conditions for suppression of convection are described, and transmission of solar radiation through the pond is discussed for each of the three membrane configurations. The steady state thermal efficiency is calculated for the horizontal sheet configuration. Thermal behavior is similar to that of salt gradient solar ponds, but much deeper heat storage layers are feasible. In some cases aquaculture farming may be suitable in the storage layer.     

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.     

Development of the gel pond technology

A review of the development of the gel pond technology is presented. First, the emergence and growth of solar pond technology since the 1950's is described. The inherent problems encountered with the conventional salt gradient ponds are discussed, leading to the concept of the solar gel pond in which the salt gradient layer in the former is replaced by a transparent polymer gel. The major work in the first phase dealt with the experimental development of a polymer gel which met certain selection criteria. The criteria considered included transmissivity, stability of physical and chemical properties, high viscosity and other physical and optical properties. The gradual development of the polymer gel through an alternating process of testing and elimination and evaluation of relevant properties of the gel has been described. Modeling and optimization studies of the solar gel pond have been presented. Bansal and Kaushik's model for a salt gradient pond has been modified for a solar gel pond, and the results of the simulation are presented in a graphical form to serve as a quick reference for estimation of pond surface area, depth and flow rate for heat extraction depending on the extreme temperature required in the storage zone and the required heat load. Then, a cost-benefit economic analysis compares the economics of a solar gel pond with a conventional salt gradient pond. The construction of an experimental gel pond (18 m²) at The University of New Mexico is described, and the results of the study are summarized. Information on commercial scale ponds at Chamberino, New Mexico (110 m²), and in Albuquerque, New Mexico (400 m²), is provided. The review of the technology demonstrates the immense potential of the gel pond as a source of alternate energy for the years ahead.     

A study on the thermal storage of the ground beneath solar ponds by computer simulation

A salt gradient solar pond is a large scale solar collector having built-in heat storage capability. This is in part due to the mass of water in the pond and in part to the ground beneath the pond. Some scholars have already paid attention to the ground thermal storage. In this work, emphasis has been put upon the un-steady state performance and the transient behavior of SGSPs. A simple computer simulation method is adopted to study the ground heat loss and the heat recovery rate under varied combinations of the depth of the underground water table, the thickness of the lower convective zone, the heat withdrawal pattern, and the thermal properties of the soil. The effect of an insulation layer between the pond and the ground is also examined.     

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.     

Optimum thickness of the nonconvective zone in salt gradient solar ponds

The nonconvective gradient zone of a salt gradient solar pond tends to more effectively transmit incident solar energy to the storage brine below as its thickness is reduced. However, that same gradient zone tends to more effectively reduce heat loss from the warm brines as its thickness is increased. Therefore, there exists an optimum gradient zone thickness for which the net rate of energy collected and retained is a maximum. This report describes a technique for using a numerical simulation model to determine the optimum thickness of the gradient zone in ponds; provided other basic design, operating and climatic factors are specified. Significant improvements in pond efficiency may be obtained if the thickness of the gradient zone is adjusted monthly, seasonally or even if maintained at the annual average optimum thickness as compared with operating the pond with other than an optimum gradient zone thickness.     

Kinetics of diffusion of salt in solar ponds

In the present communication, the kinetics of diffusion of salt in a stacked layer solar pond has been investigated by using the step function for the initial state of salt distribution and a closed form solution for the salt concentration has been obtained with the boundary conditions of an operational solar pond. It has been predicted that the time required for a two layered solar pond with non-convective zone of about 1 m depth to reach equilibrium concentration gradient is about 585 days, whereas that required for a ten layered pond is only 96 days.