Salt Gradient Solar Pond Research Articles

Design, Construction, and Initial Operation of a 3355 m2 Solar Pond in El Paso

A 3355 square meter, 3.3 m deep water storage pond in El Paso, Tex. was converted to a salt-gradient solar pond to supply industrial process heat to an adjacent food processing plant. Approximately 1.9 × 106 kg of sodium chloride salt was obtained to prepare near saturated brine for pond construction. Design and construction of the solar pond are described in detail including the lining technique, salt dissolution method, diffuser design, instrumentation, maintenance of optical clarity, and gradient establishment, including resolution of initial problems in gradient stability. The solar pond has been in continuous operation for over three years.

Performance of modified membrane-stratified solar ponds

The thermal behavior of membrane-stratified solar ponds (MSSP) with vertical tubes and square cells has been investigated theoretically. Comparisons have been made with salt-gradient solar ponds (SGSP). It is found that the modified MSSP is more effective than the SGSP because the amount of insolation transmitted through the insulating zone of the modified MSSP may be much greater than for the SGSP. The time-temperature variation in the storage zone of the modified MSSP in Tainan for the initial period without heat extraction has been estimated correctly.

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.

Development of a submersible shadowgraph for the study of interfaces in salt-gradient solar ponds

In this paper the processes of development and testing of a submersible shadowgraph are described. This instrument was devised as a tool for the study of interfaces in salt-gradient solar ponds. Tests were carried out in the solar pond of the University of Texas at El Paso. Photographs of interfaces inside the pond were taken for the first time. The submersible shadowgraph can be stationed inside the pond for time dependent studies of a given region, or it can be used to scan the pond depth.

Collection and storage of solar energy

Abstract Some exotic aspects of solar energy with reference to its collection and storage have been discussed, in that the quality of energy is emphasized at the outset in collecting and storing the solar energy. Several systems have been reviewed for storage of solar energy primarily for commercial purpose with a central power generating plant. Some principles of storage for individual units for domestic, industrial or commercial use have also been briefly outlined. In that the quality of the energy, favorable to natural circumstances and technological know-how are kept in mind. With the quality of energy in view among the large scale projects some of the collection-cum-storage devices, such as helio-hydrogravity, helio-aero-gravity, salt-gradient solar pond and ocean thermal conversion are presented briefly. Among the reflective collector devices, central receiver tower, parabolic trough, parabolic dish and solar bowl are recommended. The solar bowl is primarily highlighted for developing countries. Hot water, rock bed and phase change storage systems are also briefly described for relatively small units. Finally, hydrogen as energy carrier is argued as the most efficient means for collection, storing and transportation of solar energy.

Thermal field in a water body for solar energy storage and extraction due to a buoyant two-dimensional surface water jet

An experimental study on the thermal field arising in an enclosed water body being employed for energy storage, as sensible heat, due to a horizontally discharged, heated, water jet is carried out. Two-dimenional surface discharges are considered, with an outflow located at the far end of a water body, which is simulated in the laboratory by a tank of rectangular cross section. Energy loss occurs at the sides and at the top of the water body largely by convective transport and at the bottom by conduction to the ground. The study initially considers the transient behavior of the water body due to the input of thermal energy. A rapid transient, followed by a very gradual variation to an essentially steady-state temperature distribution, is observed. Experimental results are obtained in this steady regime for a water body of large extent, simulated by allowing a gradual outflow far from the inflow. The effects of the inflow conditions, aspect ration, and the position of the outflow on the resulting thermal stratification are then studied in detail. Several interesting trends are observed and related to the basic mechanisms that arise in such energy storage systems. These flows are of interest in the storage and the extraction of energy, as sensible heat, in systems such as salt-gradient solar ponds. The thermal field is important in the design of such storage systems, particularly with respect to the selection of inlet conditions and the location of the outflow.

The use of energy from salt-gradient solar ponds for reclamation of agricultural drainage water in California: Analysis and cost prediction

The cost of water from multi-effect distillation (MED) plants of various evaporator designs using thermal energy from salt-gradient solar ponds is calculated. The criterion U C , the heat-transfer capacity per unit cost for the evaporator (ratio of the overall heat-transfer coefficient in the evaporator, U, to the cost of the evaporator per unit area of heat-transfer surface, C) is varied between 12 and 105 kJ/h°C$ (3 and 25 kcal/h°C$) to evaluate its effect on plant capital cost and performance. Other parameters varied in the study include number of MED effects, solar pond temperature, and cost of steam from the solar pond. Compilation of the results from hundreds of computer simulation runs identify the conditions for minimum distilled water cost and corresponding plant productivity per unit area of solar pond. It is found that minimum distilled water cost results from the use of evaporators with high values of U C . In addition, it is found that distilled water cost is lowest when the brine temperature in the lower convecting zone of the pond is about 60°C (compared to the other temperatures considered of 50, 65, 70, and 80°C). For the case of brine temperature of 60°C, the results show that by increasing U C from 12 to 105 kJ/h°C$ (3 to 25 kcal/h°C$), distilled water cost is decreased from 3.16 to 1.63 $/ton (11.95 to 6.18 $/kgal), and the productivity per unit area of solar pond is increased from 11.6 to 20.2 kg/day per square meter of solar pond.

Solar transmission in salt solutions with reference to solar ponds

Much of the work on transmission of solar radiation in solar ponds is, like that in the oceans, based on measurements made in so called “pure water”, that is, either distilled water or clean sea water which has been extremely well filtered. The original calculations were published in 1908, based on transmission measurements for distilled water published in 1895 and using the original Langley spectrum dating from 1882/83. Yet a solar pond may contain 20% or more of sodium or magnesium or magnesium chloride, or a mixture. Whilst unfiltered solutions were known to exhibit very large transmission loss, it is now found that extreme filtering again achieves the transmission of “pure” water. Moreover, prolonged settling has that same effect, thus explaining why, yet again, salt gradient solar ponds work so remarkably well.

Problems encountered in operating salt gradient solar ponds in the Arabian Gulf region

The paper highlights the main problems encountered in operating salt gradient solar ponds in the Arabian Gulf region, characterized by hot, windy, and dusty environment. These problems are excessive erosion of the gradient zone, the formation of sizeable localized convective zones, the deterioration of pond water clarity and the high rates of surface evaporation. These weather-related problems severely impair the pond operation, as they lead to a serious drop in its collection and storage efficiency, and an excessive increase in salt consumption. Experience gained from operating a 1600 m 2 pond and a small-scale experimental pond is presented.

Use of solar heat to upgrade biogas plant performance

The performance of a biogas plant is temperature dependent. At higher temperatures specific gas yield rises almost linearly with temperature and throughput time is reduced. In winter, gas yield drops due to a fall in temperature of the slurry. In winter, if slurry could be heated to 50°C, more efficient operation in the thermophilic range would be possible. The source for heating the slurry has to satisfy many constraints: year-round supply at low temperatures and should have no fuel costs. A salt gradient solar pond satisfies these criteria. The heat transfer system should avoid hot spots above 55°C but ensure effective performance. A temperature above 55°C will destroy methanogenic bacteria. A vapour condensation heat transfer system based on acetone (b.p. 55°C) consisting of a sealed system in which two containers, one each in a biogas digester and solar pond, connected by a sloping pipe is suggested.

Performance of a covered small-scale solar pond with a planar reflector

A small-scale salt-gradient solar pond having 2 × 2 m aperture area and 0.5 m depth was constructed on the roof of the heat laboratory at Assiut University. It was covered with transparent polyethylene sheet. An in-pond heat exchanger was used. The tests were started in July 1986. A planar reflector was attached to the pond to improve its performance. Measurements have been carried out for vertical temperature and salinity distributions. The pond performance was evaluated by a collection efficiency based on the overall energy balance of the pond. The effects of the planar reflector tilt angle, reflection characteristics of the pond's inside walls on the pond efficiency were studied. Linear correlations similar to that of a flat plate collector were obtained for both instantaneous and average pond collection efficiencies.

Mixing Depth of a Submerged, Horizontally Injected Buoyant Jet

Tests in the laboratory were conducted to determine the effective mixing depth of turbulent buoyant fluid injected horizontally into an initially unstratified environment through a radial, deeply submerged diffuser. The diffuser was moved slowly during the experiments, at a rate much smaller than the entrainment and mixing process. The effective mixing depth is defined as the relative position between the initial injector level and the lowest position of the resulting mixed layer over which the injected fluid is deposited. Results have applications to discharge of contaminants and heat into large reservoirs (sewage disposal, power plants, OTEC systems) and establishment of a stratified region as is required for operation of salt gradient solar ponds. A functional dependence between mixing depth, scaled by the injector slot size, and injector Froude number was found. Dependence on submergence was not found for the large submergence values tested; dependence on diffuser radius was not investigated. The laboratory data compare well with results obtained in a reservoir of 3355 m2 surface area and indicate a strong dependence of effective mixing depth on the Froude number.

Double-Diffusive Fluxes in a Salt Gradient Solar Pond

The possible influence of double-diffusive stratification on the vertical transport of salt and heat in a mixed-layer simulation model for a salt gradient solar pond is examined. The study is concerned primarily with the interfacial fluxes across the boundary between the gradient zone and upper convecting zone of solar ponds, though the arguments presented should be applicable to other “diffusive” interfaces as well. In the absence of mechanical stirring in the upper convecting zone (e.g., by wind), double diffusive instabilities could govern the vertical flux of heat and salt by adjusting interfacial gradients of temperature and salinity which control transport by molecular diffusion. Because these gradients are generally too sharp to be resolved by numerical models, the fluxes can either be modeled directly or be parameterized by grid-dependent “effective diffusivities.” It is shown that when mechanical stirring is present in the mixed layer, double-diffusive instabilities will not be allowed to grow in the interfacial boundary layer region. Thus, double-diffusive fluxes become important only in the absence of stirring and, in effect, provide a lower bound to the fluxes that would be expected across the interface.

An accurate upper estimate for the transmission of solar radiation in salt gradient ponds

The objective of the present work is the estimation of maximum transmission of solar radiation within a body of natural water under the most favorable conditions, with special reference to salt gradient solar ponds. The study, based on recent data, includes two alternative analytical approaches based on the split of the solar spectrum into the appropriate number of spectral bands and numerical calculation of discrete values of integrals according to Beer's law. As far as radiation transmission properties of clear water are concerned, it was found that the broadly used absorption law, which is commonly referred to as an upper transmission limit, is derived from original work by Schmidt[23] and deficient data, employed at the time, are responsible for the appreciably lower theoretical maximum transmission then derived. The accurate upper transmission limit derived now also gives comparative higher heat collection efficiency. Comments have been made for the introduction of a water clarity dimensionless factor in terms of the upper transmission limit, describing the economical operation limits for solar pond water.

Transient Thermal Characteristics of the Convecting Solar Pond with the Semi-Transparent Multilayer Surface Insulating System

A concept of the convecting solar pond with the multilayer surface insulating system which is transparent to solar radiation but opaque to infrared radiation is proposed, and its transient thermal characteristics are discussed by solving time-dependent energy equations using meteorological data from several locations in Japan. It is shown that the convecting solar pond reaches a quasi-steady periodic state after three or four years of operation and that its thermal performance is comparable with that of the salt-gradient solar pond.

Thermal behaviour of salt gradient solar ponds

A numerical model has been developed to study the thermal behaviour of salt gradient solar ponds under different operating conditions. A cylindrical solar pond system with a thermal sink below and around the pond is also modelled to determine the heat losses to the earth. In these calculations, the solar pond is considered as a steady-state flat-plate collector. The effects of the thickness of three different zones and insulating materials below and around the pond on the overall thermal performance of the pond are studied. There is an optimum thickness of the non-convective zone for which the heat storage is maximum. The pond bottom and side heat losses are reduced using some low-cost insulating building materials such as dry dune sand, dry mica powder and dry marble dust below and around the pond. The results of a comparative study indicate that heat losses are considerably reduced with marble dust insulation and the solar thermal energy retaining capability of the pond is improved.

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.

Sidewall effects in a double diffusive system

The stability of a double diffusive system comprising of continuous gradients in temperature and salinity has been examined with respect to finite amplitude disturbances. On the basis of existing experimental data it is shown that for the particular case where salinity provides the stabilizing gradient (the “diffusive” mode) such instability will occur when the system is still stable to infinitesimal disturbances. A numerical model that incorporates the effects of the double diffusive instability together with other mixing processes that occur in natural and man‐made water bodies has been developed. Data from two salt gradient solar ponds, where the finite disturbance is provided by the differential heating that occurs at the shallow sidewalls are shown to agree well with the model results. While the exact details of the instability may eventually be shown to differ from those assumed in the model, it appears beyond question that in the presence of finite amplitude disturbances, the accepted infinitesimal stability criterion is of no real significance.

Design and optimization of the gel solar pond

AbstractThe gel pond concept is a unique modification of the conventional salt gradient solar pond. It replaces the salinity gradient zone by a transparent polymer gel and thereby eliminates all problems of instability associated with the former. A framework for the preliminary design of a gel pond to serve different purposes has been presented here.Heat extraction at constant pond storage zone temperatures of 40°–80°C at storage zone depths of 2–5 m has been considered and optimum pond dimensions have been predicted depending upon maximum and minimum heat load constraints. The computer simulations are carried out with the meteorological conditions for Albuquerque, New Mexico. For example, the simulations show that the gel pond can provide an average heat load of 107 W/m2, a typical domestic water heating requirement, at a constant pond temperature of 40°C with a storage zone depth of 4.0 m and gel thickness of 0.6 m. For the case of power generation, the pond can provide a minimum of 32 W/m2 at a constant pond temperature of 80°C with a storage zone depth of 4 m and gel thickness of 0.6 m. These factors combined with the easy operation and maintenance of the gel pond seem to make it a better alternative compared with the conventional salt gradient pond.

An investigation of a surface drift current return pipe system for application on salt gradient solar ponds

Based on the premise that wind-induced mixing could be the factor that limits the size of salt gradient solar ponds, this paper explores the notion of returning surface drift currents in pipes from the downwind end to the upwind end of the pond. An unstratified constant density pond subjected to a steady wind is assumed. A mathematical model is developed to predict the effect of such a pipe return system on drift and circulation currents. In an experimental study conducted in Utah State University's wind tunnel, set-up was measured on a shallow unstratified body of water to exposed wind at various speeds. The tests confirm that set-up may persist even when waves are suppressed. Circulation currents could be virtually eliminated with the return pipe system, but surface drift currents are increased. Larger scale tests on stratified ponds are needed.