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.     

Enhancing the thermal efficiency of solar ponds by extracting heat from the gradient layer

An alternative method of heat extraction from salinity-gradient solar ponds is investigated with the aim of increasing the overall energy efficiency of collecting solar radiation, storing heat and delivering this heat to an application. In this alternative method, heat is extracted from the non-convecting gradient layer of a solar pond as well as, or instead of, from the lower convective zone (LCZ). A theoretical analysis of combined gradient-layer and LCZ heat extraction is conducted to obtain expressions for the variation of temperature with depth in the pond, and the temperature gradient with depth. The dependence of the overall energy efficiency of the pond on thickness of the gradient-layer, temperature of delivered heat, and various combinations of gradient layer and LCZ heat extraction rates, including the limiting cases of gradient-layer heat extraction only, and LCZ heat extraction only, is then explored. This theoretical analysis suggests that heat extraction from the gradient layer has the potential to increase the overall energy efficiency of a solar pond delivering heat at a relatively high temperature by up to 50%, compared with the conventional method of heat extraction solely from the LCZ. The potential gain in efficiency using gradient-layer heat extraction is attributed to the lowering of heat losses by conduction to the upper convective (surface) zone that can be achieved with this method. Experimental investigations are proposed to test the predictions of the theoretical analysis in practice, and assess the impact of a number of idealized assumptions made on the findings reported here.     

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.     

Saturated solar ponds: 2. Parametric studies

The effects of following parameters on the performance of saturated solar ponds are studied: thickness of upper convective zone, nonconvective zone, and lower convective zone; starting time of the pond; water table depth below the pond; ground thermal conductivity; transmissivity of salt solution; incident radiation; ambient air temperature, humidity, and velocity; thermophysical properties of salt solution; pond bottom reflectivity; convection, evaporation, radiation, and ground heat losses; temperature and rate of heat removal; type of salt. Magnesium chloride and potassium nitrate salt ponds located at Madras (India) are considered for the parametric study. A comparison is also made with an unsaturated solar pond.     

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.     

Solar pond with honeycomb surface insulation system

A solar pond consisting of transparent compound honeycomb encapsulated with Teflon film and glass plates at the bottom and top surface respectively, floating on the body of a hot water reservoir is considered and analysed for the heat transfer processes in the system. A mathematical model is developed where the energy balance equation of the convective water is formulated by considering its capacity effects, various heat losses and solar energy gain through the surface insulation and is solved by the finite difference method. Transient rate of heat collection and storage characteristics are investigated. Explicit emphasis is laid on the effect of the thickness of the bottom encapsulation on the year-round thermal performance of the system and results seem to favour the minimum thickness. The annual average efficiency of the transparent honeycomb insulated solar pond is found to be higher than the conventional salt gradient pond by a factor of about 2.     

Thermal behavior of a small salinity-gradient solar pond with wall shading effect

The thermal behavior of a small-scale salinity-gradient solar pond has been studied in this paper. The model of heat conduction equation for the non-convective zone has been solved numerically with the boundary conditions of the upper and lower convective zones. The variation of the solar radiation, during a year, and its attenuation in the depth of the pond has been discussed. The wall shading area for a vertical wall square pond has been elaborated and its effect on the reduction of the sunny area has been included in the model. The temperature variation of the storage zone has been calculated theoretically and compared with the experimental results. The sensitivity analysis demonstrates the importance of the side and bottom insulation and the thickness of the non-convective zone, as well as the wall shading effect on the performance of the pond. The application of several loading patterns gives an overall efficiency of 10% for the small pond.     

Optimization of the gel solar pond parameters: Comparison of analytical models

A number of analytical models have been presented in the contemporary literature to describe and predict the thermal behavior of salt-gradient solar ponds under steady- and unsteady-state conditions. This paper presents a detailed theoretical comparison between three different analytical models proposed to simulate the thermal behavior of solar ponds. These models are slightly modified to represent the gel pond configuration. For experimental comparison, a gel pond constructed at the University of New Mexico, which has been in operation for several years, has been used as the reference pond. The gel pond differs from conventional salt-gradient ponds in that the nonconvective insulating layer is replaced by a transparent polymer gel. Numerical computations have been made to optimize the geometric and operational parameters of the pond using the three different models. The optimum thickness of the nonconvective layer (gel) and the thin upper convective layer (fresh water) and their effects on pond performance at a given ambient temperature, isolation and storage temperature have been calculated using all the models. The three models have also been used to calculate the absorptivity— transmissivity product (ατ), a parameter which represents the transparency of the nonconvective and the upper convective layers combined together. These calculated values have been compared with experimental values as measured through an actual gel layer to test the accuracy of the different models as applied to the gel pond.The results show that, under the same temperature difference between the storage zone and the ambient (20°C) and a yearly average insolation of 250 W/m², the model proposed by Wang and Akbarzadeh predicts an efficiency of approx. 32% as compared to the high values of 37.2 and 39% predicted by the Kooi model and Kaushik and Bansal model, respectively. Under the same conditions, the optimum gel thicknesses predicted by the three models are 62, 55, and 75 cm, respectively.     

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.     

Constant flow solar pond collector/storage system

This paper presents a periodic analysis of the process of heat extraction by the brine layer circulating at constant flow rate through the bottom convective zone of a solar pond. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted, as a function of time, depths of convective as well as non-convective zones and the flow rate, are derived. Extensive analytical results for the optimum performance of a pond during its year round operation are presented. In a pond with an upper convective zone depth of 0.2 m optimum heat extraction efficiencies of 24 per cent, 29 per cent and 32 per cent corresponding to heat extraction temperatures of 89, 55 and 42°C are predicted for water flow rates of 2 × 10^?4, 5 × 10^?4 and 10^?3 kg/s m², respectively. The load levelling in the extracted heat flux as well as in its temperature improves as the flow rate is lowered and the non-convective zone is over sized. An increase in the total depth of the solar pond improves the load levelling in extraction temperature, but influences the extracted heat flux differently; shifts its maximum to winter months and deteriorates the load levelling. The variability in flow rate required for the maintenance of constant temperature of the heat extraction zone is also investigated. It is found that the required variability is less for higher temperatures of the heat extraction zone and larger depths of the non-convective zone.     

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.     

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.     

Performance assessment of a magnesium chloride saturated solar pond

This paper deals with the experimental investigation of a magnesium chloride saturated solar pond and its performance evaluation through energy and exergy efficiencies. The solar pond system is filled with magnesium chloride containing water to form layers with varying densities. A solar pond generally consists of three zones, and the densities of these zones increase from the top convective zone to the bottom storage zone. The incoming solar radiation is absorbed by salty water (with magnesium chloride) which eventually increases the temperature of the storage zone. The high-temperature salty water at the bottom of the solar pond remains much denser than the salty water in the upper layers. Thus, the convective heat losses are prevented by gradient layers. The experimental temperature changes of the solar pond are measured by using thermocouples from August to November. The densities of the layers are also measured and analysed by taking samples from at the same point of the temperature sensors. The energy and exergy content distributions are determined for the heat storage zone and the non-convective zone. The maximum exergy destructions and losses appear to be 79.05 MJ for the heat storage zone and 175.01 MJ for the non-convective zone in August. The energy and exergy efficiencies of the solar pond are defined as a function of solar radiation and temperatures. As a result, the maximum energy and exergy efficiencies are found to be 27.41% and 26.04% for the heat storage zone, 19.71% and 17.45% for the non-convective zone in August, respectively.     

DESIGN AND PERFORMANCE EVALUATION OF A SOLAR POND FOR INDUSTRIAL PROCESS HEATING

This paper describes the design of a solar pond for delivering 54 m³/day of hot water at 60°C to a catering facility in Singapore. The design of the pond was carried out in two steps. First, the depths of different layers of the pond were determined by considering the maximum temperature of the storage zone and the useful energy gain. For the given load and the local meteorological conditions, the optimum depths of various layers of the pond were found as follows:

Depth of surface-mixed layer:0.32 m

Depth of the insulation zone :1.00 m

Depth of storage zone:1.00 m

Total depth of the pond:2.32 m

The minimum payback period was used as the economic figure of merit to determine the optimum area of the pond. The optimum area of the pond is 6000 m². The payback period depends on the transparency of the pond, and for the conditions considered in the study, it varies between 3 and 4.5 years, The solar fraction varies from 65% for extinction coefficient, ( μ = 1.0m^?1 to 94% for μ = 0.55 m^?1, An experimental pond with an area of 14 m² and a depth of 1.5 m was built and tested over a period of time under the meteorological condition of Singapore. These results are used to validate the mathematical equations used in the design of the solar pond. A good agreement was found to exist between experimental and analytical results.     

An underground water heater with a thermal trap cover on the upper surface of the ground

This communication presents an analytical model to predict the thermal performance of an underground water tank, having a thermal trap sheet on the glazed blackened top surface of the ground. The blackened surface acts as an absorber of solar insolation, while the tank, as well as the surrounding earth, acts as a storage system. The effect of ground depth and thickness of the thermal trap sheet on the thermal performance of this system has also been studied.     

Numerical simulation of double-diffusive dynamical model of a solar pond considering the effect of turbidity and wind

A solar pond, typical double-diffusive system, is a stable heat source that can collect and store the solar energy. When the thermal stable condition is not satisfied at the interface, the upper and lower convective zone (UCZ and LCZ) will erode the middle non-convective zone (NCZ), resulting in a drop or even a collapse of the thermal performance of solar pond. Wind strongly affects the erosion of NCZ from the entrainment of UCZ. The double-diffusion of heat and salt plays an important role in the erosion of NCZ from the entrainment of the lower-con vective zone (LCZ). The turbidity of saline water in the pond not only could lower the thermal performance of solar pond, but have effect on the entrainment mechanism. In this paper, based on the double-diffusive model along with the wind-driven turbulent entrainment model, the effects of turbidity and external wind etc. on the thermal performance of solar pond and the entrainment mechanism are analyzed with the numerical simulation.     

Saturated solar ponds: 1. Simulation procedure

The mass and energy balances on the upper convective zone, nonconvective zone, and lower convective zone of a saturated solar pond are written to yield a set of nonlinear partial differential equations. These are solved numerically to predict the thermal performance of the pond over a long period of time for various initial and boundary conditions. This model considers external parameters such as hourly variation of incident solar radiation, ambient temperature, air velocity, and relative humidity. Temperature and concentration dependence of density, thermal conductivity, specific heat, and mass diffusivity are taken into account. Heat transfer modes considered between the upper convective zone and the ambient are convection, evaporation, and radiation. Ground heat losses from the lower convective zone are also considered. This model is used to study the development of temperature and concentration profiles inside a saturated solar pond. This model can also be used to predict the long-term performance of a saturated solar pond for various heat extraction temperatures and rates.     

Transient behaviour of salt gradient stabilised shallow solar ponds

This paper presents the results of experimental and analytical investigations of the transient thermal processes in a salt gradient stabilised pond of shallow (10 cm) depth, useful for short-term storage applications. A small convective sublayer (about 3 cm thick) of uniform temperature tends to form at the bottom of the pond. The convective-non-convective zone boundary below the gradient zone exhibits movements with time of day as well as from day to day. This suggests that only local properties and local gradients are relevant to the stability condition. A thin oil layer cover at the pond surface considerably enhances the temperature in the pond and aids its stability. A simple transient thermal model of the pond is developed. The observed temperatures and depths of the zones are in close agreement with theory.     

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.     

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.