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.     

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.     

Solar ponds for space heating

Solar ponds are shallow bodies of water in which an artificially maintained salt concentration gradient prevents convection. They combine heat collection with long-term storage and can provide sufficient heat for the entire year. We consider the absorption of radiation as it passes through the water, and we derive equations for the resulting temperature range of the pond during year round operation, taking into account the heat that can be stored in the ground underneath the pond. Assuming a heating demand of 25000 Btu/degree day (Fahrenheit), characteristic of a 2000 ft² house with fair insulation, and using records of the U.S. Weather Bureau, we carry out detailed calculations for several different locations and climates. We find that solar ponds can supply adequate heating, even in regions near the arctic circle. In midlatitudes the pond should be, roughly speaking, comparable in surface area and volume to the space it is to heat. Under some circumstances, the most economical system will employ a heat pump in conjunction with the solar pond. Cost estimates based on present technology and construction methods indicate that solar ponds may be competitive with conventional heating.     

Experimental and theoretical temperature distributions in a solar pond

In this study, an experimental and theoretical investigation of temperature distributions in an insulated solar pond, particularly during daytimes and nighttimes, is presented. Several temperature sensors connected to a data acquisition are placed vertically inside and the bottom of the pond and also horizontally and vertically in the insulated side walls, and used to measure temperature changes with time and position. In addition, we model the solar pond to compute theoretical temperature distributions and compare with the experimental measurements, and hence a good agreement is found between experimental and theoretical temperature profiles. There is a large amount of heat losses between daytimes and nighttimes, depending upon the temperature difference, and these present a significant potential for energy savings and storage. During the months of January, May and August, it is found that the total heat losses from the inner surface of the pond and its bottom and side walls, as a function of temperature difference, are determined to account for 227.76 MJ (e.g., 84.94% from the inner surface, 3.93% from the bottom and 11.13% from the side walls, respectively).     

Large scale solar cooling design using salt gradient solar ponds

The future of large scale cooling is closely linked to the long term economically viable component development for collection and storage of solar energy at relatively high temperatures. As such, solar ponds at the present state of their development are undoubtedly considered as the only promising large scale solar collection and heat storage device for such applications. The present analysis, based on numerical calculations, allows a parametric investigation of solar pond design and operational characteristics to the capacity of a conventionally designed, commercially available, absorption chiller. The results can prove very useful for the rough design and pond size selection for operation of chillers of a substantial capacity, directly from solar ponds.     

Effect of bottom reflectivity on ground heat losses for solar ponds

A two-dimensional ground heat loss model is used to investigate the effect of bottom reflectivity on ground heat losses for solar ponds. In the model, convection boundary conditions are used between water and ground. The convection heat transfer coefficient is estimated using the correlations given for heated or cooled flat plates. The local rate of absorption of the solar radiation in the pond is determined for the direct and diffuse components by the exact treatment of the radiation problem. The fractions of heat adsorbed by the pond bottom that is transferred to soil and to water are investigated for different bottom reflectivities.     

Heat Loss Via Concrete Slab Floors With External Vertical Edge Insulations

Abstract

A new method was developed and validated against numerical simulations for the calculation of ground heat transfer via floors with vertical edge insulations along the external side of walls. Using the new method, heating and cooling energy demand for two typical houses in the eight capital cities of Australian state and territory were evaluated with different vertical edge insulations and full horizontal floor insulations. It was found that for tropical regions such as Darwin, both vertical edge and full horizontal floor insulation have no or little effect on house heating and cooling energy demand. In cooling dominated climates such as Brisbane, full horizontal floor insulation may increase the total heating and cooling energy demand due to the decoupling between the relatively cool ground and the rooms above. For heating dominated climates such as Melbourne, Canberra and Hobart, ground heat loss can contribute up to around half of the total house heating and cooling energy demand. Full horizontal floor insulation can be very effective in these heating dominated climates. For heating and cooling balanced climates such as Adelaide, Perth and Sydney, vertical edge insulation along the external side of the walls is more effective than full floor insulation.     

Transient performance of a two-dimensional salt gradient solar pond—A numerical study

Solar ponds have recently become an important source of energy that is used in many different applications. The technology of the solar pond is based on storing solar energy in salt-gradient stratified zones. Many experimental and numerical investigations concerning the optimum operational conditions and economical feasibility of solar ponds have been published in the last few decades. In the present study, a novel two-dimensional mathematical model that uses derived variables is developed and presented. This model utilizes vorticity, dilatation, density, temperature and concentration as dependent variables. The resulting governing partial differential equations are solved numerically in order to predict the transient performance of a solar pond in the two-dimensional domain. The boundary conditions are based on measured ambient and ground temperatures at Kuwait city. Based on the present formulation, a computer code has been developed to solve the problem at different operating conditions. The results are compared with the available experimental data and one-dimensional numerical results. Two-dimensionality effects are found to depend mainly on the aspect ratio of the pond. A parametric study is conducted to determine the optimum pond dimensions and operating conditions.     

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.     

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.     

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.     

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.     

Modeling the performance of a solar pond as a source of thermal energy

The use of solar ponds is becoming more attractive in today's energy scene. A major advantage of solar ponds over other collectors is the ability to store thermal energy for long periods of time. The solar pond comprises a hydraulic system subject to processes of heat and mass transfer. The design of this system and the related equipment requires a thorough knowledge of the pond heating-up process and expected thermohaline structure within the pond. The current study considers that convection currents in the pond are inhibited by the salinity distribution, and applies a finite difference implicit model in order to investigate the interaction among physical variables represented by various dimensionless parameters. Variables which are included in the analysis comprise the solar radiation input and absorption as it passes through the pond; diffusion and dispersion of heat within the pond; absorption of heat at the bottom of the pond; and withdrawal of heat from layers within the pond. The physical variables generate 3 dimensionless variables associated with the pond's heating-up process. A 4 dimensionless variable is associated with the heat utilization. The analysis represented in this paper concerns the interaction between these dimensionless parameters and its implications.     

Freshwater floating-collector-type solar pond

A new type of solar pond is introduced in an effort to provide an inexpensive, renewable heat source, mainly for space-heating purposes. The suggested freshwater solar pond is covered with floating collectors made of insulation boards and thin plastic sheets, and requires very little, if any, constructional work. Forced flow in the collector layer of the pond provides the reduction of the mean collector and pond temperatures and ensures high efficiency. The collectors also provide the necessary thermal and evaporation insulation so that freshwater may be used and the cumbersome and delicate task of salt-gradient maintenance is eliminated.Both simulation and experimental results are reported. A one-dimensional quasi-steady-state simple model is the basis for the simulation. The presented experimental data validate the simulations. Longterm performance characteristics as well as possible modes of seasonal storage utilization of such solar ponds are discussed.     

Thermal performance modeling of the reverse absorber shallow solar pond

Reverse absorber type shallow solar ponds are proposed as being capable of attaining higher temperatures and still higher efficiencies than the conventional type due to convection suppression and elimination of top radiative losses. The theoretical thermal analysis and simulation of the performance of two configurations of the reverse absorber shallow solar pond (RASSP); one with the top insulated and the other with top exposed, are presented. The ensuing model equations were solved to obtain the desired performance parameters. For a pond depth of 0.10 m, results of the simulations show that water temperatures up to 70°C could be obtained in a RASSP with double glass covers, higher than could be gotten in either an RASSP with top insulation or a conventional SSP of equal depth. The effect of pond depth on the proportions of the radiation incident on the RASSP that is either collected as thermal energy or lost was studied. The average transmissivity-absorptivity products, (τα), overall heat loss coefficients, UL and optimal pond depths were also computed.     

Thermal performance of a combined packed bed–solar pond system—a numerical study

Solar ponds have recently become an important source of energy that is used in many different applications. The technology of the solar pond is based on storing solar energy in salt-gradient stratified zones. Many experimental and numerical investigations concerning the optimum operational conditions and economical feasibility of solar ponds have been published in the last few decades. In the present study a modified solar pond with a rock bed inserted at the bottom is suggested and investigated. In order to conduct this study and predict the thermal performance of the combined system, a one-dimensional transient numerical model is developed. The boundary conditions are based on measured ambient and ground temperatures at Kuwait city. The model is validated for standard plain salt-gradient solar ponds and is then used to examine the thermal performance of the combined storage system for different rock material and bed geometry. It has been shown that the storage temperature is remarkably increased when low thermal diffusivity rocks (such as Bakelite) are used in the packed bed. Meanwhile, when high conductive rocks are used, the thermal storage temperature considerably deteriorates and the temperature variations amplitudes are almost flattened out. The bed geometry also plays a significant role in the storage process. As expected, an appreciable gain in the storage temperature was obtained for thicker rock beds. Low porosity rock beds, as well, produce higher storage temperatures in the storage zone.     

Effect of irreversibilities on performance of an absorption heat transformer used to increase solar pond’s temperature

Absorption thermal systems are attractive for using waste heat energy from industrial processes and renewable energy such as geothermal energy, solar energy, etc. The Absorption Heat Transformer (AHT) is a promising system for recovering low-level waste heat. The thermal processes in the absorption system release a large amount of heat to the environment. This heat is evolved considerably at temperature, the ambient temperature results in a major irreversible loss in the absorption system components. Exergy analysis emphasises that both losses and irreversibility have an impact on system performance. Therefore, evaluating of the AHT in exergy basis is a much more suitable approach. In this study, a mathematical model of AHTs operating with the aqua/ammonia was developed to simulate the performance of these systems coupled to a solar pond in order to increase the temperature of the useful heat produced by solar ponds. A heat source at temperatures not higher than 100 °C was used to simulate the heat input to an AHT from a solar pond. In this paper, exergy analysis of the AHT were performed and effects of exergy losses of the system components on performance of the AHT used to increase solar pond’s temperature were investigated. The maximum upgrading of solar pond’s temperature by the AHT, is obtained at 51.5 °C and gross temperature lift at 93.5 °C with coefficients of performance of about 0.4. The maximum temperature of the useful heat produced by the AHT was ˜150 °C. As a result, determining of exergy losses for the system components show that the absorber and the generator need to be improved thermally. If the exergy losses are reduced, use of the AHT to increase the temperature of the heat used from solar ponds will be more feasable.     

Physics of shallow solar pond water heater

This communication presents a theoretical analysis of a shallow solar pond water heater, which is in good agreement with the experiments of Kudish and Wolf (1979) and the authors. the heater consists of an insulated metallic rectangular tank with black bottom and sides and a transparent cover at the top. After the collection of solar energy during sunshine hours the heater stores a substantial amount of heat because the top glass cover is covered by an adequate insulation in the night. Analytical expressions for the transient rise of temperature of water in the tank have been derived taking into account the appropriate heat transfer processes during day and night. These experimental results as well as those of Kudish and Wolf (1979) have been found to be in good agreement with the theory presented in this paper. the effects of one more glass cover on the top, and of the thickness of the bottom and side insulation and tank depth on the water temperature have also been studied.     

A parametric study of the small-scale solar pond (SSSP)

A parametric study of salt-gradient solar ponds of size less than 100 m² is presented. The study is based on a dynamic model of the pond which takes into account the variation of solar radiation, ambient temperature and the amount of heat extracted with time. Furthermore3 it considers a small-scale pond whose top is covered by a transparent cover, thus considerably reducing the thickness of the top convective zone. The parameters investigated include: pond dimensions, depths of the different layers, starting dates for pond operation and load application, pond insulation and the value of the thermal load.