Heat losses from a salt-gradient solar pond

A theoretical approach for the calculation of heat losses from a cylindrical flat-bottomed salt-gradient solar pond is discussed. Steady-state heat losses from the sides and bottom of the pond have been estimated, when the pond is uninsulated as well as insulated. The insulating materials considered in the present study are dry sand, mud powder, dry cement, marble dust and mica powder. The effective insulation is varied by varying the thickness and interstitial air pressure of the insulating materials. We find that the losses are reduced to a minimum with a marble dust wall of thickness 0·20 m at an interstitial air pressure of 0·5 mm of mercury.     

Correlations for the yearly or seasonally optimum salt-gradient solar pond in Greece

Simple correlations and corresponding nomographs are presented, which express the maximum useful heat received from salt-gradient solar ponds throughout the year or during a specified season of the year, and the corresponding optimum depth of the nonconvective zone in terms of the thickness of the upper convective zone and the temperature under which the maximum useful heat is received. The correlations are valid for the Athens (Greece) area or for regions with a similar climate, because solar radiation and ambient temperature values for Athens have been employed, obtained by a statistical process of hourly measurements over a period of about 20 years. For other climates, it is easy to develop similar correlations using the same methodology. Development of the proposed correlations is based on a method, which simulates the transient operation of the salt-gradient pond using finite-differences, and calculates the useful heat received hourly along the typical year. Thus, the useful heat received during a period or throughout the year is calculated as a sum of hourly values. Calculations of the useful heat are performed for a great number of values of the parameters of the problem, and the combinations of values that maximize useful heat are selected and used for developing correlations and corresponding nomographs. The correlations presented may be employed in the design of the optimum solar pond under the specific requirements of each application.     

Stability,effective insulation and heat loss of a salt-gradient solar pond

A relation between salt gradient G_c and temperature gradient G_T is derived. Heat losses are estimated for a natural solar pond in a steady state and a thickness of insulating material to achieve the required insulation is suggested. Enlargement of the non-convecting zone in an unsteady state is also discussed. Predicted values are compared with the reported results and good agreement is found.     

Effect of water turbidity on thermal performance of a salt-gradient solar pond

The effect of water turbidity on the thermal performance of a salt-gradient solar pond is studied using a one-dimensional theoretical model. The theoretical model uses an empirical correlation that includes the effect of water turbidity on solar radiation penetration in water. The correlation is based on a uniform turbidity distribution in water; however, the correlation is extended to include a non-uniform turbidity distribution with respect to depth of water. The results indicate that water clarity plays a significant role on thermal performance for salt gradient solar ponds.     

On the hydrodynamics of salt-gradient solar ponds

The objective of this paper is to discuss those hydrodynamical issues that affect the performance of the solar pond as an energy collector and storage system, e.g., mass and energy balance; formation, stability, and maintenance of the gradient layer; energy extraction from the bottom mixed layer; stability of stratified fluids to shearing flows; interface dynamics; and wall effects. Many of these topics are not fully understood and the discussion focuses on the present state of knowledge, some of the engineering correlations available at this time, and the research that is still required to resolve the relevant issues.     

Two-dimensional modeling of a salt-gradient solar pond with wall shading effect and thermo-physical properties dependent on temperature and concentration

In this paper,the behavior of a salt-gradient solar pond with the square cross-section has been studied experimentally and numerically.A small-scale solar pond were designed and built to provide quantitative data.A two-dimensional,transient heat and mass transfer model has been solved numerically by using finite-control-volume method.In this study,all the thermo-physical properties are variable as the function of temperature and salt concentration.Numerical results as obtained for the experimental pond have been satisfactorily compared and validated against measured data.Furthermore,the wall shading effect has been elaborated to improve the agreement between two sets of results.The temperature of the storage zone is predicted well by the model.It also can be observed that the initial concentration profile is preserved with time.The stability of the pond in time has been investigated in order to distinguish the critical zones.Finally,the application of an energy analysis gives an efficiency of about 12%for the pond.     

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.     

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.     

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.     

Shallow solar pond: State-of-the-art

This review article deals with the various aspects of shallow solar ponds (SSP) suitable for domestic purposes and for supplying industrial process heat. The introduction gives a general idea of the status of the SSP technology among various solar energy applications. This part also surveys designs and performance of both water bag type and large scale SSP, already experimentally studied by other workers. The analysis of thermal process of SSP takes the input energy, the mechanism of thermal absorption and various losses in the system, into consideration. The methods for the improvement of the system performance suggests the materials that can be used and designs that can be incorporated in the various components of SSP. Here, a detailed study of the module, the glazing, the glazing supports and the insulation is done. The auxiliary systems viz, the mode of water transfer and water storage, play an important part in the optimum performance of the SSP systems. The “modes of operation” gives an account of the three different ways of circulating the liquid through the system-batch heating, closed cycle continuous flow heating, and open cycle continuous cycle flow heating. The theoretical analysis of the thermal performance of the SSP is done by making use of the Hottel-Willier-Bliss model for the flat plate collectors. A computation based on this model is employed in evaluating the values of monthly average daily collections efficiency with respect to the ambient temperatures for various initial water temperatures. The results obtained from the experiments conducted by Lawrence Livermore Laboratory, gives an idea of the system performance. The review also looks into the effects of various parameters, such as, the mass flow rate of the liquid, the total mass of water required per day, water depth, radiation intensity, average day-time ambient temperature, number of glazings, total heat loss coefficient, etc. The different modes of flow of the liquid are compared. An incorporation of the reflector in the SSP by various workers proved to provide a marked improvement in the system performance. The aspects such as, the cost effectiveness, maintenance and reliability of the SSP are briefly dealt with. It was felt that the review would be incomplete without a mention of the limitations and potential applications of the SSP. Based on these studies on SSP, done by various workers, the conclusion was that the performance of the system can be improved by the proper choice of the material, and by optimizing the design and the modes of operation.     

A fully coupled,transient double-diffusive convective model for salt-gradient solar ponds

A fully coupled two-dimensional, numerical model that evaluates, for the first time, the effects of double-diffusive convection in the thermal performance and stability of a salt-gradient solar pond is presented. The inclusion of circulation in the upper and lower convective zone clearly shows that erosion of the non-convective zone occurs. Model results show that in a two-week period, the temperature in the bottom of the solar pond increased from 20 °C to approximately 52 °C and, even though the insulating layer is being eroded by double-diffusive convection, the solar pond remained stable. Results from previous models that neglect the effect of double-diffusive convection are shown to over-estimate the temperatures in the bottom of the solar pond. Incorporation of convective mixing is shown to have profound impacts on the overall stability of a solar pond, and demonstrates the need to actively manage the mixing and heat transfer to maintain stability and an insulating non-convective zone.     

Steady-state modeling limitations in solar pond design

The present work, based on comparative investigation of performance between steady-state and time-dependent analyses, allows the investigation of conditions under which steady state modeling leads to results close to those derived from time-dependent calculations. The analysis, which was based on the development of numerical computer models, allows one to determine the design, operational, and climatic parameters which affect the accuracy of steady-state calculations and to quantify steady-state modeling limitations. It is shown that, although steady-state models are powerful and comprehensive design tools for latitudes up to about 30°, they may lead to significant errors and underestimation of performance for higher latitudes and heat-extraction temperatures.     

Thermal analysis of honeycomb solar pond

A solar pond consisting of honeycomb panels placed on a thin layer (～ 1 cm) of silicone oil floating on the body of a hot water reservoir is considered and analysed for the heat transfer processes in the system. An explicit expression for the transient rate of heat extraction at constant temperature is derived to obtain the annual variation of retrieved heat flux. The year-round thermal performance of the system has been investigated. For a solar pond with a 10 cm high honeycomb structure, annual average efficiencies of 65, 48, 33 and 24% are predicted for retrieved heat flux at temperatures of 40, 60, 80 and 90°C, respectively. A comparison between honeycomb solar pond and salt-gradient solar pond is also presented.     

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.     

Studies on membrane viscosity stabilized solar pond

In this nonsalt type of solar pond, the nonconvecting layer is composed of a viscous polymer solution partitioned by a number of transparent films. An advantage of partitioning is that a thinner polymer solution can be used and that the light transmittance increases. Results of experimental and theoretical investigations on the performance of this solar pond are summarized as follows:

1. 1. Ionized polyacrylamide solution was chosen as the thickener based on tests about solubility, viscosity, light transmittance and stability.

2. 2. The critical temperature difference for the onset of convection in the polymer layer (ΔT/L)_cr [°C/m] was given by the following formula based on the measurements in various thicknesses of the polymer layers (L) [m] and various concentrations (ζ) [%],

(ΔT/l)_cr=(55−185lnL)exp(4.66L^0.505lnζ

3. 3. An outdoor model pond, 200 × 150 cm surface and 100 cm depth, was constructed in Osaka. Four types of model ponds were tested, and the availability of membrane type with partition films was confirmed.

4. 4. The theoretical temperature rise of the pond using a one-dimensional model was calculated by solving the equations of the heat balance in the pond. As a result, the optimum values of thickness of polymer layer and number of films was determined

     

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.