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.     

Analytical model of solar pond with heat exchanger

This paper presents an analytical model of a three zone solar pond with heat exchange pipes laid in its bottom convective zone. Explicit expressions for the transient rate of heat extraction and the temperature at which heat can be extracted are derived as a function of geometrical and operational parameters of the system. The transfer of heat from the pond bottom convective zone to the heat exchange fluid is expressed in terms of a heat removal factor, F_R. Analytical results, characteristic of the optimum performance of the pond, are presented and the criteria for the size and heat transfer characteristics of the heat exchanger are investigated. The annual average efficiency of heat extraction exhibits the asymptotic increase with the increase of length per unit pond area of heat exchange pipe.     

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.     

Reflecting bottom solar pond

This communication presents an investigation of a three zone solar pond with a diffusely reflecting bottom. The dynamic performance and its optimization as a large scale solar energy collection and long term storage system have been discussed. Numerical computations, corresponding to the data of solar radiation and ambient air temperature of New Delhi during 1974, have been made for two modes of operation, viz (i) constant flow rate of heat removal fluid, and (ii) zone of heat retrieval remaining at constant temperature. At a typical flow rate of 10^?3kg/m²s, optimal heat retrieval efficiencies of 32.4% (at non-convective zone depth, l₂, of 1.25 m), 25.2% (at l₂ = 0.75 m) and 21.5% (at l12 = 0.6 m) are predicted for bottom reflectivities of 0.0, 0.5 and 0.8, respectively. Furthermore, the variability in flow rate required to keep the temperature of the heat extraction zone constant is less for higher temperatures of the heat extraction zone, and the effect of R is found to be insignificant. In the mode of heat retrieval at constant temperature, the calculations of Sodha et al. [Energy Res.5, 321 (1981)] for the case of a completely absorbing surface with no bottom reflection (α = 1.0, R = 0.0) have yielded the optimal heat retrieval efficiency of 35.5 and 26.5% at extraction temperatures of 40 and 100°C, respectively. However, for the realistic case of a partially absorbing bottom (α = 0.9 and R = 0.0), these efficiencies are predicted to go down to 32.5 and 23.5%, respectively. The effect of bottom reflectivity on heat collection efficiency has further revealed the fact that the heat collection efficiency decreases with an increase in bottom reflectivity. For a typical value of R(= 0.6) corresponding to the realistic case of a partially absorbing bottom (α = 0.9), optimum efficiencies decrease further to 22.3 and 12.4% at collection temperatures of 40 and 100°C, respectively.     

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.     

Linear stability analysis of double-diffusive solar pond

In this communication, the stability of the double-diffusive solar ponds has been investigated in the linear approximation. The corresponding linearized system of equations of motion is reduced to a single integro-differential equation using the Green-function technique. In contrast to the conclusions of Veronis that, initially, the instability occurs as an oscillatory mode and at a value of R_T (Rayleigh number for temperature) greater than R_S the motion becomes steady, the present analysis shows that, initially, as R_T increases from zero but remains considerably less than R_S, exponentially growing and decaying modes (steady motion) occur first; for a value of R_T more than a critical value R_T^c, the motion becomes oscillatory. This oscillatory motion may, due to the basic non-linear dynamics of the system, even become aperiodic. Further, for R_S → ∞, the minimum value of R_T for which steady motions can occur tends to $\text{K}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{·R}{}_{\mathrm{S}}$, where $\text{K}\text{= K}{}_{\mathrm{S}}\text{/K}{}_{\mathrm{T}}$ where K_S and K_T are diffusivity coefficients for salt and temperature, respectively; as a contrast, according to Veronis, $\text{R}{}_{\mathrm{T}}\text{a}\text{?}\text{σ}{}^{\mathrm{?1}}\text{R}{}_{\mathrm{S}}\text{; σ = v/K}{}_{\mathrm{T}}\text{, v}$ being the kinematic viscosity.     

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.     

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.     

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.     

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.     

Investigation of solar pond surface zone temperature assumptions

This paper presents the results of tests concerning two assumptions about the surface zone of a solar pond. The first assumption is that the surface zone temperature of the solar pond is equal to the air dry bulb temperature, and the second one is that it is equal to the air wet bulb temperature. The surface zone temperature and the storage zone temperature are predicted by using a lumped-parameter model. The experimental results of the surface zone conform well with theoretical values. The results indicate that the air dry bulb temperature is more accurate in winter-time but that the errors generated by both assumptions are almost equal in the summer-time.     

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.     

Numerical model of a solar pond

This paper points out an idealization of considerable significance in a recent numerical model of a solar pond due to Wang and Akbarzadeh [3] and outlines the refinements required in the formulation of the above model. Typical temperature calculations from teh resultant model are also presented. of the above model. Typical temperature calculations from the resultant model are also presented.     

Performance investigation of a salt gradient solar pond coupled with desalination facility near the Dead Sea

Solar ponds provide the most convenient and least expensive option for heat storage for daily and seasonal cycles. This is particularly important for a desalination facility, if steady and constant water production is required. If, in addition to high storage capacity, other favorable conditions exist, the salt gradient solar ponds (SGSPs) are expected to be able to carry the entire load of a large-scale flash desalination plants without dependence upon supplementary sources. This paper presents a performance investigation of a SGSP coupled with desalination plant under Jordanian climatic conditions. This is particularly convenient in the Dead Sea region characterized by high solar radiation intensities, high ambient temperature most of the year, and by the availability of high concentration brine. It was found that a 3000 m² solar pond installed near the Dead Sea is able to provide an annual average production rate of 4.3 L min⁻¹ distilled water compared with 3.3 L min⁻¹ that would be produced by El Paso solar pond, which has the same surface area. Based on this study, solar ponds appear to be a feasible and an appropriate technology for water desalination near the Dead Sea in Jordan.     

Effect of thermal trap on the performance of an underground shallow solar pond water heater

A simple analysis of an underground shallow solar pond water heater has been presented. The effect of a thermal trap at the top of the system has also been incorporated in the analysis. Using the model, the effect of various system parameters, viz. thermal trap thickness, heat capacity of water mass, flow rate and duration of flow rate have been studied in detail. Numerical calculations have been made for a typical winter day at New Delhi (India). It is concluded that the system with thermal trap gives better performance in comparison with a system with a movable insulation system.     

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.     

Spectral calculation of the thermal performance of a solar pond and comparison of the results with experiments

This paper deals with a method and the result of the spectroscopic calculation on heat balance of a salt-gradient solar pond under the conditions of spectral solar radiation. Furthermore, reflection of the ray incident upon the surface of the pond water, refraction of the rays within the salt-water layer and diffusion of salt in the pond water are considered. On the other hand, in order to make a clear mechanism of the heat collection and heat storage of the solar pond, we conducted an indoor experiment and a numerical analysis on a small scale model of the salt-gradient solar pond with 2 m² surface area and 1.6 m depth, under the incident rays from a Xe-lamp solar simulator. According to the above experimental analysis, we made a simulation model of thermal performance for a solar pond and carried out the calculation from the heat balance. We found that the simulation calculations correspond well to the experimental results, so that our thermal simulation model and method might be correct. We also did the thermal calculation by changing the incident rays from a Xe-lamp into natural ray (Moon’s spectrum) and Halogen lamp. As a result, it was found that the temperature distributions in the solar pond were notably different due to spectral characteristics of the incident ray. Therefore, the spectroscopic consideration for thermal performance of any solar pond is necessary to obtain a correct solution under the spectral incidence with special wavelength distributions.     

Maintenance strategy for a salt gradient solar pond coupled with an evaporation pond

In a previous study, the authors presented a simple mathematical model for predicting the ratio of the evaporation pond area to that of a salt gradient solar pond area. The evaporation pond idea provides a very attractive method of salt recycling by evaporation, especially in areas of high evaporation and low rates of rain as it is the case for North Africa.In this paper, the model was elaborated upon and applied to two types of surface water flushing (fresh water and seawater) under the prevailing conditions of Tripoli, Libya (latitude=32.86°N). All the results presented were predicted for the first three years of operation. The daily variations of brine concentration in the of Tajoura's Experimental Solar pond and those based on different designs were predicted and discussed under different scenarios. The quantities of brine provided by the evaporation pond and that required by were predicted for both cases of surface water flushing (fresh water and seawater) under the different design conditions. It was predicted that the can provide 20–40% during the first year and 45–95% during the third year depending on the design selected.     

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.     

Evaluation of a heat transformer powered by a solar pond

A single-stage heat transformer operating with the water/lithium bromide mixture was operated to demonstrate the feasibility of the use of these systems to increase the temperature of the heat obtained from solar ponds. Electrical heaters at temperatures not higher than 80°C were used to simulate the heat input to an absorption heat transformer from a solar pond. Gross temperature lifts, useful heat and coefficients of performance are plotted for the heat transformer against temperatures and solution concentrations. Gross temperature lifts as high as 44°C were obtained. The maximum temperature of the useful heat produced by the heat transformer operating with the water/lithium bromide mixture was 124°C. The maximum coefficient of performance for the unit was 0.16.