Electric power generation from solar pond using combined thermosyphon and thermoelectric modules

Salinity-gradient solar ponds can collect and store solar heat at temperatures up to 80 °C. As a result, these water bodies act as a renewable source of low grade heat which can be utilized for heating and power generation applications. In this paper, design and test result of the combined system of thermosyphon and thermoelectric modules (TTMs) for the generation of electricity from low grade thermal sources like solar pond is discussed. In solar ponds, temperature difference in the range 40-60 °C is available between the lower convective zone (LCZ) and the upper convective zone (UCZ) which can be applied across the hot and cold surfaces of the thermoelectric modules to make it work as a power generator. The designed system utilizes gravity assisted thermosyphon to transfer heat from the hot bottom to the cold top of the solar pond. Thermoelectric cells (TECs) are attached to the top end of the thermosyphon which lies in the UCZ thereby maintaining differential temperature across them. A laboratory scale model based on the proposed combination of thermosyphon and thermoelectric cells was fabricated and tested under the temperature differences that exist in the solar ponds. Result outcomes from the TTM prototype have indicated significant prospects of such system for power generation from low grade heat sources particularly for remote area power supply. A potential advantage of such a system is its ability to continue to provide useful power output at night time or on cloudy days because of the thermal storage capability of the solar pond.     

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.     

Performance evaluation of a small‐scale sodium carbonate salt gradient solar pond

In this study, thermal performance of the salt gradient solar pond (SGSP), which of density gradient is artificially with sodium carbonate solution, was tested under Karabuk prevailing weather conditions in Turkey. A small‐scale prismatic glass tank was constructed with an area of 0.45 × 0.20 m² and a depth of 0.25 m as solar pond. A series of experiments with four different density levels were conducted in July–August 2004. The variations of the temperature and density profiles were observed for each of experiment for a week. It was found that the maximum temperature difference between the bottom and surface of the pond is 21°C and maximum temperature in the lower convective zone (LCZ) has been measured as 49°C at the first experiment. The efficiency of the pond was evaluated 13.33% weekly mean radiation intensity of 524 W m^?2 for the first experiment. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

Experimental study of natural brine solar ponds in Tibet

A salinity gradient solar pond (SGSP) is a simple and effective way of capturing and storing solar energy. The Qinghai-Tibet Plateau has very good solar energy resources and very rich salt lake brine resources. It lacks energy for its mineral processes and is therefore an ideal location for the development and operation of solar ponds. In China, solar ponds have been widely applied for aquaculture, in the production of Glauber’s salt and in the production of lithium carbonate from salt lake. As part of an experimental study, a SGSP using the natural brine of Zabuye salt lake in the Tibet plateau has been constructed. The pond has an area of 2500 m² and is 1.9 m deep. The solar pond started operation in spring when the ambient temperature was very low and has operated steadily for 105 days, with the LCZ temperature varying between 20 and 40 °C. During the experimental study, the lower convective zone (LCZ) of the pond reached a maximum temperature of 39.1 °C. The results show that solar ponds can be operated successfully at the Qinghai-Tibet plateau and can be applied to the production of minerals.     

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.     

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.     

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.     

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.     

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.     

A parametric study on solar ponds

A linear relation between the efficiency of solar ponds and factor $\text{θ}{}_{\mathrm{d}}\text{H}$ which is the temperature difference between the pond bottom and the ambient divided by the average insolation is presented. This relation, which has been developed based on a steady state analysis provides valuable information on the relative importance of the parameters involved in the operation of solar ponds. It is found that the existence and the thickness of the top convective zone has a profound negative effect on the yield of solar ponds. The optimum thickness of the density gradient layer under various conditions is also presented. The effect of ground losses is discussed, and it is shown that for the case of wet soil, especially if the level of the underground water is high, the pond should be thermally insulated. It is also shown that the steady state analysis can predict with good accuracy the yearly average response of solar ponds under transient conditions.     

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.     

Performance characteristics of solar ponds operating at different latitudes

The results of a comparative study of the performance of solar ponds operating at different latitudes are presented in this paper. A mathematical tool was developed to study the behaviour of solar ponds operating under different physical and operational conditions. The hourly meteorological data of Kew and Singapore were used. The pond operating at higher latitudes, i.e. Kew, where seasonal variations are significant, must be deeper than a pond operating near the equator. The deeper pond can also act as an interseasonal storage device. A pond operating at a location in Singapore attains a fairly high temperature but the temperature requirements are also high for the desired applications such as space cooling in conjunction with a refrigeration cycle. Although a pond at Kew attains a lower temperature compared with a pond at Singapore, it can supply considerable amounts of thermal energy to support a space heating load. Both the ponds require auxiliary heating, the magnitude being dependent upon the nature of the load.     

Field testing on nonsalt solar ponds

A new type of nonsalt solar pond was investigated by field testing. The roof of the solar pond was formed using a transparent double film. Three kinds of tests were carried out under the following conditions: (1) insulating pellets were packed between the layers of the transparent double film of the roof at sunset; (2) the water surface of the pond was insulated using only the two transparent films; (3) the water surface of the pond was covered by the double film with the top surface blackened on which solar energy can be collected, while pond water was circulated using a solar cell powered submerged water pump. The warm water stored in the solar pond by the above methods was utilized as a heat source for a gas engine powered heat pump used to heat a greenhouse. In this report, the results of the field tests on the above solar ponds and greenhouse heating system are discussed. Also the utility of a combination plant using a solar pond and underground borehole storage system is evaluated.Important conclusions on performance are as follows: (1) collection efficiencies of these solar ponds become 9–54% corresponding to the weather conditions and pond temperatures; (2) maximum temperature of the pond water under weather conditions at Osaka is about 80°C; (3) the solar pond can be effectively utilized for heating a greenhouse; (4) the combination plant using the solar pond and the underground storage layer can store heat of 1119 MJ m⁻² yr⁻¹.     

The self-generation mechanism of a laboratory scale saturated solar pond

Laboratory simulation systems of saturated solar ponds using potash alum solutions were operated; the insulated columns 100 cm deep were heated at the bottom by heat exchangers with varying energy output. Self-generation of layered structure from homogeneous solution was observed. The temperature, density and supersaturation gradients, and the disappearance of convective waves indicated the relative stability of the layered structure. Attaining supersaturation of the surface zone, massive crystallization, and buildup of density gradient, which follows the temperature gradient, are the necessary conditions for the self-generation and self-maintenance of the laboratory pond.     

Mathematical modelling of salt gradient solar pond performance

This paper presents a mathematical model of the performance of the salt gradient solar pond. A lumped parameter model of the upper convective zone, non-convective zone and lower convective zone is used. This model enables the temperatures of the upper-convective zone and the lower convective zone of the solar pond to be predicted. The experimental results agree well with theoretically predicted values. The major error in the theoretical results is due to the difference between the theoretical value of the solar radiation inside the water and that observed experimentally. It is found that the experimental value of the solar radiation at a depth of 90 cm is approximately 26 per cent of the total solar radiation falling on the solar pond surface, whereas the corresponding theoretical value is found to be 33 per cent. The results conclude that the lumped parameter model can be used as a simple model to predict the performance of the solar pond.     

An investigation of rain and wind effects on thermal stability of large-area saltpan solar ponds

An investigation of the thermal stability of large area saltpan solar ponds under different climatic conditions is presented. The study focuses on time taken by the pond to reach its stable conditions with heavy rainfall and the effect of wind-mixing process for the stability of the pond. Investigations were carried out over a period of 60 days on a large-area solar pond of 90 cm deep. The temperature and density profiles obtained 34 days after filling showed that the pond had attained its stability with a bottom temperature of 63 °C. Results reveal that heavy rainfall is the prime cause for the pond to reach stability in a time period of about 30 days. Strong wind-induced mixing prevailed during the second half of the investigation, which contributed to the erosion of the nonconvecting zone is the cause for observed destabilization of the pond. The estimated critical wind speed for complete destruction of the nonconvecting zone is about 25 km/h.     

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.     

Performance of Shallow Solar Ponds

This investigation is devoted to study the performance of a shallow solar pond composed of two zones, a convective zone and a non-convective zone. Finite difference techniques and a comprehensive iterative procedure were employed to solve the realistic transient energy equations subjected to temperature dependent heat transfer coefficients. Actual solar insolation, wind speed, and ambient temperature were used to evaluate the performance of this shallow solar pond at various time intervals for Alexandria all the year round. Lastly, the effect of varying the percentage of the non-convective zone was also investigated under various environmental and operating conditions.     

Brine clarity maintenance in salinity-gradient solar ponds

Brine transparency is an important part of the maintenance of a salinity-gradient solar pond as it affects the amount of solar radiation reaching the storage zone and hence has an influence on the thermal performance. There is a wide range of factors that can hinder the transmission of light in a solar pond. Algal and microbial growths are the most common problems encountered in working solar ponds and control of their densities is essential to maintain transparency. Two different chemical treatment methods for algae growth prevention are described in this paper: chlorine and a novel chemical product – copper ethylamine complex. The latter method has never been implemented previously in a working pond. This paper discusses the theory of the algae control methods used and presents the experimental results of the chemical treatments. The results showed that Cupricide is more effective than chlorine and is therefore the recommended chemical for algae control in solar ponds; it improves the water transparency especially in the upper convective zone and lower convective zone with all measurement values less than 1 NTU. Chlorine was found to be more corrosive than Cupricide due to the acidic effect it has on the pH. The preliminary cost analysis showed that granular chlorine is the cheapest chemical. A more detailed financial analysis is nevertheless required to refine these costs.