Salinity Gradient Solar Pond Research Articles

Heat extraction and brine management from salinity gradient solar pond and membrane distillation

The problem associated with the reverse osmosis (RO) system is its brine disposal. Experimental salinity gradient solar pond (SGSP) with surface area 4.65m2 was established to extract and apply heat on laboratory scale direct contact membrane distillation (DCMD). In this study, the performance of the SGSP and DCMD system under Pakistan’s climatic condition of Islamabad was evaluated. The heat extraction was carried out using internal heat exchanger by passing fresh water through it at different flow rates in summer and winter. Maximum temperature of 37°C in summer and 28.5°C in winter was extracted. The extracted heat from SGSP can be used to pre-heat the brine for temperature driven desalination processes. Least drop in lower convective zone (LCZ) temperature of SGSP was observed at flow rate of 7.5L/min. In DCMD, two temperatures obtained from SGSP (28.5 and 37°C) at feed side were maintained to investigate the permeate flux, percentage salt rejection and total dissolve solids (TDS). Two further temperatures 50 and 60°C were maintained to investigate the DCMD performance. Flux increased as temperature difference between feed and permeate increased. 28.5°C in terms of SGSP temperature was also feasible for DCMD process as flux was maintained over time.

Experimental Analysis of Thermoelectric Heat Exchanger for Power Generation from Salinity Gradient Solar Pond Using Low-Grade Heat

Salinity gradient solar ponds act as an integrated thermal solar energy collector and storage system. The temperature difference between the upper convective zone and the lower convective zone of a salinity gradient solar pond can be in the range of 40-60°C. The temperature at the bottom of the pond can reach up to 90°C. Low-grade heat ( < 100°C) from solar ponds is currently converted into electricity by organic Rankine cycle engines. Thermoelectric generators can operate at very low temperature differences and can be a good candidate to replace organic Rankine cycle engines for power generation from salinity gradient solar ponds. The temperature difference in a solar pond can be used to power thermoelectric generators for electricity production. This paper presents an experimental investigation of a thermoelectric generators heat exchanger system designed to be powered by the hot water from the lower convective zone of a solar pond, and cold water from the upper convective zone of a solar pond. The results obtained have indicated significant prospects of such a system to generate power from low-grade heat for remote area power supply systems.

Efficiency and Thermal Analysis of a Salinity Gradient Solar Pond

Efficiency and Thermal Analysis of a Salinity Gradient Solar Pond

Sustainable zero liquid discharge desalination (SZLDD)

The main purpose of this study is to develop a sustainable zero liquid discharge desalination system. Direct contact membrane distillation unit is connected directly to salinity gradient solar pond (SGSP) to achieve zero liquid discharge desalination. The used system contains a hydrophobic microporous membrane module and a plastic pipe circulating all over the pond water surface to be used as a cooling system. The pipe also used as a wave suppression system as it is floating over the top of the pond water surface. The system is sourced by the hot and high concentrated saline water that is extracted from non-convective zone as a feed solution, then, the brine discharges at the lower convective zone of the solar pond. Therefore, if the saturated brine is used to produce salts, there will not be any brine left over which may lead to zero liquid discharge desalination. The system is modelled theoretically and solved by Matlab simulation program. It has been found that the system has the ability to deliver 52l/day of fresh water for m2 of membrane coupled with SGSP, consuming almost 11kW/m2 of thermal energy. Also, the trans-membrane coefficient of the used membrane is proved to be 0.001kg/m2/Pa/h. The results are analysed and the system performance is evaluated and presented in this paper.

A comprehensive transient model for the prediction of the temperature distribution in a solar pond under mediterranean conditions

Salinity gradient solar ponds can be used to store heat by trapping solar radiation. The heat can then be employed to drive various industrial applications that require low-grade heat. In this study, a comprehensive finite difference transient model has been developed incorporating many processes that affect the performance of a solar pond to predict the hourly temperature distribution. The model includes novel approaches to simulation of both the Heat Storage Zone (HSZ) and the Upper Convective Zone (UCZ) where in addition to convective, evaporative and radiative heat losses, the cooling effect of adding freshwater to the surface of the pond is taken into account. The HSZ is treated as one layer, with uniform temperature, in the finite difference method. A solar pond of 100m2 surface area is simulated for southern Turkey. The results indicate that, if the operation starts on the first day of June, the HSZ would take 65days to reach the boiling point while this would be 82days if the operation commences on the first day of December. The simulations highlight that 41–47l of freshwater will need to be supplied to the UCZ daily and the associated cooling effect of such addition is approximately 10 times larger than the convective heat loss in the first 65days of operation. In addition, as 22.4% of the incoming radiation in the form of long wavelength radiation, is absorbed within the top 1cm of the pond, there is a sharp increase in the temperature of the UCZ creating a hot-zone which slowly moves downwards to the Non-Convective Zone (NCZ) and eventually the HSZ. Hence, the HSZ does not initially prevail as the hottest zone in the pond. However, as the temperature rises and the pond approaches pseudo-steady state, the hot-zone slowly moves downwards and finally reaches the HSZ. This phenomenon is consistent with experimental studies and proves the imprecision of pseudo-steady state models. Furthermore, the HSZ becomes more resistant to losing the accumulated heat to the layers above as its temperature increases due to the better establishment of the NCZ as the insulator for the HSZ.

Comparatively study between single-phase and two-phase modes of energy extraction in a salinity-gradient solar pond power plant

The common process in all applications of a salinity-gradient solar pond (SGSP) is the energy extraction process using single-phase mode heat transfer with some limitations such as pumping the large amount of mass flow rate, and need for big size of heat exchanger. In every respect, two-phase mode heat transfer can be selected as an advantage due to its passive case of operation and comparatively high heat transfer capacity with rational system size. In this paper, an enhanced design of a large scale SGSP power plant using some two-phase closed thermosyphons has been simulated and compared with the single-phase mode heat transfer. The simulation results showed that the overall thermal efficiency of the solar pond power plant was the highest using both thermosyphons and heat exchangers.

Enhancing the efficiency of solar pond heat extraction by using both lateral and bottom heat exchangers

In this study, heat extraction from both the gradient and heat storage zones of a salinity-gradient solar pond (SGSP) has been evaluated. For this purpose, an experimental solar pond pilot plant was constructed in 2009 in Barcelona (Spain). The structure of the pond is a cylindrical tank of 3-m height and 8m diameter with a total area of 50m2. The main objective was to evaluate a heat-extraction system from the SGSP designed to enhance the system efficiency under different conditions. Thus, an in-pond heat exchanger covering all of the lateral wall area of the pond was installed, and its performance was compared with the traditional in-pond heat exchanger situated on the bottom of the pond. Heat extraction experiments were performed using both heat exchangers individually or both at the same time. The study covers the experiments performed at three different seasonal temperature conditions: winter (December), summer (July) and autumn (October and November). The variations of the temperature inside the pond during the heat extraction were measured and analyzed. The results demonstrated that the efficiency of the pond increases when the heat is removed from the lateral heat exchanger alone compared to either using the bottom heat exchanger or using both heat exchangers simultaneously.

New theoretical modelling of heat transfer in solar ponds

Solar energy has a promising future as one of the most important types of renewable energy. Solar ponds can be an effective way of capturing and storing this energy. A new theoretical model for a heat transfer in a salinity gradient solar pond has been developed. The model is based on the energy balance for each zone of the pond; three separate zones have been considered, namely the upper convective zone, the lower convective zones, as well as the non-convective zone. The upper and lower zones are considered to be well mixed, which means the temperatures in these zones are uniform. The model shows that the temperature in the storage zone can reach more than 90°C during the summer season whereas it can be more than 50°C in winter if the pond is located in the Middle East. In addition, the time dependent temperature for the three layers has been found. Furthermore, it is concluded that heat loss from the pond’s surface occurs mainly by evaporation, in comparison to convection and radiation. Heat loss to the ground has been calculated by using three different equations. It was found that the perimeter of the pond has a significant effect on heat loss to the ground from a small pond, while its effect is small in the case of large pond. The validity of the model is tested against experimental data for several established ponds; good agreement is observed.

Numerical Investigations of Coupling a Vacuum Membrane Desalination System with a Salt Gradient Solar Pond

This work proposes new configurations for the desalination of salt water using systems based on coupling of membrane distillation with solar energy. This study is a comparison between two coupling configurations of the vacuum membrane distillation (VMD) hollow fiber module with salinity gradient solar pond (SGSP). The first configuration is a module membrane in series with SGSP and the second one is a hollow fiber module immersed in the SGSP. Two models describing the heat and mass transfer in the hollow fiber module and in the SGSP will be developed. The coupling of the two models allows the determination of the instantaneous variation of temperature and salinity in the SGSP and the permeate flow variation. A comparison of each module production was carried out. The mathematical model shows that the immersed module production presents more than one and a half times that of the separated module, their production reached 75 kg.day-1 per m² of the membrane in the third year. Thus, immersing the module in the solar pond improves the performance of the hollow fiber module.

Simulation of Solar Air-Conditioning System with Salinity Gradient Solar Pond

In hot dry climates, due to the high demand for space air conditioning during summer and the abundance of solar radiation, solar air conditioning is a promising approach to reduce the energy consumption and negative environmental impact of buildings. Solar cooling systems have used various types of collectors to drive chillers. In this paper, a salinity gradient solar pond is suggested as a collector to drive an absorption chiller, to provide cool air for a house during hot and dry weather. A coupled simulation between MATLAB and TRNSYS has been used to solve the problem. MATLAB code was written to solve the governing equations for the salinity gradient solar pond and the ground underneath it. TRNSYS software was used to model the solar cooling system including the absorption chiller and building. The weather data used was for Baghdad in Iraq. It was found that the salinity gradient solar pond could be used to drive the absorption chiller and produce cool air for a single family house during the summer period. Different solar pond areas were tested with the same chiller capacity. It was found that a solar pond area of approximately 400 m2 was required to provide satisfactory cooling for a typical house with a floor area of approximately 125 m2.

Experimental studies on the effect of using phase change material in salinity-gradient solar pond

Performance of adding phase change material (PCM) to small salinity-gradient solar pond was studied experimentally. Two ponds with an area of 3.4m2 and depth of 1m were chosen. Horizontal cylindrical capsules containing Paraffin Wax as PCM were used in one of the pond’s lower convective zone. The capsules were made of black iron whose thermal conductivity was about 80W/m°C and its height and diameter were 0.5m and 75mm respectively. The study was covered within period of summer for 6weeks. An internal heat exchanger was used in the lower convective zone of pond. The hourly as well as daily variation of temperatures of the storage zone, surface zone, ambient, inlet and outlet of the internal heat exchanger have been measured and analyzed for ponds. It was shown the assisted PCM pond decreases the temperature difference between night and day. In addition, the pond had more thermal stability over environmental condition and more thermal and salinity stability on heat extraction. Hence the maximum temperature of the pond, outlet water temperature of heat exchanger, and, accordingly, the thermal efficiency decreased by using PCM. However, the heat exchanger outlet water temperature and, accordingly, the thermal efficiency, can be constant by increasing the length of the heat exchanger pipe. Therefore, based on this study, in such applications which one needs lower temperature difference and more uniform temperature, or where applications need a specific temperature range or controlled maximum temperature, PCM with a suitable melting point might be beneficial.

Numerical temperature and concentration distributions in an insulated salinity gradient solar pond

In this paper, the temperature and concentration distributions in an insulated salinity gradient solar pond are studied numerically in transient regime. The dimensionless governing equations are solved by finite-volume method using SIMPLER algorithm with HYBRID scheme. The pond is filled with a mixture of salt and water to form three zones: upper convective zone (UCZ), non-convective zone (NCZ) and lower convective zone (LCZ). The vertical walls and the bottom of the pond are thermally insulated. The bottom of the pond is black painted. The simulation of the insulated salinity gradient solar pond shows the existence of salt diffusion from the bottom to the free surface. This numerical study shows also that the buoyancy ratio increases and decreases the temperature and concentration in the LCZ and in the UCZ, respectively.

An experimental review on coupling of solar pond with membrane distillation

The interest of using solar powered membrane distillation systems for desalination is growing worldwide due to the membrane distillation (MD) attractive features. This study experimentally investigates the utilization of direct contact membrane distillation (DCMD) coupled to a salinity-gradient solar pond (SGSP) for sustainable freshwater production and reduction of brine footprint on the environment. A mathematical model for heat and mass flux in the DCMD module and thermal model for SGSP are developed and coupled to evaluate the feasibility of freshwater production. The experiment results on RMIT University SGSP coupled with DCMD are presented. The feed stream of 1.3% salinity is heated up by the SGSP and circulated through DCMD module then injected to an evaporation pond. Also, a thermal energy system is used to recover heat from the outlet brine stream of DCMD and use it as preheating for inlet feed water stream. Results are compared and showed that if the flow is laminar, the connecting DCMD module to the SGSP could induce a marked concentration and temperature polarisation phenomenon that reduces fluxes. Therefor turbulence has to be created in the feed stream to reduce polarisation. Also, to reduce the environmental footprint, the brine is recirculated after passing through the heat exchanger.

Thermal Heat Storage Gain of Salinity Gradient Solar Pond Using Evacuated Tube Solar Collectors

This paper investigates the capability of running a system which uses hot fluid from solar evacuated tube collectors to boost the temperature and overall heat storage of the solar pond. The system is circulated by a solar powered pump, producing heat energy entirely from the incoming solar radiation from the sun. Solar evacuated tube collectors use a renewable source of power directly from the sun to heat the working fluid to very high temperatures. Solar ponds are emerging on the renewable energy scene with the capacity to provide a simple and inexpensive thermal storage for the production of heat on a large scale. The results of the performance of the system show a significant heat energy increase into the solar ponds lower convective region, increasing the overall performance of the solar pond.

The Bubble-Greenhouse: A holistic sustainable approach to small-scale water desalination in remote regions

The Bubble-Greenhouse system combines the well established Seawater Greenhouse concept with a novel humidification–dehumidification (HD) process, based on the large air/water interface generated by bubbling air through a water filled column. Multistage bubble evaporators and multistage bubble condensers allow for effective recovery and reuse of latent heat via a heating/cooling circuit throughout all column stages. The system can operate with salinities of 5000–35,000ppm. Following the HD process, cooled vapour provides the tropical type greenhouse with a humid environment for selected crops. Additional condensation occurs along the greenhouse skin and is gravity-fed to drip line irrigation. Low grade energy options such as solar-thermal, photovoltaic, wind, geo-thermal and salinity-gradient solar ponds provide the energy for the Bubble-Greenhouse. Alternatively, waste heat from diesel power stations nearby can provide cogeneration of electricity and bubble evaporator heat and pressure requirements. Crops grown inside a greenhouse demonstrate a strongly reduced water demand and the closed environment protects crops from insects and diseases. As the technology is conceptually simple to implement, it holds great potential for community participation, empowerment, skills development and capacity building of local people in remote locations.

Extracting lithium from Tibetan Dangxiong Tso Salt Lake of carbonate type by using geothermal salinity-gradient solar pond

Given their unique resources, the carbonate salt lakes in Tibet offer significant advantages in the extraction of lithium, but some disadvantages are being exposed in production. These disadvantages include the long period of lithium extraction and the low grade of lithium carbonate caused by geographical conditions and climate factors of the Qinghai–Tibet plateau. Based on the above reasons, we rebuilt the traditional solar ponds by introducing a kind of heat exchanger in a bid to speed up the temperature rise of brine, whereby we planned to make use of the high-temperature geothermal resources around the lakes to produce lithium carbonate in large-scale applications. We carried out a small lithium-extraction experiment with hot water as the heat source for the purpose of verifying the feasibility of lithium extraction by geothermal-salinity gradient solar pond (G-SGSP). In the experiment, the contrastive analysis method was employed as a means to study the relationship between ion diffusion and temperature, as well as salinity in G-SGSP and these controllable factors’ impact on the precipitation of lithium carbonate. The results of the experiment show that it is highly feasible to extract lithium through G-SGSP, thus enhancing the efficiency of lithium extraction and generating higher-grade lithium carbonate. Thus, the experiment serves as a dramatic reference for extracting lithium from Dangxiong Tso Salt Lake through the application of geothermal resources.

Exergy variations of the gradient layer in a shallow solar pond

Abstract. A salinity gradient solar pond can act as a solar thermal energy storage source for all seasons of the year and in all weather conditions. Solar pond layers exergy can be considered as the potential for getting the maximum amount of work done. Exergy in a gradient layer was studied based on the experimental data of the years 1997 and 1998 for 229 days in a solar pond in Ferdowsi university of Mashhad. The research revealed that variations of specific exergy (exergy per unit mass) for gradient layer, is affected by temperature variations of lower and upper convective layers. The specific exergy reached its maximum in mid-August, but the exergy per pond unit area in the early period of that month reached the maximum amount, because in addition to the temperature variations of the lower and upper convective layers, the time variations of the gradient layer thickness also affect exergy variations per unit area.

A Simple Heat and Mass Transfer Model for Salt Gradient Solar Ponds

A salinity gradient solar pond is a free energy source system for collecting, converting and storing solar energy as heat. In this paper, the principles of solar pond are explained. A mathematical model is developed to describe and simulate heat and mass transfer behaviour of salinity gradient solar pond. MATLAB codes are programmed to solve the one dimensional finite difference method for heat and mass transfer equations. Temperature profiles and concentration distributions are calculated. The numerical results are validated with experimental data and the results are found to be in good agreement. Keywords—Finite Difference method, Salt-gradient solar-pond, Solar energy, Transient heat and mass transfer. I. INTRODUCTION HE solar pond is described as an artificial large body of water reservoir that collects and stores solar energy. It is about 1 to 3 meters deep, and the bottom of the pond is usually painted black. The solar radiation landing on the surface of the pond penetrates the liquid and falls on the blackened bottom which is thereby heated. If the liquid is homogeneous which means no density gradient, convection currents will be set up and the heated liquid being lighter will travel towards the surface and dissipate its heat to the atmosphere. In a solar pond these convection currents are prevented by having a concentration gradient of salt, the solution's concentration and density being highest at the bottom and lowest at the top. Typically ponds are composed of three zones as shown in Fig.1. The first zone is Upper Convective Zone (UCZ), which is cold, close to the ambient temperature; and has low salt concentration (almost fresh water).The thickness of this surface layer varies from 0.1 to 0.4m. The second zone is Non-Convective Zone (NCZ) or (insulation layer), which has salt density increasing with depth. The thickness of NCZ ranges from 0.6 to 1.0m. Hot water in one layer of NCZ cannot rise because of its high relative density (due to its salt content) and water above is lighter (low density). Similarly, water cannot fall because the water below it has a higher salt content and is heavier (high density). Therefore convection motions are hindered and heat transfer from the hot third zone,

Transient hydrodynamic, heat and mass transfer in a salinity gradient solar pond: A numerical study

The impoverishment of our planet in non-renewable energies has incited researchers to design salinity gradient solar ponds to collect and store solar energy at a lower cost. It is in this context that the present research work lies to focus on the numerical study of the transient hydrodynamic, heat and mass transfer in a salinity gradient solar pond. The problem is tackled using the dimensionless governing equations of Navier–Stokes, thermal energy and mass transfer, which are solved numerically by finite-volume method to provide the temperature, concentration and velocity fields in transient regime. The pond is filled with salty water of various salinities to form three zones of salty water: Upper Convective Zone (UCZ), Non-Convective Zone (NCZ) and Lower Convective Zone (LCZ). To prevent convective movements induced by the internal heating of salty water due to solar radiation absorption, a salinity gradient is used in the solar pond. Representative results illustrating the influence of internal Rayleigh number on the thermal performance of the pond and the effect of the aspect ratio on the distribution of temperature and velocity fields in the salinity gradient solar pond (SGSP) are discussed. In addition, results for the transient average temperature of UCZ and LCZ are presented and discussed for various parametric conditions.

Electric Power Generation from Solar Pond Using Combination of Thermosyphon and Thermoelectric Modules

Salinity-gradient solar pond is one type of solar collector with the ability to store thermal energy for long period of time and lower cost of construction compared with the other type of solar collector. It can collect and store solar heat at temperatures up to 80°C. A system in which heat from the lower zone is transferred to the hot surface of the thermoelectric modules using gravity- assisted heat pipes as thermosyphons has been investigated experimentally.The temperature difference between the lower convective zone and the upper convective zone is applied across the hot and cold surfaces of the thermoelectric modules. In the salinity gradient solar pond, a insulated solar pond with a surface area of 7 m2 and a depth of 1.3 m was built at Rajamangala University of Technology Isan Khon Kaen Campus,Thailand to conduct performance experiments. From the results of experiment by using water as working fluid, the temperature of the solar pond in lower convective zone is at 50°C.It can be seen that the thermoelectric is able to generate electricity at 36.25mV. Using R134a as a working fluid, the temperature of heat pond in lower convective zone is at 41°C.Due to this, thermoelectric generates electricity at 234.25mV.. Research results in the present work indicate that there is a significant potential for electric power generation from small solar ponds through a simple and passive device incorporating thermosyphons and thermoelectric cells.