Article

Temperature evolution of an experimental salt-gradient solar pond

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Abstract

A salt-gradient solar pond is a low-cost, large-scale solar collector with integrated storage that can be used as a source of energy in low-grade-heat thermal desalination systems. This work presents the thermal evolution of an experimental solar pond for both the maturation and heat extraction time periods. The temperature profile was measured every 1.1 cm using a vertical high-resolution distributed temperature sensing (DTS) system, with a temperature resolution of 0.041C. Temperatures of 34 and 451C were achieved in the bottom of the pond when the lights were on for 12 and 24 hours per day, respectively. Heat was extracted at a rate of 139 W from the solar pond, which corresponded to an efficiency of 29%. Stratification and mixing were clearly observed inside the solar pond using the vertical high-resolution DTS system.

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... A salt-gradient solar pond is a non-traditional solar collector that can provide long-term thermal storage and recovery for the collected energy. It is an artificially stratified water body that consists of three distinct zones (Suárez et al., 2010a): the upper convective zone, which is a thin layer of cooler and fresher water; the non-convective zone, comprised of a salt-gradient to suppress global circulation within the pond; and the lower-convective zone, in which salinity and temperature are the highest. The solar radiation that reaches the bottom of the pond is transformed into thermal energy and warms the brine in the lower convective zone. ...
... The hot brine in the lower convective zone may then be used directly for heating (Rabl and Nielsen, 1975), thermal desalination (Lu et al., 2001; Suárez et al., 2010c), or for other low-temperature thermal applications (Kumar and Kishore, 1999). To investigate sustainable freshwater production using thermal desalination powered with solar energy, Suárez et al. (2010a Suárez et al. ( , 2011) constructed a 1.0-m depth experimental salt-gradient solar pond. The pond was built inside a laboratory operated under controlled conditions, and was initially exposed to artificial lights 12 h per day. ...
... 8. Temperature profile in the salt-gradient solar pond at the beginning and at the end of a heat extraction experiment. Modified from Suárez et al. (2010a) Suárez et al. (2010a) also presented results of heat extraction in the experimental saltgradient solar pond. As shown inFig. ...
... There have been many investigations performed with small-or pilot-scale solar ponds [13][14][15][16], where more controlled conditions can be achieved (especially in indoor settings). Dah et al. [14] built a small-scale solar pond inside a laboratory to study the development of the temperature and salinity profiles in the absence of wind. ...
... In other investigation, Husain et al. [11] studied the inclusion of an additional salt-gradient zone between the non-convective zone and the upper convective zone. This additional layer had a thickness of 50 mm and comprised a sharper salt gradient than that used in typical non-convective zones [1,16]. Using an innovative design, Husain et al. [11] theoretically found that the temperatures in the lower convective zone should increase from 70 to 90°C. ...
... Investigations at smaller scales have proven to be useful when trying to improve the understanding of how different factors, such as ground heat losses, solar radiation, algae growth, heat extraction method, and salt type, affect solar pond efficiency [5,[11][12][13][14][15][16][17][18][19]30], but these investigations do not necessarily enable prediction of largescale solar pond performance. For example, heat losses through the sidewalls of large-scale solar ponds typically are negligible because the area of the sidewalls is small compared to the area of the bottom of the pond. ...
Article
Solar ponds are low-cost, large-scale solar collectors with integrated storage that can be used as an energy source in many thermal systems. Experimental solar pond investigations at smaller scales have proven to be useful when trying to understand how different factors affect the pond’s efficiency, but they do not necessarily represent the expected performance of large-scale solar ponds. Consequently, it is important to investigate how the results of small-scale solar pond experiments can be scaled up. In this work, we show how models based on laboratory-scale observations can be utilized to understand the expected performance of large-scale solar ponds. This paper presents an approach that combines high-resolution thermal observations with computational fluid dynamics to investigate how different physical processes affect solar pond performance at different scales. The main factors that result in differences between small- and large-scale solar pond performances are boundary effects, light radiation spectrum and intensity, and turbidity. Boundary effects (e.g., pond geometry, thermal insulation) reduce the energy that reaches the storage zone of small-scale solar ponds. Different types of lights result in different radiation spectrum and intensity, which affects the energy reaching the storage zone. Turbidity is typically not important in small-scale solar ponds subject to controlled environmental conditions. However, it is an important factor in outdoor solar ponds in which the pond is prone to particles that can deposit onto the water surface or become suspended in the gradient zone. In general, the combination of these factors results in less energy collected in small-scale solar ponds than in large-scale solar ponds, even though large-scale solar ponds are typically subject to more extreme environmental conditions. High-resolution thermal observations combined with numerical simulations to understand the expected performance of large-scale solar ponds seems to be a promising tool for improving both efficiency and operation of these solar energy systems.
... These heat losses can be easily assessed using the DTS system (e.g., by evaluating the thermal gradient measured near the bottom). For a detailed interpretation of the physics observed in this experiment, the reader is referred to the work of Suárez et al. (2010c). Figure 5 shows the temperature profiles using the DTS system and using EC probes for different days of the experiment. ...
... Furthermore, the vertical high-resolution DTS system shows that the interface between the non-convective and lower convective zones is located deeper than the desired level (at ∼0.65 m depth). This reduction of the lower convective zone occurred when constructing the salt gradient (Suárez et al., 2010c). If the temperatures measured with the EC probes are analyzed, the lower convective zone is not as clear as in the DTS measurements because almost all the data points along the water column are scattered between 23 and 24 • C (without considering the probe located at ∼0.4 m depth). ...
Article
Full-text available
In shallow thermohaline-driven lakes it is important to measure temperature on fine spatial and temporal scales to detect stratification or different hydrodynamic regimes. Raman spectra distributed temperature sensing (DTS) is an approach available to provide high spatial and temporal temperature resolution. A vertical high-resolution DTS system was constructed to overcome the problems of typical methods used in the past, i.e., without disturbing the water column, and with resistance to corrosive environments. This paper describes a method to quantitatively assess accuracy, precision and other limitations of DTS systems to fully utilize the capacity of this technology, with a focus on vertical high-resolution to measure temperatures in shallow thermohaline environments. It also presents a new method to manually calibrate temperatures along the optical fiber achieving significant improved resolution. The vertical high-resolution DTS system is used to monitor the thermal behavior of a salt-gradient solar pond, which is an engineered shallow thermohaline system that allows collection and storage of solar energy for a long period of time. The vertical high-resolution DTS system monitors the temperature profile each 1.1 cm vertically and in time averages as small as 10 s. Temperature resolution as low as 0.035 °C is obtained when the data are collected at 5-min intervals.
... This technology enables monitoring temperature at large extensions, offering an alternative for continuous measurements at a wide range of spatial scales. Recently, DTS methods have also been used in laboratory-scale SGSPs to monitor the thermal dynamics within the pond at high spatial and temporal resolutions (0.011 m and 5 min, respectively), with a thermal resolution on the order of ∼0.035°C (Suárez et al., 2010b(Suárez et al., , 2015Ruskowitz et al., 2014;Amigo et al., 2017). However, DTS methods have not been used as an operating tool that can help to decide how to maintain the density gradient within an SGSP and thus, to improve the operation of these systems. ...
... For instance, Suárez et al. (2010a) showed that the elevation of the UCZ-NCZ interface changed gradually but more quickly than that of the NCZ-LCZ interface, most likely due to the influence of the environmental conditions over the UCZ. On the other hand, Suárez et al. (2010b) measured an increase in the UCZ thickness of twice it initial size, whereas the thickness of the LCZ remained nearly constant. In the current experiments, as shown in Fig. 4, the temperature in the UCZ is more variable than in the LCZ. ...
Article
Salt-gradient solar ponds (SGSPs) collect and store solar radiation as thermal energy. This thermal energy is stored in the pond’s bottom because of the existence of the non-convective zone (NCZ), a layer comprised by a salinity gradient that results in a stable density profile, which suppresses global convection within the pond. As a consequence, the NCZ is the most important layer of an SGSP that must be maintained to sustain the internal structure of the pond and to allow successful thermal energy storage. The NCZ is characterized by a thermal gradient and thus, the internal structure of an SGSP can be inferred using temperature measurements. In this work, fiber-optic distributed temperature sensing (DTS) methods are systematically assessed in a laboratory-scale SGSP, and a simple interface tracking algorithm to determine the NCZ evolution – based on thermal measurements – is presented. To evaluate this algorithm, a DTS system and a discrete array of 14 point-in-space temperature loggers were used to record temperatures in the laboratory-scale SGSP. Acceptable results regarding the NCZ evolution were achieved with the discrete array of sensors. However, much better results were obtained with the DTS high-spatial resolution measurements, despite the presence of some artificial thermal oscillations in the DTS records. These oscillations did not affect the results of the interface tracking algorithm, but may be a concern on large-scale field installations. It was also found that the location of the NCZ boundaries can be determined even with low spatial resolution measurements. However, this determination can only be achieved using the proper interpolation method.
... Rowe and Shoaib, 2017). The high temperatures associated with these ponds are due to the insulation of 19 the lower convective zone of the ponds by the non-convective salt gradient zone above(Suárez et al., 2010). As a result of this, cracks develop, and the swelling of the GCL diminishes on rehydration. ...
Article
Bentonite clay is widely used as a hydraulic barrier in geosynthetic clay liners (GCLs) due to low hydraulic conductivity but under the aggressive environment, such as interaction with electrolytic solutions, higher temperature variations, and the number of wet and dry cycles, the performance of bentonite deteriorates. A polymerised bentonite clay (Hyper clay) has shown better performance than untreated bentonite under these conditions, possibly due to better water retention, thus highlighting the need to investigate the water retention capacity. Therefore, the water retention behaviour of the Hyper clay GCLs is studied and compared with untreated clay GCLs. The article presents the soil water retention curves of untreated bentonite GCL and Hyper clay GCLs treated with 8% carboxymethyl cellulose measured by filter paper test. Different electrolytic solutions were used as permeation fluid along with deionised water to represent a more aggressive environment. The results showed that the Hyper clay GCL has 7% higher water retention capacity near to air entry value than untreated clay GCL in deionized water. In other words, the Hyper clay GCL can be considered as a potential alternative to conventional bentonite GCL due to its improved water retention capacity.
... An experimental salt-gradient solar pond was constructed to investigate sustainable water production using solar energy. To have controlled conditions, the solar pond was built inside a laboratory and was subject to high-density discharge lamps that were turned on for 12 hours per day [20]. The pond was instrumented with a variety of sensors to monitor its performance, including a vertical high-resolution DTS system [16]. ...
Article
Full-text available
Hydrologic research is a very demanding application of fiber-optic distributed temperature sensing (DTS) in terms of precision, accuracy and calibration. The physics behind the most frequently used DTS instruments are considered as they apply to four calibration methods for single-ended DTS installations. The new methods presented are more accurate than the instrument-calibrated data, achieving accuracies on the order of tenths of a degree root mean square error (RMSE) and mean bias. Effects of localized non-uniformities that violate the assumptions of single-ended calibration data are explored and quantified. Experimental design considerations such as selection of integration times or selection of the length of the reference sections are discussed, and the impacts of these considerations on calibrated temperatures are explored in two case studies.
... An SGSP that was constructed as described in Suárez et al. (2010cSuárez et al. ( , 2011 was used to perform all experiments. The top water surface area was approximately 1.9 m 2 and the overall water capacity was approximately 1.5 m 3 . ...
Article
Evaporation represents a significant challenge to the successful operation of solar ponds. In this work, the suppression of evaporative losses from a salt-gradient solar pond was investigated in the laboratory. Two floating element designs (floating discs and floating hemispheres) and a continuous cover were tested; all three covers/elements were non-opaque, which is unique from previous studies of evaporation suppression in ponds or pools where increasing temperature and heat content are not desired. It was found that floating discs were the most effective element; full (88%) coverage of the solar pond with the floating discs decreases the evaporation rate from 4.8 to 2.5 mm/day (47% decrease), increases the highest achieved temperature from 34 °C to 43 °C (26% increase), and increases heat content from 179 to 220 MJ (22% increase). As a result of reduced evaporative losses at the surface, the amount of heat lost to the atmosphere is also reduced, which results in lower conductive losses from the NCZ and the LCZ and hence, increased temperatures in the NCZ and LCZ. The magnitude of evaporation reduction observed in this work is important as it may enable solar pond operation in locations with limited water supply for replenishment. The increase in heat content allows more heat to be withdrawn from the pond for use in external applications, which significantly improves the thermal efficiencies of solar ponds.
... After 45 h of heat extraction, the thermal structure inside the solar pond (green line) reveals that even though energy is being extracted from the NCZ, there is energy likely coming from the LCZ. When the temperatures at the extraction depth are cooler than some threshold, the fluid at this depth become heavier and thus, sinks into the bottom of the pond, which has been shown to produce a rise of the warmer brine [37]. ...
Article
Desalination powered by renewable energy sources is an attractive solution to address the worldwide water-shortage problem without contributing significant to greenhouse gas emissions. A promising system for renewable energy desalination is the utilization of low-temperature direct contact membrane distillation (DCMD) driven by a thermal solar energy system, such as a salt-gradient solar pond (SGSP). This investigation presents the first experimental study of fresh water production in a coupled DCMD/SGSP system. The objectives of this work are to determine the experimental fresh water production rates and the energetic requirements of the different components of the system. From the laboratory results, it was found that the coupled DCMD/SGSP system treats approximately six times the water flow treated by a similar system that consisted of an air–gap membrane distillation unit driven by an SGSP. In terms of the energetic requirements, approximately 70% of the heat extracted from the SGSP was utilized to drive thermal desalination and the rest was lost in different locations of the system. In the membrane module, only half of the useful heat was actually used to transport water across the membrane and the remainder was lost by conduction in the membrane. It was also found that by reducing heat losses throughout the system would yield higher water fluxes, pointing out the need to improve the efficiency throughout the DCMD/SGSP coupled system. Therefore, further investigation of membrane properties, insulation of the system, or optimal design of the solar pond must be addressed in the future.
... SGSPs are water bodies that capture and accumulate solar energy for long time periods [2][3][4][5][6]. These are artificially stratified by dissolving salts with different concentrations to form three characteristic zones ( Figure 1): the upper convective zone (UCZ), the non-convective zone (NCZ) and the lower convective zone (LCZ), which is also known as the storage zone. ...
... To validate the mathematical model that describes the performance of DCMD driven by solar ponds, we utilized the experimental data of Suárez et al. (2015), which investigated DCMD driven by a laboratory-scale solar pond. The description of this pond is well documented in the scientific literature (Suárez et al. 2010c(Suárez et al. , 2014b; thus, here we only describe the details relevant to the experiments where the extracted energy was used to drive thermal desalination. ...
Article
Full-text available
Although the costs of desalination have declined, traditional desalination systems still need large amounts of energy. Recent advances in direct contact membrane distillation can take advantage of low-quality renewable heat to desalinate brackish water, seawater, or wastewater. In this work, the performance of a direct contact membrane distillation (DCMD) system driven by salt-gradient solar ponds was investigated. A mathematical model that couples both systems was constructed and validated with experimental data available in the scientific literature. Using the validated model, the performance of this coupled system in different geographical locations and under different operational conditions was studied. Our results show that even when this coupled system can be used to meet the future needs of energy and water use in a sustainable way, it is suitable for locations between 40°N and 40°S that are near the ocean as these zones have enough solar radiation, and availability of excess water and salts to operate the coupled system. The maximum freshwater flow rates that can be obtained are on the order of 3.0 L d−1 per m2 of solar pond (12.1 m3 d−1 acre−1), but the expected freshwater production values are more likely to be on the order of 2.5 L d−1 per m2 of solar pond (10.1 m3 d−1 acre−1) when the system operates with imperfections. The coupled system has a thermal energy consumption of 880 ± 60 kWh per m3 of distillate, which is in the range of other membrane distillation systems. Different operational conditions were evaluated. The most important operating parameters that influence the freshwater production rates are the partial pressure of air entrapped in the membrane pores and the overall thermal efficiency of the coupled system. This work provides a guide for geographical zone selection and operation of a membrane distillation production system driven by solar ponds that can help mitigate the stress on the water-energy nexus.
... Several studies that required a higher spatial resolution wound the fiber-optic cable to a coil [Ciocca et al., 2012;Euser et al., 2014]. Many of these studies used PVC tubes as a support for these coils [Selker et al., 2006b;Su arez et al., 2010aSu arez et al., , 2011Vogt et al., 2010Vogt et al., , 2012Vercauteren et al., 2011]. Hilgersom et al. [2016b] show that wrapping cables to narrow coils and the use of PVC support tubes can have adverse effects on the measurements. ...
Article
Distributed temperature sensing has proven a useful technique for geoscientists to obtain spatially distributed temperature data. When studies require high-resolution temperature data in three spatial dimensions, current practices to enhance the spatial resolution do not suffice. For example, double-diffusive phenomena induce sharp and small-scale temperature patterns in water bodies subject to thermohaline gradients. This article presents a novel approach for a 3-D dense distributed temperature sensing setup, the design of which can be customized to the required spatial resolution in each dimension. Temperature is measured along fiber-optic cables that can be arranged as needed. In this case, we built a dense cage of very thin (1.6 mm) cables to ensure that interference with flow patterns was minimal. Application in water bodies with double-diffusion-induced sharp temperature gradients shows that the setup is well able to capture small-scale temperature patterns and even detects small unsuspected seeps and potential salt-fingers. However, the potential effect of the setup on the flow patterns requires further study. This article is protected by copyright. All rights reserved.
... Salt-gradient solar ponds store thermal energy from shortwave solar radiation, which penetrates the water body, in their hypersaline bottom layer (Suárez et al., 2010). The top fresh layer in the stratied water body functions as an insulator. ...
Data
Full-text available
A non-hydrostatic free-surface model was set up to simulate salt and heat transport in a solar pond in order to: 1) investigate the added value of free-surface models for these types of simulations, and 2) assess the importance of heat transport along a sloping side wall. This data set presents the source code, the raw measurement and model data, and several movies comparing the vertical two-dimensional (2DV) model simulations to the measurements. To demonstrate the added value of a free surface approach, this data set includes model results for both free surface and rigid-lid simulations. The presented model code is an extension of the SWASH non-hydrostatic model, which is briey introduced in this document. A complete discussion of the model results and conclusions are provided in an accompanying article.
... Finite difference Method 40 Koufi et al. [132] Square Cavity Parween et al. [135] Wavy square enclosure ...
Article
Over the years, the lattice Boltzmann method (LBM) has been evolved as a substitute and efficient numerical tool to mimic the single/multiphase fluid flow and transport problems. LBM has been mainly advantageous in multi-physics and multiphase flow applications. On the other hand, double-diffusive convection has extensive occurrence in domestic and industrial activities. The type of convection in which the combined effect of temperature and concentration gradients (resulting in the density variation) on fluid's hydrodynamic and thermal characteristics is called double-diffusive convection (DDC). The importance of DDC has been recognized in various engineering applications, and it has thoroughly been investigated experimentally, theoretically, and numerically. This paper is proposed to deliver a brief review of double-diffusive convection by computational approach (mainly lattice Boltzmann method and Navier–Stokes equation-based solvers). This review explores the illustration of some of the practical applications of DDC, studies of DDC in various heated cavities. The paper also gives insights into LBM formulation of DDC under various external force conditions. A table compromising various empirical correlations of the average Nusselt numbers and average Sherwood numbers as a function of different governing parameters has been discussed.
... A solar pond is a water body that is warmed by solar radiation and that can provide long-term heat storage (Rabl and Nielsen, 1975;Suárez et al., 2010a,b;Valderrama et al., 2011;Sayer et al., 2016). These ponds are artificially stratified so the heat is stored in its bottom (Suárez et al., 2010c;Alcaraz et al., 2016). A typical salt-gradient solar pond is comprised by three characteristic layers. ...
Article
Salt-gradient solar ponds are water bodies that act as solar collectors with integrated storage that are a promising renewable energy source for low-temperature applications. Evaporation is an important challenge for efficient operation of solar ponds, especially in arid locations without water sources to replenish the evaporative losses. In this work, transparent partial covers were used to investigate how evaporation suppression affects both the water and energy balance of a laboratory-scale solar pond. In our experiments, the evaporation reduction efficiency was related to the cube root of the relative covered area. This evaporation reduction efficiency is smaller than that of natural water bodies because of the influence of the warm lower convective zone. Also, as the covered fraction of the surface area increased, the thickness of the non-convective zone decreased and the heat losses through this zone increased. As a result, the temperature in the lower convective zone did not increase as the covered fraction increased. Nonetheless, the heat content within the solar pond slightly increased, demonstrating that the reduction of evaporation improves the heat storage capacity of the pond. Moreover, an economic analysis showed that although the evaporation reduction efficiency in solar ponds is smaller than that of natural water bodies or reservoirs, the benefits related to the additional energy collected in the solar pond when reducing evaporation overcome the smaller water saving benefits. Therefore, suppressing evaporation in solar ponds not only is valuable in locations with no water to replenish the evaporative losses but also improves its economic benefits.
... beneath the SGSP were measured with a vertical high-resolution distributed temperature sensing (DTS) system similar to that presented by Su arez et al. [39,40], with a temperature resolution of ±0.05 C when 5-min integration intervals were chosen. The DTS system allowed measuring temperatures every 1.1 cm within the water column and the ground beneath it (Fig. 2(a)). ...
Article
Salt-gradient solar ponds are cost-effective long-term solar collectors that can store low-grade heat and deliver it continuously. For design and operation purposes, it is important to develop computational tools that can represent energy fluxes at the interface between the bottom of the pond and the ground beneath it. In this study, a robust one-dimensional transient model is developed to represent the thermal evolution of a salt-gradient solar pond and the ground that surrounds it. The model was evaluated under different contrasting scenarios: buried or unburied ponds, artificially or naturally heated ponds and for deep or shallow groundwater tables. Experimental data from an indoor laboratory-scale solar pond were used for the development, calibration and validation of the model. A good agreement between experimental and modeled results was observed, with a root mean square error (RMSE) of 1.21 °C and 1.54 °C for the upper and lower convective zones respectively, during a 28-days validation period. Further, the model was validated using experimental data from three outdoor salt-gradient solar ponds obtaining RMSE's that ranged between 1.5 and 6.5 °C. Results show that dividing the ground into multiple layers contributes to the robustness of the model, as it allows the representation of the ground heat storage.
... Over the last years, one novel method that has been taking quite strongly and that has demonstrated to be able to provide energy for MD is the SGSP [23,84,147]. An SGSP is an artificially stratified water body that captures and accumulates solar energy for long time periods [152][153][154]. Recent studies indicate that in the solar ponds, 16% of the incoming radiation could be extractable for heating applications [155]. ...
... In spite of the simplicity of solar ponds, this technology seems to be a candidate for a seasonal storage system. Recent developments by Suarez et al (2010) confirm the importance of solar ponds, although more work is needed especially concerning questions about their long-time stability. ...
Preprint
We present a numerical study of double-diffusive convection characterized by a stratification unstable to thermal convection while at the same time a mean molecular weight (or solute concentration) difference between top and bottom counteracts this instability. Convective zones can form in this case either by the stratification being locally unstable to the combined action of both temperature and solute gradients or by another process, the oscillatory double-diffusive convective instability, which is triggered by the faster molecular diffusivity of heat in comparison with that one of the solute. We discuss successive layer formation for this problem in the case of an instantaneously heated bottom (plate) which forms a first layer with an interface that becomes temporarily unstable and triggers the formation of further, secondary layers. We consider both the case of a Prandtl number typical for water (oceanographic scenario) and of a low Prandtl number (giant planet scenario). We discuss the impact of a Couette like shear on the flow and in particular on layer formation for different shear rates. Additional layers form due to the oscillatory double-diffusive convective instability, as is observed for some cases. We also test the physical model underlying our numerical experiments by recovering experimental results of layer formation obtained in laboratory setups.
... In spite of the simplicity of solar ponds, this technology seems to be a candidate for a seasonal storage system. Recent developments by Suarez et al [50] confirm the importance of solar ponds, although more work is needed especially concerning questions about their long-time stability. ...
Article
Full-text available
We present a numerical study of double-diffusive convection characterized by a stratification unstable to thermal convection, while at the same time a mean molecular weight (or solute concentration) difference between top and bottom counteracts this instability. Convective zones can form in this case either by the stratification being locally unstable to the combined action of both temperature and solute gradients or by another process, the oscillatory double-diffusive convective instability, which is triggered by the faster molecular diffusivity of heat in comparison with that one of the solute. We discuss successive layer formation for this problem in the case of an instantaneously heated bottom (plate) which forms a first layer with an interface that becomes temporarily unstable and triggers the formation of further, secondary layers. We consider both the case of a Prandtl number typical for water (oceanographic scenario) and of a low Prandtl number (giant planet scenario). We discuss the impact of a Couette like shear on the flow and in particular on layer formation for different shear rates. Additional layers form due to the oscillatory double-diffusive convective instability, as is observed for some cases. We also test the physical model underlying our numerical experiments by recovering experimental results of layer formation obtained in laboratory setups.
... DTS has been widely used as an in situ logging technique in oil and gas wells, being the only system that offers a data profile that can be used to identify flow patterns and changes in fluid properties, as well as to monitor the overall integrity of the borehole without intervention [29]. Since the 1990s, this technology has had various uses in geosciences [30,31], environmental sciences [32][33][34][35], ecology [36], glaciology [37], hydrology [38][39][40], hydrogeology [41][42][43], engineering [44,45], and industrial applications [29]. ...
Article
Full-text available
Fiber-optic distributed temperature sensing (DTS) has been widely used since the end of the 20th century, with various industrial, Earth sciences, and research applications. To obtain precise thermal measurements, it is important to extend the currently available DTS calibration methods, considering that environmental and deployment factors can strongly impact these measurements. In this work, a laboratory experiment was performed to assess a currently available duplexed single-ended DTS calibration algorithm and to extend it in case no temperature information is available at the end of the cables, which is extremely important in geothermal applications. The extended calibration algorithms were tested in different boreholes located in the Atacama Desert and in the Central Andes Mountains to estimate the geothermal gradient in these regions. The best algorithm found achieved a root mean square error of 0.31 ± 0.07 °C at the far end of a ~1.1-km cable, which is much smaller than that obtained using the manufacturer algorithm (2.17 ± 0.35 °C). Moreover, temperature differences between single- and double-ended measurements were less than 0.3 °C at the far end of the cable, which results in differences of ~0.5 °C km−1 when determining the geothermal gradient. This improvement in the geothermal gradient is relevant, as it can reduce the drilling depth by at least 700 m in the study area. Future work should investigate new extensions of the algorithms for other DTS configurations and determining the flow rate of the Central Andes Mountains artesian well using the geothermal profile provided by the DTS measurements and the available data of the borehole
... Furthermore, the vertical high-resolution DTS system shows that the interface between the non-convective and lower convective zones is located deeper than the desired level (at ∼0.65 m depth). This reduction of the lower convective zone occurred when constructing the salt gradient (Suárez et al., 2010c). If the temperatures measured with the EC probes are analyzed, the lower convective zone is not as clear as in the DTS measurements because almost all the data points along the water column are scattered between 23 and 24 @BULLET C (without considering the probe located at ∼0.4 m depth). ...
Article
Full-text available
In shallow thermohaline-driven lakes it is important to measure temperature on fine spatial and temporal scales to detect stratification or different hydrodynamic regimes. Raman spectra distributed temperature sensing (DTS) is an approach available to provide high spatial and temporal temperature resolution. A vertical high-resolution DTS system was constructed to overcome the problems of typical methods used in the past, i.e., without disturbing the water column, and with resistance to corrosive environments. This system monitors the temperature profile each 1.1 cm vertically and in time averages as small as 10 s. Temperature resolution as low as 0.035 °C is obtained when the data are collected at 5-min intervals. The vertical high-resolution DTS system is used to monitor the thermal behavior of a salt-gradient solar pond, which is an engineered shallow thermohaline system that allows collection and storage of solar energy for a long period of time. This paper describes a method to quantitatively assess accuracy, precision and other limitations of DTS systems to fully utilize the capacity of this technology. It also presents, for the first time, a method to manually calibrate temperatures along the optical fiber.
Article
Desalination powered by renewable energy sources is an attractive solution to address the worldwide water-shortage problem without contributing significant to greenhouse gas emissions. A promising system for renewable energy desalination is the utilization of low-temperature direct contact membrane distillation (DCMD) driven by a thermal solar energy system, such as a salt-gradient solar pond (SGSP). This investigation presents the first experimental study of fresh water production in a coupled DCMD/SGSP system. The objectives of this work are to determine the experimental fresh water production rates and the energetic requirements of the different components of the system. From the laboratory results, it was found that the coupled DCMD/SGSP system treats approximately six times the water flow treated by a similar system that consisted of an air-gap membrane distillation unit driven by an SGSP. In terms of the energetic requirements, approximately 70% of the heat extracted from the SGSP was utilized to drive thermal desalination and the rest was lost in different locations of the system. In the membrane module, only half of the useful heat was actually used to transport water across the membrane and the remainder was lost by conduction in the membrane. It was also found that by reducing heat losses throughout the system would yield higher water fluxes, pointing out the need to improve the efficiency throughout the DCMD/SGSP coupled system. Therefore, further investigation of membrane properties, insulation of the system, or optimal design of the solar pond must be addressed in the future.
Chapter
Solar energy gained momentum due to energy security threats and climate change issues and pulled the attention of policymakers and researchers. Solar thermal collectors have been widely studied, and various new designs were reported. To improve the performance of these solar devices, it is essential to understand the heat transfer behavior of the systems. Because the heat transfer concepts help the researcher and designer to have a proper understanding of the losses associated and their identification. In this work, heat transfer mechanisms involved in solar thermal devices, such as flat plate collector, evacuated tube collector, solar concentrating collectors, solar pond, solar distillation, solar dryer, and solar refrigeration are discussed and important observations made by various researchers are also presented. Furthermore, this chapter also incorporates different aspects of heat transfer that are important for the improvement of solar collector designs.
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The three-zone salt gradient solar ponds are theoretically analysed taking them as steady state flat plate collectors. A numerical model has been developed to determine the thermal efficiency of a salt gradient solar pond under different operating conditions. The model simulates the thicknesses of the upper convective zone, non-convective zone and lower convective zone, convective and radiation losses from the surface, heat loss to the atmosphere due to evaporation, upward heat conduction and ground (bottom and side) heat losses from the pond. Numerical calculations are made using expression derived for heat losses from a flat-bottomed cylindrical solar pond, to investigate the optimization of geometrical and operational parameters of the solar pond. Calculations have also been done for the heat losses from the pond for different insulating materials at the bottom and the sides of the pond. The use of different insulating materials at the bottom and the side of the pond shows that dry marble dust is a better insulator to increase a thermal efficiency of the pond.
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Thermal desalination by salinity-gradient solar ponds (SGSP) is one of the most promising solar desalination technologies. Solar ponds combine solar energy collection with long-term storage and can provide reliable thermal energy at temperature ranges from 50 to 90°C. Solar-pond-powered desalination has been studied since 1987 at the El Paso Solar Pond Project, El Paso, Texas. From 1987 to 1992, the search mainly focused on the technical feasibility of thermal desalination coupled with solar ponds. Since 1999, the research has focused on long-term reliability, improvement of thermodynamic efficiency, and economics. During this period, a small multi-effect, multi-stage flash distillation (MEMS) unit, a membrane distillation unit, and a brine concentration and recovery system (BCRS) were tested over a broad range of operating conditions. The most important variables for the MEMS operation were flash range, concentration level of reject brine, and circulation rate of the first effect. The brine concentration and recovery system is part of the goal of developing a systems approach combining salinity-gradient solar pond technology with multiple process desalination and brine concentration. This system approach, called zero discharge desalination, proposes concentrating brine reject streams down to near NaCl saturated solutions and using the solution to make additional solar ponds. In addition to presenting the test results on the MEMS and BCRS units, this paper also presents a summary of solar pond operation experiences obtained from the 16-year operation at the El-Paso solar pond.
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The diffusive regime of double-diffusive convection is discussed, with a particular focus on unresolved issues that are holding up the development of large-scale parameterizations. Some of these issues, such as interfacial transports and layer-interface interactions, may be studied in isolation. Laboratory work should help with these. However, we must also face more difficult matters that relate to oceanic phenomena that are not represented easily in the laboratory. These lie beneath some fundamental questions about how double-diffusive structures are formed in the ocean, and how they evolve in the competitive ocean environment.
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Terminal lakes are water bodies that are located in closed watersheds with the only output of water occurring through evaporation or infiltration. The majority of these lakes, which are commonly located in the desert and influenced by human activities, are increasing in salinity. Treatment options are limited, due to energy costs, and many of these lakes provide an excellent opportunity to test solar-powered desalination systems. This paper theoretically investigates utilization of direct contact membrane distillation (DCMD) coupled to a salt-gradient solar pond (SGSP) for sustainable freshwater production at terminal lakes. A model for heat and mass transport in the DCMD module and a thermal model for an SGSP were developed and coupled to evaluate the feasibility of freshwater production. The construction of an SGSP outside and inside of a terminal lake was studied. As results showed that freshwater flows are on the same order of magnitude as evaporation, these systems will only be successful if the SGSP is constructed inside the terminal lake so that there is little or no net increase in surface area. For the study site of this investigation, water production on the order of 2.7 x 10(-3) m(3) d(-1) per m(2) of SGSP is possible. The major advantages of this system are that renewable thermal energy is used so that little electrical energy is required, the coupled system requires low maintenance, and the terminal lake provides a source of salts to create the stratification in the SGSP.
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Salt-Gradient Solar Ponds for Renewable Energy, Desalination and Reclamation of Terminal Lakes
  • F Suá Rez
Suá rez, F. 2010 Salt-Gradient Solar Ponds for Renewable Energy, Desalination and Reclamation of Terminal Lakes, PhD thesis, University of Nevada, Reno.