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Temperature evolution of an experimental salt-gradient solar pond
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.
... 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. ...
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.
... 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. ...
... 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. ...
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.
... 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). ...
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.
... 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. ...
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.
... Finite difference Method 40 Koufi et al. [132] Square Cavity Parween et al. [135] Wavy square enclosure ...
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.
... 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]. ...
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
... 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. ...
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 (2010) confirm the importance of solar ponds, although more work is needed especially concerning questions about their long-time stability. ...
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.
... 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]. ...
... 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)). ...
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.
... 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. ...
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.
... 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. ...
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.
... 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. ...
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.
... 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. ...
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.
... 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. ...
... 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]. ...
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.
... 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 . ...
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.
... 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]. ...
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.
... 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). ...
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.
Over the last 37 years, particular interest has been directed towards exploring the characteristics of the optical environment for sensing, giving rise to what would now be one of the largest applications of well-known optical fibers, typically employed to transmit data at high rates. Sensing temperature, pressure, liquid level, deformation, and other physical parameters utilizing optical fibers has become a growing branch of research and a business competing with well-established electrical sensors in the industry. Optical fiber sensors have all the inherent characteristics of a fiber optic cable, such as electromagnetic immunity, small size and weight, multiplexing, and so on. These exclusive features have made fiber sensors so versatile as to become a transformative technology by enabling several industrial processes to be carried out with higher reliability. Nowadays, there are several optical sensors, including fiber Bragg grating, interferometric, polarimetric, polymer fiber, distributed, and several others. Specifically, Raman-based distributed temperature sensor (RDTS) is a class of fiber optic sensors broadly employed in temperature measurement of large structures such as oil and gas wells, tunnels, and pipelines. Since 1985, many techniques have been proposed to break through the barriers of exploring Raman scattering as a distributed temperature measurement method. Range, spatial and temperature resolutions have been the most investigated parameters. In this perspective, this paper presents a comprehensive review focused on the progress of the RDTS technology over the past 37 years (1985–2022), covering an analysis of over 500 journal papers. First, a brief introduction to fiber optic sensor technology is presented as a theoretical basis, discussing the emergence of distributed sensors. Subsequently, Raman scattering in optical fibers is introduced, as well as how this nonlinear effect can be used to build temperature sensors. Next, RDTS technology is detailed, followed by a discussion of its applications and evolution over nearly four decades of development. Lastly, future perspectives are addressed in this review for the advancements in distributed temperature sensor technologies.
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.
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.
Instruments for distributed fiber-optic measurement of temperature are now available with temperature resolution of 0.01°C and spatial resolution of 1 m with temporal resolution of fractions of a minute along standard fiber-optic cables used for communication with lengths of up to 30,000 m. We discuss the spectrum of fiber-optic tools that may be employed to make these measurements, illuminating the potential and limitations of these methods in hydrologic science. There are trade-offs between precision in temperature, temporal resolution, and spatial resolution, following the square root of the number of measurements made; thus brief, short measurements are less precise than measurements taken over longer spans in time and space. Five illustrative applications demonstrate configurations where the distributed temperature sensing (DTS) approach could be used: (1) lake bottom temperatures using existing communication cables, (2) temperature profile with depth in a 1400 m deep decommissioned mine shaft, (3) air-snow interface temperature profile above a snow-covered glacier, (4) air-water interfacial temperature in a lake, and (5) temperature distribution along a first-order stream. In examples 3 and 4 it is shown that by winding the fiber around a cylinder, vertical spatial resolution of millimeters can be achieved. These tools may be of exceptional utility in observing a broad range of hydrologic processes, including evaporation, infiltration, limnology, and the local and overall energy budget spanning scales from 0.003 to 30,000 m. This range of scales corresponds well with many of the areas of greatest opportunity for discovery in hydrologic science.
Solar ponds are shallow bodies of water in which an artificially maintained salt concentration gradient prevents convection. They combine heat collection with long-term storage and can provide sufficient heat for the entire year. We consider the absorption of radiation as it passes through the water, and we derive equations for the resulting temperature range of the pond during year round operation, taking into account the heat that can be stored in the ground underneath the pond. Assuming a heating demand of 25000 Btu/degree day (Fahrenheit), characteristic of a 2000 ft2 house with fair insulation, and using records of the U.S. Weather Bureau, we carry out detailed calculations for several different locations and climates. We find that solar ponds can supply adequate heating, even in regions near the arctic circle. In midlatitudes the pond should be, roughly speaking, comparable in surface area and volume to the space it is to heat. Under some circumstances, the most economical system will employ a heat pump in conjunction with the solar pond. Cost estimates based on present technology and construction methods indicate that solar ponds may be competitive with conventional heating.
In shallow aquatical thermohaline environments, such as salt-gradient solar ponds (SGSPs), it is important to measure temperature on fine spatial and temporal scales to detect stratification or different hydrodynamics regimes (e.g., salt fingering, oscillatory motion). The spatial variability of interest in SGSPs is observed on vertical scales of centimeters, whereas the temporal variability can be on time scales ranging from minutes to months, or even years. Measuring temperature at these scales can be difficult because of the cost involved, especially when diel cycles needs to be reported during a long period of time. The Raman spectra distributed temperature sensing (DTS) is an approach available to provide coverage of both ``in space'' and ``in time'' for use in aquatic systems. Recently it has been used to measure temperature in streams, lakes, air, snow, and other hydrologic or environmental applications. This work investigates the thermal behavior of an experimental SGSP using a high-vertical resolution DTS system. In the experimental SGSP and DTS system, the vertical temperature profile is monitored each 1.1 cm within the SGSP without disturbing the water column. The air temperature profile over the water surface is also measured with the same spatial resolution. Temperatures collected in electrical conductivity probes are used to compare the DTS measurements. The temperatures measured using the DTS system during the first two weeks of experimentation showed an increase of the temperature from 17 to 34 °C in the lower zone of the SGSP. Also, mixing and stratification were observed in the different zones of the SGSP. In the surface zone, the water mixed during the night and stratified during the day. On the other hand, in the lower zone the mixing was observed during the day and stratification occurred during the night. The results show that DTS systems are highly suitable for measuring temperatures in SGSPs, with resolutions smaller than 0.03 °C on fine spatial and temporal scale.
While soil heat flux is traditionally directly measured in any land surface energy study, measuring heat flux into and out of lakes and ponds is complicated by water column mixing processes, differing radiation adsorption coefficients, turbidity variation and heat flux through the sediment-water interface. High resolution thermal profile, to assess heat storage changes in aquatic systems is both time consuming and challenging using traditional thermister or thermocouple strings or casts. Recent advances in Raman spectra distributed temperature sensing (DTS) offer the opportunity to measure, at high spatial and temporal resolution, the thermal storage changes occurring in lakes and ponds. Measurements of thermal storage using DTS are presented from two distinct environments; a strongly density stratified solar pond and a deep cavern system (Devils Hole in Death Valley National Park), demonstrating the effectiveness of high resolution temperature measurements. In the solar pond environment, closure of the energy budget using direct measurements of evaporation and net radiation was greatly improved by incorporating transient thermal measurements, and the development of a cooling boundary layer easily shown. At Devils Hole, variations in shading of the water surface produced small but measureable horizontal gradients in water column temperature for short periods of the day, which impact both pool evaporation and the metabolism and behavior of aquatic organisms
A one dimensional transient mathematical model for predicting the thermal performance of the salt gradient solar pond is developed and presented. In this paper, the natural solar ponds and different artificial solar pond systems found in the literature are introduced. Necessary modifications are made on the experimental stand located in Istanbul Technical University, the experimental stand is introduced and natural phenomena produced in the pond by the different solar pond variations under natural conditions are observed. In the theoretical work based on a one dimensional unsteady state heat conduction model with internal heat generation, the energy and mass balance equations for the upper convective zone, the non-convective zone and the lower convective zone, all of which form the solar pond, are written in terms of differential equations. These equations are solved analytically and numerically. The results obtained from the analysis are compared with the experimental results. The temperature and the concentration profiles are separately presented in the figures.
An experimental study on the evolution of the temperature and salinity profiles in a salinity gradient solar pond was executed using a small model pond. The body of the simulated pond is a cylindrical plastic tank, with 1 m height, 0.9 m diameter. The tank was insulated by 150 mm of polyurethane. The salinity gradient was established in the laboratory tank by using the salinity redistribution technique[1] The base of the tank is black painted. Solar radiation was simulated by a 2000 W light projector that presents a spectrum similar to the solar one. The measurements were taken during a period of 29 days of experimentation. This period of time allowed to show the existence of salt diffusion from the storage zone to the surface. The temperature profile was established in the small model pond after 5 days of heating. The maximum temperature attained in the storage zone was 45°C carrying out a difference in temperature between the bottom and the surface of the pond of 23°C when the projector is put of and 17°C when it is put on.A comparison between the temperature and salinity profiles obtained experimentally in the model pond and those calculated by the empirical relation of Newell show a good agreement.
In this paper we present an experimental and theoretical investigation of the exergetic performance of a solar pond (with a surface area of 4 m2 and a depth of 1.5 m) which was built at Cukurova University in Adana, Turkey. The system was filled with salty water to form three zones (e.g., upper convective, non-convective and heat storage) accordingly. A data acquisition device was used to measure and record the temperatures hourly at various locations in the pond (distributed vertically within and at the bottom of the pond, and horizontally and vertically within the insulated side-walls). An exergy model is developed to study the exergetic performance of the pond and its three zones in terms of exergy efficiencies which are then compared with the corresponding energy efficiencies. The reference environment temperature is specified for exergy analysis as the average representative temperature of each month of the year (for example, it is taken as an average temperature of 28 °C for August). Thus, the highest energy and exergy efficiencies are found for August to be: 4.22% and 3.02% for the upper convective zone, 13.80% and 12.64% for the non-convective zone, and 28.11% and 27.45% for the heat storage zone, respectively.
The ability to understand and monitor the distributed behaviour of extended, critical structures is recognized as a matter of increasing importance. Optical fibres offer unique advantages for spatially distributed measurement.
This article reviews the field of distributed optical-fibre sensing (DOFS), tracing its origins and its evolution, and discussing the present and future positions.
Examples of the use of both linear and non-linear optics in pursuit of DOFS development are presented, together with details of a range of systems which have been investigated for particular applications.
a b s t r a c t A fully coupled two-dimensional, numerical model that evaluates, for the first time, the effects of double-diffusive convection in the thermal performance and stability of a salt-gradient solar pond is presented. The inclusion of circulation in the upper and lower convective zone clearly shows that erosion of the non-convective zone occurs. Model results show that in a two-week period, the temperature in the bottom of the solar pond increased from 20 °C to approximately 52 °C and, even though the insulating layer is being eroded by double-diffusive convection, the solar pond remained stable. Results from previous models that neglect the effect of double-diffusive convection are shown to over-estimate the temperatures in the bottom of the solar pond. Incorporation of convective mixing is shown to have profound impacts on the overall stability of a solar pond, and demonstrates the need to actively manage the mixing and heat transfer to maintain stability and an insulating non-convective zone.
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.
Solar ponds are probably the simplest technology available for the useful conversion of solar energy. The basic technology is proven. Solar ponds have been shown to be technically feasible and economically viable for many applications, particularly for thermal use. The electrical conversion and use of solar energy via solar ponds is still questionable, in general, for economic viability. By putting the untapped sources together in the South Plains region, it looks promising economically both for thermal and electrical conversions and applications. There are a number of alkaline lake basins randomly scattered in the South Plains region of the U.S.A. In that area, there are thousands of crude oil producing wells that produce brine in abundance. The selection of suitable alkaline lake basins as a solar pond site and as depository sites of brine from oil wells and the using of this brine and salty water from alkaline lakes makes the solar pond economically viable for both thermal and electrical demands in the area.
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.
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.
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.
Engineered processes that cleverly exploit osmosis may provide just the answer to the global need for affordable clean water and inexpensive sustainable energy.
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.