Extreme salt-resisting multistage water treatment

Abstract

A multistage water treatment system that requires minimum energy and can be powered by low-grade energy sources, such as sunlight, is disclosed. The system may include multiple stages, where each stage comprises a hydrophobic membrane, a confined saline layer, and a condenser. Using a saline layer as the evaporator, as confined by a hydrophobic membrane, strong thermohaline convection is initiated to mitigate salt accumulation and enhance heat transfer. The multistage water treatment system is salt-resistant during continuous desalination of nearly saturated saline. Such a system enables water treatment by low-grade heat with water production exceeding the single-stage limit and ultrahigh salinity. This system is useful for water treatment, including seawater desalination, wastewater reclamation, residential water softening, and salt production from brine / seawater.

Description

[0001]EXTREME SALT-RESISTING MULTISTAGE WATER TREATMENT This application claims priority to U.S. Provisional Patent Application Serial No. 63 / 515,360, filed July 25, 2023, the disclosure of which is incorporated herein by reference in its entirety. Background Freshwater shortage is a great challenge for humanity due to the continuously increasing gap between global water demand and supply. Solar desalination provides a passive, decentralized, and zero-emission solution to sustainable water production, which can greatly increase the accessibility to freshwater. Over the past few years, significant progress has been made to promote the efficiency of solar-to-vapor conversion, relying on innovations of materials and device designs to localize thermal energy at the evaporating interface. In particular, recent advances in thermally localized multistage solar distillation promise ultrahigh-performance water production. By recovering the latent heat released from vapor condensation, the multistage configuration exceeds the thermodynamic limit of single-stage solar distillation (≈ 1.45 kg m-2h-1) and enables a multifold enhancement of water production. To enable broad deployment of multistage solar distillation globally, mitigating salt accumulation is of equal importance to increasing water production, which has received much less attention. On the one hand, salt accumulation is a major fouling mechanism during desalination, which fundamentally limits the long-term reliability of a solar desalination device. Typical solar evaporators can resist salt accumulation under only a few kWh m-2continuous solar irradiance. If efficient salt rejection can be achieved under ~100 kWh m-2continuous solar irradiance, the lifetime of a solar desalination device is expected to be extended from a few weeks to more than one year, leading to at least a one order of magnitude reduction in cost for the produced water. On the other hand, to build sustainable water economy, proper treatment of hypersaline discharge from seawater desalination is required to maximize freshwater recovery and mitigate environmental impact. In both contexts, it is essential to pursue a solution that enables extreme salt rejection to facilitate long- term reliability and widespread adoption of multistage solar distillation. However, mitigating salt accumulation is fundamentally challenging because salt diffusion is four orders of magnitude slower than molecular diffusion (e.g., water diffusion). Even with a dilute solar flux, the salt accumulation rate at the evaporating interface can be easily higher than the salt rejection rate driven by diffusion. In addition, to achieve the simultaneous thermal localization and passive water supply, conventional multistage solar distillation relies on a capillary wick evaporator. One such wick structure-based solar distillation (WSD) system is shown in FIGs. 1A-1B. A capillary wick is used to bring saltwater into a compartment, where heat evaporates the saltwater. The water vapor passes through a membrane, leaving behind the salt. However, the porous structure of the capillary wick has been recognized as a key bottleneck of efficient salt rejection because it creates additional resistance to salt transport and lowers the barrier of salt crystallization. FIG. 1B shows the salt accumulation induced by the significant transport resistance through the capillary wick and the unidirectional capillary flow away from the reservoir. Despite several strategies to manage salt, such as separating the crystallization area from the evaporation area and enhancing salt transport by macroscopic channels and liquid flows, these approaches either cannot be easily integrated with the multistage configuration or only work for low salinity conditions, which cannot fully address the critical issue of salt accumulation in long-term operation. Therefore, it would be beneficial if there were a system and method that overcame these issues. Summary A multistage water treatment system that requires minimum energy and can be powered by low-grade energy sources, such as sunlight, is disclosed. The system may include multiple stages, where each stage comprises a surface to be heated, a hydrophobic membrane, a confined saline layer, and a condenser. Using a saline layer as the evaporator, as confined by a hydrophobic membrane, strong thermohaline convection is initiated to mitigate salt accumulation and enhance heat transfer. The multistage water treatment system is salt-resistant during continuous desalination of nearly saturated saline. Such a system enables water treatment by low-grade heat with water production exceeding the single-stage limit and ultrahigh salinity. This system is useful for water treatment, including seawater desalination, wastewater reclamation, residential water softening, and salt production from brine / seawater. According to one embodiment, a membrane distillation device that uses thermohaline convection is disclosed. The membrane distillation device comprises a surface to be heated; a hydrophobic membrane separated from the surface to be heated; an evaporation chamber defined by the surface to be heated and the hydrophobic membrane; an inlet and an outlet in communication with the evaporation chamber; and a condenser; wherein the membrane distillation device is disposed at a tilt angle with respect to a horizontal direction; such that the inlet is disposed at a lower end of the evaporation chamber and the outlet is disposed at an upper end of the evaporation chamber; and wherein water vapor produced in the evaporation chamber diffuses through the hydrophobic membrane to the condenser. In some embodiments, a water collector is connected to the condenser. In some embodiments, a heat sink is disposed adjacent to the condenser. In some embodiments, the tilt angle is between 10° and 90°. In some embodiments, a spacing between the surface to be heated and the hydrophobic membrane is between 1 and 10 mm. In some embodiments, the surface to be heated comprises a solar absorber. In some embodiments, the surface to be heated comprises the condenser from an adjacent membrane distillation device. According to another embodiment, a system for water distillation is disclosed. The system comprises the membrane distillation device described above; an inner vessel, in which the membrane distillation device is disposed; and a reservoir in which the inner vessel is disposed; wherein the inlet is in communication with an opening in the inner vessel to allow fluid to pass from the reservoir to the evaporation chamber via gravity driven flow. In some embodiments, a water level of the reservoir is higher than the evaporation chamber, such that water from the reservoir enters the evaporation chamber via the gravity driven flow. According to another embodiment, a multi-stage membrane distillation device that uses thermohaline convection is disclosed. The multi-stage membrane distillation device comprises a plurality of stages, each stage comprising: a surface to be heated; a hydrophobic membrane separated from the surface to be heated; an evaporation chamber defined by the surface to be heated and the hydrophobic membrane; an inlet and an outlet in communication with the evaporation chamber; and a condenser; wherein each stage is disposed at a same tilt angle with respect to a horizontal direction; such that the inlet of each stage is disposed at a lower end of the evaporation chamber and the outlet of each stage is disposed at an upper end of the evaporation chamber; and wherein water vapor produced in the evaporation chamber of each stage diffuses through the hydrophobic membrane to the condenser. In some embodiments, the surface to be heated of a first stage comprises a solar absorber. In some embodiments, heat of condensation warms the condenser, and wherein the condenser of the first stage serves as the surface to be heated for an adjacent second stage. In some embodiments, the plurality of stages comprises three or more stages. In some embodiments, a water collector is connected to each condenser. In some embodiments, a heat sink is disposed adjacent to the condenser of a last stage. In some embodiments, the same tilt angle is between 10° and 90°. In some embodiments, a spacing between the surface to be heated and the hydrophobic membrane of each stage is between 1 and 10 mm. In certain embodiments, the spacing is reduced for a second stage as compared to a first stage. According to another embodiment, a system for water distillation is disclosed. The system comprises the multi-stage membrane distillation device described above; an inner vessel, in which the multi-stage membrane distillation device is disposed; and a reservoir in which the inner vessel is disposed wherein the inlet of each stage is in communication with one or more openings in the inner vessel to allow fluid to pass from the reservoir to each evaporation chamber via gravity driven flow. In some embodiments, a water level of the reservoir is higher than each evaporation chamber, such that water from the reservoir enters each evaporation chamber via the gravity driven flow. Brief Description of the Drawings For a better understanding of the present disclosure, reference is made to the accompanying drawings, in which like elements are referenced with like numerals, and in which: FIGs. 1A-1B show a conventional thermally localized multistage solar distillation with a capillary wick serving as the evaporator; FIGs. 2A-2B show the disclosed thermohaline convection enhanced solar membrane distillation (TSMD) device according to one embodiment; FIG. 3 shows an exploded view of one stage of the TSMD device; FIGs. 4A-4D show a three stage TSMD device and the associated stages, respectively; FIG. 5A shows the experimental setup for the TSMD device to perform temperature testing; FIGs. 5B-5H show the results of various temperature tests of the TSMD device; FIG. 6A shows the results of an additional temperature test of the TSMD device; FIGs. 6B-6D show a comparison of the TSMD device and a wick structure-based solar distillation (WSD) device in terms of mass change, production rate and efficiency, respectively; FIG. 7A shows the experimental setup for the TSMD device to collect distilled water; FIGs. 7B-7D show the performance of the TSMD device in terms of temperature, production rate and sodium concentration, respectively, as a function of salinity; FIG. 8 shows a comparison of the present TSMD device and other devices in the literature; and FIGs. 9A-9C show the results of an extended test. Detailed Description FIG. 2A shows a highly efficient multistage solar membrane distillation with extreme resistance to salt accumulation. To mitigate salt accumulation, thermohaline convection, which is the primary driving force for the energy and species transport in deep ocean, is used. Thermohaline convection is a type of natural convection caused by the density gradient induced by both temperature and salinity. By utilizing thermohaline convection, a thermohaline convection-enhanced solar membrane distillation (TSMD) device may be created. In this TSMD device, as shown in FIG. 2A, a thin saline layer 10 is confined by a hydrophobic membrane 20. As shown in FIG. 2B, an inner vessel 100 is disposed within a reservoir 110 such that the bottom of the inner vessel 100 is below the water level of the reservoir 110. In other words, the inner vessel 100 is sized and weighted such that, when placed in the reservoir 110, it remains buoyant, but reaches a depth that allows the evaporation chamber of the TSMD device to be below the water level in the reservoir 110. The TSMD device is placed in the inner vessel 100 and the inlet 31 of the TSMD device passes through a hole in the inner vessel 100 so as to be in communication with the reservoir 110. By placing the TSMD device in the inner vessel 100, such as is shown in FIG. 2B, passive liquid supply is enabled by a gravity driven flow. By properly engineering the TSMD device, the temperature and salinity gradient across the confined saline layer 10 may be manipulated and hence initiate a strong thermohaline convection to accelerate salt rejection. Meanwhile, the thermohaline convection enhances heat transfer across the confined saline layer 10, ensuring high water production rate comparable to that of the conventional wick structure-based solar distillation (WSD) devices. The strong thermohaline convection in the confined saline layer 10 is confirmed with detailed thermofluidic characterization and simulation. FIG. 3 shows the structure of one stage of the TSMD device according to one embodiment. FIG. 3 shows the top stage of a multi- stage TSMD device, or may represent a single-stage TSMD device. The TSMD device may include a frame, which may be divided into a top frame 30 and a bottom frame 35. The frame may be made from nylon or another thermally insulating material. One part of the frame includes at least one inlet 31 (see FIGs. 2A-2B) to receive saline. In some embodiments, there may be two or more inlets 31, which may be parallel to each other. One part of the frame includes at least one outlet 32 to maintain a hydraulic head. The inlet 31 and the outlet 32 may be formed in the same part of the frame, or in different parts. The at least one inlet 31 and the outlet 32 may be located on opposite sides of the frame. Disposed within the frame, adjacent the top frame 30, is a glass plate 40. Beneath the glass plate 40 is a solar absorber 50. The solar absorber 50 is designed to receive the sun’s energy and convert this solar energy to heat. In certain embodiments, the solar absorber 50 may comprise a spectrally selective coating applied to a metal plate. In some embodiments, a gap is created between the glass plate 40 and the solar absorber 50 to serve as a thermal insulator. This gap may be any desired thickness, such as between 1 and 10 mm, for example. A hydrophobic membrane 20 is disposed in the frame behind the solar absorber 50. The hydrophobic membrane 20 is spaced apart from the solar absorber 50 so as to create a spacing therebetween. The sides of the frame, the hydrophobic membrane 20 and the solar absorber 50 define an evaporation chamber 60. Note that the saline that fills this evaporation chamber 60 is referred to as the confined saline layer 10. The one or more inlets 31 and the outlet 32 are also in communication with the evaporation chamber 60. The spacing between the solar absorber 50 and the hydrophobic membrane 20 may be fairly small, such as between 1 and 10 mm. Additionally, the length and width of the evaporation chamber 60 may be any desired size, such as 10 cm x 10 cm or a larger size, such as 1 m x 1 m. The inlet 31 and the outlet 32 may have an inner diameter of between <1 mm and 9 mm, depending on the thickness of the evaporation chamber 60. Specifically, the inner diameters of the inlet 31 and the outlet 32 may be smaller than the spacing between the solar absorber 50 and the hydrophobic membrane 20. As described below, the TSMD device is tilted, such that the one or more inlets 31 is disposed at the lower end of the evaporation chamber 60, and the outlet 32 is disposed at the upper end of the evaporation chamber. The hydrophobic membrane 20 may have a thickness of about 100 μm, and may contain pores having a diameter of 1 μm with a porosity of 0.8. The hydrophobic membrane 20 may be polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polypropylene (PP), polyethylene (PE) or another suitable material. Located behind and spaced apart from the hydrophobic membrane 20 is a condenser 70, which is used to remove heat from the vapor passing through the hydrophobic membrane 20 and convert it to distilled water. In some embodiments, the condenser 70 may be a metal plate, such as an aluminum plate. The frame may include a collection port 80, located in the space between the hydrophobic membrane 20 and the condenser 70 to collect the distilled water (see FIG. 2B). The condenser 70 may also be attached to the bottom frame 35. Additionally, one or more seals may be disposed between these components. Having defined the structure of a single stage TSMD device, its operation will be described in more detail. With reference to FIG. 2B, the TSMD device is placed within an inner vessel 100, which is located below the water level of the reservoir 110. The inner vessel 100 is positioned such that the entirety of the evaporation chamber 60 of the TSMD device is below the water level of the reservoir 110. Further, the TSMD device is disposed at a tilt angle. This tilt angle may be between 10° and 90°, although other tilt angles are also possible. For example, the tilt angle may be between 30° and 90° in some embodiments. In operation, gravity drives saline water into the inlets 31, filling the evaporation chamber 60 with a confined saline layer 10. The saline water also appears in the outlet 32, such that the water level in the outlet 32 is the same as that of the reservoir 110. The sun’s rays strike the solar absorber 50, thereby heating it. The saline water within the evaporation chamber 60 is then heated by the solar absorber 50. Heating of the saline water decreases its density, causing it to rise within the evaporation chamber 60. Further, this heat causes the warmer saline water to evaporate, and the water molecules pass through the micropores of the hydrophobic membrane 20. The passage of this water vapor reduces the amount of water, with no corresponding decrease in the amount of salt. In other words, the passage of water vapor causes the salinity of the water near the hydrophobic membrane 20 to increase, which increases its density and causes a downward movement of this salty water. In other words, the upward movement of warmed saline results in the downward movement of cooler, more salty water, as depicted in the insert of FIG. 2B. Furthermore, the inlet 31 is also able to allow the more salty water to diffuse downward and out of the evaporation chamber 60. Thus, with proper tilt of the TSMD device, thermohaline convection may be initiated. Driven by the buoyancy force, low-density saline beneath the solar absorber 50 flows upward whereas high-density saline above the hydrophobic membrane 20 flows downward, inducing a circulation within the confined saline layer 10. Depending on the magnitude of thermohaline convection, multiple clockwise vortices may be created, which significantly enhance the heat and mass transfer in the confined saline layer 10. Water vapor that passes through the hydrophobic membrane 20 is cooled and returns to liquid state in the condenser 70. This distilled water may then be routed away from the TSMD device via the collection port 80. Note that the condensation of the water vapor is an exothermic reaction, which imparts heat to the condenser 70. This fact may be exploited to create a multistage membrane distillation device. In FIG. 4A, a three stage TSMD device is shown. FIGs. 4B-4D show exploded views of each of the three stages. FIG. 4A shows a three stage TSMD device having a first stage 200, a second stage 210 and a third stage 220. A heatsink 230 may be attached to the back surface of the third stage 220. FIG. 4B shows the first stage 200, which is identical to that shown in FIG. 3 and will not be described again. The second stage 210 is shown in FIG. 4C. Note that the back surface of the condenser 70 for the first stage 200 serves as the upper boundary in the evaporation chamber 60 of the second stage 210. In other words, the glass plate 40 and the solar absorber 50 from the first stage 200 are not present in the second stage 210 and the third stage 220. Rather, these components are replaced by the condenser 70 of the immediately adjacent stage. Further, note that the condenser for the third stage 220 may be incorporated into the heatsink 230. In summary, the evaporation chamber 60 of the first stage 200 is bounded by the frame, the solar absorber 50 and the hydrophobic membrane 20. The evaporation chamber 60 of the second stage 210 is bounded by the frame, the condenser 70 of the first stage 200 and the hydrophobic membrane 20. The evaporation chamber 60 of the third stage 220 is bounded by the frame, the condenser 70 of the second stage 210 and the hydrophobic membrane 20. As noted above, the condenser 70 of the third stage 220 may be incorporated in the heatsink 230, which is affixed to the third stage 220. Thus, in this device, each evaporation chamber 60 is formed by a heated front surface. This may be a solar absorber 50, a condenser 70 or a surface that is actively heated, such as by an electric heater. The rear surface of the evaporation chamber 60 is a hydrophobic membrane 20, through which water vapor may pass. These two surfaces are connected using a frame. The frame also has one or more inlets 31 disposed on the lower end and one or more outlets 32 disposed on the upper end. The inlets 31 and outlets 32 are all in communication with the evaporation chamber 60. Additionally, each stage is tilted to facilitate thermohaline convection, and the stages may all be tilted at the same angle. In some embodiments, the thickness of the evaporation chamber 60 for each stage is the same. However, in other embodiments, the thickness of the evaporation chamber 60 may decrease for each successive stage. In one particular example, reducing the thickness of the subsequent evaporation chambers 60 may reduce total thermal mass and heat losses. In this embodiment, the thickness of each stage may be between 1 and 10 mm. Note that, in this embodiment, the inner diameters of the inlets 31 and outlets 32 for each stage may decrease accordingly. Note that the TSMD device may have more or fewer than 3 stages, if desired. For example, a ten-stage device was fabricated and tested. The heatsink 230 may be attached to the final stage. The heatsink 230 may be a metal component, and may be submerged, at least partially, in water to facilitate cooling. Each stage of the TSMD device is able to produce distilled water due to the energy from the sun’s rays. The first stage receives these rays directly, while the subsequent stages receive this energy in the form of heated water vapor condensing as liquid water on the condenser of the previous stage. Thus, a multi-stage TSMD device takes advantage of both latent heat recovery and thermohaline convection. A record-high water production rate of 2.11 kg m-2h-1for the three-stage TSMD device was demonstrated when desalinating 3.5 wt% saline, without degradation of performance during continuous operation. Further, a record-high water production rate of 3.82 kg m-2h-1for a ten- stage TSMD device was also demonstrated when desalinating 3.5 wt% saline. More importantly, the TSMD device enables an unprecedented regime of solar desalination with high water production (>1.45 kg m-2h-1exceeding the single-stage limit) and high salinity (> 3.5 wt%, higher than the seawater salinity). For example, a continuous desalination of 20 wt% saline with a stable production rate of 1.51 kg m-2h-1was demonstrated for a three-stage device. Owing to the superior salt rejection capability, the long-term reliability of the TSMD device was confirmed by demonstrating stable desalination of 20 wt% saline under 60 kWh m-2continuous solar irradiance, where the amount of salt rejected from the TSMD device is equivalent to the total salt accumulation during approximately 80-day desalination of seawater. This demonstrates an approach that has extreme resistance to salt accumulation to address the bottleneck toward the practical adoption of highly efficient solar desalination. The TSMD device not only substantially increases the long-term reliability of multistage solar distillation, but also broadly impacts various emerging applications associated with hypersaline wastewater. The following details the various testing that was performed. Some of this testing was performed using the three stage TSMD device of FIG. 4A. FIG. 5A shows an experimental setup used to confirm that thermohaline convection plays a significant role in the confined saline layer 10. In addition to the three stage TSMD device, a solar simulator, a confined saline test rig, an infrared (IR) camera, a data acquisition equipment (DAQ), and a computer (PC) are used. An NaCl solution with 10 wt% salinity was used. Three K-type thermocouples were inserted to measure the temperature close to the outlet (T1), middle (T2), and inlet (T3) of the confined saline layer 10. The three stage TSMD device was tilted by 30owith respect to the horizontal plane to initiate thermohaline convection and ensure sufficient normal solar flux. FIG. 5B shows the temperature of the confined saline layer 10 as a function of time measured by three thermocouples. Under one-sun illumination (1000 W m-2), the temperature of the confined saline layer 10 rapidly increased within the first 20 minutes and reached above 40 ℃ in steady state. As expected, the temperature of the saline increases further from the inlet. More interestingly, a periodic fluctuation of confined saline layer temperature with an amplitude of about 2 ℃ in steady state was observed, which is a sign of the strong thermohaline convection. This periodic temperature fluctuation originated from the formation of multiple vortices within the confined saline layer 10. The evolution of vortex patterns was observed using the IR camera. FIG. 5C shows that six vortex patterns were generated within the confined saline layer 10. Each two adjacent vortex patterns were separated by a low-temperature boundary (white-dashed lines 1-5). Within each individual vortex, the temperature distribution was uniform due to the thermohaline convection enhanced heat transfer. At the boundary of two neighboring vortices, however, the interaction of vortices created a stagnation flow region with weak convection (see side view illustration in FIG. 5C), resulting in the low- temperature boundaries. Driven by the buoyancy force, these vortices slowly migrated upward with about 1.5 mm min-1rising velocity and continuous generation of new vortices from the inlet. To identify the role of the salinity gradient in thermohaline convection, the same characterization was performed using pure water evaporation, where no salinity gradient could be created. In general, a similar temperature response was found. However, no temperature fluctuations were observed in steady state of the pure water evaporation, indicating that the salinity gradient is essential to initiate thermohaline convection. The role of tilt angle in the formation of thermohaline convection was also investigated. To ensure the same input heat flux at different tilt angles, the solar absorber 50 was replaced by a constant heat source, as shown in FIG. 5D, which provided 1000 W / m2heat flux. An aluminum plate was used as a condenser and integrated with the confined saline test setup. By measuring the amount of water collected from the condenser, the water production rate at different tilt angles may be quantified. Two K-type thermocouples were inserted in the middle of the confined saline layer (Ts) and on the condenser (Tcon), respectively, to characterize the impact of thermohaline convection on heat transfer. FIG. 5E shows the steady-state temperatures of the confined saline layer and condenser in the horizontal and 30otilt angle configurations. NaCl solution with 10 wt% salinity was used for both configurations. In the horizontal configuration, the steady-state temperature of the confined saline layer was about 58℃ and no periodic temperature fluctuations were seen, indicating the thermohaline convection was insignificant. Compared to the horizontal configuration, a lower steady-state temperature of the confined saline layer (about 52 ℃) was shown in the 30otilt angle configuration, indicating a more uniform temperature distribution due to the thermohaline convection. The difference of confined saline layer temperatures in two configurations can be explained by the change from a conduction to convection dominated heat transfer regime with the tilt, because in the conduction dominated regime, about 10℃ temperature difference can be created across a 5 mm thick confined saline layer according to Fourier’s law. FIG. 5E also shows the correlation between simulated results and experimental results for both the horizontal and tilted configurations. Note that no fluctuation was seen in the simulated horizontal configuration, while a temperature fluctuation was seen in the simulated tilted configuration. FIG. 5F shows the corresponding water production rate as a function of time. A higher water production rate of about 0.9 kg m-2h-1was obtained in the 30otilt angle configuration, as compared with the horizontal counterpart (about 0.7 kg m-2h-1). Due to the higher water production rate, more latent heat was released from vapor condensation, which explains the higher condenser temperature measured in the 30otilt angle configuration (see FIG. 5E). This improved water production was attributed to thermohaline convection. Specifically, thermohaline convection reduced the thermal resistance of the confined saline layer 10 between the solar absorber 50 and the evaporating interface of the hydrophobic membrane 20. Meanwhile, enhanced salt rejection enabled a low salinity at the evaporating interface, which is highly desirable to maintain a high vapor pressure and hence promote evaporation. In addition, due to the evaporation induced salt accumulation, water production rate in the horizontal configuration started to decrease after 20 minutes, associated with a slight increase of the confined saline layer temperature (about 1℃). To further support the experimental characterizations, a 2D numerical simulation of the confined saline layer was performed by coupling the salt transport with heat transfer and fluid flow. FIGs. 5G-5H show the flow velocity, salinity, and temperature profiles of the confined saline layer in the horizontal and 30otilt angle configurations, respectively. As expected, thermohaline convection was not developed in the horizontal configuration, as seen in the velocity profile of FIG. 5G. Salt and thermal transport relied on diffusion only, leading to high salinity at the evaporating interface and nonuniform temperature distribution across the saline layer, as seen in the salinity and temperature profiles of FIG. 5G. In contrast, the numerical simulation confirmed the strong thermohaline convection in the 30otilt angle configuration, as shown in FIG. 5H. Six clockwise vortices were shown in the simulated velocity profile, which agrees well with the IR characterizations, shown in FIG. 5C. Boundaries between adjacent vortex patterns, represented by the stagnation flow regions (1-5 in FIG. 5H), were also well-captured by the simulation. The magnitude of vortex velocity reached 1 mm s-1, which corresponds to a salt transport Péclet number (i.e., the ratio of salt convection and diffusion rates) Pes~ 103, indicating strong thermohaline convection. As a result, salinity distribution in the confined saline layer became uniform and salt accumulation at the evaporating interface was largely mitigated, as shown in the salinity profile of FIG. 5H. In addition, the simulated steady- state temperature of the confined saline layer, shown in the temperature profile of FIG. 5H, showed a reasonable agreement with the measurement (FIG. 5E), where a similar periodic temperature fluctuation due to the evolution of vortices was calculated. Despite a similar amplitude (about 2℃) captured by simulation, the experiment shows a longer periodicity of the temperature fluctuation (about 10 min) compared with that of the simulation (about 3 min), indicating a lower rising velocity of vortices in the experimental condition. The discrepancy between experiment and simulation may be attributed to the 2D simplification as well as the capillary pinning effect and the sensible heating of the PTFE membrane that were not captured by the simulation. To verify tilt angle in another way, a theoretical framework was developed to determine the effective range of the tilt angles using a dimensionless number called Rayleigh number (Ra), which is determined based on gravitational acceleration, the coefficient of fluid volume expansion, the average temperature difference between the hot and cold side of the confined saline layer, kinematic viscosity and thermal diffusivity. Rayleigh numbers greater than 1000 are effective for generating thermohaline convection. Under one sun illumination (1000 W m-2), strong thermohaline convection can be formed when the tilt angle exceeded 10º, and is less than 90°. However, as the solar irradiance decreases from 1000 W m-2to 250 W m-2, the critical tilt angle to initiate thermohaline convection increases from 10º to 30º. There is a large optimization space to simultaneously initiate thermohaline convection and maximize solar irradiance. For example, any tilt angle greater than the critical angle and less than 90° may be used. To measure water production, the configuration shown in FIG. 7A was used. This is similar to the configuration shown in FIG. 2B, however, in this configuration, the inner vessel 100 was also filled with saline. The inlet 31 of the TSMD device was connected with the reservoir 110 through holes in the inner vessel 100, forming a communicating vessel. The saline water level in the reservoir 110 was above the evaporation chamber of the membrane distillation device, flowing into the outlet 32 and creating a hydraulic head for the liquid supply. Saline water in the inner vessel 100 was used for cooling the condenser, which was maintained below the membrane distillation device. Distilled water flowed into a water collector 150. Water production rate was determined from the mass change of the water collector 150 using a digital balance 160. FIG. 6A shows the temperature response of the single-stage TSMD device when desalinating 20 wt% saline under one-sun illumination. Temperatures of the solar absorber 50 at two locations, the confined saline layer 10 at three locations, and condenser 70 during a five-hour operation were measured using six K-type thermocouples. Similar to the temperature response of the confined saline test setup (see FIG. 5B), the evaporator temperature of the TSMD device rapidly increased to above 40 ℃ within 20 minutes, followed by a slow temperature rise during the remaining four hours (about 2.5 ℃). This slow temperature rise was attributed to the accumulation of condensed water, which increased the thermal resistance of the condenser (bottom line in FIG. 6A). A periodic temperature fluctuation of the confined saline was also observed, indicating a strong thermohaline convection developed in the membrane distillation device. The temperature difference between the solar absorber and confined saline layer was insignificant (< 1.5℃), confirming a highly uniform temperature distribution in the confined saline layer due to thermohaline convection. To demonstrate the enhanced reliability of TSMD, the confined saline evaporator was replaced by a capillary wick (cotton tissue) to create a conventional single-stage WSD device for comparison. Except for the choice of evaporator, the other components in the TSMD and WSD devices were identical. Five-hour solar desalination tests were performed on both devices in the same laboratory condition (one-sun illumination and 22-23℃ ambient temperature). The total solar irradiance of the test was 5 kWh m-2, which was comparable to a common level of averaged daily solar irradiation (4.5 kWh m-2day-1). NaCl solutions with salinity of 3.5 wt%, 10 wt%, and 20 wt% were used for the tests, which represent the salinities of seawater, hypersaline wastewater, and nearly saturated solution (saturating point is 26 wt%), respectively. The starting point of the tests was defined as the moment that the solar simulator was turned on. FIG. 6B shows the real-time mass changes of the water collector during a five-hour operation of the TSMD and WSD devices. In general, for all tests, more than 30 g water was collected using these devices, indicating a highly efficient solar distillation. Compared with the WSD device, the TSMD device took approximately 10 minutes longer to initialize, and water collection started at about 25 minutes. This delay was because the confined saline layer 10 had a larger thermal mass than the capillary wick. Despite a slower initialization, after the five-hour desalination of both 3.5 wt% and 10 wt% saline, the total water collection for the TSMD device was slightly higher than that of the WSD device, primarily due to the thermohaline convection enhanced heat transfer and salt rejection. This result highlights the significant potential of TSMD for the long-term operation. FIG. 6C shows the corresponding water production rates of the TSMD and WSD devices, which were extracted from the mass changes of the water collector every 30 minutes. In general, higher salinity led to lower water production rate, which originates from the reduced water activity with the presence of salts. As shown in FIG. 6C, the water production performance of the single-stage WSD device shows a degrading trend due to salt accumulation. Specifically, for the 3.5 wt% saline, the water production rate of the WSD device decreased from about 1 kg m-2h-1to about 0.9 kg m-2h-1after the five-hour operation. With a higher salinity (10 wt%), the degradation of WSD performance became more significant, where the water production rate decreased from about 0.85 kg m-2h-1to about 0.7 kg m-2h-1. In contrast, the water production rates of both 3.5 wt% and 10 wt% saline for the TSMD device remained stable (about 1 and 0.85 kg m-2h-1, respectively) throughout the entire test. Moreover, the significantly enhanced salt rejection allows the TSMD device to approach the limit of salinity where the WSD device cannot be operated. A stable water production rate of about 0.7 kg m-2h-1was achieved for the nearly saturated saline (20 wt%), demonstrating a highly reliable desalination performance. To better quantify the performance of water production, the solar-to-water efficiency is defined as: ^^ ൌ^^^ ^ℎ^^^^ᇱᇱ ^ (1)^ ^where ^^^^, ℎ^^, latent heat of , and effective solar absorbing area (9.4 × 9.4 cm2), respectively. Note that in contrast to the conventional definition of efficiency based on the evaporation rate, this definition quantified the water production using the actual water collection. Compared with the evaporation rate, water collection rate is a more realistic metric for the performance of a practical device because it accounts for all potential losses during vapor condensation and water collection. Due to various loss mechanisms, it has been shown that approximately 25% produced vapor cannot be collected even for state-of-the-art solar desalination devices. FIG. 6D summarizes the solar-to-water efficiencies of the WSD and TSMD devices at the second and the fifth hour. A clear degradation of the WSD performance can be seen from the reduction of its solar- to-water efficiency, with 2.5% and 7% decreases of ^^ for 3.5 wt% and 10 wt% saline, respectively. In comparison, owing to the superior salt rejection performance, no degradation of ^^ was observed for the TSMD device. In fact, ^^ became slightly higher after five-hour operation for 3.5 wt%, 10 wt%, and 20 wt% saline, which can be explained by the fully developed thermohaline convection and additional water collection due to the removal of initially pinned droplets from the condenser. For another experiment, the setup shown in FIG. 7A is used. Three K-type thermocouples were inserted into the confined saline layers of three stages (Ts,1, Ts,2, and Ts,3), respectively. Additional three thermocouples were used to measure the condenser temperature of each stage (Tcon,1, Tcon,2, and Tcon,3). FIG. 7B shows the temperature response of the three-stage TSMD device when desalinating 20 wt% saline. The test was performed for four hours under one-sun illumination. In steady state, the temperatures of confined saline layers reached about 60℃ for the first stage (Ts,1), about 50℃ for the second stage (Ts,2), and about 40℃ for the third stage (Ts,3) as shown in FIG. 7B. Periodic temperature fluctuations were seen in the confined saline layer, indicating strong thermohaline convection in all three stages. The temperature difference between the condenser and the confined saline layer in the next stage was less than 2℃, demonstrating an efficient heat transfer and latent heat recovery. FIG. 7C shows the steady-state water production rates when desalinating 3.5 wt%, 10 wt%, and 20 wt% saline. In addition, using clean water (0 wt% saline) as the source water, a record-high water production rate of 2.5 kg m-2h-1among three-stage solar distillation devices was demonstrated, which is 2.5 times greater than the water collection of state-of- the-art single-stage devices. This result indicates that replacing the capillary wick by a confined saline layer not only substantially enhances salt rejection but enables a superior evaporation rate. With the increase of salinity, water production rate gradually decreased from 2.11 kg m-2h-1(3.5 wt%) to 1.51 kg m-2h-1(20 wt%), due to the reduction of water activity. For the 20 wt% salinity, however, the water production rate of the three- stage TSMD device was more than twice that of the single-stage TSMD device (which is about 0.7 kg m-2h-1). To understand the desalination performance of the TSMD, the Na+concentration was measured before and after solar distillation of the 3.5 wt%, 10 wt%, and 20 wt% saline using an inductively coupled plasma mass spectrometry. The results are shown in FIG. 7D. For all conditions, salinities were reduced by more than four orders of magnitude after desalination, which were more than one order of magnitude lower than the drinking water standard of the World Health Organization (200 mg L-1, WHO). FIG. 8 summarizes the demonstrated solar-to-water efficiency and the corresponding salinity of the present three-stage TSMD device and recent representative works. In general, record-high solar-to-water efficiency was achieved for three-stage solar distillation devices, spanning the full range of salinity. For all salinities, the solar-to-water efficiencies of the three-stage TSMD device were above the thermodynamic limit of single-stage distillation (100%). Note that the solar-to-water efficiency for most of the previous studies was determined from the evaporation rate, i.e., the mass loss rate of the reservoir, which can overestimate the efficiency because the losses during water collection were not considered. In addition, the long-term reliability in typical multistage devices either was not achieved or relied on introducing an active flow to carry the accumulated salts out, which requires additional energy input or reduces water production. As compared, the TSMD device enabled a reliable operation via the passively initiated thermohaline convection, which simultaneously improves water production by enhancing heat transfer. More importantly, the three-stage TSMD device accessed an unprecedented performance regime: desalination of hypersaline brine (> 3.5 wt% salinity) with above 100% solar-to-water efficiency (shadowed region in FIG. 8). Owing to the modular design, it is possible to pursue even higher water production by further increasing the number of stages. To demonstrate the long-term resistance to salt accumulation enabled by the TSMD device, a 60-hour desalination test of 20 wt% saline under one-sun illumination was conducted. The corresponding cumulative solar irradiance was 60 kWh m-2, equal to the total solar irradiance of more than two weeks. Note that in conventional reliability tests, a cycling operation was adopted. In each cycle, there was a three- to six-hour operation under one-sun illumination, followed by a 21 to 18-hour waiting period without solar illumination to emulate the nighttime condition and allow the salt rejection. However, in the present reliability test, a more stringent procedure was created by performing a 60-hour continuous test and removing all waiting periods. Considering 20 wt% saline used for the test, the total amount of salt to be rejected during the 60-hour operation is equivalent to the accumulated salt in seawater desalination (3.5 wt%) throughout about 80-day cycling operations. Moreover, under the same solar illumination, the salt production rate in this test was six times higher than that in the seawater desalination because of the high salinity (20 wt%). FIG. 9A shows the temperature response of a single-stage TSMD device during a 60-hour continuous desalination of 20 wt% saline. The temperature of the confined saline layer remained stable throughout the test, indicating reliable heat transfer performance. FIGs. 9B-9C show the mass change of the water collector and the resulting water production rate as a function of time, respectively. The good linear profile of mass change during 60-hour operation indicates a stable water production rate without degradation in performance. While many of the experiments highlighted a three-stage TSMD device, it is understood that devices having more or fewer stages may be created using this modular approach. As noted above, a ten- stage TSMD device was also fabricated and tested. The present system has many advantages. A multistage TSMD device exhibiting highly efficient water production and extreme resistance to salt accumulation is disclosed. This significantly improved solar desalination performance is due to the thermohaline convection within the confined saline layer, which enhances heat transfer and salt rejection simultaneously. With a modular design, a record-high performance three-stage TSMD device with production rates of 2.11 kg m-2h-1for 3.5 wt% saline and 1.51 kg m-2h-1for 20 wt% saline is possible, enabling a new regime with highly productive hypersaline desalination. Meanwhile, the enhanced salt rejection substantially improves the long-term reliability of solar desalination. With the developed TSMD device, continuous desalination of 20 wt% saline under 60 kWh m-2solar irradiance was performed without degradation of water production rate. The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.

Claims

What is claimed is:

1. A membrane distillation device that uses thermohaline convection, comprising: a surface to be heated; a hydrophobic membrane separated from the surface to be heated; an evaporation chamber defined by the surface to be heated and the hydrophobic membrane; an inlet and an outlet in communication with the evaporation chamber; and a condenser; wherein the membrane distillation device is disposed at a tilt angle with respect to a horizontal direction; such that the inlet is disposed at a lower end of the evaporation chamber and the outlet is disposed at an upper end of the evaporation chamber; and wherein water vapor produced in the evaporation chamber diffuses through the hydrophobic membrane to the condenser.

2. The membrane distillation device of claim 1, further comprising a water collector connected to the condenser.

3. The membrane distillation device of claim 1, further comprising a heat sink disposed adjacent to the condenser.

4. The membrane distillation device of claim 1, wherein the tilt angle is between 10° and 90°.

5. The membrane distillation device of claim 1, wherein a spacing between the surface to be heated and the hydrophobic membrane is between 1 and 10 mm.

6. The membrane distillation device of claim 1, wherein the surface to be heated comprises a solar absorber.

7. The membrane distillation device of claim 1, wherein the surface to be heated comprises the condenser from an adjacent membrane distillation device.

8. A system for water distillation, comprising: the membrane distillation device of claim 1; an inner vessel, in which the membrane distillation device is disposed; and a reservoir in which the inner vessel is disposed; wherein the inlet is in communication with an opening in the inner vessel to allow fluid to pass from the reservoir to the evaporation chamber via gravity driven flow.

9. The system of claim 8, wherein a water level of the reservoir is higher than the evaporation chamber, such that water from the reservoir enters the evaporation chamber via the gravity driven flow.

10. A multi-stage membrane distillation device that uses thermohaline convection, comprising: a plurality of stages, each stage comprising: a surface to be heated; a hydrophobic membrane separated from the surface to be heated; an evaporation chamber defined by the surface to be heated and the hydrophobic membrane;an inlet and an outlet in communication with the evaporation chamber; and a condenser; wherein each stage is disposed at a same tilt angle with respect to a horizontal direction; such that the inlet of each stage is disposed at a lower end of the evaporation chamber and the outlet of each stage is disposed at an upper end of the evaporation chamber; and wherein water vapor produced in the evaporation chamber of each stage diffuses through the hydrophobic membrane to the condenser.

11. The multi-stage membrane distillation device of claim 10, wherein the surface to be heated of a first stage comprises a solar absorber.

12. The multi-stage membrane distillation device of claim 11, wherein heat of condensation warms the condenser, and wherein the condenser of the first stage serves as the surface to be heated for an adjacent second stage.

13. The multi-stage membrane distillation device of claim 10, wherein the plurality of stages comprises three or more stages.

14. The multi-stage membrane distillation device of claim 10, further comprising a water collector connected to each condenser.

15. The multi-stage membrane distillation device of claim 10, further comprising a heat sink disposed adjacent to the condenser of a last stage.

16. The multi-stage membrane distillation device of claim 10, wherein the same tilt angle is between 10° and 90°.

17. The multi-stage membrane distillation device of claim 10, wherein a spacing between the surface to be heated and the hydrophobic membrane of each stage is between 1 and 10 mm.

18. The multi-stage membrane distillation device of claim 17, wherein the spacing is reduced for a second stage as compared to a first stage.

19. A system for water distillation, comprising: the multi-stage membrane distillation device of claim 10; an inner vessel, in which the multi-stage membrane distillation device is disposed; and a reservoir in which the inner vessel is disposed; wherein the inlet of each stage is in communication with one or more openings in the inner vessel to allow fluid to pass from the reservoir to each evaporation chamber via gravity driven flow.

20. The system of claim 19, wherein a water level of the reservoir is higher than each evaporation chamber, such that water from the reservoir enters each evaporation chamber via the gravity driven flow.

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