Extreme salt-resisting multistage solar distillation with thermohaline convection

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https://techxplore.com/news/2023-09-desalination-freshwater-cheaper.html

MIT researchers have designed a new solar desalination system that takes in saltwater and heats it with natural sunlight. The system flushes out accumulated salt, so replacement parts aren’t needed often, meaning the system could potentially produce drinking water at a rate and price that is cheaper than tap water. Credit: Jintong Gao and Zhenyuan Xu

Engineers at MIT and in China are aiming to turn seawater into drinking water with a completely passive device that is inspired by the ocean, and powered by the sun.

In a paper appearing in the journal Joule, the team outlines the design for a new solar desalination system that takes in saltwater and heats it with natural sunlight.

The configuration of the device allows water to circulate in swirling eddies, in a manner similar to the much larger "thermohaline" circulation of the ocean. This circulation, combined with the sun's heat, drives water to evaporate, leaving salt behind. The resulting water vapor can then be condensed and collected as pure, drinkable water. In the meantime, the leftover salt continues to circulate through and out of the device, rather than accumulating and clogging the system.

The new system has a higher water-production rate and a higher salt-rejection rate than all other passive solar desalination concepts currently being tested.

The researchers estimate that if the system is scaled up to the size of a small suitcase, it could produce about 4‚Äď6 liters of drinking water per hour and last several years before requiring replacement parts. At this scale and performance, the system could produce drinking water at a rate and price that is cheaper than tap water.

"For the first time, it is possible for water, produced by sunlight, to be even cheaper than tap water," says Lenan Zhang, a research scientist in MIT's Device Research Laboratory.

The team envisions a scaled-up device could passively produce enough drinking water to meet the daily requirements of a small family. The system could also supply off-grid, coastal communities where seawater is easily accessible.

Zhang's study co-authors include MIT graduate student Yang Zhong, and Evelyn Wang, the Ford Professor of Engineering, along with Jintong Gao, Jinfang You, Zhanyu Ye, Ruzhu Wang, and Zhenyuan Xu of Shanghai Jiao Tong University in China.

A powerful convection

The team's new system improves on their¬†previous design‚ÄĒa similar concept of multiple layers, called stages. Each stage contained an evaporator and a condenser that used heat from the sun to passively separate salt from incoming water.

That design, which the team tested on the roof of an MIT building, efficiently converted the sun's energy to evaporate water, which was then condensed into drinkable water. But the salt that was left over quickly accumulated as crystals that clogged the system after a few days. In a real-world setting, a user would have to place stages on a frequent basis, which would significantly increase the system's overall cost.

Outdoor test of the prototype under natural sunlight. Credit: Jintong Gao and Zhenyuan Xu

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In a follow-up effort, they devised a solution with a similar layered configuration, this time with an added feature that helped to circulate the incoming water as well as any leftover salt. While this design prevented salt from settling and accumulating on the device, it desalinated water at a relatively low rate.

In the latest iteration, the team believes it has landed on a design that achieves both a high water-production rate, and high salt rejection, meaning that the system can quickly and reliably produce drinking water for an extended period.

The key to their new design is a combination of their two previous concepts: a multistage system of evaporators and condensers, that is also configured to boost the circulation of water‚ÄĒand salt‚ÄĒwithin each stage.

"We introduce now an even more powerful convection, that is similar to what we typically see in the ocean, at kilometer-long scales," Xu says.

The small circulations generated in the team's new system is similar to the "thermohaline" convection in the ocean‚ÄĒa phenomenon that drives the movement of water around the world, based on differences in sea temperature ("thermo") and salinity ("haline").

"When seawater is exposed to air, sunlight drives water to evaporate. Once water leaves the surface, salt remains. And the higher the salt concentration, the denser the liquid, and this heavier water wants to flow downward," Zhang explains. "By mimicking this kilometer-wide phenomena in small box, we can take advantage of this feature to reject salt."

Tapping out

The heart of the team's new design is a single stage that resembles a thin box, topped with a dark material that efficiently absorbs the heat of the sun. Inside, the box is separated into a top and bottom section. Water can flow through the top half, where the ceiling is lined with an evaporator layer that uses the sun's heat to warm up and evaporate any water in direct contact.

The water vapor is then funneled to the bottom half of the box, where a condensing layer air-cools the vapor into salt-free, drinkable liquid. The researchers set the entire box at a tilt within a larger, empty vessel, then attached a tube from the top half of the box down through the bottom of the vessel, and floated the vessel in saltwater.

In this configuration, water can naturally push up through the tube and into the box, where the tilt of the box, combined with the thermal energy from the sun, induces the water to swirl as it flows through. The small eddies help to bring water in contact with the upper evaporating layer while keeping salt circulating, rather than settling and clogging.

The team built several prototypes, with one, three, and 10 stages, and tested their performance in water of varying salinity, including natural seawater and water that was seven times saltier.

From these tests, the researchers calculated that if each stage were scaled up to a square meter, it would produce up to 5 liters of drinking water per hour, and that the system could desalinate water without accumulating salt for several years. Given this extended lifetime, and the fact that the system is entirely passive, requiring no electricity to run, the team estimates that the overall cost of running the system would be cheaper than what it costs to produce tap water in the United States.

"We show that this device is capable of achieving a long lifetime," Zhong says. "That means that, for the first time, it is possible for drinking water produced by sunlight to be cheaper than tap water. This opens up the possibility for solar desalination to address real-world problems."

Jintong Gao 1 3, Lenan Zhang 2 3, Jinfang You 1, Zhanyu Ye 1, Yang Zhong 2, Ruzhu Wang 1, Evelyn N. Wang 2, Zhenyuan Xu 1 4

https://doi.org/10.1016/j.joule.2023.08.012 Get rights and content

https://www.cell.com/joule/fulltext/S2542-4351(23)00360-4

Context & scale

Utilizing sunlight to drive freshwater production is a promising way to realize sustainable water supply. Recent advances in multistage solar distillation attract particular interest because it enables a multifold enhancement in water production rate. However, existing solar-powered multistage distillers suffer from reliability and limited lifetime. The undesirable reliability makes the distilled water cost much higher than the tap water price, which significantly impedes the practical adoption. A key underlying limiting factor of device reliability is the evaporator fouling induced by salt accumulation. Inspired by a natural phenomenon, thermohaline convection in the deep ocean, we demonstrate multistage solar distillation with both record-high water production efficiency and extreme resistance to salt accumulation via temperature and salinity gradient manipulation across a novel confined-saline-layer evaporator. This promises a real-world impact of solar distillation technologies.

Summary

Recent advances in¬†multistage¬†solar distillation¬†are promising for the sustainable supply of freshwater. However, significant¬†performance degradation¬†due to salt accumulation has posed a challenge for both long-term reliability of solar¬†desalination¬†and efficient treatment of hypersaline discharge. Here, inspired by a natural phenomenon, thermohaline convection, we demonstrate a solar-powered¬†multistage¬†membrane distillation¬†with extreme salt-resisting performance. Using a confined saline layer as an¬†evaporator, we initiate strong thermohaline convection to mitigate salt accumulation and enhance heat transfer. With a ten-stage device, we achieve record-high solar-to-water efficiencies of 322%‚Äď121% in the salinity range of 0‚Äď20 wt % under one-sun illumination. More importantly, we demonstrate an extreme resistance to salt accumulation with 180-h continuous¬†desalination¬†of 20 wt % concentrated seawater. With high freshwater production and extreme salt endurance, our device significantly reduces the water production cost, paving a pathway toward the practical adoption of passive solar desalination for sustainable water economy.

Graphical abstract

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Introduction

Freshwater shortage is a grand challenge for humanity due to the continuously increasing gap between global water demand and supply.12¬†Solar distillation technology, including solar still and passive solar membrane distillation (MD), provides a passive, decentralized, and zero-emission solution to sustainable water production, which can largely increase the accessibility to freshwater.34567¬†However, performance of conventional systems with volumetric heating of bulk water is highly limited due to the significant heat loss.89¬†Over the past few years, localized heating at evaporating interface by integrating novel photothermal materials and components into solar stills has been demonstrated as an effective approach to significantly improve the efficiency of solar-to-vapor conversion.10111213141516¬†In particular, recent advances in thermally localized multistage solar distillation promise ultrahigh-performance water production (Figure¬†1A).171819¬†By recovering the latent heat released from vapor condensation, the multistage configuration exceeds the thermodynamic limit of single-stage solar distillation (‚Čą1.47¬†kg m‚ąí2¬†h‚ąí1)2021¬†and enables a multifold enhancement of water production.22232425

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.1926¬†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‚ąí2¬†continuous solar irradiance.2223272829¬†If efficient salt rejection can be achieved under ‚ąľ10¬†kWh m‚ąí2¬†continuous solar irradiance, the lifetime of a solar desalination device is expected to be extended from a few months to more than 1 year, leading to a 1 order of magnitude reduction in cost for the produced water.19¬†On the other hand, to build sustainable water economy, it requires proper treatment of hypersaline discharge from seawater desalination to maximize freshwater recovery and mitigate environmental impact.303132¬†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 4 orders of magnitude slower than molecular diffusion (e.g., vapor diffusion).33 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.34 In addition, to achieve the simultaneous thermal localization and passive saline supply, the thermally localized multistage solar distillation commonly relies on a capillary wick evaporator (Figure 1A).22232435363738 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 (Figure 1B).3339 Several strategies have been proposed to mitigate salt accumulation, including self-rotating evaporator,3040 Janus evaporator,27283141 ionic rejection,4243 localized crystallization,44454647 and enhanced salt transport through water channels or liquid flows.2933394849505152 However, most state-of-the-art approaches focus on a type of evaporator with solar-thermal conversion and vapor escaping on the same side of the evaporator, which is not fully compatible with the multistage configuration.53 Moreover, the salt rejection is only demonstrated with low-salinity brine during short-term operation in many studies, which cannot fully address the significant salt accumulation associated with the distillation of hypersaline brine in long-term operation.

Here, we demonstrate a highly efficient solar-powered multistage membrane distillation (MD) with extreme resistance to salt accumulation (Figure¬†1C). Our solution to mitigate salt accumulation is inspired by a natural phenomenon known as the thermohaline convection, which is the primary driving force for the energy and species transport in deep ocean.545556¬†Thermohaline convection is a type of natural convection due to the density gradient induced by both temperature and salinity.5758¬†We replace the capillary wick by a thin layer of saline, which is confined by a hydrophobic membrane (Figure¬†1D). Learning from the MD and diffusion gap distillation,59606162¬†an air gap separates the membrane with the condenser to reduce conductive heat loss and vapor diffusion resistance.236364¬†Passive saline supply is enabled by interfacing the MD module with a floating communicating vessel (Figures¬†1D and S1), which creates a hydraulic head within the MD module. By properly engineering the device, we can manipulate the temperature and salinity gradient across the confined saline layer and hence initiate a strong thermohaline convection to accelerate salt rejection (close-up illustration of Figure¬†1D). Meanwhile, the thermohaline convection enhances heat transfer across the confined saline layer, ensuring high water production rate comparable to that of the conventional wick structure-based solar distillation (WSD). We confirmed the¬†strong thermohaline convection in the confined saline layer with detailed thermofluidic characterization and simulation. Taking advantage of both latent heat recovery¬†and thermohaline convection, we constructed a ten-stage solar MD prototype. Freshwater production rates of 4.74, 3.82, 2.86, and 1.78¬†kg m‚ąí2¬†h‚ąí1¬†(solar-to-water efficiencies of 322%‚Äď121%) were demonstrated under one-sun illumination (1,000¬†W m‚ąí2) with pure water, 3.5 wt % saline, 10 wt % saline, and 20 wt % saline, respectively. This indicates that our device achieved record-high solar-to-water efficiencies for passive solar distillation, covering the full range of salinity from 0 to 20 wt¬†%. With our thermohaline convection-enhanced solar MD (TSMD), an unprecedented regime of passive solar distillation with high-efficiency water production (exceeding the single-stage limit) and ultrahigh salinity (20 wt %, nearly saturated) was enabled. Owing to the superior salt rejection capability, we further confirm the long-term reliability of TSMD by demonstrating stable desalination of 20 wt % concentrated seawater in a 180-h continuous test, where the amount of salt rejected from the TSMD is equivalent to the total salt accumulation during approximately 229-day desalination of seawater. This work demonstrates an approach that has extreme resistance to salt accumulation to address the bottleneck toward the practical adoption of highly efficient solar desalination. Our 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.

Section snippets

Thermohaline convection in the confined saline layer

Figure 1D illustrates the physical origin of thermohaline convection. By removing the capillary wick, a thin saline layer is confined between a solar absorber and a hydrophobic membrane. Heat is transferred from the solar absorber to the confined saline layer, which drives evaporation through micropores of the membrane. As a result, the saline layer has higher temperature close to the solar absorber and exhibits higher salinity near the membrane, leading to a density gradient across the

Discussion

We have demonstrated a multistage TSMD approach exhibiting highly efficient water production and extreme resistance to salt accumulation. 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, we show record-high solar-to-water efficiencies of 322%‚Äď121% for the whole salinity range of 0‚Äď20 wt %, enabling a new regime with highly

Lead contact

Further information and requests for resources and materials should be directed to and will be fulfilled by the lead contact, Zhenyuan Xu (xuzhy@sjtu.edu.cn).

Materials availability

This study did not generate unique materials.

Design and fabrication of the prototypes

The prototype frames were 3D printed using nylon 7500 (WeNext Technology). To simultaneously achieve reliable sealing and flexible modular design, the frames were designed with twelve screw holes to connect adjacent stages together. A hollow structure was adopted for enhancing thermal insulation

Acknowledgments

We gratefully acknowledge the funding support from the National Natural Science Foundation of China (52376200, 51976123).

Author contributions

Z.X., L.Z., and E.N.W. conceived the initial idea. J.G. and Z.X. developed the experimental device. J.G. and J.Y. conducted the experiments. J.G. and L.Z. performed numerical simulation. Z.X., J.G., L.Z., and E.N.W. analyzed the results. L.Z. and J.G. wrote the manuscript with input from all authors. Z.X., E.N.W., and R.W. supervised the research. All authors discussed the

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