MIT Researchers build a portable desalination unit that generates clear, clean drinking water without the need for filters or high-pressure pumps.
MIT researchers have developed a portable desalination unit, weighing less than 10 kilograms, that can remove particles and salts to generate drinking water.
The suitcase-sized device, which requires less power to operate than a cell phone charger, can also be driven by a small, portable solar panel, which can be purchased online for around $50. It automatically generates drinking water that exceeds World Health Organization quality standards. The technology is packaged into a user-friendly device that runs with the push of one button.
Unlike other portable desalination units that require water to pass through filters, this device utilizes electrical power to remove particles from drinking water. Eliminating the need for replacement filters greatly reduces the long-term maintenance requirements.
This could enable the unit to be deployed in remote and severely resource-limited areas, such as communities on small islands or aboard seafaring cargo ships. It could also be used to aid refugees fleeing natural disasters or by soldiers carrying out long-term military operations.
“This is really the culmination of a 10-year journey that I and my group have been on. We worked for years on the physics behind individual desalination processes, but pushing all those advances into a box, building a system, and demonstrating it in the ocean, that was a really meaningful and rewarding experience for me,” says senior author Jongyoon Han, a professor of electrical engineering and computer science and of biological engineering, and a member of the Research Laboratory of Electronics (RLE).
Joining Han on the paper are first author Junghyo Yoon, a research scientist in RLE; Hyukjin J. Kwon, a former postdoc; SungKu Kang, a postdoc at Northeastern University; and Eric Brack of the U.S. Army Combat Capabilities Development Command (DEVCOM). The research has been published online in Environmental Science and Technology.
Watch: YouTube Videp
Filter-free technology
Commercially available portable desalination units typically require high-pressure pumps to push water through filters, which are very difficult to miniaturize without compromising the energy-efficiency of the device, explains Yoon.
Instead, their unit relies on a technique called ion concentration polarization (ICP), which was pioneered by Han’s group more than 10 years ago. Rather than filtering water, the ICP process applies an electrical field to membranes placed above and below a channel of water. The membranes repel positively or negatively charged particles — including salt molecules, bacteria, and viruses — as they flow past. The charged particles are funneled into a second stream of water that is eventually discharged.
The process removes both dissolved and suspended solids, allowing clean water to pass through the channel. Since it only requires a low-pressure pump, ICP uses less energy than other techniques.
But ICP does not always remove all the salts floating in the middle of the channel. So the researchers incorporated a second process, known as electrodialysis, to remove remaining salt ions.
Yoon and Kang used machine learning to find the ideal combination of ICP and electrodialysis modules. The optimal setup includes a two-stage ICP process, with water flowing through six modules in the first stage then through three in the second stage, followed by a single electrodialysis process. This minimized energy usage while ensuring the process remains self-cleaning.
“While it is true that some charged particles could be captured on the ion exchange membrane, if they get trapped, we just reverse the polarity of the electric field and the charged particles can be easily removed,” Yoon explains.
They shrunk and stacked the ICP and electrodialysis modules to improve their energy efficiency and enable them to fit inside a portable device. The researchers designed the device for nonexperts, with just one button to launch the automatic desalination and purification process. Once the salinity level and the number of particles decrease to specific thresholds, the device notifies the user that the water is drinkable.
The researchers also created a smartphone app that can control the unit wirelessly and report real-time data on power consumption and water salinity.
Beach tests
After running lab experiments using water with different salinity and turbidity (cloudiness) levels, they field-tested the device at Boston’s Carson Beach.
Yoon and Kwon set the box near the shore and tossed the feed tube into the water. In about half an hour, the device had filled a plastic drinking cup with clear, drinkable water.
“It was successful even in its first run, which was quite exciting and surprising. But I think the main reason we were successful is the accumulation of all these little advances that we made along the way,” Han says.
The resulting water exceeded World Health Organization quality guidelines, and the unit reduced the amount of suspended solids by at least a factor of 10. Their prototype generates drinking water at a rate of 0.3 liters per hour, and requires only 20 watts of power per liter.
“Right now, we are pushing our research to scale up that production rate,” Yoon says.
One of the biggest challenges of designing the portable system was engineering an intuitive device that could be used by anyone, Han says.
Yoon hopes to make the device more user-friendly and improve its energy efficiency and production rate through a startup he plans to launch to commercialize the technology.
In the lab, Han wants to apply the lessons he’s learned over the past decade to water-quality issues that go beyond desalination, such as rapidly detecting contaminants in drinking water.
“This is definitely an exciting project, and I am proud of the progress we have made so far, but there is still a lot of work to do,” he says.
For example, while “development of portable systems using electro-membrane processes is an original and exciting direction in off-grid, small-scale desalination,” the effects of fouling, especially if the water has high turbidity, could significantly increase maintenance requirements and energy costs, notes Nidal Hilal, professor of engineering and director of the New York University Abu Dhabi Water research center, who was not involved with this research.
“Another limitation is the use of expensive materials,” he adds. “It would be interesting to see similar systems with low-cost materials in place.”
The research was funded, in part, by the DEVCOM Soldier Center, the Abdul Latif Jameel Water and Food Systems Lab (J-WAFS), the Experimental AI Postdoc Fellowship Program of Northeastern University, and the Roux AI Institute.
Alharbawi Naseer Tawfiq Alwan assembled a prototype of the distiller in the UrFU workshop. Credit: Ilya Safarov.
Distillation of water using solar energy is considered one of the most popular desalination methods today.
Power engineers at Ural Federal University (UrFU), together with colleagues from Iraq, have developed a new desalination technology, which is claimed to be much more effective than others, by incorporating a rotating cylinder.
The method proposed by the UrFU power engineers will significantly reduce the cost of desalination and will increase production volumes by four times.
The experimental new solar distiller incorporates a rectangular basin, inside of which is a horizontally oriented black steel cylinder. The basin is filled with undrinkable water, and the cylinder is slowly rotated by a solar-powered DC motor.
The rotating hollow cylinder inside the solar distiller accelerates water evaporation in the vessel by forming a thin film of water on the outer and inner surface of the cylinder, which is constantly renewed with each turn. As the film is so thin, the water film quickly evaporates due to the rapid transfer of heat from the surface of the cylinder to the adjacent water film. To increase the temperature of water under the cylinder, the engineers used a solar collector.
prototype was tested on a rooftop in the Russian city of Ekaterinburg for several months (June-October, 2019). As part of the experiment, the rotation speed of the cylinder inside the solar distiller was 0.5 rpm. This intensity and time are enough to evaporate a thin film of water from the surface of the cylinder.
The tests showed the high efficiency and reliability of the developed device. In addition, the scientists noted that the relatively high intensity of solar radiation and low ambient air temperature also contributed to the performance of water distillation.
“The performance improvement factor of the created solar distiller, compared to traditional devices, was at least 280% in the relatively hot months (June, July, and August) and at least 300% and 400% in the cooler months (September and October), at the same time, the cumulative water distillation capacity reached 12.5 l/m2 per day in summer and 3.5 l/m2 per day in winter,” commentedAlharbawi Naseer Tawfik Alwan, a research engineer at the Department of Nuclear Power Plants and Renewable Energy (UrFU).
The desalination technology created in the UrFU with a simple design and low cost may be especially in demand in the Middle East and Africa – in countries with a high potential for solar energy and a shortage of freshwater.
In the future, scientists plan to improve the technology and increase the performance of the solar distiller at the lowest possible capital and operating costs for different climatic conditions.
The authors acknowledge the support received from the National Science Foundation through the Engineering Research Center for Nanotechnology-Enabled Water Treatment (EEC1449500) and via Grant CBET 1437630. The authors also acknowledge funding from the National Science Foundation Graduate Research Fellowship Program under grant no. DGE-1752134, awarded to C.J.P. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
ABSTRACT
Transmembrane protein channels, including ion channels and aquaporins that are responsible for fast and selective transport of water, have inspired membrane scientists to exploit and mimic their performance in membrane technologies. These biomimetic membranes comprise discrete nanochannels aligned within amphiphilic matrices on a robust support. While biological components have been used directly, extensive work has also been conducted to produce stable synthetic mimics of protein channels and lipid bilayers.
However, the experimental performance of biomimetic membranes remains far below that of biological membranes. In this review, we critically assess the status and potential of biomimetic desalination membranes. We first review channel chemistries and their transport behavior, identifying key characteristics to optimize water permeability and salt rejection.
We compare various channel types within an industrial context, considering transport performance, processability, and stability. Through a re-examination of previous vesicular stopped-flow studies, we demonstrate that incorrect permeability equations result in an overestimation of the water permeability of nanochannels. We find in particular that the most optimized aquaporin-bearing bilayer had a pure water permeability of 2.1 L m–2 h–1 bar–1, which is comparable to that of current state-of-the-art polymeric desalination membranes.
Through a quantitative assessment of biomimetic membrane formats, we analytically show that formats incorporating intact vesicles offer minimal benefit, whereas planar biomimetic selective layers could allow for dramatically improved salt rejections. We then show that the persistence of nanoscale defects explains observed subpar performance. We conclude with a discussion on optimal strategies for minimizing these defects, which could enable breakthrough performance.
Biomimetic Desalination Membranes
As stressors like population growth, industrialization, and climate change threaten to deplete and contaminate our freshwater resources, larger bodies of saline water could provide a vast supply of water for drinking, agricultural, and industrial use.(1) However, desalination of these waters requires more energy and financial resources than traditional freshwater purification methods.(2) Currently, the state-of-the-art technology for desalination is reverse osmosis (RO) using thin-film-composite (TFC) polyamide membranes.(3,4) Fully aromatic TFC-RO membranes are readily produced at industrial scale through interfacial polymerization, whereby a rapid reaction occurs at the interface of immiscible organic and aqueous phases to form a highly cross-linked polyamide selective layer on a porous support(5) (Figure 1). With the advent of the TFC-RO membrane and energy recovery devices, seawater desalination energy requirements for the RO stage have drastically reduced from ∼15 kWh m–3 using the original cellulose acetate membranes of the 1970s down to only ∼2 kWh m–3, only ∼25% above the practical minimum energy.(2)
Figure 1. Transition in desalination research from focusing on dense polymers that reject salt by a solution-diffusion mechanism to considering sub-nanometer channels capable of molecularly sieving ions. In the solution-diffusion panel (left), common reactants for TFC-RO membranes are represented, which rapidly react at an organic–aqueous interface during interfacial polymerization to form a cross-linked, fully aromatic polyamide selective layer with characteristic ridge-valley morphology. Salt rejection determined by a solution-diffusion mechanism results from the higher partitioning and/or diffusion rates of water over ions. In the ion sieving panel (right), common molecular sieves that have been considered for desalination are shown, with ideal water pathways illustrated. In pores similar in size to water, single-file water transport is induced. Nanotubes and nanochannels can be synthetic (e.g., carbon nanotubes) or biological (e.g., aquaporins). To produce nanoporous sheets, sub-nanometer pores where only a few atoms are vacant have been etched in single-layer graphene using chemical oxidation, electron beam irradiation, doping, and ion bombardment.(12) For 2D laminates, the water pathway is through interlayer spaces between sheets. Studies so far have primarily considered graphene oxide nanosheets for 2D laminates.(13,14) The molecular sieving mechanism for ion rejection is by size exclusion, where highly uniform pores exclude larger solutes and ideally transport only molecules similar in size to water.
Reduced Operational Costs, Improved Reliability and Efficiency and Enhanced Product Water Quality
Despite the substantial reduction in energy consumption and overall cost, seawater RO still has room for improvement. While current water permeabilities enable near-optimal performance, increased water-solute selectivity would allow for reduced operational costs, improved reliability and efficiency, and enhanced product water quality.(4) For example, TFC-RO membranes inadequately retain chloride and some small neutral solutes, such as boron in seawater desalination and trace organic contaminants in wastewater reuse, necessitating extra purification steps which increase the cost of desalination.(4)Transport through the polyamide layer is well described by the solution-diffusion model, in which permeants (i.e., water and solutes) partition into the dense polyamide layer and diffuse through it (Figure 1).(6)
The resultant permselectivity of the membrane is attributed to differences in abilities and rates of species to dissolve into and diffuse through the polyamide membrane material.(7) Although intrinsic water permeability can far exceed salt permeability during solution diffusion, as it does for polyamide, historical data suggest that it will be difficult to significantly advance performance with polymeric systems. Commercial desalination and water purification membranes typically exhibit a permeability–selectivity trade-off, similar to the Robeson plot for polymeric gas separations.(8−11)
Furthermore, despite many decades of extensive research, no polymeric material has yet surpassed the desalination performance (i.e., water permeability, water-salt selectivity, and cost-effectiveness) of fully aromatic polyamide.
To overcome the limitations of the solution-diffusion-based polyamide membranes, research focus has shifted toward the development of desalination membranes that remove solutes via molecular sieving. In this mechanism of ion rejection, highly uniform, rigid pores that are smaller than the diameter of hydrated salt ions transport water and nearly completely reject ions by size exclusion (Figure 1). Recent formats of molecular sieves considered for desalination include nanotubes and nanochannels, two-dimensional (2D) laminates, and nanoporous sheets (Figure 1).(14)
However, these top-down efforts have failed so far to achieve adequate salt rejection due to the persistence of defects coupled with the daunting challenges of tuning interlayer spacing or pore size.(12,13,15)
Biomimetic membranes, or composites comprising an amphiphilic matrix with discrete, aligned nanochannels on a robust support, may provide a platform for industrial-scale molecular sieves that overcome the limitations of solution-diffusion-based polyamide membranes.
After over 3.5 billion years of evolution,(16,17) the cell membranes of modern organisms can perform an array of highly complicated functions, which rely on a system of complex transmembrane proteins aligned within the amphiphilic lipid bilayer. In this system, water and only select ions pass through channel pores and pumps, depending on the energy and nutrient needs of the cell.(18,19) In pioneering work, Preston et al. determined that an integral membrane protein formed a biological channel that selectively transports water in and out of many types of cells. This protein was the CHannel-forming Integral Protein of 28 kDa (CHIP28),(20,21) later called the aquaporin. For these discoveries, Peter Agre and Roderick MacKinnon shared the 2003 Nobel Prize in Chemistry. Through additional biophysical studies, ion channels also showed impressive selectivity, inspiring the design of synthetic ion channels.(22−25)
Eventually, researchers realized the potential implications of these channels for industrial-scale water purification, especially aquaporin in the use of desalination, and attempted to produce biomimetic membranes, or materials that mimic the structure and performance of biological membranes.(26−35)
While much of the work has focused on water-solute separations, the biomimetic membrane format also presents opportunities to develop membranes with tunable selectivity based on a chosen channel type.However, translating biological mechanisms into industrial-scale technology necessitates scale-up by orders of magnitude—from the microscopic size of a cell membrane to tens of square meters.(36) For industrial relevance, the synthesis of a biomimetic membrane would need to be cost-effective and simple. Simultaneously, such a membrane would need to be mechanically stable under RO pressures exceeding 70 bar and chemically stable during repeated membrane cleaning and usage.(37) Notably, even at the lab scale, sufficiently high-salt rejection has not yet been achieved for biomimetic desalination membranes after over a decade of research.(37,38)
Therefore, channels, selective layer formats, synthesis strategies, and support layer types must be carefully considered to attain the capabilities of this technology. While certain aspects of biomimetic desalination membranes have been reviewed recently,(37−42) a critical analysis of their performance and their potential application in water-treatment processes remains necessary.In this critical review, we examine efforts toward biomimetic desalination membranes for water purification in order to identify the best strategies to realize their full potential for both desalination and solute–solute selectivity. We first examine molecular transport, contrasting solution-diffusion with molecular sieving and assessing transport through the mixed matrix of biomimetic selective layers. We next identify the key characteristics of the aquaporin that explain its ultraselectivity and fast water transport, comparing this biological channel to several synthetic channels and placing each in industrial context. Using corrected analysis of reported permeability measurements, we then show that the water permeabilities of many channels have been overestimated. Subsequently, we predict best-case-scenario outcomes for common biomimetic formats, including membranes with intact vesicles and membranes with planar biomimetic layers. For the more promising planar format, the biggest challenge is the presence of nanoscale defects. Through mathematical models, we estimate the defect density for several reported biomimetic membranes. We then discuss synthesis pathways that could limit both the presence of defects and the effect of defects on transport performance.
We conclude with a discussion on the practicality of biomimetic desalination membranes and how to best exploit the strengths of discrete nanochannels as molecular sieves in other applications beyond desalination.
Figure 2. Channel types for biomimetic membranes, including biological as well as bioinspired and bioderived. (a) Single channel water permeability versus pore interior diameter. The pore interior diameter here is defined as the inner diameter of the most constricted region. For PAH[4], the pore diameter shown refers to the average width of dynamic voids that formed in channel clusters.
Channel permeabilities from stopped-flow data and simulations were adjusted to 25 °C and corrected for any previous errors in permeability calculation (see SI, Section S1 for details). (b) (left) Overhead view of AqpZ tetramer and (right) side view of single AqpZ channel with characteristic hourglass shape. (c) GramA dimer as it exists in biological and vesicular environments. In organic solvents, the monomers can intertwine to form a parallel or antiparallel helix. AqpZ and GramA diagrams were drawn using PyMOL(69) with protein sequences from the Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB-PDB, https://www.rcsb.org/), PDB-ID codes 1RC2(70) and 1NRM.(71) (d) Dependence of water diffusivity through pores on the number of interior hydrogen-bond points.
Data extracted from ref (56). (e) Cyclic peptide nanotubes. (left) Hydrogen-bonding pattern of pore-forming, stacked cyclic peptides. A polyglycine structure is shown for simplicity; side residues would typically be present. (right) Modified cyclic peptide with interior peptide-mimicking functional groups. The analogous unmodified cyclic peptide is radially symmetrical with a fourth primary amine side chain (cyclo[(d-Ala-Lys)4]).(72) (f) Single-walled carbon nanotube porins (wide and narrow) with armchair pattern. Number of carbons approximate those of wCNTP and nCNTP. (g) (left) PAP[5] and (right) PAH[4] nanochannels with peptide appendages that form interarm hydrogen bonds. (h) (left) Aquafoldamer subunits with “sticky” ends. Differences in end groups that comprise Aqf1 and Aqf2 subunits are illustrated. (right) Weak hydrogen-bonding pattern of pore-forming, stacked aquafoldamers. Six subunits are needed to cross a DOPC membrane. (i) Pure water permeability versus total channel areal coverage in a DOPC bilayer. Single channel permeabilities from (a) were divided by channel cross-sectional area and converted into channel water permeability (A) coefficients using eq 3. Overall biomimetic layer A coefficients were calculated for various densities of channels within a DOPC bilayer using eq 7. DOPC bilayer hydraulic permeability was taken as 0.15 L m–2 h–1 bar–1.(73) The shaded region indicates the water permeability of current commercial TFC-RO membranes, an adequate range for desalination performance. Permeabilities are listed in Table S1.
** Read the Complete Paper and Conclusions from AZ Nano by Following the Link Below
Cassandra J. Porter: Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut
Jay R. Werber: Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut and Department of Chemistry, University of Minnesota, Minneapolis, Minnesota
Mingjiang Zhong: Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut
Corey J. Wilson: School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States
Menachem Elimelech: Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut
A new carbon membrane could someday be used in commercial desalination plants
Leiden chemists have created a new ultrathin membrane only one molecule thick. The membrane can produce a hundred times more power from seawater than the best membranes used today. The researchers have published their findings in Nature Nanotechnology.
Thin and porous
When fresh and saltwater meet, an exchange of salt and other particles takes place. A membrane placed in water is able to harness energy from particles moving from one side to the other. A similar process can also be used to desalinate seawater. Leiden chemists have developed a new membrane that can produce a hundred times more energy than classic membranes and known prototype membranes in scientific literature.
How much power is generated depends on the thickness of the membrane and how porous it is. Researchers were able to create a carbon based membrane that is both porous and thin. That is why it can produce more energy than current membranes, which are either porous or thin, but not both.
Credit: Xue Liu
To create this new membrane, Xue Liu and Grégory Schneider spread a large number of oily molecules on a water surface. These molecular building blocks then form a thin film on their own. By heating the film, the molecules are locked in place, creating a stable and porous membrane. According to Xue Liu, the membrane can be adapted for specific requirements. Liu: “The membrane we’ve created is only two nanometers thick and permeable to potassium ions. We can change the properties of the membrane by using a different molecular building block. That way we can adapt it to suit any need.”
Graphene
The new carbon membrane is similar to graphene, a large flat membrane made up of only carbon atoms. But according to Grégory Schneider, this new membrane is in a whole different category. Schneider: “When making a membrane, a lot of researchers start out with graphene, which is very thin, but not porous. They then try to punch holes in it to make more permeable. We’ve done the reverse by assembling small molecules and building a larger porous membrane from those molecules. Compared to graphene, it contains imperfections, but that’s what gives it its special properties.”
This new membrane combines the best of both worlds. Schneider: “Much of the research in this field was focused on creating better catalysts, membranes were somewhat of a dead end. This new discovery opens up whole new possibilities for power generation, desalination and for building much more efficient fuel cells.”
More information: Xue Liu et al. Power generation by reverse electrodialysis in a single-layer nanoporous membrane made from core–rim polycyclic aromatic hydrocarbons, Nature Nanotechnology (2020). DOI: 10.1038/s41565-020-0641-5
Oceanus Water & Power design for environment-friendly desalination plant
The contraption, reminiscent of Rube Goldberg, would produce two of Southern California’s most precious and essential resources: water and electricity.
The electricity would be renewable. And the drought-proof, desalinated ocean water could prove more environmentally friendly — and cheaper — than the water produced from three other desalters proposed for Southern California.
The idea, developed by Silicon Valley-based Neal Aronson and his Oceanus Power & Water venture, caught the attention of the Santa Margarita Water District. The agency quickly saw the project’s viability to fill a void.
“Somebody looked at a problem differently than anybody has in the past,” said district General Manager Dan Ferons. “It’s really creative and got us excited about it. … It could become a primary source of water for south Orange County.”
While Oceanus’s proposals at locations in Mexico and Chile have advanced to the preliminary engineering phases, it remains in the conceptual stage for a plant in northern Camp Pendleton.
But because south Orange County is almost entirely dependent on imported water — and vulnerable to shortages during droughts and earthquakes that can disrupt imported flows — the Santa Margarita Water District last summer signed a non-binding memorandum of understanding with Oceanus for possible participation.
“You don’t want to have just one source of water,” Ferons said, noting that the district is also interested in desalted water from two other plants proposed for the region.
“We thought, ‘Here’s another opportunity to increase our resilience.’ It made sense to encourage and support the project while they pull it together.”
How it works
The Oceanus process begins by pumping water from the ocean to a reservoir some 1,000 feet above sea level, using solar and wind energy to power the two-way pump turbines during daylight hours when those renewable sources are plentiful. During the evening and early morning, water would be released to run downhill, most of it churning through turbines to create electricity when solar and wind energy are unavailable.
Additionally, a portion of the downhill water would be diverted into a desalination operation, where gravity would force it through reverse osmosis membranes that remove the salt.
Relying on gravity rather than electricity to push water through the filters is key to making it cheaper than the water that someday could be produced at desalination plants proposed for Huntington Beach, Dana Point and El Segundo.
Meanwhile, the salty brine byproduct would be mixed with the other outward-bound seawater, greatly diluting it before entering the ocean. Concerns linger among some environmentalists about the harm that the brine would inflict on marine life at other proposed plants, something that would be minimized by the Oceanus approach.
“No one has done this type of project anywhere in the world,” said Oceanus CEO Aronson, whose background is in real estate development and renewable energy projects. “It’s climate resilient. And we’re not planning to use any energy beyond pumping water from the ocean. … I’m a huge believer in the value this integration can bring.”
Inspiration and invention
The idea of creating energy by releasing water to drive turbines is hardly a new one — that’s how hydroelectric dams work. Even the idea of pumping water into reservoirs when electric rates are low and releasing it when they spike — a process known as pumped-storage hydroelectricity — has been used for over a century, Aronson said.
He recalled seeing such plants while vacationing with his parents in Switzerland and France as a child.
Neal Aronson, CEO of Oceanus Power & Water. (Courtesy of Oceanus Power & Water)
And then, in 2014, while developing a solar farm project near the San Luis Reservoir in Merced County, he sized up his project sitting in the shadow of the reservoir and began imagining future sites for pumped-storage hydroelectricity.
Solar and wind are great but what do you do at night?” Aronson said. “Chemical batteries aren’t the right solution for large-scale energy storage.”
While batteries increasingly are being used in California to store solar and wind energy for use during off hours, Aronson points out that those batteries have a limited life span, are still expensive, and can have negative environmental consequences as they have a carbon footprint and are not yet recyclable.
After Aronson began thinking about pumped-storage hyrdoelectricity plants, his focus sharpened to the possibility of building such plants along the coast, using ocean water — something that was only being done in Japan.
Next, the desalter portion of the plant clicked into place.
“In conversations with engineers, one kind of flippantly said, ‘If you’re going to stick a straw in the ocean and suck water out, why don’t you desalinate it while you’re at it,’ ” Aronson recalled. “And we figured out, yeah, you can do that.”
Next steps
Oceanus, founded in 2015, is farthest along with its plans in Chile, where Aronson said he may have all permits necessary to break ground within two years.
The proposal in Sonora, Mexico, on the Sea of Cortez sounds more tentative. While Oceanus has a site, a feasibility study is still underway by the Binational Desalination Work Group. If the U.S.-Mexico entity decides to go forward, Oceanus would likely compete with other bidders. The water would go to both Mexico and to southernmost U.S. states that depend on increasingly uncertain water supplies from the Colorado River.
Camp Pendleton is farther off still. Aronson said he has had talks with the base, the Navy and the Department of Defense, but the decision to go forward has not yet been made. A selling point for the military is that the plant could help the base become more resilient and self-sufficient in terms of water and electricity, which is of particular interest to the Department of Defense, Aronson said.
“The first step is to get them to draft and issue a solicitation for something like this, and we would bid into it,” Aronson said. He declined to speculate on how long it might take for the project to get off the ground, but Santa Margarita Water District’s Ferons estimated a minimum of five years.
While Aronson envisions building a solar farm to power a plant in Sonora, he said a Camp Pendleton plant would likely use power from the state electrical grid during hours when it’s being fed by renewable sources.
Tests on an MIT building rooftop showed that a simple proof-of-concept desalination device could produce clean, drinkable water at a rate equivalent to more than 1.5 gallons per hour for each square meter of solar collecting area. Images courtesy of the researchers
System achieves new level of efficiency in harnessing sunlight to make fresh potable water from seawater.
A completely passive solar-powered desalination system developed by researchers at MIT and in China could provide more than 1.5 gallons of fresh drinking water per hour for every square meter of solar collecting area. Such systems could potentially serve off-grid arid coastal areas to provide an efficient, low-cost water source.
The system uses multiple layers of flat solar evaporators and condensers, lined up in a vertical array and topped with transparent aerogel insulation. It is described in a paper appearing today in the journal Energy and Environmental Science, authored by MIT doctoral students Lenan Zhang and Lin Zhao, postdoc Zhenyuan Xu, professor of mechanical engineering and department head Evelyn Wang, and eight others at MIT and at Shanghai Jiao Tong University in China.
The key to the system’s efficiency lies in the way it uses each of the multiple stages to desalinate the water. At each stage, heat released by the previous stage is harnessed instead of wasted. In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation.
The device is essentially a multilayer solar still, with a set of evaporating and condensing components like those used to distill liquor. It uses flat panels to absorb heat and then transfer that heat to a layer of water so that it begins to evaporate. The vapor then condenses on the next panel. That water gets collected, while the heat from the vapor condensation gets passed to the next layer.
Whenever vapor condenses on a surface, it releases heat; in typical condenser systems, that heat is simply lost to the environment. But in this multilayer evaporator the released heat flows to the next evaporating layer, recycling the solar heat and boosting the overall efficiency.
“When you condense water, you release energy as heat,” Wang says. “If you have more than one stage, you can take advantage of that heat.”
Adding more layers increases the conversion efficiency for producing potable water, but each layer also adds cost and bulk to the system. The team settled on a 10-stage system for their proof-of-concept device, which was tested on an MIT building rooftop. The system delivered pure water that exceeded city drinking water standards, at a rate of 5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area. This is more than two times as much as the record amount previously produced by any such passive solar-powered desalination system, Wang says.
Theoretically, with more desalination stages and further optimization, such systems could reach overall efficiency levels as high as 700 or 800 percent, Zhang says.
Unlike some desalination systems, there is no accumulation of salt or concentrated brines to be disposed of. In a free-floating configuration, any salt that accumulates during the day would simply be carried back out at night through the wicking material and back into the seawater, according to the researchers.
Their demonstration unit was built mostly from inexpensive, readily available materials such as a commercial black solar absorber and paper towels for a capillary wick to carry the water into contact with the solar absorber. In most other attempts to make passive solar desalination systems, the solar absorber material and the wicking material have been a single component, which requires specialized and expensive materials, Wang says. “We’ve been able to decouple these two.”
The most expensive component of the prototype is a layer of transparent aerogel used as an insulator at the top of the stack, but the team suggests other less expensive insulators could be used as an alternative. (The aerogel itself is made from dirt-cheap silica but requires specialized drying equipment for its manufacture.)
Wang emphasizes that the team’s key contribution is a framework for understanding how to optimize such multistage passive systems, which they call thermally localized multistage desalination. The formulas they developed could likely be applied to a variety of materials and device architectures, allowing for further optimization of systems based on different scales of operation or local conditions and materials.
One possible configuration would be floating panels on a body of saltwater such as an impoundment pond. These could constantly and passively deliver fresh water through pipes to the shore, as long as the sun shines each day. Other systems could be designed to serve a single household, perhaps using a flat panel on a large shallow tank of seawater that is pumped or carried in. The team estimates that a system with a roughly 1-square-meter solar collecting area could meet the daily drinking water needs of one person. In production, they think a system built to serve the needs of a family might be built for around $100.
The researchers plan further experiments to continue to optimize the choice of materials and configurations, and to test the durability of the system under realistic conditions. They also will work on translating the design of their lab-scale device into a something that would be suitable for use by consumers. The hope is that it could ultimately play a role in alleviating water scarcity in parts of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.
“This new approach is very significant,” says Ravi Prasher, an associate lab director at
Lawrence Berkeley National Laboratory and adjunct professor of mechanical engineering at the University of California at Berkeley, who was not involved in this work. “One of the challenges in solar still-based desalination has been low efficiency due to the loss of significant energy in condensation. By efficiently harvesting the condensation energy, the overall solar to vapor efficiency is dramatically improved. … This increased efficiency will have an overall impact on reducing the cost of produced water.”
The research team included Bangjun Li, Chenxi Wang and Ruzhu Wang at the Shanghai Jiao Tong University, and Bikram Bhatia, Kyle Wilke, Youngsup Song, Omar Labban, and John Lienhard, who is the Abdul Latif Jameel Professor of Water at MIT. The research was supported by the National Natural Science Foundation of China, the Singapore-MIT Alliance for Research and Technology, and the MIT Tata Center for Technology and Design.
Concentrating the sunlight on tiny spots on the heat-generating membrane exploits an inherent and previously unrecognized nonlinear relationship between photothermal heating and vapor pressure. Credit: Pratiksha Dongare/Rice University
Rice University’s solar-powered approach for purifying salt water with sunlight and nanoparticles is even more efficient than its creators first believed.
Researchers in Rice’s Laboratory for Nanophotonics (LANP) this week showed they could boost the efficiency of their solar-powered desalination system by more than 50% simply by adding inexpensive plastic lenses to concentrate sunlight into “hot spots.” The results are available online in the Proceedings of the National Academy of Sciences.
“The typical way to boost performance in solar-driven systems is to add solar concentrators and bring in more light,” said Pratiksha Dongare, a graduate student in applied physics at Rice’s Brown School of Engineering and co-lead author of the paper. “The big difference here is that we’re using the same amount of light. We’ve shown it’s possible to inexpensively redistribute that power and dramatically increase the rate of purified water production.”
In conventional membrane distillation, hot, salty water is flowed across one side of a sheetlike membrane while cool, filtered water flows across the other. The temperature difference creates a difference in vapor pressure that drives water vapor from the heated side through the membrane toward the cooler, lower-pressure side. Scaling up the technology is difficult because the temperature difference across the membrane—and the resulting output of clean water—decreases as the size of the membrane increases. Rice’s “nanophotonics-enabled solar membrane distillation” (NESMD) technology addresses this by using light-absorbing nanoparticles to turn the membrane itself into a solar-driven heating element.
Rice University researchers (from left) Pratiksha Dongare, Alessandro Alabastri and Oara Neumann showed that Rice’s ‘nanophotonics-enabled solar membrane distillation’ (NESMD) system was more efficient when the size of the device was scaled up and light was concentrated in ‘hot spots.’ Credit: Jeff Fitlow/Rice University
Dongare and colleagues, including study co-lead author Alessandro Alabastri, coat the top layer of their membranes with low-cost, commercially available nanoparticles that are designed to convert more than 80% of sunlight energy into heat. The solar-driven nanoparticle heating reduces production costs, and Rice engineers are working to scale up the technology for applications in remote areas that have no access to electricity.
The concept and particles used in NESMD were first demonstrated in 2012 by LANP director Naomi Halas and research scientist Oara Neumann, who are both co-authors on the new study. In this week’s study, Halas, Dongare, Alabastri, Neumann and LANP physicist Peter Nordlander found they could exploit an inherent and previously unrecognized nonlinear relationship between incident light intensity and vapor pressure.
Alabastri, a physicist and Texas Instruments Research Assistant Professor in Rice’s Department of Electrical and Computer Engineering, used a simple mathematical example to describe the difference between a linear and nonlinear relationship. “If you take any two numbers that equal 10—seven and three, five and five, six and four—you will always get 10 if you add them together. But if the process is nonlinear, you might square them or even cube them before adding. So if we have nine and one, that would be nine squared, or 81, plus one squared, which equals 82. That is far better than 10, which is the best you can do with a linear relationship.”
In the case of NESMD, the nonlinear improvement comes from concentrating sunlight into tiny spots, much like a child might with a magnifying glass on a sunny day. Concentrating the light on a tiny spot on the membrane results in a linear increase in heat, but the heating, in turn, produces a nonlinear increase in vapor pressure. And the increased pressure forces more purified steam through the membrane in less time.
Researchers from Rice University’s Laboratory for Nanophotonics found they could boost the efficiency of their solar-powered desalination system by more than 50% by adding inexpensive plastic lenses to concentrate sunlight into “hot spots.” . Credit: Pratiksha Dongare/Rice University
“We showed that it’s always better to have more photons in a smaller area than to have a homogeneous distribution of photons across the entire membrane,” Alabastri said.
Halas, a chemist and engineer who’s spent more than 25 years pioneering the use of light-activated nanomaterials, said, “The efficiencies provided by this nonlinear optical process are important because water scarcity is a daily reality for about half of the world’s people, and efficient solar distillation could change that.
“Beyond water purification, this nonlinear optical effect also could improve technologies that use solar heating to drive chemical processes like photocatalysis,” Halas said.
For example, LANP is developing a copper-based nanoparticle for converting ammonia into hydrogen fuel at ambient pressure.
Halas is the Stanley C. Moore Professor of Electrical and Computer Engineering, director of Rice’s Smalley-Curl Institute and a professor of chemistry, bioengineering, physics and astronomy, and materials science and nanoengineering.
NESMD is in development at the Rice-based Center for Nanotechnology Enabled Water Treatment (NEWT) and won research and development funding from the Department of Energy’s Solar Desalination program in 2018.
More information: Pratiksha D. Dongare et al, Solar thermal desalination as a nonlinear optical process, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073/pnas.1905311116
Researchers at Swinburne, the University of Sydney and Australian National University have collaborated to develop a solar absorbing, ultra-thin graphene-based film with unique properties that has great potential for use in solar thermal energy harvesting.
The 90 nanometre material is said to be a 1000 times finer than a human hair and is able to rapidly heat up to 160°C under natural sunlight in an open environment.
The team stated that this new graphene-based material may also open new avenues in:
thermophotovoltaics (the direct conversion of heat to electricity)
It could possibly lead to the development of ‘invisible cloaking technology’ through developing large-scale thin films enclosing the objects to be ‘hidden’.
The researchers have developed a 2.5cm x 5cm working prototype to demonstrate the photo-thermal performance of the graphene-based metamaterial absorber. They have also proposed a scalable manufacturing strategy to fabricate the proposed graphene-based absorber at low cost.
“This is among many graphene innovations in our group,” says Professor Baohua Jia, Research Leader, Nanophotonic Solar Technology, in Swinburne’s Center for Micro-Photonics.
“In this work, the reduced graphene oxide layer and grating structures were coated with a solution and fabricated by a laser nanofabrication method, respectively, which are both scalable and low cost.”
“Our cost-effective and scalable graphene absorber is promising for integrated, large-scale applications that require polarisation-independent, angle insensitive and broad bandwidth absorption, such as energy-harvesting, thermal emitters, optical interconnects, photodetectors and optical modulators,” says first author of this research paper, Dr Han Lin, Senior Research Fellow in Swinburne’s Center for Micro-Photonics.
“Fabrication on a flexible substrate and the robustness stemming from graphene make it suitable for industrial use,” Dr Keng-Te Lin, another author, added.
“The physical effect causing this outstanding absorption in such a thin layer is quite general and thereby opens up a lot of exciting applications,” says Dr Bjorn Sturmberg, who completed his PhD in physics at the University of Sydney in 2016 and now holds a position at the Australian National University.
“The result shows what can be achieved through collaboration between different universities, in this case with the University of Sydney and Swinburne, each bringing in their own expertise to discover new science and applications for our science,” says Professor Martijn de Sterke, Director of the Institute of Photonics and Optical Science.
“Through our collaboration we came up with a very innovative and successful result. We have essentially developed a new class of optical material, the properties of which can be tuned for multiple uses.”
Researchers at The University of Manchester’s National Graphene Institute in the UK have succeeded in making artificial channels just one atom in size for the first time. The new capillaries, which are very much like natural protein channels such as aquaporins, are small enough to block the flow of smallest ions like Na+ and Cl- but allow water to flow through freely. As well as improving our fundamental understanding of molecular transport at the atomic scale, and especially in biological systems, the structures could be ideal in desalination and filtration technologies.
** See More About Graphene (YouTube Video) and Desalination at the end of this article **
Researchers at The University of Manchester’s National Graphene Institute in the UK have succeeded in making artificial channels just one atom in size for the first time. The new capillaries, which are very much like natural protein channels such as aquaporins, are small enough to block the flow of smallest ions like Na+ and Cl- but allow water to flow through freely. As well as improving our fundamental understanding of molecular transport at the atomic scale, and especially in biological systems, the structures could be ideal in desalination and filtration technologies.
“Obviously, it is impossible to make capillaries smaller than one atom in size,” explains team leader Sir Andre Geim. “Our feat seemed nigh on impossible, even in hindsight, and it was difficult to imagine such tiny capillaries just a couple of years ago.”
Naturally occurring protein channels, such as aquaporins, allow water to quickly permeate through them but block hydrated ions larger than around 7 A in size thanks to mechanisms like steric (size) exclusion and electrostatic repulsion. Researchers have been trying to make artificial capillaries that work just like their natural counterparts, but despite much progress in creating nanoscale pores and nanotubes, all such structures to date have still been much bigger than biological channels.
Geim and colleagues have now fabricated channels that are around just 3.4 A in height. This is about half the size of the smallest hydrated ions, such as K+ and Cl-, which have a diameter of 6.6 A. These channels behave just like protein channels in that they are small enough to block these ions but are sufficiently big to allow water molecules (with a diameter of around 2.8 A) to freely flow through.
The structures could, importantly, help in the development of cost-effective, high-flux filters for water desalination and related technologies – a holy grail for researchers in the field.
Credit: University of Manchester
Atomic-scale Lego
Publishing their findings in Science the researchers made their structures using a van der Waals assembly technique, also known as “atomic-scale Lego”, which was invented thanks to research on graphene. “We cleave atomically flat nanocrystals just 50 and 200 nanometre in thickness from bulk graphite and then place strips of monolayer graphene onto the surface of these nanocrystals,” explains Dr. Radha Boya, a co-author of the research paper. “These strips serve as spacers between the two crystals when a similar atomically-flat crystal is subsequently placed on top. The resulting trilayer assembly can be viewed as a pair of edge dislocations connected with a flat void in between. This space can accommodate only one atomic layer of water.”
Using the graphene monolayers as spacers is a first and this is what makes the new channels different from any previous structures, she says.
The Manchester scientists designed their 2-D capillaries to be 130 nm wide and several microns in length. They assembled them atop a silicon nitride membrane that separated two isolated containers to ensure that the channels were the only pathway through which water and ions could flow.
Until now, researchers had only been able to measure water flowing though capillaries that had much thicker spacers (around 6.7 A high). And while some of their molecular dynamics simulations indicated that smaller 2-D cavities should collapse because of van der Waals attraction between the opposite walls, other calculations pointed to the fact that water molecules inside the slits could actually act as a support and prevent even one-atom-high slits (just 3.4 A tall) from falling down. This is indeed what the Manchester team has now found in its experiments.
Measuring water and ion flow
“We measured water permeation through our channels using a technique known as gravimetry,” says Radha. “Here, we allow water in a small sealed container to evaporate exclusively through the capillaries and we then accurately measure (to microgram precision) how much weight the container loses over a period of several hours.”
To do this, the researchers say they built a large number of channels (over a hundred) in parallel to increase the sensitivity of their measurements. They also used thicker top crystals to prevent sagging, and clipped the top opening of the capillaries (using plasma etching) to remove any potential blockages by thin edges present here.
To measure ion flow, they forced ions to move through the capillaries by applying an electric field and then measured the resulting currents. “If our capillaries were two atoms high, we found that small ions can move freely though them, just like what happens in bulk water,” says Radha. “In contrast, no ions could pass through our ultimately-small one-atom-high channels.
“The exception was protons, which are known to move through water as true subatomic particles, rather than ions dressed up in relatively large hydration shells several angstroms in diameter. Our channels thus block all hydrated ions but allow protons to pass.”
Since these capillaries behave in the same way as protein channels, they will be important for better understanding how water and ions behave on the molecular scale – as in angstrom-scale biological filters. “Our work (both present and previous) shows that atomically-confined water has very different properties from those of bulk water,” explains Geim. “For example, it becomes strongly layered, has a different structure, and exhibits radically dissimilar dielectric properties.”
Water, one of the world’s most abundant and highly demanded resources for sustaining life, agriculture, and industry, is being contaminated globally or is unsafe for drinking, creating a need for new and better desalination methods. Current desalination methods have high financial, energy, construction, and operating costs, resulting in them contributing to less than 1% of the world’s reserve water supplies. Advances in nanoscale science and engineering suggest that more cost effective and environmentally friendly desalination process using graphene is possible …
Credit: Xue Liu
Tests on an MIT building rooftop showed that a simple proof-of-concept desalination device could produce clean, drinkable water at a rate equivalent to more than 1.5 gallons per hour for each square meter of solar collecting area. Images courtesy of the researchers
Credit: University of Manchester
MIT: Optimizing carbon nanotube electrodes for energy storage and water desalination applications
Genesis Nanotechnologyo and l o 
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