All it takes is a pinch of salt, a dash of sunlight, a solar panel, and a thirsty polymer
It’s a warm summer evening in Saudi Arabia, a relief from the heavy heat of a sun-scorched day. Renyuan Li looks down at the glass of water in his hand, a wide smile spread across his face. He has summoned fresh water from thin air.
Per UNICEF, two-thirds of the world’s population experiences severe water scarcity for at least one month each year, and half of the world’s population could face water scarcity by 2025.
As climate change dries Saudi Arabia’s already parched spaces and steals moisture from the soils of cities and villages across the world, an urgent question resounds across farms, government buildings, and homes everywhere: “Where do I get my water?”
Li and his team of researchers at the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia have an answer.
“Conventional liquid water resources such as river, lake, or seawater – these are no longer the limitation,” Li explains. “Atmospheric water can be replenished by the ambient atmospheric cycle itself.”
Their creation, the water-electricity-crop co-production system – or the WEC2P – offers communities in arid and semi-arid climates access to sustainable water production and more. In one day, depending on its setting, the WEC2P can produce about 600 milliliters of water, or 586 watt-hours of electricity per square meter, and simultaneously grow 60 plants.
The light at the end of the tunnel
Li couldn’t see the WEC2P in his future as an undergraduate studying material science engineering.
“I was not supposed to be a scientist or a researcher… I have a very bad grade in my undergraduate study,” Li says, “I [didn’t] know what to do when I graduated from university.”
He was certain he couldn’t get into graduate school, and he didn’t really know what sort of work he wanted to pursue. But in 2013, his career started coming into focus. Li was in his third year at the Beijing University of Chemical Technology when he traveled to an annual science conference in Beijing. There, he met Peng Wang. Wang’s research in water filtration efforts struck and amazed Li, and to Li’s surprise, Wang wanted Li to join him in his work.
“I don’t know why Professor Wang decided to recruit me at the time,” Li says.
But after their one chance meeting, Wang became Li’s mentor and advocate, bending Li’s future toward success. Under Wang, Li spent a gap year as a visiting student at KAUST’s Visiting Student Research Program (VSRP), and when Li was ready, Wang and the program director of the VSRP battled KAUST’s resistant admission committee until Li and his low grades were admitted to KAUST’s graduate program for environmental science engineering.
A few years later, Li had completed the first of many products that would evolve into the WEC2P. It was a flat, five or six centimeter disc with colorful salt atop synthetic material that he had developed. It sat in a petri dish, and when it worked, a few drops of condensed water clung to the walls of the dish.
Li calls this time the “Stone Age” of the WEC2P.
Evolution of the WEC2P
Today, the WEC2P is a little more complex. It uses solar cell technology to squeeze electricity from the sun and harnesses heat waste produced by the solar panel to drive clean water production. It has a plant-growing unit (PGU) that reduces plants’ water intake.
The heart of the WEC2P’s water capture system is its hydrogel. It consists of two parts: the top is calcium chloride, a salt; the bottom is a custom polymer made from materials that have been freeze-dried and soaked in a solution. These are separated by an anti-corrosive film so that the salt can’t damage the polymer.
Water vapor pressure is key to the hydrogel’s utility. The air around us, even in the desert, always contains some water vapor but the hydrogel itself has very little water vapor. This means water vapor tends to flow into it.
Initially, Li’s team used an expensive carbon nanotube base instead of the polymer, but since the WEC2P is intended to be broadly accessible, they got more creative.
“We [wanted] to find some alternative…a very cheap one,” Li says. This hydrogel’s low cost and easy production make it the ideal material for water capture in the WEC2P.
The hydrogel’s low water vapor pressure makes water absorption possible. The air around us contains some water vapor, even in the desert. That water vapor always has some pressure. If you’ve ever taken a road trip up to a high elevation and noticed that your chip bag was about to pop, you’ve seen how air in the bag expands in the low pressure air to try and reach equilibrium. The same is true for water vapor: it moves from high to low pressures.
The hydrogel’s water vapor pressure remains lower than the vapor pressure in the surrounding air because of the calcium chloride salt layer. Positively charged parts of the water are attracted to negatively charged ions in the salt and vice versa. This draws water vapor to the salt, encourages the water to stay with the salt, and in turn, keeps pressure within the salt water solution low. Seeking equilibrium, water vapor continues to move from ambient air into the hydrogel, increasing pressure there until all salt ions are paired.
“The salt has an extraordinary affinity for water,” Li says. It holds “maybe five times its own volume in water.” So with a lot of salt, the hydrogel can hold a lot of water.
This part of the science isn’t news. Researchers including Li have been working to draw water from ambient air for a few years now. In 2017, a popular academic paper describing the process opened the field to widespread research. Reading it for the first time as a master’s student, Li remembers that the technology “[felt] like magic.” He wanted to be a part of making it.
Most other researchers in the field use the sun as a direct heat source to collect water from air. Li and his team relied on recycled solar panel heat waste to do the job.
When the WEC2P is closed, heat waste from the solar panel drives evaporation. Heat increases the temperature of the salt, causing the water in it to evaporate and move into the air within the system. Once it cools down, this vapor condenses into water, which the system collects in a metal chamber enclosing the hydrogel before it’s funneled into a glass bottle.
About 80% of sunlight that hits the solar panel is normally converted into heat rather than electricity. In other scenarios, when solar panels are used only for producing electricity, that thermal energy goes unused, but not in the WEC2P.
In fact, because this heat waste is directed away, toward the hydrogel, the solar panel also cools, improving its efficiency by nearly 10%. The WEC2P also helps produce crops alongside electricity and water.
While Li credits his multidisciplinary team for putting together the project, he was the glue that brought agronomy, material science, and engineering perspectives together.
Ideas sprout in gardens, too
Today, Li is an avid gardener. When he first moved into KAUST’s graduate student housing, he came into a spacious backyard, one that was muggy and bug-filled, but brimming with potential. He cleared the space and planted vegetables. Passion for his new hobby grew as steadily as his seedlings did. (He likes his luffa plants the most.) Somewhere along the line, it began making sense to integrate crop growth into the WEC2P, too.
“Think about how water contributes to our daily life,” Li suggests. “When you’re in places like the middle of the desert, islands, or remote areas [where fresh water may be scarce], probably food is another concern in your life.”
In 2020, up to 811 million people went undernourished. These communities in the economic “bottom billion” are the same groups that lack access to safe drinking water and electricity. By incorporating crop growth into the WEC2P, Li and his team plan to improve accessibility to the most fundamental resources necessary for quality of life and sustainable development.
Li decided to try growing water spinach with as little water as possible. Water spinach in the plant-growing unit sprouted from holes in a covered tray, sitting inside of a hole-covered box, shrouded in a sunlight-shielding net. By housing the spinach, he could minimize water lost through plant transpiration, and holes in the housing kept temperatures low enough for plants to survive. When his two-week test was through, his plants in the PGU had a 95% survival rate. None in the control group given the same amount of water survived.
Making it viable
Growth extended beyond the plant-growing unit. Li and his team saw plenty of it. It took five years and six permutations of their project to get to this point.
“I enjoy the sense of maturement after overcoming the challenge,” Li says. “Each challenge for me is a chance to improve myself, to reach the limit of myself.”
Li recalls one moment when the team first began integrating components of the WEC2P. The system was dry during its first dry run. There was no water, and no one knew why. Li spent a week scouring the system for anything that could account for the failure. Eventually, he found a gap that was leaking precious water vapor-filled air. He reconfigured the condensation subsystem until the problem went away.
The team also bumped up against the occasional ominous Microsoft update that erased all data and forced it to restart collection.
For the most part, Li and his team steadily integrated new components and tested different materials and layouts until they found the right balance between utility, material efficiency, and low cost to make the WEC2P possible for global distribution.
Leave the world more nourished than you found it
The team now aims to test the WEC2P in a high, central province of Saudi Arabia, where the land and air are far drier. They will adjust the WEC2P until it is effective in all climates and environments, including the most arid.
Li will complete his postdoctoral research at KAUST at the end of the year and hopes to get a faculty position thereafter. He looks forward to seeing the WEC2P on the market in the next few years, and to seeing it impact communities around the world.
By doing this work, he says “You are shaping the future. You are leaving a… small mark on the history of mankind.”
And that’s something to feel excited about.
Elle Bernbaum is a freelance writer with a bachelor’s in physics from the University of Washington. She loves this stuff
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