Water is all around us. The only problem is that it remains trapped in the atmosphere until the right conditions release it as rain or snow. Now Omar Yaghi, a chemistry professor at the University of California, Berkeley, has helped find a way to grab that water anytime we need it.
Yaghi says it is possible using something called a metal-organic framework (MOF). This is basically a sponge-like compound that looks like sand to the naked eye, and which is extremely dense. Depending on the materials involved, the MOF can be constructed to harvest different chemicals, from industrial emissions to natural gas.
Yaghi and his team at U.C. Berkeley found that a metal-organic framework that includes the element zirconium will capture water out of the air at night, store it, then release it during the daytime upon exposure to the heat of sunlight – no electricity needed. They collaborated with researchers from the Massachusetts Institute of Technology to build the water-collecting device and are working to develop a version with the capacity to serve a single-family home at a competitive price.
Other devices are capable of capturing water from the air in humid environments, like so-called “fog catchers” tested successfully around the world. What makes their invention so remarkable is that it works especially well in arid environments, like those that exist throughout the West. Water Deeply recently interviewed Yaghi to learn more.
Water Deeply: What was your inspiration for this water-harvesting device?
Omar Yaghi: We were actually studying the trapping of carbon dioxide using metal-organic frameworks from post-combustion gases, which include water. Water presents a challenge since it competes with carbon dioxide for the adsorptive sites in the pores of MOFs.
We noticed that some MOFs exhibit a unique uptake of water. A highly cooperative phenomenon seems to dominate the uptake and result in a sharp water permeation of the MOF at very low relative humidity. In essence, inside these MOFs, one has solid water in hot weather.
Water Deeply: And once inside the MOF, how do you get the water out?
Yaghi: To get the water out, as the goal is liquid water, one simply needs to heat the materials to a mere 45C (113F). This is the temperature [needed] inside the enclosed device, which can be a lot higher than the ambient outdoor temperature – similar to the greenhouse effect. This provides enough energy to break those weak bonds and release the water from the pores. This was very surprising, but also exciting. It meant that certain MOFs can be used to trap water from the atmosphere especially in arid climates, and then be released easily for collection.
However, we quickly realized that the only way to accelerate the development of this water-harvesting technology is through worldwide collaborations and competitions. Before that can happen, the world needs to visualize this chemistry in action first.
Water Deeply: And how did you make that happen?
Yaghi: We teamed up with mechanical engineers to design and build a simple harvester around the MOFs. Together with our collaborators, we demonstrated that a MOF-based device is capable of delivering clean water even in typical desert climates. Remarkably, the device operates without any power input, aside from ambient sunlight.
The MOF is exposed to the atmosphere, bringing water from dry air into its pores and concentrating it. Then, upon enclosing the MOF into a container, which is exposed to sunlight, the container heats up and water is released from the MOF into the container as high humidity. This, in turn, is condensed by virtue of the temperature difference between the warm interior of the container and room temperature.
The water harvester works passively and can be placed in the desert to trap water at night and release it during the day when the temperature rises. This technology has just been showcased at the 2017 World Economic Forum as one of the top 10 emerging technologies to change the world.
Water Deeply: Can you briefly describe how a metal-organic framework works, and what it looks like?
Yaghi: Metal-organic frameworks are materials made by stitching together organic and inorganic units into porous frameworks, which can have extremely high internal surface areas. One gram of a MOF the size of a sugar cube has an internal surface area equivalent to an entire football field.
A magnified image of a metal-organic framework of the sort that Omar Yaghi developed to harvest water from the air. (Photo Courtesy Omar Yaghi)
This high surface area ensures high capacity for the captured water. The ability to modify the pores chemically by changing the chemical characteristics, and physically by changing the pore size and shape, leads to MOFs tailored for specific functions such as water capture, carbon dioxide capture and conversion to fuels, methane storage for automobile fuel tanks and so on.
MOFs are solids which appear to the naked eye no different than sand, where each granule is riddled with holes into which gases and molecules of interest can be selected, brought in and compacted, making it possible to store voluminous amounts of gases in smaller containers. However, if you look at the MOF crystals under the microscope, many of them can be quite beautiful.
Water Deeply: Will it harvest water in all types of climates, or only desert environments?
Yaghi: There are over 70,000 different MOFs made to date, and each MOF has its own unique function and capability. We can design and make different MOFs capable of such efficient water uptake at humidity levels as low as 5 percent and as high as 80 percent. I believe the necessary temperature inside the device (113F) can be achievable in many regions other than the deserts.
Water Deepy: Does the water need to be made potable after being harvested?
Yaghi: No, the water being harvested is clean and pure. It is no different than rain water or distilled water.
Water Deeply: How large would this device have to be to serve an average home?
Yaghi: We are currently working on the next generation MOF water-harvester. I think after further improvement, a MOF water harvester the size of a washing machine could satisfy the most basic needs of a household.
Water Deeply: Do you envision a time when a device like this becomes commonplace on homes? How long might that take?
Yaghi: Yes, my vision is to achieve “personalized water,” where people have a device at home running on ambient solar, delivering water that will satisfy the needs of a household. There are certainly many advances still required to bring this technology to the kind of utility and accessibility to make his vision come true. Nevertheless with our invention, it is very possible now.
Water Deeply: Is it possible to scale this up to serve, say, an apartment building or even a small farm?
Yaghi: Yes. Our research team at U.C. Berkeley is going to unveil very soon a next-generation MOF water harvester that is larger in scale and of higher efficiency.
Water Deeply: I gather you’re also developing a cheaper version using aluminum instead of zirconium. What is the current cost of the device? And what might the cost be for a residential-sized unit once it’s market ready?
Yaghi: It is very difficult to say precisely how much it will cost at this point, but I am confident it will be relatively inexpensive because the major components are plastic and the MOF. In our next-generation MOF water-harvesting system, which is going to be published in early June, we have replaced the copper mesh of the device with much cheaper materials.
Also, we are going to introduce an aluminum-based MOF that can deliver more than twice the amount of water but at the same time is at least 150 times cheaper than the zirconium-based MOF. The progress of innovation of this technology is fast, and the cost is going down every day. I believe in two to three years, this water-harvesting technology will be ready for commercial production.