Although the Sun’s heat is critical to solar-powered water purification, researchers from the University of Buffalo found a counter-intuitive way of improving the filtration process: keeping things cool.

By using carbon-dipped paper to weaken light intensity, the new system increases efficiency and provides a more affordable water-treatment system. The university team hopes the technology will soon support disaster-relief in countries impacted by natural disasters, as well as remote villages with no access to electricity or water-purification utilities.

Keeping Things Cool

One common way to get clean water from the Sun is a condensation trap known as a solar still.

With the help of the Sun’s heat, the still evaporates the water, leaving behind salt, bacteria, dirt and other contaminants. After the vapor cools and returns to a liquid state, the water is collected in a clean container.

The stills, however, often have a flat surface and take a direct hit from the Sun’s rays, leading to an overheated device. When the water evaporates, energy is frequently wasted as heat, and therefore lost to the surrounding environment.

Thanks to a unique design, the engineers from the University of Buffalo are purifying water at higher rates.

The purifier, shaped like a birdhouse, weakens the light’s intensity, helping to speed vapor and water generation in experiments.

By draping carbon-covered, fabric-like paper in the shape of an upside-down “V,” the sheet’s bottom edges hang in a pool, soaking up the water. The carbon coating simultaneously absorbs solar energy and transforms it into heat for evaporation.

Because most of the paper remains under room temperature, the material pulls in heat from the surrounding area, compensating for the regular loss of solar energy that occurs during the vaporization process.

The system's V-shaped architecture speeds vapor and water generation in experiments. The paper's bottom edges soak up water, while the carbon coating absorbs solar energy and transforms it into heat for evaporation. (Image Credit: Huaxiu Chen)

Electric engineering professor at the University of Buffalo Qiaoqiang Gan led the research in the “evaporative cooling” technology.

“The process is like this: If we have a natural wet material in a dark environment, with no light or heating, its temperature will always be lower than the ambient, since natural water evaporation will consume energy from the environment,” said Gan.

Think of your home humidifier. Because the humidifier’s moisture consumes the heat and takes energy from the environment, the area in front of the device will feel cooler than the ambient temperature.

Employing a “cold vapor generation” method, Gan and his team intentionally keep the surface temperature of the fabric below the ambient temperature. Because the system is cooler than the ambient air, the system can take heat from the environment rather than lose heat energy.

“As long as the surface temperature is not higher than the ambient, the system will continuously take energy from the environment. In this case, the incident light energy can only be used for evaporation,” said Gan. “There are no other loss channels.”

The water project, funded by the National Science Foundation (NSF), was a collaboration between the University of Buffalo, Fudan University in China, and the University of Wisconsin-Madison. UB electrical engineering PhD graduate Haomin Song and PhD candidate Youhai Liu were the study’s first authors.

Pure Possibilities

With the new purifier, the researchers evaporated the equivalent of 2.2 liters of water per hour for every square meter of area illuminated by regular sunlight. Medical professionals estimate that a person requires at least 2 liters of water per day.

“If all this moisture can be collected, we can produce approximately 10 liters of clean water after 8 to 10 hours of operation,” said Gan.

Gan, Song, and other colleagues have launched a startup, Sunny Clean Water, to bring the invention to people and communities in need. With support from the NSF Small Business Innovation Research program, the company is integrating the new evaporation system into a prototype of a solar still.

Gan is also currently working with two non-governmental organizations (NGOs) to perform field test in remote areas and areas affected by natural disasters, including Puerto Rico and the Philippines.

"The most exciting potential is that we may deliver a portable and low-cost system for individuals and families in remote areas, islands, and areas affected by natural disasters where utility-level water treatment systems are not affordable,“ said Gan.

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