Inspired by both nature and a classic novel, researchers from the University of Texas at Austin created a water purifier that, at first glance, looks like a black rose in a jar. The design choice – not an entirely artistic one – enables a more efficient filtering process.
In addition to removing contamination from heavy metals and bacteria, the system extracts salt from seawater, producing clean water that meets drinking standard requirements set by the World Health Organization.
The team, led by associate professor Donglei (Emma) Fan in the Cockrell School of Engineering’s Walker Department of Mechanical Engineering, advances a water-production method known as solar steaming – a technique that uses sunlight energy to separate salt and other impurities from water through evaporation.
A typical solar steaming device mainly consists of three components: the solar steaming material, a container told hold the contaminated water, and a condensation cover.
The UT Austin team integrated a pump to the solar steaming device, reducing the pressure inside the collection chamber such that the solar steaming and vapor collection performance could be improved.
“We designed the purification-collection unisystem to include a connection point for a low-pressure pump to help condense the water more effectively,” said Weigu Li, a Ph.D. candidate in Fan’s lab and lead author on the paper. “Once it is condensed, the glass jar is designed to be compact, sturdy and secure for storing clean water.”
Attached to a stem-like tube that collects untreated water from any water source, the 3D rose shape improves the structure’s ability to collect and retain more liquid. Water collects through the capillary, “feeding” the flower-shaped structure atop the device.
The water purifier's flower-inspired design also improves filtering capabilities. The UT Austin solar-steaming system is made from petal-shaped paper sheets. The filtered black paper, coated with a special type of polymer known as polypyrrole, converts solar light into thermal heat.
Once water finds its way to the petals, the polypyrrole material coating the flower turns the water into steam. Impurities naturally separate from water when condensed in this way.
Fan and her team experimented with a number of different design shapes to achieve optimal water retention levels. The researchers began by placing single, round layers of the coated paper flat on the ground under direct sunlight – a worthy effort, but the single sheets only collected water in small amounts.
Then, Fan thought about a book she read in high school.
Although not about roses, The Black Tulip by Alexandre Dumas inspired the UT Austin researcher to try a flower-like shape. A rose structure, with more internal reflections than other floral shapes, allowed more direct sunlight to hit the photothermic material and provided an enlarged surface area for water vapor to dissipate from the material.
Each flower-like structure costs less than 2 cents and can produce more than half a gallon of water per hour per square meter.
Researcher Weigu Li spoke with Tech Briefs about the surprising advantages of a rose-like structure.
Tech Briefs: What is solar steaming, and how is your system a new take on the technique?
Weigu Li: The solar steaming is a water purification technology that converts the solar light into thermal heat and then vaporizes the water at the interface.
The rose-like solar steamer provides both high energy conversion efficiency and high water evaporation rate. The fabrication of our material is also low-cost, which is feasible for large-scale industrial manufacturing. Finally, we introduced a low-pressure system for solar steaming and collection, which improves the water collection by 54%.
Tech Briefs: How does a flower-like structure lead to better filtration/purification techniques?
Weigu Li: First, the multiple folded petals can generate multiple light reflections inside the rose, which improves the optical absorption by 5% compared to the planar structure. This helps obtain a high solar-thermal conversion efficiency.
The rose-like structure also provides an enlarged specific surface area, which is ten times that of a planar structure. As a result, the evaporation rate can be greatly improved.
Tech Briefs: If the water ends up in the jar, where do the impurities go in your system?
Weigu Li: The contaminated water is drawn to the surface of the rose-like steamer through capillary force, which then evaporates under the solar illumination. The impurities will stay and precipitate on the polypyrrole materials. This means we need to recover the solar steaming device after use for several times. This is one of the research directions for solar steaming at this moment.
Tech Briefs: How is your system more effective than conventional methods?
Weigu Li: Compared to conventional thermal distillation for sea water desalination, the solar steaming is more effective in energy conversion.
Conventional thermal distillation usually heats up the entire water to generate steam, which suffers low thermal efficiency of ~40%. Our solar steaming method employs interfacial water evaporation, which allows the air-liquid interface to be heated rather than the bulk water, resulting in the ultrahigh solar-thermal conversion efficiency of up to ~90%.
Tech Briefs: How does the low-pressure solar steaming result in better efficiency?
Weigu Li: When further controlling the pressure in the steaming-collection system to ~ 0.17 atm, the water collection rate improves dramatically by 52%. Although particle energy is utilized to obtain low pressure, evaluations and calculations show that the overall energy efficiency is still enhanced remarkably in the low-pressure system compared to that in ambient pressure.
The mechanisms of low-pressure-induced solar steaming/collection improvement can be understood from the perspective of kinetics. In a closed system, evaporation and condensation are reversible reactions. Water molecules tend to evaporate into vapor until the vapor pressure reaches the saturation vapor pressure. Hence, a reduced pressure condition drives the phase transition from water to vapor, facilitating the evaporation rate and thereby improving the condensation.
Tech Briefs: Where have you tested this technology, and what did the results demonstrate?
Weigu Li: We tested this technology in our lab using the solar simulator as the Sun source. We used this technology to treat the water from Colorado River the Gulf of Mexico. It can effectively alleviate alkalinity, reduce hardness, and remove bacteria from the water of the Colorado River. It also lessens the content of various metal ions like sodium, magnesium, and calcium in the seawater from the Gulf of Mexico to meet the drinking standard set by the World Health Organization.
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