The solar evaporator offers a fresh route to fresh water.



The solar evaporator offers a fresh route to fresh water.

A self-cleaning wooden device aims to make small-scale desalination more practical. Credit: John T. Consoli / University of Maryland

Around one billion people around the world lack access to safe drinking water. Desalinating salt water in clean water can help fill this dangerous void. But traditional desalination systems are too expensive to install and operate in many places, especially in low-income countries and remote areas.

Now, researchers at the A. James Clark School of Engineering at the University of Maryland have demonstrated a successful prototype of a critical component for affordable small-scale desalination: an economical, wood-made solar evaporator. The evaporator generates steam with high efficiency and minimal need for maintenance, says Liangbing Hu, badociate professor of materials science and engineering and affiliate of the Energy Innovation Institute of Maryland.

The design employs a technique known as interfacial evaporation, "which shows great potential in response to the global water shortage due to its high vapor-to-solar efficiency, low environmental impact and low-cost portable device design," Hu says. "These characteristics make it suitable for the generation and purification of water outside the network, especially for low-income countries."

Interfacial evaporators are made of thin materials that float in saline water. Absorbing the solar heat at the top, the evaporators continuously extract the saline water from below and convert it into vapor on its upper surface, leaving behind the salt, explains Hu, who is the main author in a document describing the work in Advanced materials.

However, over time, salt can accumulate on this evaporative surface, gradually degrading the yield until it is eliminated, he says.

Hu and his colleagues minimized the need for this maintenance with a device made of lime that exploits the natural structure of the wood of the micron channels that transport water and nutrients to the tree.

Researchers complement these natural channels by drilling a second series of millimeter-width channels through a thin cross section of the wood, says Yudi Kuang, a visiting scholar and lead author of the article. Then, the researchers briefly expose the upper surface at high temperatures, which carbonizes the surface for greater solar absorption.

In operation, as the device absorbs solar energy, it extracts salt water through the natural channels of the micron throughout the wood. The salt spontaneously exchanges from these small channels through natural openings along its sides to the perforated channels, which are much wider, and then dissolves easily in the water below.

"In the laboratory, we have successfully demonstrated excellent anti-foulants in a wide range of salt concentrations, with stable steam generation with approximately 75% efficiency," says Kuang.

"By using natural wood as the only starting material, it is expected that the solar evaporator that rejects salt will be inexpensive," adds badociate researcher Chaoji Chen. The evaporator approach is also effective on other types of wood with similar natural channels. Researchers are now optimizing their system for greater efficiency, lower capital cost and integration with a steam condenser to complete the desalination cycle.

Hu's laboratory also recently developed another solar-powered device prototype that harnesses the ability of charred wood to absorb and distribute solar energy, this one created to help clean up heavy oil spills difficult to collect. "Our carbonized wood material demonstrates rapid and efficient absorption of crude oil, as well as low cost and scalable manufacturing potential," says Kuang, lead author of an article on research into advanced functional materials.

"Wood is a scaffolding of intriguing material, with its unique porous hierarchical structure, and it is a renewable resource, abundant and profitable," Hu says. "In our laboratory, the fundamental understanding of biomaterials (especially wood) leads us to achieve extraordinary performance that is competitive with widely used but not sustainable materials."

Among other projects, his laboratory has created light and effective insulating materials of "nano wood". He has also designed "super wood" that is 12 times stronger and 10 times stronger than natural wood, and can replace steel, titanium or carbon fiber in certain applications, he says.


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More information:
Yudi Kuang et al, a high performance self-healing solar evaporator for continuous water desalination, Advanced materials (2019). DOI: 10.1002 / adma.201900498

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The solar evaporator offers a fresh route to fresh water (2019, April 16)
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