Wood based solar evaporator could provide safe water

An inexpensive solar evaporator made of wood could provide safe drinking water to people in regions where desalination systems are too expensive to install and operate.

solar evaporator
A self-cleaning device made of wood aims to make small-scale desalination more practical (Pic: John T. Consoli/University of Maryland)

Developed and demonstrated by researchers at the University of Maryland’s A. James Clark School of Engineering, the evaporator is said to generate steam with high efficiency and minimal need for maintenance.

The design employs interfacial evaporation, which Liangbing Hu, associate professor of materials science and engineering and affiliate of the Maryland Energy Innovation Institute, said shows great potential in response to global water scarcity because of its high solar-to-vapour efficiency, low environmental impact, and portable device design with low cost.

“These features make it suitable for off-grid water generation and purification, especially for low-income countries,” said Hu.

Interfacial evaporators are made of thin materials that float on saline water. Absorbing solar heat on top, the evaporators continuously pull up the saline water from below and convert it to steam on their top surface, leaving behind the salt, said Hu, who is senior author on a paper describing the work in Advanced Materials.

Over time salt can build up on this evaporative surface, gradually degrading performance until it is removed, he said.

Hu and his colleagues are said to have minimised the need for this maintenance with a device made from basswood that exploits the wood’s natural structure of the micron-wide channels that carry water and nutrients up the tree.

The researchers supplement these natural channels by drilling a second array of millimetre-wide channels through a thin cross-section of the wood, said Yudi Kuang, a visiting scholar and lead author on the paper. The investigators then briefly expose the top surface to high heat, which carbonises the surface for greater solar absorption.

In operation, as the device absorbs solar energy, it draws up salty water through the wood’s natural micron-wide channels. Salt is spontaneously exchanged from these tiny channels through natural openings along their sides to the vastly wider drilled channels, and then easily dissolves back into the water below.

“In the lab, we have successfully demonstrated excellent anti-fouling in a wide range of salt concentrations, with stable steam generation with about 75 per cent efficiency,” said Kuang.

“Using natural wood as the only starting material, the salt-rejecting solar evaporator is expected to be low-cost,” said research associate Chaoji Chen. The evaporator approach is claimed to be effective in other types of wood with similar natural channels. The researchers now are optimising their system for higher efficiency, lower capital cost, and integration with a steam condenser to complete the desalination cycle.