Floating, blackened wood could bring cheap, drinkable water where needed

A simple wooden device could provide an alternative to expensive desalination, potentially helping bring safe drinking water to the roughly one billion people without access to it.

Researchers at the University of Maryland’s A. James Clark School of Engineering created the prototype for affordable small-scale desalination, an inexpensive solar evaporator that reportedly generates steam with high efficiency and minimal maintenance. Traditional desalination systems are “far too expensive to install and operate in many locations, especially in low-income countries and remote areas,” the team said.

The new design uses a technique called interfacial evaporation. The method “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," said associate professor Liangbing Hu. "These features make it suitable for off-grid water generation and purification, especially for low-income countries."

Interfacial evaporators are made of thin materials that float on saline water, such as sea water. Absorbing heat from the Sun on top, the evaporators pull up the saline water from below and convert it to steam on their top surface, leaving behind salt. Over time, the salt can build up on the evaporative surface, gradually degrading performance until it is removed.

Senior author Hu and his colleagues minimised the need for this maintenance with a device made out of basswood, similar to British lime or linden, which has a natural structure of micron-wide channels that carry water and nutrients up the tree. The researchers supplemented these natural channels by drilling a second array of millimetre-wide channels through a thin cross-section of the wood. They then carbonised the surface of the wood for greater solar absorption.

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 much wider drilled channels, before easily dissolving back into the water below.

Using wood as the only starting material, the researchers expect the evaporator to be low-cost. They are optimising it for higher efficiency, lower capital cost and integration with a steam condenser to complete the desalination cycle.

The research was published in Advanced Materials.