Desalination: Solar power tapped to provide clean water from oceans

A water vendor at Miritini, Mombasa County. Scientists say that desalinating ocean water using affordable solar technology can help end perennial drinking water shortages for communities along the coast. FILE | NATION

Scientists are on the verge of developing sustainable and affordable technology that could turn oceans into an abundant supply of fresh drinking water.

About a third of the world’s surface is covered by oceans, yet according to lifewater.org, roughly one in 10 people lack access to clean water.

Instead of reverse osmosis, the new technology uses solar power. In reverse osmosis, pressure is applied to water, moving it through a semi-permeable membrane designed to remove salts from water.

However, this technology is costly because the pumps used to induce pressure are expensive to make, transport and operate. The cost is especially high for water with high salinity.

Reverse osmosis also uses a lot of electricity and needs quality infrastructure to keep the pumps going, thereby locking out poor communities who live near oceans.

Researchers are working with Nanotechnology Enabled Water Treatment (NEWT) Systems to develop inexpensive, sustainable water-treatment systems that can work without electrical power. NEWT came up with a desalination technique known as membrane distillation.

POWERED BY THE SUN
In contrast to reverse osmosis, membrane distillation is powered by heat, not artificially produced pressure in water. Saltwater is injected with heat from various sources, which can include waste heat produced in another industrial process. As the water flows into contact with a heated membrane, it evaporates. The water vapour diffuses through the membrane, condensing on the cold side, while the salts remain on the hot side.

“The materials are broadly similar to specially designed semi-porous waterproof coats, which allow your sweat and water vapour to pass through the coat as it is hotter, but cannot allow rain to go through because it is colder,” said Akshay Deshmukh, a member of the research team and PhD candidate working on desalination technologies at Yale University in the US.

The hot side of the membrane where the salty water is directed is coated with carbon black nanoparticles. These particles absorb light and convert the resulting energy into thermal energy which heats the water and enables desalination to take place.

This process, called nanophotonics-enabled solar membrane distillation (NESMD), can be fully powered by the sun.

At initial tests, the membrane was capable of desalinating six litres of water per square metre of membrane, every hour, using a simple lens that concentrated sunlight to around 17.5 kilowatts of power. Scientists expect forthcoming trials to desalinate 10 litres of water per hour, meaning that a square metre of membrane could provide as much as 240 litres of water per day.