How sunlight can make seawater drinkable for coastal people

A summer of extreme heat and drought across the world has been a reminder that water scarcity is an urgent issue and will only get worse with climate change. More than two billion people worldwide no longer have easy access to safe drinking water, according to the World Health Organization (WHO)

For some countries, desalination plants offer a solution — removing salt from seawater to satisfy their freshwater needs. The Middle East has the highest concentration of these in the world.

But these plants, still powered mainly by fossil fuels, are energy intensive and the process creates an extremely salty effluent known as brine, which can damage marine ecosystems and animals when it is pumped back into the sea.

That’s why some startups and researchers are upgrading century-old solar distillation technology, which uses only sunlight to purify water.

While the technology is still a long way from producing the volume of freshwater generated by desalination plants, it could be valuable to off-grid or coastal communities.

floating solar stills

Abu Dhabi-based startup Manhat, founded in 2019, is developing a floating device that distills water without needing electricity or creating brine.

It consists of a greenhouse structure that floats on the ocean’s surface: sunlight heats and evaporates the water beneath the structure — separating it from the salt crystals that are left behind in the sea — and as temperatures cool, the water condenses into fresh water and is collected inside.

“It’s really similar to the natural water cycle,” said Saeed Alhassan Alkhazraji, founder of the company and an associate professor at Abu Dhabi’s Khalifa University. He says solar evaporation has long been used for this purpose, but it typically involves putting water in a basin where, once the water has evaporated, the salt is left behind.

Unlike traditional solar stills, the Manhattan device floats in the ocean, drawing water directly from the sea. Salt does not accumulate in the device and the angle of the collecting cylinder prevents water droplets from evaporating back into the sea, says Alhassan.

Earlier this year, Manhattan’s patented technology won the Water Europe Innovation award for small and medium-sized companies with innovative solutions in the water sector, lauded for its ability to produce fresh water with “zero carbon footprint and zero brine rejection”.

The startup plans to leverage its technology on floating farms, which would use its desalination devices to provide freshwater irrigation for crops without the need to transport water and its associated emissions.

This would benefit arid coastal areas where land is intensively farmed, says Alhassan. “If you produce (fresh) water on the sea surface and use it for agriculture, you can effectively allow arable land to be rejuvenated,” he says, adding that the technology could work well for countries like the Maldives, which have little land available for desalination plants.

Others are also innovating with solar stills. In 2020, researchers at the Massachusetts Institute of Technology (MIT) developed a floating desalination unit consisting of a multi-layer evaporator that recycles the heat generated when water vapor condenses, increasing its overall efficiency.

While field trials are ongoing, it has been touted as a technology that could “potentially serve arid coastal areas off-grid to provide an efficient, low-cost source of water.” Researchers have suggested that it could be configured as a floating panel at sea, supplying fresh water through pipes to the shore, or it could be designed to serve a single home, using it on top of a seawater tank.

Enlargement

Geoff Townsend, who works on water scarcity innovations for the treatment and hygiene company Ecolab, believes that while solar innovations are unlikely to replace conventional desalination, they could “supplement existing technology, reducing the company’s overall carbon footprint.” desalination”.

But he cautions that “desalination typically needs to provide a very predictable supply of water” and that “there will be potential concerns about the extent to which diurnal (daily) and seasonal changes in performance may affect the ability to meet the minimum production requirement.”

An even greater challenge for this type of technology is scale. “One downside is their low intrinsic efficiency,” says Townsend, adding that they tend to take up a lot of space for the small amount of water they produce.

The MIT device was found to produce about five liters of fresh water per hour for every square meter of solar collection area. The current Manhattan floating prototype, which covers 2.25 square meters but is only one square meter open to the water, produces 1.5 liters of fresh water a day — a drop in the ocean, considering the World Health Organization estimates that an average person needs at least two liters a day to be healthy.

Alhassan says Manhat is working to increase that volume to five litres, optimizing materials and design, with the long-term goal of reaching at least 20 litres. The startup has raised $130,000 in funding so far, predominantly through a collaboration with Abu Dhabi Ports, but with increased investment, he is confident these goals can be achieved.

A pilot of the floating farm concept will begin next year. By connecting multiple modular devices in a grid formation, Manhat believes his current technology can provide enough desalination to grow less water-intensive crops like mushrooms, and as the devices improve, they could start to target other crops like mushrooms. lettuce or tomato.

Despite the challenges, Alhassan believes that solar stills will one day become an important source of fresh water. “We have to accept the fact that seawater must be a key factor in the supply of fresh water,” he says. “But we need to have a solution that minimizes CO2 emissions and completely eliminates brine.”