How sunlight could turn seawater into fresh water for coastal communities

For some countries, desalination plants offer a solution: removing salt from seawater to meet their freshwater needs. The Middle East has the highest concentration of these in the world. But such plants, still mostly fueled by fossil fuels, are energy-intensive and the process creates an extremely salty wastewater known as brine, which can damage marine ecosystems and animals when pumped back into the sea.

That’s why some start-ups and researchers are updating the century-old photovoltaic technology, which uses only sunlight to purify water. While the technology is still a long way from producing the volume of fresh water generated by desalination plants, it could prove invaluable to off-grid or coastal communities.

Abu Dhabi-based startup Manhat, founded in 2019, is developing a floating device that distills water without requiring electricity or creating brine. It consists of a greenhouse structure that floats on the surface of the ocean: sunlight heats and evaporates the water under the structure – separating it from the salt crystals that remain in the sea – and when temperatures cool down, the water condenses into fresh water and is collected inside.

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

Unlike traditional solar stills, the Manhat 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, Alhassan says.

Earlier this year, Manhat’s patented technology won the Water Europe Innovation Award for Small and Medium-Sized Businesses with Breakthrough Water Solutions, praised for its ability to produce “zero carbon and zero carbon” fresh water. brine waste “.

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

This would benefit arid coastal areas where land is intensively cultivated, says Alhassan. “If you produce (fresh) water on the sea surface and use it for agriculture, you can actually enable rejuvenation of arable land,” he says, adding that the technology could work well for countries like the Maldives that have little land available for planting. desalination.

Manhat wants to use his desalination devices for agriculture, creating floating farms surrounded by multiple devices for on-site irrigation, as shown in this render. Credit: Manhat

Others have also innovated with solar stills. In 2020, researchers from the Massachusetts Institute of Technology (MIT) developed a floating desalination unit consisting of a multilayer evaporator that recycles the heat generated when water vapor condenses, increasing its overall efficiency.

While field tests are ongoing, it has been touted as a technology that could “potentially serve off-grid arid coastal areas to provide an efficient, low-cost water source.” The researchers suggested that it could be configured as a floating panel on the sea, delivering fresh water through pipes to the shore, or it could be designed to serve a single family, using it above a seawater tank.

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Geoff Townsend, who works on innovations in water scarcity for water treatment and sanitation company Ecolab, believes that while solar innovations are unlikely to replace conventional desalination, they could “complement existing technology, reducing the footprint of overall carbon of desalination “.

But he warns that “desalination in general must provide a very predictable water supply” and that “there will be potential concerns about the extent to which diurnal (daily) and seasonal changes in performance could affect the ability to achieve minimum production requirements. . ”

An even bigger challenge for this type of technology is scale. “One downside is their inherently low efficiency,” Townsend says, 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. Manhat’s current floating prototype, which covers 2.25 square meters but has only one square meter open to the water, produces 1.5 liters of fresh water per day – a drop in the ocean, considering the World Organization of Healthcare estimates that the average person needs at least 50 per 100 liters per day to stay healthy

Alhassan says Manhat is working to increase this volume to five liters by optimizing materials and design, with the long-term goal of reaching at least 20 liters. The startup has raised $ 130,000 in funding so far, mostly through a partnership with Abu Dhabi Ports, but with more investment it 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 could provide enough desalination to grow water-less crops, such as mushrooms, and as devices improve, they may begin targeting others. crops such as lettuce or tomatoes.

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

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