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That’s why some startups and researchers are updating centuries-old solar distillation technology, which uses only sunlight to purify water. While the technology is a long way from producing the volume of freshwater produced by desalination plants, it could be valuable for off-grid or coastal communities.
Abu Dhabi-based startup Manhattan, founded in 2019, is developing a floating device that can distill water without electricity or making brine. It consists of greenhouse structures that float on the surface of the ocean: sunlight heats and evaporates the water beneath the structure – separating it from the salt crystals that remain in the sea – and as the temperature cools, the water condenses into fresh water and collects inside.
“It’s very similar to the natural water cycle,” said Dr. Saeed Alhassan Alkhazraji, the company’s founder and associate professor at Khalifa University in Abu Dhabi. Solar evaporation has long been used for this purpose, but usually by putting the water in a basin, and once the water evaporates, the salts are left behind, he said.
Unlike traditional solar stills, Manhattan’s device floats in the ocean, pumping water directly from it. Salt doesn’t accumulate in the device, and the angle of the collection cylinder prevents water droplets from evaporating back into the sea, Alhassan said.
The startup plans to use its technology in a floating farm that will use its desalination equipment to provide freshwater irrigation to crops without the need for water transport and its associated emissions.
This will benefit dry coastal areas where land is intensively farmed, Alhassan said. “If you produce (freshwater) water on the sea surface and use it for agriculture, you can effectively revitalize arable land,” he said, adding that the technology could work well in countries with little land, such as the Maldives . Desalination plant.
While field testing is underway, it is touted as a technology that “could potentially provide an efficient, low-cost source of water for off-grid dry coastal regions.” The researchers suggest it could be configured as a floating panel at sea to pipe fresh water to shore, or it could be designed to serve a single household, using it on seawater tanks.
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Geoff Townsend, who works on water scarcity innovations at water treatment and sanitation company Ecolab, believes that while solar innovations are unlikely to replace traditional desalination, they could “complement existing technologies and reduce the overall carbon footprint of desalination”.
But he cautioned that “desalination typically requires a very predictable water supply” and that “there are potential concerns that diurnal (daily) and seasonal variations in performance may affect the ability to meet minimum production requirements.”
The bigger challenge for this technology is scale. “One downside is their inherent inefficiency,” Townsend said, adding that they tend to take up a lot of space to produce small amounts of water.
Manhattan is working to increase that volume to 5 liters by optimizing materials and design, with a long-term goal of at least 20 liters, Alhassan said. The startup has raised $130,000 in funding so far, mostly through a partnership with Abu Dhabi Ports, but with more investment he believes these goals can be achieved.
Pilots of the floating farm concept will begin next year. By connecting multiple modular devices in a grid, Manhat believes its current technology could provide enough desalination to grow less water-intensive crops like mushrooms, and as the devices improve, they could start targeting lettuce or other crops such as tomatoes.
Despite the challenges, Alhassan believes that solar stills will one day be an important source of fresh water. “We have to accept the fact that seawater should be the key to providing fresh water,” he said. “But we need a solution that minimises CO2 emissions and eliminates salt water completely.”
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