How sunlight can turn seawater into fresh water for coastal communities

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

That’s why some startups and researchers are updating the centuries-old technology of solar distillation, which uses only sunlight to purify water. While this technology is still a long way from producing the volume of fresh water generated by desalination plants, it may be of value to off-grid or coastal communities.

Abu Dhabi startup Manhat, founded in 2019, is developing a floating device for distilling water without the need for electricity or brine production. It consists of a greenhouse structure that floats on the ocean’s surface: sunlight heats and evaporates the water under the hull – separating it from the salt crystals left behind in the sea – and as temperatures cool, the water condenses into fresh water and is collected inside.

“It’s like a really natural water cycle,” says Dr. Saeed Al-Hassan Al-Khazraji, 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 it usually involves placing the water in a basin where, once the water has evaporated, the salt is left behind.

Unlike traditional solar stills, the Manhatt floats in the ocean, drawing water directly from the sea. No salt builds up in the device, and the angle of the collecting cylinder prevents water droplets from evaporating back into the sea, says Hassan.

Earlier this year, Manhat’s patented technology won the Water Europe Innovation Award for SMEs with innovative solutions in the water sector, lauded for its ability to produce fresh water “with zero carbon emissions and no brine rejection”.

The startup plans to harness its technology on floating farms, which will use desalination devices to provide fresh water irrigation for crops without the need for water transfer and associated emissions.

Al-Hassan says this would benefit the arid coastal regions where land is intensively cultivated. “If you produce (fresh) water at the sea surface and use it for agriculture, you can allow the arable land to be regenerated effectively,” he says, adding that the technology could work well for countries like the Maldives that have few land desalination plants. water.

Manhat wants to use desalination devices in agriculture, creating floating farms surrounded by multiple devices for instant irrigation, as shown in this demo. Credit: Manhattan

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

While field tests are underway, it has been promoted as a technology that could “serve off-grid, arid coastal regions to provide an efficient, low-cost water source.” The researchers suggested that it could be configured as a floating panel on the sea, to deliver 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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Jeff Townsend, who works on innovations in water scarcity for water treatment and hygiene, Ecolab, believes that while solar innovations are unlikely to replace conventional desalination, they could “complement existing technology, reducing the footprint total carbon dioxide for desalination.

But he cautioned that “desalination usually needs to provide a very predictable water supply,” and that “there will be potential concerns about the extent to which daily (daily) and seasonal variations in performance will affect the ability to achieve minimum production requirements.”

The biggest challenge for this type of technology is scale. “The drawback is their intrinsically 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 per square meter of solar collecting area. The current floating prototype of the manhat, which covers 2.25 square meters but has only one square meter open to water, produces 1.5 liters of fresh water per day – a drop in the ocean, considering WHO estimates that the average person needs at least 50 to 100 liters daily to be healthy

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

A pilot project for the floating farm concept will begin next year. By connecting several modular devices to form the grid, Manhatt believes its current technology can provide enough desalination to grow crops that consume less water, such as mushrooms, and as the devices improve it could begin to target other crops such as lettuce or tomatoes.

Despite the challenges, El Hassan believes that solar footage will one day become an important source of fresh water. “We have to accept the fact that seawater should play a major role in providing fresh water,” he says. “But we need a solution that reduces CO2 emissions and eliminates brine completely.”

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