How sunlight can turn seawater into fresh water for coastal communities


A summer of extreme heat and drought around the world has served as a reminder that water scarcity is a serious problem and will worsen with climate change. Already, more than two billion people worldwide do not have easy access to clean water, according to the World Health Organization (WHO).

For some countries, desalination plants offer a solution: removing salt from seawater to meet fresh water needs. The Middle East has the highest concentration in the world. But such plants, which are still mostly fueled by fossil fuels, are energy intensive and the process produces highly salty wastewater called brine, which can harm marine ecosystems and animals when it returns to the sea.

That’s why some startups and researchers are updating the centuries-old solar water technology, which uses only sunlight to purify water. While the technology is far from producing the volume of fresh water produced by desalination plants, it can be valuable for off-grid or coastal communities.

Abu Dhabi-based startup Manhat, founded in 2019, is developing a floating device that distills water without the need for electricity or the creation of brine. It consists of a greenhouse structure floating 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 as the temperature cools, the water turns into fresh water and collects inside.

“It’s really similar to the natural water cycle,” says Dr. Saeed Alhassan Alkhazraji, the company’s founder and associate professor at Abu Dhabi’s Khalifa University. Solar evaporation has long been used for this, he says, but it usually involves putting water into a basin where, once the water evaporates, the salt is left behind.

Unlike traditional solar stills, Manhattan’s device floats in the ocean, drawing water directly from the sea. Salt does not build up in the device and the angle of the collection cylinder prevents water droplets from evaporating into the sea, says Alhassan.

Earlier this year, Manhat’s patented technology won the Water Europe Innovation Award for small and medium-sized enterprises with breakthrough solutions in the water sector, praised for its ability to produce fresh water with “zero carbon footprint and brine waste”.

The startup plans to leverage its technology on floating farms, which would use its desalination devices to irrigate crops with fresh water, without the need to transport water and without the associated emissions.

This would benefit dry coastal areas where land is intensively farmed, says Alhassan. “If you can produce (fresh) water on the surface of the sea and use it for agriculture, you can effectively rejuvenate arable land,” he says, adding that the technology could work well in land-scarce countries like the Maldives. desalination plants

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

While field trials are underway, it was presented as a technology that could work in “dry off-grid coastal areas” to provide an efficient and affordable source of water. The researchers suggested that it could be configured as a floating panel in the sea, piped fresh water to the coast, or could be designed to serve a single household, using it on top of a seawater tank.

Geoff Townsend, who works on water scarcity innovations for water treatment and hygiene company Ecolab, believes that while solar-side innovations won’t replace conventional desalination, they “can complement existing technology, reducing the overall carbon footprint of desalination.”

But he warned that “desalination should normally provide a very predictable water supply” and “there will be potential concerns that day-to-day (daily) and seasonal variations in performance may affect the ability to achieve the minimum production requirement.”

An even bigger challenge with this type of technology is scale. “One drawback is their inherent low efficiency,” Townsend says, adding that they take up a lot of space for the small amount of water they produce.

MIT’s device was found to produce five liters of fresh water per square meter of solar collection area. Manhattan’s current floating prototype, which covers 2.25 square meters but has only one square meter of open water, produces 1.5 liters of fresh water a day – one drop in the ocean, given that the World Health Organization estimates that the average person needs at least 50 and that he needs 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 so far, mainly through a partnership with Abu Dhabi Ports, but with more investment it is confident that these goals can be met.

The pilot concept of the floating farm will start next year. By connecting multiple modular devices in a network formation, Manhat believes that today’s technology can provide enough desalination to grow crops that use less water, such as mushrooms, and as the devices improve, they can start targeting other crops, such as lettuce or tomatoes.

Despite the challenges, Alhassan believes that solar panels will one day become an important source of fresh water. “We have to accept that sea water must be a key factor in supplying fresh water,” he says. “But we have to have a solution that will minimize CO2 emissions and completely eliminate brine.”