Fog Harvesting: A Low-Energy Solution

What if we could harvest water without relying on electricity? The simplest technique involves capturing fog, a method utilized by ancient civilizations, including the Incas. Archaeological findings suggest that similar devices were employed in regions like Israel and Egypt. Recently, this concept has been revitalized, with researchers from the Virginia Institute of Technology introducing a simple fog-harvesting device known as the fog harp.

This invention comprises vertical steel threads spaced closely enough for fog droplets to adhere and flow into a collection tank. The design was inspired by nature itself.

> “Coastal sequoias meet a third of their water needs by collecting mist,” notes Professor Brook Kennedy from Virginia Tech. “These trees, adapted to the foggy California climate, have evolved needle arrangements that mimic pine trees.”

Advanced Hydrogels for Water Recovery

While fog and dew collection are beneficial, they may not suffice to meet human demands. Innovative materials, such as a calcium chloride-based hydrogel developed at Saudi Arabia's King Abdullah University of Science and Technology (KAUST), hold promise.

This hydrogel can absorb its weight in water at night when temperatures drop, and it releases the stored moisture as vapor during the day. Researchers have infused it with carbon nanotubes, enhancing its ability to absorb solar energy and accelerate evaporation. Remarkably, just 2.5 hours of sunlight can recover all the stored water. With a small-scale system yielding 3 liters daily, the material cost is only half a cent per day, as the inventors confirm.

> “The hydrogel's high efficiency and affordability are its standout features,” emphasizes researcher Renyuan Li.

Innovative Greenhouses and Space Applications

Inspired by similar principles, a team from the National Institute of Singapore has created a self-fertilizing micro-greenhouse named SmartFarm. This device opens at night to capture atmospheric moisture and utilizes sunlight during the day to evaporate stored water, which then condenses on the greenhouse's interior surfaces. Specially designed wipers help collect these droplets, which nourish the plants below. Researchers successfully grew water spinach (Ipomoea aquatica), a tropical plant, within this setup.

> “SmartFarm significantly reduces irrigation water needs and has potential applications for large-scale rooftop farming in urban environments. This is a crucial advancement in conserving water and improving food accessibility,” asserts project leader Prof. Tan Swee Ching.

The team is also exploring using this technology in space, having tested the gel at the Hawaii Space Exploration Analog and Simulation (HI-SEAS) base for Mars and lunar mission simulations. Such devices are ideal for space exploration, where space constraints for plant cultivation are minimal.

> “In Singapore, approximately 45 million cubic meters of water evaporate from reservoirs annually. Our hydrogel can recover this vapor without requiring external energy,” stresses Prof. Swee-Ching Tan.

This gel can store four times its weight in water and release it when exposed to sunlight, offering an alternative to energy-intensive desalination processes. The innovation could benefit floating farms and rural communities facing challenges in accessing clean water.

> “We aim to mitigate the global water crisis through innovative solutions,” concludes Prof. Tan.

Metal-Organic Frameworks (MOFs) for Water Extraction

Researchers at MIT and the University of California at Berkeley have developed a device capable of extracting water from desert air with just 10% relative humidity. The technology utilizes metal-organic frameworks (MOFs), which consist of metal atoms surrounded by intricate organic compounds, boasting an impressive surface area of thousands of square meters per gram. In experimental setups heated solely by solar energy, the researchers successfully extracted 0.25 liters of water per kilogram of MOF. By incorporating energy sources like solar cells, these devices can function independently of day-night cycles.

> “Our goal is to create a system capable of producing large quantities of water,” states Prof. Evelyn Wang, co-author of the project.

Cactus-Inspired Designs for Water Harvesting

Innovators at the California Institute of Technology are developing advanced structures for water collection, drawing inspiration from cacti. Their proposed solution involves micro-trees featuring a gel membrane covered with nanoscale structures resembling cactus spines, which are adept at surviving arid conditions.

> “Our microspikes attract tiny water droplets suspended in the air, guiding them to the base where they merge into larger droplets, ultimately forming a water reservoir,” explains co-author Ye Shi.

This device can function in two ways: collecting dew at night and purifying evaporated water during the day.

Harnessing Water from Air in Sunlight

Remarkably, water can also be extracted from the air during the day without energy consumption. Researchers at ETH Zurich have developed a glass plate with a unique coating that reflects sunlight while emitting heat rays. An additional shield helps prevent heat from re-entering the system. Initial tests on a rooftop have yielded promising results, with a 10 cm diameter plate producing 4.6 ml of water daily.

> “We are nearing the theoretical maximum of 60 ml per hour, which cannot be surpassed,” notes Ivan Hächler, one of the device's developers.

Interestingly, a beetle native to the Namib Desert employs a similar technique, radiating heat to condense water on its body, but only at night. The Swiss innovation, however, surpasses nature, showcasing how engineering can potentially address global water scarcity.