Waste heat produced by solar cells undermines their performance, but the race is on to harness it for useful purposes. Researchers have found a way to tap into that heat to collect water out of the air, and have demonstrated the effectiveness of the idea by growing spinach in the Arabian desert, one of the driest places on Earth,

Recent years have seen many technologies touted to turn water vapor in the air into fresh water, some of them inspired by animals that had the same approach first. Unfortunately, the amounts of water these can produce per square meter are modest, limiting their applications unless they can be paired with another use. The most effective often rely on technology that may be too expensive for the places that need them most.

In Cell Reports Physical Science, a team at King Abdullah University of Science and Technology describes a possible solution, applying a hydrogel to the back of solar panels. The idea relies on turning one of photovoltaic cells' weaknesses – that they only turn about 20 percent of the Sun's energy into electricity – into a strength.

As no solar cell is 100 percent efficient, they produce a lot of waste heat. This can degrade panel performance in hot conditions and is one of the reasons solar is increasingly being built on reservoirs and canals, where the panels not only reduce evaporation but are kept cool by the water beneath.

In Thuwal, Saudi Arabia, researchers coated the backs of two solar panels with a hydrogel that absorbs water from the air, and evaporates it when hot.

At night, the concentrated evaporated water condensed into a box below and was used to grow spinach, a crop that struggles more than most without regular watering. Of 60 seeds planted in the heat of summer, 57 grew to harvestable size, a score a farmer in more temperate climates would be proud of.

The hydrogel could work when placed on the underside of other materials as well – after all, anything will get hot when exposed to the sunlight of the Arabian desert. However, when the water evaporates it cools the panels, by up to 17º C (30.6º F) in the trial. The coated panels produced nearly 10 percent more electricity respectively than an uncoated panel nearby.

The combination of simplicity, extra electricity, and water where it is in short supply could make the combined system, called WEC²P, economic where other forms of atmospheric water gathering are not.

“Our design makes water out of air using clean energy that would’ve been wasted and is suitable for decentralized, small-scale farms in remote places like deserts and oceanic islands,” senior author Professor Peng Wang said in a statement. 

On the other hand, where some water-from-air designs are labor-intensive to produce, and therefore might not scale well, WEC²P could also suit larger operations

“Our goal is to create an integrated system of clean energy, water, and food production, especially the water-creation part in our design, which sets us apart from current agrophotovoltaics,” Wang said.

The same team have previously used waste heat from solar panels to desalinate seawater, but the new version could work without an ocean nearby.