Solar stills provide a clever and simple means of purifying dirty or salty water, but they work at a rather slow rate. A new material has been shown to boost their performance, and it's made from fruit waste which would otherwise be discarded.
In its most basic form, a conventional solar still consists of a basin of undrinkable water that is set beneath a transparent cover. The water evaporates as it's heated by the sun, condensing on the inside surface of the cover. That condensation – which is pure, clean water – trickles down the cover and is collected in a separate receptacle for drinking.
In order to warm the dirty/salty water at a faster rate, scientists have developed materials that float on its surface, converting sunlight into heat. And while those materials have been made from a number of different ingredients, they commonly use carbon obtained from coal.
Seeking a less costly and more eco-friendly alternative, Asst. Prof. Edison Ang and colleagues at Singapore's Nanyang Technological University looked to something that doesn't have to be mined, is free for the taking, and would otherwise simply be disposed of – fruit waste. More specifically, the scientists tried out coconut husks, orange peels and banana peels.
In a simple two-stage carbonization process, the fruit waste was heated at 850 ºC (1,562 ºF) for a few hours, and mixed with a molybdenum reactant. Doing so transformed the waste into sheets of two-dimensional molybdenum carbide, which belongs to a family of metals known as MXenes. Among other things, MXenes are hydrophilic (water-attracting) and they have a very high light-to-heat conversion efficiency.
When tested in a small solar still, square sheets of the photothermal molybdenum carbide proved to be very effective at converting sunlight into heat, causing the underlying simulated seawater water to evaporate much more rapidly than it would otherwise. And because the material is very porous, the water vapor droplets were able rise right through it, subsequently condensing on the inside of the still's cover.
Material derived from the coconut husks worked best, as it converted sunlight to heat with an efficiency rate of 94%.
Prof. Ang and his team are now developing the technology further, and are seeking industry partners to help with its commercialization.
Source: Nanyang Technological University
