Researchers from MIT and China have created a solar desalination device that can provide a household of four with all of the drinking water they require — and it only costs $4 to build.
Solar desalination: Although water covers more than two-thirds of the Earth's surface, 97 percent of it is saltwater. That water's salt concentration is simply too high for human systems to handle; if you drank too much of it, you'd become dehydrated and die.
We need freshwater, which has a salt concentration of less than 1%, and because this water is scarce, people have devised numerous methods for removing enough salt from saltwater to make it drinkable.
Solar desalination is one of them.
While the details vary, the essential notion is that the heat of the sun is applied to saltwater. The water evaporates, leaving the salt behind, and the water vapor is collected and condensed, resulting in liquid freshwater.
Engineers have built a plethora of solar desalination systems, but for the devices to make a difference in the locations where they are most needed, they must be not only cheap and efficient, but also resilient and long-lasting.
The issue is that salt accumulates in these devices, notably on the wick used to draw water through the device. Those wicks are difficult to clean, and the salt accumulation means they must be replaced over time.
"The trouble has been the salt fouling issue, which people haven't really addressed," MIT News researcher Evelyn Wang explained. "So, we see these really appealing performance numbers, but they are frequently limited due to lifespan." Things will deteriorate over time."
The concept: MIT and Shanghai Jiao Tong University researchers have developed a wick-free solar desalination device that employs gravity-driven convection to transfer water.
Natural convection is a type of movement that happens when the density of a liquid is not uniform. In this case, the less dense liquid will ascend while the denser liquid will fall. (The same is true for gases: hot air rises while cold air falls because it is less dense.)
The new solar desalination apparatus is made up of many layers. The top layer is sprayed with salt water. The sun's heat is drawn to that layer by the black paint, resulting in the desired formation of water vapor for collection.
Any water that remains in that stratum is now highly salty and thus extra thick.
More saltwater is found in the device's very bottom layer. The layers between it and the top are meant to maintain heat in the top layer while allowing super-dense seawater to travel downward and mix with regular density saltwater via natural convection.
Looking ahead: The device ran for a week without showing any signs of salt accumulation, but we still don't know how long it could run for.
The best news, according to the MIT researchers, is that it would only cost $4 to create a solar desalination device large enough to cover the drinking water demands of a family of four. If it proves to be robust, it could be a relatively low-cost method of delivering freshwater to people.
