Researchers from MIT have developed a new material that converts sunlight into steam much more efficiently than currently available alternatives. This material acts sort of like a two-way sponge: it can attract and hold sunlight for heat and can also continually draw up water in order to create a constant supply of steam. The research was led by Hadi Ghasemi and the results were published in Nature Communications.
“Steam is important for desalination, hygiene systems, and sterilization,” Ghasemi said in a press release. “Especially in remote areas where the sun is the only source of energy, if you can generate steam with solar energy, it would be very useful.”
The system works in layers. Graphite flakes that have been treated with microwave radiation are positioned on top of a layer of carbon foam, which is porous, hydrophilic, and is an effective insulator.
“On the left, a representative structure for localization of heat; the cross section of structure and temperature distribution. On the right, a picture of enhanced steam generation by the DLS structure under solar illumination.” -Courtesy of the researchers
When it is exposed to sunlight, the graphite flakes heat up. This causes a difference in pressure in the system, and the water is drawn through the porous material where it is evaporated off as steam on the surface. The insulating properties of the foam keep the heat on the surface where steam is created and also prevents it from heating up the entire water reservoir below, where it would be wasted. The material generates steam from 85 percent of the sunlight it receives with minimal heat loss, making it astoundingly efficient.
Solar-powered steam generation isn’t anything new, but this material is markedly more efficient. Traditionally, devices have been designed using mirrors and lenses to focus the incoming light several times over and use it to heat a tank of water, which eventually generates steam at the surface. Unfortunately, this method is not very efficient and a great deal of heat is lost before the liquid water is converted into steam.
Another new approach to generating steam has used nanoparticles within water. When exposed to an extreme amount of sunlight (about 1,000 times more than is produced on an average day), the nanoparticles heat the water from within, creating steam very rapidly. The large amounts of sunlight required to generate the steam make it somewhat impractical, as the equipment required to concentrate the solar energy drives the price up.
The new carbon-based material from Ghasemi’s team only needs sunlight to be concentrated about 10 times, which is considerably lower than the other solar-powered systems. This will allow the system to function even on days when the sunlight isn’t as strong, and the simpler concentration equipment results in reduced operational costs.
“This is a huge advantage in cost-reduction,” Ghasemi explained. “That’s exciting for us because we’ve come up with a new approach to solar steam generation.”