The sponge is made of a layer of graphite flakes and an underlying carbon foam – is a porous, insulating material structure that floats on water. When sunlight hits the structure’s surface, it creates a hotspot in the graphite, drawing water up through the material’s pores, where it evaporates as steam. The brighter the light, the more steam is generated.
The new material is able to convert 85 per cent of incoming solar energy into steam – a significant improvement over recent approaches to solarpowered steam generation. What’s more, the setup loses very little heat in the process, and can produce steam at relatively low solar intensity. This would mean that, if scaled up, the setup would likely not require costly systems to concentrate sunlight.
Hadi Ghasemi, a postdoc at Massachusetts Institute of Technology, says the spongelike structure can be made from relatively inexpensive materials -a particular advantage for a variety of compact, steam-powered applications. “Steam is important for desalination, hygiene systems, and sterilisation,” says Ghasemi, who led the development of the structure. “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 researchers reported the details in Nature Communications.
FROM SUN TO STEAM
The approach itself is relatively simple: Since steam is generated at the surface of a liquid, Ghasemi looked for a material that could both efficiently absorb sunlight and generate steam at aliquid’s surface. He settled on a thin, double-layered, disc-shaped structure. Its top layer is made from graphite that the researchers exfoliated by placing the material in a microwave. The effect is just like popcorn: The graphite bubbles up, forming a nest of flakes. The result is a highly porous material that can better absorb and retain solar energy. The structure’s bottom is a carbon foam that contains pockets of air to keep the foam afloat and act as an insulator, preventing heat from escaping to the underlying liquid. The foam also contains very small pores that allow water to creep up via capillary action.
As sunlight hits the structure, it creates a hotspot in the graphite layer, generating a pressure gradient that draws water up through the carbon foam. As water seeps into the graphite layer, the heat concentrated in the graphite turns the water into steam. The structure works much like a sponge that, when placed in water on a hot, sunny day, can continuously absorb and evaporate liquid.
They tested the structure by placing it in a chamber of water and exposing it to a solar simulator – a light source that simulates intensities of solar radiation. They found they were able to convert 85 per cent of solar energy into steam at a solar intensity 10 times that of a typical sunny day.