New spongelike structure generates steam by soaking up sun

MIT researchers have developed a new spongelike structure that converts solar energy into steam.
The structure - 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 structure is able to convert 85 per cent of incoming solar energy into steam - a significant improvement over recent approaches to solar-powered steam generation.
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 complex, costly systems to highly concentrate sunlight.

Hadi Ghasemi, a postdoc in Massachusetts Institute of Technology's Department of Mechanical Engineering, said the spongelike structure can be made from relatively inexpensive materials - a particular advantage for a variety of compact, steam-powered applications.

The researchers looked for a material that efficiently absorbs sunlight and generate steam at a liquid's surface.
After trials with multiple materials, they 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, said mechanical engineering department head Gang Chen, 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 layer 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 through the structure 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.

The researchers tested the structure by placing it in a chamber of water and exposing it to a solar simulator - a light source that simulates various 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.
The study was published in the journal Nature Communications.