Novel sponge creates steam using ambient sunlight

Scientists have invented a bubble-wrapped, sponge-like device that soaks up sunlight and heats water to boiling temperatures, generating steam through its pores, an advance that may lead to low cost alternatives to wastewater treatment and medical tool sterilisation.
The "solar vapour generator" requires no expensive mirrors or lenses to concentrate the sunlight, but instead relies on a combination of relatively low-tech materials such as bubble wrap to capture ambient sunlight and concentrate it as heat.
The heat is then directed toward the pores of the sponge, which draw water up and release it as steam.
Researchers at Massachusetts Institute of Technology (MIT) in the US found the structure heated water to its boiling temperature of 100 degrees Celsius, even on relatively cool, overcast days. The sponge also converted 20 per cent of the incoming sunlight to steam.

The design may provide inexpensive alternatives for applications such as desalination, residential water heating, wastewater treatment and medical tool sterilisation.
The current design builds on a solar-absorbing structure they developed in 2014 - a similar floating, sponge-like material made of graphite and carbon foam, that was able to boil water to 100 degrees Celsius and convert 85 per cent of the incoming sunlight to steam.

To generate steam at such efficient levels, researchers had to expose the structure to simulated sunlight that was 10 times the intensity of sunlight in normal, ambient conditions.
In ambient sunlight, while the black graphite structure absorbed sunlight well, it also tended to radiate heat back out into the environment.

To minimise heat lost, the team used spectrally-selective absorber - a thin, blue, metallic-like film that is commonly used in solar water heaters and possesses unique absorptive properties.
They then mounted the structure on a thermally-insulating piece of floating foam.
Researchers use bubble wrap, as a cost-effective way to prevent heat loss by convection. This approach would let sunlight in through the material's transparent wrapping, while trapping air in its insulating bubbles.
"I thought it was not a high-performance material. But we tried the clearer bubble wrap with bigger bubbles for more air trapping effect, and it turns out, it works," said George Ni, an MIT graduate student.

"Now because of this bubble wrap, we don't need mirrors to concentrate the Sun," said Ni.
The bubble wrap, combined with the selective absorber, kept heat from escaping the surface of the sponge.
Once the heat was trapped, the copper layer conducted the heat toward a single hole, or channel, that the researchers had drilled through the structure.
When they placed the sponge in water, they found that water crept up the channel, where it was heated to 100 degrees Celsius, then turned to steam.
The research was published in the journal Nature Energy.