Sponge Makes Steam Using Natural Sunlight

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Instead of a kettle or a flame, MIT engineers have boiled water using a sponge-like device that generates steam through its peers.

The researchers have called their device a “solar vapor generator,” and it does not need any expensive devices to power it.

Low-tech solar sponge boils water

In fact the solar sponge using relatively low tech materials to turn sunlight into heat. The scientists found that the sponge was capable of heating water to 100 degrees Celsius even on relatively overcast days.

It is thought that the low-tech solar sponge could be used in desalination, residential water heating and waste water treatment. The full results were published in the journal Nature Energy, and the research team was led by MIT graduate student George Ni.

The solar-absorbing structure is a development of a design that was first made in 2014. This latest model is capable of converting 20% of incoming sunlight into steam.

Special material absorbs sunlight and traps heat

It uses a spectrally-selective absorber because they trap solar energy well and radiate little heat back into the environment. The absorber is a thin, blue, metallic-like film that is often found in solar water heaters.

The material traps radiation in the visible range of the electromagnetic spectrum, while not radiating infrared. This means that is absorbs sunlight while trapping heat.

This spectrally-selective absorber was wrapped around a thin sheet of copper, selected for its ability to conduct heat, and the structure was then placed onto a piece of floating foam.

Bubble wrap to the rescue

Another obstacle was heat loss due to convection, as air molecules from wind would cool the surface of the device. The inspiration for a solution was one of the researcher’s daughter, who was using bubble wrap to construct a small greenhouse for a science project.

At first Ni wasn’t keen on the idea. However bubble wrap allows sunlight through its transparent surface while also trapping air in the insulating bubbles.

“I was very skeptical of the idea at first,” Ni recalls. “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. Now because of this bubble wrap, we don’t need mirrors to concentrate the sun.”

Thanks to the combination of bubble wrap and the selective absorber, heat was prevented from escaping from the surface of the sponge. After trapping the heat, the copper layer conducts it to a single channel that the scientists made in the sponge.

As water creeps through the channel it is heated to 100C before turning to steam. According to the team the method could be used to desalinate some bodies of water, or treat waste water.

The low tech approach means that the device does not need expensive maintenance or spare parts. It would also function for 1-2 years before a replacement was needed.

“Even so, the cost is pretty competitive,” Ni says. “It’s kind of a different approach, where before, people were doing high-tech and long-term [solar absorbers]. We’re doing low-tech and short-term.”

“What fascinates us is the innovative idea behind this inexpensive device, where we have creatively designed this device based on basic understanding of capillarity and solar thermal radiation. Meanwhile, we are excited to continue probing the complicated physics of solar vapor generation and to discover new knowledge for the scientific community,” said study co-author TieJun Zhang says.

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