We noticed you're browsing in private or incognito mode.

To continue reading this article, please exit incognito mode or log in.

Not a subscriber? Subscribe now for unlimited access to online articles.

Solar Sponge

Low-tech device turns sunlight into steam.

MIT engineers have invented a bubble-wrapped, spongelike device that soaks up natural sunlight and heats water to boiling temperatures, generating steam through its pores.

The solar vapor generator requires no expensive mirrors or lenses to concentrate the sunlight. Instead, it relies on low-tech materials to capture 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.

From their experiments—including one in which they placed the solar sponge on the roof of MIT’s Building 3—the researchers found that the structure heated water to its boiling temperature of 100 °C. The sponge can generate steam in a matter of minutes (and sometimes just seconds), even on relatively cool, overcast days.

This story is part of the November/December 2016 Issue of the MIT News magazine
See the rest of the issue
Bubble wrap helps the solar vapor generator trap the sun’s heat.

The design may provide inexpensive alternatives for desalination, residential water heating, wastewater treatment, and medical tool sterilization.

The research was led by George Ni, an MIT graduate student, and Gang Chen, a professor of power engineering and head of the Department of Mechanical Engineering.

One key component of the design is a spectrally selective absorber—a blue, metallic-like film that is commonly used in solar water heaters. The material absorbs radiation in the visible range of the electromagnetic spectrum, but it does not radiate in the infrared range, meaning that it both absorbs sunlight and traps heat, minimizing heat loss.

Since copper conducts heat efficiently, the researchers mounted a thin sheet of copper coated with the spectrally selective absorber onto a thermally insulating piece of floating foam. However, they found that even though the structure did not radiate much heat back out to the environment, heat was still escaping through convection, in which moving air molecules such as wind would naturally cool the surface.

A solution to this problem came from Chen’s 16-year-old daughter, who was then working on a science fair project—a makeshift greenhouse made from simple materials, including bubble wrap.

“She was able to heat it to 160 °F, in winter!” Chen says. “It was very effective.”

Chen proposed to Ni that the packing material could be used to prevent heat loss by convection. Covering the sponge with it would let sunlight in through the material’s transparent wrapping while trapping air in its 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.”

While the prototype converted just 20 percent of the incoming sunlight to steam, the researchers plan to explore other materials, including different configurations of bubble wrap, to improve performance of the low-cost device.

Become an MIT Technology Review Insider for in-depth analysis and unparalleled perspective.

Subscribe today
Bubble wrap helps the solar vapor generator trap the sun’s heat.
Next in MIT News
Want more award-winning journalism? Subscribe to Print + All Access Digital.
  • Print + All Access Digital {! insider.prices.print_digital !}*

    {! insider.display.menuOptionsLabel !}

    The best of MIT Technology Review in print and online, plus unlimited access to our online archive, an ad-free web experience, discounts to MIT Technology Review events, and The Download delivered to your email in-box each weekday.

    See details+

    12-month subscription

    Unlimited access to all our daily online news and feature stories

    6 bi-monthly issues of print + digital magazine

    10% discount to MIT Technology Review events

    Access to entire PDF magazine archive dating back to 1899

    Ad-free website experience

    The Download: newsletter delivered daily

You've read of three free articles this month. for unlimited online access. You've read of three free articles this month. for unlimited online access. This is your last free article this month. for unlimited online access. You've read all your free articles this month. for unlimited online access. You've read of three free articles this month. for more, or for unlimited online access. for two more free articles, or for unlimited online access.

MIT News = for alumni only.

Are you an MIT alum?
Sign in now to read all MIT alumni news and class notes— or to manage your magazine subscription.

Sign in and read on