To be of use, solar energy must be converted into heat, electricity or mechanical power.

Heat (a.k.a. "solar thermal conversion") is one of the most popular methods for solar energy harvesting. The process involves collecting the sun's energy through optical concentrators (including mirrors or lenses), and, as the units heat up, heat is then transferred to the underlying water. Once the water temperature reaches 100°C., it converts to useful steam.

However, researchers at the Massachusetts Institute of Technology (MIT) have found a way to eliminate the need for expensive optical concentrators, replacing them with low-cost absorbers, thermally insulated foam sponges, and, of all things, bubble wrap.

The researchers found that the units, which they call "solar vapor generators," were able to work on the roof of an MIT building, even on relatively cool and overcast days, with the sponge converting 20 percent of the incoming sunlight to steam.

These low-cost units are not designed to compete against the more expensive optical concentrating systems in terms of efficiency per area. However, the units are likely to become appealing for their low cost, meaning accessibility to solar thermal systems will increase.

The MIT researchers expect the new technology, once scalable, will be most suitable for the water industry, especially desalination and wastewater treatment, where relatively low temperatures can be used.

Who came up with the idea? One of the researchers' 16-year-old daughters. She was working on a science fair project in which she constructed a makeshift greenhouse from simple materials, including bubble wrap.

"She was able to heat it to 160°F in winter," said Gang Chen, professor of power engineering at MIT, one of the researchers.

Dr. Chen proposed the idea of bubble wrap to George Ni, an MIT grad student in the department of mechanical engineering, and lead author of the research report.

"I was very skeptical of the idea at first," Ni said. "I thought it was not a performance material. But we tried clearer bubble wrap with bigger bubbles for more air trapping effect, and it turns out that it worked. Now, because of this bubble wrap, we don't need mirrors to concentrate the sun."