A team of engineers from the Massachusetts Institute of Technology (MIT)
has created a unique device that uses a bubble-wrap-encased sponge to boil water fueled by nothing but sunlight. The inexpensive nature of the device makes it an ideal candidate for use in applications such as wastewater treatment, residential water heating and medical tool sterilization, and continues to pave the way for the utilization of sponge-like materials for steam creation.

Also referred to as a "solar vapor generator," the device is extremely cost-efficient due to the lack of any mirrors or lenses, instead using its sponge-like design to utilize ambient sunlight to heat water to the boiling point of 100° C (212°F).

The device in the current study used a solar-absorbing structure that the same MIT team created back in 2014 as a guideline for the new design. In the 2014 study, the sponge-like material was made from graphite and foam and was also able to heat water to temperatures of 100° C. To achieve this, they exposed the device to sunlight that was approximately 10 times more intense than natural sunlight. The new device achieves the same thing using ambient sunlight.

"It was relatively low optical concentration," says senior author Gang Chen of MIT speaking of the original invention. "But I kept asking myself, 'Can we basically boil water on a rooftop, in normal conditions, without optically concentrating the sunlight? That was the basic premise."

Chen and his team determined that although black graphite is effective at sunlight absorption, it also has an unwanted tendency to release heat back into the environment, prompting them to search for a different material that would be more efficient at trapping sunlight.

The new design utilizes a thin sheet of copper coated with a spectrally-selective absorber, which is unique in its ability to absorb radiation in the visible range of the electromagnetic spectrum while not radiating in the infrared range. This allows for the effective absorption of sunlight without significant heat loss. After wrapping the material around a thermally insulating piece of floating foam, the team integrated bubble wrap into the design to prevent heat from escaping through the process of air motion, also known as convection.

Each time steam is produced from the device, it is released from either one hole, or a number of smaller holes that line the surface of the foam device. Determining which route to take depends on whether you want to create high-temperature steam (100°C) or high rates of vapor.

"We have a small wick through the thermal foam, which is connected to the opening in the copper sheet," says Ni. "This draws water to the opening. Because water is evaporating in this opening, it cools the surrounding areas, similar to when someone sweats to cool off. This cooling sucks heat from the surrounding copper, which is generating heat from the sun." The copper sheet array continues to funnel solar heat to the opening, however, so that the steam production continues.

As the steam makes its way into the atmosphere, the air pockets in the bubble wrap minimize convection and reduce heat loss, in turn increasing the system's operating temperature. Ni likens the device to a hefty winter coat that traps air, although in this case the bubble wrap is of particular use due to its transparency, which allows sunlight into the system.

"I was very skeptical of the idea at first," says Ni. "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."

Through numerous experiments using the device, the team discovered that it could heat water to a boil even on cooler and overcast days.

Although the bubble wrap in the current study encompasses the entire device, in the future the team hopes that larger devices with increased insulation will only require the top of the device to be covered in wrap.

"For now, we are envisioning a large system, so the sides are not as important to insulate," says George Ni, an MIT graduate student and first author of the study. "In such a case, the bubble wrap can go on only the top of the system. However, for smaller systems, the sides are important to insulate as well, so in those cases the bubble wrap should wrap the entire structure."

The device is an ideal method of short-term solar steam generation due to the elimination of an optical concentrator, which greatly reduces the cost. Although the short-term, low-tech nature of the device means that replacement would likely be needed every one to two years, the wide availability of the selective absorber and low price of the bubble wrap offset this short lifespan with lower maintenance costs.

"It's kind of a different approach where before, people were doing high-tech and long-term (solar absorbers)," says Ni. "We're doing low-tech and short-term."

The findings were published in the journal Nature Energy.