A team of engineers from MIT have ignored the tradition flame and kettle method of boiling water, and developed a bubble-wrapped sponge-like device that absorbs sunlight to heat water up to boiling temperatures, creating steam through pores.
The model - called a “solar vapor generator” - uses a combination of low-tech materials to absorb sunlight and concentrate it to generate heat, instead of relying on expensive lenses or mirrors. The heat generated is directed to the pores in the sponge, which pulls water up and releases it as steam.
From the tests conducted - including one where they placed the sponge on the roof of Building 3 of MIT - the scientists discovered that the solar vapor generator could heat water to its boiling temperature (100oC) even on cool and overcast days. Nearly 20% of the sunlight captured by the device was also converted into steam.
Due to the low-tech materials used, the device can serve as an inexpensive substitute for many applications including residential water heaters, desalination, medical tool sterilization and wastewater treatment, among others.
The findings of the team’s research have been published in the August 23 issue of Nature Energy. The study was headed by MIT graduate student George Ni, and the Carl Richard Soderberg Professor in Power Engineering and the head of the Department of Mechanical Engineering, Gang Chen.
The researchers collaborated with TieJun Zhang and his team members Hongxia Li and Weilin Yang from the Department of Mechanical and Materials Engineering at the Masdar Institute of Science and Technology, in the United Arab Emirates.
Building up the Sun
The current design of the researchers is modeled on a similar solar-absorbing structure they created in 2014. The sponge-like floating material was made of carbon foam and graphite, and was capable of boiling water to 100oC and converting 85% of the captured sunlight to steam.
The structure had to be exposed to stimulated sunlight that had 10 times more intensity than the sunlight in ambient conditions, to produce steam at such efficient rates.
It was relatively low optical concentration, 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.
Gang Chen, Professor, MIT
Despite its efficient absorptive features, the researchers found that at ambient environments the black graphite design radiated heat back into the atmosphere. In order to reduce the heat loss, the researchers searched for materials that can trap solar energy more efficiently.
A Bubbly Solution
The researchers adopted a spectrally-selective absorber, for their recent design. A spectrally-selective absorber is a thin, blue, metallic-like film which, often used in solar water heaters, has special absorptive properties. The material absorbs sunlight, traps heat and reduces heat loss, as it absorbs heat in the electromagnetic spectrum’s visible range while not radiating in the infrared range.
A thin sheet of copper, coated with the spectrally-selective absorber, was used by the researchers due to its heat-conducting abilities. The device was then placed on a thermally-insulating piece of floating foam. However, the researchers observed that loss of heat, though not radiated by the device, was caused due to convection, where wind and other mobile air molecules naturally cooled the surface.
The researchers found a remedy to the problem from an unexpected source. Chen’s 16 year-old daughter was building a makeshift greenhouse, for a science project, with various simple materials, including bubble wrap.
“She was able to heat it to 160 degrees Fahrenheit, in winter!” Chen says. “It was very effective.”
Chen suggested the bubble wrap as a cost-effective method of preventing the heat loss caused by convection. Using the packing material would ensure that sunlight enters the material through its transparent surface and air is trapped in its insulating bubbles.
I was very skeptical of the idea at first. 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.
George Ni, Graduate Student, MIT
The heat was prevented from escaping the sponge with the combination of the selective absorber. After the heat was trapped, the heat was directed to the single channel or hole drilled by the researchers through the device, by the copper layer.
When the researchers placed the device on water, they found that the water entered the device through the channel. It was then heated to a 100oC and converted to steam.
Due to the use of low-tech materials, the device will be made more affordable for a wide variety of applications, says Tao Deng, professor of material sciences and engineering at Shanghai Jiao Tong University.
This device offers a totally new design paradigm for solar steam generation. It eliminates the need of the expensive optical concentrator, which is a key advantage in bringing down the cost of the solar steam generation system. Certainly the clever use of bubble wrap and commercially available selective absorber also helps suppress the convection and radiation heat loss, both of which not only improve the solar harvesting efficiency but also further lower the system cost.
Tao Deng, Professor, Shanghai Jiao Tong University
Ni and Chen state that the general design can be used to develop solar absorbers that can be employed as large sheets to treat wastewater or desalinate small water bodies. Solar-based technologies that depend on optical-concentrating technologies, though need maintenance and expensive parts, are often developed to serve for one or two decades, says Ni. He states, that this new low-cost design can work for a couple of years before requiring replacements.
“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,” says Zhang. “Meanwhile, we are excited to continue probing the complicated physics of solar vapor generation and to discover new knowledge for the scientific community.”
The study was financially supported in part by an agreement between MIT and the Masdar Institute of Science and Technology; and by the Solid-State Solar Thermal Energy Conversion Center, an Energy Frontier Research Center financially aided by U.S. Department of Energy.