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Bubbly solution to water heating

By Jennifer Chu – MIT News

Massachusetts Institute of Technology (MIT) researchers and others from the Masdar Institute of Science and Technology, in the United Arab Emirateshave invented a device that absorbs natural sunlight and heats water to boiling temperatures.

The bubble-wrapped, sponge-like device soaks up ambient sunlight and heats water to boiling temperatures, generating steam through its pores.

How do you boil water? Do you usually use an electric or stovetop kettle? Not at MIT. This design, which the researchers call a “solar vapour generator”, does not require expensive mirrors or lenses to concentrate the sunlight. Instead, it relies on a combination of relatively low technology materials to capture ambient sunlight and concentrate it as heat. The heat is then directed towards the sponge’s pores, which draw water up and release it as steam.

From their experiments — including one in which they simply placed the solar sponge on the roof of MIT’s Building 3 — the researchers discovered that the structure heated water to its boiling temperature of 100°C even on relatively cool, overcast days. The sponge also converted 20% of the incoming sunlight to steam.

This low technology design may provide inexpensive alternatives for applications ranging from desalination and residential water heating, to wastewater treatment and medical tool sterilisation.

The team published their results in the Nature Energy journal in September. The research was led by George Ni (an MIT graduate student) and Gang Chen (the Carl Richard Söderberg Professor of Power Engineering and head of the Department of Mechanical Engineering at MIT), in collaboration with TieJun Zhang and his group 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.

Looking for answers

The researchers’ current design builds on a solar-absorbing structure they developed in 2014: a similar floating, sponge-like material made of graphite and carbon foam, which was able to boil water to 100°C and convert 85% of the incoming sunlight to steam.

To generate steam at such efficient levels, the researchers had to expose the structure to simulated sunlight that was 10 times the intensity of sunlight in normal, ambient conditions. “It was relatively low optical concentration,” Chen says. “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.”

In ambient sunlight, the researchers found that, while the black graphite structure absorbed sunlight well, it also tended to radiate heat back out into the environment. To minimise the amount of heat loss, the team looked for materials that would better trap solar energy.

The secret’s in the bubbles

In their new design, the researchers settled on a spectrally selective absorber: a thin, blue, metallic-like film that is commonly used in solar water heaters and possesses unique absorptive properties. 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, thereby minimising heat loss.

The researchers used a thin sheet of copper, chosen for its heat-conducting abilities, and coated it with the spectrally selective absorber. They then mounted the structure on a thermally insulating piece of floating foam. However, they found that even though the structure did not radiate much heat back out into 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 an unlikely source: Chen’s 16-year-old daughter, who at the time 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 [71.11°C], in winter!” Chen says. “It was very effective.”

Chen proposed the packaging material to Ni as a cost-effective way to prevent heat loss by convection. This approach would let sunlight in through the material’s transparent wrapping, while trapping air in its insulating bubbles.

“I was very sceptical 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, as it turns out, it worked. Now because of this bubble wrap, we don’t need mirrors to concentrate the sun.”

The bubble wrap, combined with the selective absorber, prevented heat from escaping the surface of the sponge. Once the heat was trapped, the copper layer conducted the heat towards a single hole (or channel) that the researchers had drilled through the structure. When they placed the sponge in water, they found that water crept up the channel, where it was heated to 100°C and then turned to steam.

Tao Deng, professor of material sciences and engineering at Shanghai Jiao Tong University in China, says the researchers’ use of low-cost materials will make the device more affordable for a wide range of applications.

“This device offers a totally new design paradigm for solar steam generation,” says Deng, who was not involved in the study. “It eliminates the need for 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.”

Chen and Ni say that solar absorbers based on this general design could be used as large sheets to desalinate small bodies of water or to treat wastewater. Ni says other solar-based technologies that rely on optical-concentrating technologies typically are designed to last 10–20 years, though they require expensive parts and maintenance. This new, low technology design, he says, could operate for one to two years before needing to be replaced.

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

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

This research was funded, in part, by a co-operative agreement between the Masdar Institute of Science and Technology and MIT, as well as by the Solid-State Solar Thermal Energy Conversion Centre, which is an Energy Frontier Research Centre funded by the US Department of Energy.


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