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Steam generator: Graphite flakes and carbon foam float on water
A team of US engineering researchers has developed a material structure that generates steam by soaking up the sun.
The structure – a layer of graphite flakes and an underlying carbon foam – is a porous, insulating material that floats on water. When sunlight hits the structure’s surface, it creates a hotspot in the graphite, drawing water up through pores, where it evaporates as steam.
The brighter the light, the more steam is generated. The material is able to convert 85% of incoming solar energy into steam – a significant improvement over recent approaches to solar-powered steam generation. What’s more, the set-up loses very little heat in the process, and can produce steam at relatively low solar intensity. So, if scaled up, the set-up would not require complex, costly systems to concentrate sunlight.
Hadi Ghasemi, a postdoc in the department of mechanical engineering at Massachusetts Institute of Technology, which has developed the material, said the spongelike structure could be made from relatively inexpensive materials.
“Steam is important for desalination, hygiene systems, and sterilisation,” said Ghasemi. “In remote areas where the sun is the only source of energy, if you can generate steam with solar energy, it would be very useful.”
Today, solar-powered steam generation involves vast fields of mirrors or lenses that concentrate incoming sunlight, heating large volumes of liquid to high temperatures. But these complex systems can experience significant heat loss, leading to inefficient steam generation.
Engineers have explored ways to improve the efficiency of solar-thermal harvesting by developing new solar receivers and by working with nanofluids. The latter approach involves mixing water with nanoparticles that heat up quickly when exposed to sunlight, vaporising the surrounding water molecules as steam. But initiating this reaction requires intense solar energy – 1,000 times that of an average sunny day.
By contrast, the MIT approach generates steam at a solar intensity 10 times that of a sunny day — the lowest optical concentration reported so far. The implication, the researchers say, is that steam-generating applications could function with lower sunlight concentration and less-expensive tracking systems.
“This is a huge advantage in cost-reduction,” said Ghasemi. “That’s exciting for us because we’ve come up with a new approach to solar steam generation.”
Ghasemi said the structure could be designed to be even more efficient, depending on the materials used.
“Different combinations of materials can lead to higher efficiencies at lower concentration.”