• On the left, a representative structure for localization of heat; the cross section of structure and temperature distribution. On the right, a picture of enhanced steam generation by the DLS structure under solar illumination.

    On the left, a representative structure for localization of heat; the cross section of structure and temperature distribution. On the right, a picture of enhanced steam generation by the DLS structure under solar illumination.

    Courtesy of the researchers

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  • The DLS that consists of a carbon foam (10-mm thick) supporting an exfoliated graphite layer (B5-mm thick). Both layers are hydrophilic to promote the capillary rise of water to the surface.

    The DLS that consists of a carbon foam (10-mm thick) supporting an exfoliated graphite layer (B5-mm thick). Both layers are hydrophilic to promote the capillary rise of water to the surface.

    Courtesy of the researchers

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Steam from the sun

New spongelike structure converts solar energy into steam.

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A new material structure developed at MIT generates steam by soaking up the sun.

The structure — a layer of graphite flakes and an underlying carbon foam — is a porous, insulating material structure that floats on water. When sunlight hits the structure’s surface, it creates a hotspot in the graphite, drawing water up through the material’s pores, where it evaporates as steam. The brighter the light, the more steam is generated.

The new material is able to convert 85 percent of incoming solar energy into steam — a significant improvement over recent approaches to solar-powered steam generation. What’s more, the setup loses very little heat in the process, and can produce steam at relatively low solar intensity. This would mean that, if scaled up, the setup would likely not require complex, costly systems to highly concentrate sunlight.

Hadi Ghasemi, a postdoc in MIT’s Department of Mechanical Engineering, says the spongelike structure can be made from relatively inexpensive materials — a particular advantage for a variety of compact, steam-powered applications.

“Steam is important for desalination, hygiene systems, and sterilization,” says Ghasemi, who led the development of the structure. “Especially 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.”

Ghasemi and mechanical engineering department head Gang Chen, along with five others at MIT, report on the details of the new steam-generating structure in the journal Nature Communications

Cutting the optical concentration

Today, solar-powered steam generation involves vast fields of mirrors or lenses that concentrate incoming sunlight, heating large volumes of liquid to high enough temperatures to produce steam. However, these complex systems can experience significant heat loss, leading to inefficient steam generation.

Recently, scientists 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, vaporizing the surrounding water molecules as steam. But initiating this reaction requires very intense solar energy — about 1,000 times that of an average sunny day.

By contrast, the MIT approach generates steam at a solar intensity about 10 times that of a sunny day — the lowest optical concentration reported thus far. The implication, the researchers say, is that steam-generating applications can function with lower sunlight concentration and less-expensive tracking systems.  

“This is a huge advantage in cost-reduction,” Ghasemi says. “That’s exciting for us because we’ve come up with a new approach to solar steam generation.”

From sun to steam

The approach itself is relatively simple: Since steam is generated at the surface of a liquid, Ghasemi looked for a material that could both efficiently absorb sunlight and generate steam at a liquid’s surface.

After trials with multiple materials, he settled on a thin, double-layered, disc-shaped structure. Its top layer is made from graphite that the researchers exfoliated by placing the material in a microwave. The effect, Chen says, is “just like popcorn”: The graphite bubbles up, forming a nest of flakes. The result is a highly porous material that can better absorb and retain solar energy.

The structure’s bottom layer is a carbon foam that contains pockets of air to keep the foam afloat and act as an insulator, preventing heat from escaping to the underlying liquid. The foam also contains very small pores that allow water to creep up through the structure via capillary action.

As sunlight hits the structure, it creates a hotspot in the graphite layer, generating a pressure gradient that draws water up through the carbon foam. As water seeps into the graphite layer, the heat concentrated in the graphite turns the water into steam. The structure works much like a sponge that, when placed in water on a hot, sunny day, can continuously absorb and evaporate liquid.

The researchers tested the structure by placing it in a chamber of water and exposing it to a solar simulator — a light source that simulates various intensities of solar radiation. They found they were able to convert 85 percent of solar energy into steam at a solar intensity 10 times that of a typical sunny day.

Ghasemi says the structure may be designed to be even more efficient, depending on the type of materials used.

“There can be different combinations of materials that can be used in these two layers that can lead to higher efficiencies at lower concentrations,” Ghasemi says. “There is still a lot of research that can be done on implementing this in larger systems.”

Topics: Solar, Energy, Desalination, Materials science, Mechanical engineering, Research, Nanoscience and nanotechnology


Love the idea. Great work guys and girls!!

Seems like a key use for this technology would be to create water, ¥es?

Why don't you use graphene?

Wow! This type of steam generation is amazing.

Two Questions:
How do I make it?
Where can I get it?

This is pretty incredible - I wonder what's the survival of the material over the medium term?

More proof of where our reserch funds should go--to renewables!

How can I most closely replicate this at home for under $20?

Impurities in the water build up in the top layer and soon begin to interfere with the whole setup. Am I right? This means a much more complex setup needed for desalination and purification. Right?

Very interesting? Have you tried running this process with saline or brackish water? Are there issues with the pores becoming clogged with salts or other contaminants?

So, if you build a massive field of this sponge and just leave it on the ocean, it will create clouds more "quickly" ?

It'll work great where you don't need heat.

If used to purify water, would the carbon sponge eventually become clogged with salt or contaminants?

Doesn't capturing the steam impede the sunrays from directly hitting the material and thus produce less steam?

Isn't that water vapour? Any steam would be invisible.

Wow, this doesn't sound like it would be too expensive to produce. Hopefully we can see this material used soon so everyone in the world can lift a bottle, cup, or glass and take a drink of clean water.

No mention of efficiency at typical incoming energy intensities. If this requires an order of magnitude more solar energy than is typically available on a sunny day, then would it not also require mirrors or other optical amplification to see useful benefit? How does the porous material fare in the filthy water conditions expected in the parts of the world this kind of passive solar energy is most desperately needed? How is the steam captured without affecting access to incoming solar energy (light)?

I'm not sure I understand the problem this is solving. Water is already an excellent absorber of infrared. How is this better than, say, a trough reflector or the like?

Can this make rain if it's used in a bigger scale? Lets say we put 20 million of these little sponges in the ocean. What do you think?

Great research.
Dr.A.Jagadeesh Nellore(AP),India

I wonder what the potential for an array of these sponges combined with a Stirling engine could be?

Just the capillarity effect increasing thousands of times the water surface.

If the goal is just atmospheric pressure steam for cleaning, distillation, etc, 100 deg. C is sufficient. To run a practical steam engine, however, requires at least 200 deg. C. How many suns of intensity would be necessary for this?

So, what if someone were to take a petawatt laser to this? It being the brightest light in the universe, wouldn't a very substantial amount of energy be created?

So, what if a petawatt laser was taken to this material? Wouldn't a very substantial amount of energy be at anyone's disposal? Does this material work in full spectrum or is it like the solar panels of old; restricted to a predetermined wavelength?

This is a joke right? Water evaporates from all kinds of things, that doesn't mean they creat steam. For you to have steam the water must reach 212 degrees F. I expect better from MIT. Heck even Wiki gets it Steam is the technical term for the gaseous phase of water, which is formed when water boils

I hope this material becomes available for tinkerers. If issues with saltwater can be dealt with, I have some ideas this might make possible.
How hot is the vapor released from this material?

An image from another web site (not mine!). This appears to be a product that would be easy to manufacture and reproduce. Assume that what you see here is the size of a common kitchen sponge. For small applications (e.g. personal water purification) you use a few. For larger uses--residential heat, power generation, etc., use a much larger number. Small pieces like this could be added to a system as need and checkbook allow. I presume the material can also be made in larger pieces for large-scale projects.

If indeed this is collecting energy at 85% efficiency, it has the potential to be four times more efficient than photovoltaic cells (though one must allow for some loss if using it to run a generator). That is a quantum leap forward in solar technology.

I too hope it becomes available to those with tinkering in the blood. Or perhaps, as more details emerge, it will prove to be something that can be made by the average garage inventor. I look forward to seeing further information.

Solar intensity ten times greater than a typical sunny day just with that amount of heat everything would be boiling anyway probably melting. how does it perform on a normal sunny day ?

why is it necessary to use a graphite layer?

What are the materials and how did you make it? I want to make it before May 20th(want to use it for a school solar project)

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