Researchers report major advance in using sunlight to produce steam without boiling water

Nov 19, 2012
New solar steam technology developed at Rice University uses nanoparticles so effective at turning sunlight into heat that it can produce steam from icy-cold water. Credit: Jeff Fitlow/Rice University

(Phys.org)—Rice University scientists have unveiled a revolutionary new technology that uses nanoparticles to convert solar energy directly into steam. The new "solar steam" method from Rice's Laboratory for Nanophotonics (LANP) is so effective it can even produce steam from icy cold water.

Details of the solar steam method were published online today in ACS Nano. The technology has an overall energy efficiency of 24 percent. Photovoltaic solar panels, by comparison, typically have an overall energy efficiency around 15 percent. However, the inventors of solar steam said they expect the first uses of the new technology will not be for electricity generation but rather for sanitation and water purification in developing countries.

"This is about a lot more than electricity," said LANP Director Naomi Halas, the lead scientist on the project. "With this technology, we are beginning to think about solar thermal power in a completely different way."

The efficiency of solar steam is due to the light-capturing nanoparticles that convert sunlight into heat. When submerged in water and exposed to sunlight, the particles heat up so quickly they instantly vaporize water and create steam. Halas said the solar steam's overall energy efficiency can probably be increased as the technology is refined.

"We're going from heating water on the macro scale to heating it at the nanoscale," Halas said. "Our particles are very small—even smaller than a wavelength of light—which means they have an extremely small surface area to dissipate heat. This intense heating allows us to generate steam locally, right at the surface of the particle, and the idea of generating steam locally is really counterintuitive."

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To show just how counterintuitive, Rice graduate student Oara Neumann videotaped a solar steam demonstration in which a test tube of water containing light-activated nanoparticles was submerged into a bath of ice water. Using a lens to concentrate sunlight onto the near-freezing mixture in the tube, Neumann showed she could create steam from nearly frozen water.

Steam is one of the world's most-used industrial fluids. About 90 percent of electricity is produced from steam, and steam is also used to sterilize medical waste and surgical instruments, to prepare food and to purify water.

Most industrial steam is produced in large boilers, and Halas said solar steam's efficiency could allow steam to become economical on a much smaller scale.

People in developing countries will be among the first to see the benefits of solar steam. Rice engineering undergraduates have already created a solar steam-powered autoclave that's capable of sterilizing medical and dental instruments at clinics that lack electricity. Halas also won a Grand Challenges grant from the Bill and Melinda Gates Foundation to create an ultra-small-scale system for treating human waste in areas without sewer systems or electricity.

"Solar steam is remarkable because of its efficiency," said Neumann, the lead co-author on the paper. "It does not require acres of mirrors or solar panels. In fact, the footprint can be very small. For example, the light window in our demonstration autoclave was just a few square centimeters."

Another potential use could be in powering hybrid air-conditioning and heating systems that run off of sunlight during the day and electricity at night. Halas, Neumann and colleagues have also conducted distillation experiments and found that solar steam is about two-and-a-half times more efficient than existing distillation columns.

Halas, the Stanley C. Moore Professor in Electrical and Computer Engineering, professor of physics, professor of chemistry and professor of biomedical engineering, is one of the world's most-cited chemists. Her lab specializes in creating and studying light-activated particles. One of her creations, gold nanoshells, is the subject of several clinical trials for cancer treatment.

For the cancer treatment technology and many other applications, Halas' team chooses particles that interact with just a few wavelengths of light. For the solar steam project, Halas and Neumann set out to design a particle that would interact with the widest possible spectrum of sunlight energy. Their new nanoparticles are activated by both visible sunlight and shorter wavelengths that humans cannot see.

"We're not changing any of the laws of thermodynamics," Halas said. "We're just boiling water in a radically different way."

Explore further: Pinpoint laser heating creates a maelstrom of magnetic nanotextures

More information: Solar Vapor Generation Enabled by Nanoparticles, dx.doi.org/10.1021/nn304948h

Abstract
Solar illumination of broadly absorbing metal or carbon nanoparticles dispersed in a liquid produces vapor without the requirement of heating the fluid volume. When particles are dispersed in water at ambient temperature, energy is directed primarily to vaporization of water into steam, with a much smaller fraction resulting in heating of the fluid. Sunlight-illuminated particles can also drive H2O-ethanol distillation, yielding fractions significantly richer in ethanol content than simple thermal distillation. These phenomena can also enable important compact solar applications such as sterilization of waste and surgical instruments in resource-poor locations.

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grondilu
not rated yet Nov 19, 2012
Wow. A potential desalinization technique using light with a 82% efficiency?? Seems almost too good to be true.

PS. Hang on. The overall efficiency is rather 24 percent. My bad.
Sonhouse
not rated yet Nov 19, 2012
24% is a non-optimized number as the article states. The question I have is what concentration of sunlight produced the 82% part? Direct sunlight with no concentration? That would be a game changer if so. It would say perhaps better efficiency could be made with concentrated sunlight. It is surprising such particles can absorb light since it sounds like they are using particles about 50 nanometers or so, much much smaller than the wavelength of the ambient light hitting them. I wonder if it has to be a group effort to achieve that adsorption percentage.

Those numbers also beg the question as to where they are losing 3/4 of the heat as it stands. I guess that is the next question they will want answered. It is clear, assuming there is not much degradation of the nano dots you just feed in water and steam comes out and you re-use the old dots which don't get emitted into the steam. If that were the case, you can see a camp stove made like this to steam veggies on a camping trip.
ScooterG
1.5 / 5 (8) Nov 19, 2012
How is precipitate dealt with? Since there's no heat in the liquid, maybe it could be contained in a throw-away plastic liner?

Cool!
antialias_physorg
5 / 5 (1) Nov 19, 2012
Sounds good. Must be used in a closed system, though. It would be an environmental hazard to have these nanoparticles mixed in with any steam that escapes into the atmosphere.
Aryeh_Z
1.3 / 5 (3) Nov 19, 2012
I don't want to rain on anyones parade but this would make an extremely powerful naval weapon. Imagine dumping a few tons of this into the sea in the middle of a formation and cooking up the hulls of the ships.
nkalanaga
4.5 / 5 (2) Nov 19, 2012
Sonhouse: I would imagine that the 76% waste heat went directly into water molecules or into the container. But even so, boiling water at 24% efficiency with no artificial fuel source would be very handy.
Sean_W
2 / 5 (10) Nov 19, 2012
I don't want to rain on anyones parade but this would make an extremely powerful naval weapon. Imagine dumping a few tons of this into the sea in the middle of a formation and cooking up the hulls of the ships.


I am not sure that would be practical. The vapour constantly forming around the particle might make it difficult for it to adhere to the hulls and so the heat would be carried away with the steam. as the steam rises it would quickly lose energy to the surrounding sea air causing an updraft of warm air and condensing droplets. You might affect the local weather but you wouldn't fry the ships.

I wonder if that principal could be used to raise the steam up an inclined but insulated pipe to get moisture from the coast to high altitude deserts; like the natural water cycle only directed to more targeted locations.
antialias_physorg
4.9 / 5 (7) Nov 19, 2012
I don't want to rain on anyones parade but this would make an extremely powerful naval weapon. Imagine dumping a few tons of this into the sea in the middle of a formation and cooking up the hulls of the ships

Try it. It would take giagtonnes of the stuff to even obscure vision a tiny bit (not that naval vessels rely on vision these days). And steel hulls are quite immune against steam. You'd need something a LOT hotter to affect them.

Then again: Close to the hull the hull shadows the water: so no steam at all.

Then again: the stuff would disperse/sink immediately (which renders it ineffective since only stuff at the surface would receive light)

Then again: naval vessels MOVE. So how long would any effect last? Ten seconds till you've moved past the spot? Then what?

Then again: how do you even get it close to a ship? If you can get gigatonnes that close then a mine/missile is WAY more effective than a bit of water vapour.

Nah. This isn't a weapon.
Jeddy_Mctedder
1.3 / 5 (12) Nov 19, 2012
nanoparticles. i love it. same old nonsense. slip in a buzzword as a proxy for explaining any details.
Sonhouse
not rated yet Nov 19, 2012
Sonhouse: I would imagine that the 76% waste heat went directly into water molecules or into the container. But even so, boiling water at 24% efficiency with no artificial fuel source would be very handy.


What is the efficiency level of a typical home stove, say an electric stove that boils water in a one gallon container, never thought much about actual thermal efficiency there. I did note when I would boil water at home if the electric burner is sized close to the bottom of the pan there is not much thermal radiation leakage, at least what I can feel with my hand, compared with the radiation felt by a bare electric burner.
lengould100
1.8 / 5 (5) Nov 19, 2012
The key question left unanswered here is "what use is the steam?". Steam at atmospheric pressure is pretty much usless. What efficiency effects of pressurizing the boiling vessel to 10 bar (useful steam for space heating) or 100 bar (useful to drive a turbine). I expect the efficiency would drop dramatically, since steam at high pressure is at much higher temperature and contains a lot more energy than steam at atmospheric pressure.
drhoo
3 / 5 (2) Nov 19, 2012
It's the pressure that steam is under in a generating plant that creates mechanical energy, the steam itself is somewhat secondary.
For instance you could heat air to raise its pressure and drive a turbine, of course not very efficient.
Steam is just water vapor, to me the gee wiz of the story is missing.
nkalanaga
5 / 5 (2) Nov 20, 2012
I doubt that this would be of any use for industrial scale steam power. It would work for anything needing low pressure steam, which includes distillation and disinfecting, where all that's needed is to raise the temperature. They note that they've already done both of those. For small scale high pressure steam it would also work, as long as the boiler could be made of glass or other transparent material.

Sonhouse: I have no idea of the thermal efficiency of a pot on a stove, but matching the burner to the pot is a long-established method of increasing that efficiency. I would guess that it would be fairly high, as the only direct losses would be from the sides of the pot, by radiation and convection to the air, and the top of the water, through evaporation and convection before the water boils.
unknownorgin
1.5 / 5 (6) Nov 20, 2012
Carbon is known to convert most of the electromagnetic spectrum into heat so if you want to experiment india ink made from soot should work for this.
Sonhouse
5 / 5 (1) Nov 20, 2012
I think the effect they are after is heavily dependent on the size of the nano particles. Do you know the size of the india ink particles? I made a quick calc of the size they are using and it seems to be around 50 nanometers. That is pretty small. India ink maybe be more like 1000 nm or 1 micron would be my guess.
jyro
1 / 5 (5) Nov 20, 2012
Not to rain on a parade but how would you contain the nanoparticle pollution. If this got into water as pollution, wouldn't it be a major game changer for weather?
A_Paradox
not rated yet Nov 20, 2012
I doubt that this would be of any use for industrial scale steam power. It would work for anything needing low pressure steam, which includes distillation and disinfecting, where all that's needed is to raise the temperature. They note that they've already done both of those. For small scale high pressure steam it would also work, as long as the boiler could be made of glass or other transparent material.


I agree; I think maybe the water purification potential will be the 'killer app' for this. If you hark your mind back to images I am sure you have seen of little African kids scooping water to drink out of wheel ruts in a road strewn with buffalo shit, the life saving potential of this is obvious.

A_Paradox
not rated yet Nov 20, 2012
I think the effect they are after is heavily dependent on the size of the nano particles. .... I made a quick calc of the size they are using and it seems to be around 50 nanometers. That is pretty small. ...


I agree; the size of the particles is the 'nitty gritty' issue I think. I would like to know if the effect is still available to nano particle of requisite type affixed to a metal surface in air?
Reason: Motive power sourced from sunlight does not need steam; an appropriately designed Stirling engine could do just as well with far less complexity.

Storage of concentrated solar [heat] energy just needs a pile of rocks inside an insulated enclosure. Such a heat store, if provided with appropriately shaped and positioned metal heating surfaces will be able to provide heat for cooking; and a removable insulated lid for each when not in use would optimise the set up. Heat could be transferred to the store via simple hot air.
Shinobiwan Kenobi
1.8 / 5 (5) Nov 21, 2012
nanoparticles. i love it. same old nonsense. slip in a buzzword as a proxy for explaining any details.


Willful ignorance. I love it. Same old nonsense. Slip in dots for the reader to connect on their own and the inept blame the experts for poor reading comprehension.
PhotonX
not rated yet Nov 24, 2012
Whatever the merits of this piece of tech, Rice University certainly seems to be on the ball. Either that or they have better PR than a lot of schools. Seems like every other week I read about something from Rice, from single-pixel imaging to this. I don't have a kid to send off to college, but if I was in Texas I would give Rice a good look.
ValeriaT
1 / 5 (5) Dec 03, 2012
IMO if you have carbon black collector, it will absorb as much of energy, like the water filled with these particles. The lost of heat with radiation will remain the same too. And the particles will indeed cause flocking and sediments and another mechanical problems in boiling circuit. Not to say, the walls of boiler must be made of brittle transparent glass, which reflects light, prone to rupture due the heat expansion a lot and it's difficult to machine into required shape. Such a technology appears elegant at the paper - but the reality may be way less convenient.

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