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Solar Steam
August 15, 2014
There's an interesting device called a
Crookes radiometer, named after its
inventor,
Sir William Crookes. Crookes, who practiced both
physics and
chemistry, invented this device, shown in the photograph, during the course of his
experiments in 1873.
A Crookes radiometer
Its inventor, Sir William Crookes, was also the discoverer of the element, thallium.
Crookes found thallium in residues of sulfuric acid production. He noticed thallium's intense 535.04 nanometer green emission line by flame spectroscopy in 1861.
(Via Wikimedia Commons.)
The radiometer has a set of
pinwheel vanes that
rotate in response to
light. One side of each vane is black, and the other side is either white, or the bright, bare
metal. Crookes thought that his device was responding to the
radiation pressure of light; that is,
photons would be
reflected from the white or
mirror faces, but they would be
absorbed by the black faces. This explanation appeals to many an
undergraduate physicist, even today.
The true mechanism is
thermodynamic. The dark side of the vanes is slightly hotter than the other side, so
air molecules in their vicinity strike those vanes with more
energy, and the vanes rotate in the direction that this little push imparts. The importance of air molecules is confirmed by the fact that the vanes won't rotate at too low a
pressure (about 10
-6 torr). The motion of the vanes is impeded if there's too much air, so the device works best at about 10
-2 torr.
The Crookes radiometer is a somewhat elaborate demonstration that black absorbs radiation better than white. Black is also better at
radiating energy. Black
coffee will radiate heat faster than coffee with
milk or
cream added, but the
thermodynamics of cooling coffee is more complex than that, as I wrote in a
previous article (Coffee Thermodynamics, June 17, 2011).
A team of
engineers from the
MIT Department of Mechanical Engineering, the
Purdue University School of Mechanical Engineering, and the Micro and Nanotechnology Programme of the
Middle East Technical University (Ankara, Turkey) have used this basic radiation absorption concept in a device that converts
solar energy into
steam.[1-3]
Their black absorber is a
porous composite of
graphite flakes and
carbon foam that floats on
water. When concentrated
sunlight is focused on this
material, water is drawn up through the
hockey puck pores and is converted to steam.[2]
The solar steam generator consists of a layer of exfoliated graphite supported by a centimeter thick piece of carbon foam. Both materials are hydrophilic, so water is transported through them by capillary action.
(MIT image courtesy of the research team.)[2)]
Explains
Hadi Ghasemi, a
postdoc in the MIT Department of Mechanical Engineering,
"Steam is important for desalination, hygiene systems, and sterilization... 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."[2]
One approach to solar steam generation has been
nanofluids, which are
nanoparticles in water that heat rapidly when exposed to sunlight. However, solar energy needs to be concentrated a thousand times for nanofluids to work. The MIT solar steam hockey puck needs just a ten times concentration, so it will function with simple
solar concentrators.[2]
The
exfoliated graphite, which is used as the solar energy absorbing layer, is produced by placing graphite in a
microwave oven. The graphite bursts apart, forming a nest of highly porous flakes that are excellent solar energy absorbers.[2] A
Brunauer-Emmett-Teller absorption experiment gave a
surface area of 320
square meters per
gram for the exfoliated graphite, as compared to 10 before exfoliation.[3]
Optical measurements show that 97±1% of solar energy is absorbed by the exfoliated graphite layer.[3]
The carbon foam
substrate contains pockets of air, and these have the two-fold purpose of making the material less
dense than water, so it will float, and act as a
thermal insulator to keep the
heat in the
liquid. The foam has a porous structure that allows water from below to rise through the material by
capillary action.[2] The pore diameters are in the range of 300-600
μm.[3]
Steam generation in a laboratory beaker.
(MIT image courtesy of the research team.)
In operation, sunlight heats the top surface of the structure,
evaporating water, and generating a
pressure gradient that transports water through the carbon foam. This water enters the exfoliated graphite layer where the heat turns it into steam. In tests with a
solar simulator, the structure was found to convert 85 percent of solar energy into steam with just a ten-fold concentration of
solar radiation; that is, at a
radiant flux of 10
kilowatts per square meter.[1-2]
Says Ghasemi,
"There can be different combinations of materials that can be used in these two layers that can lead to higher efficiencies at lower concentrations... There is still a lot of research that can be done on implementing this in larger systems."[2]
References:
- Hadi Ghasemi, George Ni, Amy Marie Marconnet, James Loomis, Selcuk Yerci, Nenad Miljkovic, and Gang Chen, "Solar steam generation by heat localization," Nature Communications, vol. 5, article no. 4449 (July 21, 2014), doi:10.1038/ncomms5449.
- Jennifer Chu, "Steam from the sun - New spongelike structure converts solar energy into steam," MIT Press Release, July 21, 2014.
- Supplementary Information for Ref. 1.
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