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.
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.
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."
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.
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. 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. Optical measurements show that 97±1% of solar energy is absorbed by the exfoliated graphite layer.
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. The pore diameters are in the range of 300-600 μm.
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]
"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."
- 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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