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newsoffice 00227.txt

#Getting a charge out of water droplets Last year MIT researchers discovered that when water droplets spontaneously jump away from superhydrophobic surfaces during condensation they can gain electric charge in the process. Now the same team has demonstrated that this process can generate small amounts of electricity that might be used to power electronic devices. The new findings by postdoc Nenad Miljkovic associate professor of mechanical engineering Evelyn Wang and two others are published in the journal Applied Physics Letters. This approach could lead to devices to charge cellphones or other electronics using just the humidity in the air. As a side benefit the system could also produce clean water. The device itself could be simple Miljkovic says consisting of a series of interleaved flat metal plates. Although his initial tests involved copper plates he says any conductive metal would do including cheaper aluminum. In initial testing the amount of power produced was vanishingly small just 15 picowatts or trillionths of a watt per square centimeter of metal plate. But Miljkovic says the process could easily be tuned to achieve at least 1 microwatt or millionth of a watt per square centimeter. Such output would be comparable to that of other systems that have been proposed for harvesting waste heat vibrations or other sources of ambient energy and represents an amount that could be sufficient to provide useful power for electronic devices in some remote locations. For example Miljkovic has calculated that at 1 microwatt per square centimeter a cube measuring about 50 centimeters on a side about the size of a typical camping cooler could be sufficient to fully charge a cellphone in about 12 hours. While that may seem slow he says people in remote areas may have few alternatives. False-color time-lapse images captured#via#high-speed imaging show a droplet jumping (colored green) from a superhydrophobic copper oxide fin to a hydrophilic (water-attracting) copper fin (colored orange. Courtesy of Nenad Miljkovic and Daniel J. Preston) There are some constraints: Because the process relies on condensation it requires a humid environment as well as a source of temperatures colder than the surrounding air such as a cave or river. The system is based on Miljkovic and Wang s 2013 finding in attempting to develop an improved heat-transfer surface to be used as a condenser in applications such as power plants that droplets on a superhydrophobic surface convert surface energy to kinetic energy as they merge to form larger droplets. This sometimes causes the droplets to spontaneously jump away enhancing heat transfer by 30 percent relative to other techniques. They later found that in that process the jumping droplets gain a small electric charge meaning that the jumping and the accompanying transfer of heat could be enhanced by a nearby metal plate whose opposite charge is attractive to the droplets. Now the researchers have shown that the same process can be used to generate power simply by giving the second plate a hydrophilic surface. As the droplets jump they carry charge from one plate to the other; if the two plates are connected through an external circuit that charge difference can be harnessed to provide power. In a practical device two arrays of metal plates like fins on a radiator would be interleaved so that they are very close but not touching. The system would operate passively with no moving parts. For powering remote automated environmental sensors even a tiny amount of energy might be sufficient; any location where dew forms would be capable of producing power for a few hours in the morning Miljkovic says. Water will condense out from the atmosphere it happens naturally he says. The atmosphere is a huge source of power and all you need is a temperature difference between the air and the device he adds allowing the device to produce condensation just as water condenses from warm humid air on the outside of a cold glass. Chuanhua Duan an assistant professor of mechanical engineering at Boston University who was involved not in this research says This work provides a new approach for energy harvesting which can be used to power microelectromechanical devices and small electronic devices. He adds Getting power from a condensation process is definitely a novel idea as condensation is used mainly for thermal management.##Recent studies of condensation on superhydrophobic surfaces have extended its applications in self-cleaning and anti-icing but no one has correlated condensation with energy harvesting before. The research which also included MIT graduate student Daniel Preston and former postdoc Ryan Enright now at Lucent Ireland Ltd. was supported by MIT s Solid-state Solar-Thermal energy Conversion Center (S3tec) funded by the U s. Department of energy; the Office of Naval Research; and the National Science Foundation n

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