This process can generate small amounts of electricity.
Last year, MIT researchers discovered that when water droplets spontaneously jump away from superhydrophobic (water-repelling) surfaces during condensation, the droplets can gain electric charge in the process.
Now the same team has demonstrated that this process can generate small amounts of electricity, which could lead to devices that can 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, says MIT postdoc Nenad Miljkovic, Department of Mechanical Engineering NanoEngineering Group, consisting of a series of interleaved flat metal plates. Although his initial tests involved copper plates, any conductive metal would do, including cheaper aluminum, he says.
How it works
The system is based on a 2013 finding by Miljkovic and associate professor of mechanical engineering Evelyn Wang 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 (water-attracting) 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.
Camping-cooler-size device could charge a cellphone in 12 hours
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.
That’s 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.
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.
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.
“There are other innovations for electrical energy harvesting based on vibration, solar, thermal, and wind energies,” Miljkovic said in an email to KurzweilAI. (Other methods for power-harvesting covered on KurzweilAI include ambient wireless signals, brain glucose, roadways, body movements, and charges from graphene captured by flowing water.)
“In terms of efficiency, other approaches right now may be better, and we do not claim to beat the competition,” continued Miljkovic. “However, with further development, we hope to increase the performance of our system to compete (if not surpass) competition.
“We’ve filed a patent on the technology. It’s up to the market to decide when the technology is mature enough to develop further into a commercial product. However right now, a timeline of probably five years is required to fully develop a proper commercial entity, do more research and development, and bring the product to market.”
The new findings are published in the journal Applied Physics Letters. The research 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.