Monday, April 17, 2006
A high-tech way to defrost
Humans have been getting rid of ice the wrong way for centuries, it turns out.
Dartmouth College engineering professor Victor Petrenko, not to be confused with one of the Champions on Ice, has devised a way to use a burst of electricity to remove ice caked on walls or windows. For surfaces coated with a special film, the jolt gets rid of ice in less than a second, far less time than it takes to hack at it with an ice scraper.
While drivers might find easy-cleaning windshields convenient, the technology--called thin-film pulse electrothermal de-icing, or PETD--could have significant economic impact if widely deployed. It could, for example, cut the costs of repairing power lines downed by ice storms and keep plane windshields frost-free, decreasing fuel consumption.
In Sweden, civil engineers have tested PETD and decided to cover the Uddevalla Bridge in a 12-millimeter-thick PETD foil to keep it from icing over.
"Frost-free refrigerators can approximately reduce energy consumption by a factor of two. Billions of dollars are spent each year on running refrigerators and air conditioners. If you can cut that, it's great," Petrenko said. "In ice makers, we can cut the ice-harvesting cycle and increase the productivity of ice makers by 30 (percent) to 40 percent."
A refrigerator for the residential market sporting PETD will likely come out soon. The technology will also be incorporated into the windshield of an upcoming commercial jet, according to Petrenko. Aerospace parts supplier Goodrich, an investor in and one of the seven licensees of Petrenko's Ice Engineering company, is also promoting the concept among utilities as a way to keep wind turbines de-iced.
PETD can go in reverse, too. By varying the electric pulse, the technology can cause ice to stick better to surfaces. That could help snowboarders and skiers better manage the friction with the slope, for greater or lesser traction, as needed.
The technology essentially takes advantages of the inherent properties of ice. Ice, it turns out, is a semiconductor, meaning that it conducts an electrical charge under certain circumstances. Unlike silicon, which conducts negatively charged electrons, ice conducts protons, the core of hydrogen atoms that are part of the water molecules.
"(An) ice surface has an enormously high electric charge," Petrenko said.
As a result, ice doesn't simply cake onto surfaces--it bonds to them in three ways: via the hydrogen atoms themselves, via an electrostatic bond caused by the current, and via comparatively weak van der Waals forces.
PETD works by breaking the first two bonds. An electric charge lasting a few milliseconds heats the surface buried in ice just long enough to melt about a micron or two of the surface of the ice. Once the ice is melted, the hydrogen and electrical bonds break. The resulting water then acts as a lubricant, allowing the mass of ice to slide away.
"With short pulses, the heat doesn't have time to diffuse. It is all released on the interface," Petrenko said.
To get ice to stick to a surface, the pulse is shortened--first the ice melts, then refreezes. The resulting bond between the material and the ice is even stronger than before.
Why hasn't anyone already come up with this?
"I don't know," he said. "It is a very common story: People for centuries miss a very simple principle. When it's found, people say, 'How could we miss it?'"
Traditional ice removal methods don't address how to reverse the electrical bonds, which explains why they don't work that well. Ice scrapers essentially tear away ice from the outside. Material to repel ice also fails because ice will invariably bond. Companies have thrown money at trying to develop ice-resistant surfaces, but the results have been mediocre.
Petrenko himself worked on a project funded by a generous federal grant. "We concluded that it is against the laws of nature to have an ice-phobic material," he said. "Ice is very strong glue. It is a universal adhesive."
The difficulty with PETD lies in power delivery. The surface only has to be heated to about 1 to 2 degrees Celsius, but a broad surface has to be heated simultaneously.
Still, an ordinary car, while running, could provide enough energy to remove the ice. It also takes less energy than heating the windshield.
The intellectual property at Ice Engineering mostly concerns developing power distribution systems and thin films, which coat the surface and conduct heat to the ice material interface. The composition of the films varies. In the case of windshields, Ice Engineering employs a layer of clear indium oxide. "It is the same thing on laptop displays," Petrenko said.
Ice machines and refrigerators, meanwhile, can rely on titanium or carbon fiber composites, which are more durable, because transparency isn't an issue.
The research, so far, has yielded 14 U.S. patents, and several more are pending. Dartmouth owns the patents but markets them through Ice Engineering.
Petrenko came to studying ice by accident. For years, he worked as a semiconductor researcher at Moscow's Institute of Physics and Technology. While on an exchange at Britain's University of Birmingham, he happened upon that school's ice research department. His life changed after that.
"We built a solar cell made of ice," he recalled. "While it is not as efficient as a silicon solar cell, it costs a penny a square mile."
source:http://news.com.com/A+high-tech+way+to+defrost/2100-11395_3-6061333.html
