Grabbing infrared energy with nanoantennas

Researchers at the Idaho National Laboratory are looking at solar energy in a new light, developing a flexible plastic sheet of embedded nanoantennas that are able to absorb energy in the infrared band of radiation.

Traditional solar photovoltaic panels absorb the visible light of the sun, but there's more to the big ball of fire in the sky than just what we can see, and the Idaho lab's nanoantenna's could lead to the production of flexible panels that would continue to work day and night.

"When the sun sets, all that energy that's been stored by the Earth absorbing it is re-radiated, and we harvest it," Dale Kotter, an engineer at the Idaho National Laboratory, told the Cleantech Group.

The nanoantennas target mid-infrared rays, which the planet gives off as heat. Infrared radiation is also generated by industrial processes, including coal-fired plants.

Methods to convert the energy into usable electricity still need to be developed, but the nanoantennas have the potential in the short term to act as cooling devices, drawing waste heat from buildings or electronics without using electricity.

The Idaho lab is one of 11 national laboratories run by the U.S. Department of Energy, conducting research and development on energy efficiency and renewable energy technologies (see Saving energy at the U.S. DOE).

The Idaho National Laboratory is working on the nanoantennas with partners at Cambridge, Mass.-based MicroContinuum and the University of Missouri. The researchers will report their findings later this week at the American Society of Mechanical Engineers' 2nd International Conference on Energy Sustainability in Jacksonville, Fla.

The nanoantennas are small gold spirals set in a specially treated form of polyethylene. "Our design goal was to get away from silicon wafers and rigid substrates," said Kotter. "Our current techniques only allow us to make a master template about six inches in diameter."

"We've had to stamp six inch regions, stamp and repeat, and mosaic them together to get a larger sheet."

Take a look at a sheet from the Idaho National Laboratory here >>

MicroContinuum is working on a roll-to-roll manufacturing process that could print the sheets at a rate of several yards per minute.

Kotter said the nanoantenna research always had the end-use in mind. "We chose to design our antennas at these longer wavelengths for proof of concept because they're easier to manufacture for larger devices at that scale."

"It has to be affordable," he said. "We're a national lab. Our goal is to get technology out to the private sector, and we want to get technology that's going to be affordable."

The nanoantennas have been shown to collect greater than 80 percent of the photon energy from the sun, and although a rectifier component to convert the energy to direct current has yet to be built, the efficiency of the complete system could reach record levels.

"Generation one of commercial solars cells right now are about 12 percent efficient at taking solar energy to power," said Kotter. "Our design shows that we can leapfrog that and go up to about 50 percent efficiency."

There's an average of about 168 watts per square meter of photonic energy from the sun hitting the Earth that can be captured by these antennas, meaning a potential of about 75 watts that could be collected by each square meter of the flexible technology.

And it could apply to any industrial waste stream where heat is generated. "Even by an automobile engine. The exhaust just goes out as waste. So the concept is, instead of letting it just go to ambient air is convert it back to electricity."

The technology to convert the energy in electricity is still a few years away, with no commercial technology available today that will work at the wavelengths needed for these nanoantennas, but there is potential in energy efficiency applications in the short-term.

"Antennas can absorb energy, and antennas can transmit energy," said Kotter. "We're working on some designs where you do thermal energy management. They're like heat sinks, sponges that can absorb certain wavelengths of energy."

For manufacturers that are trying to cool down electronic circuitry that puts off a certain wavelength of energy, the antennas could be designed to be very efficient at absorbing all of that energy and moving it away from the electronics.

Kotter said the lab is already in the process of getting partners together to go into the development of prototypes for those types of applications.

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