Incandescent lighting could make a comeback with a new type of filter that “recycles” infrared photons and improves efficiency, an innovation that also could enable solar cells to convert heat into electricity more efficiently than conventional photovoltaic technology.
“The biggest disadvantage of incandescent lighting has been a lack of energy efficiency,” said Peter Bermel, an assistant professor in Purdue University‘s School of Electrical and Computer Engineering. “The way incandescent lights work is that you heat a filament to a certain temperature and it emits a broad band of light, but only about one in 20 photons or so is actually visible to the human eye; the other 19 photons are essentially just wasted as heat.”
Now researchers have developed a potential solution: a new type of filter to recycle wasted photons that is made out of alternating layers of materials such as silicon dioxide and tantalum dioxide, each with thicknesses less than 1/100th that of a human hair. This approach could improve incandescent lighting efficiency by 10 times, making it more efficient than commercial compact fluorescent and LED lighting, said Bermel, who worked with researchers at the Massachusetts Institute of Technology to develop the filter.
The team’s findings are detailed in a paper appearing on Monday (Jan. 11) in the journal Nature Nanotechnology.
The efficiency of the new lighting source already approaches that of some fluorescent and LED bulbs, and could approach 40 percent, surpassing all existing energy-efficient lighting sources. Commercially available fluorescent lighting has an efficiency range from 7 percent to 13 percent and LED lighting products range from 5 percent to 13 percent, while more advanced LED lamps under development may range as high as 29 percent.
The Nature Nanotechnology paper was authored by MIT postdoctoral associate Ognjen Ilic; Bermel; and also from MIT: Gang Chen, head of the Department of Mechanical Engineering and Carl Richard Soderberg Professor of Power Engineering; John D. Joannopoulos, the Francis Wright Davis Professor of Physics; principal research scientist Ivan Celanovic; and Marin Soljačić, a professor of physics.
The selective filter designed and built by the researchers allows the passage of visible photons but not infrared photons, which reflect back to the incandescent source and are essentially recycled.
“That was the trick, really, because it’s relatively easy to make something that reflects everything back or that transmits everything, but to only reflect the infrared and transmit all of the visible at the same time is harder,” said Bermel, who is the lead author of a recent U.S. patent application on the new filter concept.
When the filter reflects the infrared photons, they are eventually absorbed by the incandescent filament, causing its temperature to rise.
“You can send those infrared photons back to the emitting source as many times as you need until they get reabsorbed,” Bermel said. “Each photon has a certain amount of energy associated with it, so you can reclaim that energy as heat. The net effect when you do that many times is that you can maintain a higher temperature and brightness using much less electric power than would ordinarily be required.”
The researchers have performed both detailed numerical simulations and laboratory experiments to confirm the findings.
However, some practical questions remain, such as the ultimate performance, thermal stability and lifetime of the design.
“Further research is needed to measure the long-term performance and production costs of these devices. Fortunately, the basic materials used in our experiment are both abundant and non-toxic,” he said.
In photovoltaic cells, electrons in a semiconductor occupy a region of energy called the “valence band” while in darkness, but shining light on the material causes the electrons to absorb energy, elevating them into a region of higher energy called the “conduction band.” The region between both bands is called the “band gap. “
“In thermophotovoltaics you have a radiating heat source, not unlike an incandescent filament, and then you shine that light on a photovoltaic cell to generate electricity,” Bermel said. “The filter can be used to select only photons with energy levels corresponding to the semiconductor band gap of the material in the solar cell for maximally efficient conversion.”