[Editors] MIT revamps energy system for more fuel-efficient cars

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT revamps energy system for more fuel-efficient cars
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For Immediate Release
MONDAY, MAY 22, 2006
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

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CAMBRIDGE, Mass.--MIT researchers are trying to unleash the promise 
of an old idea by converting light into electricity more efficiently 
than ever before.

The research is applying new materials, new technologies and new 
ideas to radically improve an old concept -- thermophotovoltaic (TPV) 
conversion of light into electricity. Rather than using the engine to 
turn a generator or alternator in a car, for example, the new TPV 
system would burn a little fuel to create super-bright light. 
Efficient photo diodes (which are similar to solar cells) would then 
harvest the energy and send the electricity off to run the various 
lighting, electrical and electronic systems in the car.

Such a light-based system would not replace the car's engine. Instead 
it would supply enough electricity to run subsystems, consuming far 
less fuel than is needed to keep a heavy, multi-cylinder engine 
running, even at low speed. Also, the TPV system would have no moving 
parts; no cams, no bearings, no spinning shafts, so no energy would 
be spent just to keep an engine turning over, even at idle.

"What's new here is the opportunity for a much more effective energy 
system to be created using new semiconductor materials and the 
science of photonics," said Professor John Kassakian, director of the 
Laboratory for Electromagnetic and Electronic Systems (LEES), where 
the work was conducted. The idea is to create intense light, let it 
shine on new types of photo diodes to make electricity, and bounce 
any excess light back to the light source to help keep it 
glowing-hot. In theory, Kassakian said, efficiency could be as high 
as 40 percent or 50 percent.

Kassakian is a professor in the Department of Electrical Engineering 
and Computer Science (EECS). His research colleagues are EECS 
Professor David Perreault and LEES principal research engineer Thomas 
Keim, plus EECS graduate students Ivan Celanovic and Natalija 
Jovanovic.

At the heart of their energy system would be a cylindrical element, 
such as tungsten, etched with tiny pits -- nano-holes -- so it emits 
intense light at selected wavelengths when heated to a high 
temperature, perhaps 2,200 degrees Fahrenheit (1,500 Kelvin). Special 
light-sensing cells, made of a new material such as 
gallium-antimonide, would surround the glowing element, picking up 
the radiated light. A highly specialized filter, set between the two, 
would let the most useful light wavelengths pass through to hit the 
photo diodes, while reflecting light of less useful wavelengths back 
to the heating element, pumping up the temperature.

The relatively high efficiency, compared to photovoltaic systems in 
use today, is expected to come from scientists' new ability to 
fine-tune all three main parts of this system. This includes the 
light emitter, the cells that respond well to that tuned light, and a 
way to scavenge light at wavelengths that might otherwise be wasted.

"This new technology is what makes it a very attractive system," 
Kassakian said. "There are the new materials that let us build more 
appropriate photo diodes" that convert light into electricity. 
"There's our new understanding of photonics that lets us build the 
selective emitters" to glow brightly at specific wavelengths. "And 
there's the photonic band-gap filter, made of thin silicon and 
silicon-dioxide layers that act as selective mirrors, letting the 
desired wavelengths through and reflecting back the rest."

Of course, numerous engineering problems remain to be solved. 
Kassakian said the light-collecting cells will have to be cooled: 
"We'll want to run as hot as we can, but not melt everything." Also, 
different materials are being tested to see which work best in terms 
of light emissions, light harvesting and light reflection.

"This whole concept is simple and not new," he added. Back in the 
late 1960s and early 1970s, much research was done on TPV and 
light-harvesting technology, first to create solar energy systems for 
spacecraft, and then in response to energy shortages that spurred an 
intense burst of research into various alternative energy 
technologies, he said.

The first focus of this MIT research "is for an automotive system 
that will take excess heat from the TPV system and use it to drive 
the car's heating and air conditioning systems," Kassakian said. "And 
what this would do is replace the present alternator and air 
conditioner, both of which are now run by the engine."

In addition, new TPV systems might mesh nicely with hybrid automobile 
technology, in which fuel is saved by shutting down the engine when 
the car is stopped, say at traffic signals. In the future, 
conventional cars may operate the same way. Providing electricity and 
air conditioning with the engine off will be a necessity.

He added that such a system, once commercialized, might also be 
applied to other problems, such as supplying the power to run large 
semi trucks' lighting, air conditioning and electronic systems, 
eliminating the need to run the diesel engine all night long while 
the driver rests. TPV-generated power might also be ideal for uses in 
remote places, distant from power lines, similar to what is being 
done now with solar collectors and fuel cells.

Recent papers on this work have appeared in Physical Review B and the 
Journal of Applied Physics. Initial funding for the research was from 
the MIT/Industry Consortium on Advanced Automotive 
Electrical/Electronic Components and Systems.

The work is presently funded in part by Toyota, but Toyota has made 
no decision to develop this technology for automobiles.

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Elizabeth A. Thomson
Assistant Director, Science & Engineering News
Massachusetts Institute of Technology
News Office, Room 11-400
77 Massachusetts Ave.
Cambridge, MA  02139-4307
617-258-5402 (ph); 617-258-8762 (fax)
<thomson at mit.edu>

<http://web.mit.edu/newsoffice/www>
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