"Sun-Free" Photovoltaics!

A new photovoltaic energy-transition system developed at MIT can be catered by heat alone, producing electricity with no sunlight at all. Although the principle called for is not new, a new way of directing the surface of a material to convert heat into precisely tuned wavelengths of light, selected to equate the wavelengths that photovoltaic cells can best convert to electricity, makes the new system much more effective than previous versions.

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A variety of silicon chip micro-reactors!

The fundamental idea to this calibrated light emission, described in the journal Physical Review A, rests in a material with billions of nano-scale pits inscribed on its surface. When the material draws heat, whether from the sun, a hydrocarbon fuel, a decaying radioisotope or any other heat emitting source, the pitted surface beams energy mainly at these carefully chosen wavelengths. Based on this technology, researchers at MIT have created a button-sized power generator fired by butane that can run three times longer than a lithium-ion battery of the same weight; the device can then be recharged right away, just by breaking in a tiny cartridge of fresh fuel. Other device, fueled by a radio-isotope that steadily creates heat from radioactive decay, could produce electricity for 30 years without refueling or servicing, an ideal source of electricity for spacecraft steered on long missions away from the sun.

As per the U.S. Energy Information Administration, 92 percent of  the entire energy we use involves converting heat into mechanical energy, and then often into electricity, such as using fuel to boil water to turn a turbine, which is attached to a generator. But nowadays' mechanical systems have comparatively low efficiency, and can't be surmounted down to the small sizes required for devices such as sensors, smartphones or medical monitors. "Being able to convert heat from various sources into electricity without moving parts would bring huge benefits," says Ivan Celanovic ScD '06, research engineer in MIT's Institute for Soldier Nanotechnologies (ISN), "especially if we could do it efficiently, relatively inexpensively and on a small scale."

It has been known for long that photovoltaic (PV) cells need not constantly run on sunlight. About half a century ago, researchers built up thermo-photovoltaics (TPV), which couple a PV cell with any source of heat. For example, a burning hydrocarbon heats up a material called the thermal emitter, which radiates heat and light onto the PV diode, producing electricity. The thermal emitter's radiation consists of far more infrared wavelengths than come in the solar spectrum, and "low band-gap" PV materials devised less than a decade ago can engage more of that infrared radiation than standard silicon PVs can. But much of the heat still remains unused, so efficiencies continued to remain comparatively low.

Celanovic says the solution is to design a thermal emitter that radiates only the wavelengths that the PV diode can engage and change over into electricity, while inhibiting other wavelengths. "But how do we find a material that has this magical property of emitting only at the wavelengths that we want?" asks Marin Solja?i?, professor of physics and ISN researcher. The answer: Create a photonic crystal by taking a sample of material and create some nanoscale features on its surface, say, a regularly repeating pattern of holes or ridges, so light propagates through the sample in a dramatically dissimilar way. "By choosing how we design the nanostructure, we can create materials that have novel optical properties," Solja?i? says. "This gives us the ability to control and manipulate the behavior of light."

The team, which also comprises Peter Bermel, research scientist in the Research Laboratory for Electronics (RLE); Peter Fisher, professor of physics; and Michael Ghebrebrhan, a postdoc in RLE; employed a slab of tungsten, organizing billions of tiny pits on its surface. When the slab heats up, it produces bright light with an manipulated emission spectrum because each pit acts as a resonator, capable of giving off radiation at only specific wavelengths. This approach, co-formulated by John D. Joannopoulos, the Francis Wright Davis Professor of Physics and ISN director, and others, has been employed a great degree to improve lasers, light-emitting diodes and even optical fibers. The MIT team, endorsed in part by a seed grant from the MIT Energy Initiative, is now working with collaborators at MIT and elsewhere to use it to create several novel electricity-generating devices.

Celanovic and Solja?i? emphasize that building practical system demands integrating many technologies and fields of expertise. Celanovic says, "It's a really multidisciplinary effort. And it's a neat example of how fundamental research in materials can result in new performance that enables a whole spectrum of applications for efficient energy conversion."