A variety of silicon chip micro-reactors developed by the MIT team. Each of these contains photonic crystals on both flat faces, with external tubes for injecting fuel and air and ejecting waste products. Inside the chip, the fuel and air react to heat up the photonic crystals. In use, these reactors would have a photovoltaic cell mounted against each face, with a tiny gap between, to convert the emitted wavelengths of light to electricity. Photo: Justin Knight |
A
new photovoltaic energy-conversion system developed at Massachusetts Institute
of Technology (MIT) can be powered solely by heat, generating electricity with
no sunlight at all. While the principle involved is not new, a novel way of
engineering the surface of a material to convert heat into precisely tuned
wavelengths of light—selected to match the wavelengths that photovoltaic cells can
best convert to electricity—makes the new system much more efficient than
previous versions.
The
key to this fine-tuned light emission, described in Physical Review A, lies in a material with billions of nano-scale
pits etched on its surface. When the material absorbs heat—whether from the
sun, a hydrocarbon fuel, a decaying radioisotope, or any other source—the
pitted surface radiates energy primarily at these carefully chosen wavelengths.
Based
on that technology, MIT researchers have made a button-sized power generator
fueled by butane that can run three times longer than a lithium-ion battery of
the same weight; the device can then be recharged instantly, just by snapping
in a tiny cartridge of fresh fuel. Another device, powered by a radioisotope
that steadily produces heat from radioactive decay, could generate electricity
for 30 years without refueling or servicing—an ideal source of electricity for
spacecraft headed on long missions away from the sun.
According
to the U.S. Energy Information Administration, 92% of all the 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 today’s mechanical systems have relatively low
efficiency, and can’t be scaled down to the small sizes needed 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 long been known that photovoltaic (PV) cells needn’t always run on
sunlight. Half a century ago, researchers developed thermophotovoltaics (TPV),
which couple a PV cell with any source of heat: A burning hydrocarbon, for
example, heats up a material called the thermal emitter, which radiates heat
and light onto the PV diode, generating electricity. The thermal emitter’s
radiation includes far more infrared wavelengths than occur in the solar
spectrum, and “low band-gap” PV materials invented less than a decade
ago can absorb more of that infrared radiation than standard silicon PVs can.
But much of the heat is still wasted, so efficiencies remain relatively low.
An ideal match
The solution, Celanovic says, is to design a thermal emitter that radiates only
the wavelengths that the PV diode can absorb and convert into electricity,
while suppressing 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: Make a photonic crystal by taking a sample of material and create some
nano-scale features on its surface—say, a regularly repeating pattern of holes
or ridges—so light propagates through the sample in a dramatically different
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 includes Peter Bermel, research scientist in the Research
Laboratory for Electronics (RLE); Peter Fisher, professor of physics; and
Michael Ghebrebrhan, a postdoc in RLE—used a slab of tungsten, engineering
billions of tiny pits on its surface. When the slab heats up, it generates
bright light with an altered emission spectrum because each pit acts as a
resonator, capable of giving off radiation at only certain wavelengths.
This
powerful approach—co-developed by John D. Joannopoulos, the Francis Wright
Davis Professor of Physics and ISN director, and others—has been widely used to
improve lasers, light-emitting diodes and even optical fibers. The MIT team,
supported 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.