Sun-free photovoltaics: Materials engineered to give off
precisely tuned wavelengths of light when heated are key to new high-efficiency
generating system.
A new photovoltaic energy-conversion system developed at 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 the
journal Physical Review A, lies in a material with billions of nanoscale 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
percent 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 Soljačić, professor of physics and ISN
researcher. The answer: Make 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 different way.
"By choosing how we design the nanostructure, we can
create materials that have novel optical properties," Soljačić 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.
Mike Waits, an electronics engineer at the Army Research
Laboratory in Adelphi, Md., who was not involved in this work, says this
approach to producing miniature power supplies could lead to lighter portable
electronics, which is "critical for the soldier to lighten his load. It
not only reduces his burden, but also reduces the logistics chain" to
deliver those devices to the field. "There are a lot of lives at
stake," he says, "so if you can make the power sources more efficient,
it could be a great benefit."
The button-like device that uses hydrocarbon fuels such as
butane or propane as its heat source — known as a micro-TPV power generator — has
at its heart a "micro-reactor" designed by Klavs Jensen, the Warren
K. Lewis Professor of Chemical Engineering, and fabricated in the Microsystems
Technology Laboratories. While the device achieves a fuel-to-electricity
conversion efficiency three times greater than that of a lithium-ion battery of
the same size and weight, Celanovic is confident that with further work his
team can triple the current energy density. "At that point, our TPV
generator could power your smartphone for a whole week without being
recharged," he says.
Celanovic and Soljačić stress that building practical
systems requires integrating many technologies and fields of expertise.
"It's a really multidisciplinary effort," Celanovic says. "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."
Science news source:
Massachusetts Institute of Technology
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