Tech Briefs Magazine - March 2022 - 25

temperature. It enables capture of energy
that is otherwise trapped in the near-field
of the emitter - called near-field thermophotovoltaics
(NF-TPV) - and uses custom-built
photovoltaic cells and emitter designs
for near-field operating conditions.
The technique exhibited a power density
almost an order of magnitude higher
than that for the best-reported near-fieldTPV
systems, while also operating at six
times higher efficiency, paving the way
for future near-field-TPV applications. In
the future, near-field-TPVs could serve
as more compact and higher-efficiency
power sources for soldiers, as these devices
can function at lower operating temperatures
than conventional TPVs.
The efficiency of a TPV device is
characterized by how much of the total
energy transfer between the emitter and
the photovoltaic cell is used to excite the
electron-hole pairs in the photovoltaic
cell. While increasing the temperature
of the emitter increases the number of
photons above the bandgap of the cell,
the number of sub-bandgap photons
that can heat up the photovoltaic cell
need to be minimized.
This was achieved by fabricating thinfilm
TPV cells with ultra-flat surfaces and
with a metal back reflector. The photons
above the bandgap of the cell are efficiently
absorbed in the micron-thick
semiconductor, while those below the
bandgap are reflected back to the silicon
emitter and recycled.
The researchers grew thin-film indium
gallium arsenide photovoltaic
cells on thick semiconductor substrates
and then peeled off the very thin semiconductor
active region of the cell and
transferred it to a silicon substrate. The
researchers also performed theoretical
calculations to estimate the performance
of the photovoltaic cell at each
temperature and gap size and showed
good agreement between the experiments
and computational predictions.
For more information, contact the Army Research
Laboratory Public Affairs Office at
public_affairs@arl.army.mil; 301-394-3590.
Charging Room System Provides Electricity Over the Air
In-wall capacitors power lights, phones, and laptops without wires.
University of Michigan, Ann Arbor, MI, and University of Tokyo, Japan
R
esearchers have developed a system
to safely deliver electricity over the
air, potentially turning entire buildings
into wireless charging zones. The technology
can deliver 50 watts of power using
magnetic fields.
In addition to untethering phones
and laptops, the technology could also
power implanted medical devices. Heart
implants, for example, require a wire
that runs from the pump through the
body to an external power supply. The
new technology could eliminate that,
reducing the risk of infection and improving
patients' quality of life.
The technology could also open
new possibilities for mobile robotics in
homes and manufacturing facilities. The
team is also working on implementing
the system in spaces that are smaller
than room-sized; for example, a toolbox
that charges tools placed inside it.
The team demonstrated the technology
in a purpose-built aluminum
test room measuring approximately
10 feet by 10 feet. They wirelessly powered
lamps, fans, and cellphones that
could draw current from anywhere in
the room regardless of the placement of
people and furniture.
The system is a major improvement
over previous attempts at wireless
charging systems, which used potentially
harm ful microwave radiation or required
devices to be placed on dedicated
charging pads. Instead, it uses a conductive
surface on room walls and a conductive
pole to generate magnetic fields.
De vices harness the magnetic field with
wire coils, which can be integrated into
electronics like cellphones. The system
could easily be scaled up to larger structures
like factories or warehouses while
still meeting existing safety guidelines
for exposure to electromagnetic fields.
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Tech Briefs Magazine - March 2022

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