Tech Briefs Magazine - December 2021 - 39

distance between the charger and the
moving object changed. But that initial
system wasn't efficient enough to be
practical. The amplifier used so much
electricity internally to produce the
required amplification effect that the system
only transmitted 10% of the power
flowing through the system.
In the new work, the researchers
boosted the system's wireless-transmission
efficiency to 92%. The key was to
replace the original amplifier with a far
more efficient switch mode amplifier.
Such amplifiers aren't new but they will
only produce high-efficiency amplification
under very precise conditions.
The new prototype can wirelessly
transmit 10 watts of electricity over a distance
of two or three feet. There aren't
any fundamental obstacles to scaling up
a system to transmit the tens or hundreds
of kilowatts that a car would need
and the system is more than fast enough
to resupply a speeding automobile. The
only limiting factor will be how fast the
car's batteries can absorb all the power.
The wireless chargers shouldn't pose a
health risk because even those that are
Thermochromic Photovoltaic Windows
The windows reduce the need for air conditioning and simultaneously generate electricity.
National Renewable Energy Laboratory, Golden, Colorado
eat generated by sunlight shining
through windows is the single
largest contributor to the need for air
conditioning and cooling in buildings.
Because residential and commercial
buildings use 74% of all electricity and
39% of all energy in the United States,
the shading effect from tinting windows
helps buildings use less energy.
The technology, termed " thermo -
chromic photovoltaic, " allows the window
to change color to block glare and
reduce unwanted solar heating when
the glass gets warm on a hot, sunny day.
This color change also leads to the formation
of a functioning solar cell that
generates onboard power. Thermo -
chromic photovoltaic windows can help
buildings turn into energy generators,
increasing their contribution to the
broader energy grid's needs.
Samples of perovskite-based window technology. (Photo by Dennis Schroeder, NREL)
The research builds upon previous work
into a thermochromic window that darkened
as the Sun heated its surface. As the
window shifted from transparent to tinted,
perovskites embedded within the material
generated electricity. Perov skites are a crystalline
structure shown to have remarkable
efficiency at harnessing sunlight.
The first-generation solar window was
able to switch back and forth between
Tech Briefs, December 2021
Cov
transparent and a reddish-brown color,
requiring temperatures between 150 and
175 °F to trigger the transformation. The
latest iteration allows a broad choice of colors
and works at 95 to 115 °F, a glass temperature
easily achieved on a hot day.
By using a different chemical composition
and materials, the researchers also
were able to rapidly speed up the color
transformation. The time was reduced
to about seven seconds from the three
minutes it took during the proof-of-concept
thermochromic photovoltaic window
demonstrated previously. The scientists
sandwiched a thin perovskite film
between two layers of glass and injected
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ToC
vapor. The vapor triggers a reaction that
causes the perovskite to arrange itself
into different shapes, from a chain to a
sheet to a cube. The colors emerge with
the changing shapes. Lowering the
humidity returns the perovskite to its
normal transparent state.
One area of additional research is the
number of times the thermochromic window
can be cycled into an electricity-generating
operating mode and return to transparent.
The efficiency of converting sunlight
into electricity will also be explored.
For more information, contact David
Glickson at David.Glickson@nrel.gov;
303-275-4097.
39
H
powerful enough for cars would produce
magnetic fields that are well within
established safety guidelines. The magnetic
fields
can transmit
through people without them feeling a
thing.
Though it could be years before wireless
chargers become embedded in highways,
the opportunities for robots and drones
are more immediate. It's much less costly
to embed chargers in floors or on rooftops
than on long stretches of highway.
For more information, contact Shanhui
Fan at shanhui.fan@stanford.edu.
electricity
Researchers have developed a nextgeneration
thermochromic window that
enables myriad colors and a broader
range of temperatures that drive the
color switch. This increases design flexibility
for improving energy efficiency as
well as control over building aesthetics
that is highly desirable for both architects
and end users.
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Tech Briefs Magazine - December 2021

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