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Article
Daytime radiative cooling using near-black infrared emitters
Jun-long Kou, Zoila Jurado, Zhen Chen, Shanhui Fan, and Austin J. Minnich
ACS Photonics
,
Just Accepted Manuscript
• DOI: 10.1021/acsphotonics.6b00991
• Publication Date (Web): 03 Feb 2017
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Daytime
radiative
co oling
using
near-black
infrared
emitters
Jun-long
Kou,
Zoila
Jurado,
Zhen
Chen,
Shanhui
Fan,
and
Austin
J.
Minnich
,
Division
of
Engineering
and
Applied
Science,
California
Institute
of
Technology,
Pasadena,
California
91125,
USA.
Ginzton
Laboratory,
Department
of
Electrical
Engineering,
Stanford
University,
Stanford,
California
94305,
USA.
E-mail:
aminnich@caltech.edu
Abstract
Recent
works
have
demonstrated
that
daytime
radiative
co oling
under
direct
sun-
light
can
b e
achieved
using
multilayer
thin
lms
designed
to
emit
in
the
infrared
atmo-
spheric
transparency
window
while
reecting
visible
light.
Here,
we
demonstrate
that
a
p olymer-coated
fused
silica
mirror,
as
a
near-ideal
blackb o dy
in
the
mid-infrared
and
near-ideal
reector
in
the
solar
sp ectrum,
achieves
radiative
co oling
b elow
ambient
air
temp erature
under
direct
sunlight
(8.2
C)
and
at
night
(8.4
C).
Its
p erformance
ex-
ceeds
that
of
a
multilayer
thin
lm
stack
fabricated
using
vacuum
dep osition
metho ds
by
nearly
3
C.
Furthermore,
we
estimate
the
co oler
has
an
average
net
co oling
p ower
of
ab out
127
Wm
-2
during
daytime
at
ambient
temp erature
even
considering
the
signif-
icant
inuence
of
external
conduction
and
convection,
more
than
twice
that
rep orted
previously.
Our
work
demonstrates
that
abundant
materials
and
straight-forward
fab-
rication
can
b e
used
to
achieve
daytime
radiative
co oling,
advancing
applications
such
as
dry
co oling
of
thermal
p ower
plants.
1
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Keywords
passive
radiative
co oling,
thermal
radiation,
infrared
emitters
Manipulating
thermal
emission
from
surfaces
by
thermal
photonic
design
has
received
great
attention
in
recent
years.
111
In
particular,
passive
radiative
co oling
schemes
that
do
not
require
external
active
devices
such
as
fans,
air
conditioners
or
thermo electrics
are
of
much
interest
b ecause
of
their
p otential
to
reduce
energy
consumption.
1216
Radiative
co oling
refers
to
the
physical
pro cess
by
which
a
b o dy
dissipates
heat
to
another
b o dy
of
lower
temp erature
via
thermal
radiation.
The
coldest
known
heat
sink
is
the
universe
with
a
temp erature
of
around
3
K,
and
radiative
thermal
contact
can
b e
made
with
this
thermal
reservoir
by
exchanging
energy
through
the
transparency
window
of
the
atmosphere.
Historically,
radiative
co oling
during
nighttime
has
b een
widely
studied
and
employed
for
ro oftop
co oling.
14,1720
However,
radiative
co oling
during
daytime
is
more
useful
as
co oling
demand
p eaks
during
daytime
hours.
Recently,
a
passive
radiative
co oling
scheme
has
b een
rep orted
by
Raman
et
al.
that
achieves
this
goal
by
radiating
energy
through
the
main
atmospheric
transparency
window
in
the
range
of
8
-
13
μ
m
while
reecting
incident
sunlight.
6
Their
radiative
co oler
consisted
of
seven
alternating
layers
of
SiO
2
and
HfO
2
on
top
of
a
silver
back
reector,
resulting
in
97%
reection
of
solar
illumination
and
an
average
emissivity
of
ab out
0.65
in
the
transparency
window.
With
a
relatively
simple
exp erimental
apparatus,
Raman
et
al.
was
able
to
achieve
a
5
C
degree
reduction
b elow
the
ambient
air
temp erature
under
direct
sunlight.
Subse-
quently,
Chen
et
al.
was
able
to
demonstrate
an
average
temp erature
reduction
of
37
C
b elow
ambient
by
combining
a
selective
emitter
with
an
apparatus
consisting
of
a
vacuum
chamb er.
21
Related
to
these
exp eriments,
there
have
b een
other
recent
theoretical
works
in
designing
various
photonic
structures
for
radiative
co oling
purp oses.
12,2225
Most
of
these
radiative
co olers
are
designed
to
emit
only
in
the
atmospheric
transparency
window
to
avoid
exchanging
radiation
with
the
atmosphere.
This
requirement
leads
to
complex
photonic
designs,
for
instance
consisting
of
multilayer
stacks
that
require
vacuum
2
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dep osition
metho ds.
It
is
interesting
to
consider
whether
emitting
and
absorbing
outside
of
the
main
atmospheric
transparency
window
is
necessarily
detrimental.
If
not,
materials
that
are
naturally
visibly
transparent
yet
emit
strongly
over
a
broad
bandwidth
in
the
mid-
infrared,
such
as
glasses,
could
p erform
as
well
as
other
more
complex
photonic
structures
rep orted
previously.
Here,
we
exp erimentally
demonstrate
passive
radiative
co oling
under
direct
sunlight
and
at
night
using
only
a
p olymer-silica-mirror
consisting
of
a
fused
silica
wafer
coated
with
a
p olymer
top
layer
and
a
silver
back
reector.
This
simple
scheme
achieves
daytime
co oling
temp erature
dierentials
of
8.2
C
under
direct
sunlight
and
8.4
C
at
night,
nearly
3
C
larger
than
that
achieved
by
the
nanophotonic
structure
in
daytime.
Our
work
demonstrates
that
inexp ensive,
abundant
materials
can
b e
used
for
applications
in
energy
such
as
dry
co oling
for
p ower
plants
by
realizing
daytime
radiative
co oling
without
need
for
complex
photonic
structures.
We
exp erimentally
examine
the
radiative
co oling
p erformance
of
the
p olymer-silica-mirror
by
coating
a
4-inch
fused
silica
wafer
of
500
μ
m
thickness
with
a
100
μ
m
thick
p olydimethyl-
siloxane
(PDMS)
lm
as
a
top
layer
and
120
nm
thick
silver
lm
as
a
back
reector.
The
silver
lm
is
dep osited
by
electron
b eam
evap oration
metho d
under
high
vacuum.
The
PDMS
lm
is
spin-coated
for
60
seconds
followed
by
degassing
for
10
minutes
and
curing
for
one
hour
at
80
C.
The
p erformance
of
the
device
is
tested
on
the
ro of
of
a
building
in
Pasadena,
California
by
exp osing
it
to
the
sky.
A
picture
of
the
setup
and
surroundings
is
shown
in
Figure
1a.
To
exp erimentally
achieve
co oling
b elow
ambient,
sp ecial
care
needs
to
b e
taken
in
the
measurement
setup
to
reduce
the
parasitic
conduction
and
convection
from
the
ambient.
In
our
measurement,
the
device
is
placed
on
a
low
thermal
conductivity
aerogel
blanket
which
is
attached
to
the
inner
side
of
a
p etri-dish.
The
p etri-dish
is
supp orted
by
three
glass
ro ds
to
susp end
it
ab ove
the
ro of.
The
top
of
the
p etri-dish
is
covered
by
a
p olyethylene
lm,
acting
as
a
convection
shield
that
is
transparent
to
all
the
radiative
wavelengths
of
interest.
The
temp eratures
of
the
device
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and
ambient
air
are
recorded
by
K-typ e
thermo couples.
Figure
1:
(a)
Image
of
the
samples
under
eld
test
on
the
ro of
of
a
building
in
Pasadena,
California.
The
device
sits
on
top
of
an
aerogel
blankets
attached
to
the
b ottom
surface
of
a
p etri-dish
with
full
access
to
the
sky.
The
p etri-dish
is
supp orted
by
three
glass
ro ds,
susp ending
the
p etri-dish
from
the
ro of.
The
top
of
the
p etri-dish
is
covered
by
p olyethylene
lm,
acting
as
a
convection
shield
that
is
transparent
to
all
the
radiative
wavelengths
of
interest.
(b)
Schematic
of
the
test
setup.
The
input/output
energy
balance
is
lab eled
with
P
rad
,
P
sun
,
P
atm
and
P
con
denoting
the
radiated
p ower
from
the
co oler,
absorb ed
p ower
from
the
sun,
absorb ed
p ower
from
the
atmosphere,
and
conduction/convection
p ower
loss,
resp ectively.
The
inset
in
(b)
shows
the
cross
section
of
the
co oler
structure
consisting
three
layers.
The
measured
temp eratures
of
the
p olymer-silica-mirror,
silica-mirror
without
p olymer
coating
and
the
ambient
air
are
shown
in
Figure
2.
The
p olymer-silica-mirror
maintains
a
temp erature
that
on
average
is
8.2
C
b elow
the
ambient
air
temp erature
throughout
the
p erio d
when
it
is
exp osed
to
the
sun.
At
night,
the
device
achieves
8.4
C
b elow
ambient
air
temp erature
without
sun
irradiation.
The
daytime
temp erature
dierential
is
1.0
C
larger
than
the
silica-mirror
and
nearly
3
C
larger
than
that
of
a
prior
rep ort.
6
For
comparison,
we
also
include
the
eld
test
results
of
a
dop ed
silicon
wafer
(resistivity
of
8
-
12
-cm)
measured
under
the
same
conditions.
Its
temp erature
increases
signicantly
after
exp osure
to
sunlight,
reaching
nearly
57
C
under
the
p eak
solar
irradiation.
Interestingly,
the
dop ed
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silicon
wafer
also
exhibits
radiative
co oling
of
ab out
5
C
b elow
ambient
air
temp erature
after
sunset,
indicating
the
co oling
ability
of
silicon
solar
cells.
Here,
the
infrared
absorption
and
emission
is
due
to
free
carriers
intro duced
by
the
doping.
Figure
2:
(a)
Temp erature
measurement
of
the
p olymer-silica-mirror
(orange),
silica-mirror
(red),
ambient
air
temp erature
(blue)
and
bare
dop ed
silicon
wafer
(purple)
during
a
24-hour
cycle.
(b)
Zo om-in
of
the
temp erature
measurement
when
the
device
is
under
direct
solar
irradiation.
The
p olymer-silica-mirror
achieves
a
temp erature
that
is
8.2
C
b elow
ambient
air
temp erature
under
these
conditions.
To
understand
these
observations,
we
measure
the
emissivity
of
the
samples
over
the
vis-
ible
and
infrared
wavelength
ranges
using
an
ultraviolet/visible/near-infrared
sp ectrometer
and
Fourier
transform
infrared
sp ectroscopy
(FTIR).
The
result
is
shown
in
Figure
3.
Due
to
the
transparency
of
fused
silica
and
PDMS
as
well
as
the
high
reectivity
of
silver
from
the
visible
to
the
near-infrared,
the
absorption
for
these
wavelengths
is
minimal.
However,
a
sig-
nicant
p ortion
of
the
ultraviolet
light
is
absorb ed
by
the
samples,
resulting
in
ab out
23
Wm
-2
absorption
p ower
density
for
the
p olymer-silica-mirror.
The
emissivity
approaches
unity
for
infrared
wavelengths
longer
than
4.5
microns
due
to
absorption
of
PDMS
and
silica.
Here,
PDMS
is
added
to
the
design
to
counteract
the
large
absorption
dip
of
fused
silica
around
wavelengths
of
9
microns,
shown
as
the
red
line
in
Figure
3.
Counterintuitively,
despite
the
fact
that
the
sample
has
a
high
absorption
outside
the
main
atmospheric
transparency
window,
we
observe
radiative
co oling
p erformance
exceeding
that
of
the
nanophotonic
co oler
designed
to
emit
only
within
the
atmospheric
transparency
window
of
Raman
et
al.
6
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