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Every summer, when I was growing up,

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I would fly from my home in Canada
to visit my grandparents,

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who lived in Mumbai, India.

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Now, Canadian summers
are pretty mild at best --

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about 22 degrees Celsius
or 72 degrees Fahrenheit

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is a typical summer's day and not too hot.

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Mumbai on the other hand,
is a hot and humid place

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well into the 30s Celsius
or 90s Fahrenheit.

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As soon as I’d reach, I'd ask,

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"How could anyone live, work
or sleep in such weather?"

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To make things worse, my grandparents
didn't have an air conditioner.

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And while I tried my very, very best,

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I was never able
to persuade them to get one.

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But this is changing.

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And fast.

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Cooling systems today
collectively account for 17 percent

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of the electricity we use worldwide.

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This includes everything,
from the air conditioners

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I so desperately wanted
during my summer vacations,

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to the refrigeration systems
that keep our food safe and cold for us

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in our supermarkets,

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to the industrial scale systems
that keep our data centers operational.

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Collectively, these systems
account for eight percent

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of global greenhouse gas emissions.

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But what keeps me up at night

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is that our energy use for cooling
might grow six fold by the year 2050.

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Primarily driven by increasing usage
in Asian and African countries.

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I've seen this firsthand.

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Nearly every apartment in and around
my grandmother's place now

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has an air conditioner.

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And that is emphatically a good thing

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for the health, well-being
and productivity

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of people living in warmer climates.

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However, one of the most
alarming things about climate change

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is that the warmer our planet gets,

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the more we're going to need
cooling systems.

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Systems that are themselves large
emitters of greenhouse gas emissions.

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This then has the potential
to cause a feedback loop,

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where cooling systems alone
could become one of our biggest sources

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of greenhouse gases later this century.

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In the worst case,

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we might need more than 10 trillion
kilowatt hours of electricity every year,

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just for cooling, by the year 2100.

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That's half our electricity supply today.

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Just for cooling.

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But this also point us
to an amazing opportunity.

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A 10 or 20 percent improvement
in the efficiency of every cooling system

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could actually have an enormous impact
on our greenhouse gas emissions,

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both today and later this century.

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And it could help us avert
that worst-case feedback loop.

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I'm a scientist who thinks a lot
about light and heat.

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In particular, how new materials
allow us to alter the flow

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of these basic elements of nature

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in ways we might have
once thought impossible.

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So, while I always understood
the value of cooling,

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during my summer vacations,

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I actually wound up
working on this problem

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because of an intellectual puzzle
that I came across about six years ago.

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How were ancient peoples
able to make ice in desert climates?

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This is a picture of an ice house,

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also called a Yakhchal,
located in the southwest of Iran.

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There are ruins of dozens
of such structures throughout Iran,

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with evidence of similar such buildings
throughout the rest of the Middle East

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and all the way to China.

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The people who operated
this ice house many centuries ago,

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would pour water in the pool
you see on the left

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in the early evening hours,
as the sun set.

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And then something amazing happened.

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Even though the air temperature
might be above freezing,

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say five degrees Celsius
or 41 degrees Fahrenheit,

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the water would freeze.

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The ice generated would then be collected
in the early morning hours,

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and stored for use in the building
you see on the right,

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all the way through the summer months.

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You've actually likely seen
something very similar play

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if you've ever noticed
frost form on the ground

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on a clear night

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even when the air temperature
is well above freezing.

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But wait.

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How did the water freeze
if the air temperature is above freezing?

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Evaporation could have played an effect

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but that's not enough to actually
cause the water to become ice.

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Something else must have cooled it down.

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Think about a pie
cooling on a window sill.

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For it to be able to cool down,
its heat needs to flow somewhere cooler.

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Namely, the air that surrounds it.

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As implausible as it may sound,

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for that pool of water its heat
is actually flowing to the cold of space.

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How is this possible?

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Well, that pool of water,
like most natural materials,

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sends out its heat as light.

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This is a concept known
as thermal radiation.

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In fact, we're all sending out our heat
as infrared light right now,

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to each other and our surroundings.

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We can actually visualize this
with thermal cameras

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and the images they produce,
like the ones I'm showing you right now.

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So that pool of water
is sending out its heat

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upward towards the atmosphere.

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The atmosphere and the molecules in it

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absorb some of that heat and send it back.

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That's actually the greenhouse effect
that's responsible for climate change.

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But here's the critical thing
to understand.

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Our atmosphere doesn't absorb
all of that heat.

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If it did, we'd be
on a much warmer planet.

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At certain wavelengths,

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in particular between
eight and 13 microns,

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our atmosphere has what's known
as a transmission window.

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This window allows some of the heat,

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that goes up as infrared light,

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to effectively escape,
carrying away that pool's heat.

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And it can escape to a place
that is much, much colder.

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The cold of this upper atmosphere

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and all the way out to outer space,

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which can be as cold
as minus 270 degrees Celsius,

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or minus 454 degrees Fahrenheit.

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So that pool of water is able
to send out more heat to the sky

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than the sky sends back to it.

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And because of that,

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the pool will cool down below
its surroundings' temperature.

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This is an effect known
as night-sky cooling

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or radiative cooling.

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And it's always been understood
by climate scientists and meteorologists

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as a very important natural phenomenon.

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When I came across all of this,

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it was towards the end
of my PhD at Stanford.

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And I was amazed by its
apparent simplicity

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as a cooling method.

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Yet really puzzled.

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Why aren't we making use of this?

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Now, scientists and engineers
had investigated this idea

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in previous decades.

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But there turned out to be
at least one big problem.

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It was called night-sky
cooling for a reason.

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Why?

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Well, it's a little thing called the Sun.

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So, for the surface
that's doing the cooling,

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it needs to be able to face the sky.

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And during the middle of the day,

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when we might want
something cold the most,

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unfortunately that means
you're going to look up to the Sun.

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And the Sun heats most materials up

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enough to completely counteract
this cooling effect.

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My colleagues and I
spend a lot of our time

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thinking about how
we can structure materials

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at very small length scales

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such that they can do
new and useful things with light.

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Length scale smaller
than the wavelength of light itself.

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Using insights from this field
known as nanophotonics,

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or metamaterials research,

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we realized that there might be a way
to make this possible during the day

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for the first time.

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To do this, I designed
a multilayer optical material

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shown here in a microscope image.

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It's more than 40 times thinner
than a typical human hair.

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And it's able to do
two things simultaneously.

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First, it sends its heat out

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precisely where our atmosphere
lets that heat out the best.

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We targeted the window to space.

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The second thing it does,

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is it avoids getting heated up by the sun.

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It's a very good mirror to sunlight.

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The first time I tested this
was on a rooftop in Stanford

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that I'm showing you right here.

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I left the device out for a little while,

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and I walked up to it after a few minutes

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and within seconds I knew it was working.

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How?

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I touched it and it felt cold.

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(Applause)

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Just to emphasize how weird
and counterintuitive this is.

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This material and others like it

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will get colder when we take them
out of the shade,

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even though the sun is shining on it.

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I'm showing you data here
from our very first experiment,

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where that material stayed
more than five degrees Celsius

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or nine degrees Fahrenheit colder
than the air temperature,

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even though the sun
was shining directly on it.

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The manufacturing method we used
to actually make this material

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already exists at large volume scales.

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So I was really excited,

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because not only
do we make something cool,

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but we might actually have the opportunity
to do something real and make it useful.

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That brings me to the next big question.

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How do you actually
save energy with this idea?

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Well, we believe the most direct way
to save energy with this technology

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is as an efficiency boost

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for today's air conditioning
and refrigeration systems.

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To do this, we've built
fluid cooling panels,

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like the ones shown right here.

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These panels have a similar shape
to solar water heaters,

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except they do the opposite --
they cool the water, passively,

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using our specialized material.

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These panels can then
be integrated with a component

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almost every cooling system has,
called a condenser,

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to improve the system's
underlying efficiency.

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Our start-up, SkyCool Systems,

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has recently completed a field trial
in Davis, California, shown right here.

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In that demonstration,

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we showed that we could actually
improve the efficiency

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of that cooling system
as much as 12 percent in the field.

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Over the next year or two,

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I'm super excited to see this
go to its first commercial-scale pilots

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in both the air conditioning
and refrigeration space.

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In the future, we might be able
to integrate these kinds of panels

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with higher deficiency
building cooling systems

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to reduce their energy
usage by two-thirds.

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And eventually, we might actually
be able to build a cooling system

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that requires no electricity input at all.

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As a first step towards that

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my colleagues at Stanford and I

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have shown that you could
actually maintain

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something more than 42 degrees Celsius
below the air temperature

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with better engineering.

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Thank you.

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(Applause)

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So just imagine that --
something that is below freezing

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on a hot summer's day.

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So, while I'm very excited
about all we can do for cooling,

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and I think there's a lot yet to be done,

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as a scientist I'm also drawn
to a more profound opportunity

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that I believe this work highlights.

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We can use the cold darkness of space

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to improve the efficiency

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of every energy-related
process here on Earth.

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One such process
I like to highlight, are solar cells.

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They heat up under the sun

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and become less efficient
the hotter they are.

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In 2015 we showed that
with deliberate kinds of micro structures

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on top of a solar cell

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we could take better advantage
of this cooling effect

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to maintain a solar cell passively
at a lower temperature.

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This allows the cell
to operate more efficiently.

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At the lab I now run
at the University of Pennsylvania,

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we're probing these kinds
of opportunities further.

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We're asking whether
we can use the cold of space

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to help us with water conservation.

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Or perhaps with off-grid scenarios.

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Perhaps we could even directly
generate power with this cold.

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There's a large temperature difference
between us here on Earth

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and the cold of space.

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That difference, at least conceptually,

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could be used to drive
something called the heat engine

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to generate electricity.

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Could we then make a nighttime
power generation device

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00:12:06,831 --> 00:12:09,228
that generates useful
amounts of electricity

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when solar cells don't work?

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Could we generate light from darkness?

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Central to this ability
is being able to manage

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00:12:19,522 --> 00:12:22,633
the thermal radiation
that's all around us.

250
00:12:22,657 --> 00:12:25,457
We're constantly bathed in infrared light.

251
00:12:25,903 --> 00:12:28,355
If we could bend it to our will

252
00:12:28,379 --> 00:12:31,109
we could profoundly change
the flows of heat and energy

253
00:12:31,133 --> 00:12:33,866
that permeate around us every single day.

254
00:12:34,427 --> 00:12:37,768
This ability, coupled
with the cold darkness of space,

255
00:12:37,792 --> 00:12:41,101
points us to a future
where we, as a civilization,

256
00:12:41,125 --> 00:12:46,363
might be able to more intelligently manage
our thermal energy footprint

257
00:12:46,387 --> 00:12:48,187
at the very largest scales.

258
00:12:49,141 --> 00:12:51,355
As we confront climate change,

259
00:12:51,379 --> 00:12:53,982
I believe having
this ability in our toolkit

260
00:12:54,006 --> 00:12:55,806
will prove to be essential.

261
00:12:56,665 --> 00:12:59,863
So, the next time
you're walking around outside,

262
00:12:59,887 --> 00:13:06,231
yes, do marvel at how the Sun
is essential to life on Earth itself,

263
00:13:06,255 --> 00:13:10,931
but don't forget that the rest of the sky
has something to offer us, as well.

264
00:13:11,770 --> 00:13:12,921
Thank you.

265
00:13:12,945 --> 00:13:16,514
(Applause)