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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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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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the thermal radiation
that's all around us.
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We're constantly bathed in infrared light.
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If we could bend it to our will
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we could profoundly change
the flows of heat and energy
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that permeate around us every single day.
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This ability, coupled
with the cold darkness of space,
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points us to a future
where we, as a civilization,
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might be able to more intelligently manage
our thermal energy footprint
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at the very largest scales.
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As we confront climate change,
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I believe having
this ability in our toolkit
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will prove to be essential.
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So, the next time
you're walking around outside,
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yes, do marvel at how the Sun
is essential to life on Earth itself,
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but don't forget that the rest of the sky
has something to offer us, as well.
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Thank you.
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(Applause)