Every summer when I was growing up, I would fly from my home in Canada
to visit my grandparents, who lived in Mumbai, India. Now, Canadian summers
are pretty mild at best -- about 22 degrees Celsius
or 72 degrees Fahrenheit is a typical summer's day,
and not too hot. Mumbai, on the other hand,
is a hot and humid place well into the 30s Celsius
or 90s Fahrenheit. As soon as I'd reach it, I'd ask, "How could anyone live, work
or sleep in such weather?" To make things worse, my grandparents
didn't have an air conditioner. And while I tried my very, very best, I was never able
to persuade them to get one. But this is changing, and fast. Cooling systems today
collectively account for 17 percent of the electricity we use worldwide. This includes everything
from the air conditioners I so desperately wanted
during my summer vacations, to the refrigeration systems
that keep our food safe and cold for us in our supermarkets, to the industrial scale systems
that keep our data centers operational. Collectively, these systems
account for eight percent of global greenhouse gas emissions. But what keeps me up at night is that our energy use for cooling
might grow sixfold by the year 2050, primarily driven by increasing usage
in Asian and African countries. I've seen this firsthand. Nearly every apartment
in and around my grandmother's place now has an air conditioner. And that is, emphatically, a good thing for the health, well-being
and productivity of people living in warmer climates. However, one of the most
alarming things about climate change is that the warmer our planet gets, the more we're going to need
cooling systems -- systems that are themselves large
emitters of greenhouse gas emissions. This then has the potential
to cause a feedback loop, where cooling systems alone could become one of our biggest sources
of greenhouse gases later this century. In the worst case, we might need more than 10 trillion
kilowatt-hours of electricity every year, just for cooling, by the year 2100. That's half our electricity supply today. Just for cooling. But this also point us
to an amazing opportunity. A 10 or 20 percent improvement
in the efficiency of every cooling system could actually have an enormous impact
on our greenhouse gas emissions, both today and later this century. And it could help us avert
that worst-case feedback loop. I'm a scientist who thinks a lot
about light and heat. In particular, how new materials
allow us to alter the flow of these basic elements of nature in ways we might have
once thought impossible. So, while I always understood
the value of cooling during my summer vacations, I actually wound up
working on this problem because of an intellectual puzzle
that I came across about six years ago. How were ancient peoples
able to make ice in desert climates? This is a picture of an ice house, also called a Yakhchal,
located in the southwest of Iran. There are ruins of dozens
of such structures throughout Iran, with evidence of similar such buildings
throughout the rest of the Middle East and all the way to China. The people who operated
this ice house many centuries ago, would pour water
in the pool you see on the left in the early evening hours,
as the sun set. And then something amazing happened. Even though the air temperature
might be above freezing, say five degrees Celsius
or 41 degrees Fahrenheit, the water would freeze. The ice generated would then be collected
in the early morning hours and stored for use in the building
you see on the right, all the way through the summer months. You've actually likely seen
something very similar at play if you've ever noticed frost form
on the ground on a clear night, even when the air temperature
is well above freezing. But wait. How did the water freeze
if the air temperature is above freezing? Evaporation could have played an effect, but that's not enough to actually
cause the water to become ice. Something else must have cooled it down. Think about a pie
cooling on a window sill. For it to be able to cool down,
its heat needs to flow somewhere cooler. Namely, the air that surrounds it. As implausible as it may sound, for that pool of water, its heat
is actually flowing to the cold of space. How is this possible? Well, that pool of water,
like most natural materials, sends out its heat as light. This is a concept
known as thermal radiation. In fact, we're all sending out our heat
as infrared light right now, to each other and our surroundings. We can actually visualize this
with thermal cameras and the images they produce,
like the ones I'm showing you right now. So that pool of water
is sending out its heat upward towards the atmosphere. The atmosphere and the molecules in it absorb some of that heat and send it back. That's actually the greenhouse effect
that's responsible for climate change. But here's the critical thing
to understand. Our atmosphere doesn't absorb
all of that heat. If it did, we'd be
on a much warmer planet. At certain wavelengths, in particular between
eight and 13 microns, our atmosphere has what's known
as a transmission window. This window allows some of the heat
that goes up as infrared light to effectively escape,
carrying away that pool's heat. And it can escape to a place
that is much, much colder. The cold of this upper atmosphere and all the way out to outer space, which can be as cold
as minus 270 degrees Celsius, or minus 454 degrees Fahrenheit. So that pool of water is able
to send out more heat to the sky than the sky sends back to it. And because of that, the pool will cool down
below its surroundings' temperature. This is an effect
known as night-sky cooling or radiative cooling. And it's always been understood
by climate scientists and meteorologists as a very important natural phenomenon. When I came across all of this, it was towards the end
of my PhD at Stanford. And I was amazed by its apparent
simplicity as a cooling method, yet really puzzled. Why aren't we making use of this? Now, scientists and engineers
had investigated this idea in previous decades. But there turned out to be
at least one big problem. It was called night-sky
cooling for a reason. Why? Well, it's a little thing called the sun. So, for the surface
that's doing the cooling, it needs to be able to face the sky. And during the middle of the day, when we might want
something cold the most, unfortunately, that means
you're going to look up to the sun. And the sun heats most materials up enough to completely counteract
this cooling effect. My colleagues and I
spend a lot of our time thinking about how
we can structure materials at very small length scales such that they can do
new and useful things with light -- length scales smaller
than the wavelength of light itself. Using insights from this field, known as nanophotonics
or metamaterials research, we realized that there might be a way
to make this possible during the day for the first time. To do this, I designed
a multilayer optical material shown here in a microscope image. It's more than 40 times thinner
than a typical human hair. And it's able to do
two things simultaneously. First, it sends its heat out precisely where our atmosphere
lets that heat out the best. We targeted the window to space. The second thing it does
is it avoids getting heated up by the sun. It's a very good mirror to sunlight. The first time I tested this
was on a rooftop in Stanford that I'm showing you right here. I left the device out for a little while, and I walked up to it after a few minutes, and within seconds, I knew it was working. How? I touched it, and it felt cold. (Applause) Just to emphasize how weird
and counterintuitive this is: this material and others like it will get colder when we take them
out of the shade, even though the sun is shining on it. I'm showing you data here
from our very first experiment, where that material stayed
more than five degrees Celsius, or nine degrees Fahrenheit, colder
than the air temperature, even though the sun
was shining directly on it. The manufacturing method we used
to actually make this material already exists at large volume scales. So I was really excited, because not only
do we make something cool, but we might actually have the opportunity
to do something real and make it useful. That brings me to the next big question. How do you actually
save energy with this idea? Well, we believe the most direct way
to save energy with this technology is as an efficiency boost for today's air-conditioning
and refrigeration systems. To do this, we've built
fluid cooling panels, like the ones shown right here. These panels have a similar shape
to solar water heaters, except they do the opposite --
they cool the water, passively, using our specialized material. These panels can then
be integrated with a component almost every cooling system has,
called a condenser, to improve the system's
underlying efficiency. Our start-up, SkyCool Systems, has recently completed a field trial
in Davis, California, shown right here. In that demonstration, we showed that we could actually
improve the efficiency of that cooling system
as much as 12 percent in the field. Over the next year or two, I'm super excited to see this go
to its first commercial-scale pilots in both the air conditioning
and refrigeration space. In the future, we might be able
to integrate these kinds of panels with higher efficiency
building cooling systems to reduce their energy
usage by two-thirds. And eventually, we might actually
be able to build a cooling system that requires no electricity input at all. As a first step towards that, my colleagues at Stanford and I have shown that you could
actually maintain something more than 42 degrees Celsius
below the air temperature with better engineering. Thank you. (Applause) So just imagine that -- something that is below freezing
on a hot summer's day. So, while I'm very excited
about all we can do for cooling, and I think there's a lot yet to be done, as a scientist, I'm also drawn
to a more profound opportunity that I believe this work highlights. We can use the cold darkness of space to improve the efficiency of every energy-related
process here on earth. One such process
I'd like to highlight are solar cells. They heat up under the sun and become less efficient
the hotter they are. In 2015, we showed that
with deliberate kinds of microstructures on top of a solar cell, we could take better advantage
of this cooling effect to maintain a solar cell passively
at a lower temperature. This allows the cell
to operate more efficiently. We're probing these kinds
of opportunities further. We're asking whether
we can use the cold of space to help us with water conservation. Or perhaps with off-grid scenarios. Perhaps we could even directly
generate power with this cold. There's a large temperature difference
between us here on earth and the cold of space. That difference, at least conceptually, could be used to drive
something called a heat engine to generate electricity. Could we then make a nighttime
power-generation device that generates useful
amounts of electricity when solar cells don't work? Could we generate light from darkness? Central to this ability
is being able to manage the thermal radiation
that's all around us. We're constantly bathed in infrared light; if we could bend it to our will, we could profoundly change
the flows of heat and energy that permeate around us every single day. This ability, coupled
with the cold darkness of space, points us to a future
where we, as a civilization, might be able to more intelligently manage
our thermal energy footprint at the very largest scales. As we confront climate change, I believe having
this ability in our toolkit will prove to be essential. So, the next time
you're walking around outside, yes, do marvel at how the sun
is essential to life on earth itself, but don't forget that the rest of the sky
has something to offer us as well. Thank you. (Applause)
I think that’s one of the coolest technologies I’ve ever heard of. I hope they get enough attention etc. to take this all that way
This will be so cool, if it happens commercially.
Roger Penrose pointed out that the sun is not the source of all life. It's the interplay between the sun and the heat sink of the rest of the universe.
Take a look at OTEC (Ocean Thermal Energy Conversion) energy generation. It uses a refrigerant to generate electicity through a turbine where you have mild temperature differences. This could be used for that... have an ice generator, and another heat sink, and you can use it to generate electricity at any time of day using the OTEC cycle.
That was awesome! I can't believe something so 'simple' and common as frozen grass in the morning actually wasn't understood too well, since we're only inventing this now.
Fascinating - this is a beautiful philosophy and a great energy idea.
Amazing.