Translator: Robert Tucker
Reviewer: Denise RQ We use a lot of energy as a planet. By 2050, our average power consumption
will be 28 terawatts, that's 12 zeros after 28. The only resource that's available
to supply to this demand is the sun. We have about 100,000 terawatts striking at the Earth's surface
when the Sun is shining. And after we subtract away
the ocean, mountains, and so forth, the usable energy is about 600 terawatts. So that's still in far excess
of our utilization. But there's a problem with the Sun; it doesn't shine all the time,
and it doesn't shine everywhere. This is a picture, a photo,
from the International Space Station showing the United States
half in daytime and half in nighttime. This is one problem of the Sun. The second problem
is that the Sun doesn't shine where we need it to shine. We have here London, Tokyo, and Chicago. So if you've been to these places
or lived in those places, you know that the sun[light]
is not abundant. Yet, these are giant metropolises
in which we have huge population centers. So you may ask, "How about Texas?"
There's plenty of sun in Texas, right? That's not entirely true. Even in the summer, you have thunder storms
that limit the availability of the Sun. So the big problem with solar
is that it is not available when and where it is needed,
at least not all the time. So the vision we have is
to make energy available when and where it's needed. So, roughly speaking,
we can divide it into several processes. One, we have a carbon-free
source, like the sun. We have to first capture it, then we have to think
about how to store it - and that's going to be
the bulk of my talk today - we have to deliver it,
and we have to utilize it. We already do this today, pretty well. We can take solar panels
as a way to capture sunlight, turn that into electricity, we can store it in batteries,
like our iPhones or electric cars, we can deliver it using
the conventional electric grid, and we can use it. But the problem lies with storage.
It is not a perfect mechanism. With batteries it's rather
expensive, and it's heavy, we're carrying away dead weight
with batteries most of the time, we're not carrying the energy we need. It's mostly just things that are inactive,
you're not storing the energy. Moreover, battery does not store
electricity for a long period of time. If you look at your iPhone and so forth, it only lasts for maybe 30 days,
or 60 days if we don't charge it. So it will lose charge over time. What we need is a medium to store energy
that is long-lasting, dispatchable, so we can bring it
to wherever it is needed, anytime, whether the Sun
is shining or not. So, I want to introduce you to the concept
of what we call solar fuels. Fuel such as ethanol, methane - which is the biggest
component in natural gas - or hydrogen. It's a great way to store energy, you can dispatch it whenever you want,
it is very high energy density, and it can be derived
directly from the Sun. You can imagine we can take molecules
like water or carbon dioxide, we can put the sun to it,
take them apart, reassemble them into these energetic
molecules, such as ethanol, we can store it in their form,
we can transport it in the pipelines, we can use it. And when we'll burn these materials,
what we'd get? Water and CO2. And it goes back right to the top
of the loop, where we start again. It is a carbon-neutral energy cycle. So this is were we aim to be,
but we're pretty far from it now, but this is the way of the future. So let me talk a little bit about how
to turn water into hydrogen and oxygen. Here hydrogen is your fuel. We call this sometime
the reverse combustion process. Combustion is the other way round, you take hydrogen
or any other form of fuel, you put it with air,
and you burn it, OK? And you can get power out of it. So this is the reverse process. You can imagine
that it is an uphill process; you're spending energy, you're pushing the water molecule uphill,
as shown on the slide. And you want to get over this barrier;
it's about 1.2 volt, this is roughly the voltage
of an AA battery that you have. It doesn't sound like a lot,
but it is very hard to achieve, to get this 1.2 volt needed to dissociate
water into hydrogen and oxygen, so that we can use it
when and wherever we want. So in this process of dissociating water
and taking it into a form of the fuel, it's inefficient. And I'm going to talk
about a couple of things that we're doing at Stanford
to make this a reality. One of the biggest challenges with taking
sunlight and storing it as the fuel is we can't use the entire
solar spectrum very well. On the top of the screen you see
the various colors of the Sun. You have UV light, after violet light, you have the visible light,
and you have infrared. Solar cells today can take
the visible light very well. They can also take
the ultraviolet light very well. But they can't take the infrared light which is actually a bulk part
of the solar spectrum. And if you take a look at the availability
of power as a function of the color, the wavelength of light, you will see what the problem is. Solar cells today can only take
a very small portion of it like the one shown in red. Everything else is lost as heat. And because solar cell efficiency
decreases with heat, you'll have to cool it
in order to maintain efficiency. So all this energy that is not being used
and is now turning up as heat is discarded in the system. So we can't use that very well. But we now have developed
a new system at Stanford to help us take not only the light energy
but also the thermal energy. So we can take the entire solar spectrum, whether it's coming as light,
or being absorbed as heat, and put all that energy toward rolling
that water molecule up the hill, so that we can dissociate it
into hydrogen, so it can be used as a fuel,
stored and dispatched. Another big problem with solar fuel is it often takes very rare materials
to perform the process. Often, it takes materials
like platinum or iridium, and these are among the rarest
materials on the planet, to carry out this pushing
uphill process with light. What is happening when you shine
light on these materials is the electrons start moving around, and the electron is zapping
the water molecules, and allowing it to be dissociated
into hydrogen and oxygen. But you want to do it
with a material that is abundant. And what we're looking at
is using material, iron oxide. So this is essentially a form of rust. It gives rise to the red color of rocks
in Southwest United States, it's one of the most abundant materials, the problem with this material is that these electrons, when you shine
a light on them, don't move very fast. So you can't extract energy
very efficiently. But it turns out that if you give it
a little bit of heat, these electrons start moving
much and much faster, so that it can eventually reach
the water molecules and turn that into storable fuel. So that's what we've been looking at
for the past couple of years, on taking sunlight, water,
and sometimes even carbon dioxide, and turning that into a fuel so we can use it
when and whenever we want. You might ask one question: "Well, how do we get both heat
and light at the same time?" After all, if we stand outside,
we don't get to a very high temperature. But it turns out
that if we take a magnifying glass, we can focus sunlight to a smaller spot. And that allows us
to achieve simultaneously the intensity of light and also heat. This is a conventional solar cell. You take light, and you shine it on it,
and you convert it to electricity. But imagine now you have a way
to focus the light with mirrors, and now you have a smaller solar cell, so you decrease the cost
because you're doing more with less. But the problem
with conventional solar cells is you have to cool it in the process
because the heat decreases the efficiency. But now imagine using a process that can positively be enhanced
with both light and heat. Then you can take both, convert it
to useful energy without the cooling. So now you eliminate the part
of the system which was limiting the cost. So I'm hopefully giving you some idea of the possibility to store
sunlight in terms of fuel, turning water, carbon dioxide into fuel, and making the Sun available
when and where it's needed. Thank you. (Applause)
