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 sunlight 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 where 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.