NATHAN S. LEWIS: When humans exist, we are glowing 100-watt light bulbs. A 2,000-calorie diet per day is equivalent to us glowing like a 100-watt light bulb. Now, it takes 10 times the amount of energy to bring that food from the farm and to put it into a truck and put it into a refrigerator and get it into your mouth. If you just add up the energy it takes to put food on the table and eat, that’s already 10 trillion watts. And then we actually have to do other things like get to work, like turn on the lights. As population grows, we’re going to need between 20 and 25 trillion watts of energy. So where are we going to get 20 to 25 trillion watts of carbon-free energy within our lifetimes?

SOSSINA HAILE: We can think about lots of renewables, like tidal and wind and geothermal and so on, but those are not significant energy inputs.

LEWIS: Nuclear could be a slice of this puzzle. But even to be a slice... we would have to build 10,000 nuclear reactors over the next 50 years.

HAILE: Do we want to put nuclear power plants in countries that we’re already saying, “Don’t do this”?

LEWIS: You could get to maybe piecing together a little puzzle that barely gets you to half of what you might need, but if you want to bridge that gap, we need to go to the sun.

More energy from the sun hits the earth in one hour than all the energy consumed by humans on our planet in an entire year. That means even if we only took a tiny fraction of what hits the earth, we would still make more energy than we could ever dream of using as a civilization.

HAILE: We love solar because we have so much of it.

JONAS PETERS: The only solution that seems abundantly clear is to somehow use sunlight to power the planet.

LEWIS: The problem with the sun is it has this nasty habit - it goes out locally every single night. And people need energy at night, too.

So unless you can store sunlight, you can never use it to make primary energy for the people when they need it and where they need it.

PETERS: Solar cells are one part of the solution. So solar cells provide electricity.

LEWIS: But unless we store it, that means we can only provide people energy between 10 a.m. and 4 p.m. on a sunny day in Arizona or in California. What happens when it rains in Boston?

PETERS: There’s a big chunk of energy that won’t be derived from solar cells. But it might be derived from something hooked up to, you know, sort of a creative adaptation of solar cell that can also be used to then make fuel.

HAILE: What that means is taking that solar energy and converting it into a chemical fuel rather than converting it to electricity.

PETERS: And that fuel in its simplest form is hydrogen. The sun already powers the planet through photosynthesis, and we all see that every day. The sun shines on the earth, and plants take sunlight, water, and carbon dioxide and turn that into a fuel.

It sounds pretty simple, but the machinery that they use to do that is an incredibly complex chemical machine. The question is, can you extract the essence of how it works and provide a fuel source for the planet for the long haul? And that would be a pretty cool problem to solve.

LEWIS: What we’re trying to do is artificial photosynthesis, in essence, build a mechanical version of a leaf. Find a system that takes sunlight and water as the only two inputs, and out comes hydrogen as the fuel.

PETERS: In essence, take H2O and strip the hydrogen out of H2O, which takes energy, and the energy to pull that H2 out of H20 is just sunlight.

So what we want to build is an artificial photosynthesis device that has four components. It has sunlight, a membrane, catalysts, and water. What you want to do is split water. And so you need catalysts that facilitate splitting water.

HAILE: A catalyst is something that makes something else happen faster.

So let’s say you’re sitting there in bed and you need to go to school, and your mom comes in and tells you, “Get out of bed, because you need to go to school.” That’s the catalyst during that scenario. Ok? This is something that was going to happen anyway but just needed to be speeded up. So that’s what the catalyst does.

PETERS: When you split water, you remove electrons and protons from water, and those

protons and electrons flow across the membrane and get recombined on the other side of the membrane to form hydrogen. That is your ultimate fuel source.

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