It’s being billed as a triumph for solar power, but the Solar Impulse solar-powered airplane could also be seen as an illustration of just how amazing liquid fuels like jet fuel are, and how far solar power and battery technology would need to go to challenge them. A far better idea than solar-powered flight is nuclear-powered flight, although I don’t mean putting nuclear reactors on airplanes as the U.S. government once proposed (see these two pdfs). Let’s use fission to make low-carbon fuel.

The Solar Impulse plane, powered by a combination of crystalline silicon solar panels and lithium ion batteries, made news last year when it made the world’s first solar-powered intercontinental flight. And it’s scheduled to fly across the United States this summer. But the continents it travelled between—the plane flew from Spain to Morocco–nearly touch. And the plane will make the trip across the U.S. in five legs. The plane carries only one person, yet its wingspan is equal to that of a jumbo jet, which provides the needed lift and the area for the solar panels.

There are some things solar panels are good for, but they’re not good for passenger aircraft. The energy in sunlight is too diffuse. A square-meter solar panel generates less than 200 watts in full sunshine. In comparison, a small, half-meter-wide, gasoline-powered generator can generate 3,500 watts. It can run for 24 hours–at half power–on less than 12 gallons of fuel. The solar panel, of course, stops working at night. If it can generate some extra power during the day, that could be stored in batteries. But batteries store only about 1/100^th as much energy as gasoline. (Incidentally, one of the attractions of nuclear power is that you can get about a million times more energy out of uranium in a power plant than you can get out of diesel in an engine.)

Solar panels could be great for some niche aircraft. For example, lightweight military drones—the kinds that look like model airplanes and that soldiers launch by throwing them into the air—are now being outfitted with high efficiency solar cells that allow them to stay in the air much longer than they could with batteries alone. But even doubling or tripling solar panel efficiency and battery storage capacity won’t allow these sources to provide the kind of energy needed to heave a jumbo jet from San Francisco to Beijing.

If we’re going to have low carbon emissions airlines, we’ll need to make liquid fuels from low-carbon sources. Biofuels made from sugar or grass could play some role in lowering emissions. But their role is limited given the energy needed for farming and the large amounts of land required (okay, except in the case of seaweed).

The lowest carbon route could be making fuels from carbon dioxide and water using energy from land-based solar power or nuclear power plants. Eventually high temperature nuclear power plants now in development might be an even better option–their heat could be used to facilitate the necessary chemical reactions more efficiently than electricity could (see “A Better Way to Get Hydrogen from Water,” “Safer Nuclear, At Half the Price,” and “Developing Nations Put Nuclear on Fast-Forward”).

As an aside, solar on cars will run into the same problems as solar on airplanes. One could imagine using solar panels on cars to slowly charge batteries while the cars are parked. Maybe you could get a few kilowatt hours a day this way, enough for short commutes on sunny days. You couldn’t rely on it for long trips. That’s what liquid fuels are for.