Apr 9 JDN 2460042

Air travel is currently one of the most carbon-intensive activities anyone can engage in. Per passenger kilometer, airplanes emit about 8 times as much carbon as ships, 4 times as much as trains, and 1.5 times as much as cars. Living in a relatively eco-friendly city without a car and eating a vegetarian diet, I produce much less carbon than most First World citizens—except when I fly across the Atlantic a couple of times a year.

Until quite recently, most climate scientists believed that this was basically unavoidable, that simply sustaining the kind of power output required to keep an airliner in the air would always require carbon-intensive jet fuel. But in just the past few years, major breakthroughs have been made in using hydrogen propulsion.

The beautiful thing about hydrogen is that burning it simply produces water—no harmful pollution at all. It’s basically the cleanest possible fuel.

The simplest approach, which is actually quite old, but until recently didn’t seem viable, is the use of liquid hydrogen as airplane fuel.

We’ve been using liquid hydrogen as a rocket fuel for decades; so we knew it had enough energy density. (Actually its energy density is higher than conventional jet fuel.)

The problem with liquid hydrogen is that it must be kept extremely cold—it boils at 20 Kelvin. And once liquid hydrogen boils into gas, it builds up pressure very fast and easily permeates through most materials, so it’s extremely hard to contain. This makes it very difficult and expensive to handle.

But this isn’t the only way to use hydrogen, and may turn out to not be the best one.

There are now prototype aircraft that have flown using hydrogen fuel cells. These fuel cells can be fed with hydrogen gas—so no need to cool below 20 Kelvin. But then they can’t directly run the turbines; instead, these planes use electric turbines which are powered by the fuel cell.

Basically these are really electric aircraft. But whereas a lithium battery would be far too heavy, a hydrogen fuel cell is light enough for aviation use. In fact, hydrogen gas up to a certain pressure is lighter than air (it was often used for zeppelins, though, uh, occasionally catastrophically), so potentially the planes could use their own fuel tanks for buoyancy, landing “heavier” than they took off. (On the other hand it might make more sense to pressurize the hydrogen beyond that point, so that it will still be heavier than air—but perhaps still lighter than jet fuel!)

Of course, the technology is currently too untested and too expensive to be used on a wide scale. But this is how all technologies begin. It’s of course possible that we won’t be able to solve the engineering problems that currently make hydrogen-powered aircraft unaffordable; but several aircraft manufacturers are now investing in hydrogen research—suggesting that they at least believe there is a good chance we will.

There’s also the issue of where we get all the hydrogen. Hydrogen is extremely abundant—literally the most abundant baryonic matter in the universe—but most of what’s on Earth is locked up in water or hydrocarbons. Most of the hydrogen we currently make is produced by processing hydrocarbons (particularly methane), but that produces carbon emissions, so it wouldn’t solve the problem.

A better option is electrolysis: Using electricity to separate water into hydrogen and oxyen. But this requires a lot of energy—and necessarily, more energy than you can get out of burning the hydrogen later, since burning it basically is just putting the hydrogen and oxygen back together to make water.

Yet all is not lost, for while energy density is absolutely vital for an aircraft fuel, it’s not so important for a ground-based power plant. As an ultimate fuel source, hydrogen is a non-starter. But as an energy storage medium, it could be ideal.

The idea is this: We take the excess energy from wind and solar power plants, and use that energy to electrolyze water into hydrogen and oxygen. We then store that hydrogen and use it for fuel cells to run aircraft (and potentially other things as well). This ensures that the extra energy that renewable sources can generate in peak times doesn’t go to waste, and also provides us with what we need to produce clean-burning hydrogen fuel.

The basic technology for doing all this already exists. The current problem is cost. Under current conditions, it’s far more expensive to make hydrogen fuel than to make conventional jet fuel. Since fuel is one of the largest costs for airlines, even small increases in fuel prices matter a lot for the price of air travel; and these are not even small differences. Currently hydrogen costs over 10 times as much per kilogram, and its higher energy density isn’t enough to make up for that. For hydrogen aviation to be viable, that ratio needs to drop to more like 2 or 3—maybe even all the way to 1, since hydrogen is also more expensive to store than jet fuel (the gas needs high-pressure tanks, the liquid needs cryogenic cooling systems).

This means that, for the time being, it’s still environmentally responsible to reduce your air travel. Fly less often, always fly economy (more people on the plane means less carbon per passenger), and buy carbon offsets (they’re cheaper than you may think).

But in the long run, we may be able to have our cake and eat it too: If hydrogen aviation does become viable, we may not need to give up the benefits of routine air travel in order to reduce our carbon emissions.