carbon footprint

Will hydrogen make air travel sustainable?

pnrj current events, futurism, public policy , , , , , , ,

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.

What will we do without air travel?

pnrj futurism, market failure, public policy , , , , , , , , , , , ,

August 6, JDN 2457972

Air travel is incredibly carbon-intensive. Just one round-trip trans-Atlantic flight produces about 1 ton of carbon emissions per passenger. To keep global warming below 2 K, personal carbon emissions will need to be reduced to less than 1.5 tons per person per year by 2050. This means that simply flying from New York to London and back twice in a year would be enough to exceed the total carbon emissions each person can afford if we are to prevent catastrophic global climate change.

Currently about 12% of US transportation-based carbon emissions are attributable to aircraft; that may not sound like a lot, but consider this. Of the almost 5 trillion passenger-miles traveled by Americans each year, only 600 billion are by air, while 60,000 are by public transit. That leaves 4.4 trillion passenger-miles traveled by car. About 60% of US transportation emissions are due to cars, while 88% of US transportation is by car. About 12% of US transportation emissions are due to airplanes, while 12% of US passenger-miles are traveled by airplane. This means that cars produce about 2/3 as much carbon per passenger-mile, even though we tend to fill up airplanes to the brim and most Americans drive alone most of the time.

Moreover, we know how to reduce emissions from cars. We can use hybrid vehicles, we can carpool more, or best of all we can switch to entirely electric vehicles charged off a grid that is driven by solar and nuclear power. It is theoretically possible to make personal emissions from car travel zero. (Though making car manufacturing truly carbon-neutral may not be feasible; electric cars actually produce somewhat more carbon in their production, though not enough to actually make them worse than conventional cars.)

We have basically no idea how to reduce emissions from air travel. Jet engines are already about as efficient as we know how to make them. There are some tweaks to taxi and takeoff procedure that would help a little bit (chiefly, towing the aircraft to the runway instead of taking them there on their own power; also, taking off from longer runways that require lower throttle to achieve takeoff speed). But there’s basically nothing we can do to reduce the carbon emissions of a cruising airliner at altitude. Even very optimistic estimates involving new high-tech alloys, wing-morphing technology, and dramatically improved turbofan engines only promise to reduce emissions by about 30%.

This is something that affects me quite directly; air travel is a major source of my personal carbon footprint, but also the best way I have to visit family back home.
Using the EPA’s handy carbon footprint calculator, I estimate that everything else I do in my entire life produces about 10 tons of carbon emissions per year. (This is actually pretty good, given the US average of 22 tons per person per year. It helps that I’m vegetarian, I drive a fuel-efficient car, and I live in Southern California.)

Using the ICAO’s even more handy carbon footprint calculator for air travel, I estimate that I produce about 0.2 tons for every round-trip economy-class transcontinental flight from California to Michigan. But that doesn’t account for the fact that higher-altitude emissions are more dangerous. If you adjust for this, the net effect is as if I had produced a full half-ton of carbon for each round-trip flight. Therefore, just four round-trip flights per year increases my total carbon footprint by 20%—and again, by itself exceeds what my carbon emissions need to be reduced to by the year 2050.

With this in mind, most ecologists agree that air travel as we know it is simply not sustainable.

The question then becomes: What do we do without it?

One option would be to simply take all the travel we currently do in airplanes, and stop it. For me this would mean no more trips from California to Michigan, except perhaps occasional long road trips for moving and staying for long periods.

This is unappealing, though it is also not as harmful as you might imagine; most of the world’s population has never flown in an airplane. Our estimates of exactly what proportion of people have flown are very poor, but our best guesses are that about 6% of the world’s population flies in any given year, and about 40% has ever flown in their entire life. Statistically, most of my readers are middle-class Americans, and we’re accustomed to flying; about 80% of Americans have flown on an airplane at least once, and about 1/3 of Americans fly at least once a year. But we’re weird (indeed, WEIRD, White, Educated, Industrialized, Rich, and Democratic); most people in the world fly on airplanes rarely, if ever.

Moreover, air travel has only been widely available to the general population, even in the US, for about the last 60 years. Passenger-miles on airplanes in the US have increased by a factor of 20 since just 1960, while car passenger-miles have only tripled and population has only doubled. Most of the human race through most of history has only dreamed of air travel, and managed to survive just fine without it.

It certainly would not mean needing to stop all long-distance travel, though long-distance travel would be substantially curtailed. It would no longer be possible to travel across the country for a one-week stay; you’d have to plan for four or five days of travel in each direction. Traveling from the US to Europe takes about a week by sea, each way. That means planning your trip much further in advance, and taking off a lot more time from work to do it.

Fortunately, trade is actually not that all that dependent on aircraft. The vast majority of shipping is done by sea vessel already, as container ships are simply far more efficient. Shipping by container ship produces only about 2% as much carbon per ton-kilometer as shipping by aircraft. “Slow-steaming”, the use of more ships at lower speeds to conserve fuel, is already widespread, and carbon taxes would further incentivize it. So we need not fear giving up globalized trade simply because we gave up airplanes.

But we can do better than that. We don’t need to give up the chance to travel across the country in a weekend. The answer is high-speed rail.

A typical airliner cruises at about 500 miles per hour. Can trains match that? Not quite, but close. Spain already has an existing commercial high-speed rail line, the AVE, which goes from Madrid to Barcelona at a cruising speed of 190 miles per hour. This is far from the limits of the technology. The fastest train ever built is the L0 series, a Japanese maglev which can maintain a top speed of 375 miles per hour.

This means that if we put our minds to it, we could build a rail line crossing the United States, say from Los Angeles to New York via Chicago, averaging at least 300 miles per hour. That’s a distance of 2800 miles by road (rail should be comparable); so the whole trip should take about 9 and a half hours. This is slower than a flight (unless you have a long layover), but could still make it there and back in the same weekend.

How much would such a rail system cost? Official estimates of the cost of maglev line are about $100 million per mile. This could probably be brought down by technological development and economies of scale, but let’s go with it for now. This means that my proposed LA-NY line would cost $280 billion.

That’s not a small amount of money, to be sure. It’s about the annual cost of ending world hunger forever. It’s almost half the US military budget. It’s about one-third of Obama’s stimulus plan in 2009. It’s about one-fourth Trump’s proposed infrastructure plan (that will probably never happen).

In other words, it’s a large project, but well within the capacity of a nation as wealthy as the United States.

Add in another 500 miles to upgrade the (already-successful) Acela corridor line on the East Coast, and another 800 miles to make the proposed California High-Speed Rail from LA to SF a maglev line, and you’ve increased the cost to $410 billion.
$410 billion is about 2 years of revenue for all US airlines. These lines could replace a large proportion of all US air traffic. So if the maglev system simply charged as much as a plane ticket and carried the same number of passengers, it would pay for itself in a few years. Realistically it would probably be a bit cheaper and carry fewer people, so the true payoff period might be more like 10 years. That is a perfectly reasonable payoff period for a major infrastructure project.

Compare this to our existing rail network, which is pitiful. There are Amtrak lines from California to Chicago; one is the Texas Eagle of 2700 miles, comparable to my proposed LA-NY maglev; the other is the California Zephyr of 2400 miles. Each of them completes one trip in about two and a half days—so a week-long trip is unviable and a weekend trip is mathematically impossible. Over 60 hours on each train, instead of the proposed 9.5 for the same distance. The operating speed is only about 55 miles per hour when we now have technology that could do 300. The Acela Express is our fastest train line with a top speed of 150 miles per hour and average end-to-end speed of 72 miles per hour; and (not coincidentally I think) it is by far the most profitable train line in the United States.

And best of all, the entire rail system could be carbon-neutral. Making the train itself run without carbon emissions is simple; you just run it off nuclear power plants and solar farms. The emissions from the construction and manufacturing would have to be offset, but most of them would be one-time emissions, precisely the sort of thing that it does make sense to offset with reforestation. Realistically some emissions would continue during the processes of repair and maintenance, but these would be far, far less than what the airplanes were producing—indeed, not much more than the emissions from a comparable length of interstate highway.

Let me emphasize, this is all existing technology. Unlike those optimistic forecasts about advanced new aircraft alloys and morphing wings, I’m not talking about inventing anything new here. This is something other countries have already built (albeit on a much smaller scale). I’m using official cost estimates. Nothing about this plan should be infeasible.

Why are we not doing this? We’re choosing not to. Our government has decided to spend on other things instead. Most Americans are quite complacent about climate change, though at least most Americans do believe in it now.

What about transcontinental travel? There we may have no choice but to give up our weekend visits. Sea vessels simply can’t be built as fast as airplanes. Even experimental high-speed Navy ships can’t far exceed 50 knots, which is about 57 miles per hour—highway speed, not airplane speed. A typical container vessel slow-steams at about 12 knots—14 miles per hour.

But how many people travel across the ocean anyway? As I’ve already established, Americans fly more than almost anyone else in the world; but of the 900 million passengers carried in flights in, through, or out of the US, only 200 million were international Some 64% of Americans have never left the United States—never even to Canada or Mexico! Even if we cut off all overseas commercial flights completely, we are affecting a remarkably small proportion of the world’s population.

And of course I wouldn’t actually suggest banning air travel. We should be taxing air travel, in proportion to its effect on global warming; and those funds ought to get us pretty far in paying for the up-front cost of the maglev network.

What can you do as an individual? Ay, there’s the rub. Not much, unfortunately. You can of course support candidates and political campaigns for high-speed rail. You can take fewer flights yourself. But until this infrastructure is built, those of us who live far from our ancestral home will face the stark tradeoff between increasing our carbon footprint and never getting to see our families.