Examining the Challenge of Sustainable Long-Distance Travel
Hippy activists may tell you humans are killing the earth— they’re wrong. Earth is fine, and we’re the ones in trouble. The cold, dark universe will remain indifferent to our climate and ecological dilemmas. Here I will explain why we won’t be using rail or electric airplanes, and why hydrogen is a worthwhile alternative to the use of fossil fuels in jets.
First off, taking trains or driving electric cars for long distance travel is absurd once you look closely at the consequences. Conventional rail is way too slow for transcontinental travel in the 21st century, competing with cars for speed.
It’s easy for small nations like Germany and Japan to use electric trains when the distance from Berlin to Munich for example is only 300 miles; Houston to El Paso is more than doubled that distance alone, two cities in Texas, a state which accounts for less than 9% of land in the contiguous 48 states.
In the fastest bullet train, it would take 15 hours to travel from Seattle to Miami, as opposed to 6 hours via airplane. We happen to have unlimited natural obstacles including mountains and mile-wide rivers which complicate building these high-speed rail networks, driving up the cost. At $70 million per rail mile, a route from Seattle to Miami would cost on the order of $250 billion, or close to 7,800 times what it costs to build a single Boeing 737.
The American aerospace industry is working on developing supersonic aircraft for commercial airlines, and NASA is designing aircraft that produce 70% less noise-pollution, two innovations which will hopefully kill any remaining attraction towards rail and other ground options. High-speed rail has only gotten more expensive, and is significantly speed-constricted due to high air-resistance at low-altitudes. Most densely populated areas are at or near sea-level, so this problem is unavoidable.
The fastest passenger train is slower than the slowest passenger airliners in widespread service today. There is no competition. Rail sucks.
International air travel produced an estimated 665 million tons of carbon emissions in 2018, which, while small compared to global emissions, can still cause trouble in the long-term. There have been a few proposals for the decarbonization of air travel, including using hydrogen fuel cells with electric motors. To the best of my knowledge, there are no major companies seriously considering battery-electric aircraft, as they are simply uneconomical and a step back in virtually every relevant regard.
For those curious about battery-electric aircraft, lithium-ion batteries have roughly fifty times the density and weight of jet fuel per unit of energy, making their use untenable for the purpose of air travel, as planes must be light and compact to reduce drag and maintain positive lift.
There is some merit to hydrogen fuel cell aircraft, which is something a few companies and NASA have been researching. Hydrogen has quite a few advantages; it’s fully renewable, it’s very efficient, it has nearly tripled the specific energy per mass-unit of jet fuel today, and hydrogen can be produced anywhere in the world.
Hydrogen is pretty great, but it does come with some caveats. The storage and transportation of hydrogen is more difficult thanks to its physical nature; hydrogen has a boiling point of -252.9°C, and it’s no less flammable than its fossil fuel counterparts. The real trouble is in the process of producing hydrogen, which currently relies on fossil fuels directly through steam reformation, or indirectly through electrolysis of water.
According to the Department of Energy, hydrogen fuel cells are typically 40-60% efficient, which, along with the efficiency of the typical electric motor, should allow them to compete well with jet engines. The Boeing-777 turbine for example is 36% energy efficient. Since hydrogen has three times the energy and twice the price per mass-unit, it should be about half as expensive to fuel aircraft with.
Since economies of scale have a tendency to bring prices down, and since energy is expected to become even cheaper and greener, we should expect the cost of producing hydrogen to decline, further increasing its value as an alternative to existing jet fuels.
The big challenge for hydrogen will be producing viable electric propulsion, as well as making space in future aircraft to accommodate a roughly three-times greater volume of fuel. The Soviet Tupolev design bureau built the first hydrogen-powered aircraft in 1988, designated the Tu-155, a variant of the Tu-154. This aircraft could travel rather quickly, but had something like a third lower passenger capacity because its fuel took up so much space.
Another challenge is the lack of infrastructure and limited availability of hydrogen with great enough flow and in quantities necessary for refueling jets. This problem can likely be rectified with some reasonably limited investment.
In conclusion, hydrogen aircraft offer some hefty advantages at relatively minor cost. With pressure from voters on Congress to combat climate change, we can expect funding for hydrogen infrastructure, as well as a real effort from aerospace companies to develop hydrogen-fueled aircraft. Until then, fly safe and keep filing noise complaints with the FAA!