Decarbonizing Aviation Is Not As Hard As We Think
By now, all of us have heard a compelling case for the “electrify everything” approach to decarbonization. The easiest and cheapest forms of zero-carbon energy are solar and wind generated electricity and therefore, in order to get to a zero-carbon society, we need to shift all stoves, furnaces, and road vehicles to the grid and meet the increased electricity demand with photovoltaics and wind turbines. For the most part, the electrify everything plan is a good one, but when it comes to commercial air travel, it has no solution beyond suggesting that people ultimately… just need to fly less. Unfortunately, people are not flying less; in fact, quite the contrary. Today, aviation represents at least 2.5% of global CO2 emissions, and its contribution is projected to continue accelerating. By 2050, aviation emissions could contribute close to 10% of current global emissions, making it a very significant threat to international emissions targets.
The electrify everything approach offers no solution to decarbonizing air travel because the technology for an electric commercial airplane does not yet exist, nor is likely to exist any time soon. Simply put, today’s lithium-ion batteries are too heavy to be used in a long-distance airborne craft. In illustration, the cutting-edge battery pack of the Tesla Model 3 electric car has a gravimetric density of 168 Wh/kg. If we apply this energy density to the requirement for a passenger aircraft such as a Boeing 747 flying from JFK to Heathrow, a single flight would need approximately 3881 metric tons of battery packs. A 747’s maximum take-off weight is 333 metric tons, one tenth the weight of the batteries needed to power it. It is going to require generational advancements in electricity storage before a battery-powered commercial plane ever crosses the Atlantic Ocean.
Looking at these numbers, it is entirely understandable that air travel has become the elephant in the room when it comes to global climate action. However, by considering options other than electrification, one realizes that air travel may offer a significant near-term emissions reduction opportunity.
Rather than replace the fuel tanks with electro-chemical batteries, aircrafts could instead use carbon-neutral jet fuel. Jet fuel is an umbrella term used for a number of different kerosene-like blends of hydrocarbons that are synthesizable using carbon dioxide and hydrogen. Provided that the energy used in the synthesis process comes from renewably generated electricity, and that the CO2 is captured from the atmosphere or from another emissions source, this synthetic jet fuel could be entirely carbon-neutral and allow existing aircrafts to continue operating as normal for their full 30+ year lifespan.
Based on a recent analysis by the Kleinman Center, water electrolysis – to produce hydrogen – and the direct air capture (DAC) of CO2 contribute greater than 90% of both the energy demand and capital costs of synthetic fuel production. Electrolysis, for example, demands approximately 52.6 kWh per kg of hydrogen produced. Given the energy demand of a Boeing 747 , electrolysis would consume 109.4 kWh per mile of air travel. Using an average US electricity costs of 12 cents/kWh, this equates to $13.13 per mile. The $0.50 per kg capital cost of the electrolysis facility is comparatively small once levelized over a lifetime of 40 years, and equates to an additional $1 per mile traveled. DAC of CO2, assuming it can recycle heat from the electrolysis process, demands hardly any additional energy input, but costs an additional $1.39 per mile in levelized capital costs. This means that the total fuel cost per mile travelled by a Boeing 747 would increase from about $8.55 using conventional fuel to about $15.52 using carbon-neutral fuel. On a flight from JFK to Heathrow (3,443 miles), assuming a 747 carrying 400 passengers, this would increase each passenger’s fuel costs from $73.6 to $133.6.
Transatlantic plane tickets cost, on average, about $700. Most of this cost is attributable to airport taxes, maintenance costs, and crew salaries, and relatively little represents the cost of fuel. As a result, a near doubling of fuel costs translates to only an 8.5% increase in the cost of the average ticket. Compared to the doubling of utility bills or gasoline costs, this makes aviation a much more accommodating sector to the use of synthetic fuels than are home heating or automobiles. This cost structure also means that decarbonization of commercial air travel is not a far-off possibility dependent on ground-breaking advancements in energy storage, but an actionable and pragmatic approach to achieving global emissions targets. In addition, by seizing this opportunity to decarbonize aviation using synthetic fuel, the world can continue to develop and improve electrolysis and DAC technologies, both of which will play a vital role in the future of climate action.
