Flying on the Grid
Decarbonizing aviation will require massive amounts of renewable electricity. The demand for electricity from aviation alone in 2050 could equal the world’s entire production of renewable electricity in 2019. It’s not that electric aircraft are the future of aviation; they are still in development and will likely be viable only for short flights. Instead, aviation’s appetite for electricity emerges from the power needed to produce clean fuels that can replace conventional jet fuel, known as Jet A.
Two of the most promising alternatives to Jet A are e-kerosene and liquid hydrogen (LH2). Each requires large amounts of electricity to produce, with energy conversion efficiencies of 56-63% for LH2 and 51-58% for e-kerosene. This means that producing these fuels requires 1.6-2.0 times as much energy as the fuels contain, which could translate to more emissions overall unless their production is managed carefully. Carefully, in turn, means that two conditions must be met: the energy used to create alternative fuels must be renewable and it must be additional. How essential are these qualifiers to the goal of decarbonizing aviation? Spoiler alert: extremely.
Renewable means that the process of generating electricity emits no carbon, for example when solar or wind are the energy source. Additional requires that the electricity come from new renewable energy generation capacity, above and beyond existing plans for grid decarbonization. This ensures that the energy used to produce these fuels is not diverted from existing uses. Failing to enforce additionality through policy measures such as power purchase agreements, plus proof that the producer receives no subsidies or other policy support, could incentivize construction of new natural gas power plants to meet increased energy demand.
This blog post focuses on the renewable requirement. To demonstrate renewability, let’s compare GHG emissions of alternative fuels produced using grid electricity (often generated using a mix of renewable and nonrenewable energy), and those produced using renewable electricity exclusively. We can then compare the results to the GHG emissions associated with using Jet A. We’ll limit the scope of emissions calculations to the life-cycle GHG emissions of electricity from renewable and nonrenewable sources, and Jet A.
To calculate GHG emissions associated with grid electricity generation, we convert grid generation mixes into grid-averaged carbon intensity, measured in grams of CO2 equivalent emitted per kilowatt-hour of electricity generation (gCO2e/kWh). Since LH2 and e-kerosene are unlikely to be used much in aviation before 2035, we use projections of grid carbon intensity. These projections are difficult to make and are often inaccurate due to rapidly changing policies and incentives. To cast a wide net of possible outcomes, let’s examine two bookend scenarios developed at the International Energy Agency (IEA): the Stated Policies Scenario (STEPS), and the Sustainable Development Scenario (SDS). STEPS consists of projections based on current policy statements, while SDS represents the trajectory required to meet a “well below two degrees” Paris Agreement target.
Borrowing from previous ICCT work on electric cars, Table 1 lists the lifecycle GHG intensity of the grid in the US and EU in 2035 and 2050 under the STEPS and SDS scenarios in gCO2e/kWh. For renewable energy, the carbon intensity of building the power plants is included and is based on a 50/50 mix of wind and solar production.
|Life-cycle GHG intensity of electricity consumption (gCO2e/kWh)|
Table 1. Life-cycle greenhouse gas (GHG) intensity of the electric grid under various scenarios
So, we have the carbon intensity of electricity production, and of Jet A fuel. To compare the alternative fuels and Jet A across a variety of aircraft we use a common metric known as revenue passenger kilometers (RPK). It is the product of the number of passengers carried by an aircraft, and the distance it travels. To normalize emissions by RPK, we compare the performance of these fuels on a common reference mission. Let’s use a narrowbody aircraft on a 3,200 km flight carrying 165 passengers, a mission that could be flown by either a LH2-powered or an e-kerosene-powered aircraft.
Our recent analysis shows that an LH2-powered aircraft needs 0.97 MJ/RPK. Dividing that energy requirement by the fuel production efficiency (56-63%) gives us an input energy requirement of 1.54-1.73 MJ/RPK or 0.43-0.48 kWh/RPK. An equivalent jet-fueled aircraft, such as the A320neo, is slightly more efficient, requiring 0.82 MJ/RPK. However, the production of e-kerosene is less energy efficient (51-58%), so the input energy requirement to fuel the flight with e-kerosene is 0.39-0.45 kWh/RPK. These values are close enough to justify using a value of 0.44 kWh/RPK to represent both e-kerosene and LH2.
On the fossil-fueled side, the standard carbon intensity for jet fuel is 89 grams of carbon dioxide equivalent per MJ (g CO2e/MJ). Multiplying the energy requirement and the carbon intensity of the fuel gives us a 73 g CO2e/RPK for aircraft fueled with Jet A.
The figure below shows the carbon intensities of aircraft fueled under the different scenarios. The brown bar represents the carbon intensity of fossil-fueled aircraft and includes an annual 0.5% reduction in fuel consumption due to efficiency improvements. This is compared to the carbon intensity of aircraft running on an alternative fuel, such as LH2 or e-kerosene, produced using grid electricity under STEPS for the US (red) and EU (blue), and using renewable energy (green). The error bars extend from the carbon intensities under STEPS down to SDS. Under currently stated policies (STEPS), in 2035 grid-produced fuels could be more than twice as carbon-intensive as jet fuel in the US, and just as bad as fossil fuels in the EU. Fuels produced using renewable energy would provide a carbon emission reduction of > 75% compared to the fossil-fueled alternative, even when considering the embodied energy and emissions associated with building new renewable energy projects.
Figure. Life-cycle carbon intensity of aircraft fuels under different production scenarios
Weaning aviation off its dependence on fossil fuel requires the use of sustainable alternative fuels produced using additional renewable energy. Failing to enforce both requirements through policy could increase aviation’s GHG emissions and necessitate the building of more fossil-fueled power plants to meet the increased global energy demand.