What to expect when expecting electric airplanes
Our just-released ICCT report on electric aircraft concludes that electric models will be limited to short range flights (< 500 km) in the foreseeable future. Despite leaps-and-bounds improvements in battery technology in the past three decades, batteries remain inadequate to the task of electrifying most of passenger aviation. Our paper explored the capabilities of electric aircraft given current and projected battery technology. Now we address the inverse: how good do batteries need to be to power most flights?
As any economy passenger wedged into the 28 inches between seat rows will attest—after paying the surcharge for an 8 kg carry-on—mass and volume on an aircraft come at a premium. This is just as true for the mass and volume of an aircraft’s battery pack, whose key metrics are the pack-level specific energy (eb, the amount of energy stored per unit of battery mass), and the energy density (vb, the energy stored per unit volume). eb is measured in watt-hours per kilogram (Wh/kg) and vb is measured in watt-hours per liter (Wh/L).
In our paper analyzing electric aircraft, we derived the electric range equation and modified it to account for reserve requirements. Here we rearrange the equation to calculate the eb needed to power flights of different stage lengths, using assumptions for a few aircraft parameters. Aircraft-specific parameters, such as the battery mass fraction (BMF, or battery mass as a share of the total mass of the aircraft), and loiter velocity of the aircraft, are calculated according to methods outlined in the paper. Similarly, structural, aerodynamic, and electric efficiencies, along with reserve requirements, are kept identical to those in the paper.
To determine the vb required, the total battery energy is divided by the available volume in the aircraft. We assume the available volume is 10% of the fuselage volume of a representative aircraft—the cargo hold’s share of the fuselage of an Airbus A350.
What kind of aircraft do we want to electrify? All of them! Passenger aircraft can be broadly classified into 4 types based on size: commuter, regional, narrowbody, and widebody. Each successive type can carry more passengers over greater distances and will require more powerful batteries to electrify. To find representative missions for each aircraft class we rely on 2019 airline data from ICCT’s Global Aviation Carbon Assessment (GACA) database.
To ensure that we cover the vast majority of routes flown in each aircraft class, we define a representative mission as those lying at the 90th percentile for distance and passenger capacity. For example, 90% of all narrowbody routes are less than 3000 km long and 90% of them carry fewer than 191 passengers. So the representative route of narrowbody aircraft carries 191 passengers 3000 km. For the maximum takeoff mass (MTOM) of commuter class aircraft, we use the European Aviation Safety Agency’s Part 23 weight limit. For the other aircraft classes, we use representative (round) numbers. Table 1 lists the parameters that define the aircraft and the missions we are trying to electrify.
Table 1. Parameters defining the aircraft classes and representative missions
|Aircraft class||Passengers||Stage length (km)||MTOM (kg)||BMF||Wing area (m2)||Available volume (m3)|
Before calculating the resulting battery requirements, let’s describe the current capabilities of batteries. Today’s best-in-class lithium-ion batteries achieve eb = 250 Wh/kg and vb¬ = 500 Wh/L. This level of power can enable a 140 km flight carrying 9 passengers. Aircraft running on fossil jet fuel fly much farther with more people because they are dramatically more powerful: fossil jet fuel has a specific energy nearly 50 times higher (12,000 Wh/kg) and an energy density about 20 times higher (9,700 Wh/L).
Using the values identified, we can calculate the battery requirements for representative missions of the four aircraft classes. Table 2 presents the pack-level battery requirements to decarbonize each aircraft class and includes the improvements required relative to state-of-the-art batteries.
Table 2. Pack-level battery requirements to decarbonize each aircraft class
|Passengers||Stage length (km)||eb
|Regional||104||1500||1500 (6x)||860 (2x)|
|Narrowbody||191||3000||2300 (9x)||1800 (4x)|
|Widebody||375||8800||5100 (20x)||3600 (7x)|
Comparing the required battery parameters with what is currently achievable highlights the difficulty in electrifying anything but commuter aircraft. Replacing regional, narrowbody, and widebody aircraft would require roughly 6x, 9x, and 20x improvements in the specific energy of the battery pack. In the 25 years from 1991 to 2015, the specific energy and energy density of lithium-ion batteries improved by a factor of 3. Assuming the same exponential growth (3x increase in 25 years), it will be 2090 before widebody aircraft can be electrified. However, this is impossible with current lithium-ion batteries or solid-state batteries, because of the physical limits of the chemistry of these technologies. The specific energy at the pack level for these batteries might not exceed 400-500 Wh/kg. New battery chemistries would need to be developed.
Does this daunting picture mean we should throw our hands up and stop developing electric aircraft? Not at all! The energy efficiency and zero-emission benefits of electric aircraft merit their adoption for short-hop commuter flights (9-19 passengers for < 200 km) wherever feasible. For example, short-hop flights are responsible for a disproportionate amount of local pollution from aircraft, so electric aircraft, which are zero-emission, could contribute to cleaner air in some regions. While these flights account for a sliver of aviation’s emissions, every electrified route represents a reduction in aviation’s climate impact and is a worthwhile investment.