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AIAA SciTech Forum
American Institute of Aeronautics and Astronautics
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entrance into larger, more powerful aircraft. Other research projects, such as NASA’s SCEPTOR program [7], are investigating integrated airframe-propulsion technologies (in this case, distributed electric propulsion) as a way to further increase the cruise efficiency available to electric aircraft, enabling higher speeds and longer ranges.
A. Barriers to Adoption of Electric Aircraft Limits of the component technologies needed for electric aircraft are often cited as a reason that electric
propulsion will remain a niche market for some time to come [8]. While there are certainly improvements needed within the electric aircraft system, the successful adoption of electric aircraft requires consideration of not just the aircraft-specific components, but the environment and infrastructure in which the aircraft must operate. For this paper, the authors consider three major barriers that constrain the application of electric propulsion to aircraft, both within and external to the individual aircraft: 1) specific energy of onboard energy storage, 2) refueling and support infrastructure, and 3) certification.
1. Specific Energy of Onboard Energy Storage
The first major barrier is the specific energy of the onboard energy storage system. Today’s civil aircraft almost exclusively use refined petroleum products for onboard energy storage. The most common fuel for light aircraft reciprocating engines is a high-octane leaded aviation fuel [9] (i.e. 100 octane “low lead,” or 100LL) that is related to high-octane automotive pump gasoline found at most filling stations (albeit with tetraethyl lead added to maintain compatibility with older, higher-power engines). Aircraft powered by gas turbine engines (and some compression- ignition reciprocating engine-powered aircraft) use “jet fuel” [10] (i.e. Jet A or Jet A-1) that is much like kerosene or diesel fuel. The specific energy of 100LL and Jet A is approximately 43 MJ/kg.
Batteries are the predominant form of electricity storage for electrically-powered aircraft and other vehicles. Given that electric motors are often rated in Watts, the energy of batteries is typically quoted in terms of Watt-hours. Advanced battery formulations such as lithium-polymer yield hobby applications that approach 200W-hrs/kg [11]. Current in-production vehicles, such as Tesla’s Roadster [12] and Pipistrel’s Taurus Electro motorglider [13], yield pack-level specific energy values of 132 and 113 W-hrs/kg, respectively. Hence, the amount of energy per unit mass in a modern high-energy battery is about 60-100 times less than that in the equivalent mass in aviation fuels (by comparison, the specific energy values of 100LL and Jet A are approximately 12,000 W-hrs/kg). While significant research and development is being conducted for higher-energy batteries, the low specific energy associated with batteries requires a very steep improvement to reach parity with the energy content of modern fuels on a mass basis.
Using mass of fuel vs. battery systems is not necessarily a fair comparison, however. From a pessimistic perspective, batteries are typically not fully discharged; this reduces the number of charge cycles they can experience or otherwise reduces their specific energy capacity. An 80% depth of discharge is a commonly used number to ensure sufficient charge/discharge cycles over the lifetime of the battery. In addition, fuel-burning aircraft that fly long distances benefit from the reduction in weight as fuel is depleted, as they need to produce less lift and therefore less induced drag for a given flight condition. Batteries are more or less constant-weight devices (with some exceptions, such as lithium-air batteries). However, from an optimistic perspective, the batteries are used to power electric motors that can have three times (or more) greater efficiency than gasoline-powered engines, so only one-third of the total onboard energy is needed (neglecting mass effects) for the same range. Finally, many aircraft do not fly missions that require the use of all of the onboard fuel – commercial short-haul commuters and many light personal aircraft fly for only an hour or less to complete their missions, though they may have a fuel capacity for many hours of flight [14]. Hence, the 60-100 times deficit that batteries have in energy per unit mass may well only be a deficit of 10-20 times or less, depending on the mission. Other system-level benefits, such as reduced cooling mass, or the benefits associated with distributed electric propulsion, can further reduce the impact of this deficit. Still, it remains a daunting challenge, and one in which most current electric vehicle efforts are placing their focus (either through design of higher-energy batteries or focus on configurations/missions that use less energy).
