A number of hybrid and turboelectric airplane concepts have been explored through a combination of NASA, industry, academia and other government agencies. For regional jets and larger planes, the design configurations generally fall into three categories: partially turboelectric, fully turboelectric, and hybrid electric.
Turboelectric or partially turboelectric systems store all energy in fuel and convert some or all of it to electrical power to drive propulsors. Hybrid electric systems store a portion of the energy in fuel and the remainder in a battery or equivalent energy storage system with a wide range of possible implementations.
Hybrid & Electric
The Boeing SUGAR Freeze airplane concept looks at many advanced technologies which combine to provide over 70% reductions in Carbon Dioxide emissions. It is an example of a partially turbo-electric architecture. This plane uses liquid natural gas instead of jet fuel, and generates electricity in flight by integrating a solid oxide fuel cell with the turbine engine. The electrical energy is then used to drive an aft propulsor at the tail of the plane in order to energize the boundary layer and reduce drag.
The ECO-150 concept developed by Empirical Systems Aerospace, Inc. is an example of a turbo-electric architecture which uses electrical generators on the wing-mounted turbofan engines to power electrically-driven fans. This plane distributes many smaller motors and fans along the wing in a split-wing, which gives the wing and propulsion system better performance, reducing the total energy required for the trip.
Single-Aisle Turbo-Electric Aircraft
This partially turboelectric concept, dubbed STARC-ABL, has high-power generators integrated with each of the two wing-mounted turbofan engines. In addition to producing thrust, the engines generate megawatts of electricity which can be used to power the airplanes normal electric systems, such as cabin cooling and instrumentation, and is also routed via high voltage cables to the aft of the vehicle. Below the tail is an electric motor and aft fan which provides thrust while reducing drag by re-energizing the low momentum boundary layer flow.
The N3-X is a fully turbo-electric, superconducting airplane with many advanced technologies which combine to provide over 70% reduction in fuel burn in a large, twin aisle passenger aircraft. Liquid hydrogen fuel is used by the wing-mounted turbo-generators to produce over 30 megawatts each of power, which is distributed to the 14 motor-driven fans at the aft of the vehicle. These distributed propulsors help reduce fuel burn by efficiently producing thrust while re-energizing the aft boundary layer..
SUGAR Volt is a hybrid electric airplane concept. This plane uses on-board batteries to drive a motor that is attached to the turbofan engine, augmenting the shaft speed during the cruise phase of its mission. This approach reduces the amount of jet fuel required to produce thrust from the engine, and therefore reduces emissions.