According to Renewable Energy Perspectives for Aviation research paper, there are Three paths for renewable energy perspectives in aviation.
1) Renewable Drop-in Fuels
The use of drop-in fuels which have equal characteristics as conventional kerosene requires in general no adaptation of the fuel distribution infrastructure, on-board fuel systems, or combustion engines. e.g. Biofuels, Solar Drop-in Fuels.
One way to reduce the carbon footprint of aviation is to introduce sustainable, carbon-neutral drop-in fuels that can be used to substitute conventional kerosene in present motive power systems and could, therefore, represent an elegant way to fulfill the International Air Transport Associations (IATA) target of carbon-neutral growth starting in the year 2020. Certified drop-in fuels based on different resources exist and enter long-term testing phases (Lufthansa, KLM).
2) Renewable Non-Drop-in Fuels
As opposed to fossil or solar kerosene, nondrop-in fuels are not compatible with today’s transportation fuel systems and therefore cannot be used without major adaptations in infrastructure including fuel production, distribution and storage, and motive power system of the aircraft. e.g. Solar Hydrogen.
3) Electric Motive Power Systems:
An electric aircraft is an aircraft powered by electric motors. Electricity may be supplied by a variety of methods including batteries, ground power cables, solar cells, ultracapacitors, fuel cells, and power beaming.
The first commercially available, non-certified production electric aircraft, the Alisport Silent Club self-launching glider plane, flew in 1997. It is optionally driven by a 13 kW (17 hp) DC electric motor running on 40 kg (88 lb) of batteries that store 1.4 kWh of energy.
The first certificate of airworthiness for an electric-powered aircraft was granted to the Lange Antares 20E in 2003. Also an electric, self-launching 20-meter glider/sailplane, with a 42-kilowatt DC/DC brushless motor and lithium-ion batteries, it can climb up to 3,000 meters with fully charged cells. The first flight was in 2003. In 2011 the aircraft won the 2011 Berblinger competition.
Solar Impulse 2 is powered by four electric motors. Energy from solar cells on the wings and horizontal stabilizer is stored in lithium polymer batteries and used to drive propellers. In 2012 the first Solar Impulse made the first intercontinental flight by a solar plane, flying from Madrid, Spain to Rabat, Morocco. Completed in 2014, Solar Impulse 2 carried more solar cells and more powerful engines, among other improvements. In March 2015, the plane took off on the first stage of a planned round-the-world trip.
SolarStratos is an aeronautical project aimed at flying a solar-powered airplane for the first time to the stratosphere. The SolarStratos airplane is the first commercial two-seater solar-powered aircraft in history, it will also be the first manned solar-powered aircraft to enter the stratosphere.
Currently, battery-powered electric aircraft have much more limited payload, range and endurance than those powered by internal combustion engines. It is hence only suitable for small aircraft (for large passenger aircraft, an improvement of the energy density by a factor 20 compared to li-ion batteries would be required.
Hybrid electric aircraft
A hybrid electric aircraft is an aircraft with a hybrid electric powertrain, needed for airliners as the energy density of lithium-ion batteries is much lower than aviation fuel. By May 2018, there were over 30 projects, and short-haul hybrid-electric airliners were envisioned from 2032. The most advanced are the Zunum Aero 10-seater, the Airbus E-Fan X demonstrator, the VoltAero Cassio, UTC is modifying a Bombardier Dash 8, while an Ampaire prototype first flew on 6 June 2019.