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There seem to be several options for aircraft R&D regarding cleaner fuels for the time when fossil fuels are no longer economically viable.

Some of these include:

  • Algae. A drop-in fuel. These fuels are being used and developed in co-operation with the USAF. They have had several successful test flights.
  • Batteries. Doesn't seem viable at this stage.
  • Nuclear reactor on board. Doesn't sound safe.
  • Hydrogen. This wiki page says they could be built by 2020. That's only a year away now...

I've found a few examples of test prototypes & flights using fuel mixtures that improve emissions and gasoline consumption, but are there any commercial aircraft in development, with an expected launch date that utilize 100% renewable energy?

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    $\begingroup$ Please do not confuse energy sources with energy storage. Rechargeable batteries and hydrogen are storing energy. Charging batteries or producing hydrogen with electric power that is generated from fossil fuels to replace fossil fuel is counterproductive. $\endgroup$ – bogl Oct 30 '18 at 13:43
  • $\begingroup$ zunum.aero $\endgroup$ – abelenky Oct 30 '18 at 14:02
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    $\begingroup$ I wouldn't call a nuclear reactor a renewable energy source... $\endgroup$ – Bianfable Oct 30 '18 at 15:04
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    $\begingroup$ @Bianfable: Certainly a nuclear reactor can be renewable. See "breeder reactor" for details. Now hydrogen, OTOH, is (in current implementations) about as far from renewable as you can get. $\endgroup$ – jamesqf Oct 30 '18 at 18:18
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    $\begingroup$ Not commercial, but I'm surprised no one's mentioned the Solar Impulse in the comments yet. It's 100% solar, has circumnavigated the world, and ...sounds pretty miserable to fly. theguardian.com/environment/gallery/2016/jun/23/… $\endgroup$ – esmail Nov 2 '18 at 14:33
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Contrary to one clause of your question, there are a number of battery-electric light aircraft (or what the USA calls "light sport" class) in development, with prototypes flying. This includes both fixed-wing conventional types with single or multiple electric powered propellers, and "drone" style craft using what amounts to thrust hover with electric powered propellers. Flight time is limited, but better with the fixed wing types -- enough so that I'd call an electric motor glider a very feasible design.

Hydrogen has limited usefulness, for the same reasons it doesn't work well for cars: it's technologically difficult to store enough for reasonable range. Either extreme pressure, extreme cold, or a chemical storage method that limits the rate at which you can draw fuel are reqired.

A submarine type nuclear reactor was operated aboard a modified B36 in the 1950s -- the project, intended to lead to a strategic bomber with a flight time of weeks or months, was dropped due to lack of need as well as cost and safety concerns (most of the safety concerns, such as "roll-up" explosions, were later resolved or found to be overblown). This kind of power isn't "fully renewable" anyway -- fission fuel must be mined, it isn't grown.

Algae fuels, as drop-in replacements for kerosene, may have some usefulness, but there are storage issues (hard to stockpile something that can mold or rot) -- as you note, however, testing is in progress, and we'll know in a few years.

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Oct 31 '18 at 6:54
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    $\begingroup$ Algae fuel is basicly vegetable oil, could be mixed with diesel for storage. $\endgroup$ – Robert DiGiovanni Nov 3 '18 at 3:54
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Yes, there are projects to power the jet engines by oil from plants: canola (rapeseed), coconuts and soybeans. This fuel is also cleaner (less sulfur) and even freezing point (-47 degrees C) is acceptable.

While not an immediate certified replacement, these oils can work as jet fuel without radically re-designing the engine. Some refining technology and suitable engines are under development.

Planes with piston diesel engines could also probably run on this fuel.

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Batteries are indeed more feasible then you give them credit for. For short hop flights they are actually within reason:

https://arstechnica.com/tech-policy/2018/10/scotlands-orkney-islands-may-see-electric-plane-service-by-2021/

...the added benefit of electrifying the Orkney Islands flights is that there's an abundance of renewable energy, especially wind energy, already on the grid in the area.

Though replacing the jet fuel burned by small island-hopping planes in a remote part of the world may seem like a drop in the bucket in efforts to reduce emissions, starting small is often a path to thinking big. If the shortest flights in the world can be flown successfully, gradually longer and longer commercial electric flights may someday be attempted.

Norway is also setting a goal of entirely electric-powered short-haul flights by 2040: http://www.bbc.com/future/story/20180814-norways-plan-for-a-fleet-of-electric-planes

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