Regarding aviation: worldwide and in the EU, transportation accounts for about 25% of energy consumption [1].

Aviation accounts for 10-13% of transportation [2]. (global data looks similar, although I don't have a link at hand)

So while 2.5% is not nothing, it is only a small fraction of energy consumption and could remain Kerosene based in the medium term.

Also, a lot of flights could theoretically be shifted to high speed rail.

[1] https://en.wikipedia.org/wiki/World_energy_consumption#By_se...

[2] (2016) https://www.eea.europa.eu/data-and-maps/indicators/transport...

If we can use electricity to turn atmospheric CO2 back into jet fuel it could still be net zero emissions. At that point the planes are just using hydrocarbons as a means of dense energy storage.

Technologically-speaking, been done; the economics aren’t quite there yet, but there are some promising noncarbonization paths that potentially could scale to price-parity, depending on global fuel costs.

Going into the future, I think the assumption that one needs to hit exact parity with existing fossil fuels at their current price point will be re-examined. The economic comparison should be conventional extraction fossil fuels + externality costs vs cost of a scaled carbon neutral jet fuel production.

One could already see maybe small scale operations capturing niches - maybe if a corporation that operates a private jet wants to further a reduction in their carbon footprint.

Completely agree that the best and most market-friendly path is to take socialized CO2 costs and apply them to point-of-emission prices — though I doubt entrenched interests of the iron triangle variety will allow that to happen. Nonetheless, if we assume Johnson and Hope (2012)’s range of $55-$266 social cost per ton of CO2, even current capture-and-sequester methods look price-competitive, which is encouraging!

Politically, I wonder if the best solution is the approach the other end of the development cycle, with dynamically scaling investment incentives that expand your "credit limit" to lend more if your projects successfully reduce carbon, but overall scale back as total carbon emissions reduce. Sort of a Green New Deal Fannie Mae type entity. Add in incentives to retire stranded fossil fuel centric assets like fracking rights, etc. This creates a psychologically different environment, more like a Fear of Missing Out on the development rush instead of a Fear of getting a Big Stick regulation.

Sounds like an approach that would coexist nicely alongside technology funding efforts! ARPA-E and EERE have done some great work in that regard; thankfully and a bit surprisingly Congress has so far declined to defund them, despite the Trump administration’s consistent efforts to shut them down, so hopefully they’ll continue to support innovative and incremental advances. Adding carbon incentives to a tax-and-rebate scenario (should we ever get to that) would seem prudent.

I not intimately familiar with ARPA-E and EERE funding programs, but if the follow the common model of selection of funding to a expert tech evaluators, they are a bit different from what I was musing about. They aren't bad, just far too constrained for what we need.

Instead, I'm for transferring funding decisions under the program to a much more open set of actors to originate investment loans (much like mortgage brokers). The investment performances are evaluated on their successes/feasibility, the loans purchased from their originators with high feasibility (i.e. we're scaling known limits), as well as opening up higher origination limits (if they prove out better carbon negative/netural tech). It's more of a financial tooling solution than a tech centered solution. This increases the possibility of failures, but provides a regulated scale out needed with a optimization criteria to keep scaling along successful lines with more organizations rewarded who make successful carbon investments. Experts like ARPA-E could definitely be involved in assessment, but the scale would be completely different then what has gone on before.

The economics are extremely attractive if you have an abundance of electricity production at unpredictable times with build electricity rates tending negative occasionally. Turn that excess electricity into kerosene for jets and ocean travel and turn a profit!

This is discussed in this headline article on carbon capture developments on the BBC today. https://www.bbc.com/news/science-environment-47638586

Check out carbon engineering[1]. They are using direct air capture and hydrogen to create synthetic fuels using a fischer-tropsch like process.

https://www.carbonengineering.com/

Could we power airplanes with ethanol? I've heard that we couldn't realistically grown enough corn/canes to produce ethanol for all cars, but if cars are electric, could we potentially produce enough ethanol for airplanes?

If we can't make enough ethanol for all air travel, I'm still optimistic. It's likely that battery densities will become good enough for us to have electrically powered regional jets within the next decade. Then only long flights would need liquid fuels. As a last option, we could also switch to high-speed trains for continental travel.

Piston-engined aircraft, which are basically just light general aviation aircraft, can and have used ethanol.

Most commercial aircraft use jet engines. Other biofuels that are chemically closer to kerosene can be used in most existing jet engines in blends; use of 100% biofuels for hasn't been extensively tested-- but it has been tested![0] This is also the highest-hanging fruit and I wouldn't worry about it in 2019.

[0]https://www.navy.mil/submit/display.asp?story_id=96702

Turbine engines in theory can run on almost any flammable liquid. However in practice they would need substantial re-engineering. Also ethanol is less energy-dense than kerosene/jet fuel.

Have high-power turbocharged piston engines been run on ethanol in service? I’m not aware of any. GA piston fleet has the common property where 20% of the fleet burns 70+% of the fuel as they are working airplanes. (Think CapeAir and similar in passenger service.) Those highly stressed turbocharged engines have lower detonation margins, currently requiring tetraethyl lead additive to meet worst case detonation margins.

The US DOD has a project which qualifies what is essentially biodiesel (bio-derived JP8) for use in jet aircraft.

There's not a lot of work to do, just the details of making a mil-spec standard and then doing the testing in each engine type. In other words, everyday blocking and tackling not scientific breakthroughs.

A lot of companies are working on electric planes for short haul flights:

https://techcrunch.com/2018/07/08/the-electric-aircraft-is-t...

And what fraction of aviation emissions are short haul? My hunch is not much. Factor of 50x in energy density really matters in the air.

It’s closer to 19x when you consider engine efficiency. Heat is not that useful on it’s own.

Considering jets already do 9,500 mile flights a 500 mile range seems viable. But, if you start talking significant amounts of electric aircraft, very short landings for battery swapping are also possible. That's not going to get you to Hawaii, but it's still faster than taking a high speed train.

PS: You can also push these numbers as electricity is much cheaper than jet fuel. Electric engines also weigh less than batters which allows for a higher mass fraction for fuel. Shorter flights also have lower penalties for going slower. So, an optimized electric aircraft could probably make 800 miles with similar costs to current jets without any major breakthroughs.

Yes, and it could be viable for short flights with mountains between, where it's hard to build high speed rail, like flights LA<->Las Vegas.

Ops: Electric engines also weigh less than jet engines which allows for a higher mass fraction for batteries.

Really? I thought turbines were really excellent in terms of power to weight?

Currently you can get 10kw/kg turbines and 10kw/kg electric motors so they are close.

However, the turbines require more infrastructure around the turbine. Put another way, simply comparing fuel vs batteries ignores the weight of fuel tanks, fuel pumps, etc. We care about the system not just individual components. That said, the fact that burned fuel reduces aircraft weight is a huge advantage over batteries.

Other than being super explosive would liquid hydrogen work for an airplane?

Even if the safety issues could be resolved, the cooling requirements and high volume make liquid hydrogen impractical for commercial aviation.

The most likely carbon neutral solutions for aviation fuel are either algae produced bio fuels, or synthetic hydrocarbons manufactured using renewable energy.

*Manufacturer using low-carbon energy.

At the moment, biofuels are renewable but high-carbon and nuclear is low-carbon but not (considered) renewable.

I know nothing about the topic, but there seems to be a nice Wikipedia article that your question prompted me to find: https://en.wikipedia.org/wiki/Hydrogen-powered_aircraft

I'm suddenly imagining a hydrogen balloon-plane at take off that uses up the hydrogen in the balloon as it flies, until the balloon has been fully reeled back into the plane and it lands on a runway like a normal plane, with reserve hydrogen in its wing-tanks.

If you time it right, it just slowly drifts to the ground and lands vertically with a slight jolt.

Depends on your definition of aeroplane: https://en.wikipedia.org/wiki/Skylon_%28spacecraft%29#SABRE_...