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The Future of Flight – Three energy futures (Electric, Hydrogen or just less?)

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John Armstrong's picture
Chief Operating Officer BPA

John Armstrong is an engineer whose career has spanned the extremes of the energy industry – giving him a front-row seat on the energy roller-coaster. He began his career constructing oil...

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  • Jan 15, 2020
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In my blog on the future of energy I made a very bold statement about he future of flight being cut by 50% by 2030. A lot of comments on the article informed me I was probably wrong – and this got me thinking. With my other predictions there is a reasonably clear de-carbonisation pathway i.e. heat moving to heat pumps and automotive to electric batteries (with some hydrogen for freight). Air travel just doesn’t seem so clear – right now there is no readily available and technically proven option for air travel.

Every minute 84 flights take off somewhere in the world with over 4 Billion journeys being made by plane. Burning nearly 300 million tonnes of Jet Fuel annually making up 2.7 percent of global CO2 emissions and global air travel is expected to double over the next two decades! (so quite a big gap from my prediction of a reduction of 50%!) This isn't a problem that is just going to go away!

Recently I’ve been really excited to see small electric planes taking off such as Alice. Using battery technology taken from automotive these planes have been able to travel reasonable distances on one charge – carrying a couple of a passengers. There have also been some pretty bold statements about having electrically powered large aircraft by the end of the decade from the likes of easyjet. These innovation steps however are nowhere near to decarbonising a long (or even a short) haul flight.

The fundamental challenge of decarbonising flight is the energy density of the storage… The physics of batteries seem to work for smaller applications… but not necessarily for bigger aircraft. Currently Lithium Iron batteries can store around 250Whr per Kg which is 30 times less dense than Jet Fuel. So the weight of the batteries ends up limiting the ability of larger planes even to get themselves off the ground – never mind carry a payload. Research has suggested that for battery powered aircraft to work the energy density would need to be nearer to 800WHr/Kg so nearly triple that of best available technology today. At the current rate of technology improvement this kind of energy density isn’t going to be available till well after 2050.

Busy Airport

Even if the improved energy density could be achieved a huge challenge still remains – that of charging large airliners on the ground. To put it into context a 747 needs around 40 to 50 MW of power on take-off. A 747 currently has a turnaround time of 150 minutes and a 737 for the likes of Ryanair 27 minutes! The infrastructure required to be able to charge such a large battery so quickly would be quite incredible (never mind charging 1300 flights a day at Heathrow!). Even if the energy density challenge can be resolved there is still a long way to go to be able to charge the aircraft once its on the ground!

Looking to next decade what does the above mean for air travel?

Smaller electric airplanes will have a role to play

  1. The development of smaller emissions electric air-planes may mean we see an explosion of smaller regional airports with pilot less air taxis. The shorter available distances will mean that short hop aviation may become a real thing. Also smaller planes will be able to fly at lower altitudes avoiding congestion and with weight becoming critical in range removing the pilot starts to make sense – accelerating autonomous flying to enhance the economic case.

charging 51000 cars at heathrow.

  1. Electricity demand at transport hubs will become an increasing problem. With an increase in electric vehicles the demand on local electricity infrastructure will increase – Heathrow airport for example has 51,000 car parking spaces! Just charging that many vehicles will take a huge re-enforcement of electricity provision. Assuming slow chargers (3KW) and about 10% of vehicles on charge you'd need a 15MW connection just for the vehicles! – never mind if you wanted to start charging giant airliners (in quick turn around times!) as well. With smaller electric planes there will be no need to use giant hub infrastructure like now – operators will be able to move to cheaper smaller airports where charging provision is more easily provided (from local solar for example).
  2. For larger aircraft alternative fuels such as Hydrogen with higher energy densities may make more sense than electrification. The energy density of these fuels will make a switch over more logical.the development of regional Hydrogen centres presents opportunities globally however as of today no one is commercially flying on Hydrogen so there is still a long way to go.

In conclusion with current technology the only way to reduce carbon emissions is to fly less. There just isn’t a clear enough pathway to lower or zero) carbon flight. Smaller electric planes may supplement existing routes – but without significant regulatory intervention they won’t replace our existing hydrocarbon hungry fleet any time soon!

Photos by Skyler Smith and  Josue Isai Ramos Figuero on Unsplash

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Matt Chester's picture
Matt Chester on Jan 15, 2020

An important topic in the decarbonization world-- thanks for sharing John. Often air travel is given a bit of an afterthought in terms of removing emissions, likely because the technology does not appear there, but when any organization looks at its carbon footprint the air miles are anything but a footnote-- they're a HUGE driver. 

In conclusion with current technology the only way to reduce carbon emissions is to fly less. There just isn’t a clear enough pathway to lower or zero) carbon flight. Smaller electric planes may supplement existing routes – but without significant regulatory intervention they won’t replace our existing hydrocarbon hungry fleet any time soon!

It seems hard to believe that the flying less route will take hold anywhere near in the numbers that would be required-- but what do you think the likelihood that regulatory intervention (funding, incentives, etc.) comes into play sooner than later to make a dent here? 

John Armstrong's picture
John Armstrong on Jan 15, 2020

Its interesting on reduced flying. A number of countries are talking about penalising frequent flyers and flight shaming in Sweden has already taken a 4% chunk off flights this year alone. 

If people had personal carbon budgets I do think flights would be the first thing to go in favour of other carbon 'luxuries'.

Matt Chester's picture
Matt Chester on Jan 15, 2020

Interesting-- I hadn't heard of penalizing frequent flyers. That said, I think people are wont to talk a big game about being green and reducing personal carbon budgets until it becomes a real inconvenience to their life-- and flying is the biggest example of that. I know there is definitely a portion of the population that is reducing for climate reasons, but I think there's a limit to how effective that will be (until, at least, there are high speed rail or similar options that can replace them). People are unwilling to sacrifice their level of living, which is why the cleaner fuels for flying are so critical. 

John Armstrong's picture
John Armstrong on Jan 16, 2020

In the UK 15% of the people do 70% of the flights! Two political parties are pushing for a frequent flyer tax.

i do think some kind of   personal carbon. I'd get would help.

Bob Meinetz's picture
Bob Meinetz on Jan 15, 2020

John, aviation is only responsible for 2% of global GHG emissions, where electricity and terrestrial transportation are each responsible for emissions an order of magnitude higher.

Moreover, there remains no economical way to manufacture "green" hydrogen at scale, and due to its dependence on weather/time of day, establishing a reliable supply chain would be impossible.

Given these challenges, maybe working to clean air travel shouldn't be a top priority.

John Armstrong's picture
John Armstrong on Jan 15, 2020

Bob

I think this is a great point... also the economic impact of reducing air travel may be huge. 

I do think however that carbon reduction is something thats a bit like eating an elephant... one bite at a time! So you do need to tackly it on all fronts.

John

Bob Meinetz's picture
Bob Meinetz on Jan 15, 2020

True. Container shipping, which generates 50% more GHGs than aviation, is another target. The International Windship Association, which

"facilitates and promotes wind propulsion for commercial shipping worldwide and brings together all parties in the development of a wind-ship sector to shape industry and government attitudes and policies..."

http://wind-ship.org/en/grid-homepage/

believes modern container ships with computer-controlled sails could actually be cheaper and faster than the bunker-oil burning variety.

The biggest challenge for hydrogen-powered aviation may be one of public perception.

John Armstrong's picture
John Armstrong on Jan 15, 2020

Great point! There is a company in the uk which is building massive air ships!

Nathan Wilson's picture
Nathan Wilson on Jan 16, 2020

When we say "[carbon-free aviation] fuels such as hydrogen", it's worth pointing out that ammonia is the other carbon-free fuel.

Ammonia (NH3) is in many ways, a better aviation fuel than H2.  It's got 67% higher volumetric energy density than liquid hydrogen, 5 times higher density than gaseous hydrogen, and doesn't require storage at near absolute zero or in high pressure tanks.

Ammonia is much less explosive than hydrogen, and accidents like the recent case in which the airliner dumped jet fuel on Los Angeles school kids can't happen, since ammonia is lighter than air.

The main downside of ammonia as an aviation fuel is that its gravimetric energy density is only 43% that of jet fuel.  That will make trans-continental flights on ammonia impossible, but since the majority of flights are part of the hub-and-spoke system (i.e. under 1500 miles), ammonia can do the heavy-lifting.

Like H2, ammonia can be made from any energy source: from fossil fuel with CC&S, or by appropriately combining electricity, water, and air. 

Because ammonia is more easily stored than H2, it is also more easily transported by truck.  This means that ammonia refueling station are easier to establish, since H2 requires deployment of a pipeline network (although the US does already have 3000 miles of ammonia pipeline, for distribution of ammonia for use as fertilizer).

I should mention that unlike the demo-flights flown by the battery powered prototypes, all commercial flights operate under FAA rules that require a 45 minute fuel reserve.  And unlike fuel tanks which get lighter as their energy is depleted (making the plane grow in energy efficiency), batteries maintain their full weight for the entire flight.  Battery powered commercial flights are still a long ways off.

Other than the X-15 rocket plane which flew on ammonia fuel, I don't think there has been much ammonia aviation work.    But there has been work on ammonia fueled turbine engines for small power plants.  This is viable technology; the only thing missing is market demand (i.e. strong CO2 policy).

see:

https://nh3fuelassociation.org/events-conferences/

X-15 rocket plane in flight

Bob Meinetz's picture
Bob Meinetz on Jan 17, 2020

Nathan, in past years on the Energy Collective I've been critical of your advocacy for ammonia. But compared to hydrogen, ammonia is a no-brainer.
Why it hasn't taken off, no doubt, is in large part because it's isn't made from fossil fuel. In smalller part, because it's mildly toxic, and even smaller, because it smells.
Maybe it's an acquired scent, one to which we will get accustomed the more it's adopted: "The Smell of Zero Carbon Emissions and Clean Floors."

Matt Chester's picture
Matt Chester on Jan 16, 2020

I wonder what people thought of the smell of gasoline back when it was first introduced as a fuel. I drive an EV, but I admit a part of me misses the smell of stopping at the gas station!

John Armstrong's picture
John Armstrong on Jan 16, 2020

Thats certainly food for thought. I just can't see a world where we handle ammonia in large volumes at airports (there is a lot to go wrong there!)... I have expereince of ammonia in heat pumps and it takes some careful handling.

I do take your point though about rhe challenges of compressed or liquified Hydrigen.

Fantastic to debate though (and you've given me food for thought!)

This is the kind of conversation we need to be having about energy... open and interesting! 

 

 

 

Nathan Wilson's picture
Nathan Wilson on Jan 17, 2020

I think there is some faulty public perception of the ammonia safety issue.  I remember seeing a safety study a while ago comparing various fuels.  Ammonia, hydrogen, and gasoline had roughly equivalent safety (ammonia's toxicity is offset by its very low explosion risk).  The worst of the bunch was actually LPG (similar to propane), because it is explosive, and the vapors are heavier than air, thus they linger near the ground, which increases the odds they will find an ignition source.  

That said, I did stop advocating for ammonia powered cars years ago.  But I think the need for special handling is more of a concern for consumer use than for professionals.

To Bob's point about the smell: I guess there is a whole generation of people who are too young to recognize ammonia as the smell of blue-print drawings. sigh.

Roger Arnold's picture
Roger Arnold on Jan 23, 2020

In conclusion with current technology the only way to reduce carbon emissions is to fly less. There just isn’t a clear enough pathway to lower or zero) carbon flight.

You left out the main alternative that both NASA and the big airline companies are pursuing: hybrid electric flight. It's by no means a zero-carbon option, but NASA believes it has the potential for a 50% reduction in fuel consumption per passenger-mile.

The reasons for expectations of enhanced efficiency from electric propulsion can be summed up with the phrase "boundary layer control". That won't mean anything to most readers, and I'm certainly not going to try to explain it here. But the bottom line is a better overall lift to drag ratio for the aircraft. The engines don't have to work as hard to maintain flight. A side benefit is that takeoffs and landings will be much quieter. The planes should be able to use shorter runways at airports closer in to the cities they serve.

The engine that NASA is assuming for their design studies is a modified helicopter combustion turbine engine with a high power-to-weight ratio.  Of course, being NASA, they're also looking at a superconducting generator and superconducting cables to distribute power from the turbine-generator to the dozen or more electric fanjets distributed along the wing. (At least that's one concept under investigation). Including superconductors pushes the timeframe for deployment out a bit, but apparently not as far as one might expect. High temperature superconducting motors and generators have been coming along. A 3.6 MW wind turbine with superconducting generator has undergone a successful field test in Denmark.

If a successful hybrid electric airliner can be rolled out, it would be a small step to fuel the turbine with carbon neutral biofuels, or with ammonia. Hydrogen is a remote possibility, but not too likely. It would be hard to carry enough of it for long duration flights.

Matt Chester's picture
Matt Chester on Jan 23, 2020

The reasons for expectations of enhanced efficiency from electric propulsion can be summed up with the phrase "boundary layer control". That won't mean anything to most readers, and I'm certainly not going to try to explain it here. But the bottom line is a better overall lift to drag ratio for the aircraft. The engines don't have to work as hard to maintain flight. A side benefit is that takeoffs and landings will be much quieter. The planes should be able to use shorter runways at airports closer in to the cities they serve.

Respect that these comments probably won't be sufficient to explain boundary layer control, but do you have a link to a good resource to read up on it for those of us who are interested? And in what ballpark of efficiency increases does this strategy offer?

Bob Meinetz's picture
Bob Meinetz on Jan 23, 2020

Matt, I wasn't aware of it either, but Wikipedia is always a good place to start:

https://en.wikipedia.org/wiki/Boundary_layer_control

A technical paper to which the article refers concludes:

"The previous review of the state of the art reveals that the understanding of the physics governing drag reduction technologies is still limited. Furthermore, the application of active control skin friction drag reduction has seen limited success in the industrial environment."

John Armstrong's picture
John Armstrong on May 1, 2020

these points are great.... as with any decarbonisation looking to use less up front makes a lot of sense. 

Electrification is still reserved to smaller aircraft right now with reasearch centred on six seaters!

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