Wednesday, October 21, 2020

Electric aviation

Over the past few years, we've seen a flood of drones - almost all electrically powered - take flight in our skies, making up the vast majority of electric flight today.  But we're also starting to see more and more manned electric aviation in our skies, such as the recent test flight of Ampaire's 6-seat modified 337, Bye Aerospace's forthcoming 2- and 4-seat all-electric trainers, and the recently certified (in Europe) Pipistrel Velis Electro.  I think there's an interesting trend here that is well worth watching, and I'd like to weigh in here with some of my thoughts.

At a high level, I think it's worth stating up front: these are early days, there are significant challenges to solve, and fleet turnover is a very very slow process.  While I do think it's very possible we could see an electric equivalent to the Cessna 172 within a decade, I suspect that an electric transport aircraft (i.e., more than 30 people, more than 1,000nm range) is unlikely within even 20 years.  But 30?  There I'm not so sure.

I'm an amateur in this space, though, but I think I'm qualified to weigh in here nevertheless.  I am a (piston) pilot myself, so I know a little about aviation. I have one degree in physics and two in electrical engineering, so I'm something of a geek on the technology side.  I've spent the last 14 years as an "angel investor" in early stage companies, so I have at least a bit of experience trying to spot technology trends.  And I've been driving electric for almost 8 years ago, so I've experienced the liquid-fuel-to-electrons transition directly, learning a few things in the process.  

As the saying goes, though: It's hard to make predictions, especially about the future.  So below I'm going make observations based on the trends I see, with some extrapolation, rather than making outright predictions.

Why Electric Aviation?

Why is electric aviation ("EA") even interesting to consider, when existing fossil/liquid fuel based aviation (I'll refer to this as "LFA") works pretty darned well?  There are many reasons.

But first, a disclaimer: for the sake of this discussion - I'm going to make a simplifying assumption that the environmental impact of both LFA and Electric Aviation (EA) are both identically zero.  Why?  Three reasons: (a) I don't want to get into a policy/political discussion; I think the technology/economic trends are fascinating enough, (b) you don't need environmental arguments for EA to be an attractive proposition, and (c) EA is still not real enough that any statements about its environmental impact pro or con are meaningful.  Perhaps in a later post I can talk a bit about that, but for this post I will not make any argument involving emissions/environment/etc., nor about any policy to try to push EA over LFA: I'm going to assume here that policy is frozen as it is today, with the small exception of fixing obviously broken things like requiring an oil gauge for each engine in order to be more technology agnostic.

Where was I?  Oh, right: what benefits does EA offer over LFA?  Many, I think (obviously assuming that the technological and economic issues can be addressed).

The first is reliability. Electric motors have few moving parts, and very few failure modes.  They deliver torque across a wide RPM range.  You can't over or under lean; the engine doesn't need air to operate so it can deliver full power at any altitude. There is no fuel contamination, no detonation or pre-ignition, and the "TBO" is generally...well, forever.  (OK, my lack of turbine expertise is clearly showing through here; but I am comfortable asserting that electric engines are much simpler.)

Related to reliability is safety, but don't think I can make a definitive call either way on that.  Batteries have obviously had issues with thermal runaway, but equally obviously all liquid fuels are by definition highly flammable.  New battery designs have improved safety, but its hard to say whether future battery technologies will be safer - or will have different dangers to be aware of.  Something to watch...

A related advantage of EA is maintainability.  With so few moving parts, there just isn't a lot to inspect or maintain: no belts to break, hoses to burst, no oil to check/change/drain, no starter motor, no fuel filters or air filters, etc. Electric motors are lightweight and can allow for more aerodynamic designs (for example, you can for a single-engine propeller airplane you can have a narrow cowling because you don't need airflow through the engine; this also allows more of the propeller's lift to become thrust) and the "fuel" has no weight at all - you can always apply all of your useful-load to payload, take-off with "full-tanks" even with full load, and you have no fuel-related center-of-gravity issues to manage in-flight.

Electric motors are also significantly more efficient. Your car's internal combustion engine turns only about 20-35% of the gasoline's chemical engineering into forward motion, a jet engine has higher thermodynamic efficiency, turning about 55% of the fuel's energy into useful work; electric engines, on the other hand, are commonly north of 85-90% efficiency.  These latter numbers are energy-in/energy-out, so net propulsive efficiency (= energy-in / kinetic-energy of the aircraft that results) is a bit lower due to propulsion inefficiencies, but here I'm just comparing the power plant.

Electric engines have low operating costs.  This obviously varies on a lot of factors, but a joule of energy delivered electrically is often significantly cheaper than a joule of energy in liquid form.  When coupled with low maintenance costs, the impact on cost-per-mile can be significant.  By analogy, my experience with my EV has been that it's about $0.03/mile (I go about 3.3 miles on a kWh, which costs me about $0.10 where I live), whereas my previous car, a 25-mpg Audi (a lighter car, believe it or not), would cost $0.08/mile at $2/gallon (a price I think I never saw while I owned it), nearly 3x as much.

Electric aircraft are quieter.  Sure, a propeller makes noise, but take the engine out of the equation and the noise level can drop considerably.  Switch from one or two high-RPM propellers to more low-RPM propellers (see below), and the noise level drops significantly more.

But perhaps the biggest advantage of electric aviation is the most subtle: the engine is abstracted from its power source.  In other words, it doesn't know or care from where its energy source is derived.  All conventional LFA engines are intimately connected with their fuel source.  (Try putting Jet-A into your piston aircraft and let me know how that works out for you!) A huge part of the engineering of LFA engines is all about getting a very specific liquid fuel to the right place at the right time with the right mixture with the air and only then converting it to energy from which to produce thrust.  Think of carburetors and fuel injectors, air filters and ducts, mixture controls, spark plugs, valves, exhaust systems (again I'm displaying my piston familiarity and turbine ignorance here...), all of which makes the LFA engine intimate with its energy source.  

None of that is true in an EA engine.  The wires are carrying electric current.  It simply doesn't matter if the source of that energy is dead dinosaurs (yeah, I know it's probably plant-based, but that's a less interesting metaphor),  nuclear, solar, or humans-in-vats a la "The Matrix".  

That abstraction carries at least five very under-appreciated but significant benefits:

  • The engine is simpler, more reliable, more maintainable, higher efficiency, etc. precisely because it can throw out all of the stuff related to the liquid fuel and work with electrical energy in a much simpler manner.  
  • There's no notion of having the right grade of fuel.  (Look at how difficult it is to come up with a drop-in replacement for 100LL, for example!)  Electrons are electrons.
  • Because the engine is simpler/lighter, and wires are much easier to route than fuel and exhaust, non-traditional designs that improve performance and safety become more feasible.  For example, instead of just one or two big engines, you could have 10 small engines.  An engine failure in such a design is effectively a non-event.  You could even imagine designs where you - the pilot - could snap-in or remove engines on the fly (no pun intended), removing engines for efficiency in a lightly loaded flight and adding more when you need more thrust, perhaps allowing you to trade of range and useful load.
  • It enables software (rather than hardware) control, which means updates and improvements are easy and inexpensive to deploy widely.  (Over-the-air updates is one of my favorite features of my EV, by the way - and many of the enhancements simply could not be done with internal combustion).
  • Most importantly, it means that the engine - and thus the aircraft - can enjoy any improvements to the energy source.  So if you buy an electric airplane in year 1 and in year 6 a new battery technology is available that is lighter, cheaper, and higher capacity, then your aircraft can suddenly have longer range and more useful load.
Of course, allowing the engine to be ignorant of its power source just pushes that problem to the power source itself.  Which makes a great segue to...

Technology trends

With all of the above, why does LFA continue to dominate?  Two words account for almost the entire reason: energy density.  There are a few other factors I'll discuss, but this is the big one.

It's actually the exact same reason that gasoline/diesel cars continue to dominate automotive transportation.  For all of the many advantages of electric engines, having a dense power source is critical, and today, liquid fuels provide that.  This is changing, more rapidly on the ground than in the air, but the transition in ground transportation is driving innovations that will benefit EA.

First let me distinguish two kinds of energy density, both of which are important.

  • Volumetric Density is a measure of how much energy you can store per unit volume.
  • Specific Energy or Gravimetric Density is a measure of how energy you can store per unit of mass (weight).

Obviously, for aviation, both space and weight are at a premium, so you want a very high volumetric density, in order to pack as much energy as possible into whatever space you have, and a very high specific energy, to carry as much energy as possible for every bit of weight you can carry.

Batteries in 2020 have improved - dramatically - over the past 20 years, to the point that cars with more than 300 miles of useful range are economically viable.  But aviation is an energy-intensive endeavor.  How do current batteries stack up against liquid fuels?


Gravimetric Density
Volumetric Density
100LL44 MJ/kg
31.6 MJ/L
Jet Fuel43 MJ/kg
35 MJ/L
"Standard" Lithium-Ion
(actual numbers vary
based on technology)
~750 KJ/kg
~1.2 MJ/L

Wow.  So liquid fuels are about 58x more dense on a weight basis, and 25-30x more dense on a volumetric basis.  Those are truly daunting ratios.  For EA to compete head-to-head with LFA, it needs to make up a deficit of almost 60x.  Game over?

No.  And here's where I'm going to do some back-of-the-envelope math and extrapolate various trends.  I'm going to switch now to rough numbers, since we're talking trends, various technologies, and long timelines.

The first thing I'd observe is that significantly more energy is wasted as heat with liquid fuels than in an electric system.  Going electric means going from 30-50% efficiency to potentially over 90%.  It's not quite a 2x improvement, but it's close. So for the sake of rough math, let's say that the hurdle for EA to overcome comes down from roughly 60x to roughly 30x.  Still an order of magnitude.

Where is battery technology headed over the coming years?  The first place to look is to extrapolate trends to date.  They're encouraging, having more or less tripled over the last 10 years:

Of course, as they say "past performance is no guarantee of future performance": battery advances are dealing with difficult physics and chemistry, and each advance is hard fought and hard won, but the incredible growth of solar and EV's have produced a tailwind of R&D here that does not show signs of abating any time soon.

It's hard to say what that means for our 30x multiple: the first 3x is certainly easier than the next 3x.  But there are encouraging signs that there may be some step functions ahead of us.

The picture below is from a webinar in which I was a speaker in a few months ago for my angel investing group, showing various technologies on the horizon.  Current commercial Lithium-Ion Batteries (LIB) are the dark blue.  (For comparison, lead-acid, which is in your car and in your aircraft's starter motor, is the red dot in the lower left corner; the aqua oval is Nickel Metal Hydride).

All of these are shown as rough regions rather than precise dots because within each technology there are a variety of technology choices that can be made, and in the case of just about everything to the right of LIB, these are technologies that are available in a lab environment but not not yet ready for widespread commercialization.  Chemistries like Lithium-Sulfur and Lithium-Air (Li/O2) are on the horizon and can increase gravimetric density by approximately a factor of 4-5.  This alone would reduce the hurdle from 30x to about 6x.  And given the optimizations and improvements that inevitably happen once a technology leaps from the lab to being commercialized, I would be surprised to see us hit a hard wall.  

6x may sound like it's still a big gap - and it is.  It is not remotely adequate for long-haul flights, for example.  But it's also no longer a full order-of-magnitude problem; at that point, EA and LFA can find themselves in the same ballpark. And given the opportunities to play with the aerodynamics of the aircraft themselves, fine-tune L/D ratios without being constrained by one- or two- big engines, and adjust operating speeds for efficiency, it's more than sufficient for a huge range of shorter-range flights.  Boston-New York, London-Paris, or Miami-Orlando runs are quite good examples of routes flown today by 737-class aircraft but which do not need the range that aircraft provides, and which are short enough that slowing down by 25-40% wouldn't appreciably impact the value proposition of flying vs. other modes of transportation.  And as 6x becomes 5x and smaller, the set of flights for which this is interesting simply grows monotonically.

Volumetric densities don't show quite the same sort of potential increase - perhaps 2-4x improvement.  But batteries have geometric advantages over liquid fuels.  For one, since wires are easier to distribute than fuel lines, you can distribute them more broadly throughout the aircraft, which can actually help with things like center-of-gravity design.  Another advantage is that you don't have to worry about gravity/low-points, pumps, and the like.  So if the gravimetric density is high enough, you can get creative about the volumetric challenge in ways that you simply can't with liquid fuels.

The net result is that advances in batteries alone could achieve energy densities sufficient for a significant number of real-world mission profiles over the next decade or two.  

But of course, there's no requirement that energy storage be entirely in batteries.  Remember the value of abstraction of the energy source from the engine?  This means that you can get the benefits of electric propulsion with other sources of energy.  For example, low-temperature fuel-cells can provide electricity using highly energy-dense chemical (liquid) fuels, virtually eliminating any energy density issue.  Or you could pair electric propulsion with some sort of combustion (turbine or piston) power generation; this is how many modern trains work.  More fancifully as a thought experiment, if someday the Mr. Fusion from "Back to the Future" were to become reality, you could swap that in and fly indefinitely.  The bigger and more interesting point here is that going electric means you can mix-and-match to fit your mission profile and to take advantage of whatever technology advances arise on the energy source/storage side of the equation without having to completely redesign your propulsion system.

Enough about energy density, there are a few other technology challenges for electric aviation worth discussing.

One, of course, is charging.  One advantage of LFA is that you can turn around very quickly after a flight.  While fast-charging technology exists for batteries today, it is still slower than filling a tank with a liquid, and is harder on the battery itself than a slower charging rate; these are both areas of active research.  There is already one solution, though, to electric "fast charging:" battery swapping, where you keep a cache of fully-charged batteries, and when a plane lands you literally remove the depleted battery and replace it with a fully-charged one, which you then recharge at your leisure.  There is no technology challenge to this at all; rather, it presents a logistical and economic challenge.

I should note, though, that even without battery swapping EA can actually have an advantage in charging over LFA: batteries can charge unattended.  So while slow charging is hugely limiting if the mission requires fast turnarounds, it is generally not a problem for missions where you don't need a quick turnaround, such as trips where the plane flies outbound one day and returns another.  By analogy to my electric car: yes it takes longer for my car to charge than my old car took to refuel, but I personally save significant time because I don't have to be involved: I used to devote about 15 minutes out of my commute ever few days to detour to a gas station to fuel my car.  But now, I plug it in and abandon it at the end of the day and it's fully charged waiting for me in the morning.

As for charging infrastructure, I don't think there's any meaningful technology challenge to overcome; it's a matter of building it out, and that's an economic, not technical, issue.

Another technology area to watch is trends in 3-D printing and lightweight materials like carbon fiber.  Costs and efficiencies for both continue to improve dramatically as economies of scale from other industries kick in.  Both of these technologies can yield lighter weight (and often stronger!) aircraft and novel and efficient aerodynamic designs.  Of course, both LFA and EA would (and should!) benefit from these improvements, but my key observation here is that while it improves LFA in the missions that it already serves, it has a disproportionate positive impact on EA by enabling it to serve previously unachievable missions.

Finally, I recall a presentation given by NASA at Oshkosh a few years ago about research regarding an Electric 737 equivalent, and the speaker commented that the most difficult technological issue was not energy storage per se, but rather heat dissipation, from the enormous power (2.4MW!) that must be delivered in a small space during high-power operations.  In LFA, the combustion (which generates most of the heat) happens right at the motor where it can be quickly air-dissipated; in EA, the bulk of the heat is generated from resistance in the batteries and in the wires as current travels between the energy source and the engine, raising a novel engineering challenge.  Some of that heat can clearly be used for cabin temperature control and for heated airfoils for flight into icing, but this won't eliminate the heat management challenge.

Economic Trends

Even if we the technological challenges above are addressed, EA will be confined to niche applications unless it can compete economically with LFA.  As discussed earlier, EA is very likely to have lower operating and maintenance costs than equivalent LFA from the outset.  

It's the capital costs that are high today - in particular, batteries are expensive.  And they're life limited, so while an electric aircraft engine might have a TBO well beyond that of an LFA engine, the batteries likely will need to be replaced every few years.  Add in potential spare battery packs if you want to do battery swapping, and the costs - today! - can be quite large.

The key trend to watch here, though, is that battery costs have been falling.  Dramatically. Indeed over the past decade, they hit predicted prices years or even decades sooner than expected.  I love this chart from S&P Global which shows a decline of 87% over the decade:

SNL Image

This is why the original Tesla Model S and Roadster were so expensive, but now you're seeing electric cars enter the market at up-front prices that are in the ballpark of their internal combustion equivalents (in fact, depending on the study you cite, up-front unsubsidized price parity for EVs is nearly upon us).

The shape of the graph above shows an exponential decline.  The funny thing about those sorts of trends: they almost never have a sudden flattening.  Economies of scale and incremental improvements tend to keep them going for a while, and the S&P article cited above forecasts another 50-60% drop within the next 5-10 years.  And if the accuracy of previous forecasts about price is any guide to the accuracy of this one, then we should assume it is overly conservative and will in fact be a lot faster than predicted.

Newer more dense battery technologies will undoubtedly enter at the higher end of the price spectrum, but they will also benefit from the efficiencies (manufacturing, safety, supply chain, packaging, distribution, etc.) that drive the shape of the curve above, so my expectation would be that it will follow a similar shape, with faster rates of decline.

EA will also be able to draft off of the tailwinds from increasing EV penetration.  EV growth is driving battery management sophistication, charging infrastructure and methods of payment, investment in lightweight materials like carbon fiber, and so forth.  All of the benefits of traversing those learning curves will accrue to EA.

The other big challenge for EA is charging infrastructure.  LFA has a massive installed infrastructure to ensure that fuel is available to a lot of airports; today, at least, there is very little comparable charging infrastructure.  The good news here is that - as mentioned above - this is not a technology problem, or a distribution problem.  There's nothing really to invent, and just about every airport today that has fuel (and many that don't!) already has electricity.  It's an economic/business challenge: upgrades are often required to the electrical panels to support higher amperage loads, charging stations themselves have to be installed and maintained, payment/customer-support infrastructure needs to be handled, and so forth.

As an aside: the distribution of "fuel" is easier for EA than for liquid fuels.  100LL, Gasoline, and Jet A all need distinct distribution pipelines, refineries, storage, and pumps.  Adding charging infrastructure to an airport, though, can share and build on the electrical infrastructure that's already in place, and it serves a single universal energy source to any electric aircraft.

The good news is that EVs provide a great template for how to do this, and the scale of the problem for aviation is significantly smaller - both in terms of fleet size and in terms of the number of charging stations needed.  For example, the US currently has about 168,000 gas stations, but only about 4,000 airports that have fuel services of one sort or another, so the scale of the problem is significantly smaller than that for EVs.  And there's yet another benefit of the abstraction I mentioned above: in electric aviation, there's no need to find an airport with fuel appropriate to your aircraft: electricity is electricity, the only difference might be charging speeds that are available.

Conclusions

I certainly don't want to be a Pollyanna about electric aviation, particularly for long-range or high-capacity aviation.  There are significant and very real technological and economic challenges to overcome  But I also think that the trends are actually aligned for tremendous innovation as well.

Despite the significant technological hurdles, electric aviation is already "taking off" (pun intended), as evidenced by the examples at the top of this post.  Technology development is almost never a linear extrapolation of prior trends.  New technologies rarely are able to compete as drop-in replacements for existing missions: the first PCs were effectively toys; the first cell-phones didn't displace landlines, cars didn't merely replace horses and buggies, they created entirely new modes of transportation.  (Interestingly, EVs are being held to the "drop-in replacement" standard, and they're actually doing pretty well at that!).  These technologies find niches where they can compete, and grow from there rather than taking on incumbent technologies head-on from day one.  Drones are an obvious such niche, where the tradeoffs actually work very well in favor of electric already today.  And I think Bye is wise to focus on the training sector, where range/endurance is not the most critical factor.
 
And what's fascinating to me about Bye, Pipistrel, and Ampaire is that they are showing that electric manned flight is not impossible.  That's actually the critical step - after that, it's a matter of slow but steady iterative improvement that expand the set of achievable missions.
 
I believe, though, that we're going to be surprised, not just by how soon we'll be seeing electric aircraft in our skies, but perhaps even more by their application to missions we aren't even thinking about today.  For example, looking back:
  • 15 years ago nobody predicted the widespread adoption of drones; today, they outnumber manned aircraft (by a good margin).
  • EA could open up small-capacity medium-range point-to-point markets that are simply impractical today with LFA turbine aircraft, providing congestion relief from major airports and a better passenger experience (no TSA lines, shorter travel between airport and home/destination, etc.).  And when you realize that the main NIMBY opposition to new airports or expanded operations at existing ones is typically noise based, you can imagine the potential advantages of EA playing out in unexpected ways.
  • We're now seeing a lot of discussion of eVTOL (Electric Vertical Take-Off-and-Land) short-hop autonomous concepts and other applications that can be ideal for electric aviation, none of which were in the public consciousness 10 years ago. Indeed many of these simply aren't practical with LFA and can provide new niches in which EA can grow, possibly complementing rather than replacing traditional LFA.

I would bet that in 2035, we'll be discussing applications of EA that I can't even fathom from my 2020 perspective, and thus cannot list here.

Not all endeavors to achieve this will work, of course.  Indeed, I expect most will fail - it's a super hard problem, after all.  But that's a healthy and an inherent part of the "creative destruction" of capitalism and it happens as any new technology emerges (anybody remember Compaq? Lycos or AltaVista? Blackberry?).  At a high level, all of the trends are pointing in the right direction to enable this next generation of flight, which suggests that there will be winners that ultimately emerge, and for that reason more than any other I would not ignore developments in this nascent sector.