Electric battery powered planes are the future (yes really)

I just saw this video from the brilliant Minute Earth guys suggesting the future of aeroplanes is bio-fuel, and it inspired me to write a proper response.

I've seen arguments like this before and their argument (and additional arguments from other places including this minute physics video) against battery electric planes can be categorised as follows:

1. Battery density is too low.
   Jet fuel has a very high energy density, batteries are much lower (over 2 orders of magnitude). A plane simply couldn't lift or fit enough batteries to fly.
2. Batteries are too heavy and this will make the undercarriage of the plane break every time it lands (yes I've really seen this argument).
3. Batteries are too expensive.
4. Batteries take too long to refuel.

On the face of it these arguments have some truth, however all of them lack context and vision. Please allow me to add some of both in rebuttal.

Responding to the arguments against

Battery density is too low.

This argument has the most 'weight' (pun intended). Jet fuel has an energy density of around 12,000 wh/kg. The current best in class batteries are around 300 wh/kg or about 40 times less, if you measure at the pack level its even lower. So given a basic conversion of an existing plane, ripping out the fuel tanks and engines and replacing them with batteries and motors you might expect the vehicle to have a range that is 40 times less. This is the point were most people stop thinking and assume its impossible.

However this is exactly what Magnix Aero have done with a Cessna Caravan. Whilst its not yet in commercial operation its currently going through the development and approval process and a working prototype is in existence and flying. It has a range of about 100 miles which is about 12 times less than its fossil fuel powered original. So even with this basic conversion type process we see ranges better than we might expect. Why would this be and 100 miles pff, whats the use in that?

Increased efficiency

Fossil fuel powered engines are inherently inefficient. Over the years they have got a bit better but most fossil fuel engines will struggle to reach 30% efficiency most are much lower.

According to this article modern engines use about 300g of fuel per kwh of energy delivered, this translates to a usable energy density of around 3600 wh/kg or about 30% of the original amount. On the other hand electric motors can have efficiencies as high as 98%. Interestingly the example of the Cessna above matches these figures extremely well so the real energy density difference is closer to 12 than 40.

Reduced range

So the above converted aircraft will have a maximum range of only 100 miles. What is the point of that you might ask? Well according to the article previously linked, about 5% of all global air travel is less than 100 miles. Therefore even just given vehicles like the example above 5% of all air travel COULD be electrified. For reasons why you WOULD electrify as much air travel as you possible could (if you were an air transportation operator), see later in this article.

Increasing range through vehicle efficiencies

Update, I've written more about vehicle efficiency in part 3: here

When cars first started being electrified, a lot of car companies just kept their existing vehicles, removed the fuel tanks and engines and replaced them with batteries and electric motors. These vehicles were cheap on R&D but expensive to produce and didn't have very good range, typically around 100 miles or less.

Vehicles that were designed from the ground up however, whilst costing more at the R&D stage, had much better ranges (the original Tesla model S had a range of 265 miles or more than 2.5 times the 'converted' vehicles) and were cheaper to produce. (Sorry for the lack of links but you can google this for yourselves. E.g. Currently each Tesla model 3 has a profit margin of around 20% whereas 'converted' vehicles from e.g. Fiat were making a loss causing the CEO to ask his customers to stop buying them).

Retrofitting existing aircraft designs with electrified power-trains will certainly have a market for short range transportation, however existing plane designs are extremely inefficient. Its not just their engines, the entire design is based on technology that have hardly changed from the beginnings of powered flight. Using new techniques and redesigning the craft from the ground up to run on electricity will yield vehicles with greatly increased efficiency and with that more range. Aerodynamic control surfaces and stabilising fins are incredibly inefficient and not needed when you have lightweight steerable electric motors that can do thrust vectoring.

Whilst we're still waiting for commercial vehicles designed like this from the ground up, designs and prototypes are already in development. Expect to see significantly better range from these vehicles than converted ones, using the example from the automobile industry we might see ranges 2 or more times better. (For an example of one such vehicle see Lilium).

So with better designs electric aircraft can take over even more of the current market.

Increasing density of batteries.

Jet fuel energy density is not going to improve by any amount, and the engines that burn it have reached a plateau of efficiency are are unlikely to get much better. Battery technology however is still progressing rapidly. In the last 10 years energy density has doubled and the speed of improvement appears to be accelerating rather than declining. There are limits to this of course and these were discussed in the Minute Physics video mentioned at the outset, however we are not near the limit yet. Its quite possible and even likely that the energy density will double again in the next 5 years and double again in the next 5-10 years after that. In other words its highly likely that in 15 years the best in class batteries will be about 4 times better than current. This would give the Cessna example above a range of 400 miles, which would significantly extend its market share, and purpose built planes might have ranges of 800 or more miles, bringing into possibility the lions share of the existing air travel industry.

This begs the question, how much range do electric planes need before they replace all the existing vehicles? But before we look at that lets quickly look at the other 3 arguments

Battery planes will break the undercarriage

Whilst the people making this argument are probably well educated and intelligent they must suffer from terrible lack of vision. 

Firstly, this is an engineering problem. Planes are engineered to have undercarriages that can handle a plane landing only when the fuel is mostly empty because that's the situation most of the time. There would be no point in engineering a plane to be able to land with all its fuel as that is almost never required, and as such would be a waste of weight (in terms of upgraded components) to build it to take the full weight. (As a side note, this is another win for electric planes. Currently if you're on a flight and they have to come down early you might find your plane circling for hours burning off fuel, or sometimes they are able to dump it overboard. One is a terrible passenger experience and the other is terrible for the environment and both are terrible for the operators pocket).

Electric powered planes (or converted ones like the Cessna) will have undercarriages designed for the weight of the vehicle, which will of course not change during flight. Likely the newer, more efficient designs will also be vertical take off and landing (VTOL) vehicles (like the Lilium jet for example) and these will only require very simple and lightweight landing "springs".

Batteries are too expensive

There are a multitude of reasons why this will not stop the electrification of air travel.

The first is that battery prices have been falling consistently over time, and fell by 85% in the last 8 years. There is currently no sign that this price decline is going to stop any time soon.

The second is that the rest of the power-train is likely cheaper. Electric motors are also getting lighter and cheaper over time, and the ancillary components needed to run them (inverters etc) are also improving and falling in cost and (excluding batteries) an electric power-train is already cheaper than a fossil fuel one. With the cost improvements continuing its likely new electric vehicles designed from the ground up, will in time, cost less to buy than the fossil fuel powered alternative.

The second is the operating cost, or the Total Cost of Ownership. This will be discussed in the section below and as you will see, no aircraft operator is going to care about a higher upfront cost even at today's prices.

Batteries take too long to refuel.

This is again an engineering problem with multiple answers.

The first is that battery recharge times are based on the cell chemistry and battery cooling technology. Charging quickly generates heat, the amount of heat per unit of energy is related to the cell chemistry and also the cell design. Its possible to engineer a cell that can charge very quickly if you sacrifice some other aspect of it. In any case improvements to cell chemistry and design are improving this situation. In addition you can design the pack cooling system to be able to handle larger amounts of heat if so desired.

However neither of the above are likely to be problems for the airline industry. Unlike cars, which are mostly owned by private individuals that drive all over the place and driven by people who are averse to swapping part of their vehicle out, planes are mostly owned by commercial operators. They always run from known fixed locations to other known fixed locations (airports) and commercial operators will almost always have infrastructure at all airports they operate from. Therefore there is no compelling argument against designing the battery packs to be quickly swappable. In fact, this is even happening with personal motor vehicles, (albeit with less popularity than the recharging model). The currently in development battery planes all seem to feature the battery swap model too. See: Wright Electric, Pipistrel, and Lilium.

With those arguments out of the way we really need to look at the biggest issue, why airlines will be falling over themselves to electrify as much of their fleet as quickly as possible.

How much range do we need

Or, "how much range do planes need before they will replace most if not all of the existing vehicles"?

To answer that question you need to look at the economics.

Economics

Jet fuel is expensive. 

Given the example I previously linked of the Cessna, a 100 mile journey in the traditionally fuelled vehicle would cost about $300 in fuel. It would only cost about $6 to charge the electric version. That's nearly 2 orders of magnitude cheaper or about 50 times cheaper to put it another way. 

Fossil fuel engines are expensive to maintain.

Traditional vehicles require constant inspections and maintenance. Whilst we don't have firm numbers yet on how much cheaper it will be to maintain an electric power-train, we do know it will be much cheaper. 

The article on the Cessna for example guesses at the total cost of running the electric vehicle will be between 40 and 80 times cheaper. 40 times cheaper would assume it costs more to maintain (as the fuel is 50 times cheaper) but 80 times cheaper indicates a huge saving on maintenance. For some inspiration we can look to the electric car world.

There are numerous examples of electric cars running for hundreds of thousands of miles with only minor maintenance required. The batteries were seen as the weak link with older designs only lasting a few years however the newest designs are already running into the hundreds of thousands of miles before needing replacement and soon we are expecting the batteries to outlast the rest of the already very reliable vehicle, quite possibly meaning electric cars will run for 1 million miles with no major repairs to the power-train needed.

If we assume that running an electric plane costs 65 times less than its traditional counterpart, that means if an airline is currently spending $1 million on fuelling and maintaining their fleet, if they can switch it to electric it will only cost them $15,000. Assuming most airlines are running on tight margins, the first airlines to do this will be able to slash their prices (NB: See update at end or article) by half and still make about 30 times more profit than they previously were. 

Let me say that again, they can halve their ticket prices and still improve profits by 30 times. Not 30%, 30 times, or 30000%. 

OK this assumes they have no other overheads and we're only discussing the craft running part of their business (which is likely a significant part), so the numbers would be a little lower but still, even if running the planes is only half their outgoings they can still half the ticket price and improve their margins by several orders of magnitude. The first airlines to do this will quickly start crushing all their competition easily able to undercut them on prices and be massively profitable. This is why all airlines run by sane individuals will electrify as much of their fleet as quickly as they can. Every route they electrify will gain this cost saving. Any small increase in the upfront cost of the vehicle (if any) will be quickly repaid, and likely buying new vehicles even many years before they need to retire their old ones due to the pay back in profitability being so quick. 

Electric vehicles are quieter

Electric planes make much less noise than traditional ones. Currently most airports impose time limits on when flights can come in and out of the airport based on noise restrictions to limit disturbances to local residents. We don't know for sure, but its quite likely that some or all of these restrictions will be eased or completely removed for pure electric flights.

If so this will drastically increase the number of flights per day that an operator can use their vehicle for, and thus improve the profitability of the vehicle even further.

I therefore expect that a lot of currently flying planes will be grounded and scrapped significantly ahead of their currently expected retirement dates and absolutely no-one will be buying a traditionally fuelled vehicle if there is an electrically fuelled one that can accomplish the route it needs. Which brings me to the actual point of this section.

We don't need all that range.

With the economics of electric flight so much more compelling than traditionally fuelled vehicles, there are a number of routes that whilst not possible on a single leg, could be done with a brief layover and battery swap. 

Assuming the Cessna example, I've assumed it flies at a maximum speed of about 200 mph (I couldn't find the details, but this is the speed of the traditional model). Assuming it takes 10 minutes to take off and land but otherwise flies at its maximum speed (this is a poor assumption) it would take about 1 hour and 10 minutes to fly 200 miles. If we now say it needs to make a layover to get a battery swap at 100 miles, and the swapping process takes another 10 minutes, the entire journey will now take 1 hour and 30 minutes. OK so the entire journey is nearly 50% slower, but if you're given the choice of the two, and the longer flight is say, 30 times cheaper so $10 instead of $300, many people would likely choose the cheaper option. Probably most. 

This is a rubbish example as I don't see these converted planes being used for the majority of flights. They will have a niche, especially as they will arrive first, but the purpose built crafts, with longer ranges will be much more attractive. The currently in development Wright Electric jet for instance has a predicted range of 300 miles and in any case, there is another factor:

Electric jets could be faster

The higher you go the less air resistance there is. The less air resistance the less fuel you need to push through it, and the faster you can go with the same amount of resistance. Planes therefore usually fly as high as they can based on the distance of their journey, the cost to raise their altitude, and the maximum fight altitude of the vehicle. This is not just the most efficient, its also the fastest.

Then there is also the speed of sound to contend with. Breaking the sound barrier creates a shock wave known as a 'sonic boom'. There are also a number of engineering challenges to create a vehicle that can safely be controlled through the sound barrier. Many of these problems are trivial if you have redesigned the vehicle to not have aero control surfaces but instead use thrust vectoring and a much improved more aerodynamic body shape. I.E, exactly the kind of vehicles now being designed for electric flight. 

Current planes are limited in their maximum height by the ability of their engines to ingest enough oxygen to efficiently burn the fuel (as less air also means less oxygen). Electric jets will not have this restriction.

Given these 2 points, and this is pure conjecture on my part, but I believe its at least likely that future electric jet aircraft will be supersonic and will fly significantly higher and thus faster than any previous (Concord) or current jet was or is able to.

Given this extra speed, having to make a stop halfway (for flights over land) will be a complete non-issue, in addition the improved efficiency of flying higher (supersonic or not) will further improve the efficiency and range of electric planes.

Summary

Without giving a specific date, I would be surprised if most if not all aircraft routes are not completely 100 % electric in 20 years from today. There are some routes that are very long haul where the requirement to stop for midway battery swaps could be an issue for the passengers or logistically difficult, however given the reduced cost I doubt there will remain a market for fuel burning vehicles with passengers and airline operators preferring the 30 to 60 times cheaper (possibly) slightly longer journeys.

Of course that's assuming that long haul flights are still a thing, but that's a topic for another day (Starship, other non rocket orbital launch technologies, hyperloop,....).

Please see part 2 for some additional thoughts and examples.

Update:

I recently learned that fuel costs are about (on average) 19% of the total cost of running an airline and maintenance costs are around 20%. Therefore (unless governments start adding a carbon tax on aircraft fuel), the total 30x savings of an EV plane will only apply to about 39% of the airlines total costs. 

This is still a huge saving for an airline e.g.

British Airways Opex around £6.3 billion with fuel and maintenance being around £1.6 billion (25%). 

Divided by 30 would give a cost of £53 million or a saving of more than £1.5 billion per year.