Jun 232016
 
Artist's impression of an electric plane concept being investigated by NASA.

Artist’s impression of an electric plane concept being investigated by NASA.

We all love hybrid and electric vehicles, right?  Those alluring claims of 100 miles per ‘gallon equivalent of petrol’ (whatever that actually means), the extraordinary sportiness of the Tesla S, or the sturdy economy of a Toyota Prius and its 50+ mpg (of real gasoline, not ‘equivalent’ gas).  No-one can deny, with the next generation of all-electric cars due to come out later this year offering close to ‘normal’ pricing and close to ‘normal’ range between charges (ie a net cost of about $30,000 and 200 mile range with the Chevrolet Bolt) that electric cars are evolving from an impractical oddity to a sensible and appealing alternative to traditionally powered gas and diesel vehicles.

This writer is as entranced by electric vehicle technology as anyone else, and – as long as I can manage to maintain sufficient myopia to overlook the slow drip drip drip of bad news about Tesla’s vehicles – would dearly love to have a Tesla, too.  Indeed, I’ve even ‘converted’ to a battery powered lawnmower, while struggling to ignore its challenges (heavier than a regular mower and battery life 5 minutes shorter than the time it takes to mow the lawn), and I’ve a set of solar panels (which, alas, never generate even 1/10th their rated power due to a subtle but significant bit of fine print in their performance claims that makes them useless in my environment).  So I’m a keen, albeit realistic, advocate of electric solutions.

So when we all read a headline in the usually fairly staid and sober Wall St Journal ‘Plans for Electric Planes Gain Momentum’ and when we recall how much airlines obsess over the price of jet fuel (except for when, as at present, it goes way down), the thought of electric planes seems incredibly appealing, right?  If we can now have electric powered lawnmowers, cars and even buses with performance comparable to normal vehicles, it would seem, on the face of it, that the time is right to extend the same technology to planes, too.  If electric cars are more efficient than gas powered cars, shouldn’t electric planes be another slam-dunk application for electrical power, too?

You probably will have difficulty reading the article unless you are a subscriber, but if you try opening the link in ‘incognito mode’ that sometimes works.  Alternative, search for the headline “Plans for Electric Planes Gain Momentum” on Google and then click a link from there – those links are usually allowed.

It seems the article was built on a press release from NASA about its progress in developing an electric plane.  Here’s a quick summary.

Unfortunately, as lovely as the notion of battery powered flight sounds, it is no more practical for commercial airplanes than powering airplanes by asking all the passengers to help out by turning sets of pedals on the floor of their seats, geared to the propellers outside.  And, when you apply a more logical analysis to the difference between gas and battery power, and the difference between airplanes and cars, the many reasons why become obvious to everyone (except the starry eyed researchers and the Wall St Journal reporters who write about them).

Batteries are Too Heavy

Airlines are obsessed by the weight of their planes, and quite rightly so.  As a rule of thumb, the plane and everything on it requires a 3% equivalent weight of jet-fuel to be burned per hour of regular flight.  The 10 hour flight to Europe on a, say, 750,000 pound 777 requires a bit less than 225,000 lbs of jet fuel – call it 30,000 gallons.  If the airline could trim a mere 5% from the weight of the flight, then it could save 1500 gallons of fuel each way, and that starts to add up very quickly if you’re operating a hundred similar flights (or a thousand, etc) every day.  Taking even 100 lbs of weight off the plane would save 30 lbs of fuel each way, and so on.

So we see the airlines spending huge amounts of money to replace seats with lighter weight seats, inflight electronics with lighter versions of the same, even going to the lengths of more carefully matching the drinks on the beverage carts to the expected consumption so as not to be carrying any more weight in drinks than necessary, and reducing/eliminating such seemingly not-heavy things as pillows, blankets, and magazines.

So, let’s now think about this obsession with weight and how it applies to what happens if the jet fuel is replaced by rechargeable batteries?  Are batteries heavier or lighter than jet fuel?

To understand that, it is necessary to know the ‘energy density’ of jet fuel compared to batteries.  In other words, if you have a pound of jet fuel, how much energy can you get from it?  And if you have a battery weighing a pound, how much energy can you get from that?  The more energy per pound, the less the weight of the fuel or battery needed (although different efficiencies of converting from the stored energy to the net energy moving the plane through the sky are also a factor).

Jet fuel has an energy density in the order of 46 MJ/kg.  Lithium ion batteries have energy densities in the order of 0.4 – 0.9 MJ/kg.  In other words, you need between 50 and 100 times greater weight if you want your plane to be fueled by batteries.

Let’s think what that means.  Looking at the same notional 750,000 lb 777, it has one third of that weight allocated towards a nearly full load of fuel stored on board.  In other words, you couldn’t simply increase the weight of the fuel by 50-fold.

But that’s not all.  There’s a nasty loop – the greater weight of batteries means more weight is needed for a stronger plane structure, which requires more power for the plane to fly, which requires more weight for batteries, which requires more power for the extra weight, which requires an even stronger plane structure which weighs more, and around and around that loop you go.

That is why the airlines are so obsessed over saving a pound or two, because the pound they save in drinks then saves them half a pound in fuel, and the half pound in fuel saves them also the extra fuel needed to fly the half pound of fuel, and then with less fuel, the plane can either carry more ‘payload’ or be built lighter, saving more fuel, and so on and so on.

One more thing about batteries.  Lithium ion batteries are dangerous, as was famously demonstrated in the early days of the 787.  The solution was not to make the batteries safe, but rather to pack the batteries in very solid safety boxes so that if a battery goes wild, the heat and fire and fumes doesn’t spread to other batteries and the rest of the plane.  That is arguably acceptable (but far from optimum) when you just have one or two batteries for emergency backup power, but think about the additional extra weight and systems required to have the enormous number of batteries to actually power the entire plane through hours of flight, all contained in special safety boxes.  The weight penalty for using batteries has just become even more extreme.

It is possible there might be some weight savings in the design of electrical motors on the wings, but those weight savings would be trivial compared to the weight penalties of the batteries and their containment devices.

Batteries Don’t Get Lighter when Discharged

There’s another nice thing about jet fuel.  When you burn it, it has gone.  The 777 that starts its flight with 250,000 lbs of jet fuel might end the flight with only 25,000 lbs of jet fuel.  So it has averaged about 138,000 lbs of jet fuel over the flight.

But batteries don’t lose weight as they are used.  All that happens is that electrons travel from one side of the battery to the other.  There’s no weight loss at all.  If by some magical stroke, you could end up with a 250,000 lb battery that had as much power as 250,000 lbs of jet fuel, you’d need a further stroke of magic to allow the battery to actually reduce down in weight still more to 138,000 lbs.

This points to another interesting problem.

Landings

Let’s look again at our 777 that we’re using as an example.  This chart shows you specifications for the entire family.

Note that while a 777-200LR has a maximum take-off weight of 766,000 lbs, it has a maximum landing weight of a much lighter 492,000 lbs.  That is usually not a problem – the plane starts its flight with a pre-planned amount of fuel (and everything else) so that at the end of the flight, it will have burned off enough fuel to come down under the maximum landing weight figure.

In the case of an emergency, it shouldn’t land again until it has somehow lost as much as 250,000 lbs of weight – ie fuel.  It does this either by circling anxiously, burning fuel for hours until light enough to land, or by dumping the fuel out drain ports to allow for a faster reduction in weight.  Sure, you can land a plane with any amount of weight, but the heavier it is, the more likely that tires will burst, landing gear will collapse, brakes will burn out and/or lock, and other generally bad things happen.

But what will happen with a battery powered plane?  They don’t get lighter.  Plus, in an emergency, tossing them overboard would not only be difficult to arrange, but might alarm people on the ground if they had a solid 50lb battery land on their head (or car or house, etc).

So planes would either be limited to being loaded only to the maximum landing weight, or would need to be built with massively stronger landing gear in order to permit heavier weight landings.  And, ooops, ‘massively stronger’ is a polite way of saying ‘much heavier’ and ‘more expensive’.

The massively stronger approach is the only viable one, as unappealing as it is.  If you are to remove 250,000 lbs of payload from the 777 so that is maximum take off weight is the same as its maximum landing weight, you end up with a plane that is totally useless and unable to be flown profitably on any basis at all, because its load capacity had disappeared (as has most of its battery capacity, too!).

Batteries Take Up Too Much Space

There certainly are some fuel sources with a much higher energy density than jet fuel (and greater than batteries too, of course) – hydrogen, for example.  But there is also a second related consideration and constraint when designing an optimum airplane.  Not only is weight a critical concern, but as we all know from the last time we were in coach class, and the attack on our knees and shoulders while seated there, space is a critical factor, too.  The airlines go to equally extreme measures to maximise their utilization of every cubic inch of space – the space we see passengers crammed into, and the under-floor cargo space we don’t see for freight.

Once again, jet fuel trumps lithium-ion rechargeable batteries, by a factor of about 20:1.  Even if we could make the batteries lightweight enough, there’d be nowhere on the plane to put them!  Every bit of space would be full of batteries, leaving no room for passengers, their bags, or any other freight.

Recharging Time and Power Availability

Battery power is wonderful, until the point where the batteries need recharging.  Another thing airlines are focused on is getting the maximum amount of ‘utilization’ out of their planes – they want to turn them around at the gates as quickly as possible, and keep them flying as much of every day as they can schedule.

On average, it seems most commercial jets have about 11 ‘block hours’ of utilization each day – the time notionally from when the wheel blocks are removed prior to a flight starting to when they are replaced when the plane has pulled in and parked at the end of a flight.  This site has fascinating data on that and many other aspects of airline operations.

Is half a day enough time to charge a plane for the other half a day it is in service?  The answer to that is ‘it depends’ (but probably yes), at least in theory.  But, in reality, this consideration uncovers an entire new range of problems.

Superficially, being able to recharge planes would require massive (ie costly) infrastructure investments at airports to bring in prodigious amounts of power.  But, now let’s look at exactly how much power, and the problem takes on a surprising new dimension.

If a plane is loading 150,000 lbs of fuel, that is the same as about 870 MWhr of electricity.  Let’s say it will charge over two hours, which means it needs 430 MWhr per hour of electricity flowing into the plane.  What does that mean?  That is the same as 290,000 heaters (each 1500 watts) all operating simultaneously.  That is the same amount of power as is used by 350,000 houses (how many houses in your town or city?).  Now, let’s say that the airport has 20 or 30 planes, all simultaneously wanting to recharge between flights.  We’re talking about a power load equivalent to perhaps 10 million households.  Or, to put it yet another way, each plane needs to be charged at a rate 2,000 times faster than the Tesla supercharger very fast charging rate for Teslas.

Now, let’s think about more than one airport.  Let’s multiply that by 50 (and when you keep in mind there are over 30,000 flights a day in the US, to have 1,000 planes refueling at the same time seems totally realistic and probably too low).  We’re now talking about a need to supply more power to planes than is used by every household in the country.  Note – it might be that the energy in a battery is more efficiently converted to propulsive power than the energy in jet fuel, so maybe not so much power is needed if coming from batteries than from jet fuel, and these numbers might be off by a factor of two or more.  But it doesn’t matter, the sheer impossibility of powering our nation’s air fleet by electricity seems starkly apparent at either level.

Guess what, folks.  We don’t have that much surplus generating capacity.  And if airlines start pushing up the demand for power, what is going to happen to our electricity cost?  It sure isn’t going to go down?  Not only will our personal electricity costs rise, but so too will the costs of energy for the airplanes, reducing the value proposition that may or may not support a switch to electric power.

And where will all that extra power generating capacity suddenly come from?  Not from solar or wind although that would create a whole new series of weather related excuses for airlines, wouldn’t it – ‘We’re sorry, our flight will be delayed for another 90 minutes due to weather – there’s not enough wind to blow the wind turbines to recharge our plane’.

On the other hand, if we put more oil fired power stations online, what have we actually achieved?  We’re burning hydrocarbons at the power station to convert them into electricity there, we’re then accepting inefficiencies in transmission losses to get the power to the airport and into the plane’s batteries, and then more inefficiencies to convert the electric power to motive force.  Why not simply transport the fuel to the airport, load it onto the plane, and directly burn it efficiently in the engines, as needed?  And we’ve effectively neutralized the ecological argument in favor of using electricity too, if all we are doing is shifting the act of burning hydrocarbons from a plane’s engines to a power station.

Of course, some people would say that the answer is to deploy more nuclear power plants.  But sadly, nuclear power has a bad, albeit entirely undeserved, reputation, and the people arguing against hydrocarbons are sadly not advocating a switch to clean green nuclear power.

Some Sensible Science

Airplanes aren’t as well suited to power-recovery techniques as cars.

One of the major benefits of a battery-electric vehicle is that it recovers the energy spent in speeding up when you brake.  In a regular car, that energy is wasted in your brake linings and as heat; in an electric vehicle, the electric motors swap to become electric generators and recover much of the energy to re-use the next time you accelerate again.

But an airplane speeds up and then stays at that speed, possibly for many hours.  If it does slow down, it usually does so not by ‘braking’ but simply by reducing power and letting the speed slow down naturally.  It is only for a minute or two while coming in to land that the air brakes might be activated, and it would be unthinkably difficult to develop some sort of way that the energy lost through the air brakes could be readily converted into energy.  It doesn’t have nearly the same energy wastage that a car has (and let’s not forget, as well, that a fully loaded plane is a much more energy efficient means of transportation than a partly loaded car).

When the plane touches down, there’s an intense 15 – 30 seconds of braking.  But that braking is so intense that even if there were generators on the wheels, they’d only capture part of the energy and the rest would still need to go through regular braking.

Some Stupid Science

If you want to see a stunning example of scientific illiteracy, please read this line in the WSJ article :

Another promising idea: Harvest the kinetic energy from spinning wheels on takeoffs and landings in a generator that could charge the plane’s batteries.

Sorry, but that’s not a promising idea at all.  Rather, it is a scary indication of the lunatic non-science that may be underpinning much of the entire concept of bringing batteries to commercial passenger jets.

When the plane is taking off, it needs every possible bit of energy to be spent on accelerating down the runway and quickly taking off.  It has none to spare.

Besides which, there’s the inconvenient concept of the impossibility of perpetual motion machines.  To suggest that it would be effective to use energy to then recapture it and store it is impossible and is as sensible as suggesting that a sailing ship could go faster if the wind was used to power a turbine generator which was then used to drive a fan to blow wind into the ship’s sails.

In reality, to do a complete ‘cycle’ – to use energy to power the plane forward, then to take some of that energy, convert it to electricity, store it, and subsequently re-use it – that would see a loss of perhaps 30% – 50% as between the energy originally expended and the energy available the second time around.

The last thing a plane wants, when taking off, is to simultaneously have some of that energy being drained off into inefficient energy recovery, because none of the energy at take-off is spare.

We’ll concede that a little energy could be recovered when landing, but that is beyond inconsequential in terms of the overall energy needed for any length of flight.  And it would require extra weight for the generators to be added to all the airplane axles, etc etc.  There have been some proposals for planes to have a separate battery/electric system purely for taxiing on the ground – that is a different issue and revolves around the fact that the jet engines aren’t very efficient to drive a plane on the ground at 15 mph, and of course are wasting energy while waiting in line for take-off, etc.

Technological Breakthroughs

To make battery power an alternative to jet fuel, we’d have to make batteries that have 50 times greater energy density in terms of weight, and 20 times more density in terms of size.  At present, it seems that lithium ion battery technology is improving at a rate of perhaps 5% a year, so unless an extraordinary breakthrough suddenly transforms things, we’re not going to see batteries as a viable solution in our lifetimes – or those of our children, either.

And even when that problem finally does get solved, there’s the other problem – where will we get all the electricity from to replace the jet fuel?

Summary

Jet-fuel might seem old-fashioned and low tech, but it is also about as good a source of energy as currently exists.  Well, there is one other source that has about 20 million times greater energy per pound, and debatably better energy per unit of volume as well – nuclear power.  Although its energy density is superb, other factors and raw emotion has rather frozen that out of the realm of consideration.  A flying nuclear reactor is all well and good until it crashes.

We don’t doubt that we’ll see some niche airplanes appear with battery/electric power.  But they will be vanity ‘feel good’ projects without any commercial underpinning, and such special/limited use small planes do not mark a trend that will smoothly carry over to commercial passenger jets and air freighters.

Not even Elon Musk and Tesla can promise us an electric plane.  Absent an unexpected revolution in battery technology, and the necessary boost in terrestrial power generating capacity, you’ll not see them in the skies, and neither will your children.

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