Electric (non-boogaloo) airplanes 2: supercaps

Factual Questions
1

Last year, I started a thread on electric airplanes. I recently had a thought about them that didn’t seem to be covered there. Rather than make everyone read through that long thread, er … topic, I figured I’d start a new one.

My thought was that E-planes could use supercaps for takeoff and initial climb to altitude, when there’s a high demand for power. Then when they get to cruising altitude (and the supercaps are expended) use batteries for the low-power-demand cruise part of the trip.

One big advantage is that supercaps recharge really fast compared to batteries, so they should be able to completely recharge during a relatively short period at the terminal. The battery can be partially recharged at the same time, assuming they don’t need a full charge for the expected upcoming flight.[*] That would be for a series of short flights. For long flights, they’d need a full charge, but I expect they could get a significantly longer flight for about the same weight by replacing some of the battery with supercaps.

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[*] Note that electric ferries do this. They start the day with a full charge, and only partially recharge at each end of their route.

2

Another thing one could add is a catapult launch system, a longer and softer one like used on aircraft carriers, but one that could also deliver electric power. That could get you to takeoff speed for free (plane power speaking), while the plane is still on shore power, and perhaps even with batteries still being topped off, then after takeoff perhaps a handoff to the supercaps which were also topped off during the launch to help with the initial climb.

However, while the launch system does help as the plane should be totally ‘full’ at takeoff this way, and not use battery power to get up to takeoff speed, I am failing to see and advantage of the super caps, as long as the battery itself can produce max power for the motors already. It’s not like the super caps will make the motors ‘better’. The only differences is if the super caps can store energy more weight and space efficient, or can produce more power then the batteries can AND the motors can use that extra power for thrust.

3

Supercaps are a very rapidly-advancing technology, and we don’t know of any practical limit to the amount of energy they can store. So it may well be that, at some point in the future, we will have airplanes powered partly or entirely by supercaps.

But we’re not there yet. Batteries aren’t advancing as rapidly as capacitors, and they do have real fundamental limits that we’re not all that far from, but they also have a huge head start. Even though you can get the energy out of a capacitor very quickly, you just can’t store enough energy in them to make a difference, right now.

4

I can’t help wondering if we would first get to a point where it is more effective* to use electricity to manufacture hydrocarbon fuels from scratch, then use these in conventional jet engines.

*NB I didn’t say ‘efficient’ - for a purpose like this, being able to leave the inefficient part on the ground, might still be worthwhile.

5

Not sure how you could manufacture hydro carbons from scratch, irrespective of energy input. I suggest you’d still likely need a base fuel, like oil or coal.
Generating hydrogen is far more practical through electrolysis of water. We just need to advance efficiency of hydrogen burning portable engines whilst increasing the storage and transportation efficiency of hydrogen

6

dtilque,

Interesting, so how do available sources compare? How much energy is available from a pound of super-cap vs a pound of Lipo vs a pound of jet fuel?

7

Did a little Googling and found that Lipos deliver around 100 killowatt hours per Kg and supercaps closer to 8 Kwh per Kg. I suspect Lipos can discharge faster than supercaps so what would the super caps add in exchange for their greater weight?

8

https://www.pnas.org/content/116/20/9693

9

Ooops, missed the edit window - should be Watt hours per Kg. But the comparison still holds.

10

Better zoom WAY out if you want that chart to include jet fuel, liquid hydrogen, etc.:

11

Yeah, there’s a lot of energy in jet fuel!

12

Super caps can safely discharge much more rapidly than pretty much any battery. We use them (this one specifically, https://www.maxwell.com/products/ultracapacitors/16v-large-modules) for starting gensets and in order to “boost” them if they are depleted you have to use some sort of charge controller.
One of the biggest adbvantages in this configuration is that they pretty much ignore temperatures that can cripple batteries, are rugged, and have a reasonably long life.
This cell can deliver 1.9 K Amps max and 18 Wh total in a package that weighs 5.5 kg. Much lighter than a typical automotive style start battery.

13

alternate link:
File:Energy density.svg - Wikimedia Commons

The chart is Joule/Litre (volumetric density). Which is interesting in it’s own right – the problem with suggested hydrogen-fueled planes is the most of the body would be taken by the hydrogen tanks – but not directly comparable to the first chart.

14

A typical automotive start battery is still good for “cost density” - J/$
But it’s loosing it’s edge. One of the advantages over other battery technologies is that it’s inherently a voltage regulator. That makes charge, discharge, and application easier. But that edge is being eroded by electronic charge,discharge and inverter technology. Anyway, a supercap is a known way of changing (improving) the discharge characteristic of a lead-acid battery, but it’s advantage is less clear for Li-xx batteries.

15

This was exactly my point in the OP. Airplanes need a huge amount of energy to take off and climb to cruising altitude. That takes only about 15 minutes or so of a multiple-hour flight. And after the plane is in cruise, it uses relatively little energy to stay there. So what I’m thinking is to use the supercaps for takeoff and then switch to batteries for most of the flight.

There are buses that are already doing this. Well, buses don’t have to take off, but they do have to accelerate frequently from stops. So they use supercaps for that acceleration and batteries for the rest of the time. And then use regen braking to recharge the supercaps. (Yes, eventually, they’ll have to recharge from outside the bus, but they can get away with doing this for quite a few stops.)

That’s Energy density (Wh/kg), but supercaps have about 10 times more Power density (W/Kg) than Li-ion batteries of any kind. Some of them a lot more. (Look on the chart you posted, it’s right there.) When you need a lot of power quickly, Power density is more important.

16

I wonder if it would be practical to have a sort of ‘booster’ module containing the supercaps that, once it had served its purpose of getting the plane to cruising altitude, detaches (reducing the weight of the plane) - the module could glide back to a ground base autonomously.
Probably a whole bunch of safety reasons why that isn’t a good idea I suppose

17

Thanks for the clarification. They are indeed SUPERcaps!

18

I think ‘huge amount of energy’ of climb compared to cruise is overstating it a bit. For example, an O-540 is a 250hp engine used in larger singles and light twins. At 100% power and full rich (takeoff settings), it will consume about 25 gallons per hour (gph) typically.

Once you are in a climb you would throttle back to 75% and lean the engine, which gets you about 16-17 gph. Finally, at 65% cruise at altitude you might be burning 11 gph or so. Absolute economy cruise can get you down to somewhere around 9 gph.

So let’s look at a typical flight:
Takeoff (call it 5 mins): 25gph * 5 min = 2.1 gal
Climb (15 min * 16 gph) = 4 gal
Cruise (2 hr * 9 gph) = 18 gal
VFR Reserve (45 min * 9 gph) = 12 gal

So for a legal 2 hour flight, we’d need 36 gallons of fuel. If we used a supercapacitor to replace gasoline for takeoff, we reduce our fuel need to 33.1 gal. Without the reserve requirement (which doesn’t affect charging time) we go from 24 gal to 21.9. Not a huge difference.

A gallon of gas has about 33.75 kwh of energy. So all else being equal, you would need 1215 kwh of batteries. The gas has a weight of about 6.3lbs/gal, so the gas weighs about 227 lbs. The best energy density I’ve seen for Lithium Ion is about 260 Wh/kg So if we wanted to replace the gas in the plane with batteries, al, else being equal you’d need 4673 kg of batteries for that flight.

Going to supercapacitors only lowers the battery weight by 259 kg, minus the weight of the supercapacitors. Maybe a 5% difference in the end.

A typical single that uses an O-540 light Beech Bonanza might have an empty weight of 2000-2500 lbs. So the batteries alone weigh more than 4 times the weight of the entire airplane.

Of course this is just the starting point, and you have to adjust for higher efficiency of electric motors, etc. But the ratios don’t really change.

Supercapacitors are very useful for certain applications. Perhaps the higher instantaneous power will allow for more efficient electric motors or something. But they won’t save electric planes, which still require major breakthroughs in efficiency and energy density to be practical for all but niche applications.

The two applications I’ve seen so far: A very light electric training plane for use in teaching takeoffs, landings, and pattern work. It has maybe a half hour to 45 minute duration, with no reserve (which it doesn’t need since it’s staying around the airport), and a conversion of a De Havilland Beaver which is used for island hoppjng in BC with a typical flight duration of only 15-30 mins.

Now if you could power one of those Beavers for 15 mins on supercapacitors, you could cut the charge time in half for each trip, which might make it worthwhile. But again, that’s a niche application.

19

One significant difference between batteries and supercaps is the discharge profile. Batteries discharge at a nearly flat voltage, but the output of supercaps is a linear drop, so I think you have to find a way to compensate for that.

20

Supercaps aren’t useful in a pure electric plane. Yes, they have high energy density, but with the number of cells that you’d need for a practical electric plane, there is still plenty of power available.

There’s actually a pretty easy way of thinking about this: the “C” rating. 1C means discharging a 1 W-h battery at 1 W, or a 1 kW-h battery at 1 kW, etc. 10C means 1 W-h at 10 W.

Lithium ion cells are content to discharge at 5C very comfortably. A high-end Tesla Model S makes >500 kW power with a 100 kW-h battery. Not a problem at all, and with some cell chemistries you can go higher.

That would deplete the battery in 12 minutes (60/5=12 min), so you wouldn’t sustain that. But it does mean that your plane can handle very high power bursts for takeoff if it has enough energy for even an hour or so of flight. Adding supercaps to this wouldn’t get you anything, and only decrease the energy density.

Supercaps are great if you need to expend all your energy in seconds or a few minutes. But that’s not the case here.