Originally published at: MIT Team Releases Tempting Report on Electric Aircraft Technology - AVweb
New tech could triple the energy density of current lithium-ion batteries.
Maybe, maybe not, then again maybe, or maybe not. A concept needs to bump up against the real world before a judgement can emerge, however flawed it may be. This might just be the Edsel of batteries, or perhaps the model “A”.
Hopefully something is lost in the translation or reporting. Apparently fuelled with sodium metal which reacts with air exothermically and ends with caustic soda that reacts exothermically with aluminium to produce flammable hydrogen.
Liquid sodium inside my airplane? I don’t think so. Why not focus on something that makes sense, like lightweight diesel aircraft engines that burn our glorious fossil fuel, kerosene, diesel, or Jet-A, with its very high energy density, low flammability, easy to store, available globally in limitless supply. And it can be made from coal via synfuel.
1000 Watt Hours / Kilogram… Hmmm… My airplane can hold 60 gallons of fuel, or 360 pounds, or 163.3 Kilograms. So, at 1000 WH/Kg, that would be 163,300 Watt hours equivalent to my full fuel load. 163,300 Wh, or 163.3 KWH would be 222 horsepower hours… or a little over an hour, no reserve, at full throttle (200HP in my case).
Full fuel gets me around 6 hours in cruise with 100LL.
Not quite there yet. Anyone see anything wrong with my math?
@AvidFlyer: Solid math. But here’s an additional benefit, as avgas is burned, the aircraft gets lighter. Even with reserve, you’re probably using about 300 pounds per flight. That’s a drop of more than 80% in fuel weight over the mission. Less weight means less drag, better climb, more range. Performance improves as you go.
Batteries don’t work that way. You carry 100% of the battery weight from takeoff to touchdown, whether it’s full or flat.
That is not just a technical detail. It’s an operational advantage. Avgas gives you power and sheds weight along the way. Batteries can’t.
UPDATE:
The Good:
- Big energy numbers — over 1,500 Wh/kg.
- Sheds weight like avgas as it runs.
- Sodium is cheap, abundant, and not tied to rare earth mining.
- Possible COâ‚‚ capture from the byproduct.
The Bad:
- Needs constant heat — sodium must stay above 208°F.
- Complex system — heaters, insulation, and chemical handling.
- Not rechargeable — think fuel, not battery.
The Ugly:
- Byproduct is caustic soda (NaOH) — corrosive and hazardous.
- Environmental and safety concerns if it leaks or vents.
- A long, rough road to certification and real-world aviation use.
What you are missing is overall efficiency. A Lycoming or Continental engine will probably have a real world efficiency of about 20%. The overall efficiency of a battery electric powertrain is typically over 80%. I am the advisor for a university student electric race car team (FormulaSAE). We have an EMRAX motor that has a peak efficiency of 98%. These motors were originally designed for self-launching gliders. It also makes 80 hp with a weight of about 17 lb so the power train weight is also considerably less.
Actually, the system that the MIT research group is suggesting is more like a fuel cell and is losing weight as is expends energy, The sodium is going to sodium oxide and then reacts with moisture in the air to form sodium hydroxide which is dumped overboard. I am not sure I like this system but it does shed weight as it flies. I think that some of the lithium sulfur battery chemistries are a better answer for electric aviation as they probably have a real world potential for about 900 Whr/kg (why can’t we use Joules instead or Whr?).
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Indeed - a lot of confusion from inadequate wording in the article.
Spitting-out high pH sodium hydroxide solution (commonly known as caustic soda or lye) at take-off and landing will not be acceptable; so the open-loop option with the exhaust mixing with atmospheric CO2 for decarbonation benefits is a nice concept but won’t fly.
IMHO I believe that a yet-to-be-industralized fuel cell system will in the mid-term future overtake batteries for range applications. It will have to be POC-demonstrated on land vehicules, so don’t hold your breath just now.
Note that sodium is a liquid above 208 degF, so the fuel cell will have to keep the sodium hot whether it is being used or not. So it will be discharging NaOH all the time.!!
As a reference, the net equivalent energy for avgas is about 4,000 whr per kg, when accounting for the 90% eff of electric drives vs 25% for ic engines. So, this isn’t a very practical energy source for aircraft.
That’s true. On the other hand, nobody flying an electric airplane is going to crash because they forgot to switch tanks, or the carb iced up, or the engine swallowed a valve, etc. And the “fuel” gauges will actually be accurate. Battery technology isn’t where it needs to be to make these aircraft practical, but there are a lot of people investing a lot of research into making better batteries. If they succeed, electric propulsion will have a lot of advantages.
All superficial things considered :
If you’re judging by energy per pound, ease of use, and flight performance, avgas is king for most flying.
Batteries are cleaner and simpler, sure, but they come up short on range and carry full weight from takeoff to touchdown.
MIT’s sodium–air fuel cell looks promising on paper, but once you add in the corrosive byproducts, constant heating needs, and complex systems to manage it all, it starts to feel more like a lab project than something ready for the ramp.
Battery/ electric propulsion systems are not simple.
As an example, a Tesla 3 has 3 cooling system loops, to cool the motor[s], the controller and the battery. In cold weather the battery has to heat itself to maintain an operating temperature.
An E-aircraft will be as complex with more cooling requirements than an E-car, since the motor, controller and battery will be operating at much higher power levels than in an E-car.
The plot thickens. No free lunch. Thank you Jim.
I’m an electrical engineer, pilot and Cessna owner…it’s what I know.!!
Sounds like it could catch on. Maybe some amount of time after I am too old to fly anything in need of propulsion, or perhaps long after my demise. I don’t think I’ll worry about this…
So, liquid sodium? What’s the downside of liquid sodium besides being self igniting in the presence of…air? Chemists, please comment.