Lithium-Sulfur Battery Project Aims to Double the Range of Electric Airplanes | Hacker News Reader

Lithium-Sulfur Battery Project Aims to Double the Range of Electric Airplanes

spectrum.ieee.org

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Electric aviation always strikes me as an odd idea, given the low power densities that batteries achieve.

By contrast, biofuels are available today and can be made from agricultural waste products (2nd gen), in bioreactors using salt water algae (3rd gen) or electrochemically (4th gen).

They're potentially a drop in replacement for regular fuel - for planes, but also for cars.

Due to the lower energy requirements for building ICE cars versus batteries, a biofuel powered car will be much better for CO2 emissions than an electric one, which you need to drive for many years before it beats a similar sized gasoline fueled car on sum CO2 emissions.

So why are we not heavily investing in biofuel infrastructure? Why are electric motors and batteries hyped up as being the tech of the future? That's an honest question, since I've yet to hear a well-founded physical or engineering argument against it.

Don't biofuels take up way too much land to be economical? A large portion of US crop area is for corn and soybeans for ethanol and biodiesel. I can see the reasoning behind biofuels for aircraft, but I don't think biofuels are sustainable if people continue driving as much as they do now.

> Don't biofuels take up way too much land to be economical?

Can we turn solar energy into hydrocarbons? (Water and CO2 are total feedstocks.)

Yes.

Hydrogenate CO2 to methanol with electrolytic hydrogen produced from clean electricity [1]. Reform methanol to hydrocarbons [2] that can burn in existing engines. (Or burn methanol itself in slightly modified engines. This offers somewhat lower energy density per tankful of fuel, higher total efficiency from electricity-to-motion.)

Synthetic fuel makes sense for fueling aircraft, rockets, long distance shipping, collectible historic cars, and other niches. It doesn't make sense for fueling everyday ground transportation. Too much of the original energy is lost in the chain of chemical transformations and the chemical processing requires facilities on the scale of conventional oil refineries. The total cost is lower to electrify ground transportation and supply it with clean electricity than to transform clean electricity into liquid fuels for equivalent legacy vehicles.

[1] https://www.frontiersin.org/articles/10.3389/fchem.2018.0044...

[2] https://en.wikipedia.org/wiki/Gas_to_liquids

> Synthetic fuel makes sense for fueling aircraft, rockets, long distance shipping, collectible historic cars, and other niches

What's driving the emphasis on lightweight batteries for aviation over better synthesisation?

For UAVs, battery/electric systems don't just reduce fueling costs. They deliver features unobtainable from hydrocarbon engines:

- Extremely long flight endurance without refueling (in large solar/battery UAVs)

- Low noise

- Odorless

- Simpler and cheaper manufacturing

The advantages are much less obvious to me for aircraft that transport passengers or significant cargo loads. I suppose that the companies developing them are hoping to make the lifetime total cost of operation cheaper than existing aircraft serving the same niche. That seems much tougher to achieve.

EDIT: Bye Aerospace, the company described in the IEEE article, says they are targeting a particular niche:

https://byeaerospace.com/eflyer/

They intend to replace trainer aircraft that burn leaded aviation gasoline. This polluting niche fuel costs significantly more than kerosene. Maybe the economics can work out for this case.

Thank you. Incredibly clarifying.

There's no emphasis on lightweight batteries for passanger aircrafts, both batteries and biofuels are pursued and both are marginal compared to traditional fossil fuels.

Biofuels always sounded wasteful to me. But this post makes a lot of good points on its merits that I hadn’t considered.

How efficient is producing biofuels using renewable energy as a way of storing power long term?

Alternatively, we can use electricity and heat from renewable power sources to synthesize kerosene from CO2 and water.

For me, the experience of operating an electric vehicle is so much better than biofuels: refueling at home, simplicity of the drivetrain, and quietness/smoothness all are huge improvements over ICE vehicles.

For your comparison between ICE and electric vehicles, are you counting the fuel processing and delivery infrastructure? Electric delivery infrastructure has very low emissions once it's in place, and production continues to get better (especially with distributed models like rooftop solar). Also, do you have a citation for "...lower energy requirements for building ICE cars versus batteries"? Yes, battery production is fairly intensive (economically and resource use), but (like I said above) the drivetrain is vastly simpler.

I can't see how electric general aviation will become widespread without a monumental breakthrough. But it's hard to beat electric for ground-based transportation, IMHO.

This article addresses your questions:

https://www.carbonbrief.org/factcheck-how-electric-vehicles-...

Check out the "lifecycle greenhouse gas emissions" figures. It's really interesting in how it shows how different the total emissions are depending on which country you charge your electric car in. Also, depending on which electric car model you have, the lifecycle emission improvement over a conventional one might be relatively slim, depending on where and how much you drive it.

By and large, electrics are at a substantial disadvantage due to both battery production and fuel cycle emissions, which then gets compensated over time as you drive the car.

Edit: There's also this super interesting article on Wikipedia: https://en.wikipedia.org/wiki/Environmental_aspects_of_the_e... which is largely consistent with the Carbonbrief article.

I've seen the graph there and I find one thing troubling:

Lifecycle greenhouse gas emissions for conventional and electric vehicles (by country) in grammes CO2-equivalent per kilometre, assuming 150,000 kilometres driven over the vehicle lifetime.

That's less than 100,000 miles - rarely do cars travel less than that during their lifetime.

Specifically in my country that would mean using the car for ~10 years, meanwhile the average passenger vehicle age is 14 years!

As for biofuels: land and water usage are major hindrances. For example you need ~1800 liters of water to grow 1kg of soybean. Sure, you can use waste products for fuels, but they need to be waste from something.

My gut feeling is electric cars may very well last significantly longer than internal combustion ones. Assuming a 100,000 mile lifetime range smells to me like someone putting their thumb on the scale.

> Due to the lower energy requirements for building ICE cars versus batteries, a biofuel powered car will be much better for CO2 emissions than an electric one

What's the GHG emissions for the different types of biofuels ? What's the land use needed ? How abundant ? Etc.

For general aviation (small prop planes), it's because so much of the cost of running one comes from the engine. The most common piston engines are supposed to make it 1500-2000 hours between overhauls, and an overhaul runs around $16K, so you have to save $10-15 per flight hour in engine reserve to pay for the overhaul when it comes due and to cover any unexpected maintenance between overhauls. Plus, there are consumables that aren't cheap -- oil changes, spark plugs, and so forth. A brushless electric motor has a lot fewer moving parts, and a lot less vibration, so overhauls could be cheaper and with a larger interval between them, and regular maintenance would be cheaper as well. Plus, no leaded avgas to mess with, no worries about reduced power at higher altitudes where the air is thinner, no worries about carburetor icing or adjusting the air/fuel mixture. Sounds like a win to me.

Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against.

This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor).

That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well.

I’m not entirely sure how you’ll cool the engines at high altitudes, combustion engines can use their exhaust to cool the engine.

Using large radiators on an electric engine kinda defeats the purpose especially when they can’t be effectively be built into the body of the engine.

I'm not sure why largeish passive radiators are a problem. If you're worried about the aesthetics you can put them inside of ducts and pass the air over them. This doesn't seem like it should be that big of an issue.

Plus you'll probably want to recover some of that waste heat to heat the cabin.

How exactly does one use exhaust to cool the engine? This is something I've never heard of in the context of cars of motorcycles, where exhaust is generally removed as soon as possible, and if it's used for anything, it's for running turbochargers that make the engine even hotter.

Not sure what GP was referring to, but they might have had in mind the practice to run engines a bit too rich, so that fuel runs through the system with insufficient oxygen to burn, ie as a liquid, which cools the engine.

There are two strategies for keeping cylinders cool in cruise: push the red mixture knob forward for a rich fuel/air mixture setting that uses unburned fuel to lower cylinder head temperatures, or pull the mixture knob out to such a lean setting that an abundance of air and reduction in engine power and heat cools the cylinders. Running aircraft engines “lean of peak” (LOP) typically reduces airspeed about 5 percent in cruise while lowering fuel consumption about 20 percent—but not all engines are capable of LOP operations, and doing it wrong can cause permanent and costly engine damage.

source: https://www.aopa.org/news-and-media/all-news/2019/may/flight...

Nope, combustion engines generate MUCH more heat than electric engines. Also they don't use exhaust to cool the engine, with all due respect, your argument is entirely backwards. It will be easier to cool electric engines and they will need smaller radiators (if any, have you seen radiators on engines in drones?). More probable is that you will need to heat batteries so that they don't freeze at higher altitudes.

But no worse of a problem than it currently is for fossil fuel engines, right?

I don't math good, so I'll just ask :

Does the RPM of the electric engine matter if the RPM of the propeller can't exceed a certain rate (because the tips of the prop will break the speed of sound, which you generally don't want for comfort reasons)? I mean this in terms of power generation and pushing air.

Gas-powered aircraft don't have transmissions, and gas engines tend to only be efficient in a narrow band, so I'd guess that the RPM limitations of an electric motor (which can generally tolerate a wider range) wouldn't be an issue as long as it's a reasonably-engineered motor.

Beyond a certain RPM range electric motors lose torque. Eventually you'd reach the limits of the bearings or the maximum centrifugal force that the rotor can withstand, but that's usually at a higher RPM than gas engines are made to survive. I would expect an electric motor designed for an aircraft would be optimized for maximum power/efficiency at whatever RPM band is optimal for the propeller, and the battery would be optimized for whatever voltage is optimal for the motor at that RPM range.

> Unfortunately the reduced power is still a problem at higher altitudes but for different reasons. Instead of oxygen starvation, the props will have less to push against.

Yes, but air-density-altitute problems are identical no matter what is spinning the prop, righ?

> This is still less of an issue as the electric motor can spin faster due to the reduced resistance. Higher speeds will create higher resistances from within the motor itself (which will in turn be increase the heat generated in the motor).

OK, so do the math for me here, I am a bit confused. I am not a motor whiz, but torque at the output converted to watts is going to be power at the input less i^2*r losses. The back-EMF will go up as the motor speeds up, so it may be at a somewhat less efficient point on the RPM-versus-torque curve, but the resistance of the actual wires doesn't change.

> That heat will also be harder to dissipate in lower pressures, but I suppose alternative cooling could be added as well.

I think that is all true. Seems to me you design cooling around the cooling capacity of the air flow at max operating altitude. Is that really a huge design constraint?

As another commenter noted, propeller speeds are limited by multiple factors, and generally complex aircraft use a constant-speed propeller that changes pitch. Because of this the increased electric speed likely isn't a factor.

Cooling the motors isn't a big problem. Their heat output is around 10% of the shaft power, compared to combustion engines that put out 200% of shaft power as heat. While piston engines need a lot of cooling air flow and often have cowls that are mostly open at the front, an electric propellor drive can have a small cooling vent.

The batteries may produce as much heat as the motor, and they have a more limited temperature range. Their service life decreases when run above 40 C. Tesla batteries are liquid-cooled.

Could low temperatures especially at higher altitudes be a problem? Aircraft probably get exposed to higher temperature gradients and in much shorter periods of time than a typical Tesla would encounter. Not sure about Lithium Sulfur batteries but reduced capacity at lower temperatures is common for many battery types.

It's certainly not a problem if the batteries are in the climate-controlled fuselage. It might be tricky to keep them insulated inside the wings (where fuel typically is.)

And the motor _should_ be a lot more reliable.

But discouragingly, just last August, in Norway, a Pipistrel Alpha Electro lost power shortly after takeoff and crashed. https://aviation-safety.net/wikibase/228207

Hopefully, these folks in Fresno, CA will get their electric aircraft flight school up and running sometime soon. But summertime in California's central valley is challenging: https://sustainableaviationproject.com/2018/10/28/we-hit-the...

"A brushless electric motor has a lot fewer moving parts"

Not to mention it makes a lot less weight to lift; the difference is huge.

But the battery would be much heavier than a fuel tank. You can't have an electric engine without a battery, so...

Well, there are fuel cells (that turn fuel into electricity).

Combustion engines are quite inefficient and so are fuel synthesis processes. batteries have a high energy round-trip efficiency. The energy density does not matter as much in cars, which after mobile compute have been the primary drivers for technology. The density may make more sense for planes.

Additionally 1st gen biofuels have poisoned the well to some extent due to competing with food crops.

The only reason we have electric cars being pushed as much as they are is because of Tesla, specifically Elon Musk. It is not some government program. If it weren't for them we would have the Nissan Leaf and that is about it. And it wouldn't have half the performance that even it has today.

Short answer: Monied interests don't really care, or don't think it is possible or feasible.

Electric motors and batteries are very efficient, and therefore extremely cheap to operate.

Biofuels make sense I think for aviation because you need that energy density to get a usable range, but the fuel is likely to be expensive for a long time. If you can figure out how to make liquid fuel for about fifty cents a gallon or so, it might be competitive with electricity for car use.

I don't know what the energy requirements are for modern batteries, but I expect it varies a lot depending on battery chemistry. Battery sizes vary a lot, too.

For very short distance flights, electric aircraft are already viable. For example, Harbour Air Seaplanes operates a 20 minutes flight between Vancouver, BC and Victoria, BC. They use float planes because both cities are on the coast. Their first electric aircraft should take off in a couple months.

One nice thing about electricity is that it's much cheaper than biofuels. This is because motors are much more efficient than engines, and because large generating stations are more efficient than small aircraft engines. BC is 95% hydroelectric which makes electricity even cheaper.

>So why are we not heavily investing in biofuel infrastructure?

I think the problems are more economics than physics. There was a huge investment in algal biofuels but it never proved economic. (eg https://www.greentechmedia.com/articles/read/lessons-from-th...)

I was a fan of Algenol for a while who were supposed to have algae that gave off ethanol cheaper than normal fossil fuel but it never worked out. Maybe something like that in the future will work.

This reminded me of a graph I thought was really interesting from a thesis on a solar powered UAV that shows energy density and power per weight for different sources of energy, I've highlighted the 500wh/kg line which is being claimed in the article:

https://i.imgur.com/nZSWpW1.png

Should be very interesting if they can deliver that level of performance.

The source document for that graph is here: https://ethz.ch/content/dam/ethz/special-interest/mavt/robot...

And described here: https://spie.org/news/1649-designing-solar-airplanes-for-con...

24,965,421,631,578 wh/kg (antimatter)

23,222,915,000 wh/kg (plutonium 239)

https://en.wikipedia.org/wiki/Energy_density#Table_of_energy...

If your goal is to use exciting power sources then those two have a flaw, they can be scaled down to fit the power needs of the plane, which would limit the spectacle you could achieve. The penrose process may be a better choice since there's a miniaturization limit.

You can, in fact, put a functional nuclear reactor on a plane: https://en.wikipedia.org/wiki/Tupolev_Tu-95LAL . Of course, it's a terrible idea, but it is feasible.

The holy grail would be to put antimatter on a spaceship.

Instead of the pyramid of fuel mass needed to escape a gravity well, the capsule would basically be constant mass.

An antimatter-powered spaceship could conquer the solar system or even the universe.

Russia sure does like mobile nuclear reactors

Reading one of the Feynman books, probably "Surely You're Joking, Mr. Feynman" I remember the section about nuclear-powered patent ideas, like nuclear-powered spaceship, plane, boat, ... and submarine. We only did the boat and submarine. I guess we did the spaceship too.

They are still working on it for cruise missiles https://en.wikipedia.org/wiki/9M730_Burevestnik

No lithium-sulfur battery (so far) survives enough charge-discharge cycles to be attractive for portable electronics or electric cars. But higher energy density makes it potentially attractive over lithium ion batteries for aircraft. Even if you have to replace the batteries after e.g. only 100 cycles, that could enable multi-month missions for solar powered high altitude UAVs.

The Airbus Zephyr was using lithium-sulfur batteries back in 2015:

https://cosmosmagazine.com/technology/lighter-cheaper-satell...

Flying at higher altitudes reduces drag and increases efficiency due to the thinner air. Ideally, we'd be flying planes a lot higher than we do today. The problem is combustion engines need oxygen and the higher you go, the less oxygen you have available. Electric planes don't have that problem which could be the secret to faster global travel.

The SR-71 and U-2 flew plenty high and had enough oxygen. When you’re high enough that there’s no oxygen, you have no atmosphere, and then your wings and turbines don’t do anything useful. That means it’s rocket time.

Also, depressurization happens, and the risks and complexity of workarounds increase with altitude.

> The SR-71 and U-2 flew plenty high and had enough oxygen

Actually none those two planes had fully pressurized cockpits. I guess they were pressurized at 26k feet altitude pressure. That's why the pilots were literally wearing spacesuits.

https://images.app.goo.gl/KjqGARoyiddhDxAE6

GP was referencing oxygen in regards to combustion, not the cockpit.

The space suits weren't because of the cockpit altitude (when the cockpit was pressurized), they were because the crew were operating well above the Armstrong Limit and no amount of oxygen could keep them awake in the event of depressurization without the suit.

Unless you're thinking supersonic travel, the "coffin corner" (difference between stall speed and Mach 1 (or Vne)) also narrows as you get higher. In an extreme case, the U-2 cruises at 70,000 ft, and at that altitude the stall speed is about 10 knots below the maximum speed.

Turbos go a long ways to overcoming the thinner air problem with ICE engines. There is another, arguably bigger reason flying above 10k feet is uncommon—oxygen requirements. Few small planes are pressurized and O2 systems are expensive extras which require additional maintenance and filling. Systems which service passengers as well even more-so.

I flew a Cessna 206U for a few years and it was fine up to 15k feet or more, but I'd almost always skim just below 10,500 so I didn't have to bring along supplemental oxygen. I suspect that's where most of these guys will live, in the 7,000-10,000 foot range. It's much faster then flying near sea level but without the problems with O2.

Why can’t planes just pressurize the outside air for breathing? No need to bring your own o2?

Big planes do this. In a little plane, that's expensive and heavy and you still need to have a backup system in case it fails.

Because for a small aircraft, the weight and energy requirements of a compressor is greater than the weight of a small tank full of oxygen.

And the increased structural and sealing requirements of being several PSI above ambient. That will add cost, complexity and weight.

Most planes use turbine bleed air to pressurize the cabin. This of course implies that the plane has a turbine and a cabin that can hold positive pressure. They are usually $500/h+ planes.

Is the speed of sound barrier higher too?

The speed of sound decreases as altitude goes up, unfortunately.

Sometimes. Speed of sound decreases with temperature. At any flight level above 300 speed of sound stays the same and the increases with altitude.

Good to know, thanks for the correction.

No, it's lower, speed of sound is 38.94 * sqrt(temperature) with the speed in knots and the temperature in Kelvin.

Every airliner notices this when climbing, you get closer to the Mmo speed (maximum Mach number for operating) while at the same time the IAS decreases and you get closer to a stall due to decreasing airdensity. At some point you cannot go faster due to maximum Mach while you also cannot go slower due to minimum IAS to avoid a stall. At that point you cannot climb higher even if your engines have the power.

Flying at higher altitudes also increases chance of developing cancer due to cosmic radiation.

https://www.ncbi.nlm.nih.gov/pubmed/12862322 https://www.cdc.gov/niosh/topics/aircrew/cosmicionizingradia... https://www.cdc.gov/nceh/radiation/air_travel.html

Couldn't this be curbed by better radiation shielding on the plane itself?

Not necessarily. Photons at Compton-dominant energies (200-5000 keV) can’t be effectively shielded because every material (including, eg, lead) absorbs them about the same. Also, shielding is inevitably heavy.

The first study:

> METHODS: A cohort of 10,051 male and 160 female airline pilots [...] was followed for cancer incidence [...].

> RESULTS: Among male pilots, there were 466 cases of cancer diagnosed vs. 456 expected.

So, a heightened risk of (466-456)/10051 = 0,001% if I read this correctly.

Or am I misunderstanding something here? I guess so, because this seems close to negligible.

You forgot to multiply by 100 when you went to %, so it's actually 0.1% higher. Still pretty low.

ah, thanks, silly mistake! Anyway, still a surprisingly low risk. Seems within the margin of error to me.

> Electric planes don't have that problem which could be the secret to faster global travel.

However, to make it feasible for commercial air travel, you need much faster charging times, because airlines want to have as short of a turnaround as possible for their planes.

But I can totally see how electric planes would work for hobby piloting in the first place.

They could also make the battery swappable. That is more of a logistic problem, but perhaps easier.

Also doable. Either way, don't underestimate the simplicity and energy transfer rate of a fuel truck.

On short routes, passenger loading time dominates. Electric could work for going between islands for Hawaii, Isle of Mann, New Zealand, and Japan. It could connect Martha's Vineyard or Long Island with various nearby places.

It's a ferry replacement.

The majority of air passed through modern jets is bypass air. I don't think the availability of oxygen is the clincher.

Things like refueling speed, the weight of the batteries themselves (which don't burn up as you consume them like combustibles, and the overall better energy density of combustibles probably make the whole thing more attractive at today's tech.

Flying at higher altitudes in lower density air also reduces lift, though [1]. A plane does get extra benefit from flying higher as drag on the fuselage is reduced, but there is a limit — the plane has to go faster to make up for the reduction in lift due to the reduction in air density. An electric plane ends up being able to fly a little higher, but not by much. See these notebooks for details:

https://github.com/gusgordon/atmosat/blob/master/atmosat.ipy...

https://github.com/gusgordon/electric_jet/blob/master/plane....

[1]: https://en.wikipedia.org/wiki/Lift_(force)#Lift_coefficient

This is not correct. Here are the formulae:

Lift = weight = 0.5 * \rho * v^2 * S * C_L

Power = drag * speed = 0.5 * \rho * v^3 * S * C_D

where \rho is the air density. A good assumption for cruise is (C_L/C_D)_max. If you do the math, you realize that for a constant weight, the power output must be proportional to:

Power \propto \rho^{-0.5}

For example, to fly a plane at 10km above sea level you need twice as much power than sea level. This is totally independent of your propulsion system.

It's not just that they need oxygen. Cold air is good for thermal efficiency. Even if the heat source were nuclear (as in https://en.wikipedia.org/wiki/Project_Pluto engines) there would be a problem with the thermal efficiency. (see https://en.wikipedia.org/wiki/Thermodynamic_cycle and https://en.wikipedia.org/wiki/Carnot_cycle for why) Air temperature rises from about 217 Kelvin to about 270 Kelvin as you climb up the stratosphere. The temperature goes back down again as you get up to around 50 km (80 miles) but that is really high altitude.

There is an upside though. Subsonic aircraft can go faster as the temperature rises because the speed of sound increases. It goes from about 295 m/s to 329 m/s. The extra 11% is nice.

Graph of the air by altitude: https://upload.wikimedia.org/wikipedia/commons/9/9d/Comparis...

> They said the Oxis battery would provide “in excess” of 500 Wh/kg, a number which appears to apply to the individual cells, rather than the battery pack, with all its packaging, power electronics, and other paraphernalia.

For comparison the energy density of gasoline is 12,200 Wh/kg

https://xtronics.com/wiki/Energy_density.html

Although it's more complicated than that, because IC engines are only have about 15% efficiency, whereas electric motors are up at about 80%.

Real world efficiency for modern gasoline engines is closer to 20%. In hybrid applications they can get up to 35%.

>In hybrid applications they can get up to 35%.

But surely that is largely due to the electric element of the combined powertrain? The ICE part hasn't magically become more efficent?

The whole point of a hybrid is to run the ICE only in its most efficient region. At low loads the electric battery drives the car. If the battery gets too low, then the ICE runs, but rather than running at a low (and inefficient) load, it runs at its sweet spot and the excess power it generates charges the battery. When the demand is high, rather than running the ICE hard, it is kept at its sweet spot and the electric motor boosts power instead.

So yes, the ICE part is more efficient in a hybrid, though it isn't magic.

The ICE is more efficient because the design constraints have changed.

Here's a single example with the Prius:

1. The intake valve stays open for part of the compression stroke.

2. (1) means that the compression ratio for the compression stroke is lower than the compression ratio for the power stroke.

3. The efficiency of the engine is limited by the compression ratio of the power stroke

4. The compression ratio of the compression stroke is limited by engine knocking (if you compress a mixture of fuel and air too much, it will spontaneously combust)

5. So this engine can be made more efficient than an engine that closes the valve for the entire compression stroke

6. Some (non-hybrid) engines have variable valve timing and can do (1) some of the time, to a small extent.

7. Doing (1) to a larger extent makes the low-RPM torque very poor.

8. Electric motors have excellent low-RPM torque, so (7) is compensated for by having an electric motor run at low speeds.

Somewhat, by putting an intermediate step between the gas/diesel engine and the work the engine can be run more efficiently as well, instead of having to speed up and down at the whims of the driver it can run at a more steady and constant pace and the electric engine also allows regenerative brakes to be used which can recapture a lot of the energy lost in a normal diving situation during braking.

https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio...

Over 30% efficiency for gasoline, and over 50% for diesels.

You cannot look at the energy density of the fuel by itself.

Electric motors are lighter than ICE engines per HP and torque unit.

Emrax 268 (electric engine designed for aircraft) - 6.12 HP/lb

GE 90-115B (jet engine used in 777) - 6.10 HP/lb

Junkers Jumo 205A (diesel engine used in Ju 86C-1) - 0.66 HP/lb

Electric also scale down better so for light weight UAV a higher energy density battery will open up a bunch of use cases.

Absent notes to the contrary, I'm going to assume that's just the raw energy density. However, by my understanding the typical gasoline engine in a passenger car has an efficiency of something in the low-to-mid-30s percent. So, assuming 33%, that's 4026 Wh/kg of actually-useful energy density, the rest of the energy wasted as exhaust heat.

Looking things up, I see different numbers for electric motor efficiency, ranging from the low 70s to the low 90s percent. As an overly simplified example, let's assume an electric motor in a car has an efficiency of 80% (in reality, they may be better). At that efficiency, a battery pack only has to get to 5032.5 Wh/kg to achieve the same practical energy density as gasoline, less than half the actual raw energy density. That is probably an easier number to reach than trying to achieve the same raw energy density of gasoline.

Nobody has made this argument in this thread (yet, as I type this), but I've seen it made before and made it myself. Yes, at those energy densities, batteries are highly dangerous if something goes wrong (like a crash) and they release all their energy at once, but so is gasoline. At practical ranges, you're potentially sitting on a pile of high-explosives either way. But, I'd hope a 5032.5 Wh/kg battery is easier to make safer in a crash than 12,200 Wh/kg gasoline is.

It's worth noting that the latest generation of the Prius has reached 40% thermal efficiency [1]

[1] https://newsroom.toyota.eu/new-2019-toyota-prius-with-intell...

I've also read good things about israeli aquaris engine. they've optimized it to brim for electricity generation for series EV applications & made it quite light/cheap. we'll see.

On the other hand a plane loses weight as it burns fuel, this isn't true for batteries.

It is losing weight but only very very little.

If we want to be all pedantic then certain battery types (metal-air) gain weight as they are depleted.

thats not true, fuel weight is about 1/3 of total plane weight.

I assume he's talking about the mass defect in discharged batteries

I agree with most of what you said, however, internal combustion engine is nowhere near as efficient or complete combustion as a jet engine which can get to high 60s with heat recovery. I with somebody had thought of making a series hybrid with a small but optimal turbine based battery charger.

the other benefit is the weight of gas goes down as the flight progresses which is about 1/3 of the total weight so make the design a bit worse but I believe it would still be compensated by cheapness of solar electricity based charging.

on a related note, IMO this is the reason the whole fuel cell shtick from toyota is BS. Because if it were anywhere near economical & scalable then we would be seeing fuel cell powered drones all over.

My internal pedant compels me to note that a turbine-based engine (which all aviation jet engines are, AFAIK) is still an internal-combustion-engine. Not all internal combustion designs rely on reciprocating pistons. Though you are quite correct that turbine-based designs can achieve efficiencies that most piston-based designs typically can't.

I was thinking in terms of passenger cars for some reason (despite the word "airplane" being in the title), where piston engines are far more common than turbine engines. That said, I stand by the main point of my post, which is that you can't just consider the energy density of your energy source in isolation, you have to also account for the efficiency by which you convert that energy source to useful work.

A thing I find myself wondering about now is the viability of an arrangement similar to that of cargo trains. IIRC, the locomotive of cargo trains is a hybrid of sorts. An internal combustion engine running at relatively high efficiency drives a generator, which in turn powers electric motors which drive the wheels. This sound similar to your hybrid idea, perhaps with the addition of batteries.

I think one of the biggest challenges to hybrid systems is to make sure the overall system efficiency doesn't drop below that of the current typical systems which it is intended to replace. Say you have a plane with turbojet engines running at 60% efficiency. And we replace it with a charging turbine-engine at 70% efficiency, coupled to an 80%-efficient charging system, coupled to a 90%-efficient electrical engine. In isolation, each of those numbers sound better than the 60% efficiency of the turbojet engine. However, efficiencies combine multiplicatively, and so 70% * 80% * 90% = 50.4%. That's overall worse than the turbojet we started with. This new system better be much lighter than a turbojet to be worth it.

Yes a turbine engine is a internal combustion engine, specifically a Brayton cycle engine vs a Otto or Diesel cycle piston engine.

Turbines are not used in cars because they have high rpm and low torque requiring large transmissions to gear them down properly. They also are not very responsive to throttle requiring spin up, very noisy and have have an abundance of hot exhaust to deal with. You can't muffle them as easily.

Series hybrid diesel electric trains are actually less efficient than diesel mechanical drive trains. They are used because of the need for precise traction control to prevent wheel slip, they sacrifice efficiency at speed though to attain it.

Double conversion from mechanical to electrical back to mechanical will always be less efficient than a straight through mechanical drive once up to speed, this is why nearly all hybrid cars are parallel and go full mechanically coupled at highway speed.

So, how would you design a hybrid powerplant for an airplane? Electric engine at low prop/fan shaft speeds, combustion engine takes over at higher speeds?

5032.5 Wh/kg

As far as I know, there are simply no rechargeable battery chemistries that have any (even theoretical) hope of achieving this level of specific energy.

https://en.wikipedia.org/wiki/Zinc%E2%80%93air_battery

That's looking at the energy density of the fuel itself, not counting the weight of the engine, several gallons of oil, air filters, fuel pumps, etc etc. The weight of equipment required to extract that energy from gasoline is a lot higher than the weight of an electric motor.

90something% efficient electric motors vs 40-60% efficient gas engines however.

If you can make up in other areas like drag in the design because of the different power plant it's not such a mental gap to close.

Have they solved the problems of

1. sulfur plating the anode? 2. The cathode swelling during charge and mechanically damaging the cell?

If so, I'm excited! But last I checked thier solution to #2 was to clamp the cell between metal plates, which nullified the gains in energy density when considering the mass of the clamps.

1 & 2. Not really and not really.

Curious, have you ever heard of Boron-Nitride Nanotubes (BNNT)? Basically a new nanomaterial that is similar to carbon nanotubes except it has the rare property of being a high energy band-width semiconductor whilst having excellent thermal conductivity (basically solves problems 1. and 2.). Only problem is that its really hard to produce in mass quantity... well maybe... have a look at the company PPK

Whoever creates a better battery, which moves us away from Li, will be a trillionaire.

I do not think lithium or a chemical battery is the right solution. Too expensive and polluting. I am hoping for a silicon battery that will take over the world.

> Whoever creates a better battery, which moves us away from Li, will be a trillionaire.

So now it should be a golden age to be a chemical engineer or a physicist specialising in battery technology research I suppose.

> I am hoping for a silicon battery that will take over the world.

Chemical Engineers and Battery Technologists will be the new 'Software Engineers'.

As a chemical engineer who did my Ph.D in Li-ion battery anodes, I switched to data science immediately afterward. So I wish you were right, but I wouldn’t count on it any time soon.

uh... unless that data science is on like cancer research or something please go back

Lithium is here to stay for a while because it has the best ion mobility of any reactive metal. Sodium is much fatter and slower, but might be good for grid storage.

I can imagine two really amazing effects from electric airplanes

1. Noise pollution could be significantly reduced . So many flight paths over homes.

2. Cost efficiencies could be amazing with fuel anywhere from 25% - 35% fluctuations, https://www.statista.com/statistics/591285/aviation-industry...

I imagine the operating cost of electric would be even less due to the simpler design of engines for electric vs gas

In my dreams airplanes could be designed to be more comfortable due to fixed operating costs... that would be nice

See also lithium-air (aka lithium oxygen) battery:

https://en.wikipedia.org/wiki/Lithium–air_battery

I've been following this company since their announcement of "300Wh/kg soon" a few years ago.

The pace of their progress is not jaw-dropping, but if anyone has a shot at bringing this chemistry to the market, it's them.

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