Comment by zaarn | Hacker News Reader

Comment by zaarn | Hacker News Reader

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The simple reason ICE's will remain in use is energy density, no matter how much you wish it wasn't true.

Diesel packs 48 Megajoules per Kilogram of Mass, Jet Fuel 43 and plain old wood does 16.

Accumulators/Batteries manage betweed 0.25 and 0.9 MJ per Kilogram, exception being LiPo which can do 1.8 MJ per KG.

ICE's usually can do a 30% efficiency, even in crude designs, so even accounting for that you get like 16 MJ per KG of fuel out of a diesel engine while LiPo even at 90% efficienty manages a 1/10th of that.

If you're weight constrained like when you want to fly an aircraft, there isn't much choice but to have a combustion engine of sorts.

If you wanted to replace an aircraft gas turbine (usually a turbojet or turbofan) with an electric one, you'd have to make the engine and battery combo that weighs no more than a jet fuel engine while having the same power output.

For raw numbers

A Boeing 737 MAX has an engine that weighs 2'780 kg. Two of them. It has a TSFC of 0.012kg/kN*s which works out to 70 MW on continous thrust. The fuel weighs 19 tons at full load.

So the electric replacement for this engine now needs to output twice 70MW of power for up to 7'000 kilometers of range. At normal cruising speed that's 10 hours.

140MW over 10 Hours is 1400MWh / 2.5 Gigajoules. The best battery tech we have will result in a 2777.8 ton battery. If the electric motor can do 100% efficiency.

2777.8 tons vs 19 tons. (If my numbers are correct, which should be the case)

NASA did a study in about 2010 of electric aircraft where they concluded that battery energy density needed to be just 3 times the current state of the technology before it would become economically viable. [1]

[1]https://ntrs.nasa.gov/search.jsp?R=20110011311

(this is not the actual paper I remember but it is very similar)

The batteries need to weigh less than the fuel because fuel weight disappears as it’s used.

Yup, I didn't account for fuel weight going down overtime, in part because I forgot and in part because it complicates the equations.

Even not accounting for that, the fuel tank wins over batteries, which would be like stapling a third engine on the aircraft that doesn't work, yet wanting the same distance and speed performance from the aircraft.

I also tried to figure out how much energy you could pull from solar panels to cover energy usage and reduce the battery use but A) doesn't work for overnight flights and B) the energy output is abysmal for various and dozens of reasons even if you magically doubled the surface area that the sunlight reaches. By over-the-fist estimations, the weight you save on the battery would be lost on the solar panels.

If the numbers and units are correct in your earlier comment then it would be

like strapping another 1000 engines that don’t work on the aircraft

Because you wrote the engine weighs 2700ish kg and the theoretical battery would way 2700ish tons.

Ah, yes, correct. 1000 deadweight engines.

But the difference between kg and Ton doesn't matter much, in both cases the aircraft won't fly very far.

The difference is dramatic. I mean to say: in the drama of it.

An aircraft with a third engine isn’t an altogether uncommon sight, or wasn’t not too long ago.

An aircraft with one thousand extra engines goes a long way to dramatically demonstrate how ridiculous battery powered flight is with today’s, or any near-future, battery technology.

If travel speed isn't a concern could blimps be powered by batteries and solar?

Yes, blimps would be fairly eco friendly.

Though they do have other drawbacks, namely that the only good carrier gas is hydrogen and that has... a rather explosive nature. Helium is only half as good.

The carrier capacity of a blimp is kinda abysmal.

But what bit of the battery are you weighing?

If you look at how Tesla do their batteries there is the actual chemistry bit on some thin foil that is separated with some thin plastic. This is then put into a metal cylinder. This is then wired up in a module that holds the cells. This is then put in a tray that holds all of the modules. Then these trays are placed into a semi-structural floor unit which then has a titanium protection shield under the car.

So now the chemical bits that could probably fit in a beer bottle now weigh a metric tonne and are too big for all but American roads (Tesla cars are boat sized in Europe, too wide for garages and country lanes or city streets).

Meanwhile ICE juice is measured without the matryoshka dolls of containers within containers. Nobody takes the weight of the container into consideration.

Hopefully someone will one day invent structural batteries where the whole thing is a battery that neither zaps you or boils you to death. Then adding a few seats will add to the range. If you have to have a cabin anyway then, if it had dual use as energy storage, the actual energy density would not have to be in the same league as highly explosive hydrocarbons.

There should be easy wins in this, e.g. a bicycle. The current generation of electric bikes go for the bracket + holder + container of containers mounted as high as possible for poor centre of gravity. I am sure there are exceptions but ultimately the battery will end up in the frame tubes and you could double energy density plus power to weight just by taking the integrated approach.

I know nothing about engineering and why batteries have to be ultra bulky but I hope that we don't just stick with ICE because petrol is more energy dense.

I took the numbers from wikipedia which to my knowledge refer to the raw chemical energy density of the battery without any additional control circuit.

The weight of a fueltank is not that much. Modern airplanes use the wings for fuel storage, you get the mostly for free. That goes for other storage tanks too, it's largely just reused space. You could replace that with battery space but...

>Hopefully someone will one day invent structural batteries where the whole thing is a battery that neither zaps you or boils you to death.

A battery that has the same energy density as jet engine fuel will, upon thermal runaway, behave similar to jet fuel being set on fire. That is the cruel reality of fuels.

The chemical reactions happening in jet fuel are immensely more exotherm than batteries. Batteries need to put out their chemical energy into electricity without being exotherm (ideally) but that is inherently less efficient.

And as mentioned in another comment, batteries have an inherent and unfixable flaw; they do not get lighter when used.

A ~2700 Ton battery will weigh 2700 Tons on takeoff and 2700 Tons during landing.

19 Tons of Fuel weigh 19 Tons on takeoff and maybe a couple hundred kilos during landing. Not having to bring all the fuel all the way saves a lot of energy.

>the actual energy density would not have to be in the same league as highly explosive hydrocarbons.

As mentioned above, if your battery has the same energy density as highly explosive hydrocarbons, it's mostly going to behave like highly explosive hydrocarbons once it goes into runaway.

The reason for that is rather simple; there is no battery material (that I'm aware of) that provides high density without also being able to release the same energy via method of rapid unplanned combustion.

>I know nothing about engineering and why batteries have to be ultra bulky

For an airplane, every kilogram matters immensely. Jet fuel is energy dense and thusly you can safe a lot of fuel. On a modern plane fuel is also saved wherever possible, ie by riding jet streams, by reducing high thrust sitations, etc.

Batteries have to be bulky for several reasons; any LiIon battery needs to have the charger hooked up to every battery cell. If you don't do that you run into a potentially detonating battery cell ones it is overcharged (which will happen if you don't balance cells)

On top of that, battery chemistry is largely very simple. Take an oxidizer and a reductant (oxidizers don't need to contain oxygen here). The properties must be such that during oxidation they give of electric charge.

Ideally the reaction is auto-reversable by applying a current to the battery.

This process must run very cool and slow when using up electricity but the reality of the matter is that it doesn't. It'll always run a bit hot.

If it gets too hot, it will almost always result in the battery switching from electric to combusting once it reaches the exothermic threshold (ie, the temperature at which the reaction becomes fully self-sustaining).

Batteries WANT to be discharged. Every battery on this planet is longing to be discharged instantly in a big hot explosion. There is an energy difference that needs to be released.

If your battery has the same energy density as jet fuel, this reaction wants to happen even harder than already and if it goes nuclear it'll behave like jet fuel.

Except worse because lithium-fires are hard to extinguish if not impossible unless you can cool it fast enough (which is impossible once it reaches a certain size)

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The sad reality is; Petrol fuels won't be going away. They are energy dense. They are relatively easy to combust with good efficiency. They can be stored relatively easily and safely. Same for transport.

Batteries don't offer these advantages because there is an inherent tradeoff.

We do have other alternatives. Alternative fuels with similar energy density, like Cryogenic HydroOxy or Methane. The burnoff from these is not as bad depending on what you do.

Airlines do want to save fuel but the reality is that there are some easy wins that have not been tackled. Before take-off the taxi journey to the runway is under jet power when you could have a more efficient tow truck to take the plane there. Concorde was the worst, B52s excepted, 2 tonnes of fuel just to get from the departure lounge to the runway.

Then there is the final approach. The Space Shuttle was somewhat brick shaped but they could glide that down from space to land on a runway. Obviously the safety of fare paying planet trashers coming back from their cheap flight is paramount and the peace of those living under the flight path matters not, however, planes could land space shuttle style, gliding in with no fuel being burnt. After 3000 miles of journey this ten miles of coasting might be rounding error, but what if you could catch the jet stream and, rather than take an hour off the journey time, coast as much as possible?

If the external costs of cheap flights had to be paid for then I am sure these options would become as standard as 'stop start' is with today's automobiles.

Regarding batteries destined to explode, there are ceramic batteries that do not spontaneously combust, imagine if parts of your car/plane/bicycle used such materials, e.g. if the boot liner or bumpers were made of such a lower density battery but doubled up with functionality? It can't be ruled out as possible.

I checked the ceramic batteries but all I found was replacing electrolyte with ceramic materials. This won't solve all the problems, only some that occur due to physical damage. Since the wikipedia energy density is to my knowledge based on raw chemical performance, there would be no change as ceramics would only allow you to pack existing battery packs more densely.

And we're still talking about 2700 tons of battery to carry with the plane. Even if you improved the performance of a battery by an order of magnitude overnight, you'd still perform significantly worse than petrol fuel.

Regarding the taxi fuel; that includes other fuel consumption like the APU burning until engine start and 2 tonnes would be the standard usage for larger airports, smaller ones have 1 ton.

Also consider that I calculated pure flight performance, if you'd use electric engines to taxi then you'd need an even bigger battery that you can't shed.

Taxing by a tow truck is possible but likely too slow (you have to pull an entire airplane).

Regarding landing; Spaceshuttles are hardly comparable to normal airplanes. The Spaceshuttle when landing goes a lot faster than most airplanes can safely do at that altitude and requires all brakes and a parachute to slow down on an extra long runway. Airplanes already minimize fuel consumption when there is no engine really needed but for landing a normal airplane you need the engines 999 out of 1000 times.

In case you were wondering, here's a light-weight fuel cell made from aluminum. It holds 20 gallons of fuel and weighs about 22lbs according to the description.

https://www.amazon.com/20-Gallon-Lightweight-Polished-Alumin...

This is just some random fuel cell I found on Amazon. There are probably even lighter options out there. Racers are always looking to shed weight.

There are also lots of examples where the frame of a vehicle can perform double-duty and hold fuel as well. I don't think that any regular production car will ever do this as it's probably not safe, but the idea isn't a new concept. Here's just an example:

https://bicyclemotorworks.com/product/built-in-fuel-tank-bik...

I think you read my comment backwards, petrol always has a simple tank, even in F1 where a lot of baffles and what not go into the design for safety and performance reasons.

However, on electrified transport from the humble bicycle upwards there is a lot of extra brackets and boxes getting added.

It is like cars from before the monocoque era, a chassis where none is needed. With the bicycle example you could put the batteries inside the frame instead of in a box mounted on the rear rack. These 'chassis style' products are not fine engineering and set a bad example.

Oh yeah I see what you're saying. It's an afterthought on so many electrified vehicles. Makes sense and I agree. We have a lot of the Limebikes in our area, and the battery is just bolted to your standard bike rack. Always seemed like an afterthought to me too. Like you're saying, even in a simplistic design, it seems like you could fit a lot 18650 cells inside a tubular frame.

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