4 ms·
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 ca
by korethr 7y ago
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.
- ProfessorLayton 7y agoIt'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-intelligent-all-wheel-drive/ https://newsroom.toyota.eu/new-2019-toyota-prius-with-intell...
- DesiLurker 7y agoI'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.
- the8472 7y agoOn the other hand a plane loses weight as it burns fuel, this isn't true for batteries.
- mrpurple_ 7y agoIt is losing weight but only very very little.
- the8472 7y agoIf we want to be all pedantic then certain battery types (metal-air) gain weight as they are depleted.
- DesiLurker 7y agothats not true, fuel weight is about 1/3 of total plane weight.
- the8472 7y agoI assume he's talking about the mass defect in discharged batteries
- DesiLurker 7y agoI 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.
- korethr 7y agoMy 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.
- hcknwscommenter 7y ago5032.5 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.
- rini17 7y agohttps://en.wikipedia.org/wiki/Zinc%E2%80%93air_battery https://en.wikipedia.org/wiki/Zinc%E2%80%93air_battery