Question About Electric Aircraft Propulsion

[cjl]

Yeah, that's definitely an interesting approach. I'd have thought that the extra induced drag at cruise would be a downside to increasing the wingloading that much, but I'll have to keep an eye on their results going forwards. It's definitely an interesting aircraft, and it's completely counter to the trends in commercial aviation, which is currently going towards lower wing loading and simpler high lift devices with each generation.

 

[skystare]

That's interesting, and I'll be curious to see more details about the design. However, this statement from the article:

"Because electric motor systems are more compact with fewer moving parts than internal-combustion engines, they are simpler to maintain and weigh much less "

is simply false. As discussed earlier in this thread, one downside of electric motors is that they weigh substantially more than turbine engines for a given power output. They are (based on the numbers given by @essenmein earlier in this thread) a bit lighter than piston engines, but not much lighter, and certainly not enough lighter for it to be a large advantage.

Electric motors fare even worse against jets and fan jets, as an e-jet also requires the weight of the compressors and fans to be added.
Against small piston engines though, they are the clear winner, being of comparable weight and having a huge maintenance/reliability advantage. This shows more in aircraft, where engine maintenance and replacement can easily cost as much as fuel burn, especially in privately owned and intermittently used craft. If battery weights ever really approach gasoline weight, all lightplanes will be electric.

 

[essenmein]

NIMBYism plays a large part, although I think that's more of an issue for spent fuel.

What you call superstition I would just call ignorance. People who don't understand science (read "most people") cannot / will not take the time to understand the issues and are only aware of the most sensationalist media reports on things like Three Mile Island and Chernobyl.

Its funny, I went from growing up in NZ, nuclear free rainbow warrior era, to now, where my position is not NIMBY, but the much better IMBSP wrt nuclear reactors (In My Basement Soon Please).

 

[gmax137]

my position is not NIMBY, but the much better IMBSP wrt nuclear reactors (In My Basement Soon Please).

When I lived in the snow belt I repeatedly volunteered to take home some spent fuel; I wanted to grind it up and mix it with asphalt to pave my driveway. No more plowing snow or chipping ice . Don't worry, I'm just kidding.

 

[essenmein]

When I lived in the snow belt I repeatedly volunteered to take home some spent fuel; I wanted to grind it up and mix it with asphalt to pave my driveway. No more plowing snow or chipping ice . Don't worry, I'm just kidding.

Someone else mentioned it, if its highly radioactive, that means there is still a lot of energy there. So instead of putting spent fuel in cooling pools for a few years and wasting all that free heat, put those somewhere near large populations, eg downtown. Voila, free low grade heat for buildings and keeping roads/sidewalks snow free in the winter.

IMO actually, if we embrace the nukes with both arms, put SMR's in building basements, heat, hot water, electricity. Put pipes in the roads to keep the whole city ice free in the winter (now live in Canada, so snow is a real thing...). Could you imagine the quality of life increase? No more clogged city, no more plow trucks, ice free side walks would have a large impact on winter life for elderly, disabled etc. No more having to repave every couple of years due to frost heave if your roads never freeze...

(getting way OT...)

 

[KDan]

1) Roughly 20% of airline operating budgets are spent on fuel. Electric energy currently costs about 1/3rd as much as chemical energy stored in Jet-A (after accounting for the <40% efficiency explained by someone else earlier in this thread), and I suspect that number will drop to 1/30th over the next 15 years as roll-to-roll thin film solar advances and gets picked up.
1b) If it takes 2x as much energy to move people, and you can only move them half as far and half as fast, every airline in the world would swap to electric instantly. It is a business necessity. You, the consumer, won't have a choice.
2) In terms of energy storage density, please remember that current planes are designed for Jet-A; a plane designed for a battery requiring more volume might have a slightly different shape, but the volume isn't problematic.
3) In terms of specific energy storage density, Li-ion isn't going to get there. Therefore anyone working on Li-ion batteries isn't working on this problem, unless their work is translatable to a different technology. I've seen Li-air and one of the Sulphur batteries mentioned in this thread.
4) The battery that would move a plane, i.e. one with a specific energy density of 4x today's SOTA (950Wh/kg) and thus ~15x today's electric car batteries: how would that change electric cars? If electric cars had a range of 4,000 miles between necessary fill-ups, that would eliminate one of the final barriers for many people, and could also be used to solve some of the distribution problems associated with widespread electric car adoption. It would also enable electric personal air vehicles. The impact of this battery development on CO2 would be far more than the emissions from air travel, and the environmental reasons are the least of why this should be a priority.

 

[jrmichler]

Electric airplanes are here. Pipistrel has an FAA certified two seat electric airplane in production: https://www.pipistrel-usa.com/alpha-electro/. It has a one hour, 100 mile, range plus reserve. It is intended for flight training, where the one hour range is acceptable.

Another company is flying a full electric Cessna 208: https://www.aviationtoday.com/2020/...avan-showcases-maturity-of-electric-aviation/. The Cessna 208 is a nine passenger / cargo airplane used on short haul routes. The conversion is based on a business case where they expect profitable operation on shorter routes.

The electric conversion company also has a full electric DeHavilland Beaver flying. This article has good comments on the business case: https://www.aopa.org/news-and-media...er/10/harbour-air-flies-first-electric-beaver.

All of that is using existing technology. A factor of two improvement in battery energy density will really increase opportunities for electric aircraft.

 

[russ_watters]

1) Roughly 20% of airline operating budgets are spent on fuel. Electric energy currently costs about 1/3rd as much as chemical energy stored in Jet-A (after accounting for the <40% efficiency explained by someone else earlier in this thread)

I don't follow - do you have a source or did someone actually say that, that you can quote? A lot of our electricity is generated in engines that are close siblings of airplane engines. If the engine efficiencies are similar, the electric cost has to be higher. Unless you fundamentally change the power plant and plane and make a compromise. Or if different fuels with vastly different prices are used. But then, that would just mean we should be switching to powering the planes with natural gas.

...and I suspect that number will drop to 1/30th over the next 15 years as roll-to-roll thin film solar advances and gets picked up.

I think that's extremely unlikely as even if solar panel costs drop to nothing, it is still cost a lot of money to collect and convert the electricity to a usable form. At this point I think the cost of the panels already isn't a majority of the cost of solar energy.

 

[Tom.G]

For electrical generating plant cost and operating comparison table, see PDF pgs 28,29 of:
https://www.eia.gov/analysis/studies/powerplants/capitalcost/pdf/capital_cost_AEO2020.pdf

The report is 5.5MB, 212 pgs. and has a cost breakdown for 25 different technologies. Solar Photovoltaic (case 24) looks pretty good.

Cheers,
Tom

 

[russ_watters]

For electrical generating plant cost and operating comparison table, see PDF pgs 28,29 of:
https://www.eia.gov/analysis/studies/powerplants/capitalcost/pdf/capital_cost_AEO2020.pdf

The report is 5.5MB, 212 pgs. and has a cost breakdown for 25 different technologies. Solar Photovoltaic (case 24) looks pretty good.

Thanks. It's a relatively new report so I'll probably go through it, but a quick look at the table on pages 28-29. It looks ok, but it is really tough to make a useful comparison because different plants have different purposes/capacity factors. So an obvious example is that if you are using solar for peaking and replace a low capacity factor gas turbine, it could be competitive. But if the gas turbine installation is expected to have a high capacity factor (say, it is replacing a base loaded coal plant), it's not close, since solar's capacity factor has a hard limit between 20-30% (depending on location and system configuration).

Also, the future prospects of solar are clouded by it competing with itself as the installed capacity rises. It doesn't even take a double-digit total generation fraction for solar to have times when solar provides more than 100% of the needed power.

In any case, I don't think that solar should be a very relevant part of this discussion. This idea that suddenly electricity in general is going to become extremely cheap is just hope, without real basis. The problems with electric planes aren't from the electricity cost. [so perhaps I should have let that one go...]

 

[Stormer]

The issue is that jet engines don't rely on a moving prop to generate their thrust (with the exception of turboprop engines), they rely on the heating and expansion of gas powered by the combustion of fuel. How would you do something similar using electric power?

No. The gas turbine part of a modern turbofan engine is not the main contributer to thrust. It is mainly just to drive the fan in the front and that gives the vast majority of the thrust.
And you can get supersonic exhaust from a electric ducted fan moving at subsonic tip speed. Just by using diverging / expanding nozzles. And that is also how a modern military supersonic jet with supercruise works. It get its thrust form the fan (just like in a passenger jet) moving at subsonic speed and then makes use of geometry in the engine/nozzle to accelerate the exhaust gass to supersonic speed.

Yeah that's true but you wouldn't get the volume expansion ratio of burning liquid fuel to produce combustion gasses. Perhaps you could use the electric power to boil water instead of heat air. I agree, not sure it is really doable.

Yes you do. That is how a nuclear powred jet/gass turbine works. It does not inject any fuel, it just heats up the air. And heating up air makes it expand.

 

[BillW]

I like turboprops!

 

[cjl]

No. The gas turbine part of a modern turbofan engine is not the main contributer to thrust. It is mainly just to drive the fan in the front and that gives the vast majority of the thrust.
And you can get supersonic exhaust from a electric ducted fan moving at subsonic tip speed. Just by using diverging / expanding nozzles. And that is also how a modern military supersonic jet with supercruise works. It get its thrust form the fan (just like in a passenger jet) moving at subsonic speed and then makes use of geometry in the engine/nozzle to accelerate the exhaust gass to supersonic speed.

A couple things here (yeah, this is nearly a year old, but I don't want to let it just sit):

1) I incorrectly made a similar assumption about the fan several pages back, but then I went back through my textbooks and did some math, and at cruise, the core is actually contributing substantially (35-50%) to the thrust. Sure, you could just drive a big fan with an electric motor, but you'd need a substantially larger and/or faster fan to achieve the same performance compared to current jet engines. That's the trend for modern jets anyways (larger, not faster), but it still does have some important implications for weight and aircraft design. In addition, the fan contribution at takeoff is much larger - the core thrust is important at cruise, but at takeoff, the fan really does do the vast majority of the work. If you were willing to sacrifice some cruise performance compared to modern jets (more akin to the cruise speed of a turboprop), you could get away with electric ducted fans much closer in size to the current engines (for the same size/weight of aircraft).

2) I don't know of a single modern turbofan (or old school one, for that matter) that runs a subsonic fan. Not a one. Typical airliners are running a bit over mach 1.5 at the blade tips at takeoff, and a bit less at cruise (but still well supersonic). Military jets run just as fast. Actually, the funny thing is that most jets run exactly backwards vs your statement - they run a subsonic exhaust (military being the exception here, obviously, though even military tends to only run a supersonic exhaust on afterburner or maybe at full dry thrust), but a supersonic fan. You could absolutely design an engine that would have a supersonic exhaust despite being powered by a subsonic electric fan, but it'd be loud and there's not much reason to use a converging-diverging nozzle unless you're working with a heated gas (since it cools a lot going through a C-D nozzle during the supersonic acceleration).

 

[JackCatDaily]

Well, theoretically you could use a giant electric heater to heat the air in the "combustion chamber", but I'm not sure if that's really doable.

Put wings on a Train and have a prop attached to the boiler. Let's see if it works.

 

[RonL]

That's very unlikely in the near to moderate future in my opinion. Fossil fuels are extremely energy dense, which is one of the features that makes them useful over other energy sources, and it's very difficult for batteries to even approach their energy density. Jet fuel has an energy density of around 43 MJ/kg, while lithium ion batteries are about 0.8 at the top end, so we're talking roughly a fifty-fold difference between them.
The issue is that jet engines don't rely on a moving prop to generate their thrust (with the exception of turboprop engines), they rely on the heating and expansion of gas powered by the combustion of fuel. How would you do something similar using electric power?

Had a hard time deciding which post to reply to, you mention energy density of fuel and batteries, consider the efficiency of generated electricity straight to heat, almost 100% where you need it.
I can't attempt to lay out a complete design, but considering the sum total of air comes in the front of the jet tube, a certain amount of ram energy can be used here, but the main idea is to generate electrical energy and the power turbine at the rear powers a compressor that supplies compressed air into the combustion chamber.
The shaft that goes from the rear to the front of the engine can be designed to house one or more generators that power a number of quartz type heating elements in the combustion chamber.
Some of the reasoning for these thoughts is based on the use of an electric halogen cooktop stove that has been in use for over 20 years in my home, it has one heating pad that has a ring of 30 small bulbs, (about 6mm X 25mm), when the control knob is turned on, an almost instant glow through the darkened glass top becomes very bright and hot.
Just for a size reference and frame of mind, a 250,000 Watt mass of heat elements would not require much more diameter and length than a 5 gallon bucket. All the heat has to move with the air flow through a jet engine.
At the very least, this might be a way to eliminate a very large quantity of fuel consumed during flight. Not much additional weight to the engine (mostly small high speed generators that produce high voltage and high frequency electricity).

So yes I think there is a way to go electric inside a jet engine.

 

[sophiecentaur]

consider the efficiency of generated electricity straight to heat, almost 100% where you need it.

I can't see that using a 'heat engine' as an intermediate stage of energy path could possibly help with efficiency. The maximum efficiency of any heat engine is limited to the difference in operating temperatures of the gas (air). The majority of the power from a modern 'jet' engine comes from the fan and a could be rotated by electric motors, once a more dense energy storage system can be invented.

 

[cjl]

At the very least, this might be a way to eliminate a very large quantity of fuel consumed during flight. Not much additional weight to the engine (mostly small high speed generators that produce high voltage and high frequency electricity).

So yes I think there is a way to go electric inside a jet engine.

Out of curiosity, is your suggestion to add generators to the jet engine and use those to power heating elements in the core? If so, that's not going to gain you anything, since any extra power you get from the extra heat is going to be needed to spin the generators in the first place. This is like trying to power a fan by using a wind turbine that the fan is pointing at.

(If this isn't what you're saying, then I'm a bit confused as to what your suggestion is - could you clarify a bit more?)

 

[anorlunda]

Why in the world would anyone consider such complexity? A simple electric motor plus a simple propeller is just fine at appropriate speeds. Only if you think faster speeds outweigh efficiency would you consider anything else.

 

[RonL]

Out of curiosity, is your suggestion to add generators to the jet engine and use those to power heating elements in the core? If so, that's not going to gain you anything, since any extra power you get from the extra heat is going to be needed to spin the generators in the first place. This is like trying to power a fan by using a wind turbine that the fan is pointing at.

(If this isn't what you're saying, then I'm a bit confused as to what your suggestion is - could you clarify a bit more?)

Well I'm in over my head, but in my thoughts if you can extract power for the compressor, then why not a bit larger and stronger power turbine that will cover the generators output converted to pure heat.
In normal use motors and generators are governed by mechanical limits, but in this application a generator system can be much greater than the needs of the engine air flow and it seems to me an almost complete recycle of energy from heat back to mechanical force.
I'm sure I didn't say that correctly, but I'm convinced there is room for invention here. :)

 

[jrmichler]

This thread was off to a good start with discussions of battery electric aircraft, but has deteriorated into attempts to violate the laws of thermodynamics. Thread closed.