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Electric powered airplanes
August 30, 2010 8:54 PM   Subscribe

Electric airplanes are not a new idea, but there have been recent developments in building affordable production electric sport aircraft. They are super quiet, low vibration, highly reliable, simple and gas-free.

The Chinese made Yuneec e430 (videos) is probably farthest along, along with the eSpyder. The ElectraFlyer (videos) is also in development. The first commercial electric aircraft was the Silent Club (video) in 1997, a self launching glider. Another electric self launching glider is the Antares 20E and Arcus E. (Previously).
posted by stbalbach (52 comments total) 3 users marked this as a favorite

 
What kind of range do they have?
posted by empath at 9:00 PM on August 30, 2010


ElectraFlyer.com says one to two hours charge 'without lift'. Does that mean that take-off isn't included?

These really look gliders with motors. Better than a glider without a motor, but not exactly a plane for actually getting anywhere, unless you like living dangerously.
posted by eye of newt at 9:05 PM on August 30, 2010


Boeing suggests that it could (possibly maybe) have an electric commercial aircraft in the air in 2035.
posted by wilful at 9:12 PM on August 30, 2010


The great thing about gasoline is its high power to weight ratio. A car with 15 gallons of gas and 25 mpg can get 375 miles before running out. The Prius (according to http://en.wikipedia.org/wiki/Toyota_Prius#Plug-in_version) can get only 14.5 miles (!) on it's electric battery (15 gallons is about 124 lbs, and the battery weights about the same, again according to the pedia).

Trains use electric motors as well, but those are in turn powered by diesel engines that burn gasoline. San Francisco buses use electric motors (and the streets are quieter for it), but those things have to be connected to the grid constantly (I've seen the modern versions go about a block while disconnected during a special event).

Trains, buses and cars aren't nearly as weight sensitive as airplanes, so I feel like this is going to be a solar / glider / drone / ultralight thing for a little while longer. The e430 claims to be a Light Sport Aircraft, but with an empty weight of 392 lbs, it's only 138 pounds heavier than an ultralight.
posted by Phredward at 9:58 PM on August 30, 2010 [4 favorites]


Boeing suggests that it could (possibly maybe) have an electric commercial aircraft in the air in 2035.

Boeing suggest that it could (possibly maybe) have the 787 shipped by 2008.
posted by eriko at 10:00 PM on August 30, 2010 [4 favorites]


The Yuneec page proudly reminds us that it has "no emissions, no fuel, [...] environmentally friendly". Just where exactly do they think the electricity came from to charge those batteries? Most likely coal or natural gas. Are people really that intellectually lazy that they would somehow convince themselves that this is a green idea?
posted by Rhomboid at 10:03 PM on August 30, 2010


The Yuneec e430 can carry 1 passenger and has a cruise speed of 90 km/h and a range of 227 km.

I can easily see electric planes being used for scenic flights as soon as they can carry a pilot and 2 passengers - both in places where gas powered planes aren't allowed now because of noise and in places where gas powered scenic flights are currently allowed because it would be a more pleasant experience.

But as far as A --> B flights? Not without a big change in battery technology.
posted by vapidave at 10:16 PM on August 30, 2010


Most likely coal or natural gas.

On the other hand, a gasoline powered plane is guaranteed fueled by petroleum. "Maybe petroleum from a high-efficiency source" trumps "guaranteed petroleum from a low-efficiency source."
posted by Jimmy Havok at 10:17 PM on August 30, 2010 [3 favorites]


Rhomboid: I would. 100% hydro.
posted by five fresh fish at 10:19 PM on August 30, 2010 [1 favorite]


The Yuneec page proudly reminds us that it has "no emissions, no fuel, [...] environmentally friendly". Just where exactly do they think the electricity came from to charge those batteries? Most likely coal or natural gas. Are people really that intellectually lazy that they would somehow convince themselves that this is a green idea?

If you don't have things that run on batteries, you don't have the incentive to build the infrastructure for charging said batteries from solar/wind/what-have-you good energy. This seems like a step in the right direction - I wouldn't dismiss its greenness so quickly.
posted by ivancho at 10:36 PM on August 30, 2010 [1 favorite]


"Maybe petroleum from a high-efficiency source" trumps "guaranteed petroleum from a low-efficiency source."

Even if the power plant had ideal efficiency, you still end up losing about 7 percent for distributing it from the plant to you and another 10 to 20 percent converting it back to mechanical energy. The petrol-powered plane doesn't have to pay those penalties so it doesn't have to be quite as efficient to still come out ahead.
posted by Rhomboid at 10:50 PM on August 30, 2010


Jimmy Havok On the other hand, a gasoline powered plane is guaranteed fueled by petroleum. "Maybe petroleum from a high-efficiency source" trumps "guaranteed petroleum from a low-efficiency source."

Gas turbine engines are highly efficient for their power-to-weight ratio, and in many cases the turbines used to generate power at plants using boilers (i.e. most coal/gas/nuclear) are designed similarly. GE develops turbines for aircraft propulsion, then produces derivatives for power generation use. Jet engines are not a type of motor where fuel efficiency has been an afterthought. Commercial and general aviation has for the most part been driven by a financial bottom line. Fuel costs for aircraft are a much higher proportion than that for autos. Carriers need low-operating cost (fuel efficient) aircraft to keep fares low. This is unlike the auto industry, where most purchases come from individual consumers who want a sweet ride.

An electric aircraft might produce a net increase in non-renewable energy use, because of the power losses through the grid after being generated by a turbine (versus the power being generated by the turbine integrated with the aircraft).
posted by mnemonic at 11:12 PM on August 30, 2010 [2 favorites]


The Yuneec page proudly reminds us that it has "no emissions, no fuel, [...] environmentally friendly". Just where exactly do they think the electricity came from to charge those batteries? Most likely coal or natural gas. Are people really that intellectually lazy that they would somehow convince themselves that this is a green idea?

You're being intellectually lazy to assume this old myth ever had legs and somehow no serious engineers ever noticed.

For example, an electric car can be powered by electricity that was generated by burning the gasoline from the same gas station pump that regular cars run on, and that car can still emit a fraction of the emissions as regular cars do, and do the same amount of work or more. The line losses of electricity pale next to the multiple layers of efficiency gains. Hybrids don't even use grid electricity and add weight to a vehicle and yet still manage to increase the amount of useful energy that a car can get from burning the same amount of gasoline.

(Also, suggesting that electricity comes from dirty sources is kind of a provincial American response - US electricity generation may be notoriously dirty, but a lot of these technologies are will end up in (and/or are being developed in) countries with much cleaner generation. And as they mature, perhaps USA energy will be getting cleaner too)

Even using dirty generation, electric is greener than gasoline.
posted by -harlequin- at 11:21 PM on August 30, 2010 [8 favorites]


The Yuneec page proudly reminds us that it has "no emissions, no fuel, [...] environmentally friendly". Just where exactly do they think the electricity came from to charge those batteries? Most likely coal or natural gas. Are people really that intellectually lazy that they would somehow convince themselves that this is a green idea?

Are you retarded? Obviously electricity doesn't need to be generated by coal or natural gas. In a lot of places, it's possible to specify the electricity you use is renewable.

In any case, this thing is just for entertainment, not as any kind of realistic transportation. But in general an electric whatever is going to be better for the environment then a fossil fuel powered one. First off it doesn't require fossil fuel infrastructure to stay in place. If everyone switched to electric cars, gas stations and refiners could go away.

And secondly, and immediately, electricity is a mix of renewable, nuclear, and fossil fuels. Even if the energy efficiency of a gas engine was higher, you would need to divide the energy cost for electric by (the reciprocal of) the proportion of actual greenhouse gas production.

And then, of course, you're probably not even figuring in the CO2 generated by the refinery that converts crude oil into gasoline.

The whole "electric X is just as bad for the environment as a gas burning one!" is a just a brain-dead right-wing trope.
posted by delmoi at 11:24 PM on August 30, 2010 [4 favorites]


A dirty generator runs at optimal efficiency. A vehicle engine runs at crazy bad efficiency - it has to, it is designed to, it has to be able to cope with rapid and insane shifts of load and revs, a worst-of-both-worlds solution, while the generator not only sits in the ideal band always, but it's efficient band is far more efficient.

The losses from using combustion engines for anything other than generation are staggering.
posted by -harlequin- at 11:26 PM on August 30, 2010 [1 favorite]


Even if the power plant had ideal efficiency, you still end up losing about 7 percent for distributing it from the plant to you and another 10 to 20 percent converting it back to mechanical energy. The petrol-powered plane doesn't have to pay those penalties so it doesn't have to be quite as efficient to still come out ahead.

Again, you generate more then the equivalent of 7% CO2 refining oil to gasoline. And you lose about 60% of the energy generated by a motor as straight up heat. Electrical motors are much more efficient then gasoline ones.

And on top of that, as I said, you have to factor in the mix of CO2 producing to non-CO2 producing energy. In a country like France, about 70% of the energy is produced by nuclear power.
posted by delmoi at 11:28 PM on August 30, 2010


A dirty generator runs at optimal efficiency. A vehicle engine runs at crazy bad efficiency - it has to, it is designed to, it has to be able to cope with rapid and insane shifts of load and revs, a worst-of-both-worlds solution, while the generator not only sits in the ideal band always, but it's efficient band is far more efficient.

Rev shifts won't be as big of a deal on an airplane, which should mostly be flying at the same speed all the time.
posted by delmoi at 11:29 PM on August 30, 2010


Rev shifts won't be as big of a deal on an airplane, which should mostly be flying at the same speed all the time.

Less of a problem, but you still can't build a very efficient engine, because it has to be capable of sustaining the far higher load during take-off than the band it will be operating in during 98% of the flight.

Even if you build entirely separate engines, each specialized for a stage of flight, and so more efficient, you're still always lugging around the weight of the one you're not using.
posted by -harlequin- at 11:39 PM on August 30, 2010


Also, dirty generators put all the generation in one spot, which is ideal for infrastructure, so they can have huge bulky heavy emission scrubbers installed on site. Vehicles cant carry effective emission scrubbers. Cars have barely more than just a catalytic converter. Planes have what... nothing?
posted by -harlequin- at 11:45 PM on August 30, 2010


Obviously electricity doesn't need to be generated by coal or natural gas.

But for the most part in the United States in 2010, it does, which is why I have a problem with saying that a vehicle powered from the grid has zero emissions and is green.

And you lose about 60% of the energy generated by a motor as straight up heat. Electrical motors are much more efficient then gasoline ones.

With the electric motor you still have those same exact losses of a heat engine, it's just happening at the power plant that burns coal or natural gas rather than at the airplane engine. I specifically left that out because both sides have to pay for it. Yes, of course this doesn't apply if you have hydro or wind power but, again, that is not the reality for the vast majority of people in the US.
posted by Rhomboid at 11:48 PM on August 30, 2010


Planes have what... nothing?

General aviation aircraft are still using gasoline with lead levels that were illegal for cars in the mid-1970s. Lawn mowers have more stringent emissions regulations.
posted by ryanrs at 11:59 PM on August 30, 2010 [1 favorite]


With the electric motor you still have those same exact losses of a heat engine, it's just happening at the power plant that burns coal or natural gas rather than at the airplane engine.

No, they do not have the same losses. For example, the power plant doesn't have to fucking fly around in the sky.
posted by ryanrs at 12:07 AM on August 31, 2010 [4 favorites]


With the electric motor you still have those same exact losses of a heat engine, it's just happening at the power plant that burns coal or natural gas

No, you don't have the same heat losses because engines at power plants can run more efficiently, since they only run at one speed. It's less true for a light aircraft then it would be for a car (and of course on a jetliner the engines will probably be running at a close to optimal level)
posted by delmoi at 12:12 AM on August 31, 2010


For example, the power plant doesn't have to fucking fly around in the sky.

The power plant is still a heat engine and is thus limited by the Carnot cycle, which puts a fundamental limit on the efficiency as a function of high side temperature, which is determined by what material you choose to make the turbine blades out of (e.g. Inconel.) Just because a plant is fixed doesn't change the materials science aspect of that problem -- the turbines in power plants are remarkably similar to those in planes. No matter how much you optimize for a fixed speed you can't get around this limit, which means that coal plants hit about 33% efficiency and natural gas up to 50%.
posted by Rhomboid at 1:12 AM on August 31, 2010


The power plant is still a heat engine and is thus limited by the Carnot cycle, which puts a fundamental limit on the efficiency as a function of high side temperature

No one is arguing that power plants are 100% efficient. Just that they're a little more efficient then engines you'd find on your average sports plane. And again, even if only 10% of the energy is generated without CO2, it's still going to more then make up for transmission losses.

And of course not all locations get the same mix of power. There are places in the U.S. where most of the energy comes from hydroelectric, or nuclear power. Plus the plane is being made in China, presumably for people all over the world. In France, 70% of the power comes from nuclear energy. You can request that your energy comes from renewable in many places, you can setup your own solar panels, etc. Given how often the average sports plane is actually used you could probably charge it up with solar panels and take it out once a month or something.

The main environmental benefit is that with electricity, there is a huge number of options about where the energy comes from. You don't get that flexibility with a fossil fuel burning engine.
posted by delmoi at 1:27 AM on August 31, 2010


(actually I just checked, in France electricity is actually 78% nuclear)
posted by delmoi at 1:29 AM on August 31, 2010


So I was talking to acquaintance the other day. Smart guy. Helped design last season's winning car in Formula 1. He now works as an engineer on a range of alternative energy vehicle projects.

He was flat out clear hydrogen won't be the answer. And then he told me about their electric car project. Apparently the battery malfunctioned. Within 10 seconds the entire building was alight, such was the ferocity of the blaze. He expressed amazement that the issue of battery safety wasn't perceived as a bigger deal.

So, um, I'd like a few more tests done on electric planes, even if the whole weight/power/range things becomes viable.
posted by MuffinMan at 1:36 AM on August 31, 2010


Within 10 seconds the entire building was alight, such was the ferocity of the blaze.

Link?
posted by Jimmy Havok at 1:55 AM on August 31, 2010 [1 favorite]


Surely the contemporary niche for electric powered flight has to be in conjunction with solar power (like some of the earlier links in the FPP)? Not suitable for those who want to go a long way in a hurry but great if you have a drone what needs to be in the air for a long period - or an airship like this one.
posted by rongorongo at 2:24 AM on August 31, 2010


Within 10 seconds the entire building was alight, such was the ferocity of the blaze.

Link?


You want me to link to a conversation I was having with someone?
posted by MuffinMan at 2:54 AM on August 31, 2010 [1 favorite]


Even if you build entirely separate engines, each specialized for a stage of flight, and so more efficient, you're still always lugging around the weight of the one you're not using.

That's not necessarily true. For example, the Space Shuttle doesn't carry the weight of its rockets for the entire duration of its flight. A jet fighter taking off from an aircraft carrier doesn't carry the entire weight of the power system needed to take off.

Now, I haven't the foggiest if it's possible, or a bad idea, to have planes flying about that can't take off independently or which don't carry all their propulsion engines all the time, but technically it isn't absolutely necessary for them to do it.
posted by MuffinMan at 3:00 AM on August 31, 2010


FWIW, I recall back in the 80's someone converting a Piper 140 to electric, using a torpedo motor and some sort of hydroxide battery where the 'fuel' was cycled from a fresh tank to a used tank (one in each wing) so that the weight/balance stayed the same throughout the flight. This predates Google a bit, and I haven't gone looking for it, but my reads at the time including 'Flying' magazine so it may have been there.

Regardless, I think electric flight is worth pursuing, if only because it is a goal requiring advancement on several fronts... high density power storage, efficient motors, materials.

It wasn't all that long ago that ultralights were impractical. Now, we have carbon fiber composites that make the aluminum framed UL from the 80's look like steel by comparison.

Anyone who has ever been in an ultralight or in a realllllly long small plane flight can appreciate the promise of quiet, vibration free flight. Noisiest damned things in the world.
posted by FauxScot at 3:34 AM on August 31, 2010


The power plant is still a heat engine and is thus limited by the Carnot cycle

General aviation aircraft don't use Carnot heat engines. They use ancient and horrible gasoline piston engines (carburetors and leaded gas, ugh). The notion that they operate anywhere near theoretical thermodynamic efficiency is crazy.

[The Carnot cycle] puts a fundamental limit on the efficiency as a function of high side temperature

Actually, the ratio of high and low side temperatures. A power plant can achieve a lower temperature sink than an aircraft engine by using evaporative cooling or natural . But again, neither power plants nor aircraft engines are reversible heat engines.

No matter how much you optimize for a fixed speed you can't get around this limit, which means that coal plants hit about 33% efficiency and natural gas up to 50%.

Real fossil fuel thermal power plants can beat the theoretical Carnot efficiency limit with cogeneration. The Carnot limit also doesn't apply to non-thermal fossil fuel power plants such as natural-gas fuel cells. Then there are the non-fossil fuel energy sources like nuclear, hydro, wind, and solar. None of those are amenable to airborne operation.
posted by ryanrs at 4:20 AM on August 31, 2010 [3 favorites]


Now, I haven't the foggiest if it's possible, or a bad idea, to have planes flying about that can't take off independently or which don't carry all their propulsion engines all the time

Are you kidding? It's a fantastic idea!
posted by tss at 4:32 AM on August 31, 2010


One of the problems I have heard about with electric airplanes (and I'm thinking specifically about aircraft you actually want to get somewhere in, not light sports or gliders) is that the FAA doesn't really know how to certify them. With a traditional combustion engine, range is pretty easy to determine - you run out of range when you run out of gas. The power curve on electrics don't have that same sharp break when you "run out of fuel"; rather, the batteries will slowly provide less and less power until the engine can't produce enough thrust to keep you in the air. If you ever had an RC car as a kid, you know that it doesn't just die as the battery runs out, it slows down and creeps along before it finally gives out.

The other concern is more of a human factors one, namely that the pilot can tell pretty easily when he's run out of gas (engine stops making lots of noise), but it's much more subtle - especially if you're already at speed in the air - when the batteries are too low. The natural tendency would be to notice that you're losing altitude and continue to pull up while losing airspeed, which will put you into a stall. Training can overcome this, but that's just one more thing that needs to get worked out before these move beyond the experimental stage.

Also, batteries are really heavy. I'd just like to see them stop using leaded gasoline at this point.
posted by backseatpilot at 4:40 AM on August 31, 2010


because it has to be capable of sustaining the far higher load during take-off than the band it will be operating in during 98% of the flight

Not as much as you might think. Unlike automobile engines, airplane piston gasoline engines are happy to run at 75% power all day, or even higher, and are normally operated this way when used for travel (as opposed to farting around low and slow, which is fun too).
posted by exogenous at 5:00 AM on August 31, 2010


Lithium batteries are certainly capable of causing BIG fires but bear in mind that a regular plane is carrying around a massive load of, y'know, jet fuel.
posted by unSane at 5:02 AM on August 31, 2010 [1 favorite]


The great thing about gasoline is its high power to weight ratio. A car with 15 gallons of gas and 25 mpg can get 375 miles before running out. The Prius (according to http://en.wikipedia.org/wiki/Toyota_Prius#Plug-in_version) can get only 14.5 miles (!) on it's electric battery (15 gallons is about 124 lbs, and the battery weights about the same, again according to the pedia).

Why are you weighing the fuel in one case but the gas tank (i.e. the batteries) in the other?

Yes, battery weight is a problem. But let us at least compare apples to apples.
posted by DU at 5:03 AM on August 31, 2010


Why are you weighing the fuel in one case but the gas tank (i.e. the batteries) in the other?


Because the weight of fuel is a standard operational concern in flight. With that said, typically we assume about 6 pounds a gallon, so 15 gallons would weigh about 90 pounds. Coming up with a battery that can hold the same energy intensity as gasoline with the same energy release rate as gasoline combustion would be necessary to match current flight profiles with electric technology. We are along way off. Still, electric for self-launching gliders seems way cool and good application for this.
posted by meinvt at 5:20 AM on August 31, 2010


Why are you weighing the fuel in one case but the gas tank (i.e. the batteries) in the other?

Because the weight of fuel is a standard operational concern in flight.

My question is not "why are you concerned with weight". My question is "why are you weighing different things when comparing weight". Gas is fuel. Batteries are not fuel. When the plane flies, the batteries do not disappear. At the end of the flight, the batteries weigh the same as they did at the beginning.

Batteries are the gas tank. To really compare gas and electricity in terms of power vs weight, you need to weight the entire fuel system together. gas + tank + fuel lines + injector/carburettor + etc vs batteries + wires. And also the difference in weight between a gas engine and an electric motor of the required powers.

I will completely grant that batteries are mad heavy and seem to be the limiting factor in at least some electric vehicle situations.
posted by DU at 5:38 AM on August 31, 2010 [1 favorite]


With a traditional combustion engine, range is pretty easy to determine - you run out of range when you run out of gas. The power curve on electrics don't have that same sharp break when you "run out of fuel"

It would be trivial to implement a "sharp break" in the power controller, if that's really what you want (it probably isn't).

the pilot can tell pretty easily when he's run out of gas (engine stops making lots of noise), but it's much more subtle - especially if you're already at speed in the air - when the batteries are too low

I bet they notice when the instruments go out and cabin pressure starts to drop!

Actually, it's very easy to calculate the state of charge of lithium-ion batteries. Ah in = Ah out, to within 0.5%. So whatever you put in, you can rely on getting back out. The battery meter on your mp3 player is probably less reliable since it attempts to account for the total energy in the cell rather than just the energy added during the last recharge. By "rolling over" the unused charge each cycle, errors accumulate. You wouldn't do this with an aircraft battery (at least not for more than a few cycles). If a particular flight plan required the full capacity of an aircraft's batteries, the batteries would need to be fully discharged, then recharged. You couldn't rely on the previous flight "leaving some fuel in the tank".
posted by ryanrs at 5:44 AM on August 31, 2010 [1 favorite]


An electric airplane will never be able to take off from a treadmill.
posted by five fresh fish at 8:01 AM on August 31, 2010 [4 favorites]


At the end of the flight, the batteries weigh the same as they did at the beginning.

That's a good point. Batteries will need significantly higher specific energy than jet fuel to match the performance characteristics of existing aircraft.

I believe lithium-sulfur batteries have the highest specific energies today. Li-S have demonstrated 1.3 MJ/kg, with 2 MJ/kg promised in the near future. Li-S chemistry has a theoretical maximum specific energy of 9 MJ/kg.

Still a long way from Jet-A's 43 MJ/kg, though.
posted by ryanrs at 9:18 AM on August 31, 2010


Lithium air batteries have a theoretical energy density of 40.1MJ/kg, assuming the use of atmospheric oxygen, which is almost as good as fossil fuels. In practice, the first generation of lithium air batteries is expected to manage 3.6MJ/kg. An order of magnitude less than fossil fuels but a substantial improvement over existing batteries.
posted by jedicus at 9:34 AM on August 31, 2010


Batteries are a real problem if you demand that they be solid and rechargeable. Most of those use metals which kill the power to energy storage ratio. On the other hand, electric motors are hugely efficient, better than 90% at turning electricity into shaft rotation.

On the other hand, heat engines like turbines overcome their relatively poor efficiency as engines by using a very energy-dense fuels source, hydrocarbons. HC's are probably the most energy-dense substance which won't take your hand off if you look at them funny and can be handles without special equipment.

So what one needs is a system which can generate electricity without the penalty of a huge, metal battery. This exists: fuel cells. A fuel cell is essentially a battery with renewable chemical storage media. As the cell depletes the stored chemical energy, the chemical contents of the cell are renewed to regenerate the power available.

A storage tank, fuel-cell, electric motor is potentially the highest efficiency, low-mass chemically-powered locomotion system we know about today. Fuel cells have been demonstrated in the lab that have better than 66% conversion efficiencies. The problems with fuel cells right now is that they are very hard to make cheaply and repeatedly and break down at the slightest provocation. Just like gasoline engines in 1905. Some models also need huge, heavy extra infrastructure like heaters or gas-crackers to work properly. Others require exotic, hard to handle, hard to store fuels like gaseous hydrogen or molten sulphur.

In twenty years, I'm sure most vehicles, cars, planes, what not, will be battery driven. those batteries, however, will be open, fuel cell designs, I suspect, not the closed-cell battery-types being developed. It's not like the current work is a waste though---there's still a lot of useful work to be done on engine and vehicle design in advance of the fuel cell technologies being mature enough to use. I don't expect to ever see a really decent vehicle based on a closed-cell battery design though. The physical chemistry of batteries doesn't seem to work well enough.
posted by bonehead at 10:58 AM on August 31, 2010 [1 favorite]


Hmm, no one's mentioned solar impulse yet? It's a fully solar plane that has been able to fly for up to 24 hours. Recharging during the day (while in the air) and storing the power for night. We had a post on it a while back but that was before it had actually been in the air for a long time. here's a video of it taking off from their youtube channel
posted by delmoi at 11:12 AM on August 31, 2010


Lithium air batteries have a theoretical energy density of 40.1MJ/kg, assuming the use of atmospheric oxygen, which is almost as good as fossil fuels

But after you burn the fossil fuels, you dump the combustion products into the atmosphere. With Li-air, you have to carry the Li2O to your destination. By the time it has discharged, your battery will have doubled in mass. So the weight of a Li-air battery averaged over the entire flight is roughly 50% higher than the takeoff weight, whereas the average weight of jet fuel is 1/2 the takeoff weight. Based on this, I would argue that the practical specific energy of Li-air batteries is only half that of fossil fuels.

It would be a more even fight if you required the jet engine to sequester its carbon emissions.
posted by ryanrs at 12:26 PM on August 31, 2010


A storage tank, fuel-cell, electric motor is potentially the highest efficiency, low-mass chemically-powered locomotion system we know about today. [..] Some models also need huge, heavy extra infrastructure like heaters or gas-crackers to work properly. Others require exotic, hard to handle, hard to store fuels like gaseous hydrogen or molten sulphur.

The nice thing about the big, heavy, 1000°C fuel cells is their efficiency. Unlike low temperature hydrogen fuel cells, the big boys can eat carbon. And carbon is the key to efficient use of energy-dense liquid fuels.

The reason the carbon fuel cells run so hot is because they rely on oxygen transport rather than hydrogen. So instead of the hydrogen-permeable plastic used in classic fuel cells, they use ceramics that are oxygen-permeable at high temperatures. Wikipedia has some nice diagrams: solid oxide fuel cell and molten carbonate fuel cell.

Don't be fooled by stuff like the reformed methanol fuel cell. Although it runs on methanol and emits CO2, it's really just a hydrogen fuel cell in disguise. A separate reformer pre-processes the methanol, stripping away the carbon before it hits the hydrogen membrane. No electricity is produced fom the carbon, it is just turned into heat.

Anyway, if you're starting out with electricity (e.g. from solar or nuclear), then all fuel cells have crap efficiency compared to batteries. But if your input is fossil fuel or ethanol from sugarcane, then the high temperature oxygen fuel cells are quite efficient. Definitely better than thermal engines.

As for the hydrogen fuel cells, I've never seen the appeal. They have the efficiency of a thermal engine and the fuel handling problems of 5,000 psi compressed hydrogen. They're the worst of both worlds. But they're small and cute, so they make nice desk toys for scientists. For actual, practical, real-world power generation, oxygen fuel cells are where it's at.

[this comment powered by solid oxide fuels cells fom Bloom Energy]
posted by ryanrs at 1:32 PM on August 31, 2010


Hrm. I may have messed up the chemistry here. I forgot that the reformer creates another H2 when it oxidizes CO to CO2, so that energy isn't wasted. Not sure how it compares to oxidizing the CO at the anode. I guess since the products are the same, it's equivalent.
posted by ryanrs at 4:42 PM on August 31, 2010


As for the hydrogen fuel cells, I've never seen the appeal. They have the efficiency of a thermal engine and the fuel handling problems of 5,000 psi compressed hydrogen

I wonder why there isn't more talk of using hydrogen with conventional thermal engines? seems like a pretty efficient way to store energy. Maybe there are a lot of losses in generating it, or something.
posted by delmoi at 6:49 PM on August 31, 2010


i. Hydrogen is very hard to store. It has to be compressed gas (using conventional techniques). The liquid form is far to cold to be practical. As a gas, it migrates through metal at a fairly decent rate and through plastics like it wasn't there.
ii. Hydrogen is dangerously combustable. It has a wide explosive range and is hard to detect---it's invisible, odourless and hard to see even with instruments. It burns with an ultraviolet flame; you can't see it even when it burns.
iii. Compared to hydrocarbons, hydrogen is low energy density, about 70% energy by weight compared to natural gas.

None of these problems are really deal breakers (though i. is close), but they do make it a bastard fuel to handle.
posted by bonehead at 5:28 AM on September 1, 2010


[hydrogen] seems like a pretty efficient way to store energy

Oh god, it sucks.

Some definitions and data:
  Specific energy: energy per mass, MJ/kg.
  Energy density: energy per volume, MJ/Liter.
MJ/kg MJ/L
  143    6  Hydrogen gas @ 10,000 psi
   54    9  Natural gas @ 3,600 psi
   20   16  Methanol
   46   34  Gasoline (diesel and jet fuel are similar)
More: energy content of many fuels.

Hydrogen has super-high specific energy because it is very reactive and very lightweight. But its energy density is very low, even when compressed to extreme pressures. In fact, hydrogen's energy density is so low that fuel cell vehicles have the same issues with range as electric vehicles (Honda FCX Clarity 280 miles, Tesla Roadster 244 miles).

The thing with compressed hydrogen, though, is that it's hard to store. You can't use steel tanks because the hydrogen weakens the metal. Basically, hydrogen atoms diffuse through metal walls and react with carbon in the steel to form methane. But methane does not diffuse through steel, so it collects in microscopic cracks and voids, building up tremendous pressure. This greatly weakens the container. So compressed hydrogen requires expensive aluminum-lined carbon fiber tanks.

So compressed hydrogen needs to be stored in special tanks, but we can do that. But the energy density is still so low that the tank needs to hold five times the volume of an equivalent gasoline tank. But the fancy carbon fiber tanks have to be round, so instead of one big rectangular tank, you have several small round tanks. That takes up more space. And money. But again, it's doable.

Finally, having conquered the myriad technical difficulties of storing compressed hydrogen, you end up with something like Quantum Technologies' hydrogen storage tanks. Their carbon fiber composite tanks are probably the best commercially available tanks for vehicle applications. Many current fuel cell demonstration vehicles use them. Each tank stores 3 kg of hydrogen at 10,000 psi. The tank, plumbing, and associated support equipment has a "system volume" of 100 L and weighs 50 kg.

Net result: 8.6 MJ/kg and 4.3 MJ/L.

Fuck hydrogen.
posted by ryanrs at 9:56 AM on September 1, 2010 [3 favorites]


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