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On Flying Cars and Funky Batteries


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I’ve had a little extra free time on my hands lately, and instead of spending it on duteous pursuits as I ought, I’ve spent an inordinate amount of time watching YouTube DIY videos where enterprising souls are trying to make iron man suits, repulsors, lightsabers, and the like. It’s very entertaining.

Some of these videos set me on a rabbit trail concerning battery specific energy and specific power. As we likely all know, one of the challenges in trying to get electric planes and electric flying cars is the competition against the terrific specific energy of hydrocarbons. The weight of the batteries required to generate enough power for sustained flight is just about prohibitive, and incremental advances in lithium-ion technology are unlikely to change that any time soon.

One promising alternative to lithium-ion batteries is the zinc-air battery, where zinc reacts with air using a catalyst to produce zinc oxide and electrical energy. Because the air is one ion source, it has a very good specific energy (5-6 times greater than lithium-ion) and reasonably good specific power (about 30-50%). They do have some odd side effects, though. For example, because zinc oxide is heavier than zinc, the batteries get heavier as they discharge; for another thing they are tricky to recharge over many cycles and usually must have their zinc “fuel” reprocessed rather than simply being plugged into the wall.

Now, if there was a way to catalyze a faster uptake of oxygen atoms by zinc air battery, then presumably you could increase specific power. If you could quadruple specific power, then zinc air batteries would dramatically outperform lithium ion batteries, perhaps to the point that they would become a viable alternative to hydrocarbons. It is worth noting that zinc air batteries are much cheaper, much safer, and much more environmentally friendly than lithium ion batteries.

But how exactly would one quadruple the reaction rate of a battery? Here’s where it gets interesting. I freely confess I am no chemist, so this could be absolute BS, but it seems to me that if zinc and air reacts together at rate R, and air is only 21% oxygen, then zinc and pure oxygen should react together at 4-5R. If the reaction rate is 4 to 5 times faster, then presumably the specific power would also be significantly higher.

Bringing canisters of pure oxygen along doesn’t seem like the greatest idea, though, for a number of reasons. So we will have to get our oxygen somewhere else. Remember all those silly “water fueled car” hoaxes that pop up from time to time? They purport to use electrolysis from a vehicle alternator to split water into hydrogen and oxygen, increasing vehicle gas mileage. Of course this is nonsense; whatever negligible power is added by burning trace amounts of oxygen and hydrogen in the engine is outweighed by power lost to the alternator to split the water in the first place. There is no free lunch.

But we don’t need a free lunch. We are looking for something different. What if we bring along water and use electrolysis from the zinc-air battery to split the water into hydrogen and oxygen, then feed that oxygen back into the zinc air battery to increase the rate of the reaction? As an added bonus, electrolysis is a self-pressurizing reaction, meaning we can feed the pure oxygen into the zinc air battery at pressures that would otherwise require a heavy turbine or compressor, thus further increasing the reaction rate.

Of course, since the discharge of the zinc air battery is now being tapped to provide current for our electrolysis, the actual output of our battery has gone down, not up. It seems this exercise has been a waste. But is it? No. Because the system is producing some thing else: pure hydrogen. Pure hydrogen can be fed into a hydrogen fuel cell which reacts with ordinary air to recuperate roughly 60% of that lost energy, and hydrogen fuel cells have a specific power in order of magnitude higher than lithium ion batteries.

There’s no free lunch happening here, of course. This system would simply take advantage of the fact that the reaction between hydrogen and air happens much, much more quickly than the reaction between zinc and air, and so it’s an acceptable trade to lose some specific energy in order to feed pure pressurized oxygen to the zinc-air battery. This only works if doing so will linearly increase the reaction rate. That is where my chemistry knowledge fails.

Operationally, you would merely need to refill your tanks with water between trips/flights, and swamp out a zinc coil for reprocessing every couple of weeks. A reprocessing system wouldn’t be very large at all and could run off of ordinary grid electricity.

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Posted (edited)
12 minutes ago, sevenperforce said:

I’ve had a little extra free time on my hands lately, and instead of spending it on duteous pursuits as I ought, I’ve spent an inordinate amount of time watching YouTube DIY videos where enterprising souls are trying to make iron man suits, repulsors, lightsabers, and the like. It’s very entertaining.

Some of these videos set me on a rabbit trail concerning battery specific energy and specific power. As we likely all know, one of the challenges in trying to get electric planes and electric flying cars is the competition against the terrific specific energy of hydrocarbons. The weight of the batteries required to generate enough power for sustained flight is just about prohibitive, and incremental advances in lithium-ion technology are unlikely to change that any time soon.

One promising alternative to lithium-ion batteries is the zinc-air battery, where zinc reacts with air using a catalyst to produce zinc oxide and electrical energy. Because the air is one ion source, it has a very good specific energy (5-6 times greater than lithium-ion) and reasonably good specific power (about 30-50%). They do have some odd side effects, though. For example, because zinc oxide is heavier than zinc, the batteries get heavier as they discharge; for another thing they are tricky to recharge over many cycles and usually must have their zinc “fuel” reprocessed rather than simply being plugged into the wall.

Now, if there was a way to catalyze a faster uptake of oxygen atoms by zinc air battery, then presumably you could increase specific power. If you could quadruple specific power, then zinc air batteries would dramatically outperform lithium ion batteries, perhaps to the point that they would become a viable alternative to hydrocarbons. It is worth noting that zinc air batteries are much cheaper, much safer, and much more environmentally friendly than lithium ion batteries.

But how exactly would one quadruple the reaction rate of a battery? Here’s where it gets interesting. I freely confess I am no chemist, so this could be absolute BS, but it seems to me that if zinc and air reacts together at rate R, and air is only 21% oxygen, then zinc and pure oxygen should react together at 4-5R. If the reaction rate is 4 to 5 times faster, then presumably the specific power would also be significantly higher.

Bringing canisters of pure oxygen along doesn’t seem like the greatest idea, though, for a number of reasons. So we will have to get our oxygen somewhere else. Remember all those silly “water fueled car” hoaxes that pop up from time to time? They purport to use electrolysis from a vehicle alternator to split water into hydrogen and oxygen, increasing vehicle gas mileage. Of course this is nonsense; whatever negligible power is added by burning trace amounts of oxygen and hydrogen in the engine is outweighed by power lost to the alternator to split the water in the first place. There is no free lunch.

But we don’t need a free lunch. We are looking for something different. What if we bring along water and use electrolysis from the zinc-air battery to split the water into hydrogen and oxygen, then feed that oxygen back into the zinc air battery to increase the rate of the reaction? As an added bonus, electrolysis is a self-pressurizing reaction, meaning we can feed the pure oxygen into the zinc air battery at pressures that would otherwise require a heavy turbine or compressor, thus further increasing the reaction rate.

Of course, since the discharge of the zinc air battery is now being tapped to provide current for our electrolysis, the actual output of our battery has gone down, not up. It seems this exercise has been a waste. But is it? No. Because the system is producing some thing else: pure hydrogen. Pure hydrogen can be fed into a hydrogen fuel cell which reacts with ordinary air to recuperate roughly 60% of that lost energy, and hydrogen fuel cells have a specific power in order of magnitude higher than lithium ion batteries.

There’s no free lunch happening here, of course. This system would simply take advantage of the fact that the reaction between hydrogen and air happens much, much more quickly than the reaction between zinc and air, and so it’s an acceptable trade to lose some specific energy in order to feed pure pressurized oxygen to the zinc-air battery. This only works if doing so will linearly increase the reaction rate. That is where my chemistry knowledge fails.

Operationally, you would merely need to refill your tanks with water between trips/flights, and swamp out a zinc coil for reprocessing every couple of weeks. A reprocessing system wouldn’t be very large at all and could run off of ordinary grid electricity.

 

Hmmm.... you may be on to something.... have any connections with aerospace engineers who would be willing to test this out?

Either that or it's ALL you, which is arguably harder.

Using the air around in creative ways should make all the difference though.

I never was much convinced anything other than project Orion could SSTO with a meaningful (40 tons cargo or more) payload.

 

The problem with SSTO is twofold.

1. Reaching orbit without staging requires a LOT of energy expenditure.

2. Deorbiting will damage the hull on each go so much that I doubt you could do multiple reentries without major hull repair. In fact the only way to avoid fiery reentry is powered deobiting, which is also possible using project orion. Landing the monstrousity would require a significant amount of chemical propellant though, preferably on it's belly since landing on the plate is not advisable.

Edited by Spacescifi
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Zinc-air batteries (and other x-air batteries like iron-air) can offer many solutions but like stated specific power is lacking. There was a bus company in California (can't remember the name) that was doing research into bus scale zinc-air batteries. They discovered the graphite side (oxygen gathering) limited the O2 into the battery and not the quantity of O2 in the air. They theorized that a different configuration of carbon was needed, like graphene. I'm not sure they solved that, but that was not the biggest problem. The best electrolyte they found was KOH (common in alkaline batteries) but as large scale zinc-air batteries discharged the potassium reacted with with the air(mostly the CO2) and the electrolyte was precipitating out of the solution (pretty sure they haven't solved that one yet).  The best part of most x-air batteries is the ease of recycling (also , if you want another rabbit hole to fall down, zinc-air can be made into a hybrid flow battery). Used zinc for instance just needs heated and it will reject the oxygen. You just got to solve the other problems and you could be a zillionaire.

I once built a crappy zinc air battery mostly because the materials are cheap and I like to tinker.  Zinc from boat sacrificial anodes(or rub the copper off a penny), KOH from the chemical store (KCl for water softeners is also usable), and graphite from the art supply store, plus a cup and you could make your own too. Amps per cell is very low but ~1.2V (1.65v/cell is what it was supposed to have) isn't bad.

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2 hours ago, sevenperforce said:

Some of these videos set me on a rabbit trail concerning battery specific energy and specific power. As we likely all know, one of the challenges in trying to get electric planes and electric flying cars is the competition against the terrific specific energy of hydrocarbons. The weight of the batteries required to generate enough power for sustained flight is just about prohibitive, and incremental advances in lithium-ion technology are unlikely to change that any time soon.

One promising alternative to lithium-ion batteries is the zinc-air battery, where zinc reacts with air using a catalyst to produce zinc oxide and electrical energy. Because the air is one ion source, it has a very good specific energy (5-6 times greater than lithium-ion) and reasonably good specific power (about 30-50%). They do have some odd side effects, though. For example, because zinc oxide is heavier than zinc, the batteries get heavier as they discharge; for another thing they are tricky to recharge over many cycles and usually must have their zinc “fuel” reprocessed rather than simply being plugged into the wall.

Are you familiar with the range equation?

https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node98.html

This equation assumes you are burning fuel, and includes the ln of the ratio of initial weight to final weight. It accounts for the airplane getting lighter and lighter as it flies. (Or alternately, you can think of it as accounting for "burn to fuel to carry fuel" penalty -- the same thing that drives the rocket equation. In fact, if you take out the lift/drag part you can pretty much see the rocket equation sitting in the range equation.)

With an all-electric airplane, the weight never changes, which makes for a simpler equation but actually gives you a lower range because the airplane does not get lighter as you fly.

If an airplane got *heavier* as you flew, this would really throw a spanner into the works. For one thing, landing weights would be heavier than takeoff weights, which would totally change how we approached things like field length limitations. For another, reserves would get really tricky, because your airplane would be heaviest at the end of the flight, so that's actually when you would need the most energy to fly, which means you would have to carry a significantly higher fraction of your takeoff energy as your final reserve energy than you do now.

All of this would really push the range of the airplanes down hard, unless it was made up for by being very significantly more efficient.

------

And somehow I really doubt that loading the plane up with water is somehow going to improve its range. You already have a source of oxygen in the air. I don't think you are going to find that it is somehow more efficient to electrolyze water than to just pull the oxygen from the ambient air.

 

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24 minutes ago, mikegarrison said:

And somehow I really doubt that loading the plane up with water is somehow going to improve its range. You already have a source of oxygen in the air. I don't think you are going to find that it is somehow more efficient to electrolyze water than to just pull the oxygen from the ambient air.

We are pulling in the oxygen from the ambient air. 

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9 hours ago, Spacescifi said:

 

Hmmm.... you may be on to something.... have any connections with aerospace engineers who would be willing to test this out?

Either that or it's ALL you, which is arguably harder.

Using the air around in creative ways should make all the difference though.

I never was much convinced anything other than project Orion could SSTO with a meaningful (40 tons cargo or more) payload.

 

The problem with SSTO is twofold.

1. Reaching orbit without staging requires a LOT of energy expenditure.

2. Deorbiting will damage the hull on each go so much that I doubt you could do multiple reentries without major hull repair. In fact the only way to avoid fiery reentry is powered deobiting, which is also possible using project orion. Landing the monstrousity would require a significant amount of chemical propellant though, preferably on it's belly since landing on the plate is not advisable.

Did you mean to post this in some other thread, since this one has nothing to do with Orion, SSTO nor deorbiting?

@sevenperforce, instead of electrolysis, perhaps an oxygen concentrator?

Something like this, but not for ants:

https://www.usa.philips.com/healthcare/product/HC0044000/everflo-home-oxygen-system

5l/min of 93% O2 at 350W, which if my math checks out, is somewhat more efficient than electrolysis.

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14 hours ago, Spacescifi said:

Landing the monstrousity would require a significant amount of chemical propellant though, preferably on it's belly since landing on the plate is not advisable.

“I have a giant shock-absorbing landing pad. Better not land on it.”

4 hours ago, Shpaget said:

@sevenperforce, instead of electrolysis, perhaps an oxygen concentrator?

Something like this, but not for ants:

https://www.usa.philips.com/healthcare/product/HC0044000/everflo-home-oxygen-system

5l/min of 93% O2 at 350W, which if my math checks out, is somewhat more efficient than electrolysis.

My guess is that the weight of a zeolite bed and pressurization system would be prohibitive at scale. There’s really no weight at all added by introducing electrolysis; the battery already requires an aqueous catalyst.

11 hours ago, AngrybobH said:

They discovered the graphite side (oxygen gathering) limited the O2 into the battery and not the quantity of O2 in the air. They theorized that a different configuration of carbon was needed, like graphene.

That’s interesting. I’ve seen amateur cells set up where the oxygen-gathering side used steel wool but I suppose carbon’s affinity for oxygen makes it a better choice.

But if increasing oxygen partial pressure doesn’t affect uptake at all then this is DOA. 

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4 hours ago, sevenperforce said:

But if increasing oxygen partial pressure doesn’t affect uptake at all then this is DOA. 

Not necessarily. My tinkering with zinc-air was nearly 10 years ago(oh sh... it was more than that, I'm feeling old now). You would think that someone has an idea of a material that can be used. It doesn't even matter if that material is super expensive because that side of the battery is not consumed. It does seem most of the research was done when lithium batteries weren't what they are today but with the better environmental impact of zinc vs lithium(and cobalt) you would think someone is still working on those. I do know there is a company still trying for iron-air batteries. I think x-air batteries can be better than li-ion just got to wait for someone with funding to figure it out.

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9 hours ago, sevenperforce said:

“I have a giant shock-absorbing landing pad. Better not land on it.”

My guess is that the weight of a zeolite bed and pressurization system would be prohibitive at scale. There’s really no weight at all added by introducing electrolysis; the battery already requires an aqueous catalyst.

That’s interesting. I’ve seen amateur cells set up where the oxygen-gathering side used steel wool but I suppose carbon’s affinity for oxygen makes it a better choice.

But if increasing oxygen partial pressure doesn’t affect uptake at all then this is DOA. 

 

Well yes... but if you ever wish to get the pad off the ground again that means rockets, and the way I see it, rocket exhaust plumes flying past the pistons I depend on to reach orbit is almost daring murphy's law to do it's thing.

 

Especially if it is intended as a reusuable SSTO.

14 hours ago, Shpaget said:

Did you mean to post this in some other thread, since this one has nothing to do with Orion, SSTO nor deorbiting?

@sevenperforce, instead of electrolysis, perhaps an oxygen concentrator?

Something like this, but not for ants:

https://www.usa.philips.com/healthcare/product/HC0044000/everflo-home-oxygen-system

5l/min of 93% O2 at 350W, which if my math checks out, is somewhat more efficient than electrolysis.

 

Somehow my mind just went there... I assumed flying car=SSTO.

 

It does not.... but I digress.

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On 5/31/2022 at 3:27 AM, mikegarrison said:

Are you familiar with the range equation?

https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node98.html

This equation assumes you are burning fuel, and includes the ln of the ratio of initial weight to final weight. It accounts for the airplane getting lighter and lighter as it flies. (Or alternately, you can think of it as accounting for "burn to fuel to carry fuel" penalty -- the same thing that drives the rocket equation. In fact, if you take out the lift/drag part you can pretty much see the rocket equation sitting in the range equation.)

With an all-electric airplane, the weight never changes, which makes for a simpler equation but actually gives you a lower range because the airplane does not get lighter as you fly.

If an airplane got *heavier* as you flew, this would really throw a spanner into the works. For one thing, landing weights would be heavier than takeoff weights, which would totally change how we approached things like field length limitations. For another, reserves would get really tricky, because your airplane would be heaviest at the end of the flight, so that's actually when you would need the most energy to fly, which means you would have to carry a significantly higher fraction of your takeoff energy as your final reserve energy than you do now.

All of this would really push the range of the airplanes down hard, unless it was made up for by being very significantly more efficient.

------

And somehow I really doubt that loading the plane up with water is somehow going to improve its range. You already have a source of oxygen in the air. I don't think you are going to find that it is somehow more efficient to electrolyze water than to just pull the oxygen from the ambient air.

 

Range calculations goes way back as in how to support armies. Great eastern was so huge so it could reach Australia with the crappy steam engines they had back in the 1850's.
But steam engines got better, the UK got more colonies who could be used for refueling and the Suez canal.  
Great eastern had a lot of the Airbus A380 problems as in it was to large. 

Now size has benefits, Iowa battleships was set up to refuel their destroyer screen as they have so much fuel. Assume carriers could too but not sure if the nuclear ones can. 
 

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Giving this a touch more thought: what about dispensing with the zinc-air “battery” concept altogether and just going straight to a metal-oxide hydrogen generator? Sodium metal will react quite spontaneously and exothermically with water to produce sodium hydroxide and hydrogen gas. The same reaction happens for any of the alkali metals. The potential energy of the sodium+water reaction is 141 kJ/mol, and burning the resultant hydrogen in an air-hydrogen fuel cell gets you another 241.8 kj/mol. Given the 60% efficiency of a hydrogen fuel cell, the sodium metal + water + hydrogen pathway yields a specific energy off the fuel cell alone of 491 Wh/kg, nearly double that of the best lithium-ion batteries. Assuming you can in some way use a portion of the sodium+water reaction energy (let’s say 20% to give a Carnot buffer), that goes up to 914 Wh/kg.

The zinc-water reaction is not nearly so exothermic or rapid, but it still produces up to 538 Wh/kg. Zinc oxide has the tremendous advantage of being thermally reduced back to zinc metal and oxygen gas. The reaction of sodium hydroxide with more pure sodium will produce sodium oxide and steam, and sodium oxide (like zinc oxide) can be thermally reduced back into sodium metal and oxygen gas.

So there should be an alloy of zinc, sodium, and/or other metals which would react rapidly with water to produce hydrogen to operate a fuel cell and yet still be thermally reducible in a simple reprocessing unit. So your car could run on water and you’d just need to swap out your metal fuel core once a week at the gas station. Or, in the case of an electric plane, swap out the metal fuel core between flights. 

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