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Old Sep 23, 2013, 08:22 PM
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Joined Sep 2013
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Idea
Hydrogen fuel cell for a hydrogen blimp

Hi guys,

My first post here I've read a lot of discussions from this forum and learned a great deal from you guys. As I'm waiting for the shipment of parts for my very first RC blimp, my brain couldn't stop daydreaming and this thought occurred to me: how about using the hydrogen lifting gas in an hydrogen blimp for fuel? It's mentioned in a post in 2007 but quickly dismissed without much reasoning. I think it deserves more thoughts now that the fuel cell tech is gradually getting there.

I always liked the concept of airships. But when it gets down to a real RC model blimp, one thing I'm very unhappy about is the battery life. Blimps don't glide like airplanes because of their sizes, so you have to constantly fight the wind drift with thrusters. Even when there's no wind, to maintain a cruising speed, you still need to pump amps into the thrusters because the blimp would quickly stop once the thrust is gone. That simply kills the idea for an RC blimp to stay aloft for tens of hours. So naturally I started asking google if there are practical workarounds. Gasoline engines are definitely one possibility, but how about fancier technologies such as fuel cells?

So I did some back-of-the-envelope calculation. According to http://anabatic.aero/twin_specifications.php their 200 cf blimp can be driven by two 200W motors (electric, I assume), for about half an hour. From info I collected from various discussions, it seems like 400W is about on the right order of magnitude of power needed to drive a blimp that size to 30km/h (their max speed), so at cruising (15km/h), the power consumption is probably going to be constantly around 100W. If it's LiPo batteries that supply that 100W for an hour (to be on the safe side), they'll take up 27oz weight out of the 200oz total lift. Now even if we're willing to carry 100oz of battery (50% of total lift), it's still merely 4 hours of cruising, or less if we have to fight the wind once in a while. That's not an awesome duration, especially considering that the ability of staying in the air without power to sustain the lift is the one advantage airships have got against airplanes.

So what about fuel cells? Chemical fuels (like hydrogen) have much higher specific energy compared to batteries, and fuel cells have rather high efficiency (~50%). Hydrogen fuel cells can be made quite lightweight (see Horizon's aeropak for a 500g fuel cell). The only problem is the handling of hydrogen. Hydrogen has higher energy/weight ratio than even gasoline, but its energy/volume ratio is terrible. For fuel cell cars, people use highly compressed (700 bar) hydrogen, but even at that pressure its energy/volume ratio is still much lower than gasoline, which is the major barrier for wide adoption of hydrogen fuel cells. But for blimps, volume is not a problem at all since the envelope is full of almost uncompressed lifting gas anyway. It'd be ridiculous for a car to run around with a gigantic air bag full of uncompressed hydrogen, but for blimps, it's part of what we are. No extra cost.

Now let's talk about energy capacity. Hydrogen's energy density is 5.6MJ per litter at 700 bar. Assuming a fuel cell efficiency of 50%, it takes only 3 cf of hydrogen to provide that 100Wh power above, that takes 27 oz of LiPo battery! Now some one please check my calculation, because I think it's very surprising that 3 cf hydrogen can provide this much power. It's just 1.5% of the envelope volume... hard to believe. If my calculations are correct, this means that storing uncompressed hydrogen in the envelope for use with fuel cells could be very profitable.

But of course there are a lot of challenges to get there, and I'd like to hear everyone's opinion. First of all, let's set the fire safety issues aside and assume that we've learned enough since Hidenburg to be able to handle hydrogen safely at this point. The hydrogen purity required by the fuel cell is pretty high, so I guess we can't just grab the lifting gas and feed it directly into the fuel cell. It'll have to be something like an airbag inside a rigid airship, that's dedicated to storing fuel hydrogen. Taking 1.5% out of the total gas volume won't affect lift that much, but taking away 15% might be a problem. The good news is, hydrogen compressed 1.5 times is still a darn good lift gas (still better than helium), so the question is, how practical is it to fill the envelope with a little pressure? Does it require a lot of extra weight in the envelope? Is it possible to just store the fuel portion of the hydrogen with a little pressure, while leaving the rest of the envelope volume uncompressed?

It's a trivial thing, but I do believe that the notion has got some value. Let's dig in it together!
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Old Sep 18, 2014, 11:33 PM
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Joined May 2014
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ID be using methane ...
making hydrogen yourself is just a waste of time, electricity and yield..
storing it is another problem.. H2 is just so small and will leak easily...
fuel cells seem like a waste of time to me...
hydrogen via electrolysis is a joke really
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Old Sep 20, 2014, 11:11 AM
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Quote:
Originally Posted by nechaus View Post
ID be using methane ...
making hydrogen yourself is just a waste of time, electricity and yield..
storing it is another problem.. H2 is just so small and will leak easily...
fuel cells seem like a waste of time to me...
hydrogen via electrolysis is a joke really
I think you are confusing hydrogen fuel cells with those home made hydrogen generators using electrolysis that people put into their cars thinking that they can beat the laws of physics and get more energy out of their system than goes into it. A hydrogen fuel cell is actually the reverse of one of those-- taking hydrogen and converting it to electricity. The fuel cell is a good idea for the same reason that electrolysis is a bad idea: while it takes a lot of electricity to generate very little hydrogen, very little hydrogen can generate a lot of electricity when the reaction is run in reverse. As for how the hydrogen is produced, that is not discussed. I agree that producing it through electrolysis would be a waste of time, although it does look like Horizon's personal hydrogen generator products do just that. Lets just assume the hydrogen is being purchased in tanks the same as helium, since a lot more would be needed than what would fill a tiny cartridge like what the Horizon generators are designed to fill.

The concept of using a gas as fuel is not new, but has not been tried on an RC blimp as far as I know. The Graph Zeppelin used a similar idea back in the 1930s, dedicating a lower section of its gas bags to holding Blau Gas, which is similar to today's propane. The advantage of Blau Gas was that it weighed almost the same as air, so lift was not dramatically effected as it was consumed. The main issue with hydrogen, other than the purity requirement mentioned, is going to be compensating for the loss of lift as the gas is consumed. You would need some kind of ballast system I think. Trying to compress a usable amount of hydrogen into a cylinder is going to add a bunch of weight, as it would take a thick walled cylinder to withstand enough pressure to pack a usable amount of gas into a small tank. For comparison, an aluminum scuba tank filled with 80cf of air weights 40 lbs and takes 3000psi to pack that much air in. Pressures that high then require heavy fittings and hoses too.

It would be an interesting thing to try though. The low energy density of batteries combined with their cost and heavy weight definitely makes research into alternatives a worthwhile endeavor.
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Last edited by pyronaught; Sep 20, 2014 at 12:45 PM.
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Old Sep 20, 2014, 12:10 PM
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I've been reading more about hydrogen fuel cells, as I think this idea has merit. It turns out a company called Jadoo has come up with a way to get around both the purity and the compression problem by using canisters that contain metal hydrides to hold the hydrogen. The metal hydrides soak up the hydrogen like a sponge, packing three times more hydrogen into a given space than just the gas alone. Since the cylinders are not under pressure, they are much lighter weight. These cartridges have to charge like a battery though, they don't fill quickly like a gas cylinder would. They still charge in about 1/6th the time it takes to charge a lithium battery though, and are less than half the weight of an equivalent battery. Since they can be charged with just standard industrial grade hydrogen, you could use the same gas that is being used for lift. Depending on what the charge rate is, you might just be able to continually charge them during flight by feeding them uncompressed gas from the envelope. There is some water generated as a byproduct, so perhaps this could help offset the change in buoyancy.

There are also further weight reduction technologies on the horizon, compared with lithium battery technology which has just stagnated for several years now. Some of these involve using water as the source of hydrogen though, which you really don't want for a blimp application. Another attractive thing about the hydrogen fuel cells also don't have a short shelf life the way lithium batteries do.

It really looks to me like hydrogen fuel cells have a shot at actually replacing lithium batteries across the board in the future. One of the road blocks that has slowed down progress is the irrational fear of hydrogen that the general public has, but this is slowly being eradicated by educational demonstrations. As more and more applications adopt them, the price will continue to drop. There are already cell phone chargers using this tech on the consumer market.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1797861/
http://www.austinkayak.com/products/...HrgaAtLX8P8HAQ
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Last edited by pyronaught; Sep 20, 2014 at 12:31 PM.
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Old Sep 20, 2014, 01:55 PM
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Joined Feb 2009
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The big question here is what kind of current capacity can a small, lightweight fuel cell support? Electric RC motors are big current hogs, with 8 to 15 amps being pretty average requirements. This is where lithium polymer batteries shine, with the ability to dish out current in large amounts on demand. The commercial fuel cell device chargers currently on the market can only supply 1 to 2 amps. How much bigger does the system have to get to supply 15 amps? A battery is just holding a charge that has already been generated, and as we all know it takes a lot longer to charge a battery than it does to discharge it. A fuel cell is generating electricity on the spot, so it is a lot like a battery charger in that respect. I suspect this current capacity is going to be the limiting issue, as fuel cell size will have to increase to get more current, and thus the weight will increase. I can't seem to find much info on the current capacity of fuel cells.
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Old Sep 20, 2014, 02:11 PM
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Joined Sep 2013
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Quote:
Originally Posted by pyronaught View Post
I've been reading more about hydrogen fuel cells, as I think this idea has merit. It turns out a company called Jadoo has come up with a way to get around both the purity and the compression problem by using canisters that contain metal hydrides to hold the hydrogen. The metal hydrides soak up the hydrogen like a sponge, packing three times more hydrogen into a given space than just the gas alone. Since the cylinders are not under pressure, they are much lighter weight. These cartridges have to charge like a battery though, they don't fill quickly like a gas cylinder would. They still charge in about 1/6th the time it takes to charge a lithium battery though, and are less than half the weight of an equivalent battery. Since they can be charged with just standard industrial grade hydrogen, you could use the same gas that is being used for lift. Depending on what the charge rate is, you might just be able to continually charge them during flight by feeding them uncompressed gas from the envelope. There is some water generated as a byproduct, so perhaps this could help offset the change in buoyancy.

There are also further weight reduction technologies on the horizon, compared with lithium battery technology which has just stagnated for several years now. Some of these involve using water as the source of hydrogen though, which you really don't want for a blimp application. Another attractive thing about the hydrogen fuel cells also don't have a short shelf life the way lithium batteries do.

It really looks to me like hydrogen fuel cells have a shot at actually replacing lithium batteries across the board in the future. One of the road blocks that has slowed down progress is the irrational fear of hydrogen that the general public has, but this is slowly being eradicated by educational demonstrations. As more and more applications adopt them, the price will continue to drop. There are already cell phone chargers using this tech on the consumer market.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1797861/
http://www.austinkayak.com/products/...HrgaAtLX8P8HAQ
Very interesting thoughts! I didn't look into metal hydrides much because the second I saw the word "metal" I assumed it'll be heavy I probably should read more into it.

I also looked into high pressure cylinders for carrying helium, reading the specs of several solutions like scuba tanks etc, and got the same conclusion -- the combined weight of tank and helium is always higher than a Lipo that carries the same watt hours. So that's probably a dead end.

Your idea of continuously charging the metal hydrides with the lifting helium is quite interesting. It's like using the metal hydrides as a purifier to prepare lifting helium for use with fuel cells. Neat.
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