Using electrolysis, how much hydrogen would you be able to acquire from from one gallon of water, and how much energy would it require? (Sorry, I know this is a simple question. I'm not too well acquainted with these types of conversions)

Thank you for viewing.

 

Enough to get the thread closed if you want to use the hydrogen in a car.

 

It's not an efficient process. I don't have exact answers for you, but I found this:

The reason this system isn't very efficient is because some of the electrical energy is converted into heat during the process. There have been reports of 50%-70% efficiency

 

Hello,

Also the wiki states that the efficiency varies widely between 50 and 80 %.
http://en.wikipedia.org/wiki/Electrolysis_of_water

The wiki also has an other article on hydrogen production:
http://en.wikipedia.org/wiki/Hydrogen_production

Bertus

 

Ok, thanks guys. And for those of you who are curious, the reason I ask is I was trying to think of ways an airship using hydrogen as a lift gas could acquire more lift gas in the event that it is depleted.

One situation I though of is a hydrogen lifted airship is caught in a storm and a hole is ripped in the envelope, all the gas escapes, and it lands in an uninhabited region. It needs a way to refill it's envelope after repairs are made.

I though perhaps a wood powered generator could acquire more hydrogen by powering an on board electrolysis device. Do any of you have any thoughts?

*EDIT* Sorry, just to clarify, I am talking about a way to independently acquire hydrogen from the environment, such as wood to power a generator, and water to be used in the devise.

 

http://books.google.com/books?id=Mr...6AEwAg#v=onepage&q=water electrolysis&f=false

in the upper right corner will be a button with a gear shape on it. You will find the option to download the pdf file there.

Nothing much has changed except the look of the apparatus in over 100 years. The procedures and methods for water electrolysis presented in this old book are just as valid today.


P.S. It takes lots of current flow(amps) to get a useful amount in a short amount of time. If you have time then leaks are your only worry and Hydrogen WILL leak from most any system over time.

 

That's an interesting premise. It reminds me of Jules Verne's novel 'The Mysterious Island'. If your island has bauxite, an aluminum ore, perhaps you could combine it with lye from wood ash and water to create hydrogen. Lye, aluminum and water is a great chemical hydrogen generator. Is this research for a book?

You could probably scavenge aluminum from the airframe of the ship within limits or from the remains of the 'LOST' plane wreck on the other side of the island.

 

I suppose scavenging could be a possible way to go about it as well.

Yes, I am quite interested in steampunk-type themes such as Jules Verne. I'm just trying to think of practical ways to overcome obstacles that a sky adventureer might come upon amid his journeys.

 

Hydrogen airships?

http://www.youtube.com/watch?v=F54rqDh2mWA

 

Okay, first off, the Hindenburg was originally designed for helium, but because of WWII, the US (The main producer of helium at the time) would not trade any to Germany because it was considered a munitions. So they had to fill it with hydrogen, whereas the systems aboard the airship were designed for the non-flammable helium. As a result of there lack of patience, it horribly malfunctioned.

Secondly, I find it quite hypocritical that people hate hydrogen airships now because of mainly the Hindenburg, but hundreds of airplanes crash year after year, but we are completely okay with them, but just one airship (I know other airships have crashed too, but this is manly because of the one) explodes because the wrong gas for the systems of the ship was used. And just a side note, back in WWI, Germany used hydrogen zeppelins all the time in war, they could take hundreds of rounds and even artillery and still fly away in one piece.

My point is, a hydrogen airships is just as dangerous as most aircraft.

 

If you are going to pour out such vitriol, at least be accurate in your reasoning.

The Hindenburg predated WWII by nearly 2.5 years.

The Hindenburg disaster took place on Thursday, 6 May 1937

By convention the start date of WWII is generally given by historians as 1 September 1939

It is interesting to note that of the hundreds of balloons at the annual Bristol Balloon Festival, none are hydrogen.
It is also interesting to note that there are a handful of crashes annually of these 'safer' types due to fire.

I doubt that there are 'hundreds of plane crashes annually'.

I remember a series of stories by one of the big name authors of SF in the late 1960s, early seventies about a planet civilisation that lived on airships.

I have been trying to remember the details for you as they were very good.

 

One situation I though of is a hydrogen lifted airship is caught in a storm and a hole is ripped in the envelope, all the gas escapes, and it lands in an uninhabited region. It needs a way to refill it's envelope after repairs are made.

I though perhaps a wood powered generator could acquire more hydrogen by powering an on board electrolysis device. Do any of you have any thoughts?

Yes. The Zeppelin to Brill has just arrived. Including onboard wood generator + crystal water vials.

 

...One situation I though of is a hydrogen lifted airship is caught in a storm and a hole is ripped in the envelope, all the gas escapes, and it lands in an uninhabited region. It needs a way to refill it's envelope after repairs are made.

The one problem with this scenario is an airship with a hole large enough to evacuate all of the lifting gas does not land.

It crashes.

 

The Hindenburg predated WWII by nearly 2.5 years.

The Hindenburg disaster took place on Thursday, 6 May 1937

By convention the start date of WWII is generally given by historians as 1 September 1939

I'm no historian. I will admit, my facts may have been flawed, but no doubt, 2.5 years before WWII, tensions were high, and that isn't even the point. The point is the Hindenburg's systems were not meant for hydrogen, greatly increasing it's chances of an accident occurring.

I just realized, I horribly digress, sorry about that. I had best get on topic before I give off any more flawed facts.

At the end of the day, if you don't want to fly a hydrogen airship, by no means will I force you. But I know the dangers. And hey, I never said I wanted anyone to fly in one, I'm just trying to think of ways to replenish it's envelope gas supply independently from nature.

 

The one problem with this scenario is an airship with a hole large enough to evacuate all of the lifting gas does not land.

It crashes.

Erm... I mean a whole that slowly releases the gas, and after it lands, the last of it escapes before it can get patched.

*EDIT* Hmm... I feel as if I am not sufficiently conveying this thought, sorry if I make it hard to understand.

 

When posting in the science forum, everyone becomes a nitpicker. It is the nature of the beast.

I suspect chemical ways to generate hydrogen would be best. You can make acids with a wide variety of materials, and most metals generate hydrogen when dissolved with an acid. The trick then becomes finding a reaction that is vigorous enough to be useful. It would probably not be practical to carry the base materials with you, so you would likely have to find indigenous materials instead.

Hydrochloric Acid (aka muriatic acid, 33% HCl) with hydrogen peroxide (commercial grade, 3%) are very vigorous when used with aluminum, for example.

 

I suppose a scavenger hunt for the appropriate chemicals could be a side journey for the landed/crashed adventurers.

Do you think it would be practical to keep an emergency supply of highly compressed hydrogen aboard the airships within holding tanks for that scenario as well?

*EDIT* But, don't you think by the time one has acquired all the materials needed for producing hydrogen, they could already have enough hydrogen from electrolysis?

 

I cannot trace the SF story I mentioned but the Wiki article on Zeppelins is interesting.

A total of 84 Zeppelins were built during the war. Over 60 were lost, roughly evenly divided between accident and enemy action. 51 raids had been undertaken,

Not such a high success rate. The story of the accidental damage to the first 4 zeppelins is also interesting.

There is much in the article that may be of use in your own story.

http://en.wikipedia.org/wiki/Zeppelin

 

In defense of zeppelins, they were a newer technology, and had issues, just like planes. I would imagine plane's success rates were not as charming either in the world wars.

 

Personally, I will at least applaud you for trying to find out how reasonable something is before throwing it into a book. Very unusual that anyone makes even a passing effort at that, whether it be a book, a TV series, or a movie.

Having said that, you do want to do a better job researching the various aspects of your topic. While hydrogen airships were initially fairly immune to being shot full of holes, this is because the gas was at only slightly higher than atmospheric pressure and so holes in the side and, particularly, bottom of the gas bag did not result in catastophic leakage. They tended to fly high enough that it was difficult to attack them from above with the fighters available early in the war. But three things caused all of that to change. They had to fly lower to actually hit anything (and even then they were never very effective, causing perhaps a few hundred casualties throughout the entire wire), the fighters rapidly evolved and were able to hit them from above, and the really big one, the Allies started used incendiary bullets which very much did cause them to become the airborne funeral pyres for their crews. Germany was forced to abandon their use for strategic bombing because they were just too vulnerable. They did, however, have better success using them in the maritime role. Interestingly enough, the U.S. used airships in WWII for maritime defense with pretty good success. In that role, their primary duty was to scout for enemy submarines and call in whatever assets were available, be it Navy, Army, or Civil Air Patrol.

Getting back to your purpose, we can be pretty flexible with the constraints since we are talking ballpark numbers for a book; so let's see if we can at least get some ballpark numbers for you. If found a reference giving the gas envelope for a "small" airship at about 60,000 cubic feet, or 1.7 million liters. Assuming that the gas is at standard pressure and temperature, then one mole of an ideal gas occupies 22.4 liters, so we are talking about at basically 76,000 moles of hydrogen. Since water is H20, a mole of water will produce one mole of H2 and half a mole of O2. A mole of water is basically 18g, so you would need about 1370 kg of water, or about 360 gallons (which is actually less than I expected, but a quick sanity check of 22.4l of H2 for 18ml of H20 shows this to be right).

So finding the water you need shouldn't be a problem, since we are only talking about a volume of water that is only 4ftx4ftx3ft.

But what about the energy. The energy it takes to liberate the hydrogen is basically the same that is given off when the hydrogen is burned to produce water again. I found that number given as 16.5kWh/gal and 237kJ/mole. Using the first, that gives us right at 6MWh, which is right about six months of consumption for an 'average' American household (right about a year's consumption in Maine). And this is for a 100% efficient conversion process. If you are in the 60% efficient range, then you need 10MWh of electricity.

You mention using wood to fire your converter. Firewood as, very roughly, 20GJ of energy per cord (128 cubic feet of wood). At 237kJ/mole and 76kmoles, you need about a cord of wood assuming 100% efficiency. If we assume 60% efficiency for the electrolysis and 35% efficiency for your wood-fired generator (and my guess is that it will be less than that given the size of the generator you will want to carry around in your airship), you are going to need about five cords of wood. That's quite a bit of wood, but not an unreasonable amount for a story such as you are proposing. For perspective, we use wood heat during the winter and my best guess is that we burn about three cords a year. So I suspect the biggest problem you are going to have, especially given that you aren't going to want a big generator and, hence, not a big electrical output, is that it will take a very long time to produce the necessary hydrogen. You want to get a feel for how well a properly patched gas envelope can hold the hydrogen. Here, you might be able to leverage the line that the envelope was originally filled with helium, which leaks much more easily through just about anything than hydrogen, and thus your survivors are able to fill the helium envelope with hydrogen over a long period (months?) without loosing too much of it. You could also play games such as expanding the gas bag externally by pulling on the attach points to create a slightly negative relative pressure in the bag, thus discouraging the hydrogen from leaking out.