the single most important question for any alternate-fuel ships?

The energy density per mass of hydrogen is greater than the energy density per mass of gasoline , for example.

If you have on board re generating capabilities , then one can create a formula relating the time averaged rate of re generating energy versus the rate of energy consumption along with the longest time that one expects to have gaps in the available ambient energy of your choice and the duration of the passages compared to the difference between mean energy capture and mean energy expenditure.

You analyze that and you' ll know how much energy storage capacity you' ll need for your particular application.

As for fuel cells there are at least four major types with many subtypes.

No , a fuel cell can be made without expendable parts (or at least very long lifetime). Certain fuel cells are also reversible , in that they can split or combine water - H2 + 0)

Yes it does take a lot of energy to compress to liquid state. Myself i am researching for eventual use for our house the details of self compressing the hydrogen and oxygen. This is when the electrolysis products feed directly (no return safety valve) to the pressure tanks. As more gets added the pressure increases and so the bubbles form slower, but do not stop. This represents the same energy loss as using some of the products to compress the rest, only more simply and it would seem , more efficiently. I am not sufficiently knowledgeable to determine this for sure but it seems to me that self compressing will stop when the pressure becomes so great as to liquefy one or both gases. By this point though the energy per volume is quite acceptable although not as compact as gasoline.

Splitting sea water itself is however and unfortunately problematic for reasons of corrosion. Hopefully better minds than mine are working on solving this practical problem.

Any extra information on self compressing electrolysis of water into respective gases would certainly be appreciated from me.

 

T^3, the self-compressing electrolysis idea sounds interesting. I'll let you know if I find anything useful on it, there are a few labs around here working on fuel cell / hydrogen research.

Indeed. If you choose to go cryogenic, there is a significant energy cost involved in the chilling; if you go compressed, there's an energy cost to compressing the hydrogen. To be fair, in either case, it's possible to recover some of that energy when you remove the hydrogen from the tanks. For cryogenic, you could use a Stirling or similar engine operating between the 20 K hydrogen you're boiling off the liquid tanks and the ambient (290 K) temperature or the fuel cell's steam exhaust to recover some of the energy used to chill it. For compressed, you could run it through a turbine instead of a throttle in the pressure regulator, thus recovering a bit of the energy used to compress it.

Fuel cells- both the PEMFC type used for cars/buses and the SOFC, molten carbonate, etc. types that would likely be used on a ship if fuel cells were to be implemented there- usually prefer pure hydrogen. The use of hydrocarbon fuels is mainly for convenience of storage; the pre-processing needed to extract the hydrogen can be a real pain- but it lets us use existing liquid fuels. (Notable exception- DMFC for small electronics.)
Extracting hydrogen from seawater is a bit tricker. There's the salt to consider, for one thing- getting rid of it is energy intensive, and leaving it in makes the electrolyzer spew chlorine gas and clog up with sodium and calcium oxides. And don't forget that pesky First Law- the energy's gotta come from somewhere, so if you're using solar panels to power the electrolysis, then using the hydrogen right away, there's absolutely no way you'll get any more power out of the fuel cell than you would by just hooking the panels straight to the motor controller. Now, if you spend most of your time at anchor, the panels might be able to produce hydrogen and fill the tanks while you lounge around on some idyllic beach- but I've run the numbers on this, and it's neither efficient nor fast.

A fuel cell doesn't store energy; all it does is convert chemical energy (in the form of stored hydrogen and atmospheric oxygen) into electrical energy. I do think that this technology holds promise for boats/ships, once the cost comes down. SOFC and other high-temp, constant-load fuel cell systems hold the promise of being cheaper and more powerful than the PEMFC used for cars, which must handle rapid swings in load. And bulkiness isn't as big a concern on a ship as it is on a car. We'll see how things go on this front.

Still, it's hard to beat a stick, some wires, and a big honkin' sheet of canvas.

 

T^3:
Do you have any information on the variance in the amount of electricity consumed as the pressure in the electrolysis product tanks rises? I'd be interested in seeing someone draw up an energy-efficiency comparison between that process & mechanical compression.

Marshmatt:
Couldn't the electrolysis problems caused by the salt (and other impurities) in seawater be reduced (or eliminated) by simply distilling the water first? (re: solar-distilling desalinization plants)
I just figure, if we're going to use the sun/wind to generate the electricity, why not use the sun to generate purified water too...that'd sure help with the corrosion concerns (and it'd be nice to not have to run the watermaker constantly, or bring along 100s of gallons of fresh drinking water.

 

I've searched the net for hours and have not found any serious info on the self compressing thing, so its been me using logic and trying to brush up on my electrochemical physics. The amount of energy going into the splitter is whatever the [insert generating system of choice here] is producing at that moment. As pressure increases a kind of resistance to splitting increases, but the energy has to go somewhere, so bubbles keep on forming , working against the pressure, so now the energy goes into splitting and increasing the pressure more.

Seeing as how little water is actually needed to produce huge amounts of H2 & O it may well be feasible to distill, using the sun, the required water as you go.

 

T^3

2 questions/problems I have with the self-compressing thing:

1. How can you keep the O2 & H2 from mixing by diffusion through the water at such high gas pressures? (or does the electrical current combat this sufficiently)

2. Won't the gasses build different pressures during splitting, resulting in unequal pressure on the water in your splitter (which would eventually cause the water to be forced completely away from one electrode, or forced down far enough for the gasses to exchange directly)?


I'm trying to see where the benefits & drawbacks of this method would be in my mind, and these are a couple of the things that keep nagging at me...."how can I fix that."

 

Very valid points.

1 a baffle that is partially immersed in the water ensures the gases from each electrode remain separate.

2 The hydrogen tank is exactly twice as voluminous as the oxygen tank.

 

1. They do.. The amounts however are aren't significan't in atmospheric pressure. Stirring water adds this mixing somewhat. It made anyway a bit lowder noise than "unstirred" test tube (oh those golden memories from the chemistry class) but nothing like when the +/- wires were changed during the process
..It might thou be a problem with pressures like 4000psi/250bar?
2. No. Hydrogen just uses double the volume of oxygen. And if an electrode runs dry there's no electrolysis either..

 

That's exactly my concern...the high pressure of the gasses raising the saturation levels in the water, thus possibly making for a more significant mixing of the gasses through the water.

I understand the electrode running dry stopping electrolysis...that would be the problem...no more electrolysis=no more pressure...the other scenario (blow-out & direct gas exchange) was what I envisioned happening if someone decided to "simply" lower the electrodes deeper into the water, and ended up with electrodes deeper than the baffle...OOPS.
I didn't know the exact ratio of compression between the H2 and the O2, but if it's EXACTLY double at all pressures we'll be experiencing, than simply using double the storage space for the H2 (as T^3 suggested) could be the solution.


(now the "Devil's Advocate" in me keeps wanting to see if I can find more flaws to make you guys solve)

 

Hydrogen can either be produced directly from water through electrolysis, or can be made from natural gas by cracking the molecules. Compression of hydrogen is a hassle, but I think the most viable solution would be mass production at a geothermal station to fill unmanned supertankers. In this way, the required energy would be irrelevant. The size of these would have to be significantly larger than conventional oil tankers, but the weight of hydrogen is much lower. Iceland should look towards implementing this system in light of their recent economic troubles.

One of the problems implementing geothermal power is the contamination of the pressurized gasses and steam by particulate matter. This can be averted by using tesla turbines made of corrosion resistant materials.

 

Also, I might mention, I suspect that the higher pressures of self-compressing electrolysis would make the process require more energy. Electrolysis is most efficient in a higher temperature solution at low or zero pressure.

 

ok ill try that i just need a recomendation from someone that isnt me i just jumped on here and asked who ever

 

Yes, I think we all agree here...the question we're running into there, is which is more efficient? Compression through high-pressure electrolysis, or low-pressure electrolysis, then refrigeration or mech. compression. Any thoughts there? (not sarcastic, it's an honest question of opinion)

 

OK, but please, in the future, try skimming through the forum first. Get a general feel for the way things are organized & it'll save you a lot of effort looking for information (and a little embarassment too)

 

For a small scale producer-consumer like an independent boat making her (the boat) own hydrogen mechanical compression is the easiast and cheapest way to go. The capacities are more of a consern. Storage tanks volume&pressure? What's enough? How long times needed for refilling? How much power needed and how?

For a normal sailor oxygen is just a waste product. For me using my boat for scuba diving and the need of oxygen for Nitrox gas was the original trigger for the thoughts of electrolysis in the first place. Then the hydrogen were the waste and going directly to fuel cell making more power for the electrolysis and reducing the amount of additional electric power and some for the cooking (today however membrane filters are the easiest way to produce on site oxygen). Anyway it should have worked..

 

Another interesting idea is simply burning the hydrogen and oxygen as a fuel in a traditional internal combustion engine with no emissions. The addition of hydrogen into the combustion allows lower grade fuels such as impure alcohol to be burned, but raises the combustion temperature, meaning more engine wear. An all ceramic engine is a possible solution.