* Explosion danger
Since mixtures of hydrogen and air or oxygen can be explosive, you
_really_ don't want to try this without previous practical experience
in handling more than small amounts of hydrogen. The amount of
hydrogen you can generate from 1 gallon of water will occupy a tank 3
ft by 3 ft by 15 ft (at one atmosphere pressure) (about 1250 gallons).
In addition to preventing leaks, you will need some way to exclude air
and oxygen from the collecting tank; displacement of water might do,
but that's a lot of water to waste. Hydrogen is both colorless and
odorless; you would need some device to test for hydrogen leaks
without using sparks or flames. If you wanted to keep the hydrogen in
a smaller tank, you would have to be concerned with methods of
compressing hydrogen and avoiding explosions there also.
* Other practical considerations:
The water needs to have some ionic compound dissolved in it so the
solution can carry the amount of current that is required. Commercial
electrolysis generally uses 25% potassium hydroxide. I did not
investigate cathode material, but I know that it must be inert, e.g.,
platinum, and not require too high an over-voltage. Another
consideration is that you would probably need a fairly large surface
area for the cathode in order to generate the hydrogen in a reasonable
amount of time. You didn't ask about electrical current requirements,
but you will be limited in that you can't do this in a day if you are
limited to household 10 amp or 20 amp circuit breakers. You probably
also need some way to add replacement water as water is being used up
- although I suppose you could start with 5 gallons and stop when you
get down to 4 gallons without too much variation in the rate of
hydrogen generation.
It's probably illegal to do this without a special permit because of
the explosion danger.
Commercial electrolysis apparatus is sold that addresses these
problems, but the commercial apparatus is expensive because it needs
to deal with all the problems encountered.
* Efficiency:
Wikipedia lists gives efficiencies of 50 - 70% and 80 - 94% and lists
two references. In my calculations below, I assume 100% efficiency;
the actual energy needs could be up to twice those amounts.
http://en.wikipedia.org/wiki/Electrolysis
* Amount of hydrogen generated from 1 gallon of water:
The approximate amount of hydrogen produced from one gallon of water
(at 1 atmosphere pressure and at 32 degrees Fahrenheit) is 1250
gallons. This volume is 4.7 m^3, or about 3 ft by 3 ft by 15 ft.
* How long the electrolysis could run continuously:
This depends on whether or not water is replaced, whether electrodes
start to lose contact with the water, whether the ionic concentration
changes too much as water is used up, etc. Assuming these issues are
taken care of and that leakage of hydrogen (including any effects from
its low solubility in water and diffusion through water) doesn't
exceed the rate of generation, the apparatus should be able to run
indefinitely if inert electrodes are used.
* Wattage needed
16.5 Kwatt-hours of energy are needed to convert 1 gallon of water to
hydrogen. The wattage needed depends on the current flow; if an
apparatus used 1000 watts, then the hydrogen could be generated in
less than a day. However, the current required is too high to do
electrolysis at this rate in normal households; an industrial line
would most likely be needed. You could generate hydrogen at a slower
rate and wattage and still get the conversion done on a 10 amp or 20
amp circuit, but it would take longer.
* Data / constants needed for calculations
1 gallon = 3785 ml
1000 mL = 1L
1 mL water weighs 1 gm
1 water yields 1 hydrogen
1 mole of water = 18 g
electrolysis of 1 mole of water requires 282.1 kJ of energy
from http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/electrol.html
1 mole of gas occupies 22.4 L at STP
1 kwatt-hour = 3600 kJ
1 m^3 = 1000 L
(for calculation of amps, you also need:
1 mole =6.023 x 10^23
2 electrons per hydrogen molecule
1 amp = 1 coulomb / sec
1 coulomb = charge on 6.24 x 10^18 electrons
* Stepwise results from calculation:
1 gallon water
= 3785 mL
= 3785 g water
= 210.3 mole of water
yields 210.3 mole of hydrogen
requires 59,320 kJ of energy
requires 16.5 kwatt-hours of energy
210.3 mole hydrogen
occupies 4710 L at STP
occupies 1245 gallons at STP
occupies 4.7 m^3
amperage results omitted here (I definitely don't want to encourage you.)
------
The site below shows a commercial electolyser; it produces 200 m^3 per
hour (vs the 4.7 m^3 you would get from 1 gallon of water)
http://www.bellona.no/en/energy/hydrogen/report_6-2002/22871.html
The site below discusses electrolysis of water and costs for
commercial electrolysers:
http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html
"The smallest 1 kWel electrolysers can cost up to 10,000 DM with the
price only falling to the 500 DM/kWel figure in the MW range.
Operating efficiencies lie in the 50-60% range for the smaller
electrolysers and around 65-70% for the larger plants."
The smallest electrolyser uses 1 kwatt and costs 10,000 Deutsche marks
(1 DM = 60 cents - but the euro has replaced DM, so these prices are
out-of-date). The cost is / was approximately $6,000. |
