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[Edit: changed name and crossing out the option for lowering
filled canisters into
the sea, which will be moved out to another idea]
For hydrogen produced with electrolysis, an important
factor
is cost of compression, to 300 or 3000 psig. (20 or 200
atm)
see links. Although running costs
of electricity are the
main factor of production costs (80%), the compression
of the Hydrogen remains an important factor. According
to the first link, industrial hydrogen production is done in
water, already under the desired pressure.
But at deep sea, this can be achieved {{strikeout|in one of
two ways: 1. Producing}}
... by producing at 200 meters or 2km below sea (assuming
~10m/atm of water depth)
[Edit: 2018, added]
Getting the wires and the weighted and water-filled canisters
down is really no problem at all at no serious effort. The
whole thing will be weighted just enough so that only when
full they'll begin to float back up. A weight detector or some
other device can trigger a signal to the surface to pull the
canister up.
The wire is set from above into the canister with a (cheap)
water tight electric connector that can be easily
disconnected.
The canisters are held upright by the weights, so the gas does
not leak while being filled and a simple pressure seal closes
on the bottom once the canister is on its way up, when the
pressure inside is greater than that ourside.
[/End of edit]
{{strikeout|2. Simply pulling the hydrogen balloon down
under sea.}}
Presumably the energy costs would be less than the
typical pressure building by pumping costs, since most of the
energy loss is
due to heat, see 2nd link.
Costs of hydrogen production by electrolysis (2009)
https://www.hydroge....gov/pdfs/46676.pdf
[pashute, Mar 22 2017]
Costs of hydrogen compression (2013)
https://www1.eere.e...workshop_report.pdf
[pashute, Mar 22 2017]
[link]
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PV = RT, which means there's no free lunch. |
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To get the compression, you have to put in the energy, because a compressed gas represents potential energy. As it expands, that expansion can be used to do work. |
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It's possible to envisage a combination RO and electrolysis cell that's lowered into the deep ocean. The high pressure would force water through the RO membrane, which could then be electrolysed. But this will cause the pressure in the cell to rise, although the oxygen can be vented; the high pressure hydrogen will need to be continuously removed. |
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There are formidable engineering challenges and the cost-effectiveness is unclear. |
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//There are formidable engineering challenges and the
cost-effectiveness is unclear// - just like
everything else on the Halfbakery... |
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I've always wondered if you could use pressure and a
vacuum chamber at the bottom to have a self sustaining
electrolysis reaction, since the gases are compressible. |
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"self sustaining" in what way ? |
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Whatever the physical environment, you still have to put in
enough energy to split the O - H bonds. That's immutable,
whatever the circumstances. You may in some cases have to put
in more that that due to losses and poor efficiency, but there's a
minimu m that can't be changed. |
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self sustaining in that the fall of water in a sufficiently tall
tube into an empty chamber while turning a turbine would
provide enough energy to split the hydrogen and then vent
the gases so the water can fall again.
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Until the ocean's emptied out, of course :) |
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" Presumably the energy costs would be less than the typical " |
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// the fall of water in a sufficiently tall tube into an empty chamber while turning a turbine would provide enough energy to split the hydrogen and then vent the gases so the water can fall again. // |
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So ... water descends a tube under gravity, at the base of which is a turbine above a reservoir which is initially empty. |
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Water passes from said reservoir into an electrolysis cell, where the rising hydrogen and oxygen bubble streams also lift surplus water (taken from the reservoir) back to the top. The water is separated and allowed to fall down the pipe again. The hydrogen is stored, the oxygen is either stored or released. |
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Go on, do the math. We dare you. |
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It needs more than one math, [8th]. |
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I may end up doing the math on this one (it would just take a single
chalkboard; I'm not in the UK where math comes in pints so you have
to use a whole bunch of them). It looks to me like you could
electrolyze the water at high pressure (deep, you don't have to
put it down there & the electricity doesn't compress) and expand the
oxygen through a turbine as you vent it to the surface. |
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Electrolysis takes extra energy at higher pressures. The extra energy required is, coincidentally, exactly the amount you could get from expanding the gas to atmospheric pressure. |
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I haven't found an electrolysis equation with a pressure term. Will
keep looking. |
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[MB], you've almost convinced me hydro-power is
impossible. |
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Let's imagine for a second that the water falls into a
previously vacuumed chamber, which opens and closes as
the operation progresses. |
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As the chamber is closed, isolating it from the water
column, what is the "extra" pressure acting on the water
that's about to be electrolized? |
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Wouldn't the key obstacle be sufficient depth to generate
the required energy -- don't really see pressure playing a
role directly on the electrolysis. You seem to be implying
the taller the column of water, the more expensive to
electrolize the cubic foot (or whatever) at the bottom? I
must be missing something. |
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//Electrolysis takes extra energy at higher pressures. The extra
energy required is, coincidentally, exactly the amount you could get
from expanding the gas to atmospheric pressure.// |
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My bogusity-checker says that this means a fuel cell produces extra
energy at higher pressures. I'm gargling that, and it don't taste right... |
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So, if I understand, you're imagining dropping water down a deep, empty well leading to the bottom of the ocean, and then electrolysing the water in the well? If so, then the electrolysis does indeed happen under atmospheric pressure, and no extra energy is needed to make it happen. |
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However, if you then want to let that hydrogen (and oxygen) bubble up to the surface, doing work as they go, then you have to push the gas out of the bottom of the well (where it was made) and into the surrounding water. And that pushing will require energy. |
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Sorry -- humor me for another anno as this has been in my
head for a long time. |
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A vertical cylinder which has water all the way except for -
- let's call it a combustion chamber -- at the bottom. Let's
imagine we were able to sufficiently protect this chamber,
and we open it at its top (which is the bottom of the
"well") |
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As we open the chamber at the top, the water from above
it rushes in and turns a turbine that, let's say, charges a
battery. The chamber closes. Now we have a cubic unit of
water. |
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We apply the current to said water, until the chamber is
full of air. We open the chamber at its top, at which point
this air what -- just compresses? No, it escapes up into the
water column. Then we close the chamber, rinse repeat. |
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Since it's a given -- I think -- that the taller the column of
water the more power -- and since the size of the chamber
is fixed, it should be that there's a depth sufficient to
electrolize the chamber. Maybe :) |
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The sharper the knife, the less pressure to cut. The right
knife for the job and "A la peanut butter sandwiches!" . |
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//We apply the current to said water, until the chamber is full of air.// You mean full of H2+O2? But unless the chamber is only very partially full of water to begin with, you'll create a high pressure, which will raise the power needed for the electrolysis. |
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// We open the chamber at its top, at which point this air what -- just compresses? // Actually, yes, the H2+O2 will compress. As the bubbles rise, they will re-expand. |
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And, in any case, you are then left with your chamber full of seawater, which has to be pumped out before the next cycle. And that pumping needs energy, proportional to the volume pumped out and the depth of the water. |
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Energy conservation always finds a way to get even. |
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" Presumably the energy costs would be less than the typical " |
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" Energy conservation always finds a way to get even " |
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The sea water becomes H and O, no? The whole point is to
convert the water coming in. No pumping |
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I'm still not sure I follow. But the bottom line is that if you want to recover energy either in the form of gas compression, or from the rising of gas through water, then you have to pay for that energy at some point. |
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[theircompetitor] takes maniacal free energy posture :) |
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MB -- if the chamber is empty at first. Then the water falling
into it charges a battery. The chamber is closed. Then the
battery energy electrolises the water in the chamber making
it gas. Then the chamber is opened chamber at the top and
water rushes in pushing gas up and again charges battery.
then close the chamber electrolize it and make it into gas. |
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Until you run out of water. |
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Pushing gas up and letting water fall amount to the same thing, so that makes life simpler. |
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Whatever energy you can harvest from falling water/rising gas will, if all your efficiencies are 100%, be sufficient to compensate for the extra energy needed to electrolyse the water in an enclosed space (ie, against the pressure that will build up as you do the electrolysis), as opposed to doing it in the open at the surface. |
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One side seawater the other two sides gas with magical
future water spliting membrane/catalyst in between. Both
Hydrogen and Oxygen gas areas bottled away and replaced
with empty bottles. |
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But then again, manufacture of the membranes might be a
bad good. |
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sorry, I'm not seeing the free energy trap, and I'm not
getting
the pressure correlates to energy cost issue. When the
chamber is closed, it is not subject to any pressure,
therefore
the electrolysis process happens just as it would at the
surface -- but with the added energy of the water's fall.
Once the chamber is full of gases and not water, it can be
opened as it is compressible, and can be vented up, while
the turbines turns. There may be a free energy trap here,
but i don't see what pressure has to do with it. |
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All I want to have is a compressible chamber at the bottom
to which the water can fall. If I supplied an external
power source to create that empty chamber -- let's say
even a pump -- I'd accept that it would cost as much as the
water's fall to do so, or perhaps even more so. |
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But if I electrolize the water instead of pumping it -- the
only question should be how efficient the electrolysis is.
And how deep the tube is. At a sufficient depth -- perhaps
Jupiter type depth -- there should be enough to
electrolize a fixed unity of water. |
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Think of it this way. If I have a hydraulic power station, it clearly does
generate power as the water falls -- this is not subject to debate :) |
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And then if I know that it takes me so many kw to electrolize a cubic
meter of water -- now I just need the head parameter of my tube such that
my power equation to be "tall enough" to electrolize the same unit of
water. |
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ok, I'm off to charge at windmills |
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//When the chamber is closed, it is not subject to any pressure, therefore the electrolysis process happens just as it would at the surface// Well, not really, because the chamber _is_ closed and therefore pressure will build up as electrolysis proceeds. And so the electrolysis will need additional power to overcome this back-pressure. |
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However, let's assume that you have only a small volume of water in a large chamber, so the pressure buildup is not significant. In that case, you won't need any extra energy to do the electrolysis (compared to the energy you would need at the surface). |
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Then you vent the electrolytic gases, and you capture the energy of the rising gas by whatever means you like. So far so good. However, in venting the gases you will also flood your chamber, and you will then have to pump out the chamber (leaving only the small volume of water to be electrolysed) before you can start your next cycle. |
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You might say "Aha! But when I vent the electrolytically-produced gases from the chamber, I don't let water flood back in - I just let the gases escape through a narrow vent, by virtue of their pressure! So my chamber doesn't flood, and I can start over again." But, for this to work, the electrolytically-produced gases will have to be at a high pressure, which means you will have had to use extra energy in the electrolysis; and with passive venting, the residual pressure in the chamber (before you start your next cycle of electrolysis) will be no less than that of the surrounding water. |
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I accept that you understand this better than I :) |
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But you keep saying I want to get the energy from the gas.
I do not. I want to get the energy from the falling water. |
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Imagine if I put an electric pump at the bottom of the
Mariana Trench, and a cylindrical pipe from the surface of
the ocean all the way down. Would you accept that I
could -- by investing the energy -- pump out water at the
BOTTOM of that tube, and then harvest energy --
ADMITEDLY less than put in -- by having the water fall into
that chamber and turn a turbine? |
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Put another way, hydraulic power only works because
there's still water at the top of the waterfall, and there's
still room at the bottom of the waterfall, right? That's
what I'm trying to recreate with electrolysis, just create a
closed water cycle |
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Yes, if you let water fall down a deep empty pipe, you can get energy from it - no problem. (As you note, though, this will be less than the energy you used to create that situation in the first place - for instance, the energy needed to push a deep, empty pipe with a closed bottom down into the sea, against bouyancy.) |
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But in your deep-sea rig, any energy you can get from falling water will come from rising gas (the two are equal and opposite). Ultimately, the water can only fall because the gas rises - directly or indirectly. |
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At the very best, your entire system will be no more efficient than just a surface, built electrolysis system. In practice, of course, it will be a good deal less efficient. |
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If I'm missing something, then help me out. List the steps in your process (1, 2, 3...) starting with the setting up of the system, going through one cycle of operation, and then returning to the starting condition ready for another cycle. |
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maybe this is where I am confused:it has to do with how to
properly calculate HEAD |
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Our pipe is not all empty. It is all full of water except for
one cubic meter at the bottom (our empty chamber). We
are not attempting to
derive energy from the
water rushing in from the ocean at the bottom into the
tube. We are attempting
to derive energy from the
water falling inside the tube into the bottom of the
chamber. |
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Let's say our pipe is 1001 meters tall and for simplicity one
meter in area, with 1000 meters of
water, and that one meter of the empty chamber, at the
bottom (where the turbine is). So the pipe is full of 1,000
cubic meters of water
when we start. |
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Let's forget about the electrolysis for a second. Let's just
imagine we can push a button, and our one cubic meter
empty chamber is open at its top (1,000 meters below the
top of the pipe), allowing the the thousand cubic meters
of
water IN THE PIPE already to
start rushing in -- of course only the first one of them will
make it :) |
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What is the equation that describes the power generated
by that fall. Is it affected by the height of the pipe (and
this the volume of water that is waiting to fall)? Or
not? |
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Are you saying HEAD is 1 regardless of the height of the
pipe? Then of course this idea doesn't work. |
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Or is head affected by the height of the pipe? I thought it
would be as clearly there's more pressure before the
chamber is open. |
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10001 meters tall, or 101 meters tall, all full of water
except for that last empty chamber. Are they generating
exactly the same amount of power when the chamber is
open, or not, because the pressure is different? |
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Ah, OK. So, you have a 1000 metre pipe (and let's say it has a 1 metre square cross section), and you have a 1 metre long empty section at the bottom of it. |
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You open the doors, and that 1000 metre column of water drops by 1 metre, yes? |
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OK, so the mass is 1000,000kg (your 1000-metre column of water), falling through 1 metre under 1g of gravity. So, the energy you get is |
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mass x height x g
=1000,000 x 1 x 10 (roughly)
=10MJ of energy. |
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BUT. How did you create that 1 metre space at the bottom? If you did it by pumping out the water in that chamber, you will have used 10MJ of energy to do so (if your pump is 100% efficient). If you did it by sealing the chamber whilst on the surface, and then lowering the pipe 1001 metres into the sea, then you'll have used 10MJ of energy to push the pipe down (since it has the bouyancy of 1m3 of air in the chamber at the end). If you did it not by pushing the pipe down, but by weighting it so that it sinks, then you've lost 10MJ of energy in the weights that carried it down to 1001metres. Or if you did it by using electrolysis to create H2+O2 and push the water out of the 1m3 chamber, then you'll have needed an extra 10MJ for your electrolysis, to overcome the pressure. |
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In other words, however you create your 1m3 void, 1000m under the ocean, you use as much energy to create it as you could possibly recover from letting water fall into it. |
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Your idea is basically the same as saying "lift a weight and then extract energy from its fall". |
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right, so you are saying you generate more power, but the
required emptying of the chamber requires more power.
That's the part I don't grasp at an emotional level :) But I
believe you. |
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I'm saying that whatever energy you get from letting water drop into the empty chamber has to be paid for if you want to get back to your starting point (of having an empty chamber). |
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OK, if it helps, imagine solid masses rather than water. Suppose I have a weight - any weight - and it has room to drop by 1 metre. By letting it drop, I can get energy. But I had to put energy in to raise the weight in the first place and, if I want to do it again, I have to use more energy to lift the weight back up again. |
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I am sure there is an old Russian nuclear sub sitting around
somewhere that needs 1m2 hole retrofitted. |
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[8'th], Back to a discussion about this (deep sea
electrolysis) idea: |
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Getting the wires down is really no problem at all. So what
energy do I have to "put in"? Getting the pressure canisters
down is not a problem either. They are filled with water.
They will be weighted just enough so that only when full
they'll begin to float back up. |
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The canisters are upright so the gas does not leak while
being filled and a simple pressure seal closes once the
canister is on its way up, when the pressure inside is
greater than that ourside. |
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