Re: High-frequency electrolyzers

On Jun 13, 11:49 pm, Robert Adsett <s...@xxxxxxxxxxxxxxxxxxxxx> wrote:
In article <1181748228.240503.113...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Williamknowsbest says...





On Jun 11, 11:19 pm, Robert Adsett <s...@xxxxxxxxxxxxxxxxxxxxx> wrote:
In article <1181545591.201413.56...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Williamknowsbest says...

On Jun 9, 5:59 pm, Robert Adsett <s...@xxxxxxxxxxxxxxxxxxxxx> wrote:
In article <1181376476.771491.24...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
says...

On Jun 8, 11:11 pm, Robert Adsett <s...@xxxxxxxxxxxxxxxxxxxxx> wrote:
In article <1181306147.880719.176...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
says...

On Jun 7, 8:20 pm, Robert Adsett <s...@xxxxxxxxxxxxxxxxxxxxx> wrote:
In article <1181259760.318128.268...@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
says...

But my variable load electrolyzers are 85% efficient in the
electrolysis step. Proton Exchange Membranes (PEM) can be even more
efficient (single step) but not by much - but the costs are
tremendously high. The advantage of PEM is that you can go either way
with fair efficiency - 80% electricity to hydrogen to electricity -
under ideal condtions -but these fall off rapidly in less ideal
conditions..

Where do you get a > 80% efficient PEM cell?

http://waterfuelcell.org/WFCprojects/Tero/series_cell_v1.2.pdf

Well it's not PEM,

That's true its just plain old stainless steel

More to the point it's an alkaline cell. In my experience with fuel
cells the alkaline units I saw were substantially more efficient than
the PEM units. Although to be fair they were not as far along the
commercialization route.

Yes.

My understanding is
that for very sound fundamental reasons PEM is far more efficient than
plain old stainless steel. If you can show plain old stainless is
more efficient than PEM I'd like to see it.

You just did. You've yet to show a PEM unit that comes close to your
claim.

Well, there are Alkaline Units, Polymer PEM, and Solid Oxide Ceramic
Exchange Membrane - it seems pretty straightforward to go look up the
best available in each of these classes wouldn't you say?

Hey, you are the one who claimed the existance of > 80% PEM cells.

Its not a claim dude.

Sure it is, you typed it in black and white just a few lines above.


Cute. So, you have no real issue with what I say so you are reduced
to making *** up just because you LIKE to argue and lose? haha..


I don't know what your problem is. You are a knowledgeable person.
Its common knowledge that under large loads and the higher
overvoltages to create them fuel cells have about 60% efficiency as
you say at their peak power density.

But under low loads with high quality oxygen and hydrogen inputs they
have greater than 90% efficiency! .

As I said I'd like some sort of reference as to where you would get such
a device.

Wait a minute. Are you saying you are unaware that nearly all fuel
cells that have 50% to 60% efficiency at peak power - you are unaware
that those very same fuel cells operate at 80% to 90% efficiency at
very low power? Are you saying this? Are you saying you need a
REFERENCE for this?

Wow.

I've not seen any PEM cell with this claim.

Dude, you know don't you that fuel cells operate at a particular
current and voltage - and that this current and voltage are related.
You also know that there is a peak power density, you can't go
above., It is at THAT current and voltage you get 50% to 60%
efficiency. Now, reduce the voltage what happens? You reduce the
current. Power density goes down. But overvoltage is reduced too.
And with it, efficiency increases. What it increases to depends on
your catalysts and membrane. But most membranes that have 50% to 60%
efficiency at peak power density, rise to over 80% some to over 90% at
low power levels.

The PEM fuel
cells I looked at for use were a lot less efficient, the alkaline was
rather higher.

Yes.

It may be that the existance of such devices is common
knowledge but I've not seen them.

Obviously. That's why its hard to point to a discussion about it.
Let me see..

http://www.fuelcelltoday.com/FuelCellToday/FCTFiles/FCTArticleFiles/Article_516_FordFocusFCV.pdf

I don't know if the Ballard 902 fuel cell stackis PEM or solid oxide
or some other technology - but you see a clear reference to how the
fuel cell efficiency varies.

Clearly with lowered membrane costs, larger membrane areas are
possible for a given investment, and higher efficiencies are
possible.


All
I was asking for was an example, I've not seen one.

You've got to know that the 60% efficienct systems that you are citing
easily run at 90% efficiency in electrolyzer mode, and run 90%
efficient when power densities are cranked down to 15%-20% peak
densities right?

No, even if I were to accept your comparison that still leaves the fuel
cell side and the question of how the efficiencies are calculated.

Well, there is an over-voltage right? You have a voltage at which the
cell operates. Increase that voltage and efficiencies drop. Higher
voltages lead to higher currents and so, you have a situation where
you must choose between fuel efficiency and capital efficiency.

had not realized until going through this that fuel cell and
electrolyzers were commonly calculated using different reference points.

Its part of what makes chemistry such a difficult subject to newbies!
haha.

However, I've not
seen a pointer even to that.

What sort of pointer are you looking for?

Product literature, a paper, a textbook.

Well, there is a vague reference to it in the Ford literature above.
Its common knowledge, sort of the elusive obvious you know? People
don't generally talk about things that everyone takes for granted.

Something that points to > 80%
fuel cell. My original request was actually for a fuel cell reference
as Graham suggested but it was not as clear as it could have been. And
since you started down the electrolysis cell route I figured we'd start
there.

Since you have said you were looking for a primer on the subject, here
is a good place to start;

http://en.wikipedia.org/wiki/Electrolysis#Electrolysis_of_water


The best actual PEM cell I've actually seen was considerably less than
50%, although getting efficiency figures out of manufacturers data
sheets can be quite an excercise.

50% at peak power density yes.

Quite a bit less than that actually.

Depends on the details and which system you're talking about
obviously.

But THOSE SAME FUEL CELLS may operate
at 90% efficiency at 1/5th this power level. This is common
knowledge. You've got to know it.

Why? In fact I would expect the efficiency to drop again as the
external load decreased and balance of plant costs started being a
significant portion of the requirements.

??? How does the cost of the plant affect the efficiency fuel is
used in a fuel cell? haha.. You've lost it here Robert. Gone off
track I think.


So, why are you being obtuse about
it?

All I asked for is an example. So far you haven't provided any.

Alright, now that I realized you truly don't know what I'm talking
about I gave you a couple of references. They're not papers per se
because papers are not written about stuff that's common knowledge you
know?

Please explain that. What did they get wrong specifically? Its all
there, if they made a mistake you should be able to tell me
specificially what the mistake is. I'm the one that scanned it and I
admit they may have made one I didn't see. But if you saw a specific
mistake, then it should be easy for you to say what it was shouldn't
it? But you didn't say. So, I'm asking you.

Specifically I'm concerned about their use of 1.48V for their
electrolyzing efficiency in the calculations. I think they are
including voltages other than those contributing to electrolysis and
getting artificially high efficiency figures as a result.

Where do you imagine these voltages are coming from and why wouldn't
it show up in their experimental apparatus?

The 1.48V is their figure not mine. As near as I can tell they should
be using 1.23V.

Alright

I have not figured out where they get their extra 250mV but I wouldn't
be surprised if it came from their choice of electrode material.

I see So, you think they're not using platinum electrodes - but some
other material?

See
for exampleftp://ftp.strath.ac.uk/Esru_public/documents/MSc_
2003/papagiannakis_i.pdf where 1.23V is used along with the note that
the voltage required is usually higher and notes some of the possible
loss sources.

Alright.

Now that is on PEM rather than alkaline so there may some
differences. I could spend some more time on it but I need some better
starting point then they've given for their determination.

Yes. I gave this paper as something that was easily accessible and
showed quite clearly the basics. You have brought up some good points
- details count.

It could be
they have the right figure, it just doesn't track with me.

Certainly.

Someone else
here probably knows.

That'd be refreshing.

Since 1.23eV is what I remember as being the
energy required for splitting a water molecule 1.23V seems a better
number to use. I'll keep working on it.

As you wish.

Now, as to your need to see support of my PEM full cycle PEAK
EFFICIENCY numbers I guess I'd refer you back to the basics. You're a
knowledgeable person - you know that the efficiencies you bandy about
are measured at PEAK POWER DENSITY.

I don't actually. The references I've seen appear to be referring to
low load cases for efficiencies in PEM of as high as 60% since they
refer to it dropping under load.

hmm..

But as I said they are kind of fuzzy
so I wouldn't use them as a reference.

Right. That's my problem. It would be an arcane subject at some
graduate school somewhere - its not something that's talked about
generally since its a pretty straightforward result. But I agree,
that people should be made aware of how things operate.

It's interesting really, I just found a reference. I've done a little
more searching and found this reference.

http://books.nap.edu/openbook.php?isbn=0309091632&page=222

referring to future(1) possible efficiency of 75%, Now that's using LHV
for hydrogen so the HHV equivalent would be around 88%. The LHV would
be a better value to use for round trip efficiencies though, They
mentiones actual achieved efficiencies of around 64% (LHV).

The problem here is that again details count. I've installed a
Ballard fuel cell and captued both the hydrogen AND oxygen at high
pressure and reuse it. I use the fuel cell to capture energy and get
that energy back at night when the sun isn't shiining. Here the HHV
makes sense since I'm capturing both O2 and H2 - and water - in a
closed system - using a fuel cell like a battery.

The same reference notes that efficiences drop (from a higher unnoted
value) to 75% at 1000A/Sq ft from what appears to be about 97% at zero
load. This doesn't include the neceassary overhead to actually run the
cell which they don't estimate separately. Note that that 97%
efficiency pretty well has to be at HHV so that gives an 82.5%
efficiency at LHV figure that would normally be used when measuring fuel
cell efficiency. In order to achive the round trip efficiency of 80%
that you mentioned the fuel cell would need an efficiency of 97%.

No to have 81% round trip efficiency it would only need to have a one
way efficiency of 90% each way. 0.9 x 0.9 = 0.81

Even
at that with the figure being a no load figure it's not of anything
other than academic interest as a limiting condition.

Between peak load and no load there are an infinite number of useful
loads. The real question is capital cost. And once you get the
basics right, you can focus your attention at reducing those.

Sure, my $50,000 Ballard fuel cell equipped with hydrogen and oxygen
storage (in an underground tube at high pressure) may store no more
than $5,000 worth of car batteries -haha - but its damned efficient
otherwise!! haha,, and once you get the basics right, your mind is
clear to focus on reducing THOSE COSTS - so that you achieve what
you've just achieved at a very high cost, but at a far far lower cost
with proper focus on technical improvements.

The same reference notes that alkaline cells are more efficient at
electrolysis than PEM cells, but notes that purification and handling is
more difficult.

Solid KOH is a bitch to handle. Its also known as drain cleaner.

Now if you can provide similar backing for fuel cell efficiency you will
have managed to back your claim, if not as broadly as your original
context made it appear(2).

As with anything there are caveats. But I stand by my claim. Full
cycle efficiencies are achievable - yes, you must capture the oxygen
as well as the hydrogen, yes, you must operate at low loads - but, you
can achieve 80% full cycle efficiency in energy storage. Now, as to
cost, and load, well,that's an issue of reducing the cost of membrane
area - and that's where I've focused my attention.

Robert

(1) It's dated 2004 so it's a few years out of date but not hugely so.
(2) Your original post appeared to indicate this was a practical round
trip efficiency to consider. Subsequent posts re-enforced that this is
an ideal no-load case with none of the necessary support.

You are being overly critical of the low load case. Between 97% no
load and 60% peak load there is 90% some load - haha - and 0.9 x 0.9
is 0.81 - 81% - which is what I quoted.

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