Re: High-frequency electrolyzers

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. Fuel cells are NOT limited by the same
thermodynamic relations that a heat engine is.

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! .

Plainly operating a fuel cell under the appropriate load to produce
high efficiencies increases efficiencies to over 90%. So, 90% or more
is possible under ideal conditions.

Now, one immediately comes up against the idea that hey, you can get
high thermal efficiencies but the capital efficiency sucks! You
drop the current density to 0.1 A/cm2 and the cost per watt increases
five times from when you run at 0.6 A//cm2!

So, you've got superlative efficiencies, but who can afford it?

Now I ask you, what if there were a membrane that was 1/2% the cost of
Nafinol available?

That would change things wouldn't it!

You'd get your cost per watt WAY down, and your optimal load would
tend to fbe less than peak power density and avor something in the 80%
range, and if energy use were cricital you could even crank it up to
90% in those cases.

So, efficiencies can be 90% - TODAY - if we wanted them to be by
dropping overvoltages to 1/6th their values at 60% efficiency which
cuts power density accordingly.
..
Reducing overvoltages by a factor of 6 cause fuel cell efficiencies to
increase from 60% range to 90% range on the power production side.
And efficiencies in electrolysis are 90% already.

Now reduce the cost of membranes by a factor of 200 - and you can make
them big enough to get energy density down to the point where you're
90% efficient each step and over 80% efficiency overall.


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?

I've seen fuzzy
references to 60% for a bare stack,

Its difficult to finjd papers that discuss what is common knowledge.
ANY stack that's reasonably efficient at peak load will be supremely
efficient at low loads. This is common knowledge.

but that becomes considerably less
when balance of plant etc... is taken into account.

True - details count. And the most important detail for high fuel
cell efficiency is current density across the membrane and the
overvoltage needed to drive it.

Very low current densities mean very low overvoltages and very high
efficiencies. Again this is plainly common knowledge, so what is your
point?

Anyone who knows anything also knows that ANY of the fuel cells that
operate at peak power density - with efficiencies of 60% or so, also
operate at 90% efficiency at power densities that are 1/6th as great.

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

What sort of pointer are you looking for? This is common knowledge.
Increase overvoltage to increase current, and you increase power
density across the membrane - and reduce efficiency. Lower
overvoltage, the current drops, power density drops, and efficiencies
go to 90%. What trouble do you have with this?
..
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. 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. So, why are you being obtuse about
it?

I'm not actively searching for a cell
at the moment so I'm not inclined to do the excercise for many
manufacturers out there, even the few you can get data sheets from. If,
however, there was an 80% cell I would be interested in it.

Any cell that operates at 60% efficiency at peak power density easily
achieves 90% efficiency in electrolysis mode and 90% efficiency in
fuel cell mode AT LOW POWER DENSITIES WITH THE ASSOCIATED LOW
OVERVOLTAGES.

it's not > 80% efficient

Yes it is. Its over 80% efficient electrolyzer and it talks about why

They claim 80%, you need quite a bit above that to meet your round trip
claim.

Yes you do - you need 90% each way to get to 81% round trip

Maybe even higher than that, I thing mentioned in my wanderings is that
convetionally Fuel cells use the lower heating value when calculating
efficiency and electrolyzers use the higher heating value. If that
holds ther's an additional nearly 15% to account for leading to a need
for on the order of 97% efficiency for each.

Then there's Gibbs free energy and chemical energies. haha.. Lots of
details count. But generally speaking operating at low overvoltages
increase efficiency, so those efficiencies that you quote - are for
peak power density - they get the most power at the least cost and
least weight - are dramatically improved in the same cells at lower
power densities. To over 90% - which is my point.

the shape of the power is important - it quite specifically talks
about actually building stuff and explains things in gory detail -
specifically answering the original poster's questions and supporting
nearly everything I said in response to it.

(and I don't trust the
figures they do give).

Why is that exactly? They go into detail relating the volume of gas
at STP to precise measurement of power they give. They lay everything
out in a lot of detail. What details did they get wrong?

They haven't done any measurements of how much hydrogen they actually
have. As opposed to say water vapour. I'd expect a fair amount of the
latter given the description. The also don't measure voltage drop to
see where it's occuring. They have made no attempt to determine leakage
current.

These are good points. Any idea how much these are likely to change
their efficiency estimates? PLUS or minus 2% perhaps?

At a guess 50% wouldn't surprise me.

Why guess when you can look at actual expeience? Fact is you're
being obtuse about my comments. Why? I don't know. You are clearly
smart enough to know that fuel cells can easily operate at 90% and
knowledgeable enough to know why it isn't reported in the literature,
and aware enough to know that the 60% efficiencies quoted in the
literature are under very specific load conditions. PEAK EFFICIENCIES
90% for these very same cells that you quote at efficiencies for at
PEAK POWER DENSITY.

Why are you trying to confuse everyone?

Finally I believe they've used the wrong figures for
determining efficiency from voltage drop even if they did have proper
figures to start with.

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?

Giving a link to a site promoting > 100%
efficient electrolysis is a VERY bad start.

First off, I didn't say anything about the site, I referenced the
paper which was quite detailed. Please show me where anyone said
anything about >100% efficiency. They didn't. They spoke of APPARENT

http://waterfuelcell.org/Peoples%20Projects.html

This isn't the paper I cited is it? haha.. NO!

It's the site you cited.

I looked at the paper not the cite.

Any post on a perpetual motion site will be
heavily discounted. Period.

The paper I cited DOES NOT claim perptual motion and DOES NOT claim
greater than 100% efficiency. It goes to great lengths to point out
efficiencies of 80% ore very high.


but you should know better.

About what precisely? These vauge dismissive comments with no
referent.

That references are judged partially on the company they keep.

I merely looked at a paper that came up when I did a google search -
because you wanted a paper. This one I thought covered the basics.
Apparently rather than read the paper for its content, you went and
did something I didn't do. Examine the web site it was found on until
you found a reason to discount it - however lame.

One has to wonder what is motivating you in being so obtuse.

I
wouldn't expect references to papers on flight that lead to a UFO site

haha.. Are you claiming this paper made claims about the paranomral
and UFOs? haha.. What a crock! What are you going to say next
about this paper? That it worships satan? lol. You're a trip Robert
you know that? haha..

to be taken highly seriously either.

We're talking electrolyzers in this paper. Earlier you were talking
about PEM fuel cells and the full cycle efficiency.

So, lets not get confused.

Any person knowledgeable in the art would look AT THIS PAPER and see
that they cover the basics of electrolysis construction. And that is
all you need to show that very simple systems can be very efficient at
creating gases. As you say there are details in heating value
calculation that can affect teh result. There are well known to any
undergrad chemistry student, and need not concern us here, to build a
case against this paper on that basis is ludicrous.

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. And you also know that by backing
off the power density you can get efficiencies exceeding 90% at each
step - giving greater than 80% efficiency full cycle

So what's your beef really?

You have none - but you act like you do.

Get a grip - and accept the fact. Fuel cells can store and retrieve
energy with 80% efficiency if operated at power densities that reduce
overvoltages to a minimum.


Robert

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