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treadwater
put seawater in drum, pull drum on long walk, obtain fresh water. | |
looking at those pull along water drums, I wondered if desalination was possible.
The mechanism would be two chambers separated by a osmotic membrane. One of the chambers would be a large threaded piston. This chamber would be weighted or sprung so as to wind the piston on the walking rotation of
the drum. Seawater is placed in this chamber, pure water collected in the other side. Appropriate gearing could be used but this idea would totally be down to whether the energy numbers stack up.
Oh, probably not for the boat, more for long walks on a desert island.
Mellon-baked
https://www.cmu.edu...-barrel-filter.html
[pertinax, Sep 05 2021]
RO Desalination
https://www.science...nce/reverse-osmosis
800 - 1000 psi [Frankx, Sep 05 2021]
Hand-powered water desalinator
https://dkutenx65dk...550277Datasheet.pdf
"Katadyn SURVIVOR 06" [Frankx, Sep 06 2021]
Yacht-sized desalinator
https://www.rainman...lectric-watermaker/
1250W, output of the order of 100 litres per hour. [Frankx, Sep 06 2021]
Reverse osmosis - apparently contravening laws of thermodynamics
Supersimple_20reverse_20osmosis
[bungston] May 07 2004 [Frankx, Sep 09 2021]
[link]
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//looking at those pull along water drums// |
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Could you provide a link so that we could look at them, too? |
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Good idea - I don’t know if it exists or not. Years
ago on “Dragons’ Den” there was someone
with a very similar concept to this. It was for
refugee camps and similar places where there is
often no source of clean water. Someone could
collect water from a dirty or polluted water source
and then drag the drum back to the camp and, by
the time they got back, the rotation of the drum
and the action of the filters etc. inside the drum
would have purified the water |
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So I looked for a link myself and, apart from ads, the first thing
to come up was [link]. |
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Hmm nice idea!
[hippo] - Hippo Roller - are you being unduly
modest? |
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If you're on a desert island you're much better off with a more traditional design that doesn't require the expenditure of calories. Or if you're spending calories pumping the water to and fro, one that doesn't also need you to move yourself to and fro. [+/-] |
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//not desalination//
True - I think the desalination aspect is enough to
label this as a new idea.
For reverse-osmosis desalination you need a fairly
high pressure ~ 800 psi [link], so you’ll need a
mechanically driven pump, driven by the rolling
motion. Not particularly difficult to derive. |
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Would need prototyping to find whether flow rates
were practical. It might need to be a *rather long*
walk. |
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As others have noted, the pressure require for desalination
is MASSIVE, so this will either take a long time & lots of
energy, or only do a small amount at a time. |
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[link] So, it's certainly feasible. That one provides only
0.89l/hr at 40 pumps per minute. It weighs just over a
kilo, but costs £1400. I'm guessing that means RO
membranes are expensive, although there's probably a
hefty mark up on low-volume yachting survival
equipment. |
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In terms of the roll-along RO desalinator - yes, feasible.
Manual-powered, you could play with mechanism design,
gearing etc to optimise output. |
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A yacht-sized 1200w unit looks to be able to produce
100l/hr [link]. |
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An adult of good fitness is likely to average between 50
and 150 watts for an hour of vigorous exercise. |
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Perhaps then 5l/hr as an output for "vigorous exercise". |
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Looking at economics, it's going to be challenging to come
up with a design that's adequately robust, simple in
construction, and affordable in the kinds of context
where lack of clean water is life-critical. Perhaps most
suitable in disaster relief efforts where [over £1000 per
unit] might be acceptable. |
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I seem to remember RO membranes are susceptible to
poisoning by some common water pollutants, so again,
usage context might dictate design changes. |
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Finally, it's only going to be useful in those areas where
seawater is near. So, as you say - desert island. |
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Your drum is also going to have to be rather heavy to hold
the pressure, and the gearing needed to generate that
pressure. |
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I think, in most places where sea water is readily available,
solar distillation is a better option. |
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If you happen to have a brine spring significantly uphill from
your settlement, than the self delivering version might
might have an application. |
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Filtration seems Baked, that's fantastic. Hope it has vital function function in the world and those whom worked on it get the credit. Sounds like it is just waiting for an osmotic membrane advancement. |
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The photo says concept but I would argue that having a thing, it has been prototyped. Well, unless the photo has been doctored. |
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A sea water drinking Stranndbeest? |
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Howabout another method? Would converting that
many calories to steam do the trick? |
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[a1] - //Even though the fresh water comes almost to the
top, you still have to use some energy to pump it up the
last part. Nothin' is free// |
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Actually, in [bungston]'s Supersimple Reverse Osmosis this
was discussed at great length, much calculation done,
and generally agreed that, given a sufficiently deep
ocean you could have an "energy free" freshwater
fountain.
Many didn't like this, because it looks like a perpetual
motion machine. It's not, but it looks like one.
And the depth of ocean required - I think 24km was
calculated, but others have said 11km. |
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I don't like the prejudice against perpetual motion. "Perpetual" must imply "infinite" to some idealists, or the word must be meaningless. An otherwise brillant system may break down when the earth's gravity or its oceans disappear, but so what? There really needs to be a naming system to describe degrees of perpetual. |
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I think it's normally accepted to mean a hypothetical
device which has an energy output but no energy input,
breaking the laws of thermodynamics. |
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In that sense a hypothetical "perpetual" machine could
run for an unlimited duration without energy input
(assuming a constant surrounding environment, ignoring
wear and tear etc) |
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Devices that are demonstrated claiming to be "perpetual
energy machines" might have, for instance, very low
friction bearings and high-energy flywheels; might have a
deliberately-hidden energy source (perhaps batteries); or
might be using a novel or poorly understood source of
energy - like the freshwater fountain. |
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//overlooking the energy required to pump the water out
of the desalinated side// |
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...that's the funny thing (which caused so much
disagreement). |
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If you had a sufficiently deep ocean (and it was
sufficiently mixed in terms of salt concentration), the
fresh water would actually 'spout' out of the desalinated
column without any pumping. |
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Yes, it could. The trick is the difference in density of
fresh and salt water at an equal temperature. If you run
a pipe 10 or so km deep, the difference in the weight of
the water column between the salt side and the fresh
side is sufficient to perform reverse osmosis and spout
the fresh out the top of your pipe. |
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In still water, this stops work rapidly, because the salinity
of the salt side increases until equilibrium is reached, and
the energy recovered in water flow is less than required
to sink the membrane. But in a free flowing situation
with active currents that both maintain salinity and flush
the membrane, it can be ongoing. |
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Of course there are some complicating factors with
pressure and salinity with depth, so the actual numbers
may be deeper than that 10km. Of course if you go deep
enough, it warms back up again. So put the Kola
borehole at the bottom of the Marianas trench, and
you've got a continual fountain. |
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Since RO membranes are weird, would an "overpressure" on
the salty side result in more pressure on the fresh side as
well, or would the membrane block up & only let the fresh
water get to a particular pressure & no more? |
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