Clever Idea for Desalinator

[AJZimm]

This is one of those ideas that seems blindingly obvious once someone else has come up with it.

Uses the hydrostatic pressure of the deep ocean itself to drive a reverse osmosis desalinator. Given that the pressure at 300 metres down is 10 Bar, I see no reason why this wouldn't work. Great to have one of these in your emergency life raft kit for offshore passages.

https://www.quenchsea.world/pages/reel

[pez_leon]

Re: Clever Idea for Desalinator

At the risk of asking an ignorant question - where does the salt go? Shouldn't the filters clog very quickly without some method of reversing flow and/or cleaning them with fresh water?

- James

[AJZimm]

Re: Clever Idea for Desalinator

At the risk of asking an ignorant question - where does the salt go? Shouldn't the filters clog very quickly without some method of reversing flow and/or cleaning them with fresh water?

- James

It's a good question. They don't explain very well but my guess is that the process is not as efficient as it might be with a higher pressure pump and therefore the brine doesn't get as concentrated. The little video animation shows the brine being poured out and they suggest the unit ideally should be cleaned after every use. I see it as being suitable for emergency use.

[P.I. Stazzer-Newt]

Re: Clever Idea for Desalinator

Ten Bar is about one hundred metres of water column.

[AndanteEd]

Re: Clever Idea for Desalinator

I've got the Quenchsea hand-pumped version. I've not tried using it, but apparently it needs about 50 bar of pressure to work.

I'd guess the innards are the same, hence the new 'deep sea' one needing needing about 500m of water to do its stuff.

There's not much info on the one I have on Youtube. The most thorough test I've seen was by a bunch of Czech sailors:

(Spoiler alert...) The water it produced was salt-free, but there was not a lot of it and it seemed like quite a lot of effort to get there.

Sinking something to 500m and then hauling it back up does seem a bit easier...

[rregge]

Re: Clever Idea for Desalinator

Question: for a pump to work you need a higher pressure on one side than the other (10-50 bar depending on which post you look at)

Whether at the surface or 500 meter below the surface the pressure on both sides would be the same.

If your plan is to connect a pipe 500 meters below the surface and use that pressure to run the pump at the surface, I don't think it will work. How much energy/pressure/work would it take to get the water from 500 meters down to the surface? Hint: if everything is 100% efficient (which it never is) I think it might be 100% of the potential energy available for the difference in pressure.

I think your 500 meter pipe would fill up with water exactly to sea level.

Then you have to factor in differences in temperature and salinity (density)

[AJZimm]

Re: Clever Idea for Desalinator

Ten Bar is about one hundred metres of water column.

You're right. Not sure how I came up with 10 bar for that depth.

[AJZimm]

Re: Clever Idea for Desalinator

Question: for a pump to work you need a higher pressure on one side than the other (10-50 bar depending on which post you look at)

Whether at the surface or 500 meter below the surface the pressure on both sides would be the same.

If your plan is to connect a pipe 500 meters below the surface and use that pressure to run the pump at the surface, I don't think it will work. How much energy/pressure/work would it take to get the water from 500 meters down to the surface? Hint: if everything is 100% efficient (which it never is) I think it might be 100% of the potential energy available for the difference in pressure.

I think your 500 meter pipe would fill up with water exactly to sea level.

Then you have to factor in differences in temperature and salinity (density)

As far as I can tell, the device does not connect a pipe from depth to the surface. It appears to be a two-sided chamber divided by the membrane, which is lowered to the desired depth. Admit seawater to the brine side at, say, 300 metres and the pressure on that side will be 30 bar. The pressure on the other side, assuming it was sealed at the surface, will only be one bar. Eventually the two sides will equalize, but at that point the osmotic pressure across the membrane will have produced fresh water on the other side.
The process, as you say, will never be 100% efficient, which is why they don't promise completely pure water, I presume.

[rregge]

Re: Clever Idea for Desalinator

As far as I can tell, the device does not connect a pipe from depth to the surface. It appears to be a two-sided chamber divided by the membrane, which is lowered to the desired depth. Admit seawater to the brine side at, say, 300 metres and the pressure on that side will be 30 bar. The pressure on the other side, assuming it was sealed at the surface, will only be one bar. Eventually the two sides will equalize, but at that point the osmotic pressure across the membrane will have produced fresh water on the other side.
The process, as you say, will never be 100% efficient, which is why they don't promise completely pure water, I presume.

So for every stroke you have to bring the "low pressure" back to the surface, release the now high pressure from the "low pressure" side reseal it and send it back down to depth, repeat.

[Thad Van Gilder]

Re: Clever Idea for Desalinator

For emergency use, why not just use a military surplus solar still....

[AJZimm]

Re: Clever Idea for Desalinator

So for every stroke you have to bring the "low pressure" back to the surface, release the now high pressure from the "low pressure" side reseal it and send it back down to depth, repeat.

The "QuenchSea Reel" unit, to which the original link points, is not a pump, per se. It has no moving parts that I can see and requires no pumping by hand. The pump shown in post #5 is made by the same company but is not the unit we're talking about.

The "QuenchSea Reel" unit seems to rely solely on the natural hydrostatic pressure difference between the seawater at depth on the seawater side of the membrane and the trapped atmospheric pressure of the surface on the freshwater side. Lower the unit down to depth, let it sit there for a bit to do its thing and bring it back up.

[AJZimm]

Re: Clever Idea for Desalinator

For emergency use, why not just use a military surplus solar still....

Don't have a lot of experience with those. How well do they work in northern latitudes, the north Pacific, say, where you can go weeks without seeing the sun?

[rregge]

Re: Clever Idea for Desalinator

The "QuenchSea Reel" unit, to which the original link points, is not a pump, per se. It has no moving parts that I can see and requires no pumping by hand. The pump shown in post #5 is made by the same company but is not the unit we're talking about.

The "QuenchSea Reel" unit seems to rely solely on the natural hydrostatic pressure difference between the seawater at depth on the seawater side of the membrane and the trapped atmospheric pressure of the surface on the freshwater side. Lower the unit down to depth, let it sit there for a bit to do its thing and bring it back up.

It may not be a mechanical pump per say, however I don't think it will work.

An example if you put a pipe 500 meters down in the ocean, the level of the water in the pipe will be at sea level. It will not gush up at 10 or 50 bars.

This reminds me of someone in college who thought they could use electricity to separate water into hydrogen and oxygen and then burn the hydrogen and produce way more electricity to than was required to separate the hydrogen and oxygen and solve the energy crisis. He (a poly sci) major would not listen to an engineer who took a boat load of thermodynamics.

[rgthom]

Re: Clever Idea for Desalinator

Reading the "how does it work" section on the website, it looks like Alex' description is right. A chamber is sealed at atmospheric pressure on the surface, then you drop the thing to 500 m with a fishing reel. Wait 5 minutes for the pressure at that depth to reverse osmosize (yes I know) 1.5 l of water into the sealed chamber. Then reel it up and have a drink. It claims 1.5 l every 5 minutes, but that does NOT include dropping and reeling in time.

[pez_leon]

Re: Clever Idea for Desalinator

I'm curious to see if this works as advertised. I have been thinking about this thing and why it's hard for me to accept that the same 50 bar of pressure that work well when provided by a pump could function just as well when provided by the pressure of the deep. My main hangup is the question of carrying the rejected salt away from the membrane. Every time this thing makes 1.5 liters of fresh water it leaves 45 grams of salt somewhere else. I only have experience with RODI units meant to purify city freshwater supplies, but I know that those produce a lot of brine - something on the order of five parts down the drain to every one part of RODI water produced. When I visualize microscopic conditions near the ocean side of this thing's membrane, I imagine an already-salt enriched brine pushing into an already salt-covered membrane surface, with no current of new seawater to carry it away. But I know very little of these things and would be delighted to be wrong.

- James