New method turns ocean water into drinking water, without waste
rochester.edu252 points by speckx 11 hours ago
252 points by speckx 11 hours ago
There is a fundamental minimum amount of energy needed to desalinate: you can't take less energy to do it,than you could gain back (from osmotic pressure) if you allowed the desalinated water to expand a cylinder containing the residual brine. This is large. This paper is a thermal method, so it doesn't have an electricity input, but to justify their efficiency claim, they should really compare against what you could do by using the same surface area for solar panels, driving a conventional setup. My (limited) understanding is that conventional reverse osmosis is not far from the theoretical optimum, energy-wise, the main difficulties being operational (the membranes need declogging). And of course RO is more expensive than rain.
This paper is interesting, however, in directly producing crystalline salt, which is lower volume than brine and easier to dispose of, maybe even valuable.
Thermal methods require energy, it seems like this substrate is effective at maintaining its solar-thermal absorbing properties better than a material that will attract salts
> Testing their solar-thermal desalination technique using samples of water from the Pacific, Atlantic, and Indian Oceans, Guo and his team were able to make the surface self-cleaning. In other words, it extracted freshwater and directed the remaining salts to the passive region where they could be later collected without reducing the panel’s efficiency.
This is not "large" this is a moderate improvement. Albedo is likely only marginally affected, and the solar power input over area is the same.
Depending on this cost of this process it could very likely be a wash in terms of NPV
If this can be applied to mine effluent, you could replace the maybe with most certainly. Sulfuric acid effluent lakes leech all sorts of valuable metals out of the ground.
> My (limited) understanding is that conventional reverse osmosis is not far from the theoretical optimum, energy-wise, the main difficulties being operational (the membranes need declogging). And of course RO is more expensive than rain.
RO is about 2-4x the theoretical minimum, depending on how much water you're willing to reject.
Brine is very easy to dispose of: you just pump it back to where it came from. Solid crystalline salt, on the other hand, is a hassle.
> Brine is very easy to dispose of: you just pump it back to where it came from.
Easy, but not necessarily good for the spot you're pumping concentrated salt back into.
If you use fat pipes that go a decent distance from shore, diluting your brine with ocean water, you’ll have a negligible impact on the ocean. The problem is if you dump lots of brine in shallow waters. Old designs did have that flaw, but it’s not that difficult to design around this constraint now that we know about it.
IMO this is an issue where NIMBYs are using environmental concerns as a smokescreen to block new desal plants from ruining the vibe at their beachfront property. Rhymes with the opposition against offshore wind farms.
The brine came from the ocean. So just dilute it back to close to ambient salinity using municipal waste water that you are discharging anyway.
> The brine came from the ocean.
Sure, and enriched uranium comes from the ground, but that doesn't mean it's safe to dump it back in after the enrichment process!
> So just dilute it back to close to ambient salinity using municipal waste water…
Wouldn't it generally be easier to process that municipal waste water, as is already fairly common?
> Sure, and enriched uranium comes from the ground
Uranium can also come from the ocean water (there is, apparently, quite a lot of it in there, relatively speaking). Japan experimented with the technology in the nineties, but it really was much cheaper to just mine it from the ground, so they abandoned it.
Japan is also barred from doing own enrichment, being a non-nuclear state. Though, there nevertheless is a dormant set of requisite facilities.
The analogy would be if you "un-enrich" it. Then it's safe. Or at least no worse than when you took it out of the ground.
> The analogy would be if you "un-enrich" it.
But you're doing that with the same water you're trying to make in the first place!
You could just dilute it using fresh seawater, if you used enough and (maybe) spread it over a wider area. The amount of water people need for drinking is a relative drop in the ocean.
Brine doesn't necessarily behave the way you imagine.