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Simple, solar-powered water desalination (2020)

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Re: Simple, solar-powered water desalination (2020)

#151

Post by shellfishgene »

dystroy wrote: post #149
throwaway894345 wrote: post #147

Earlier quoted context omitted.

Why not release the salt back to the sea? Presumably it would not increase the salt content of the ocean very much considering the vastness of the ocean and the sheer quantity of fresh water that is being added to it by melting glaciers, etc? What am I missing?

The salt you release isn't instantly diluted in the whole ocean and we're speaking, for a plant, of millions cubic meters of salt enriched water per day. The release area is just dying.

Can you provide a source for that? I dove near a desalination plant exit pipe in Saudi, the impact on the marine live did not seem to big in the area. But it was a small plant for a few thousand people and we did not do a proper assessment.

I don't doubt there is an effect, but am not convinced that the "release area just dying" is correct.

Re: Simple, solar-powered water desalination (2020)

#152

Post by shellfishgene »

kitd wrote: post #55
spockz wrote: post #36

Earlier quoted context omitted.

Yes. So it is not an issue for the device. However, the salt does end up back in the ocean where at scale the salt levels will be higher leading to the (edit: local) environmental issues mentioned by the GP.

Is the water cycle not a thing any more?

A large part of the fresh water generated may be used for watering crops or gardens, and would thus evaporate in part. The evaporated water in desert areas would probably rain back down far way, and take long to mix with the sea near the plant again. For example, the Red Sea has much higher salinity that other oceans due to high evaporation and narrow connections to the other oceans. I doubt the effect is large though.

Re: Simple, solar-powered water desalination (2020)

#153

Post by kortex »

londons_explore wrote: post #137
kortex wrote: post #133

Earlier quoted context omitted.

I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue. Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) bu…

Fouling can be dramatically reduced by putting a small electrical current through the saltwater, freeing Chlorine ions which quickly kill biological things before they can adhere to your membrane. That combined with reverse flow flushing will probably last many years. And it can all be controlled by a 10 cent microcontroller, one pump, one valve, and a pressure sensor. I don't doubt that high pressure pumps are bad..…

Heh, this is reminiscent of the infamous HN dropbox comment.

Free chlorine (technically assorted chlorine oxides like hypochlorite) attacks RO membranes, so now you have to deactivate your reactive species first. Usually UV lamps, you use sun, but now you need UV-clear tubing, not easy.

Reverse flow flushing can be done with the components you mention, that's another $20/50 plus sourcing logistics.

Yes, the theoretical expense in quality pumps is quality. There are tighter tolerances, beefier components, better polymers, and more QA. It all adds up.

It's still all fairly cheap by Western standards, but it's a tall ask for a lot of places that barely have potable water.

Re: Simple, solar-powered water desalination (2020)

#154

Post by kortex »

msandford wrote: post #140
kortex wrote: post #133

Earlier quoted context omitted.

I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue. Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) bu…

Even these numbers are too small if you're going to RO seawater. I suspect what you have in upstate NY isn't access to seawater but access to some kind of less than amazing mostly-not-saltwater. The osmotic pressure you need to overcome to perform desal is directly proportional to the concentration. So 500-1000ppm water that you don't love is a lot less challenging than seawater which is about 3.5% or 35000ppm. You c…

Exactly, there's a pressure/flow/salinity tradeoff triangle. Higher salinity needs higher pressure and/or waste bypass. This was running around 100-150 psi, was a few years back, don't quite remember.

But a few hundred bucks will still get you from either "potable but not great" to fairly pure, or brackish to potable. If you have effectively infinite seawater and power, you can still pump at lower pressure and get pure permeate, it's just less efficient per unit pump and filter lifetime. If you had a super reliable pressure system (low friction ceramic pumps for example, not cheap but last forever) and cheap first-pass filters, you can run it for quite some time. But you're spot on in that you get to a point of engineering-give-a-mouse-a-cookie and it just makes sense to optimize the whole stack.

Re: Simple, solar-powered water desalination (2020)

#155

Post by adolph »

faceplanted wrote: post #127
kwhitefoot wrote: post #75

Earlier quoted context omitted.

Efficiency is still the wrong name. What you are describing is usually referred to a coefficient of performance , abbreviated to COP . See https://en.wikipedia.org/wiki/Coefficient_of_performance

The Deja vu sensation of this conversation happening almost identically 2 days ago (everything from wondering how >100% efficiency works, someone explaining it's from the environment, and then someone else explaining efficiency is inappropriate and COP exists) is kinda wild.

Nothing to worry about. Its people alpha-testing the Copilot internet commenting plugin.

Re: Simple, solar-powered water desalination (2020)

#156

Post by jkqwzsoo »

londons_explore wrote: post #137
kortex wrote: post #133

Earlier quoted context omitted.

I have first-hand experience that a) you need positive displacement pumps of a certain quality and power for RO and the cheap Ali ones self-destruct in minutes/hours b) you still need housing, plumbing, receptacles, etc c) fouling is a non-trivial issue. Expect to pay around $300 minimum for a small RO solar plant, and a couple bucks a month for upkeep. And logistics. Granted that's USA prices (upstate NY, not SV) bu…

Fouling can be dramatically reduced by putting a small electrical current through the saltwater, freeing Chlorine ions which quickly kill biological things before they can adhere to your membrane. That combined with reverse flow flushing will probably last many years. And it can all be controlled by a 10 cent microcontroller, one pump, one valve, and a pressure sensor. I don't doubt that high pressure pumps are bad..…

Commercial RO membranes (based on the poly(m-phenylenediamine trimesic acid), a.k.a. polyamide) chemistry have zero tolerance to free chlorine and will rapidly degrade under such conditions. The chlorine tolerance is listed as “nil” on many manufacturer’s spec sheets. Even using tap water on a polyamide membrane without some kind of prefilter (like GAC) will cause noticeable reduction in both the permeance and rejection of the membrane.

There was a big push towards developing chlorine-resistant chemistries a few years ago, but as far as I can tell, that has fallen into a “researchers don’t know what they’re doing, the plants are already designed around this problem” narrative. Of course, those plants are huge investments, and maybe it’s correct that one wouldn’t be able to take advantage of chlorine-resistant membranes until a new plant was built.

Cellulose triacetate RO membranes have chlorine tolerance, but have inferior chemical stability, productivity and selectivity to polyamide membranes, so it is sometimes used where chlorine tolerance is an issue. CTA membranes are also available in hollow fiber format, while polyamide membranes are essentially exclusively found in spiral wound configuration (some operators want fibers for higher fouling/solids feed). CTA is limited to a much smaller pH window (like 4-6 versus 2-12), and are not suitable for more aggressive cleaning methods, so I’m not sure if it overall provides a benefit versus polyamide RO with more aggressive cleaning cycles.

Re: Simple, solar-powered water desalination (2020)

#159

Post by civilized »

tonyhb wrote: post #143
betwixthewires wrote: post #138

Earlier quoted context omitted.

Maybe don't spread your Malthusian bullshit like a malignant cancer. Some of us like being human.

Hey, me too! Never said I didn't. Just that it's nice to leave some parts of the world to itself.

>> malignant cancer

> it's nice

come on dude, have some self-awareness

Re: Simple, solar-powered water desalination (2020)

#160

Post by hans_castorp »

ComodoHacker wrote: post #98
crackercrews wrote: post #3

For those curious what the 385% refers to > the team’s demonstration device can achieve an overall efficiency of 385 percent in converting the energy of sunlight into the energy of water evaporation. Honestly I still don't know what that means, or how efficiency can be over 100%.

It's an example of a good old scientifically flavored clickbait. The energy efficiency of anything cannot exceed 100% (until I missed something groundbreaking in physics).

How can it be "click bait" if neither the title nor the headline contains that number? You have to read at least the first three paragraphs of the article to stumble about that number. The focus of the article is on the inexpensive design, not on its efficiency
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