Freshwater from salt water using only solar energy

http://news.rice.edu/2017/06/19/freshwater-from-salt-water-using-only-solar-energy-2/

by diggernet • 9 years ago

219 104 9 years ago

news.rice.edu

Desalinization is becoming better understood, as well as cheaper, but the biggest challenge is still what to do with the leftover salt. We can't pour it back into the ocean because it will kill everything. Hopefully someday we find a way to clean and safe way to dispose of salt.

ianai• 9 years ago

An abundance of salt hardly sounds like an environmental issue. By that I mean it's not a gas that's going off into the atmosphere nor a liquid seeping anywhere (on its own)

smhg• 9 years ago

Even assuming you have to store it like nuclear waste, the ratio of about 3.5% doesn't sound too bad to the untrained ear.

noonespecial• 9 years ago

Britain is using a huge old salt mine in Cheshire to store toxic waste right now. We could put all the salt... umm.. into old salt mines.

Now that's a job for you to tell your kid's school class about on career day. A salt un-miner.

andai• 9 years ago

Reminds me of XKCD 1119, "Undoing" https://xkcd.com/1119/

ori_b• 9 years ago

We can't pour it back into the ocean because it will kill everything.

That's simply false. The amount of water that would need to be permanently removed from the ocean in order to measurably increase the salinity would be astounding. Let alone the amount that would need to be permanently removed to kill anything.

If local salinity was a problem (which it isn't) there's an extremely easy solution: Simply pump more water, remove less salt from it. Dump it in over a wider area of shore. Let mixing take care of the rest.

rorykoehler• 9 years ago

Given our track record I'd rather we didn't try find out.

bluGill2• 9 years ago

Most of the water we desalinate gets dumped right back in the ocean, add the salt back. Even the water that doesn't get into the sewage system ends up back in the ocean again via the water cycle/rainfall. (meaning we need to worry about local salinity issues but not global)

Actually most sewage systems output water that is safe to drink so we can put that water right back into our drinking water system and forget the whole problem over an over again. (If you ban lawns there might even be water left over as food is turned into water and the food probably isn't grown with city water). People generally don't like the idea of drinking sewage though so this will never happen.

adrianmonk2• 9 years ago

Load salt on the huge cargo ships already crossing the oceans. Build machines that dump salt overboard slowly and continuously as the ships make their voyages.

The ships are probably full one direction (when traveling from countries that manufacture a lot) but relatively empty on the way back, so there's probably free space.

dotancohen• 9 years ago

This idea assumes that the product consumer is the side which needs desalination. Unfortunately, in practice usually the opposite is true.

dubyah• 9 years ago

It's not really a significant problem. Blending it with heated cooling water from power plants or treated effluent from waste water plants are common means of brine disposal. Provided you spread out the disposal in areas with good flow, even releasing the brine directly would have minimal impact.

toomanybeersies• 9 years ago

Why can't you just pour it back into the ocean?

That's what's happening to the fresh water after it's used, it goes back to the ocean. It's a closed cycle.

The scale of human water consumption to the volume of the sea is miniscule.

Global fresh water consumption is a fraction of a percent of the volume of the ocean ( https://www.wolframalpha.com/input/?i=((total+water+consumpt... )

There's no reason why we couldn't ship it into the middle of a desert if for some reason it couldn't go back into the sea, it wouldn't go very far once it was dumped. Conveniently, the places that rely/will rely on desalination the most happen to be desert countries.

coenhyde• 9 years ago

I'd imagine the problem is the concentration of salt where the dumping occurs. There would need to be a system in place to spread the salt out over a large geographical area at concentrations that is not harmful.

jccooper• 9 years ago

It's not a global problem but a local one. Locally increased salinity and temperature can be a problem. You can drop a whole bunch of hot brine in the middle of the ocean to little effect, but the same is not true of a mangrove swamp or a bay.

_ph_2• 9 years ago

Most desalinization processes don't produce "salt", but extract a certain amount of unsalted water from the in put, leaving behind more salty water. Typically reverse osmosis at best reaches 1:1, that means you need 2 units of input water and get one unit of desalinated water and one unit of salty water. For sea water desalination 1:1 is quite optimistic though. This salty water can be safely put back into the sea. With very large scale plants you might want to make sure that you don't release the salty water at a single very concentrated spot.

bjelkeman-again• 9 years ago

In the Persian Gulf there is real concern regarding the volume of brine that the desalination produces.

https://www.sciencetarget.com/Journal/index.php/IJES/article...

yardie• 9 years ago

I don't know any reverse osmosis makers that are anywhere near 1:1. Current RO systems are 9.5:1. Over 90% of the saltwater is rejected as brine.

Cthulhu_• 9 years ago

Are you saying salt water kills salt water fish?

loverofcode2• 9 years ago

of course. kind of like how oxygen can kill us.

Overtonwindow2• 9 years ago

Hey don't blame me, blame Science. Salt is a huge problem in desalinization. We can't just dump it right back into the ocean, especially if this is to be a long-term solution. https://www.scientificamerican.com/article/the-impacts-of-re...

loverofcode2• 9 years ago

Based on the article disposing of the double salty water "willy nilly" is the problem. We should be able to pipe a system that delivers it over a wide area. Nice info though!

gruturo• 9 years ago

What's wrong with dumping it back in the ocean? The desalinated water is also going back there eventually, so the equilibrium is unaffected

chrisweekly• 9 years ago

I wonder if there's a good use to which the highly-salinated solution (the "leftover salt") could be put. It's a stretch, but I vaguely recall something about storing excess solar energy thermally, in dense piles of molten salt...

jghn• 9 years ago

Part of me wonders if the water could be used for fracking as a double down on energy generation, but the other part of me says that if that were possible that people far smarter and richer than I would have already been on top of this.

logfromblammo5• 9 years ago

In general, perform reverse osmosis (RO) until the effluent brine is up to 70 g/kg salinity. Then pump it out onto a salt pan, let the rest of the water evaporate, and sell the evaporite.

Petrochem tech comes into play here, because in steam extraction of oil-sands, wastewater comes back up contaminated with silicates, and has to be treated before it can be reused. It turns out that similar processes can be used to further concentrate desalinator effluent above the 70 g/kg that regular RO tops out at, to about 130 g/kg. From there, any solar/thermal process equipment (i.e. flash distiller) can be made much smaller.

For reference, seawater is typically 35 g/kg, and the top stratum of the Dead Sea averages 315 g/kg (with significant fluctuation due to local weather history).

So Jordan and Israel can actually do RO on water from the Red Sea (40 g/kg), and pump the effluent to the Dead Sea. They don't have to worry about the hypersaline brine killing anything, because the Dead Sea is already dead (just like it says on the tin). And the pumping is easy, because the Dead Sea is below ocean level. A pair of siphoning aqueduct pipelines (40 g/kg and 70 g/kg) can supply a RO desalinator in every town from Aqaba to Potash City.

rdlecler1• 9 years ago

Solar desalination is not new. KKR invested $100b in SunDrop farms which uses solar desal that produces energy (steam) and water for greenhouses in the desert.

ogrisel• 9 years ago

Here is the link to sundrop farms for those not familiar with the concept: http://www.sundropfarms.com/facilities/

sandl• 9 years ago

*$100mm

Source: https://agfundernews.com/how-private-equity-giant-kkr-and-su...

smoyer2• 9 years ago

Solar desalination is not new ... I performed the same feat in the '70s during Boy Scout camp-outs in Virginia Beach (Fort Story) [0]. What is new is the level of efficiency!

[0] https://en.wikipedia.org/wiki/Solar_still

logfromblammo5• 9 years ago

IMHO, scout troops should teach the kids how to acquire fresh water before how to make fire, but for some odd reason the kids find fire to be more interesting.

Who wants to worry about wells and sump holes and and filters and bleach tabs when you can burn things?

dredmorbius• 9 years ago

The general problem with membrane-based desal methods is membrane fouling and lifetime.

I'm not sure that increasing the complexity of the membrane substrate itself is a positive step here, or that a complex heating mechanism offers significant wins.

The alternative of more traditional membrane reverse-osmosis processing focusing on cheap substrates, whilst provisioning power separately (conventional solar PV would be suitable, and could be located on-site or remotely) seems rather more tractable.

msl09• 9 years ago

Yeah. After the initial excitement my first thought was if using nanotechnology could make the amortized cost of a panel prohibitively expensive.

bencollier49• 9 years ago

6 litres / metre-squared per hour.

We require 130 litres / person per day in Europe. A town of 10,000 people would require 1,300,000 litres / day = ~54,000 litres / hour = 9000 metres-squared.

A facility of 100 x 100 x 1 m seems feasible. Based on those calculations, this all seems quite practical. I do wonder how frequently the filters need to be changed, though.

Kayou• 9 years ago

I thought 130 litres/person was quiet high and probably included things like water to grow vegetables, but no, one shower is between 30 et 50 litres and each time you go to the bathroom, it's 10 litres of drinkable water.

Of course, I you have saltwater and no freshwater, you would probably use saltwater in your toilets, so that could reduce the daily usage of at least 20 litres. But then, if you consider agriculture and manufacturing, I guess that the number of 130litres/person would blow up completely.

positivecomment• 9 years ago

Maybe people can also shower with saltwater followed by a quick rinse in drinkable water. That would reduce the usage drastically.

I couldn't find anything against showering with saltwater by skimming the results of a naive google search.

stult• 9 years ago

Wouldn't salt water in toilets require two sets of pipes leading into each home? Seems like an expensive proposition unless you're building a new community from scratch.

thatwebdude4• 9 years ago

We (as a society) really need to be pushing more efficient water appliances.

I looked up liters to gallons since I'm backwards and - in case anyone else is too - 10 liters is ~2.5 gallons. I replaced three toilest in my home that were flushing ~4 gallons with each flush with ones that flush with 1.28 gallons now. They are much more powerful than the previous ones I had, and use (liter-wise) less than half of the 10-liter mark.

Do your efficient toilets overseas not get down to the 4-6 liter range per flush?

The switch to more efficient toilets really made a big impact on my water bill for barely a dent to the wallet.

e12e• 9 years ago

If you have plentiful access to salt water, it might be worth it to use that for "black water" (sewage).

tolien• 9 years ago

Most of that 1.3M litres is presumably going to be within a 12 hour period, surely? Whether it’s for human use (drinking, washing, flushing) or agriculture.

I guess you can produce constantly and store - a quick search suggests that 1.3M litres would barely be known to a reservoir but you wouldn’t have to worry about the seasonality of rainfall.

Yeah, seems feasible. Almost too good to be true…

smoyer2• 9 years ago

We were each producing drinking water ... "We require" is probably a bit strong - LiveStrong says the average male needs 13 cups of water each day. If you add in the amount of water we use based on our life-style, then I can believe 130l.

peteretep• 9 years ago

    > We require 130 litres / person
    > per day in Europe

We don't require 130 litres of potable water though, which is what this produces. Salt water's fine for washing, brushing teeth, flushing toilets, washing hands, and so on.

torrent-of-ions• 9 years ago

What does this 130 litres include? Food? Meat? Clothes?

LeifCarrotson• 9 years ago

It's 6 liters per square meter of solar membrane. They increased it 25x with a Fresnel lens. From the paper:

For NEMD experiments with solar concentration, a 25.4 × 25.4 cm Fresnel lens was used to concentrate sunlight on the membrane surface by a factor of 25. The unconcentrated and concentrated solar intensities at the NESMD module surface were 0.7 and 17.5 kW·m−2, respectively.

A facility of 500x500 m^2 would probably still be feasible.

The link is to the university news department, the actual paper is here:

http://www.pnas.org/content/early/2017/06/14/1701835114.full

or here in PDF format:

http://www.pnas.org/content/early/2017/06/14/1701835114.full...

Retric• 9 years ago

You can only concentrate sunlight for a fraction of the time. The good news is water is easy to store for long periods, the bad news is utilization is very low. Sierra Suntower https://en.wikipedia.org/wiki/Sierra_SunTower is kind of a worst case senario but they got about 100 hours per year of operation.

ccalvert• 9 years ago

If this desalination system allowed a city to generate even 50% of their water needs, that would be a significant step forward. I keep seeing this metric put forward: Does X generate 100% of the needed water/electricity/heat. Why does it have to 100%? Can't three systems each contributing 33.3% be enough to reach 100%?

Our household solar power installation here in the very rainy Seattle area generates 98.15% of our electric power needs, including that for our electric car. I wish it were 100%, and if the sun shines for a few days in a row, sometimes it does generate 100% of our electric power needs for the last 365 days. It is, however, much better to meet 98% of our needs that to meet none of our needs.

ivankirigin2• 9 years ago

To hit that rate you need energy storage at your house, right?

In the context of grid power, that's an even bigger issue given cost and pollution from base load generators.

kayfox• 9 years ago

Or three systems capable of producing more than 50%.

Because infrastructure should have redundancy.

ivankirigin2• 9 years ago

Presumably you'd need to generate 1.3M liters off of only daylight hours, making it more like 130m*130m per 10K people.

Cairo has a population density of 18K people per km2, so this would mean covering 2% of the area. For contrast, streets and parking take up ~40% of most US cities.

ajarmst2• 9 years ago

Dear God, do you realize what we've done? We've invented evaporation! (Sorry, the title struck me as funny.)

Qwertious• 9 years ago

Not really - evaporation is used to trap the residue in the water, and dispose of the water itself.

This is the reverse - dispose of the residue, keep the water.

ajarmst2• 9 years ago

I appreciate your recognition of my plea for a refresher in middle-school science. Most people can't perceive my pain in the midst of the sarcasm.

SubiculumCode• 9 years ago

Frequently I hear how persons stranded at sea or on a small island die of thirst, and I wonder if they could have used the evaporation method in conjunction with the sun or fire, and a cloth set above to capture the evaporated water.

totallynotcool• 9 years ago

METTC dependant, a 'solar still' could work. With the right vegetation and a plastic bag you could even collect transpiration. Off topic, but interesting nonetheless.

[1] https://en.m.wikipedia.org/wiki/Solar_still

regularfry2• 9 years ago

Any particular reason you couldn't do a siphon-pumped multistage flash still with solar heating? That would be just glasswork.

malloryerik• 9 years ago

Dumb question: would it be possible to turn the desert green? I'm imagining something done little-by-little, through farming and maybe public trusts. If this kind of technology were to continue to fall in cost, could we have farms and forests in the Sahara?

kbart• 9 years ago

Water is not the only problem. You'd also need soil as not much can grow in a sand.

anotherbrownguy• 9 years ago

Let's not forget that solar cannot be used for anything critical where reliability is expected. It requires backup from a reliable source to be practical.

Freshwater from salt water using only solar energy

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