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Justin Gillis writes in the NYT that as drought strikes California, residents "can't help but notice the substantial reservoir of untapped water lapping at their shores — 187 quintillion gallons of it, more or less, shimmering invitingly in the sun."
Once dismissed as too expensive and harmful to the environment, desalination is getting a second look. [...] A $1 billion desalination plant to supply booming San Diego County is under construction and due to open as early as November, providing a major test of whether California cities will be able to resort to the ocean to solve their water woes. [...] "It was not an easy decision to build this plant," says Mark Weston, chairman of the agency that supplies water to towns in San Diego County. "But it is turning out to be a spectacular choice. What we thought was on the expensive side 10 years ago is now affordable."
Carlsbad's product will sell for around $2,000 per acre-foot (the amount used by two five-person U.S. households per year), which is 80 percent more than what the county pays for treated water from outside the area. Water bills already average about $75 a month and the new plant will drive them up by $5 or so to secure a new supply equal to about 7 or 8 percent of the county's water consumption.
Critics say the plant will use a huge amount of electricity, increasing the carbon dioxide emissions that cause global warming, which further strains water supplies. And local environmental groups, which have fought the plant, fear a substantial impact on sea life. "There is just a lot more that can be done on both the conservation side and the water-recycling side before you get to [desalination]," says Rick Wilson, coastal management coordinator with the environmental group Surfrider Foundation. "We feel, in a lot of cases, that we haven't really explored all of those options."
(Score: 2) by kaszz on Monday April 13 2015, @01:56AM
So how would a plant that uses solar concentrators for the thermal energy to evaporate sea water, condensate it and sterilize work out economically?
Or even more advanced. Use solar concentrators to cook water such that it's pushed through an osmosis filter?
Both solutions without using electricity as a power carrier. Nor any nuclear dirty stuff.
(Score: 2) by c0lo on Monday April 13 2015, @03:17AM
Sorry, I like my water rare - blue rare if possible.
https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
(Score: 2) by subs on Monday April 13 2015, @11:43AM
I have simplified this topic quite a bit - the details are a lot more complicated - to just focus on the basic energy requirements. Google is your friend if you want the nitty-gritty details. But in short, it boils down to your electricity vs. land cost. If land is cheap, do an evaporation-type process. If electricity is cheap, do RO. As an added bonus, if fossil fuel is cheap, do distillation-type processes (much of the middle east does this).
Or even more advanced. Use solar concentrators to cook water such that it's pushed through an osmosis filter?
I'm not aware of any direct solar-driven process which can produce pressures that would be sufficient for RO *and* be more efficient than electrical pumps (steam engines, while certainly cool, are horribly inefficient).
As you can see [wikipedia.org], both evap-type processes and RO-type processes are fairly similar in energy requirements. My original point was, there's no point in taking RO and then multiplying its energy requirements by ~4-5x by linking it with a solar power plant - i.e. if solar-powered is your goal, just use the sun directly instead of going through an unnecessary, inefficient and expensive electrical conversion step.
Nor any nuclear dirty stuff.
"Dirty" is a very relative word and there are certainly ways to address that (although we have collectively decided not to do anything about it). The beauty about heat-engine power plants is that you can desalinate sorta "for free" just using waste heat from the plant without affecting electrical generation.
(Score: 2) by kaszz on Monday April 13 2015, @12:41PM
Put it simple. If you have one container with sea water connected to an osmosis filter which is connected to an empty container. And then heat the container with water. I think that the steam pressure will force its way to the empty container until pressure equilibrium is achieved.
By repeating this process one should be able to continuously desalinate.
However I have seen a type of desalination which uses a technique which is even better (less energy, less cost) than both evaporation and osmosis. But then I have to dig deeply.
(Score: 3, Insightful) by subs on Monday April 13 2015, @12:52PM
I think that the steam pressure will force its way
If you've already invested enough energy to evaporate water to steam, why not just capture & condense the steam to get distilled (i.e. non-salty) water, instead of then adding an additional lossy step for no reason? The steam *is* your product, salts do not evaporate at 100C.