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solar still

If I replaced the foliage in this diagram with a gallon of sea water - would the resulting "fresh water" be pure enough for agriculture? For human consumption?

Is the process really that prohibitively inefficient? (like 1 gallon of sea water yielding 1 shotglass of fresh water per week)?

What gives?

How can this not be the answer to solving California's drought?

I realize I must have gaping holes in my logic. Please help me identify them.

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    Salt farms are being built in Hawaii to not only harvest the salt, but to use the freshwater evaporated for drip irrigating trees and plants. It is basically a glass topped box with glass with container of saltwater. The fresh water runs down the sloped glass to a collection trough thence into a pipe for use. The salt that eventually appears is collected and sold in specialty grocery shops. – ozhank Aug 24 '16 at 3:18
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In effect, nature does this already. The sun heats up a patch of ocean, water evaporates and rises into the atmosphere. The vapour then condenses on airborne aerosols to form clouds which are then moved by winds and given the correct atmospheric conditions, rain falls.

To produce significant amounts of water from a solar still would require either an extremely large region of ocean be covered with a condensing medium (such as a plastic sheet) or the area of ocean be covered with an extremely large number of smaller stills.

The other significant problem with this is the distribution system required to get the water from the one large still or the large number of smaller stills to where it would be required.

Other problems would be: what effect all this would have on the ocean environment and wildlife? What happens if whales, sharks, seals, birds, or other wildlife damages the stills? How do you prevent the accumulation of barnacles on the solar stills? How can the still be protected against the forces of nature, such as: wave action, winds, hurricanes and solar damage?

Then there is the economic question who is going to pay for such a system and how much will it cost? Are there cheaper alternatives and ones that are less problematic and less risky? How often would the still need to be replaced because of damage, exposure or wear-and-tear and how much is that going to cost?

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A solar still CAN convert sea water to potable water. They are commonly included in lifeboat kits. Check the Watercone or this survival guide.

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I think this is one of the "gaping holes" you are looking for:

The amount of Total Dissolved Solids (TDS) in sea water is about 40,000ppm. Thus for every 1,000L of sea water distilled, you would have 40L of solids left behind after distillation. The amount of room left for water (i.e. the capacity of the 'hole') would thus decrease by 4% each time it was used. The 'hole' would be 50% full of solids after only 17 uses and 90% full after 56 uses.

Assuming you designed your system to go through one distillation cycle every day, your holes would no longer be viable — and would need to be abandoned — roughly every two months.

To rub salt into the wound, you would leave behind hypersaline pits that would kill virtually all nearby plant life. As you dug more and more pits along the countryside, you would progressively turn the environment into a barren wasteland, devoid of life, and subject to extreme erosion.

In conclusion, the illustrated design degrades quickly, scales poorly, and has grave, long-term environmental consequences. That's why a solar sea water still could be suitable for infrequent, individual use, but would be unsuitable for long-term provision of water to the public.

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  • Why not scoop the salt out and throw it back into ocean? Surely, if you distributed it in a sane manner, the salt would not have that of an impact. – Tomáš Zato - Reinstate Monica Nov 19 '19 at 14:03
  • There are about 40-million people in California. With per-capita consumption around 500L/day that means 800,000,000 litres of hypersaline waste per day. A tanker truck holds less than 40,000L. So in excess of 20,000 tanker truck trips would need to be made each day to get rid of the waste — one trip every 4.3 seconds. The transport logistics/obstacles are mind-boggling. That's why they build desalination plants next to the ocean and pump the salt right back — creating dead zones in the process. – Tim Nov 20 '19 at 11:33
  • Surely there are other sources of water than ocean. When I said scoop out, I was directly referring to the point that the "holes" would get full. Surely there are more efficient ways, just as you said. – Tomáš Zato - Reinstate Monica Nov 20 '19 at 11:45
  • The OP questioned why a specific design (the pictured solar still loaded with sea water) wasn't a viable solution. They did not ask how the pictured design could be improved to make it more viable. Yes, changes can be made — new/different designs can be developed — that are better... but the OP didn't ask about such things. – Tim Nov 21 '19 at 12:39

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