[dalke]

Direct solar desalination as you suggest has a limit of about 4 sq. meters/liter/day. (Says http://en.wikipedia.org/wiki/Solar_desalination ). The L.A. Department of Water and Power delivers some "545,910 acre feet of water to 694,705 accounts" (http://sustainablecities.usc.edu/research/Chapter%203.%20LAD... ). That's 673 million cubic meters.

To replace that water with desal would require 7E9 sq meters or 7 379 sq. km or 2849 sq. miles, or about 4x the size of the L.A. metro area. Assuming I didn't drop a few powers of 10.

There isn't the space for that in L.A. There's also pumping costs to get sea water up and through the desal system.

Reverse osmosis seems to be the way that everyone is going. Including indirect RO through solar power. Power is a lot easier to move around than water.

[MichaelCrawford]

Thanks for the calculations.

I expect there are ways I can improve upon that though. Note that there are lots of inefficiencies in the method I described, such as the resulting fresh water being hot.

Perhaps with the use of a heat exchanger to recover the waste heat...

[dalke]

Yes, a solar still as you describe is highly inefficient. See http://en.wikipedia.org/wiki/Solar_still for a discussion. Among other points:

- The system is inefficient for how much work is put into it versus the water output

- it is advised to make a solar still to supplement another water source, such as a reverse-osmosis unit or water purification tablets

- Similar sea water stills are included in some life raft survival kits, though manual reverse osmosis desalinators have mostly replaced them

This is a well-studied problem. The question isn't if you can improve on the design in the Boy Scout Handbook, but if you can improve upon the current state of the art.

Quoting from http://www.researchgate.net/profile/Fawzi_Banat/publication/... (linked to from the Wikipedia page on 'Solar desalination'):

> In order to evaporate 1 kg of water at a temperature of 30°C about 2.4 × 106 J is required. Assuming an insolation of 250 W/m2, averaged over 24 h, this energy could evaporate a maximum of 9 L/m2/day. In practice heat losses will occur and the average daily yield which might be expected from a solar still is 4–5 L/m2/day. Today’s state-of-the-art single-effect solar stills have an efficiency of about 30–40% [25].

You mentioned a heat exchanger. That paper goes on to say:

> Multiple-effect basin stills have two or more compartments. The condensing surface of the lower compartment is the floor of the upper compartment. The heat given off by the condensing vapor provides energy to vaporize the feed water above. Multiple-effect solar desalination systems are more productive than single effect systems due to the reuse of latent heat of condensation. The increase in efficiency, though, must be balanced against the increase in capital and operating costs. Efficiency is therefore greater than for a single- basin still typically being 35% or more but the cost and complexity are correspondingly higher.

The simulated maximum for a proposed system for this seems to be 25 L/m2/day, which is 8x better than the numbers I used. I don't know if it's been tested.

However, that paper then describes indirect solar desalination were more effective. This includes a real-world desal plant running at 6–13 L/m2/day and a simulated maximum for a proposed system of 25L/m2/day.

Hence, indirect solar seems to be the way to go, and not a direct solar still like you propose. The solar collectors can be placed on cheap land, and brought to the desal plants by the ocean edge.

All of these information is available through a web search. There's little need to handwave.