
Academics build ultimate solar-powered water purifier - devinp
http://sciencebulletin.org/archives/9869.html
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philipkglass
This seems like a solid advance. There aren't any odd nanostructures or high-
maintenance components that I can see.

 _Why can 't we use these to solve drought in Australia/California/etc.?_

There are a couple of reasons.

One is that vapor based water purification is very energy-intensive compared
to reverse osmosis (where impure water is mechanically forced through membrane
pores small enough to exclude salts/viruses/bacteria). It takes at least 2258
kJ/liter to vaporize water. Reverse osmosis desalination of seawater currently
requires in the neighborhood of 10.8 kJ/L (3 kWh/m^3). There's a modest
factor-of-5 advantage for using solar thermal energy without first
transforming it to electricity that can drive pumps, but it's swamped by the
factor-of-200 higher energy requirements for evaporation. (Incidentally, this
is why even "cheap" nuclear heat sources can't beat electrically driven RO
plants when it comes to large scale desalination.)

The second reason simple thermal desalination units can't solve California's
problems is a consequence of the first reason: you just can't get enough water
from these units. They're fine for providing a bare minimum of clean drinking
water. They are nowhere near capable of providing enough fresh water for
everyone to water lawns, bathe, and wash things -- much less supply the real
800 pound Californian gorilla, agricultural irrigation.

 _If this system is so inefficient, why would anyone use it?_

The advantages are downward scalability and low infrastructure requirements.
There are a lot of parts of the world where there's enough water to irrigate
crops but it still might make people sick to drink it untreated. Solar
distillation can remove arsenic/bacteria/viruses from water and make it safe
for direct human consumption. Reverse osmosis units are advantageous only at
larger scale, they require more maintenance, and they need electricity. Solar
distillation can serve locations that lack reliable electricity and/or
maintenance workers. (Solar-PV electricity might be an option in the future,
but that would still be more complex and would require further design
iterations on RO; current units are designed to operate near continuously.)

