
Engineers boost output of solar desalination system by 50% - emptybits
https://phys.org/news/2019-06-hot-efficiency-solar-desalination.html
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cconcepts
It surprises me that we haven't worked out a more efficient solar distillation
process for desalination. Reverse Osmosis seems so energy intensive at a
fundamental level when you consider that the sun naturally distils and
desalinates water all day long.

The process described in this article ("Membrane Distillation") seems like a
hybridisation of the two techniques and the researchers have found a way to
improve efficiency by concentrating heat using lenses. Why can't this
concentration of heat be used in straightforward distillation and why does
distillation always seem to come in second best in terms of efficiency
compared to the use of membranes? Is it because of the large surface area
required?

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mikekchar
Just to fill in details of the other responses. There are multiple concept
here: latent heat, specific heat and relative humidity. The latent heat of a
substance is the amount of energy required to transform it from one phase to
another (in this case from liquid to gas). It varies on the temperature, but
basically it's about 2250 - 2500 kJ/kg. However there is only so much water
you can evaporate at a particular temperature/pressure. At room temperature
it's a lot less than at boiling. If you want to evaporate a lot of liquid,
then you need to boil it. This requires you to heat it. The specific heat of
water (the amount of energy you need to raise 1 gram of water 1 degree C) is
4.2 kJ/kg. So if we need to raise the water to boiling and then boil it, we
will need about 2600 kJ/kg.

If the sun can provide 1 kW/m2, the in 1 hour we can produce a maximum of 1
kWh/m2. That's 3600 kJ. That means we can boil 1 kg (or 1 litre) of water per
hour per square meter of sunlight at maximum efficiency. So it's just not
feasible.

I welcome fixes to the inevitable errors in the above :-P

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Hermel
You will get the latent heat back when the water condenses again.

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dotancohen
Right, but you "get it back" on the condenser surface, not in the fluid that
needs to be boiled. At it is exactly the temperature gradient that you need to
maintain, so cooling the condenser surface with the to-be-boiled fluid will
work only to a point. There are systems that cool the condenser with the water
before sending the water to the heater, but the rate at which heat flows is
much slower than the rate at which the water must flow.

That's the problem with heat. It is not a source of energy. It is a store of
energy, often one that simply needs to be disposed of.

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jacknews
I believe this could be building on a previous project from 2012:
[https://phys.org/news/2012-11-major-advance-sunlight-
steam.h...](https://phys.org/news/2012-11-major-advance-sunlight-steam.html)

They state 24% effiency there, so the new result implies 36% overall
efficiency? I find articles which only state '50% improvement' without stating
the end result, frustrating.

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sebazzz
Typical journalism. Percentages are always abused.

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agumonkey
I completely understand, but still, a 50% increase is enough to be state as
is. Usually tech progresses by much less.

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jacknews
I agree a large advance is still newsworthy, but it's just laziness not to go
on to say what that means, ag "giving a really rather good 36% overall" or
"despite the still-low overall officiency, scientists are optimistic about
future progress" etc.

Then again how are they even measuring efficiency, is there a theoretical
minimum energy required to remove the salt from water?

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mrguyorama
Would an average reader have a good concept of what "36% efficient overall"
would mean here?

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HeavenBanned
Take that, overpriced desalination system in Sim City 3000!

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Tade0
_For example, LANP is developing a copper-based nanoparticle for converting
ammonia into hydrogen fuel at ambient pressure._

This is interesting as well. Ammonia is much easier to handle than hydrogen
and the infrastructure is already there.

On top of that liquid ammonia by volume contains more hydrogen than liquid
hydrogen.

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agumonkey
May I digress a little. Anyone familiar with micro and nano filtration ? I
wonder if there's progress there (cost, technique)

