
Remote Mexican village uses solar power to purify water - Oatseller
http://news.mit.edu/2015/mexican-village-solar-power-purify-water-1008
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Figs
Does using a solar panel to run a reverse osmosis machine work better than an
evaporation-based solar still? If so, why is it more efficient?

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adevine
Yes, it is much more efficient. It takes a lot more energy to transition water
from gas to liquid state (energy which is mostly wasted when the water vapor
is converted back to liquid) than is needed just to work against osmotic
pressure.

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Scoundreller
> energy which is mostly wasted when the water vapor is converted back to
> liquid

Please tell me the condensing coils are used to pre-heat the incoming water...

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laughinghan
Unless the condensing coils transferred heat efficiently enough to _boil_ the
incoming water, that would barely make a dent in efficiency.

> [water's] enthalpy of vaporization, 40.65 kJ/mol, is more than five times
> the energy required to heat the same quantity of water from 0 °C to 100 °C
> (cp = 75.3 J/(K mol))

[https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#Therm...](https://en.wikipedia.org/wiki/Enthalpy_of_vaporization#Thermodynamic_background)

~~~
pflanze
Efficient heat transfer just means using a big exchange surface versus a small
material thickness and/or good heat conductivity of the material. But the
problem you probably have in mind is that the vapor can't drive the boiling as
for the vapor to transfer its energy it has to be condensed, at which point
the question becomes why would the vapor condense and boil the water on the
other side versus simply boil itself. The answer here should be a pump, so as
to make the pressure on the condensation side higher than the pressure on the
boiling side. The pump will need energy to maintain the cycle, but it will be
(much) less than the energy needed to boil off water in the absence of the
pump and condenser. The question becomes open again, I think, why would this
be less efficient than reverse osmosis?

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laughinghan
Sufficient pressure is actually exactly the problem, but it shouldn't be
necessary to think about that. I'm just saying, intuitively, could you imagine
pumping steam through condensing coils and the water around it _boiling_ as a
result? Like say you're boiling some water on the stove and steam is rising
out of it. If you held a cup of water in that steam, how much steam would it
take to _boil_ that cup of water?

Keep in mind water vapor is lighter than dry air of the same temperature; it
takes up about 2000x as much volume as the same mass of liquid water. The vast
majority of the steam that goes into a practical condensing coil will come
right out the other side, only a tiny amount of the steam that's in very close
proximity to the surface of the coil will actually condense, which like, if
you imagine steam going into a curly metal pipe immersed in water, I'd hope
it's intuitive that tons of steam would make it out.

You'd need crazy long and thin pipes, crazy high pressure, and crazy pipe
configurations so there's way more steam surface area than water being heated,
in order to boil water with the equivalent amount of steam.

There's other tradeoffs here, of course. Both solar panels and reverse osmosis
machines are expensive, they're like fancy machined semiconductors and
ceramics and stuff, whereas an evaporator is some metal pipes and tanks and
maybe some glass or mirrors, so depending on application they may be much more
cost effective. But in terms of efficiency, there's no comparison.

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jacobolus
This is really fantastic. I hope they can scale out to more villages in Mexico
and around the world.

In such remote places, there are regular outbreaks of serious waterborne
diseases like cholera and dysentary, hepatitis A is widespread, and amoebas,
giardia, schistosoma, various parasitic worms, etc. are endemic.

The hard part is setting up the social/institutional structures to maintain
the equipment. The technological problems per se are relatively
straightforward at this point.

Rural peasants typically don’t have the technical knowledge, connections, or
capital to set up their own water treatment facilities, and often communities
are too remote and sparsely populated to be worth piping fresh water in from a
long distance.

I particularly appreciated how the MIT engineers trained local residents to
operate and maintain the system, and they set up enough payment structure for
the plant to pay for its ongoing costs. If that can be sustained indefinitely
without too much need for additional external help, then the project should
hopefully keep people healthy for decades, on only a small up-front
investment.

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IndianAstronaut
This might have a huge impact. A lack of clean water is terrible for health
and leads to a lot of deaths. It can also he a cause of cognitve decline.

[http://www.economist.com/node/16479286](http://www.economist.com/node/16479286)

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emailgregn
I'm genuinely interested to know if the altruistic/community driven ethos of
these efforts ever last? Once the NGO leaves town and the local federalè or
whoever notices a nice little income stream?

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Gys
Read the article:

'At this price, the community reaps a profit of about 49,000 pesos, or $3,600,
per year. The community has appointed a committee to manage the incoming
funds, setting aside some money for maintenance and repair of the system, and
investing the rest back into the community.'

~~~
HiLo
How would this work in areas where local cartels already have control over
these types of assets?

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tomcam
Is this possible because PV panels are more efficient now? Pump motors? In
other words, why wasn't this done 40 years ago?

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toomuchtodo
[http://cleantechnica.com/2014/09/04/solar-panel-cost-
trends-...](http://cleantechnica.com/2014/09/04/solar-panel-cost-
trends-10-charts/)

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doguozkan
I would like some more details on the deployment and maintenance costs of this
system. Maintenance costs play a huge role in long-term sustainability and if
the initial costs are too high, many people in need of clean water might not
get the system in the first place.

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Gys
Read the article:

'She adds that the residents in La Mancalona have taken ownership of the
technology, having been trained to operate it on a day-to-day basis, from
changing out ultraviolet lights and filters to testing the water quality and
replacing batteries. They also have a list of local suppliers for replacement
parts.'

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timonoko
Wind blows 24 hours, 3 times more water and more cheaply too:
[https://youtu.be/9xYXWISWv5I?t=6m28s](https://youtu.be/9xYXWISWv5I?t=6m28s)

~~~
jws
At this scale, two solar panels, so something like 1200 watt hours of energy
per day I don't think you'll find a wind generator and tower to beat the
price.

24 hour operation may not be requirement. Clean water stores just fine.

I was going to list a counter example of a place where wind would be a better
idea, but all the cloudy places I can think of have rain, so they probably get
water from that. It's a big world though, I'm sure lots of places have bad
enough sun that the number of panels would make the cost tip in favor of wind.

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froo
> At this scale, two solar panels, so something like 1200 watt hours of energy
> per day I don't think you'll find a wind generator and tower to beat the
> price.

There are many small scale wind generators that produce more than enough
energy to run watermakers on yachts, much more cheaply solar panels for the
energy output.

This has been a solved problem for quite a few years.

