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How high can you push water vapor from solarthermal desalination?

  1. Jul 3, 2014 #1
    Hi all,

    Yet another noob question from me...

    I been thinking of Solarthermal desalination plant lately but i'm surprised that no plan seems to take into account the possibility of letting the vapor rise to a much higher level, thus creating a potential energy source. Is there something obvious i am missing? Assuming that the ground is conveniently shaped and some thermally apropriate tubing, couldn't you allow this vapor to rise to a significant altitude?
    Thank you.
  2. jcsd
  3. Jul 3, 2014 #2

    Simon Bridge

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    Law of conservation of energy I suspect.
    Note PE is not, strictly speaking, an energy source.

    You could - but why would you want to?
    The process costs more energy than would be gained in gravitational potential energy.

    Instead we use the Sun to lift our water vapor to great heights, and it comes down again in rivers which we can use to turn a water-wheel, thus claiming some of the energy.
  4. Jul 3, 2014 #3
    I don't know what "PE" is...did i mention i'm a noob?

    I was talking about Solarthermal desalination so, i already have vapor,what other energy source do i require? the set up implies that the "tubing" itself is also being heated by the sun so there shouldn't be that much loss of steam...

    That may be true where you are but i can think of "a few" comunities that would very much like to see those rivers you speak of...That is the whole point to recreate this very phenomenon if only on a very modest scale.
  5. Jul 3, 2014 #4


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    It may be counter intuitive but “wet air rises”.
    The molecular weight of H2O=18, and is much less than N2=28 and O2=32
    The density of water vapour is therefore less than air at the same temperature and pressure.
    Water vapour will rise even without heat.

    P.S. PE is potential energy.
  6. Jul 3, 2014 #5
    Well, i was concerned if the "tubing" (i need a better word) is cold then the vapor will turn back to water and fall before it reaches the desired height thus the need for a material that either doesn't allow heat to escape or allows solar heat to compensate the loss in temperature...
  7. Jul 3, 2014 #6

    Simon Bridge

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    Piping wet air from place to place does result in condensation along the pipe - before working to solve the engineering problems this produces, first answer this: what do you hope to gain by this approach to getting water-vapour to a high place? Is this an art project? Or do you hope to condense the water and let it flow back down another pipe, to drive some sort of generator or water-wheel?

    The former is one thing, but the latter has the same problems as you earlier question about hydrogen:
  8. Jul 3, 2014 #7


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    The problem is one of energy density, I would think. You would need a very large area for evaporation, which is what nature does with evaporation, clouds, rain, etc. There are more efficient ways to extract energy from insolation.
  9. Jul 4, 2014 #8


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    The problem is that water vapour is buoyant, so it has greater potential energy at the bottom.

    @willdo. How high exactly were you considering raising the air saturated with water vapour. What benefits do you expect to gain by doing that?

    If we assume you want primarily to generate fresh water from saline, then you will need a “hothouse” and a source of energy to evaporate water from the saline. You then allow the warm saturated air to flow from the top of the hothouse.

    To condense water from that air you must cool the air and then remove the latent heat energy originally invested to vaporise the water. Relatively warm wet air rises, so you may benefit if you can condense the water at a greater altitude than the hothouse. If you arrange the saturated air to rise diagonally to a pipe that runs buried? up a cool hillside, then as the airflow cools and water condenses the liquid will trickle back down that pipe. Water would be extracted from a collection sump at the bottom of the pipe. That density / thermal system would continue to run so long as the temperature of the saturated air flow reaching the top of the chimney was higher than the local air temperature. That requirement is a limitation on the length of the pipe.

    The top of the cooling pipe would be an open chimney. If that chimney was heated, then the pipe might draw more efficiently and so the system would yield more water, but once heated, no additional water could be condensed from the air in the chimney as it is no longer saturated.

    Unfortunately, the exhaust air from the system after condensation will still contain water vapour. The efficiency of the system can be improved by releasing air at the lowest possible temperature and pressure. You do not benefit by adding heat as the air rises, that just requires you go higher before condensation. I expect you are better taking the fresh water wherever you can get it.

    Putting this all together, you get saline water flowing slowly through a hothouse, with a diagonal riser to take the warm air to a hill. A buried pipe then runs up the hill cooling and condensing water from the airflow, to be vented from a solar heated chimney. Liquid water trickles back down the same pipe for collection at the bottom.

    There would be insufficient flow to recover energy from the system because all available energy is used to drive the flow through the system. There is no benefit from this system other than the production of valuable drinking water.
  10. Jul 4, 2014 #9
    Originally, i was hoping to let the newly-fresh water obtained from Solarthermal desalination in the form of vapor rise naturally from sea level to a nearby cliff where it would be allowed to condense and then use this higher position to distribute this water to the surrounding area without any artificial pumping.
    Indeed, i was looking at an abandoned salt production site with 1.1ha (2.75 acres) of bassins
    I suspect you mean that this project is much more efficient:http://aboosh.wordpress.com/tag/water-desalination/ (this is not mine and i do not know the author) and it probably is but my point is to have a very simple process without engines or pumps and thus, minimal maintenance and no hard-to-get carburant.

    With this, i found out today that evaporation is not sufficient to turn seawater into drinkable water so the whole idea seems fruitless for now, althought i do wonder if it's good enough for agriculture...
  11. Jul 4, 2014 #10


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    My suggested system is entirely solar powered. It requires no motors, fuel or pumps.

    One problem with sea water is the presence of dissolved compounds such as DMS that we can smell in very small quantities. If you pre-heat and vent the water before capturing water vapour you may eliminate those compounds. Also consider putting the condensate through a charcoal filter.
  12. Jul 4, 2014 #11

    Simon Bridge

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    Seems to be replacing maintenance of pumps etc with maintenance of the moist-air pipes ... remembering what likes to grow in warm-damp spaces. Still, that appears technically possible ... nice save ;)
  13. Jul 5, 2014 #12
    1/In your suggested system, unless i am mistaken (a clear possibility) you collect the condensed water back at the level where you started and that goes against my aim because i would then need some form of energy to move it.
    2/DMS in particular shouldn't prove to be too much of an issue since it evaporates at 37°c, a "pre-heating" bassin sounds like a feasable solution.i'm now taking some informations as to the requirements for water to be "drinkable" strangely enough it seems i'd need to add 1%...sea water...shouldn't too hard to find...

    @Simon Bridge, Yep i didn't think of "what likes to grow in warm damp places" i'll look for a solution to that as well...
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