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Solar Still

  1. May 22, 2015 #1
    I now know that an insulator will not only keep heat in but keep heat from being conducted in the first place So my solar still is almost 100% metal including the 2 valves. There is however 1 plastic pipe between the rain collector and the evaporator that is flexible.

    My solar still has several parts. It has a rain collector which has a pump in it and a pipe that starts 6 inches from the bottom. The pipe goes 6 feet up and then arches and goes back down 5 feet. Water can naturally go uphill a few inches but not 6 feet. This pipe leads to an evaporator and it has 2 valves. One is a shutoff valve near the top of the arch. The second one is a backflow preventing valve close to the evaporator(Why it is here when backflow is more commonly going to happen when water goes uphill I don't know). The sun heats up the metal surface of the evaporator which then heats up the water. This kills any microbes and leaves any dust behind since it is heat over a long period of time. It then goes up 5 feet in another pipe as a gas and goes into the condenser. This supposedly would separate atmospheric gases from water via difference in melting points and boiling points but it doesn't really, even of the nonpolar gases like O2 and N2. So the water still ends up being acidic from CO2 that naturally occurs in rainwater and is what causes rainwater to be acidic. After the condenser it goes into a spigot and then into a 10 gallon container.

    The rain collector is 6 feet tall and has a 10 ft radius. So the rain collector can hold 314.16 cubic feet or 2350.08 gallons. The evaporator is a truncated cone with a smaller radius of 5 ft, a larger radius of 10 ft, and a depth of 1 ft. This is 183.29 cubic feet or 1371.10 gallons. So the shutoff valve is there for when the evaporator is full(which it almost never would, even after a thunderstorm. But how would I know when it is full? Would I know when the flexible pipe starts bulging out near the evaporator? And if I turn the shut off valve off but the pump is still running would the flexible pipe start leaking in the rain collector causing no water to flow through the pipe even though the pump is working and there is water?

    And what if it is colder in the air and on the ground than the IR light from the sun is? Would the metal conduct cold or in other words be an insulator(since insulators keep hot things hot and cold things cold) and thus expand into a more parabolic shape causing the change in radius per inch of water to not be constant and eventually over years and years break causing metal shards to spread out like how glass breaks into shards? Now the metal that is used for all metal parts of the solar still is stainless steel which despite it being stainless, can in fact oxidize or rust(I have seen a black oxide on a stainless steel spoon which is likely Iron(II) Oxide). It just takes much longer for it to oxidize than chrome plated or galvanized steel.

    And what about the pressures? I know there would be a pressure of 2 PSI(if it is enough to pump your blood 6 feet than it is definitely enough to pump water up 6 feet) at the water pump and I know that depending on the volume of the evaporator that is water that the pressure against the water flow will vary and that the air will be pushed out because of pressure. I also know that if P2(the pressure against the water flow) > 2 PSI that the water won't flow because of too much pressure against it or it will do it at a very slow rate(Much like blood at a very high pressure taking longer to deliver oxygen to the tissues). But how will I know what P2 is without inserting a pressure gauge into the evaporator when there is the same volume all the time just with a different amount of it being air and a different amount of it being water and that varying over time?
    Last edited: May 22, 2015
  2. jcsd
  3. May 23, 2015 #2


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    A sketch would make understanding the setup so much easier. And SI units would help as well.
  4. May 23, 2015 #3
    Well I used US customary since that is what I am familiar with.

    Here is the sketch with measurements.

    The measurements in SI units would be:
    radius of rain collector: 3.048 meters
    height of rain collector: 1.829 meters
    smaller radius of evaporator: 1.524 meters
    larger radius of evaporator: 3.048 meters
    height of evaporator: .305
    Pressure from pump: 13.789 kpa
    Volume of rain collector: 8.896 kiloliters
    Volume of evaporator: 5.190 kiloliters

    Attached Files:

  5. May 23, 2015 #4
    Now that sketch is a little off because the container that the spigot fills does not hold 1.726 kiloliters but rather 37.854 liters
  6. May 24, 2015 #5


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    That setup doesn't seem to make sense.

    Metals are good conductors.
    I have no idea where you expect a parabolic shape.
    A metal container won't shatter unless you do something really stupid (like using explosives or really high pressure).
    If your pump delivers enough pressure to pump up water by 6 ft, then it will be able to pump water through, no matter how much there is in your evaporator, assuming you don't have the whole system filled with water everywhere.
  7. May 24, 2015 #6
    I expect a parabolic shape to form after years of freezing and thawing from metal conducting cold in the winter or another way to put it, act as an insulator during the winter.

    But ice expands by quite a bit(like in a normal ice cube tray it expands by 1.25 cm all the way around) and if water fills up after ice has expanded then that is going to expand. Then it will eventually melt and then it will expand again and it will just keep on doing it. After years and years of this(even longer than the parabolic shape) I expect the metal to eventually reach its breaking point.

    As far as the pressure, it would probably be enough to pump it through the whole system but if the evaporator is full wouldn't it start to bulge out like an aneurysm and if I turn the shutoff valve wouldn't pressure build up in there to the point that the flexible pipe leaks and then the volumetric flow rate near the evaporator would be 0 liters/min?

    To me the setup makes sense because there is this big cylinder that has 2 hinges, 1 on either side that I would open right before it precipitates when no flow is needed anyway. I have a flexible pipe there since it is easier to make an arc if the pipe is flexible. There is then this evaporator that will kill any microbes by heat and will leave any dust behind via evaporation(same way you get NaCl out of an aqueous solution of NaCl). Then atmospheric gases that were dissolved in the water like CO2 and O2 travel along with the water vapor to a condenser where water turns back into a liquid.

    This is supposed to separate the gases and the water via flow rate difference but it doesn't really which is my main concern. I mean the CO2 changes the taste of the water to being more acidic since CO2 when in contact with water is not only attracted to it at the 2 dipoles but also turns into carbonic acid which is H2CO3. It has 1 more proton than bicarbonate does and in fact does turn into bicarbonate via another acid base reaction, this time with water acting as a base instead of CO2 acting as a base. If it is diprotic it will(albeit very slowly) turn into carbonate. It will however rather turn back into CO2.

    The oxygen doesn't change the taste and is 40 times less likely to dissolve than CO2 is. Same goes for the nitrogen and its low solubility. However some will still dissolve. I am not as concerned with these gases as I am with the CO2. I mean nobody wants acidic water unless of course it is juice or milk(which are both acidic water with other compounds added to it). So how could I get the CO2 that was in the rainwater to begin with out of the water and not just neutralize the CO2(Which would require that I evaporate and condense the water again to get rid of the salt that formed from CO2 + a base if I wanted pure water)
  8. May 24, 2015 #7


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    Don't let your container get completely filled if ice is an issue. Or just empty it in winter, evaporation will be negligible if your water can freeze so there is no point in having it filled.

    I don't know your container, but it is unlikely that it deforms slowly. It either breaks or it does not.
    That is not true, it is even sold in bottles. Also, your rain water will have low levels of carbon to start with, so I would not worry about that.
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