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We can get really humid days sometimes. Recently I was digging around in boxes and found some old mineral specimens, and I set out a few of the salt crystal rocks. I soon found that they had pools of water accumulating underneath them, which required some cleaning and attention.

After putting them on drinkable trays, I found that the surface tension was pulling the water off the bottom of the block and causing it to collect. This got me thinking about passing the air in coming to this space through a baffle of salt blocks.

The good thing about salt blocks is that the absorbed moisture can be 'wicked' away. Whatever the solution, this needs to be a cycle that is ongoing using the material(s) installed. I am fine to imagining the installation of a plumbed drain or collection reservoir.

Is there another material or better technique for removing moisture from the air in this passive manner?

*Passive meaning 'without an energized / motorized compressor to dehumidify'. Mostly, the mechanism needs to be designed to leverage ambient energy or mechanics, and minimize maintenance cycles and materials handling.

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    $\begingroup$ You can't do what you want in a sustained way. One way or another, it takes energy to separate humid air into dry air and water. Put another way, humid air is a lower energy state than separate dry air and liquid water. You can't cheat the laws of physics. $\endgroup$
    – Olin Lathrop
    Sep 20, 2015 at 21:08
  • $\begingroup$ @OlinLathrop I am ok with that insofar as the system is designed to leverage as ambient energy, and minimize maintenance cycles and materials handling. $\endgroup$
    – New Alexandria
    Sep 21, 2015 at 3:52

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Silica gel is the standard for most purposes.

It's the desiccant you find inside those paper sachets packed with new electronic equipment.

It consists of tiny beads of silicon dioxide that are nanometres across (so that a little material has a lot of surface area), and although it's called a gel, the material itself is hard at room temperature and pressure.

The source material is very plentiful, and very cheap.

Note that in general separating water vapour from air is going to require work done: i.e. it will need continual energy inputs. In the case of a dessicant, you will need to apply dry heat to regenerate it from time to time.

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  • $\begingroup$ Sound like I would have to keep purchasing silica, or baking it and replenishing. There is energy in baking it, the handling to change it all, and/or the costs to continually buy-storage-replenish. I have updated the question, since this won't work here. $\endgroup$
    – New Alexandria
    Sep 19, 2015 at 16:30
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    $\begingroup$ @NewAlexandria in general separating water vapour from air is going to require work done: i.e. it will need continual energy inputs. $\endgroup$
    – 410 gone
    Sep 19, 2015 at 20:00
  • $\begingroup$ How would you construct this, if you were not able to remove the desiccant in order the dry it? We could close-off the desiccant from then airflow using shutters, heat it, and then reopen the shutters after treatment. (I.e. The system would then have a duty-cycle). Other ways? $\endgroup$
    – New Alexandria
    Sep 21, 2015 at 3:48
  • $\begingroup$ Would this introduce a fine dust of silica into the airflow? I haven't handle large quantities of SiO2, but I presume it will. $\endgroup$
    – New Alexandria
    Sep 21, 2015 at 3:54
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Once again, forced to answer because not enough reputation to comment.

Several answers/comments have talked about requiring energy to reduce humidity. @Chuck specifically states you need energy to convert from vapor to liquid. In fact, the reverse is true. When vapor condenses to liquid it gives up energy. For water it's approximately 1000 BTU/pound of water.

A sustainable system will require energy, either to regenerate the absorbant (salt, silica gel, other) or for a mechanical system such as a refrigeration cycle to make something cold enough to cause condensation.

If you use an absorbent, then you could end up with an analog of an industrial absorption system. In those, the working "tower" absorbs vapor from the gas (water from the air in this case) while an offline "tower" is regenerated by, for example, being heated with dry gas (air).

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  • $\begingroup$ Yes, the vapor does give up latent heat to convert from a vapor to a liquid, but it takes energy to cool a surface below the dew point in order for that phase change to occur. I worded it the way I did to emphasize the fact that the phase change is not free, you pay for it by either heating a desiccant or cooling a surface. $\endgroup$
    – Chuck
    Sep 22, 2015 at 23:29
  • $\begingroup$ Nothing is free. Operating a system will take energy, but you are not going to have to supply the energy to convert the vapor to a liquid. You may have to supply energy to generate cold, as you say. You also list a set of steps to use a naturally occurring cold source, a cave, for which you would not have to supply energy. $\endgroup$
    – JKEngineer
    Sep 24, 2015 at 3:46
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How about this: You blow your humid air through the primary side of an air-air heat exchanger, here it is cooled a bit. Then this air passes through past a cold surface: either outside or underground, depending on oyour local climate. The air cools further and moisture condenses. The cool air passes through the secondary side of our air-air heat exchanger, gets reheated (and so the relative humidity sinks) and cools the incoming air.

You would need energy input for the blower, which is small compared to a compresor in a heat pump. You would loose heat to the environment. The bigger the HX, the less heat you loose. actual design must take into account that water may condense in the HX.

IIRC correctly, this was one trick used in an energy-optimized school, in this case they used an old subterreanean bunker nearby as the sink for the moisture.

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I don't think you're going to find an answer that meets the intention of your question as I read it.

It takes energy to convert water from a vapor to a liquid. Some materials (minerals, sugar, etc.) can absorb water vapor from the air because they are hygroscopic, but it also means that there is a finite limit to the amount of water they can absorb before they need to be dried or replaced.

The closest you're going to get is the answer from mart, where you drop the air temperature, but this means that:

  1. You need a cave or other geothermal heat sink
  2. You need a blower to circulate cave-side air
  3. You need a blower to circulate room-side air
  4. You need a heater to reheat room-side air,
  5. Whatever condensate forms needs to be plumbed

and, most importantly,

  1. You can only dehumidify to a dew point of the cave temperature.

Nothing will create a sustained dehumidification by itself. You need energy input. If such a material could absorb an infinite amount of humidity, then (1) why do we still have humidity (why isn't it all absorbed already), and (2), where would that infinite quantity of water go?

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My thinking is that it can be done. I look at the dripping front window glass on the RV and it is pouring down sometimes. old boat portholes even have drip guides to collect the run off.

So if you had a black steel plate outside with a strong thermal conductor to a heatsink inside, then as the temp drops in the evening and you get to the dew point, it will condense on the coldest surface. Perhaps a little pc fan could help here with negligable current. I even made a steel 'door' for my yacht, as I would often see the roof dripping in water when stored over winter, the idea being that every temperature fall would condense water on the door and run outside via a simple little guide.

Peltier mini dehumidifiers work pretty well, but they draw about 3amps @12v which is not passive enough.

I'm currently playing with these ideas, can't see any reason they won't work, again, it does depend on temp ranges on inside vs outside.

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