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I have permanently humid walls (due to a construction error, not uncommon around here) that I need to dry out occasionally.

I live off the grid, with all power coming from solar panels. That works great, but run-off-the-mill dehumidificators would drain the system too much.

Do powerful dehumidification options exist that don't require a lot of electricity?

All I can see in shops is those small packages with some sort of salt that you can put in your closet. I need something more serious than that.

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    Thousands of pounds of silica gel?
    – bib
    Mar 8, 2015 at 22:30

2 Answers 2

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The latent heat of vaporization of water is 970 BTU. That is absolute BTU, not BTU/hr like air conditioners are rated for.

That means, to condense 1 pound (1 pint) of water, you must remove 970 BTU of heat from it. That is approximately 300 watt-hours.

Can we do this more efficiently than spending 300 watt-hours of energy? YES. Thanks to heat pumping. The best heat pumps range as high as 38 SEER, which is 12 times efficiency, so your 300 watt-hours would remove 12 pints of water. Not bad at all, you can work with that.

The #1 mistake I see (practically every time) is focusing on "alternative technologies" simply out of fear of the running watts of mechanical dehumidification. This is wrong-headed -- the goal here is to remove pints of water for the least solar power, so what matters is "watt-hours per pint".

The thing is, the dehumidifier doesn't need to run "wide open 24x7". Assuming that is why people are afraid of the "running watts" of mechanical dehumidifiers. If a 300 watt unit is removing 8 times as much water as a 150 watt unit, that's better - you want the 300-watt unit.

The second mistake is being afraid of batteries. You don't need some huge battery unless you are committed to hitting a particular humidity number 24x7 "and cost is no object". Realistically, you want to run the dehumidifier intermittently when the sun is shining - and ideally, you want to use power that would otherwise be thrown away.

If your system is properly sized, there are many times when your battery is fully 100% topped up and the charge controller must start rejecting power from the solar. Instead, start the dehumidifier. Since the battery is operating as a buffer, it's not even necessary for the solar to keep up with the dehumidifier: you simply shut off the dehumidifier when battery charge reaches a "low" setpoint e.g. 95% battery charge.

Will that be enough to reach your dehumidification goals? Don't know. But if it isn't, the cure is simple: rack more panels so the dehumidifier can run more often when the sun is out. The same battery will suffice.

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For an off-grid dehumidification system, you have two or three choices that I can think of. One is to install additional power panels to support the additional load of a "standard-type" electric refrigeration-based dehumidifier, and purchase the most efficient version of that type that you can.

The most likely alternative, which I'm not aware of being commonly available "pre-built" though it's possible that it's out there and I'm not aware of it, is to use solar heat to dry a desiccant, and then switch that desiccant into the airspace to be dried (either by switching airflow over fixed desiccant containers, or by physically moving it.) You would generally want to have at least 2 sets of desiccant, so that one set can be dried while another is absorbing water. This is fairly amenable to DIY implementation. It may need to be quite large to keep up with any significant drying load.

An outlier, which is more likely to be seen in a "commercial" or fairly large-scale offgrid setup would be to use an absorption refrigeration loop driven by solar heat to generate the "cold" for dehumidification (and possibly also for other refrigeration purposes) - the complexity of which typically makes it impractical and/or hazardous for a one-family sized situation. By the time you've gone through all that's required, option 1 looks inexpensive, normally, even though this has the potential to be more efficient in conversion of sun to cold; it's a complex and not very common setup, which can leak, has typically noxious chemistry (ammonia) and needs maintenance.

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  • Thank you, this is great. For #1 I worry about batteries, which I'd likely have to extend too if I put any more panels on the roof, and #3 would probably be too complex for me to implement. But #2 is a very interesting idea, I'll look into that!
    – Pekka
    Mar 8, 2015 at 20:40
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    For #1, one option is to only run it when the sun shines, or when the sun shines and the batteries are "full enough." But do add more panels, at least.
    – Ecnerwal
    Mar 8, 2015 at 20:59
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    If you need dehumidification when it's also hot, just air-condition - or get a mini-split type system that can cool or dehumidify (or heat, but that seems less needed, especially if the sun is shining.)
    – Ecnerwal
    Mar 8, 2015 at 21:18

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