Has anyone seen practical solar dehumidification? Say something like this: during the day, hot (and hence dry) attic air is blown through a desiccant (clay, silica gel, etc) to dry it out. Periodically, some vents switch and interior air is circulated through the desiccant - making the interior more comfortable.

Basically, I'm looking for ways to achieve comfort in hot, very humid climates where only solar power is available - it would take too many solar electric panels to power a convention A/C or dehumidifier. But a fan is possible.

Nights aren't much cooler - so thermal mass doesn't work. It's so humid that any type of evaporative cooling doesn't work well.

Another idea is to collect rain water (common for potable water anyway). This is one of the few things that is cool. This is normally stored in underground cisterns, but could be stored in a cistern adjacent to living areas - providing passive cooling.

I'm a newbie to all this, but I can offer 2 points based on what I've learned:

1. If your air is so humid that night is not much cooler than day, I would wonder if the daytime heated air really has a lower-enough relative humidity vs your interior air to do much drying of the dessicant. That is, it might be that you'd need loads of it for it to do any good. It also relies on your interior air being significantly cooler than the attic air -- which feeds into the 2nd point:

2. What other knowledgeable posters here are constantly repeating is to do insulation projects first, ideally up to the standards that are termed "PassiveHouse"; after that, the loads for heating or cooling become low enough that the actual providing of heat/coolness becomes a much easier problem to solve. For your case, if you can insulate the house to a very high degree, then a relatively small A/C unit -- small enough to power with a practical number solar PV panels -- would likely be enough to do the job. Of course, having that high an insulation level also implies near-airtightness, which means you'd also need a mechanical air-exchange facility, with humidity function. Adding that would be considered part of the insulation project. But it's still a good chance that powering both the exchanger and the A/C would be practical with PV if the house can be insulated well enough.

Actually, I wonder if your desiccant idea actually could work, after you've super-insulated the house. Basically you'd have your load of desiccant dried out during the day by hot air, collected with a thermal air panel or air from your attic (would probably have to be mechanically blown, as the mass of desiccant would probably be too great for the house to support, so the box of desiccant would have to be down on the ground). Then, overnight, air blown through that box would feed into the "fresh air" input of the heat exchanger, thus pre-conditioning the air for lower humidity, helping the air exchanger with its job of keeping humidity from entering the house.

Oh, but there might be a fatal flaw here: the desiccant will get pretty hot during the drying process. That heat won't go away quickly, especially since there will probably have to be a lot of desiccant mass. If you blow nighttime air across it for a while to try to cool it, that might add back the humidity you were trying to get rid of! So you'd wind up supplying dryer, but hotter fresh air for the house, which again would seem to defeat the purpose.

Again, I don't have any real experience with all this heat/coolness-management stuff yet, so take this with a grain of salt!

I suppose one needs 3 stages - desiccant being heated/dried by solar heat, desiccant being cooled (to ambient, ground or to stored rain water temp) and being used for interior dehumidification. Might have to alternate between several desiccant bins so that one is always available for interior use. Only minimal electricity is needed for fans to move the air around in the right ways.

I agree that an alternative is to insulate and seal well and then use enough PV panels and batteries to run A/C and dehumidification - but that is a lot of inefficient conversions (solar to electrical to chemical to electrical to rotary motion to pressure to temperature) and expensive parts to maintain.

Here is a study showing it working - and this system has the handicaps of no cool down mode and using only wood as the desiccant.

"Feasibility study of the passive solar room dehumidifying system using the sorption property of a wooden attic space through field measurement"

Using solar thermal (or natural gas burners) to recharge liquid dessicant such saturated calcium chloride/water solution is sometimes used on large HVAC systems.

This tiny version used in the University of Maryland's Solar Decathlon entry is the only single-home sized version I'm aware of: http://www.solarteam.org/page.php?id=641

It cools a little a night, but this only increases the relative humidity. In the afternoon, it's hotter but depending on the rain, the relative humidity is often lower (but overall, equally uncomfortable).

jon' has it right- the relative humidity is "relative" to the air temperature. The ABSOLUTE humidity is measured by dew point, which isn't much affected by daily outdoor temperature swings except when the air temps gets down to the dew point (100% relative humidity), and large amounts of moisture begins to condense out of the air (ground-fog/mist conditions.)

In places where the average annual temps are 55F or lower the subsoil temps can be low enough to do a modest amount of dehumidification using earth-tubes. It takes a lot of surface area to the earth tube to handle a significant seasonal latent load though, but it can work in places other than hot-humid climates. See:

http://mb-soft.com/solar/saving.html

There are similar approaches using groundwater & hydronic air coils that can work if groundwater temps are low enough.

Earth-tempering the ventilation air via earth-tubes PassiveHouse style is usually at flow rates such that the incoming air stays above the dew point though, and only the volume of air trapped a the end of a ventilation cycle can condense much humidity.

A liquid desiccant like calcium chloride might be easier - "Leaf House" used it. Pump it through an open solar panel to drive out moisture, then through a shallow underground loop to cool it, then over a waterfall inside the house. Store some extra in a tank for use at night.

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Further research shows that if you have a solar powered dehumidification system that works well, you can always trade off some of the low moisture content for cooler temperatures - ie, dry the air and then evaporate some water back into it to cool the air.