This is something I have been concerned with and thought about extensively as I will be building in a climate with hot humid summers, and cold damp winters. I have an idea but won’t be able to test for several years (until I retire baby!!!) but here’s my two cents:
Very similar to cooling tubes, using earth and convection, it would be possible to build a DIY passive dehumidifier. The idea would be to bury a length of metal tubing into the earth (on roughly a 3/1 ratio. For every inch of exposed coils, three inches of piping must be below grade.) which connect to exposed coiled copper, placed vertically, on an indoor wall. Using ethylene glycol, fill the tubing/coils and seal. Back on the interior wall, build a drip drain under the copper coils. The warmer indoor air would pass over the cold coils causing condensation. Convection would warm the liquid inside the coils creating a temperature difference, which would move and circulate the liquid. Warmer liquid would be pushed up and back into the cold earth, and cold liquid would flow into the coils. It should be able to bring down the humidity of a small sealed room. How long and how many coils you need would be dependent on many, many, factors. To make it an active system, you could have a 12 v fan blowing air at the coils. The more air that passes over them, the more effective it would be. Likewise, a 12 v pump could help circulate the ethylene glycol if convection alone doesn’t exchange warmer/colder liquid fast enough. Even you used a fan and pump, it would still require less energy than a full blown HVAC system or using standard electric dehumidifiers.
