There is nothing special about them, they are nothing more than colorized "silica gel beads" in a container with a window..
Per
HERE"Design Technology:
Engineered with non-toxic crystallized silica gel technology, the Eva-Dry small dehumidifier will quickly and efficiently absorb up to 4 to 6 oz of moisture from the air, without any leaks or spills."Silica gel works very slowly (Eva claims "quickly" which is not exactly true since they claim the 4-6 oz is over 20-30 days) to absorb moisture from the air, once they saturate, you have to place them in a very warm place to drive out the moisture or replace the beads.
Generally Silica gel beads can be placed in a oven at a low temp for several hrs to dry them out, with Eva it has a built in heater that you will have to plug it in to "renew" the beads when they reach saturation..
Those little units can only handle 4 oz- 6 oz of moisture each, that is the equivalent to a CUP to less than ONE PINT of water which is absolutely NOTHING in respect to how much a real dehumidifier can pull out of the air in a 24 hr period (typical compressor dehumidifiers are rated in pints per 24hr and small ones can pull 20 pints per 24hr up to 70 pints per 24hr period).
Just leave the windows open a slight crack, not enough to allow rain in but just enough to allow air movement. You can also do the same with the roof vents.. Cheaper and effective.