They operate by sucking air through a part of a rotating desiccant drum then exhausting it. At another spot, they heat and blow air through the desiccant, recharging it. The heavily moisture laden air is ran through a radiator where the moisture condenses out. If you have a source of low/medium grade heat, these can operate for the cost of running a few fans.

This type works actually can operate in colder climates as they can work down to near the freezing point of water, where the cooling type cannot operate when the dew point is below the freezing point of water.