There's a youtube channel where someone nicknamed Thunderf00t is making fun of a water source that is supposed to be a wind-powered dehumidifier. He makes some arguments that don't look entirely valid. He argues that the energy needed is the evaporation heat of water, or some 2000 kJ/kg. In this video he says the energy is retrieved and needs to be dissipated, in another video he says this energy has to be supplied. Clearly he's somewhat confused, and so am I. On the one hand, energy should be gained by turning vater vapor into liquid. On the other hand, water evaporates spontaneously, so putting an ideal dehumidifier into a closed loop would create a perpetuum mobile of the 2nd kind. So I was wondering, how much energy is REALLY needed to take water vapor out of the air, using an ideal heat engine, ideal heat exchangers to pre-cool the input air, etc.? Should venting be accounted for?