The ultimate ductless dryer is an outdoor* or indoor clothesline*, but there is a two-in-one machine that uses condensing technology to get the water out of the clothes. Here's a great writeup from Building Officials of Florida:
This is explicitly allowed in ASHRAE Standard 62.2 - Ventilation for Acceptable Indoor Air Quality. It is a mature technology in Europe - the two principle manufacturers of these dryers are Bosch and Miele.
The tremendous attraction for use in hot humid climates is that they provide a huge reduction in latent load while increasing the sensible load. If you are a mechanical engineer this makes your day. The energy saved on the reduction in the latent load (not exhausting 200 cfm of indoor conditioned air to be replaced with 200 cfm of unconditioned outside humid hot air) significantly offsets the
increase in sensible load (heat is now dumped into the unit rather than outside). And since the interior load increase is pure sensible load the a/c runs slightly more causing a further reduction in interior relative humidity.
The other attraction is no vents runs that fill up with lint that can catch on fire. No additional hole in the building enclosure to leak in the rain. No more need for rated shafts that go upwards.
So what is the catch? Well, they are smaller than big standard US dryers. You generally get 3 loads for every 2 loads in a standard dryer. This still wins on the energy balance but can be annoying if you do lots of laundry.
All of this applies to dryers that are designed to be ventless (i.e. they are electric condensing dryers). Venting a standard electric dryer to the interior is an incredibly dumb thing to do. You ge a huge increase in latent load as well as sensible load as well as lint.
Condensing ductless electric dryers are a great technology for hot -humid climates in Florida if you care about reducing mold and saving energy.
Joseph Lstiburek, Ph.D., P.Eng.