Is there really such a thing as windowless portable air conditioners? Not exactly. Nearly all portable air conditioners need a window in order to vent out the warm air that they take out of the room. So while there are options out there other than an actual portable window air conditioner, you will most likely need a window for venting out warm air.
Most portable air conditioners come with a window kit. This window kit usually consists of either a single or double hose that fit into a rectangular piece of plastic with holes cut out for the hoses to be placed into. This allows the warm air to be vented out of the room and outside of the building completely. Letting the warm air back into the room will only make the room seem even warmer in most cases. Even if the room is feeling cooler to you, it is still a bad idea. The air being vented into the room holds even more moisture than before and will eventually allow your room to grow mold or mildew over a given time.
If you have a room without windows that is in dire need of being cooled, there are a couple of options. Considering you have to vent the air out of the room completely, the best way to do so is by having a hole cut out of the outside wall to fit the hose into. This is also the way dryers are vented. This is the best way for you to accomplish proper venting of windowless portable air conditioners. The only downfall to this is that there will be an existing hole in your wall shall you ever rid of your portable air conditioner.
Now, if you have a room such as a server room, where an outside wall may not exist, you can quite possibly vent the portable unit through a drop ceiling. This isn’t recommended with use of most indoor portable air conditioners for the same reasons that you shouldn’t let it vent into another room. There are the risks of making the room warmer as well as the existence of mildew and mold over time. However, if this is your only option, venting it into drop ceilings has been done before. There are also special models of portable units that are designed specifically for drop ceilings and server rooms. They tend to carry a higher price tag and will have to be professionally installed. A great example is the Fujitsu AUU9RML. This unit is relatively cheaper than most but it is known to be efficient.
Another option for venting a portable air conditioner is through a sliding glass door. There are kits designed for such occasions. Other than the few ways listed, there isn’t really any other way. Windowless portable air conditioners do exist but the need for a window due to venting is essential unless you vent through the wall, ceiling, or sliding door.