Is it possible, with current technology, to make an air conditioner small enough to cool a very small area - like, say, the size of a tent (a few dozen square feet at most) - that is also relatively lightweight and portable? If so, how small would it be?
I believe there are AC units the size of a shoebox, if not a large book. It really is all in the BTUs; I suppose you could make one the size of a lipstick if it only had to cool a cubic inch.
There are also AC units that use alternate technology (not compressing and expanding a closed-loop gas) like thermoelectric junctions ([del]there’s a name for these that escapes me[/del] Peltier junctions) and adiabatic expansion. There was a surge of interest in AC units for cars using adiabatic expansion - Rotex or Rotax - a few decades back. (It works by taking a closed cell of air and increasing its volume, which drops the average temperature, and then expelling the chilled air. Sort of like a mechanical version of letting high-pressure gas out of a bottle. Some prototypes supposedly worked so well they blew frost out of the car vents.)
Solid state cooling devices (using the Peltier effect) can be quite compact and quiet for the level of heat rejection they can produce, and can be tailored to the size of the volume they’re tasked with cooling. The problem is that they are not very efficient compared to conventional heat pump refrigeration cycle coolers, the size and mass of which are largely dictated by the need for a compressor and plumbing to move the working fluid from the “cool” side to the “hot” side, where the removed heat is rejected. A solid state cooler for a tent or small cabin by ~15 °C could be made roughly the size of a couple of cinder blocks, not incluing the external power supply and a fan to force air over the cooling surfaces.
Stranger
Peltier coolers are commonly available for CPUs; you can keep a high-power CPU cool with one of these using a smaller heat sink and fan.
So basically you can make heat pumps of pretty much whatever size you want. There is, of course, the question of how you will power it. If you’re actually looking to cool a tent, note that they’re pretty poorly insulated, so the heat load will be quite high, requiring a fairly substantial air conditioner (and power supply) to keep things comfortably cool (even assuming you seal up your tent to prevent air exchange).
Dehumidification can go a long ways towards promoting comfort, so even if a mini AC unit can’t bring the temperature down much, drying the air by concentrated cooling and condensation can be an improvement.
Keep in mind that air conditioners don’t cool. They just move heat from one place to another. If you want to cool a tent, you need to have the cool side of your air conditioner inside the tent and the hot side outside the tent so that it will basically pump heat from inside the tent to the outside.
You could probably rig something up with peltier devices fairly easily. Peltiers are neat little devices. If you run electricity through them, they become itty bitty heat pumps, moving heat from one side to the other. Reverse the polarity of the electricity, and the hot side becomes the cold side and the cold side becomes the hot side. They even work in reverse. Apply a heat differential across the device (like put ice on one side and your warm hand on the other) and they’ll generate electricity.
Peltiers have their disadvantages. They are pretty inefficient and generate a lot of waste heat. You’ll need a fan or a big heat sink on the hot side to draw the heat out of the peltier, otherwise the whole thing will get hotter and hotter (both the hot and cold sides) until it overheats and fails.
Peltiers have their advantages, too. They are small and cheap, and have no moving parts (not counting any external fans you may need to add), which makes them very reliable.
Peltiers are commonly used as cpu coolers, because they are great at spot cooling like that. They are also common in portable cigarette-lighter powered picnic coolers, where their small size and light weight make them very desirable.
You may need more than one peltier device since they are fairly small, but an array of maybe four of them with a fan on either side would probably do a decent job.