I had this posted in another thread, but i thought it was worth mentioning on its own.
Although it is a crude (i am work so I can not spend all day playing it MSpaint) Here is a quick pic to explain my idea.
If i remember right co2 is heavier than most of the normal air around us. So as your fermentor produces co2 it fills the first keg with co2 while pushing the O2 and other unwanted gasses into the second collection keg. Those gasses will be forced into the next keg and then to the next. you will have to vent the last in the chain to get rid of the normal air. You will also have to keep the pressure pretty low or the stopper in your carboy will pop out.
Then replace the carboy with a water faucet (getting the hose to make a good seal on the faucet may be tricky.) You should be able to fill up 3 with water. I would then remove the kegs that are filled with water so you do not just end up with carbonated water. This will leave you with one pressurized keg and 3 with water in them.
Your fermentation should still be going, so you can empty the water kegs and start over with 3 kegs. Then 2, and then one. When you think they are all filled up you can daisy chain them together again and hook them back up to the faucet and push the co2 into just one keg again.
At some point, i am not sure how many kegs of co2 it would take, the water pressure and the co2 pressure will equalize and water will no longer flow.
I am not sure 4 kegs is enough. You could do the same thing with several half barrel kegs, and then have a corny on the end. Or do a string of 10 corny kegs. That would get more a higher pressure in the final keg.
Can someone do some math here? How much pressure should we expect?
Now we of course want to use the co2 we captured to carbonate another keg. i have an elaborate water tower design to control the pressure but it is very unrealistic. I would just buy an inline co2 regulator. You could also make a bootleg one from a normal co2 regulator.
In the end i would throw some priming sugar in a keg to carbonate the beer, then use this method to dispense the beer. This method is a lot of work for very little results. But if the place where you purchase co2 is VERY far away, or you just want to "naturally" force carbonate your beer maybe this is worth a shot. Personally I think I can't spare that many kegs, hoses and fittings.
What do you think? Is it any better than using balloons and a bike pump?