Every time I brew I watch all the CO2 from the fermentation escape into the atmosphere. And then after the fermentation is complete we take another step to carbonate. Would it be possible to create a fermentation vessel that allows pressure to build up in the vessel so that after fermentation there is a large amount of carbonation in the beer already? This way when we keg the beer it is already part of the way towards full carbonation and the time in the keg under pressure would only be to finish off the carbonation. Is this a foolish thought?
It is possible and is already something some brewers do. As you pointed out all you need is a vessel that can hold the pressure. One thing that may get considered is when the yeast is pulled out and if it does anything odd as pressure is increased. Can’t say I know the answer but something that may be a consideration and easier managed with a conical where yeast can be dropped out of the cone.
Some breweries do pressurized fermentations like this but I believe they do this to speed up the process as my understanding is the yeast work faster under these conditions. I am repeating that from information I think I’ve read somewhere so don’t quote me
I believe this is the same concept as real ale or cask conditioned ale where you serve from the same container you fermented though I believe in most cases a second fermentation is started to dial in the carbonation. Otherwise it might be tricky to get the exact carbonation you want.
That vessel already exists, it’s called a keg. Either transfer your beer about .010 before final gravity or as I have done add sugar to the keg.
If you want to ferment in the keg. Transfer wort to the keg and pitch yeast. Hook up the gas side with a length of hose stuck in a bottle of starsan like a blowoff tube. Probably only want to do 4 gallons or less this way. Also shorten the supply side tube so you can force the finished beer to a bright tank aka keg. Sounds like a lot of extra work to me. If you have money you could buy a ss conical.