Thanks to H I Sutton -There are currently over 50 AIP submarines in service around the world, with the number likely to double in the coming decade. The largest single operator is China.
Air-Independent Power (AIP) allows a submarine to run its electric motor and other electrical systems without using the batteries. It reduces the frequency with which the submarine has to put its mast above the surface to suck in air for the diesel engines to recharge the batteries. The submarine still has to snort for brief periods on most days in order to ventilate, but is much less exposed than conventional diesel-electric submarines. During operations, if the tactical situation prohibits ventilation then the submarine can delay snorting for much longer than normal. And it can revert to lighting oxygen candles (or equivalent) in dire situations. Therefore the submarine can remain submerged for much longer, giving the submarine commander much greater flexibility.
Another consideration is that most current AIP is only capable of providing power for very slow cruising – significantly less than 10kt. This is insufficient for wide-ocean transits and makes the submarine significantly slower than the task force. Therefore the regular diesel-electric power plant with snorkel cannot be done away with.