Sure,they will have to surface & they can be sunk,just like any other ship,including N-boats which have to go up after about 90 days.The point is their potential for damage in the 2 or 3 weeks they are down in wartime.What's the point in sinking a 500 million dollar ship with 30 fellows on it after it has hit a SSN with 4 times the number of men & over thrice the cost,not mentioning surface ships.
Modern AIP technology is just evolving.This Swedish 'Sterling' AIP cannot go beyond 20 days-the new German fuel cell based systems can go upto 28 days & everyone is doing research to further that endurance.
Bigger LOX tanks? I thought the Gotland was really clever when I first read of the joint exercise. But now I'm convinced that the German (fuel cell) technology is potentially better. It eliminates the necessity of dumping the combustion products of the Stirling engines' heater out of the pressure hull. It also reduces the moving parts count to a minimum.
Both the Swedish and German boats are very small and can offer only limited storage space for torpedoes and/or missiles. The Gotland was ferried to San Diego with a surface ship. It appears that these "pocket" subs are best suited to coastal defense and may have problems with "blue water" sailing. Our best defense may be to just stand off in deeper water with our missile boats.
Regards,
GtG
PS The Swedish boat is co-ed. What red blooded sewer pipe sailor could resist that duty?