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Broadly speaking, how do AIP vessels compare in performance to nuclear submarines?  Let’s consider the costs and benefits in terms of stealth, endurance, speed and cost.

Stealth:

Nuclear powered submarines have become very quiet—at least an order of magnitude quieter than a diesel submarine with its engine running.  In fact, nuclear-powered submarines may be unable to detect each other using passive sonar, as evidenced by the 2009 collision of a British and French nuclear ballistic missile submarines, both oblivious to the presence of the other.

However, there’s reason to believe that AIP submarines can, if properly designed, swim underwater even more quietly. The hydraulics in a nuclear reactor produce noise as they pump coolant liquid, while an AIP’s submarine’s engines are virtually silent. Diesel-powered submarines can also approach this level of quietness while running on battery power, but can only do so for a few hours whereas an AIP submarine can keep it up for days.

Diesel and AIP powered submarines have on more than one occasion managed to slip through anti-submarine defenses and sink American aircraft carriers in war games. Of course, such feats have also been performed by nuclear submarines.

Endurance:

Nuclear submarines can operate underwater for three or four months at a time and cross oceans with ease. While some conventional submarines can handle the distance, none have comparable underwater endurance.

AIP submarines have narrowed the gap, however.  While old diesel submarines needed to surface in a matter of hours or a few days at best to recharge batteries, new AIP powered vessels only need to surface every two to four weeks depending on type. (Some sources make the unconfirmed claim that the German Type 214 can even last more than 2 months.) Of course, surfaced submarines, or even those employing a snorkel, are comparatively easy to detect and attack.

Nuclear submarines still have a clear advantage in endurance over AIP boats, particularly on the long-distance patrols.  However, for countries like Japan, Germany and China that mostly operate close to friendly shores, extreme endurance may be a lower priority.

Speed:……..Obviously, high maximum speed grants advantages in both strategic mobility and tactical agility.  However, it should be kept in mind that even nuclear submarines rarely operate at maximum speed because of the additional noise produced.

Cost:

Who would have guessed nuclear reactors are incredibly expensive?  The contemporary U.S. Virginia class attack submarine costs $2.6 billion dollars, and the earlier Los Angeles class before it around $2 billion in inflation-adjusted dollars.  Mid-life nuclear refueling costs add millions more.

By comparison, AIP powered submarines have generally cost between $200 and $600 million, meaning a country could easily buy three or four medium-sized AIP submarines instead of one nuclear attack submarine. Bear in mind, however, that the AIP submarines are mostly small or medium sized vessels with crews of around 30 and 60 respectively, while nuclear submarines are often larger with crews of 100 or more.  They may also have heavier armament, such as Vertical Launch Systems, when compared to most AIP powered vessels.

Nevertheless, a torpedo or missile from a small submarine can hit just as hard as one fired from a large one, and having three times the number of submarine operating in a given stretch of ocean could increase the likelihood chancing upon an important target, and make it easier to overwhelm anti-submarine defenses.

While AIP vessels may not be able to do everything a nuclear submarine can, having a larger fleet of submarines would be very useful in hunting opposing ships and submarines for control of the seas. Nor would it be impossible to deploy larger AIP powered submarines; China has already deployed one, and France is marketing a cheaper AIP-powered version of the Barracuda-class nuclear attack submarine.

It is no surprise that navies that operate largely around coastal waters are turning to cheap AIP submarines, as their disadvantage are not as relevant when friendly ports are close at hand. ……..http://nationalinterest.org/blog/the-buzz/did-sweden-make-americas-nuclear-submarines-obsolete-18908?page=2