I certainly agree with this. "Air Independent" in this case means that the propulsion system doesn't need air from outside the submarine. Batteries are an "air

I certainly agree with this. "Air Independent" in this case means that the propulsion system doesn't need air from outside the submarine. Batteries are an "air independent" system but this is talking more about things like fuel cells or Stirling engines. The cool thing about Stirling engines is that there isn't a "bang" associated with the power stroke, that makes them quite (unlike diesel engines). Fuel cells obviously generate power from the catalyzed combination of chemicals, and so are also silent. As the article points out they are quiet but their range is limited so they are good for patrolling your coast, not great for going across the ocean stealthily. I was impressed that the Gothard class was getting 75kW from their Stirling engine. I had not known that there were production engines greater than 50kW. So that was new to me.