A French firm (Naval Group, formerly DCNS) has successfully tested what it calls FC2G (Fuel Cell 2nd Generation) AIP (Air Independent Propulsion) system for 18 days in a mock-up of the complete system in an 8 meter long circular structure identical to the space it would occupy in a submarine. The FC2G performed efficiently for three weeks. This test will be repeated several times as preparations are made to install FC2G in a submarine. The FC2G is safer, more efficient and easier to operate than earlier fuel cell AIP systems, including the widely used DCNS first-generation AIP.
AIP is one reason non-nuclear subs will be around for a long time. Read the entire post.