Article regarding AIPhttps://nationalinterest.org/blog/the-buzz/air-independent-propulsion-submarines-stealth-cheap-the-24245https://defencyclopedia.com/2016/07/06/explained-how-air-independent-propulsion-aip-works/
The types of AIP systems are
Closed Cycle Diesel Engines
Closed Cycle Steam Turbines
Sterling Cycle Engines
The French MESMA (Module d’Energie Sous-Marine Autonome / Autonomous Submarine Energy Module ) is the only such system available and it makes use of ethanol and oxygen as energy sources. The combustion of ethanol and oxygen under high pressure is used to generate steam. The steam generated is the working fluid and is used to run the turbine. The high pressure combustion allows the exhaust carbon dioxide to be expelled outside into the sea at any depth without making use of a compressor.
The advantage of MESMA is it’s higher power output when compared to the alternatives which allows higher underwater speeds but it’s major drawback is it’s lower efficiency. Also the rate of oxygen consumption is said to be very high and these systems are very complex. These drawbacks make several navies opt for sterling cycle and fuel cell alternatives.
Fuel cells are the most advanced and preferred AIP technology today. This is because of the major advantages they offer in stealthiness and power generation. They contribute to the stealthiness of the sub as Fuel Cells have almost no moving parts, which significantly reduces the acoustic signature of the sub. Fuel Cells can achieve an efficiency of over 80% under certain circumstances. They can also be scaled easily into large or small sizes depending on the displacement of the submarine. This is easier than developing different systems for each submarine class. Hydrogen Fuel Cells are also very environment friendly as they generate no exhaust fumes, which in turn eliminates the need to have special exhaust scrubbing and disposal machinery. The only drawback is that they are expensive and complex.
This completly makes sense, Navy and DRDO are working on something which is really cutting edge hence obviously there would have been unforseen challenges, looks like we are over the major hurdles and we are onto something good here.
Looks like Navy/ DRDO went for fuel amoung advantages it can be scaled up for Project 75 and also reasonably quickly.
And for those wanting why not Saryu- notice Japs use Stirling AIP
As of 2016, the following countries have developed their own AIP systems to be fitted on submarines.
Germany – Fuel Cell
Sweden – Stirling
Japan – Stirling
France – MESMA
Spain – Fuel Cell
India – Fuel Cell
Russia – Fuel Cell
People’s Republic of China – Stirling
LIMITATIONS OF AIP
Other than Fuel Cells, the 3 remaining technologies have a lot of moving parts which generate noise. This is not desirable as quietness is very essential for all submarines. So by using Stirling, MESMA and CCD AIP systems, submarines will be sacrificing some of their stealthiness for additional endurance.
Even though Fuel Cell AIP has many advantages, it is extremely expensive to procure and maintain them.
Submarines which use AIP need to sail at speeds of less than 10 kts in order to achieve exceptional endurance of 14-18 days as advertised. In comparison, a nuclear powered sub can travel for an unlimited distance at 30-35 kts without sacrificing endurance. So AIP equipped submarines cannot replace nuclear submarines when it comes to blue water or extended period operations.
But given our Arabian Sea shelf depth and unlike USA/USSR which dont have defend home bases, we probably need a green water Navy component, so we need SSP/SSN combo and can't go for pure SSN force.