There has been much hype since March 9, 2021, concerning a milestone reached in Indian DRDO's (NMRL's) new air independent propulsion (AIP) technology for submarines. But this is more a "soft" political "launch" to legitimise high funding involved, rather than a "hard" launch of a mature weapons system.
Proof whether this AIP is efficient or safe will require 2 or 3 years of at-sea testing. India may develop this AIP into a viable at-sea system, so this is not to denigrate or dismiss India's achievement.
India's defence establishment has been talking about DRDO AIP (scroll 1/4th way down here) since at least 2014. It must be remembered that Russia, Spain and France have been trying to develop successful second generation AIP for years - without proven at-sea progress.
On March 9, 2021 DRDO announced:
"DRDO achieved an important milestone in the development of AIP System by proving the land based prototype[artwork below] on 8 March 2021. The plant was operated in endurance mode and max power mode as per user requirements. AIP system is being developed by NMRL DRDO." .
This AIP might first be fitted into the Naval Group Scorpene variant INS Kalvari, during a scheduled, major overhaul, in 2023. The AIP will then, probably need 2 to 3 years of at-sea testing before it can be proven efficient and safe.
The AIP technology in question is called Phosphoric acid fuel cells (PAFC) with power output listed on July 2, 2020 by Defence Decode as 270kW, which compares well with a TKMS Type 214's AIP producing 240kW.
Model: "Defence Decode, July 2, 2020" carries the caption. "Indigenously developed Air independent propulsion by DRDO, through its NMRL Larsen & Toubro as a development partner (LEAD SYSTEM INTEGRATED). The AIP system is based on Indigenously developed PAFC based technology. Hydrogen and Oxygen are supplied to fuel cells to produce power."
The whole apparatus (see Model above) which will lengthen the sub from 67.6m to 77.6m looks very large (judging by the human dolls for scale) and heavy (perhaps 250 tonnes). When this is retrofitted to the Kalvari class there may be major buoyancy readjustments necessary. Many rearrangements (eg. batteries, solid ballast and air/water/diesel tanks) to rebalance the submarine, may be necessary. A big job for Naval Group and the Indian design and build companies involved.
So this is not to denigrate or dismiss India's achievement. But it will likely take 5 or 6 more years for this DRDO AIP to be fully tested in the critical, operational, at-sea environment.