Air Independent Propulsion (AIP) System

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About Submarines:

Conventional or Diesel-electric submarine: They need atmospheric oxygen to run the diesel generator which in turn charges the batteries.
Nuclear Submarine: It is a submarine powered by a nuclear reactor, but not necessarily nuclear-armed. They have considerable performance advantages over conventional submarines.

With the emergence of submarines, there was a problem finding satisfactory forms of propulsion underwater.
AIP is mostly implemented as an auxiliary source, with the traditional diesel engine handling surface propulsion. Most of these systems generate electricity, which in turn drives an electric motor for propulsion or recharges the boat’s batteries.
AIP allows longer submergence than a conventionally propelled submarine. A typical conventional power plant provides 3 megawatts maximum, and an AIP source around 10 per cent of that. A nuclear submarine’s propulsion plant is much greater than 20 megawatts.

It allows the submarines to stay for longer hours in water. The submarines need to come to the surface of the water to charge their batteries. This is reduced by AIP System.
It decreases the noise levels made by the submarines. This makes it hard to detect the submarines.

Open-cycle systems
Closed-cycle diesel engines
Closed-cycle steam turbines
Stirling cycle engines
Fuel cells
DRDO is using the fuel cells-based AIP System. It is unique as the hydrogen is generated onboard. The plant was operated in endurance mode and max power mode as per the user requirements. The system is being developed by Naval Materials Research Laboratory (NMRL) of DRDO.