Context: 

Larsen & Toubro (L&T) and the Defence Research and Development Organisation (DRDO) signed a contract for realisation of two Air Independent Propulsion (AIP) System Modules for the Kalvari Class of submarines of the Indian Navy.

About Air Independent Propulsion:

• AIP technology allows a conventional submarine to remain submerged for much longer than ordinary diesel-electric submarines.
• AIP has a force multiplier effect on lethality of a diesel electric submarine as it enhances the submerged endurance of the boat several folds. 
• Improved stealth capacity:
  • All conventional submarines have to surface to run their generators that recharge the batteries that allow the boat to function under water. 
  • However, the more frequently a submarine surfaces, the higher the chances of it being detected. 
  • AIP allows a submarine to remain submerged for more than a fortnight, compared to two to three days for diesel-electric boats.
• Indigenization of AIP: 
  • DRDO had achieved an important milestone in the development of Air Independent Propulsion (AIP) System by proving the land-based prototype.
  • India is developing AIP based on fuel cells. These cells convert chemical energy into electrical energy, recharging the batteries of the submarine.
• Around 10 countries have developed or are close to building AIP technology, and almost 20 nations have AIP submarines.

News Source: The Hindu