It seems a decision has been taken by the Indian Navy (IN) to install an air-independent propulsion (AIP) system developed by the Naval Materials Research Laboratory (NMRL), Ambernath, which is a part of the Defence Research & Development Organization (DRDO), on all six of its Scorpene class diesel-electric submarines once they become due for their first major refit. This decision is of course contingent upon NMRL’s AIP being fully proven by then as a ‘safe plug’. Given that the refit of even the first Scorpene Class submarine, the INS Kalvari, is not due for at least another six years, NMRL is confident of having refined its already advanced prototype sufficiently by that point, to satisfy IN’s requirements.
The earlier plan of installing NMRL’s AIP on the last two Scorpenes has been given up since NMRL’s AIP plug was not deemed sufficiently mature by the original equipment maker (OEM) of the Scorpene Class, France’s Naval Group, once known as DCNS. As such, it is felt by various stakeholders that it is now too late to install the AIP on the last two Scorpenes which are also at an advanced stage of construction.
However, IN has decided that even though the process of installing the AIP plug during refit will involve cutting open the hull of its Scorpene units, it would still be worth it, given that NMRL’s AIP boasts attractive features. In fact, NMRL believes that its Phosphoric Acid Fuel Cell (PAFC) based AIP solution is a step ahead of what the People’s Liberation Army Navy has obtained from Sweden in the form of a Stirling cycle based AIP for its submarines.
Original Equipment Manufactures of AIP System never usually advertise their underwater continuous usage capability but it is usually around two weeks of usage after which Submarine needs surfacing “.
Record set by a Type 212A German Submarine using a Siemens proton exchange membrane (PEM) compressed hydrogen fuel cells based Air-independent propulsion (AIP) system was for 18 days”. he further added ” If DRDO has achieved 14 days endurance with its Phosphoric acid fuel cell (PAFC) based Air-independent propulsion (AIP) system then we are par with Global leaders in AIP System ” .
Moreover, NMRL’s modular architecture scores over a composite system, since even if one of the modules fails, the control system for the PAFC stacks can reconfigure the remaining operational units to continue to supply power output, albeit at a reduced quantum. This naturally increases the survivability of the system, which is of utmost importance when being used to propel a submarine stealthily.
DRDO developed PAFC powered AIP module technology already has been transferred to Thermax Ltd in Pune for further production even though first Scorpene submarines to get indigenous AIP System will happen only in 2021-22 when they come for their first refit after which all Six submarines will get AIP module.
Navy also plans to use DRDO developed AIP System for Six next generation Diesel-Electric submarines which will be procured under Project-75I (India) soon. DRDO also has given hints that it will also start work on next-generation AIP technology which can be incorporated in future Indian-built submarines under Project-76.
DCNS’s MESMA AIP vs DRDO’s Fuel Cell AIP System
The MESMA AIP from DCNS mimcs nuclear propulsion. It uses heat generated through combustion of ethanol (grain alcohol) and stored oxygen at a pressure of 60 atmospheres to power a conventional steam turbine power plant. It doesn’t use fuel cells.
The DRDO AIP uses fuel cell technology. A fuel cell converts chemical energy from a fuel into electricity through a chemical reaction of positively charged hydrogen ions with oxygen or another oxidizing agent.
There are many fuel cell types, but the principal ones include the alkaline fuel cell (AFC), proton exchange membrane (PEM) fuel cell, direct methanol fuel cell (DMFC), molten carbonate fuel cell (MCFC), phosphoric acid fuel cell (PAFC), and solid oxide fuel cell (SOFC).
DRDO’s AIP uses PAFC, which has some advantages over other fuel cell types. PAFCs offer a broader choice of fuel being more tolerant of fuel impurities. It can operate using reformed hydrocarbon fuels (Methanol) or biogas.
PAFC operating temperatures (150 to 200 °C) are higher and as a result they generate steam as byproduct, besides electric power for propulsion. The steam can be used to meet other heating requirements, pushing up operating efficiency to as high as 70%.
DCNS initially offered its first generation MESMA AIP with the Scorpene, but the Indian Navy opted to go with a second generation DRDO developed 300-t AIP based on fuel cell technology. DCNS has since developed and demonstrated to Indian journalists a fully tested second generation operational-scale fuel cell AIP that would allow the Scorpene to stay submerged for 3 weeks, but the Indian Navy hasn’t been swayed from its decision to go with the DRDO AIP. (Without AIP, Scorpene can stay submerged for 4 days. With a MESMA AIP, it would be able to stay submerged for 2 weeks.)
One good reason why the Navy has stuck with the DRDO AIP is because it could later be reconfigured for the submarines to be acquired under Project 75I. Also, the four Scorpenes launched earlier could be retrofitted under an upgrade program.
Role of Air-independent propulsion (AIP) Explained
Modern Diesel-Electric submarines using their large network of batteries can remain submerged for 4-5 days at max before they are required to recharge them for which non-nuclear submarines have to come to periscope depth to carry out activity called “snorkelling” so that it can again use its Diesel engines to recharge their batteries.
The use of AIP on a diesel-electric submarine, greatly increases their underwater endurance, allowing them to continuously stay submerged for weeks without surfacing. Although the submarine eventually needs to surface to charge its batteries and their endurance is nowhere on-par with nuclear powered submarines, the vast increase in endurance offered by AIP gives them an advantage over non AIP equipped diesel-electric submarines. However AIP doesn’t give any advantage other than increased underwater advantage and it should not be assumed that AIP-equipped submarines will always defeat their non-AIP equipped counterparts.
In April 2006, a German Navy submarine U-32, equipped with a Siemens proton exchange membrane (PEM) compressed hydrogen fuel cell AIP, made a 2800 km uninterrupted underwater journey without surfacing/snorkeling! This is in stark contrast to non-AIP equipped submarines which can cover only 500-800 km before they have to surface and recharge their batteries by running noisy diesel generators. Comparatively, a nuclear-powered submarine has unlimited underwater endurance!
Again in 2013, U-32 set a record by traveling underwater continuously for 18 days without surfacing! Comparatively, a non-AIP diesel sub has an underwater endurance of just 4-6 days before it has to surface. This shows that AIP-equipped diesel-electric submarines are far more capable than their non-AIP equipped counterparts when it comes to endurance.
Source:- Delhi Defence Review And Defence.PK
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