Here is part of Vincent Groizeleau's Mer et Marine (all the maritime news), September 27, 2019, article (which I have translated from French and bolded some bits) at https://www.meretmarine.com/fr/content/sous-marins-le-nouvel-aip-de-naval-group-tient-ses-promesses :
“Submarines: Naval Group's new AIP delivers on its promises”
Naval Group's development of an Air Independent Propulsion (AIP) system for submarines based on a new generation of fuel cells passed a crucial milestone this year. The ground system of the Naval Group's, installed at its Indret site, close to Nantes [in France], indeed worked for 18 days in conditions representative of a real patrol. "It was a question of closely checking the functioning of the system with a profile of operational use, as well as the autonomy of diving. For 18 days, the system, connected to batteries (to be representative of actual use on a submarine) produced electricity from diesel fuel, operating at high pressure, which means that it does not limit diving depth.
[The were simulations of] Transits, very slow speeds, accelerations, fast surface rises, stops of the AIP followed by restarting, management of a possible breakdown or damage ... As on a real submarine, the system was confronted with situations to validate its performance and reliability.
...Called AIP FC-2G (Fuel Cell Second Generation - FC-2G), this system has required a decade of research. Technologically, it includes two main innovations. First, the hydrogen used for fuel cell operation is manufactured directly on board, via a chemical reforming process from diesel fuel used by diesel-generators. This process provides increased security through the absence of hydrogen storage on board. The exhaust fumes are discarded discreetly, since they dissolve instantly in the water. In parallel, Naval Group has designed a patented system to produce air by injecting nitrogen into the oxygen inlet of the cell, this oxygen is stored in cryogenic form in a tank. Synthetic air reacts with hydrogen in the fuel cell to produce electricity, which powers the submarine's batteries and the electric propulsion motor. The battery is confined in a closed and ventilated enclosure to control a possible leakage of hydrogen or oxygen.
1: the reformer is used to produce hydrogen from diesel fuel
2: the component is used to increase the hydrogen yield and eliminate the carbon monoxide produced
by the reformer
3: Purification membranes feed fuel cells with ultra-pure hydrogen
4: Fuel cells produce electricity from hydrogen and oxygen
5: the tank stores oxygen in liquid form
...Standalone operation or in tandem with batteries
Designed for a lifespan of 10,000 hours, the fuel cell has a capacity of 250 kW. It feeds the electrical plant when the submarine is in AIP mode, thus allowing the electric propulsion motor to run and provides for the [submarine's hotel load power needs]. The submarine can rely on this AIP system alone at a speed of up to about 5 knots, knowing that to obtain a range of three weeks in diving, the use profile varies between 2 and 4 knots. To do this, the power required for the AIP FC-2G is between 130 and 180 kW. Beyond that, you have to tap into the batteries, the passage from one energy source to another being done without interruption.
Many advantages over other systems
Architecturally, the Naval Group AIP FC-2G has a considerable advantage over existing AIP systems (that store hydrogen in external tanks, posing weight problems (ie. 130 to 160 tons of hydrides of which less than 2% is usable hydrogen) and other AIP systems refueling problems). Fuel cells currently in service use, in addition, pure oxygen, which generates high wear, with filters and membranes needing to be replaced very regularly. AIP FC-2G is more efficient, according to Naval Group, as AIP FC-2G offers (according to its designers) duration of use between each major maintenance approximately five times higher than that of its foreign competitors. "Our system really fits into the operational scheme of a submarine. It only requires a short interruption of three weeks each year - the only equipment to be changed during this maintenance period being the catalysts. In the meantime, there is nothing to do! Says Marc Quémeneur [AIP product manager at Naval Group].
Finally, the choice of hydrogen production from diesel fuel facilitates refueling and storage (single fuel on the submarine) while improving safety compared to systems using, for example, methanol, which are more flammable and introduce toxic products in case of leakage.
The device fully integrated in a standalone module
Like MESMA, (the first AIP developed by the French group and which equips Pakistan’s Agosta 90Bs) the entire AIP FC-2G system is grouped in a single module, autonomous from the rest of the submarine. The system is housed in a hull section about 8 meters long integrated into the submarine from the beginning or added after overhaul with a minimum of modifications to the submarine. AIP FC-2G is adaptable to submarines with a diameter of at least 6 meters, such as Scorpene or the conventional propulsion version of the Barracuda. AIP FC-2G is obviously designed not to impact overall performance, starting with diving depth and acoustic discretion.
...AIP FC-2G is therefore "ready to be marketed" and already proposed by Naval Group to a number of navies.
...While AIP FC-2G has been tested with traditional lead-acid batteries, Naval Group is also working on integration with submarines using lithium-ion battery technology, which will expand operational performance. ENDS
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See Vincent Groizeleau's full Mer et Marine report, with more illustrations, here https://www.meretmarine.com/fr/content/sous-marins-le-nouvel-aip-de-naval-group-tient-ses-promesses
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Pete Comment
Naval Group may well have timed this article with a view to India's Project 75I competition for 6 submarines with AIP. Also the Dutch Walrus submarine replacement competition may require AIP. Australia, building 12 conventional propulsion versions of the Barracuda (known as the Attack class) may also decide on AIP for the first or later batches.
Current users of Naval Group Scorpenes (Chile, Malaysia, India and Brazil) might perhaps also decide to retrofit their Scorpenes with AIP FC-2G.