German, French, Spanish, Reformer Fuel Cell AIP, Japanese LIBs

Advanced French AIP proposal. “Simplified” Layout of Diesel Fuel Processing Equipment (Diesel Fuel Autothempermal Reformer – SOFC). Layout and description courtesy Bakst Engineering.

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INTRODUCTION

The following is the section on AIP within an excellent article The Driving Factor In The SEA 1000 Choice The Submarine Propulsion Chain by submarine expert Rex Patrick, from Sydney, Australia. This was on pages 40 to 44 of the October 2015 issue (Volume 41, Number 8) of the Asia-Pacific Defence Reporter (APDR). APDR is an excellent magazine provided by subscription.

It was written before the Australian Government’s 26 April 2016 SEA 1000 decision in favour of the French DCNS Shortfin – but the article's comments on AIP still appear accurate.

Significantly the article is also published on the Siemens' website here. 

Pete has bolded for emphasis some words in the following AIP section and also added footnotes [1] and [2]. The footnotes indicate that the AIP section is still current and accurate. Japan and Lithium-ion Batteries (LIBs) are also discussed towards the end.

ARTICLE

AIR INDEPENDENT PROPULSION

The primary purpose of an AIP system is to convert stored reactant energy into electrical energy for the submarine’s main battery and to do so independently of the surface atmosphere. It provides little benefit during transits but is invaluable when operating slowly within an operational area.

TKMS will almost certainly offer Australia a reformer/ FC solution. Being a large submarine, it will demand two reformers and four 120 kW FCs.

The first element of TKMS’ AIP solution is a Methanol reformer that extracts hydrogen from methanol and feeds it directly into the FC. Methanol is selected because of its worldwide availability, high hydrogen content, low reforming temperature (250°C), reformation ease and high reforming efficiency (80 to 90%). LOX is also used in the reforming process. Sub-system waste is pressurised CO2 which can be discharged to sea down to full diving depth. The reformer is packaged in an enclosure with its own special ventilation system for cooling.

Each reformer is capable of producing enough hydrogen to supply two fully loaded 120 kW cells. It removes the need to store hydrogen on board, which is problematic from a supply availability and refuelling complexity perspective, and is also difficult on 2000+ tonne submarines because of the weight of the hydrogen’s metal hydride storage bottles.

The reformer has a two to three hour start-up time. Operationally, the idea is to start it up in the patrol area in block periods where AIP can be exploited, potentially for weeks on end, dependant on the amount of reactant stored on board.

The reformer has been in development since 1995 and a test site has been in operation for a decade, with FCs connected to it since 2010. A reformer suitably packaged for installation on board submarines is currently undergoing set-to-work in Kiel. Whilst the reformer has not been fielded on a submarine yet it is at the test bed state and therefore it attracts a low SEA 1000 project risk label.

Moving to the FC, TKMS will offer the second generation Siemens 120 kW Polymer Electrolytic Membrane (PEM) FC. The PEM FC works by feeding standard industry-grade LOX and high purity hydrogen into the cell which generate electricity in response. It does this silently and at a low temperature (80°C). It is different to a battery in that it stores no charge; it simply generates electrical energy so long as the reactants are fed into the cell. The cell is extremely (fuel burn) efficient at between 50 and 70%. Its ‘waste’ outputs are potable water, which is fed into holding tanks, and (1%) oxygen, which is fed into the submarine’s atmosphere to assist in maintaining breathable air during prolonged AIP dived periods.

The FC has been under development by Siemens since the early eighties. It was first trialled on a German Type 205 test submarine in 1988 and then contracted for supply into the German and Italian Type 212 program. The first production FC went to sea in 2002 and it is now a very mature system at sea on 24 submarines, meaning it is a minimal project risk component of the German SEA 1000 solution.

The reformer/FC system meets all of the fundamental requirements of an AIP system; high efficiency, silent, low magnetic signature, light and compact, generates no pollution or heat, reliable, relatively easy to maintain and requiring no additional operating personnel.

It is interesting that the German Defence Department funded TKMS starting in 2007 to conduct a methanol vs diesel reformer comparison, because the diesel reformer approach would negate the need for storage of an additional fuel, methanol, on board the submarine. TKMS built a small 10 kW diesel reformer to support the study. The study conclusions were instructive. The diesel reformer was less efficient because diesel has a hydrogen to carbon ratio of only two to one, whereas methanol has a hydrogen to carbon ratio of four to one. The diesel reformer also needs to run at around 850 degrees which implies heat inefficiency as compared methanol. The higher temperature also means a longer start-up time than the methanol reformer. Finally, unless the diesel carried by the submarine is sulphur free, and standard diesel is not, the required sulphur purifier at the reformer output would likely take up considerable space (as big as the reformer itself). The idea was abandoned.

Public domain information shows DCNS [now called Naval Group] have abandoned their MESMA AIP solution on the Pakistani Agosta 90s and will use a diesel reformer/FC solution on the Shortfin Barracuda. [No supporting detail seen to date.]

Further on the French FC, it appears as though two options are on the table; a PEM or Solid Oxide FC (‘SOFC’) type. If a SOFC is chosen, noting they offer good energy conversion efficiency, long life and operating cost advantages, other drawbacks need to be addressed. Most of these drawbacks relate to the high 600 to 1000°C operating temperature which brings hot exhaust issues and brittleness related shock resistance problems.

Novelty, complexity and uncertainty put this solution’s inclusion in the French package as high risk. Even if the technical challenges of the diesel reformer and FC are solved, the enemy of the DCNS development will be schedule. DCNS are believed to have started their reformer/FC work back around 2006/7 and announced it as a future solution in 2008 as part of their SMX 24 concept design. [1] It is instructive that the Germans have developed and perfected their reformer/FC solution over four decades. It is also worthy of note that the Spanish have had issues with their S-80 submarine ethanol reformer/FC solution and have announced that the first S-80 will now be fitted-for-but-not-with AIP.[2]

The Japanese will not offer up an AIP solution, rather fill any potential AIP space with additional Li-Ion batteries. It is believed this decision stems from their experience with the inefficiency of the Swedish origin Stirling AIP solution. All things considered with respect to reported Stirling engine maintenance overheads and the lack of differential between the Stirling energy density and the Li-Ion energy density, the decision is likely valid.

However, the energy-density differential between the DCNS and TKMS FC and the Japanese Li-Ion’s is large, giving the Japanese solution a poorer indiscretion ratio than the Europeans’ FC approach. It is known that the Japanese originally approached TKMS about adopting their FC AIP solution, but the adoption of the German technology was problematic for two reasons; firstly, the reformer necessary for the larger Soryu submarine was not mature at the time and TKMS/Siemens were not inclined to transfer knowledge of what they considered to be the ‘crown jewels’ of their submarine program. Whilst Li-Ion may have advantages in transit situations, which is why the French and Germans have Li-Ion as part of their solutions, the FC provides the advantage where it really counts; in the operational area. Whilst an all Li-Ion Japanese solution may have advantages with respect to transiting, it means little if the boat is then sunk upon arrival in its assigned patrol area.

FOOTNOTES

[1] The possibility of French DCNS progress is recorded in a DCNS article of 13 October 2016 for Euronaval 2016: “FC2G AIP – Fuel Cell: Second-generation Air-Independent Propulsion. DCNS has developed the 2nd generation AIP system using fuel-cell technology. FC2G AIP provides the best possible dive autonomy in total safety and easy support.”

[2] Spain's problems are covered in IHS Jane’s article of 24 January 2017, which indicates: “Spain's first S 80-class submarine will not be fitted out with [AIP] as development of the system will not be ready in time, according to the admiral in charge of Maritime Action (Almart)...He also said he was not sure which of the four new boats would be the first to be fitted with the AIP system.”